D DIM COURSES

APPLIED THERMOLUMINESCENCE DOSIMETRY

Edited by M Oberhofer and A Scharmann

Published for the Commission of the European Communities by Adam Hilger

As the use of nuclear energy increases, so too does the need for methods of radiation detection and dose assessment for a variety of purposes, the most important being personnel dosimetry and environmental monitoring.

Thermoluminescence dosimetry (TLD) is an important technique in these areas and has also found application in a wide range of different fields in medicine and biology, and industry and archaeology.

The present volume, intended to become the new standard reference on TLD and its applications, arose out of two courses held at the Joint Research Centre, Ispra, in 1977 and 1979. The edited texts of twenty lectures given by sixteen leading experts in TLD are presented. The book is divided into two parts, part I dealing with fundamentals and part II with applications.

Part I contains chapters on the historical development of TLD; theory; instrumentation; materials and their properties; measurement; and comparison of TLD to other solid state methods in dosimetry. Part II covers areas of application of TLD including personnel dosimetry; environmental monitoring; neutron dosimetry; glow-curve analysis; medicine; biology and related fields; high-level photon dosimetry in industry; reactor engineering; archaeology; and dose standardisation and intercomparison. An appendix is also included which explains the system of units adopted recently in radiation and dosimetry.

Applied Thermoluminescence Dosimetry

Ispra Courses

Applied Thermoluminescence Dosimetry

Lectures of a course held at the Joint Research Centre, Ispra, Italy, 12-16 November 1979

EDITED BY M OBERHOFER AND A SCHARMANN

Published for the Commission of the European Communities by

Adam Hilger Ltd, Bristol

EUftOP. Blblfoth.

© ECSC, EEC, EAEC, Brussels and Luxembourg 1981

Published for the Commission of the European Communities, Directorate-General Information Market and Innovation, Luxembourg.

EUR 6990 EN

LEGAL NOTICE

Neither the Commission of the European Communities nor any person acting on behalf of the Commission is responsible for the use which might be made of the following information.

British Library Cataloguing in Publication Data Applied thermoluminescence dosimetry.

1. Thermoluminescence - Congresses I. Oberhofer, M II. Scharmann, A III. Commission of the European Communities 5 35'.35 QC479

ISBN 0-85274-544-3

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior permission of the copyright holder.

Published by Adam Hilger Ltd, Techno House, Redcliffe Way, Bristol, BS1 6NX.

The Adam Hilger book-publishing imprint is owned by The Institute of Physics.

Printed in Great Britain by J W Arrowsmith Ltd, Bristol.

Lectures of a course held at the Joint Research Centre of the Commission of the European Communities, Ispra (Varese), Italy, in the framework of Ispra Courses.

Contents

List of contributors ix Preface xi

Part I: Fundamentals

1 History 3

A SCHARMANN

2 Theory

M BOHM AND A SCHARMANN 11

11 11 16 18 21 24 26 30 32 36

39

39 40 48 52 53 64

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

Introduction Excitation by radiation Thermal excitation and recombination Phenomenological analysis Kinetic models Determination of trap parameters Additional parameters Computer simulation Comparison with experiment Conclusions

3 Instrumentation

H W JULIUS

3.1 3.2 3.3 3.4 3.5 3.6

Introduction The heating system The light detecting system Special items TLD readers and systems Address list

vi Contents

4 Accessory instrumentation 67

M OBERHOFER

67 67 69 70 71 74 74 75 75 76 77 79 80

83

83 83 86 88 89 91 93 94 95

5.10 Tribothermoluminescence (or triboluminescence) 95

6 Preparation and properties of principal TL products 97

G PORTAL

97 97

106 109 111 115 118

123

123

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13

Introduction Heating planchets Gas flushing Reference light sources Powder dispensers Mechanical tweezers Vacuum tweezers Sieves Ultrasonic cleaners Annealing furnaces Annealing stands Irradiators Literature

5 General characteristics of TL materials

G BUSUOLI

5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

Introduction Linearity Response to photons Response to beta rays Response to neutrons Fading Annealing procedures Stability and reproducibility Dose rate dependence

6.1 6.2 6.3 6.4 6.5 6.6 6.7

Introduction Lithium fluoride Lithium borate Beryllium oxide Calcium fluoride Calcium sulphate Aluminium oxide

7 Operational aspects

D F REGULLA

7.1 Introduction

Contents vii

7.2 Parameters affecting precision 124 7.3 Conclusion 140

8 Precision and accuracy of TLD measurements 143

G BUSUOLI

8.1 Introduction 143 8.2 Definitions 143 8.3 Assessment of random and systematic uncertainties 143 8.4 Sources of errors in TLD 145 8.5 Precision of TL measurements 146 8.6 Accuracy of TL measurements 149 8.7 Accuracy in low-dose measurements 150

9 Reference to other solid-state methods 151

E PITT AND A SCHARMANN

9.1 Introduction 151 9.2 Radiophotoluminescence (RPL) 153 9.3 Colouring 155 9.4 Photographic processes 156 9.5 Stimulated exoelectron emission 157 9.6 Track detection 159 9.7 Change of resistance in silicon diodes 161 9.8 Scintillation dosemeter 163 9.9 Conclusions 163

Part II: Applications

10 Application of TLD to personnel dosimetry 167

E PIESCH

10.1 Introduction 167 10.2 Performance specifications 168 10.3 Detector materials and specific requirements 170 10.4 Personnel dosemeter systems 177 10.5 Special applications 182 10.6 Future trends 192

11 Application of TLD systems for environmental monitoring 197

E PIESCH

11.1 Introduction 197 11.2 Performance specifications 198 11.3 Properties of commercial TLD systems 198

viii Contents

11.4 Calibration technique for dosemeter batch and reader 214 11.5 Reproducibility and overall uncertainty of measurement 219 11.6 Interpretation of field exposures 220 11.7 Practical application 224

12 Applications of TL materials in neutron dosimetry 229

J A DOUGLAS

12.1 Introduction 229 12.2 Neutrons and dosimetry 229 12.3 Thermal neutron detectors 232 12.4 Intermediate and fast neutron dosemeters 241 12.5 Possible future developments 253

13 Glow-curve analysis 259

A C LUCAS

13.1 Introduction 259 13.2 Recording of glow curves 259 13.3 Measurement of neutron dose equivalent 261 13.4 Beta-ray measurement 265 13.5 Fading correction 266 13.6 Determination of time from exposure 268 13.7 Verification of data 269

14 Application of TLD in medicine 271

A F McKINLAY

14.1 Radiotherapy measurements 271 14.2 Diagnostic radiology measurements 271 14.3 Factors in the choice of dosemeters for clinical use 273 14.4 Radiotherapy absorbed dose measurements 279 14.5 Examples of the use of TL dosemeters in radiotherapy 283 14.6 Diagnostic radiology absorbed dose measurements 284

15 Application of TLD in biology and related fields 289

M OBERHOFER

15.1 Introduction 289 15.2 Animal experiments 289 15.3 Bone dosimetry 290 15.4 Photon radiation quality measurements 291 15.5 Toxicity determinations 292 15.6 General biology and biochemistry 293 15.7 Ecology 293 15.8 Animal habit studies 295

Contents ix

16 High-level photon dosimetry with TLD materials 297

M OBERHOFER

16.1 Introduction 297 16.2 Lithium fluoride 298 16.3 Lithium borate 306 16.4 Calcium fluoride 308 16.5 Other TLD phosphors 308 16.6 Final remarks 310

17 Application of TLD in reactor engineering 315

JRALAKEY

17.1 Introduction 315 17.2 A survey of the application of TL in reactor environments 316 17.3 Application to neutron dosimetry 327 17.4 Environmental monitoring 331 17.5 Miscellaneous applications 333 Appendix 17.1 Calculation of gamma photon absorbed dose 333 Appendix 17.2 Cavity ionisation theory 337 Appendix 17.3 The intrinsic TL response per absorbed neutron 340

18 Application of TLD for dating: a review 347

G A WAGNER

18.1 Introduction 347 18.2 Dating method 347 18.3 Dating applications 352 18.4 Conclusion 355

19 TL dating: techniques and problems 361

M J AITKEN

19.1 Introduction 361 19.2 Application 365 19.3 Recent research and outstanding problems 369

20 Application of TL dosemeters in dose standardisation and intercomparison 383

G SCARPA

20.1 Introduction 383 20.2 Dissemination of standards 383 20.3 Direct intercomparison methods 384 20.4 Characteristics of TL dosemeters used for mailed intercomparisons 386 20.5 Practical examples of mailed intercomparisons 386 20.6 Conclusions 390

x Contents

Appendix The new radiological (si) units and their conversion to the units previously used 39 j

Index 3 9 3

List of contributors

Dr M J Aitken Research Laboratory for Archaeology and the History of Art, Oxford University, 6 Keble Road, Oxford 0X1 3QJ, UK

Dr M Bohm Justus-Liebig-Universitat Giessen, I Physikalisches Institut, Heinrich-Buff-Ring 16, D-6300 Giessen, FRG

Dr G Busuoli Comitato Nazionale per l'Energia Nucleare, Laboratorio Fisica Sanitaria, Via Mazzini, 2,1-40138, Bologna, Italy

Dr J A Douglas Environmental and Medical Sciences Division, AERE, Harwell, Oxfordshire 0X11 ORA, UK

Dr H W Julius Radiologische Dienst TNO, Utrechtsweg 310, N-6812 AR, Arnhem, The Netherlands

Professor J R A Lakey Department of Nuclear Science and Technology, Royal Naval College, Greenwich, London SE10 9NN, UK

Dr A Lucas Crystal and Electronic Products Department, The Harshaw Chemical Company 6801 Cochran Road, Solon, Ohio 44139, USA

Dr A F McKinlay National Radiological Protection Board, Chilton, Didcot, Oxfordshire 0X11 ORQ, UK

xii List of contributors

Dr M Oberhofer Commission of the European Communities, Joint Research Centre, Ispra Establishment, Applied Dosimetry Research, 1-21020 Ispra (Varese), Italy

Dipl. Phys. E Piesch Kernforschungszentrum Karlsruhe GmbH, Hauptabteilung Sicherheit, Postfach 3640, D-7500 Karlsruhe 1, FRG

Dr E Pitt Justus-Liebig-Universitat Giessen, I Physikalisches Institut, Heinrich-Buff-Ring 16, D-6300 Giessen, FRG

Dr G Portal Commissariat a l'Energie Atomique, Institut de Protection et de Surete Nucleaire, Departement de Protection, BP No. 6, 92260, Fontenay-aux-Roses, France

Dr D F Regulla Gesellschaft fur Strahlen- und Umweltforschung mbH Miinchen, Institut fur Strahlenschutz, Ingolstadter Landstrasse 1, D-8042 Neuherberg, FRG

Professor G Scarpa Comitato Nazionale per l'Energia Nucleare, Centro di Studi Nucleari della Casaccia, Laboratorio di Dosimetria e Biofisica, SJ?. Anguillarese km 1 + 300,1-00100 Rome, Italy

Professor Dr A Scharmann Justus-Liebig-Universitat Giessen, I Physikalisches Institut, Heinrich-Buff-Ring 16, D-6300 Giessen, FRG

Dr G A Wagner Max-Planck-Institut fiir Kernphysik, Abt. Kosmochemie, Saupfercheckweg 1, D-6900 Heidelberg 1, FRG

Preface

With the ever increasing use of nuclear energy, particularly for power production, there is more and more need for radiation detection and dose assessment for a variety of pur­poses. Two of these are environmental dose control and personnel dose determination. These types of measurements are essential for ensuring the radiological safety of the population as a whole and of individual radiation workers. Additionally they may have great importance with respect to the legal aspects of nuclear energy.

Many radiation detectors and measuring devices have been developed over the last few decades and some are being used routinely for environmental and personnel dose control. One of them is based on the fact that some materials emit light when heated after exposure to radiation. This technique is known as thermoluminescence dosimetry (TLD). Because of its simplicity and suitability for automation much research and development work has been put into this type of dosimetry, which has also turned out to be useful in fields other than radiation protection.

The results of this research and development have been published in different scientific journals, in various conference proceedings (like the proceedings of the International Luminescence Meetings at Palo Alto, Gatlinburg, Riso, Krakow and Sao Paulo in 1965, 1968, 1971, 1974 and 1977, respectively), in some books on radiation protection and solid-state dosimetry and in the only bibliography on the subject by J R Cameron, N Suntharalingam and G N Kenney, which was published in 1968 by the University of Wisconsin under the title Thermoluminescent Dosimetry.

According to the authors of this latter publication the book was 'designed to be a comprehensive introduction to the technique giving much useful information as to instrumentation, phosphor characteristics and applications'.

For many years this was considered as the standard reference book on the subject and hence made use of by nearly all students and newcomers to the field. However, for some years now there has been an increasingly felt need for an up-dated version of the book, which sadly has not been produced.

Thus the idea was born at the Joint Research Centre (JRC) of the Commission of the European Communities, Ispra Establishment, to collect all available material in the field of TLD by organising a course.

Such a course was held within the framework of the Education and Training Program of the JRC, 14-18 November 1977 in collaboration with the I Physikalisches Institut of the Justus-Liebig-Universitat, Giessen. From the outset this Institute has contributed to the understanding of the phenomenon of TL and to the development of TLD and is still today actively engaged in many aspects of TLD research. Thirteen outstanding experts in

xiv Preface

the field of TLD agreed to present the latest state of the art. The course was such a success that it was decided to repeat the course with the aim of perfecting the material with regard to its content and presentation so that it would be suitable for later publication. Only minor refinements in the course program needed to be made, including the addition of lectures on subjects which had been missing in the first course, in order to have a com­plete treatment of the field. The second course was held at the JRC, Ispra, 12-16 Novem­ber 1979 and the contents now seemed to be worth presenting to a much larger audience than the one which attended the course.

This book contains all the lectures given at the courses in sequence of their original presentation with some changes in order to avoid too much overlap and repetition, which understandably could not be eliminated completely. This was also not desirable in order not to lose the independent character of each chapter. A number of cross-references have been inserted into the texts to give maximum information on certain aspects of TLD.

The book starts with the historical development of TLD in chapter 1. In this first chapter the reader's attention is drawn to the fact that TL is a widespread phenomenon which has been known for a very long time. Of 3000 minerals, for example, three-quarters exhibit this effect. The use of TL as a means for measuring radiation exposures or doses actually started from the observation that many minerals exhibit natural TL and also from the known uv sensitivity of manganese-activated calcium sulphate. In the late 1940s much effort was put into the development of suitable TL dosemeters, mostly for military purposes, and by 1950 many of the TL phosphors presently in use had already been dis­covered and/or rediscovered for dosimetric applications. During the 1960s a second generation of materials became available and a wide variety of commercial TLD systems were developed to more and more sophisticated levels, taking advantage of rapid progress in computer technology.

In chapter 2 an attempt is made to treat the phenomenon of TL theoretically on the basis of general physics and with the help of the energy band model of solids. It is shown that TL intensities can be simulated without any assumptions, but that other properties of the material of interest need to be studied as well in order to obtain an overall picture of the electronic processes occurring in the solid.

TLD instrumentation is the subject of chapter 3. The rather simple experimental devices used for measuring the TL from various phosphors at the very beginning of TL work have been developed into very sophisticated computerised TL readers which allow hundreds or even thousands of TL dosemeters to be read fully automatically and also include data processing. The most important common components of such a TL reader system are described and examples of commercial instruments are given without going into details of the electronics. The chapter ends with an address list of TLD instrumentation manufacturers.

Besides some basic instrumentation, TLD work requires a number of accessories, like powder dispensers, annealing stands, furnaces, etc, which are the subject of chapter 4. This completes the instrumental aspects of TLD.

Chapter 5 is dedicated to the general requirements to be fulfilled by TLD materials when intended to be used as dosemeters, in particular when worn in personal dosemeters. Among other requirements, those discussed are the sensitivity of the phosphor, its energy dependence, fading characteristics and the reproducibility obtainable.

There then follows, in chapter 6, a detailed description of each single phosphor of current interest, starting in each case with a short historical review of its actual role in the

Preface xv

field of TLD. In this chapter the reader is informed about the preparation of the phosphor and its thermal treatment afterwards, specific models are used to explain the physical properties like glow curve and emission characteristics, and the actual dosimetric proper­ties are discussed.

The aim of chapter 7 is to make the user of the materials described in chapter 6 familiar with the possibilities and limits of TL dosimetry. Experimental results obtained with currently used techniques are reported and analysed to show the sources and magni­tudes of errors. The author of this chapter shows that, besides the dependence of TL on the energy and direction of the incident radiation, operational features such as the read­out device, reading geometry, annealing cycle and dosemeter handling technique may be major factors influencing the results. At the same time suggestions are made how to minimise these effects in order to achieve maximum reliability in TL dosimetry.

While the main object of chapter 7 is to make the student familiar with sources of error in TLD measurements, chapter 8 was written to clarify the difference between accuracy and precision, to give examples and to show how error analyses are dealt with mathematically.

Before concluding the first part of the book, reference is made in chapter 9 to other solid-state methods such as radiophotoluminescence, coloration effects, the photographic effect, exoelectron emission, track detection, neutron-induced defects and scintillation which are used with success in the field of dosimetry. It turns out that, compared to all the other solid-state methods, TLD is the best developed system.

Summarising, one can say that most of the fundamentals of TL which are needed for a reasonable understanding of this phenomenon, its suitability and its advantages for dosi­metric applications are discussed in the first part of the book.

The second part of the book has been compiled with the idea of showing where TL already has been and is being applied successfully. Personnel dosimetry is such a field of application. The near-tissue equivalence for the detection of photons, the low fading and the high accuracy of a number of TL materials, coupled with the possibility of evaluating a large number of dosemeters using automatic reader systems, proved to be very advant­ageous. In chapter 10 the particular requirements for this field of application are discussed and a comprehensive survey of TLD systems and dosemeter designs for routine personnel dose control are given. Future trends of the development in this special field of applica­tion are indicated.

Environmental monitoring is another application of TLD where interest is growing rapidly. This subject is treated in chapter 11. TL dosemeters with low fading charac­teristics, low zero reading and high accuracy are very well suited and mostly much cheaper than other systems for monitoring the natural radiation background level and short-term or long-term influences of nuclear installations. Proper individual calibration of the dose­meters is of prime importance if high accuracy is to be achieved. Thus this chapter con­tains a short section on calibration techniques.

Much work has also been put into the assessment of neutron doses using TL phosphors. A review of this work is given in chapter 12.

Some knowledge of the complex processes involved when neutrons interact with matter is required for a good understanding of the neutron response of TL materials. The chapter therefore starts with a categorisation of the neutron reactions involved, a dis­cussion of the appropriate parameters necessary to monitor the effects of neutrons and some definitions of terms used throughout the chapter. The response of common TL

xvi Preface

materials to thermal neutrons and the factors affecting the measurement are dealt with in more detail than in earlier chapters.

Methods of separating the- neutron component in a mixed radiation field and of pro­ducing a high thermal neutron response by mixing a phosphor with non-luminous 6Li salt are discussed. Furthermore, techniques for increasing the intermediate and fast neutron response of phosphors by the use of proton radiators, fission foils and moderators are surveyed. The practical applications and limitations of these techniques in dosimetry are assessed and the feasibility of using the neutron activation of a constituent of a phosphor for dosimetry in therapy work or activity accidents is examined.

Finally, possible future developments in fast neutron dosimetry are considered. In all cases treated in the last three chapters valuable additional information on the

type of radiation or the time since exposure, for instance, may be obtained from the glow curves. The importance of this topic is underlined by the insertion into the course of a lecture on glow-curve analysis, which is reproduced in chapter 13. It is shown there that various analytical methods exist for complementing internal TL data with glow-curve shape analysis. The chapter enumerates five such methods.

Many advantages of TLD over other dosimetric methods have favoured its application in the medical field, where TL dosemeters are often preferred to ionisation dosemeters, for example, mainly due to their small size and thus ease of placing them singly or in large numbers within body cavities. The extent to which TL dosemeters are used today in medicine is shown in chapter 14.

Biology is another field in which TLD has been applied successfully from the beginning. Although this type of application was not included in the course the subject is treated briefly, together with related ones, in this book in chapter 15 for the sake of completeness.

The possibility of assessing very high doses of 106R or more with TL phosphors sug­gests their application for certain dosimetric problems in technological fields (material testing, electrical component testing, radiation sterilisation, etc), in the chemical indus­tries (radiation chemistry, cracking of hydrocarbons, polymerisation, vulcanisation of rubber, etc) and in food processing, for example. In chapter 16 an attempt has been made to collect TL data relevant to high dose assessment. Some other TL-related phosphor features, which in some cases may be useful for high dose measurements, have been included in the text of chapter 16.

There have been a surprisingly large number of applications of TLD in the field of reactor engineering. The published articles are widely dispersed in the literature and so are not readily available. This is why this topic is summarised in chapter 17. Many practical examples are described, such as reactor shield testing, fast reactor core measurements, reactor gamma-heat measurements, problems associated with reactor neutrons, accumu­lated activity transfer studies, etc. For a better understanding of certain peculiarities associated with TLD in reactor engineering three appendices are added. These deal with the calculation of gamma photon absorbed dose, an introduction to the cavity ionisation theory and a short description of the intrinsic TL response per absorbed neutron and its calculation.

In recent years a number of new 'atomic' tools, like radiocarbon dating, neutron activation analysis and neutron radiography, have enabled archeologists to reveal new data about ancient civilisations. A 'dating' method has been developed in parallel with these, which takes advantage of the fact that many materials exist which show TL when exposed

Preface xvii

to radiation. Materials such as rocks, ceramics, slags, bones and meteorites, for instance, can acquire significant levels of TL from 'natural' radiation. By measuring this TL the radiation dose can be determined and from that the age of the object can be obtained. This is the theme of the next two chapters which contain the contents of two unco­ordinated lectures. Chapter 18, which was presented in the 1979 course, gives a summary of the subject for the interested reader, while chapter 19, which was in the 1977 course, gives the more specialised reader full details of the various techniques and their associated problems. Although there is some overlap and repetition, the editors do not consider this a drawback bearing in mind the much higher information content of both works taken together.

The book concludes with a chapter on the applications of TL dosemeters in dose standardisation and intercomparison. This is a field where TL detectors are increasingly used as a consequence of the ease with which TL measurements can be performed and also because their small size is an advantage, especially for postal intercomparisons.

Most authors refer to literature up to mid-1978, though in some instances literature references up to the 6th International Conference on Solid State Dosimetry in Toulouse, France, from 1-4 April 1980, have been inserted into the texts during the reviewing period.

While writing their texts all of the authors were aware of the recent introduction of the Systeme International d'Unites (si) and of the adoption, by the 10th General Confer­ence of Weights and Measures, of special names for some units of this system used in the field of ionising radiation and dosimetry. In spite of this, throughout the book the 'old' units have been retained, such as the roentgen (R), the rad, the rem and the Curie (Ci). This is because those units are still widely used and will continue to be used for a while, together with the new units. They are also retained because many of the graphs selected by the authors were drawn some time ago using the old units and could not be redrawn economically and in a reasonable time using the new units. To convert from old to new units, coulomb per kilogram (Ckg-1), gray (Gy), sievert (Sv) and becquerel (Bq), the reader is referred to the Appendix.

The editors wish to thank all the authors who contributed to the courses and thus to the realisation of this book. Without their ready acceptance of the invitation to lecture at the courses and to prepare lecture notes this book could not have been published.

Furthermore, the editors would like to express their gratitude to Mr B Henry, Manager, Education and Training Program of the JRC, Ispra, who fully supported the course series and the publication of these proceedings, and to the Ispra Courses Secretarial staff for their assistance in the organisation of the courses.

We are also grateful to Adam Hilger Ltd, and in particular to its Managing Editor, Mr K J Hall, for having accepted the material of the courses for publication and for having prepared an edition, which hopefully will be widely distributed throughout the world.

Ispra Martin Oberhofer May, 1980 Arthur Scharmann

Part I: Fundamentals

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

1 History

ASCHARMANN

Even prehistoric cavemen probably observed an effect which was surely known to the medieval alchemists. Certain minerals, such as fluorite, exhibit a transient glow when they are heated in darkness [1]. This phenomenon, today called thermoluminescence, is the basis of one of the most important methods of modern integrating dosimetry.

One of the founders of modern chemistry, Sir Robert Boyle, described this effect. On 28 October 1663, he reported to the Royal Society in London the observation of a strange 'glimmering light' when he warmed a diamond in the dark of his bedroom. He said: i also brought it to some kind of glimmering light by taking it into bed with me, and holding it a good while upon a warm part of my naked body'. Boyle also did some other experiments on the shining of diamonds. He rubbed a diamond on several bodies and held it near to the flame of a candle or a piece of hot iron.

In 1705 Oldenberg [2] described the phenomenon of thermoluminescence in the mineral, fluorite. Besides this, other properties of such phosphors were also studied. In 1830 Pearsall [3] gave a description of the effects of electricity upon minerals which phosphoresce upon heating.

Henri Becquerel [4] described in his work on measurements of infrared spectra in 1883 the effect of thermoluminescence, too: 'En chauffant dans l'obscurite une substance phosphorescente a longue persistance, prealablement exposee a la lumiere, on voit la phosphorescence s'aviver, puis s'eteindre ensuite rapidement'. This means: If you heat in darkness a phosphorescent sample which was previously exposed to light, you can first see the phosphorescence becoming brighter and then being extinguished rapidly. In this work he remarked that the influence of red and infrared radiation was the same as that of a rise in temperature. In 1842, his father E Becquerel [5] had already discovered a new property of this radiation. The phosphorescence might be destroyed by red or infrared light. An explanation of these results was given by the later measurements that he carried out in the following way. Because of the influence of red and infrared radiation the stored light is released very quickly as luminescence and therefore the usually slow phosphorescence decay is no longer visible.

The fact that the spectrum consists of a part which leads to a rise in temperature was found as early as 1800 by William Herschel [6] when he performed the following experiment. He projected sunlight onto a thermometer bulb and noticed that there was a different increase of temperature in the different regions of the spectrum. The blue radiation showed only a small heating effect but, when moving towards the red, the effect became more pronounced. Even in a region beyond the visible he was able to observe maxima and minima of the heating effect.

4 A Scharmann

When it was known that there is an invisible region in the spectrum of the Sun beyond the red, the other end of the spectrum was also examined and UV radiation was discovered by its property of reducing silver salts (Ritter, Inglefield and Wollaston).

Since the time of BecquereFs first publication in 1883 on the phenomenon of lumi­nescence, several research workers, mostly physicists, have devoted their whole scientific careers to the investigation of 'fluorescence' and 'phosphorescence' [7]. The difference between the two types of behaviour lies in the decay characteristic of light emission. 'Fluorescence' has only a short lifetime, whereas 'phosphorescence' consists of a slowly decaying afterglow.

Much work was done in the field of cathodoluminescence (luminescence excited by bombardment with cathode rays), in the field of electroluminescence (luminescence excited by the application of electric fields), in the field of chemiluminescence and biological processes, in the field of triboluminescence, where luminescence is stimulated by mechanical stresses, and finally in the field of photoluminescence, stimulated by the absorption of light.

The features common to all these forms of luminescence are:

(i) the existence of some process whereby an atom, molecule, or centre is excited to higher energy states, and

(ii) the radiative transition to the ground state via the emission of a photon of appro­priate energy after a certain time delay.

As early as 1895 the physical process for the thermal release of stored radiation-induced luminescence (thermoluminescence) was used for the detection of ionising radiation by Wiedemann and Schmidt [8] in Erlangen. They irradiated a great number of minerals and inorganic compounds with cathode rays and found, among other things, that natural fluorite and manganese-activated calcium fluorite in particular show a very intense luminescence when they are heated in darkness, and there is no decay of the stored luminescence even after storage for a few weeks. Both substances are still used as thermoluminescent phosphors in solid-state dosimetry.

In the same year Rontgen [9] announced the discovery of x-rays and even in his first preliminary communication [10] he reported on the sensitivity of photographic plates to this new radiation. They enable the fixation of some phenomena and therefore a decep­tion might be better detected and 'you can take exposures with a plate covered by the paper envelope in a bright room'. Concerning the quantitative measurements he said: 'In order to get some relation between transmission and thickness of the absorbing lay:r, I took a number of exposures with photographic plates covered to some extent with tinfoil of a gradually increasing number of leafs'.

As was shown, the first measurements were done on the effect of x-rays on photo­graphic emulsions. But the influence of this radiation on thermoluminescence was reported, too. In 1925 Wick [11], from Vassar College, gave a description of the effects of x-rays in producing and modifying thermoluminescence. He observed in many substances considerable changes in TL when irradiated with x-rays. Some materials which usually showed no natural TL became thermoluminescent when excited by x-rays. He also found out that the TL after x-irradiation normally started at lower temperatures and showed higher intensities than the natural thermoluminescence of the same materials.

History 5

Today, one knows that the stability of the metastable states responsible for TL is greater for higher values of the energy necessary for the system to release charge carriers from these states. When a relatively low thermal energy is sufficient to release the charge carriers, i.e. when the observed maximum temperature is near to the excitation tempera­ture, then the charge carriers responsible for this maximum may be thermally liberated due to longer storage at the exciting temperature. Consequently, in the following glow curve this maximum cannot be observed. However, when the thermoluminescence experi­ment is performed directly after excitation this maximum can also be measured. This effect was observed by Wick but without any explanation. In 1928 he reported, together with his coworker Slattery [12], further measurements of TL in synthetic materials previ­ously excited by x-rays.

Thermoluminescence measurements in the modern sense were carried out for the first time in the Przibram Institute in Vienna by Urbach and Frisch [13]. Urbach [14] described in 1930 the luminescence of alkali halides. As well as the description of the measurement he also reported the first results of a theory. The theory for the calculation of model glow curves which is now used to estimate the trapping parameters was given in 1945 by Randall and Wilkins [15] and in 1948 by Garlick and Gibson [16]. These theories will be discussed in more detail in chapter 2.

The thermoluminescence method was hardly used for dosimetric purposes up to this time. In this field the film dosemeter reigned supreme. In the beginning the photographic dose measurement was used less in radiological protection than in other problems of medical dosimetry [10]. The reason for this is that little importance was attached to radiological protection, and the sensitivity of the available photomaterials was too small to detect the small doses then in use. Behnken [17], indeed, tried in 1922 to intensify the radiation effect on the film by means of fluorescent foils. He found, however, that there was a long-term error due to the contribution of light to the total density of the photo­graphic emulsion.

In 1926 Quimby [18] studied scattered and secondary radiation in radiation labora­tories by photographic methods, and in 1928, with increasing attention being given to the safety of the people engaged in radiation research, this method was proposed for personal dosimetry by Barclay and Cox [19]. In 1929 Eggert and Luft [20, 21] constructed a badge with several metal filters to be carried on the body of radiation workers and which was calibrated by known radiation doses. They called it a 'film dosemeter'. Bouwers and Van Der Tuuk [22] extended the arrangement of Eggert and Luft using different filter metals. In this way the photographic film was established in personal radiological protection.

With the rapidly increasing use of radiation sources and reactors in civilian as well as in military fields after World War II, far-sighted experts quickly realised that the proper­ties of photographic emulsions as a large-scale long-term dosemeter were rather limited [1]. The main reasons are the inherent problems of a strong energy dependence (film dose-meters are about 20-50 times more sensitive to 40 keV x-rays than to gamma radiation and the metal filters only partially compensate for this energy dependence), of pro­nounced fading at higher temperatures and humidities, high sensitivity to disturbing agents such as light, pressure and certain chemicals, limited lifetime, dose range and sensi­tivity, poor reproducibility, and the need for rather complex darkroom processing procedures including development, fixing and washing, involving many potential sources of error.

6 A Scharmann

In the USA especially, in the late 1940s, an intensive search therefore began for better alternatives, more suitable for the dose control of larger military units. Radiophoto-luminescence (RPL) dosemeters based on silver-activated phosphate glasses were developed (Weyl et al [23], Schulman et al [24]). The glasses later became the first mass-produced solid-state dosimetry system. More than a million units were used in the US Navy (Schul­man et al [25]). In the early 1950s most of the TL phosphors mainly used at present were discovered or rediscovered and seriously proposed for dosimetric purposes, e.g. LiF by Daniels [26], CaS04 by Kossel et al [27] and CaF2 by Ginther and Kirk [28].

The idea of dose measurements by means of TL phosphors started in two ways [7]: (i) Since the beginning of this century, the natural TL of various materials has been

studied, mostly in Europe. In 75% of about 3000 investigated minerals TL was found. (ii) Since the beginning of this century it has also been known that CaS04:Mn could

be used for the quantitative measurement of ultraviolet radiation. Originally the green luminescence of CaS04:Mn obtained by heating the material after uv exposure was evaluated visually. Later use was made of a photomultiplier for evaluation of the phos­phor. Soon it was found that this method could also be used for measuring x-rays. A practical application of this method was reported in 1951 by Purcell et al [29]. On 18 November 1948 and 17 February 1949 CaS04 and MnS04 phosphors were carried to a height of 90 and 79 miles, respectively, by means of a V2 rocket. They were then exposed to the Sun for about three minutes. Some of the phosphors were naked, while others were covered with filters of CaF2, LiF and beryllium. Upon their return the differ­ent phosphor strips were heated and the thermoluminescence was measured by a photo-multiplier. In every case a glow curve was observed and the intensities registered with the Be, CaF2 and LiF filters were about 1/10, 1/3 and 1/2 of the naked phosphor, respectively. The occurrence of thermoluminescence, even in the shielded phosphors, was evidence for a high-energy region in the Sun's spectrum. At the height where the exposures were done there was even a component in the x-ray region, proved by the thermoluminescence of the Be-covered phosphor.

Basic work on CaS04:Mn was also done by Kossel et al [27]. They described in 1954 the development of simultaneous dosimetry of radiation fields in living objects. The motivation for these investigations came from Bickenbach who was the head of the hospital in Tubingen at this time. He pointed out the urgent interest in measuring the dose distribution of radiation sources (radium and hollow anode tubes) which were introduced into small cavities of the body or even in the tissue.

It was obvious to the authors that they should use for this purpose the classical method of the light sum which was founded quantitatively by Ltnard and Hausser [30] in 1912. In this method phosphorescent substances in a low-energy state may store a light sum which is at most proportional to the exciting radiation. This light sum is measured by means of a photoelectric detector when the material is later heated. The method was useful because of the small size of such storage phosphors, which enabled convenient injection into the cavities of the body and because of the omission of leads necessary for measurements using ionisation or photocurrent methods and experiments with a counter. Another advantage of this method is the great number of phosphors which could be deposited around the radiation source. This allows simultaneous dosimetry at different places during a single radiation period. The classical phosphor, fluorite, turned out to be too insensitive to the small doses which are of interest in this special case, but CaS04:Mn,

History 7

which was suggested by Brauer, proved to be very suitabe. With this phosphor simul­taneous dosimetry could be developed. The reproducibility of the data on intensity relations was about ± 5%. The method was tested for up to 20 simultaneous working observation points.

As mentioned above, the idea of dose measurements by means of TL phosphors started in two ways. One was the early known uv sensitivity of CaS04:Mn and the other was the natural thermoluminescence observed in many minerals. Natural TL is excited in the course of geological deposition of stone formations by background radiation, in particular by naturally occurring radionuclides which are present everywhere in small quantities. Some of the minerals were more sensitive to radiation than others, i.e. under the same conditions they showed a higher TL intensity. The phosphor most investigated was the very sensitive CaF2. So, materials were discovered which were suitable for dosimetric purposes.

Since there is a close connection between TL and natural radiation, the TL measure­ments were used to investigate the thermal and radiation history of minerals. A sum­marising report on the application of thermoluminescence methods, in particular for the study of the radiation history of natural minerals for the purposes of geology, mineralogy and geological chronology but also for investigations of ceramic materials, glasses, cata­lysts, etc, was given for example by Daniels et al in their publication of 1953 Thermo­luminescence as a research tool [31].

The research work done by Houtermans et al [32] should also be mentioned here. They described in 1957 an apparatus for the quantitative measurement of glow curves with a heating rate of about 40-80 K s_1. This enormous heating rate strongly increased the TL intensity. The resolution power of the apparatus relative to the separation of the maxima, however, was nearly the same as at low heating rates. By comparing the natural glow curves with those after artificial irradiation the natural dose of 'Wolsendorfer' fluorite could be estimated. If the contents of uranium, thorium and potassium are known, the dose rate in a mineral of sufficient size could be calculated from the ionisa-tion energy of the a, 0 and y radiation of these elements. From the known stored radiation dose and dose rate the period of irradiation could be estimated. But there are still further results which could be obtained from the glow curves.

By means of annealing experiments information on the thermal history of meteorites could be obtained. So one can say that the temperature at which the natural glow curve of the material starts to increase is surely not exceeded for more than a few seconds in its thermal history during its exposure to natural radiation and in particular during its fall through the atmosphere.

In some cases it turns out that the glow curve is lower near the surface than in the interior of the meteorite and that the crust itself, which was heated up to 500 °C or more, showed no TL signal. This dependence of the natural glow curves on the depth of the material below the present crust is detectable only in the outermost layers. Deeper than 15 mm no differences between the glow curves could be observed, which proves that during the passage through the atmosphere the heat wave advanced only up to a depth of 15 mm.

In the 1950s more and more thermoluminescent materials were examined for their usefulness as dosimetric phosphors. Single crystals activated with metal ions were mostly examined, beginning with manganese-activated calcium sulphate which was occasionally

8 A Scharmann

used as mentioned above between 1895 and 1954 for radiation and uv dosimetry, e.g. in rockets. Manganese-activated calcium fluoride was also often suggested [1].

In the late 1940s studies on lithium fluoride, a material of low atomic number and therefore low energy dependence for x-rays, began at the University of Wisconsin under the guidance of Daniels. This work was interrupted between 1956 and 1960 because of the less desirable dosimetric properties of a newer material. Later works of Cameron and his coworkers in collaboration with the Harshaw Company led to the development of a material which was mainly activated with magnesium and titanium and which is now widely distributed under the name 'TLD 100'. Despite some unfavourable properties such as non-linearity at higher doses and a complex behaviour when heated, it is still the most popular TLD phosphor, and for many people knowing the field only slightly it is synonymous with the term 'solid-state dosemeter'.

Further popular TLD phosphors have been developed. Since 1957 special interest has been shown in natural fluorite, manganese-activated lithium borate, and, recently, beryl­lium oxide as an alternative to lithium fluoride for energy-independent photon measure­ments, as well as in dysprosium-activated calcium fluorite and calcium sulphate and terbium-activated magnesium orthosilicate. Thermoluminescent glasses developed in various countries, as well as infrared-stimulated detectors, have not been successful so far in large quantities.

A second generation of materials including greatly improved RPL glasses and TL phos­phors and a wide variety of commercial systems became available during the 1960s. The new techniques of exoelectron dosimetry and track etching were also explored during this period. In about 1965 an explosion of publications began which apparently has not yet reached its peak (figure 1.1). Our available knowledge on solid-state dosemeters now doubles every few years which makes it increasingly difficult for individual scientists, particularly in small institutions or developing countries, to keep their knowledge up to date. Today, scientists in about 25-30 countries are engaged in problems of solid-state dosimetry.

Figure 1.1. Approximate number of publications per year on thermoluminescence (TL) in LiF and in other materials, on radiophotoluminescence (RPL), track etching and exoelectron emission. (Based on data by Attix, Fleischer and Becker.)

History 9

References

1 Becker K and Scharmann A 1975 Einfuhrung in die Festkorperdosimetrie (Miinchen: Verlag Karl Thiemig)

2 Oldenberg H 1705 Phil. Trans. Abrdg. 3 345 3 Pearsall T J 1830/ . R. Inst. 1 267 4 Becquerel H 1883 Ann. Chim. Phys. 5e serie XXX 5 5 Becquerel E 1842 Bibl. Univ. Geneve-Arch. Set Phys. Nat. 6 HeischelW 1800Phil. Trans. 7 Oberhofer M 1973 Thermoluminescence Dosimetry, Pusat Reactor Atom Bandung PRAB: 335/

HP.40/73 8 Wiedemann E and Schmidt G C 1895 Ann. Phys. Chem. 54 604 9 Röntgen W C 1895 Verhandl. Phys.-Medizin. Akad. Wurzburg 10 Becker K 1962 Filmdosimetrie (Berlin: Springer-Verlag) 11 Wick F G 1925 Phys. Rev. 25 588 12 Wick F G and Slattery M K 1928 7. Opt. Soc. Am. 16 398 13 Scharmann A, Bohm M, Born G, Grasser R and May A 1971 Einfuhrung in die Lumineszenz

(Miinchen: Verlag Karl Thiemig) 14 Urbach F 1930 Wien. Ber. Ila 139 363 15 Randall J T and Wilkins M H F 1945 Proc. R. Soc. A 184 336, 390 16 Garlick G F J and Gibson A F 1948 Proc. Phys. Soc. 60 574 17 Behnken H 1922 Fortschr. Rontgenstr. 29 330 18 Quimby E H 1926 Radiology 7 211 19 Barclay A E and Cox S 1928 Fortschr. Rontgenstr. 38 311 20 Eggert J and Luft F 1929 Rontgenpraxis 1 188 21 Eggert J and Luft F 1929 Rontgenblätter 1 655 22 Bouwers A and Van Der Tuuk J H 1930 Br. J. Radiol. 3 503 23 Weyl W A, Schulman J H, Ginther R J and Evans L W 19497. Electrochem. Soc. 95 70 24 Schulman J H, Ginther R J, Klick C C, Alger R S and Levy R A 1951 7. Appl. Phys. 22 1479 25 Schulman J H, Shurcliff W, Ginther R J and Attix F H 1953 Nucleonics 11 (10) 52 26 Daniels F 1950 Report on 4th Symp. on Chemical Physics and Radiation Dosimetry part I (Edge-

wood, Md: Army Chemical Center) p 148 27 Kossel W, Mayer U and Wolf H 1954 Naturw. 41 209 28 Ginther R J and Kirk R D 1957 7. Electrochem. Soc. 104 365 29 Tousey R, Watanabe K and Purcell J D 1951 Phys. Rev. 83 792 30 Lenard P and Hausser W 1912 Sitzgsber. Heidelberger Akad. Wiss. Math.-Naturw. Kl. 12 Abh. 31 Daniels F, Boyd C A and Saunders D F 1953 Science 117 343 32 Houtermans F G, Jager E, Schon M and Staufer H 1957 Ann. Phys., Lpz. 20 283

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

2 Theory

M BOHM AND A SCHARMANN

2.1. Introduction

Thermoluminescence involves two steps. In the first step, the solid is exposed to the exciting radiation, such as particle or electromagnetic radiation, at a fixed temperature. In the second step the excitation is interrupted and the sample is heated. One finds that during the temperature increase the sample emits light. The intensity of luminescence as a function of temperature, which possibly exhibits several maxima, is called the thermally stimulated luminescence or thermoluminescence (TL) glow curve. In some cases glow curves of thermally stimulated conductivity (TSC) and thermally stimulated exoelectron emission (TSEE) can be observed which are usually correlated to thermoluminescence.

The theory of thermally stimulated phenomena is treated in a two-fold manner. First the excitation, the thermal stimulation, the luminescence, the electrical conductivity and the exoelectron emission are considered on the basis of general physics. Secondly, the phenomena are phenomenologically analysed without considering the various processes, such as thermal activation or recombination, from a physical point of view and without considering the atomistic structure of centres. Finally, an experimental example is theoretically analysed in this way.

2.2. Excitation by radiation

In the following the interaction of radiation with matter and defect creation are studied more closely. The radiation possibly causes numerous changes in both the indigenous lattice ion and in the impurities present. The end-products of these changes may be classified in terms of three categories of defects: (i) electronic defects, which involve changes in valence states; (ii) ionic defects, which consist of displaced lattice ions; and (iii) gross imperfections, such as dislocation loops and voids. In the latter case large macroscopic defects rarely occur and are mostly due to particle irradiation.

As for the electronic defects the valence state of both the impurities and lattice defects can be changed. The simplest radiation products arise from the impurities which are present in all samples. Thus the valence states are changed by trapping electrons and holes created elsewhere in the lattice by radiation. It means that the impurities act as traps for electrons and holes. The capture cross sections of impurities for electrons and holes vary with the type of impurity and the nature of the host lattice. The experimental proof of the change of valence state due to radiation can be obtained by studying the change in optical, dielectric and magnetic properties. For instance, different optical absorption bands or spin resonance spectra characteristic of the defect are observed.

12 M Bohm and A Scharmann

As mentioned above, imperfections in crystals can also change their nature by trapping electrons or holes. More interesting perhaps is the fact that free charges can be trapped even in perfect crystals. In alkali halides the so-called 'self-trapped holes' are obtained. Their structure can be investigated by the methods of EPR [1,2] and optical absorption [3-5] in a performed manner. The configuration of these centres is represented by figure 2.1. They consist of two nearest-neighbour < 110> halide ions that have captured a hole or have given up an electron respectively. The halide ions have moved together to form a halide molecular ion. It should be emphasised that these centres are not lattice defects in the usual sense of involving a vacancy or interstitial. The two halide ions are, however, displaced from their normal lattice sites so that they have a smaller spacing than the normal negatively charged halide ions. Also, it is noted that in order for a self-trapped hole to be produced, the accompanying electron must be trapped at some other defect, such as an impurity.

Figure 2.1. Model of the V^ centre in alkali halides.

As for the ionic defects the most important role is played by the vacancies, which are probably the best-known radiation damage products. In simple elemental crystals (e.g. metals), all vacancies are equivalent. In compounds and more complicated crystal struc­tures, there are a number of possible vacancy configurations, depending upon which elemental species is missing and which of a number of non-equivalent lattice sites are vacant.

In principle, the energy to form a vacancy can be estimated by imagining that an atom is removed from the interior of the crystal and then placed upon the surface. In the first step some energy is required to break a number of bonds; in the second step the re-establishing of a smaller number of bonds is responsible for energy becoming available. Because of relaxation of the lattice around the vacancy, it is very difficult to calculate accurately the formation energy directly from theory. However, a rough value of the order of 1 eV may be estimated.

In the case of a binary ionic compound, such as NaCl, the vacancies tends to occur in pairs in order to preserve equal numbers of alkali and halide ions. It is also true that

Theory 13

defects of one kind would leave the crystal charged or would produce high electric fields. Consequently, positive and negative ion vacancies must occur in pairs in order to guaran­tee charge neutrality. A pair of such vacancies is known as a 'Schottky defect'.

In polar compounds most of the observations of radiation defects have been made on negative-ion vacancies. In singly charged compounds, the anion vacancy which contains an electron is referred to as the F centre [6] (figure 2.2). The centre has the same charge as the anion originally present there and is consequently uncharged with respect to the perfect lattice. The electron density of the unpaired electron is not only localised to the vacancy but extends to neighbouring nuclei. The interaction with these nuclei can be detected as hyperfine structure (HFS) in the ESR or ENDOR measurements respectively.

Figure 2.2. Model of the F centre in alkali halides.

When another electron is captured by the F centre an F' centre is formed. It has a single negative charge with respect to the lattice. No ESR absorption or dispersion from an F' centre can be observed because it is a two-electron centre with a diamagnetic ground state. In alkali halides containing impurities one nearest-neighbour cation of the F centre can be replaced by an alkali ion of smaller size. A static perturbation is then applied to the F centre and the original local symmetry is reduced. The reduction in symmetry causes a splitting of the three-fold degenerate excited state into two states and therefore a splitting of the main F absorption band into two components. These centres are called FA centres [7].

There are also a number of defects which involve a positive-ion vacancy. For instance the analogue to an F centre consists of a hole located on the site of a cation vacancy [8]. This centre is called a Vjr centre. However, the symmetry of the Vp centre differs greatly from that of the F centre. The F centre has cubic symmetry in the ground state, and its wavefunction is substantially s-wave in character. In the case of the VF centre, the hole is in a p-like state. The degeneracy is removed due to a reduction in symmetry (Jahn-Teller effect).

In many cases, it is possible to produce groups of vacancies, so-called vacancy aggre­gates. For instance, aggregates of F centres which consist of two (M centre), three (R centre) or four (N centre) F centres [9,10].

14 M Bohm and A Scharmann

Other ionic defects are formed by interstitials. A lattice atom or ion is displaced from its normal site and remains in a position that is not a normal lattice site. The double defect consisting of the vacancy and interstitial is called a 'Frenkel defect'. Obviously, the energy to form a Frenkel defect is the sum of the energy to form the interstitial and the energy to form the vacancy. Such defects might be generated in the interior of the crystal by thermal vibrations. There are many possible types of interstitial centre. In compounds, there may be cation and anion interstitial ions or atoms. Moreover, interstitial ions can sit either at the centre of an interstitial site or may be drawn towards one of the lattice ions to form a centre of differing configuration such as a molecular ion. An example is shown in figure 2.3. It consists of an interstitial halide atom that has bonded itself to a lattice halide ion and shares that ion's lattice site. This molecular ion defect, called an H centre [2,11], is to be contrasted to the (halide)2-hole centre (Vk centre) in the perfect lattice, where the two anions have two lattice sites.

Figure 2.3. Model of the H centre in alkali halides.

In the following subsection, the mechanisms by which defects can be created in solids by radiation will be considered. One can distinguish three generic classes of radiation damage processes:

(i) electronic processes; (ii) elastic collisions; and

(iii) radiolysis.

The electronic class includes all processes in which an electronic state is changed or a charge is moved about by the absorption of radiant energy, but in which no ionic or atomic defects are formed. The absorption occurs somewhat differently for various types of radiation.

A heavy, energetic particle passing through matter is usually stripped of some or all of its electrons and thus represents a rapidly moving point charge which interacts with the crystal electrons. Similar considerations are valid for fast electrons. However, bombarding

Theory \ 5

electrons are not distinguishable from the crystal electrons. Moreover, since bombarding electrons, in contrast to heavy particles, are as light as the crystal electrons, they can lose an appreciable fraction of their energy in a single collision. The penetration depth of a particle in a crystal, which is very important in radiation damage, depends on the energy loss. Since the energy absorbed from a heavy particle is very much greater than that absorbed from electrons, electrons will penetrate deep into crystals while heavy particles are stopped near the surface.

Fast neutrons, since they are not charged, do not excite the crystal electronically as do charged particles. However, when a fast neutron displaces a crystal ion, the ion gives up some of its kinetic energy to the electronic structure of the crystal.

In some materials, thermal neutrons can be quite effective in producing electronic excitation indirectly. This comes about when a thermal neutron is captured by a nucleus and the excited new isotope decays.

Photons with energies in the range obtainable with x-ray or isotope sources can transfer their energy to the electronic system of a crystal by a number of processes. In the photo­electric effect, the full photon energy is transformed into ionisation and kinetic energy of one of the crystal electrons. Energy transfer increases in efficiency as the photon energy decreases until the energy becomes too small to excite K-shell electrons. If the photon transfers only a portion of its energy to an electron of the crystal the process is called the 'Compton effect'. This mechanism of energy transfer becomes important for photon energies between about 0.1 and 1 MeV. At energies above 1.02 MeV the produc­tion of electron-positron pairs ('pair production') becomes important.

In order to create defects through elastic collisions, it is necessary for the incident particle to impart sufficient energy to a lattice atom or ion to displace it through its neighbours into an interstitial site. Thus the effectiveness of an incident particle in creat­ing damage depends on the maximum amount of kinetic energy it can transfer to a lattice ion. This depends, in turn, on the energy and the mass of the incident particle and the mass of the lattice ion. In general, five types of radiation may produce displaced atoms or ions by elastic collision. These are:

(i) 7-rays, (ii) energetic electrons,

(iii) thermal neutrons, (iv) fast neutrons, and (v) energetic atoms and ions.

It is clear that the heavier particles will be much more effective in displacing lattice ions than the lighter ones. In fact, they are so effective that they produce so-called 'cascades'.

Finally, defects are created by radiolysis. This means that in certain ionic materials, defect creation is highly efficient and is most probably due to the conversion of electronic excitation energy into a form capable of manufacturing lattice defects rather than into elastic collisions. Such photochemical processes are most probably involved in the photo­graphic process and photosynthesis.

When energy is absorbed in a crystal by electronic processes as described in the first part, it appears in the form of electrons in a normally empty conduction band and holes in the normally occupied valence bands, or in the form of excitons (electron-hole pairs

16 M Bohm and A Scharmann

bound to each other) at lattice ions, impurity ions, or defects in the crystal. The excita­tion is only the first step and must be followed by processes that lead to observable electronic states. This usually involves separation of the electrons and holes, and trapping of the separated charges at impurities, defects or in the perfect lattice. The capture cross sections are determined by potential variations near the centres which can be attractive, neutral or repulsive.

Normally, the crystal as a whole remains neutral, and free electrons and holes are always created in pairs. For every electron trapping centre formed, there must also be a corresponding hole centre formed.

2.3. Thermal excitation and recombination

Supposing the solid previously excited is heated, a thermal relaxation then occurs which is the dominant mechanism in nearly all temperature-dependent processes in solids. It means that the processes are started and accelerated if energy is supplied in the form of thermal energy [12]. These thermally stimulated processes can be compared with chemi­cal reactions. The increase in the rate of such reactions with temperature can usually be expressed by an Arrhenius equation. This equation leads one to the concept of an activa­tion energy: an energy barrier which must be overcome in order to reach equilibrium.

In this special case electrons and holes may escape from metastable states during heating. These levels are known as traps. The probability of thermal excitation of a carrier, the so-called escape probability a, is assumed to be given by a Boltzmann factor:

ct = a0exp(-E/kT) (2.1)

(a0 = constant, E= thermal activation energy required to liberate a trapped electron, k = Boltzmann's constant, T = absolute temperature).

Although this expression is well supported by experiment, a detailed theoretical treat­ment is still lacking. In a semiclassical approach a trapping model with levels equidistant in energy from each other and a successive absorption of phonons is assumed. Another model is based on thermodynamic concepts as they are assumed to be valid in chemical reactions. Finally, this relation can be deduced from a simple model using the law of detailed balancing.

For this purpose one may consider traps with only one state. There are also H traps (per unit volume) and many electrons, of which n are in the conduction band and (H— n) in the traps. Denoting by a the cross section for the capture of an electron in a trap (= actual area multiplied by the probability of capture) and letting a be the probability per unit time that it escapes, from the law of detailed balancing a relation between a and a may be deduced. For a steady state one can write

n2va=(H-n)a (2.2)

(number of transitions into the traps = number of transitions into the conduction band), where v is the mean velocity of the electrons.

The concentration n of the free carriers is calculated using the methods of statistical mechanics. Thus the assumption is made that the crystal is in thermodynamic equilibrium at a fixed temperature T. With the volume V of the crystal as a second state variable the equilibrium value of n will then be given by the condition that the free energy F becomes

Theory V ^ T W T E ^ V V 1 7

a minimum:

(W73ii) r=0. | * w ^ ? / / , 0 J (2.3)

Then one obtains

n2 n-nmkTf1

F ^ - ( — ) ^-E/kT) (2

'4>

where E is the energy required to remove an electron from the trap into the conduction band. Substituting n in equation (2.2) gives

a = a0exp(-E/kT) with a0=ovNc. (2.5)

Nc is an effective density of states for electrons in the conduction band. Substituting v from (\/2)mv2 = (3/2)kT and assuming the effective cross section σ of the order of 10"lscm2 one obtains a 0 « 1011 s­1. This pre­exponential factor is of the order of the frequency of the lattice vibrations and is called the 'frequency factor'. From this model its temperature dependence is proportional to T2 (v~T112, NC~T3'2). In most cases it is neglected. The thermal activation energy, the so­called trap depth, is much smaller than the optical ionisation energy because the thermal activation, which involves a multi­

phonon process, occurs with the removal of ionic polarisation due to trapped carriers, whereas in the case of optical activation the polarisation does not have enough time to disappear according the Franck­Condon principle.

The thermal release of carriers from traps possibly gives rise to a thermally stimulated conductivity (TSC) when the sample is placed in an external electric field. Moreover, electrons liberated from traps into the conduction band also have a chance of leaving the solid if they are close to the surface and if their energy is sufficient to overcome the barrier created by the electron affinity. This phenomenon is called thermally stimulated exoelectron emission (TSEE) and according to this model it is expected to be associated with TL and TSC.

When electrons (or holes) have been thermally excited into the conduction band (or valence band) they will be captured by traps again or recombine with opposite carriers. The retrapping is influenced by the thermal velocity v in the energy band and the effec­

tive cross section a. It can be expressed by the transition coefficient ß:

P = ov. (2.6)

If recombination occurs with the emission of light, a TL glow curve can be observed. Radiationless transitions are also possible. In this process the energy is transferred to another electron (Auger recombination) or to the lattice in multiphonon processes. As is known from absorption the energy and momentum must be conserved in each recombi­

nation process. Let k be the wavenumber of the carrier in the initial state and k' in the final state. Then

hk-hk' = h/\ (2.7)

or, since the momentum of the photon (h/X, X = wavelength) is small compared with the smallest momentum of the carrier (h/a,a = lattice constant), the selection rule for optical

18 M Bohm and A Scharmann

transitions is given by

hk-hk' = 0. (2.8)

This states that direct recombination occurs only between carriers with the same wave-number. In the energy band model the carriers may only make vertical transitions. The change in momentum and energy due to electron-phonon interactions occurs in a time that is much smaller than the carriers' mean lifetime. (In other words, the phonon energy is large compared with the linewidth.) Thus a direct band-band recombination is very unlikely.

A larger probability of radiative transitions is obtained in the presence of imperfec­tions such as vacancies, interstitials or impurities. When the optical transition involves an imperfection the selection rule (2.8) is satisfied even for different wavenumbers in the conservation of momentum. In connection with luminescence these imperfections are called activators.

2.4. Phenomenological analysis

The thermally stimulated processes may be phenomenologically analysed. The physical processes such as interaction with the exciting radiation, thermal activation, charge and energy transport or recombination are not considered at all. This means that one does not investigate the kinds of transition involved and the contribution of phonons during thermal stimulation. One is also not interested in the structure of centres, such as traps and activators.

Several properties of solids may be explained by the energy band model. It involves energy states which are allowed or forbidden to be occupied by electrons. These energy levels are so closely spaced as to constitute a quasicontinuum or energy band. All the bands which represent the closed electron shells of the individual atoms are always fully occupied with electrons. The next higher-lying band contains the valence electrons and is called the valence band. In the case of the insulators in question this valence band is completely filled by the valence electrons. An energy gap without any allowed states lies between the highest state in the valence band and the lowest state of the next highest band called the conduction band. The width of the band gap is greater than 1 eV so that transitions of electrons across this gap cannot occur at normal temperatures. The conduc­tion band, normally empty, is responsible for electrical conductivity if an electron reaches this band from the valence band. For such transitions to be possible more energy from exciting radiation is necessary.

One can understand the origin of these bands from two limiting cases. In bringing free atoms together to form a crystal, the discrete levels of these atoms split up into groups of levels which then form an energy band, or by the influence of the lattice potential the continuous energy spectrum of a free-electron gas is broken at certain characteristic energies since electrons with these energies and corresponding momenta on their passage through the crystal suffer Bragg reflections from the lattice. Both types of description starting from tightly bound or completely free electrons meet in the band model of solids.

The simplified energy level scheme is commonly used to describe the non-stationary processes (figure 2.4). Wherever the perfect periodicity of the crystalline structure is dis­turbed, it is possible for carriers to take on energies which are forbidden in the perfect

Theory 19

Y

a >E

Ev

Figure 2.4. Energy level diagram for the phenomenological analysis of TL and TSC with one type of trap level and recombination level.

crystal. The presence of a defect can introduce one or more additional energy levels in the forbidden gap between the conduction and valence bands; unlike the bands them­selves, which extend throughout the crystal, the additional level is localised at the crystal defect. The activators and traps give rise to some discrete levels above the valence band and below the conduction band, respectively. In the ground state the recombination levels are occupied by electrons and the trapping levels are empty. After excitation by energetic radiation electrons or holes can be captured in traps or recombination centres, respectively. For the sake of simplicity the following consideration deals only with electrons. If sufficient thermal energy is supplied the trapped electrons may again be raised to the conduction band. The electrical conductivity can now be measured. From the bottom of the conduction band the electron may be retrapped by the traps or they may recombine with empty activators in a radiative manner. The latter process gives rise to thermoluminescence. The possible transitions are represented by arrows. The simplest model involves only a single trap level and a single type recombination level. The symbols used are h, the density of trapped charge carriers, H, the density of trap levels, n, the density of free charge carriers, and /, the density of recombination levels.

The transition probabilities are replaced by transition coefficients: a, the escape coefficient for trapped carriers, (3, the retrapping coefficient, and y, the recombination coefficient.

It is not probable that such a model accurately represents an actual situation occurring in solids since essential simplifications have been introduced. For instance:

(i) The free carriers are electrons and there is no thermal quenching. This implies that transitions of electrons from the valence band to the recombination centres are neglected.

(ii) No interaction exists between centres which excludes donor-acceptor recombina­tion and trap distributions.

20 M Bohm and A Scharmann

(iii) Only one kind of centre is involved in the recombination process; thus no killer centres are present,

(iv) Recombination of trapped electrons via excited states of defect centres does not occur.

The phenomenological analysis now deals with the kinetic balance. Some transition rates are necessary.

(i) The rate at which trapped carriers are thermally released (the rate of liberation) is proportional to the number of occupied traps h:

rate of liberation = ah.

(ii) The retrapping rate, which means the number of trapping transitions per unit time and volume, is proportional to the density of free carriers n in the conduction band and the density of empty traps (// - h):

rate of retrapping = &n{H — h).

(iii) The recombination rate, which means the number of recombination transitions per unit time and volume, is proportional to the density of free carriers n in the conduc­tion band and the density of empty recombination centres/:

rate of recombination = ynf.

In this simple model the kinetic processes involving the change in the density of trapped and free charge carriers during thermal stimulation are described by the following system of differential equations.

The rate of change of the density of trapped electrons is given by

dh/dt = -ah+pn(H-h). (2.9)

The rate of change of the density of free electrons is given by

dn/dt = ah-Pn(H-h)-ynf. (2.10)

Since the solid is electrically neutral, the density of carriers in the conduction band must be equal to the density of empty recombination centres. The condition of charge neutrality then yields

/ = n + A „ (2-11) The thermoluminescence intensity / is given by the number of radiative transitions per unit time and volume and is therefore proportional to the recombination rate:

I~ynf. (2.12)

The electrical conductivity a is determined by the free charge carriers in the conduction band when the mobility ju is assumed to be constant:

a = nen (2.13)

(e = electron charge). The purpose of phenomenological analysis is to obtain explicit functions of the carrier

density and, by that, explicit functions of the current and luminescence intensities. The

Theory 21

experimental glow curves are then fitted to the calculated ones by variation of some trapping parameters. This procedure possibly gives some physical quantities such as the thermal activation energy E and the frequency factor a0. However, all attempts to find explicit functions have been unsuccessful. Therefore one introduces some approximations or simplifying models.

2.5. Kinetic models

It is assumed that the lifetime of free carriers in the conduction band is short when com­pared with the lifetime of trapped carriers [13], which probably holds in high-resistivity solids [50]. As a result one obtains a small quasistationary electron concentration in the conduction band. This means that the density of free charge carriers is always much smaller than the density of trapped charge carriers:

n<h (2.14)

and that the time rate of change of the free carrier density is much less than the time rate of change of the trapped carrier density:

dn/dt<dh/dt. (2.15)

Also a constant heating rate q is used:

T=T0 + qt or q = dT,'dt (2.16)

(T0 ~ initial temperature). With these assumptions an analytical solution is obtained only in two special cases. In

the first case the retrapping transitions are neglected (fi = 0) [14]. In the other case the transition coefficients for retrapping and recombination are equal (fi/y = 1) [15].

Substituting equation (2.16) into (2.9) and (2.10) one obtains the following differen­tial equations for the first case (fi = 0):

qdhldT=-ah (2.17)

q dnldT=ah-ynf (2.18)

with a = a0exp(—E/kT); the temperature dependence of a0 is neglected. From equation (2.17) this case corresponds to 'first-order' or 'monomolecular' kinetics.

With the approximation (2.15) and by adding equations (2.17) and (2.18) one obtains:

qdh/dT=-ynf. (2.19)

Since the luminescence intensity is given by equation (2.12) from equations (2.19) and (2.17)

/ = constant x ah. (2.20)

The density of trapped carriers h is a function of temperature and can be calculated from equation (2.17). The integration yields

h(T) = h0exp[-f(T)] (2.21a)

22 M Bohm and A Scharmann

with

f(T) = — exp(-E/kT)dT' VJT„

(h0 = initial density of trapped carriers). With equation (2.20) the luminescence intensity is explicitly given by

I(T) = constant x a0h0 exp(-E/kT) exp [-f(T)].

(2.21b)

(2.22)

After integration by parts the function f(T) can be represented by an exponential integral function which is approximately substituted by a series. Since the initial tempera­ture T0 is far below the peak temperature the limit of integration can be replaced by zero:

I ' exp(-*/*r)«- - jl e*p(-£/*r, i <- i r J ^ (2.23)

With four terms of the semiconvergent series a glow curve of thermoluminescence is calculated from equation (2.22) represented in figure 2.5. The first exponential function is responsible for the initial increase, the second for the decay of the glow curve. A non­symmetrical shape with a steep high-temperature tail is obtained.

The error made in approximating the integral by four terms of the series can be estimated to be less than 1% [50] and is unimportant in the following calculations. This approximation is avoided by using a hyperbolic heating rate [19-21]:

1 1 = St (5= constant)

T0 T

as can be experimentally realised. Now the integral is integrable in a closed form.

(2.24)

< 10

60 70 TEMPERATURE IK)

Figure 2.5. Glow curves of TL (a) and TSC (b) calculated from equations (2.22) and (2.27) with £-=0.13eV, o„ = 2 X 10' s"\ hJH = 1 and q = 1 K s 1 .

Theory 23

In this model, however, there are some problems in calculating the conductivity glow curve. From equation (2.13) the electrical conductivity is proportional to the density of free carriers n. Using the approximation (2.14) in the condition of neutrality (2.11) one has

f*>h (2.25)

and from equation (2.19) one obtains

q dh/dT=-ynh. (2.26)

Combining this with equation (2.17) the density of free carriers is found to be

n(T) = (a0/7) exp(-E/kT). (2.27)

The result is physically absurd because there is an exponential increase in the density of conduction electrons and no maximum occurs (figure 2.5). At sufficiently high tempera­tures the condition n >h becomes valid. This is in contradiction to the initial assumption (2.14). To obtain reasonable solutions for the conductivity, too, the model must be varied, which will be discussed later.

In the second case one expects liberated electrons to be recaptured by traps with some definite probability since traps are lattice-positive. With the special assumption j3 = y from equations (2.9), (2.10) and (2.16) one obtains the differential equations

qdh/dT=-ah+yn(H-h) (2.28)

qdn/dT=ah-yn(H-h)-ynf (2.29)

and by adding these equations and using the approximations (2.14) and (2.15)

qdhldT=-ynh. (2.30)

Comparison with equation (2.28) yields

n=ah/yH. (2.31)

With that one may substitute n in equation (2.30) and one obtains

q dh/dT=-ah2/H. (2.32)

This equation describes 'second-order' or 'bimolecular' kinetics. The integration of equation (2.32) yields

ho h (T) = (2.33)

l + (h0IH)f{T) where f(T) is given by equation (2.21b). From equation (2.20), which is valid in this case also, the luminescence intensity / can be expressed by an explicit function of temperature:

a0h\ exp(-E/kT) I(T) = constant x — — . (2.34)

H [l + (h0IH)f(T)]2

Combining equation (2.32) with equation (2.31), the conductivity a (see equation (2.13))

24 M Bohm and A Scharmann

is given by

a(T) = en a0h0 exp(-E/kT) yH [\ + (h0/H)f(T)]'

(2.35)

The curves of TL and TSC calculated from this model are shown in figure 2.6. The integral of function f(T) is again approximated by four terms of the series (2.23). Above the maximum temperature the TL intensity drops more slowly than in first­order kinetics, i.e. the glow peak is more extended and more symmetrical. The conductivity glow curve is shifted to higher temperatures and shows a very slow decrease after reaching the maximum.

/

/ /

JJ

b a /■—v^

V

TEMPERATURE(K)

Figure 2.6. Glow curves of TL (a) and TSC (ft) calculated from equations (2.34) and (2.35) with £ = 0 . 1 9 e V , o0 = 7 X l O l ! s ' 1 , hJH=l, q = 1 Ks" ' and (3/7= 1.

2.6. Determination of trap parameters

The kinetic processes and the transitions of carriers are essentially influenced by the two quantities E and a0. The thermal activation energy E is of particular interest because of its importance to the lifetime of the excited state of the solid. According to the above­

mentioned models one may derive many methods for the determination of activation energy and frequency factor. Sometimes additional approximations are used, for instance, the integral in equation (2.21b) is approximated only by the first term of the series expansion (2.23).

The methods can be divided into several groups according to the experimental pro­

cedure which must be used in order to obtain the parameter. In the first approximation it is possible to calculate a formula which provides a linear dependence of the trap depth E and the maximum temperature Tm [14, 16]:

E~75kTm (2.36)

The derivation uses a frequency factor a 0 = 2 . 9 x l 0 9 s '. The rough formula is quite useful for a quick first estimation of the trap depth. However, the numerical factor in

Theory 25

equation (2.36) is dependent upon the value of a0, and hence the values of E thus obtained only have the right order of magnitude. The frequency factor, in fact, may be different for each trap in the same substance.

The position of the maximum is obtained by differentiating equation (2.22) with respect to temperature and equating to zero. Then the relation between E, a0 and Tm is given by

E/kT^ = (a0lq)exp(-E/kT). (2.37)

From this equation the peak position is influenced both by the thermal activation energy and the frequency factor (figure 2.7). A decrease of a0 or an increase of E, respectively, gives rise to a shift of the maximum to higher temperatures. If either of the parameters E and a0 are known equation (2.37) can serve as a transcendental equation which can be solved numerically for the unknown parameter.

400 500 600 MAXIMUM TEMPERATURE IK)

Figure 2.7. Thermal activation energy E as a function of maximum temperature 7 ^ of TL glow peak with the frequency factor a0 as parameter (calculated from equation (2.37) using a heating rate of 1 K s~')-

By taking the logarithm one obtains from equation (2.37)

\n(T^/q)=E/kTm + \n(a0k/E). (2.38)

Thus the plot of the left-hand side against 1/Tm is linear, having a slope E/k. E and a0 can be determined from the slope and intercept [17, 18]. From equation (2.38) we can eliminate the frequency factor for two different heating rates ql and q2 [22, 23]:

T2

E = k J m l T, 1 -<m2

7mi — T, m2 " ( - % ) ■

\Q2TlJ (2.39)

The glow curve must be measured with two different heating rates q^ and q2 which yield the different maximum temperatures Tml and 7m 2 [16, 24].

26 M Bohm and A Scharmann

Other methods use different temperatures besides the maximum temperature. If one lets T\ be the temperature corresponding to half-intensity, we can write [25]

E=\.5\kTm~—L—. (2.40)

Similar relations are obtained if the temperatures are considered where the intensity is decreased to 1/2 [26, 27], to 2/3 and 4/5 of the maximal height [28]. The inflection temperature [29] and the half-width [30] may also be utilised.

Frequently, the so-called 'initial rise method' is used to determine E. This technique is based on the fact that the function f(T) in equation (2.22) is nearly temperature-independent when the traps begin to empty as the temperature is raised.

Then, in the initial part of the glow curve the intensity is given by / ~ exp(—E/kT) or

lnI=-E/kT+ constant (2.41)

for either type of kinetics. Thus, the plots of In/ against 1/7 are linear, having slopes equal to —E/k. This provides a quick analysis of the initial ascending part of the glow peak which yields the value of E without any knowledge of a0 [31-33]. However, the peaks will become wider and shallower with increasing retrapping. The temperature limit­ing the validity of equation (2.41) is therefore decreasing [34]. Then the slope will be equal to — E/k at very low intensities where exact measurements are impossible.

From numerical methods of analysis some initial approximate values of E and a0 are chosen and they are suitably varied to determine the values giving the best fit to the experimental data [35-37]. Finally the area under the glow curve can be used for para­meter determination [38-40].

The methods may be improved by considering the temperature dependence of the frequency factor a0 [16, 24]. However, its influence on the shape of the glow curve and on the determination of activation energy is only small. It amounts to about 10% from a temperature dependence proportional to T1'2 [41] and T~" {a = 1 or 2) [27], respectively. A survey of methods from the literature is given in references [42-44].

2.7. Additional parameters

The above-mentioned models are often extended to obtain, on the one hand, a simpler mathematical treatment and, on the other, better agreement with the experimental results, mainly with respect to the conductivity glow curve. In this subsection two such models are considered in detail. Both cases show first-order conditions.

In the first case a constant lifetime of free electrons in the conduction band is assumed. This corresponds to the assumption that the density of recombination centres remains approximately constant during the thermal stimulation. So one obtains the new condition of neutrality (see equation (2.11)) [21, 41, 45-47]

n + h =/o 0* constant) (2.42)

and the balance equations

qdh/dT=-ah (2.43)

q dn/dT = ah - ynf0. (2.44)

Theory 27

By adding the differential equations and considering the approximation (2.15) one obtains

n=ahhf0. (2.45)

The density of captured carriers h as a function of temperature is calculated by integrat­ing equation (2.43) (see equation (2.21))

h(T) = h0exp[-f(T)]. (2.46)

Combining equations (2.46) and (2.45) the free carrier density is given by

n = y exp(-E/kT) exp [-f(T)]. (2.47) T/o

Since from equations (2.12), (2.42) and (2.13) both the luminescence and the conducti­vity are proportional to the density of free carriers we obtain the same temperature dependence of both intensities as is represented by equations (2.47) or (2.22). Therefore the glow curves exhibit the same shape as shown in figure 2.5. In this case the maximum of the two phenomena peaks exactly at the same temperature. This means there is no shift of the TSC glow curve to higher temperatures.

This model is used to explain the experimental results observed in x-rayed LiF crystals [48]. X-irradiation creates H centres and V centres in addition to other paramagnetic centres. The concentration of V centres is larger than that of H centres by one order of magnitude. Concerning the behaviour under thermal treatment the H centres disappear rapidly if the temperature approaches 105 K. This thermal decay is connected with a relatively small drop in the number of V centres [49]. Glow peaks are observed in TL and TSC measurements at about 107 K which show first-order conditions and nearly no temperature shift. This might suggest that the H centre decays by emitting an electron, and thus transforms into an F2 molecule. The electron in turn annihilates a Vk centre. From the above-mentioned model the observed conductivity is possibly due to this charge transfer and the thermoluminescence to the electron-hole recombination. Thereby the density of recombination centres remains approximately constant and the neutrality condition (2.42) is valid.

In the second model it is assumed that as well as the mentioned traps H there are additional traps M which are thermally disconnected and act merely as a reservoir for trapped charge carriers [50-52]. Now one obtains the neutrality condition

f=n+h+M (2.48)

and the rate equations (2.9) and (2.10) change to

qdh/AT=-ah+Pn(H-h) (2.49)

q dnjdT=ah - $n(H- h) - yn(n + h +M). (2.50)

By adding the differential equations and considering the approximations (2.14) and (2.15) one obtains

qdhldT=-yn(h+M). (2.51)

28 M Bohm and A Scharmann

Substituting q dh/dT from equation (2.19) one obtains the density of free carriers n:

ah

Combining equations (2.51) and (2.52) the following equation results:

(2.52)

dh ah(M + h) -q — = (2.53)

dT h(l-P/y) + (ply)H + M

The integration of this equation yields

l& PH\ (M + h \ i PH\ (h\ - + — In - ( 1 - — In - = / ( 7 ) (2.54)

\y yMI \M + h0) \ yM< \h0J with/(f) defined by equation (2.21b).

As it is a first-order process, retrapping is neglected and p/y < 1 is valid. In addition the assumptions H/M> 1 and $H/yM< 1 are made as a way of simulating a changing life­time of the charge carriers [50]. Then we obtain

-\n(h/h0)=f(T) (2.55)

A = A 0 e x p [ - / ( r ) ] . (2.56)

This equation yields a calculation of TL because from equations (2.12) and (2.51) the intensity is proportional to — dh/dT:

I(T) = constant x a0h0 exp(-E/kT) exp [-f(T)]. (2.57)

It is exactly the same relation as is obtained from a theoretical treatment of a first-order process without additional parameter M (see equation (2.22)).

In contrast the calculated shape of TSC shows another result. Formerly one finds that the density of charge carriers increases with constant increasing temperature according to an exponential law without reaching a maximum, whereas now from the above-mentioned relations one has

-q dh/dT n=— (2.58)

y(M + h)

and 7"1^"1ao/*oexp(-£'/fc7')exp[-/(7')] t N " (T) = (2.59)

1 + (z0H/M) exp [ - / ( D ]

where z0 = H/h0 is the fraction of initially filled traps. In the calculation of I(T) and n(T) from equations (2.57) and (2.59) the function f(T) is approximated by four terms of the semiconvergent series (2.23).

A TL glow curve and a family of TSC glow curves obtained in this way are shown in figure 2.8. The temperature function in the denominator of equation (2.59) gives rise to a shift of the TSC maxima to higher temperatures as compared with the TL maximum usually obtained by measurement. The shift increases with increasing z0H/M.

Theory 29

130 160 TEMPERATURE(K)

Figure 2.8. Calculated glow curves of TL (I) and TSC (II, III, IV) from equations (2.57) and (2.59) with E = 0.28 eV, a0 = 1010 s"', q = 0.05 K sM, and the parameter z0H/M = 1 (II), 10(111), 103(IV).

For comparison with experiment the glow curves of x-rayed LiF crystals are chosen. With a heating rate of 0.05 K s"1 the crystal shows a TL maximum at 138 K and a TSC maximum near 141 K (figure 2.9(a)). With the values of the trap depth E and the frequency factor a0 revealed from another measurement [32] and with the parameter z0H/M=3 the calculated glow curves (figure 2.9(b)) defined by equations (2.57) and (2.59) show good agreement with the experimental results. Because of the influence of the parameter z0H/M on the magnitude of the temperature shift it is in principle possible to verify the model by the dependence upon the initial occupancy z0. However, it may be very difficult to measure the low currents.

If one applies the model with additional traps M to second-order processes, there are three cases that must be distinguished [50]. In the first case (H/M< l,f3H/yM<: 1) where a constant lifetime of charge carriers is simulated, the calculated glow curves show once again a shape similar to those in first-order kinetics and consequently there is no shift of TL and TSC maxima. In the second case (H/M < 1, (3H/yM> 1) there exist relatively large half-widths. Even larger half-widths result theoretically from the third case (H/M> 1, (3H/yM< 1) where the lifetime changes during heating. Moreover, there are large shifts of TL and TSC maxima and with increasing H/M ratio the TSC shape decreases slower after reaching the maximum [50, 53].

As well as the above cases many other models are known. For instance, additional occupied traps and direct recombination of trapped electrons with free holes are con­sidered by a changed neutrality condition and an effective non-radiative transition [54]. Another model involves the transition between trap and activator in a non-radiative way. Analytical expressions for both TL and TSC intensities are obtained and the shifts of maxima can be explained [47, 55]. Now the frequency factor and the activation energy

30 M Bohm and A Scharmann

( 0 )

n \\ rs

'—n—~u

120 UO TEMPERATURE (Kl

120 HO TEMPERATURE (Kl

Figure rate: 0. eV, a„■■

2.9. (a) Glow peaks of TL and TSC obtained from an x­rayed LiF crystal (heating 05 Ks"1). (b) Calculated glow curves from equations (2.57) and (2.59) with £ = 0.27 = 5 X I O ' V , ? = 0.05Ks"' and z0H/M= 3.

for non­radiative transitions and charge transfer between an excited state and a spatially separated recombination centre are considered, too. The shape of the glow curve calcu­

lated with this model again shows first­order characteristics.

2.8. Computer simulation

In this subsection the correlated phenomena of TL and TSC will be described in simple models but without recourse to any simplifying approximations (2.14) and (2.15) and the results calculated in this way will be compared with approximate solutions. The dif­

ferential equations are solved rigorously on an electronic analog computer [57]. When the equations are amplitude­ and temperature­scaled the densities of free and captured carriers can be generated. However, one obtains no analytical solution but a simulation of TL and TSC intensities against temperature.

A typical solution in the case £"=0.13 eV, ct0 = 2 x 109s~!, /20/# = 1 and q = 1 K s"1 is shown in figure 2.10. In the TL glow curve the maximum temperature differs to some extent from that curve showing the change in the trapped carrier density (dh/dT) and is often mistaken for the TL glow curve [18, 25, 58­60]. A maximum can clearly be observed in the TSC glow curve. Moreover the exact calculation gives rise to a shift of the TSC maximum of about 2­3 K to a higher temperature when compared with the TL maximum. The break in the simulation at about 66 K is caused by the maximal limiting voltage of the analog computer. The TL glow curve calculated approximately with the above­mentioned parameters (figure 2.5) shows the same shape. However, the maxima are shifted to a somewhat higher temperature.

Theory 31

j 1 J [t n '

\

50 60 TEMPERATURE (K)

Figure 2.10. Computer simulation of TL (a), TSC (b) and the change in density of trapped carriers (c) with first-order conditions and £ = 0.12eV, ao = 2xl0 's- ' , hJH=l, q = lKs'' and 7//=0.5 cnvV'.

a

A f

1

50 60 TEMPERATURE [Kl

Figure 2.11. Computer simulation of TL (a) and TSC (b) with first-order conditions and additional thermally disconnected traps (A/); ^ O . D e V , ao=2xl0's-1 , h0/H=l, q = lKs~l

and H/M = 0.2.

Two special cases with first-order conditions described in §2.7 are also calculated without making approximations. In the first case, the density of recombination centres / and therefore the lifetime of free charge carriers are to be assumed as approximately constant during the glow experiment. This corresponds directly to the case where addi­tional thermally disconnected traps M exist and their density is assumed large in com­parison with the above-mentioned traps H [50]. This yields the neutrality condition f-n +h +M and the assumption H/M<. 1 is made. Figure 2.11 shows the glow curves simulated by analog computer calculations, again without the approximations (2.14) and (2.15). Only if the density of the additional traps is increased by a factor of five do the two glow curves have the same shape and no shift of maximum temperatures exists, as is found in the approximate calculation.

In the other case the assumptions H/M> 1 and fiH/yM< 1 are made. With these a changing lifetime of carriers is simulated (see §2.7). The exact solution of the differential equations yields the same result as is obtained in approximate calculations. The TSC curve shows a shift in the maximum temperature and a longer exponential increase at the begin­ning of the heating because of the increase in the lifetime.

Considering transitions to the traps one obtains the condition |3 = 0. From the special case with second-order conditions ()3 = 7) calculated approximately in §2.5 the glow curves of TL and TSC are exactly simulated. As a result the shape of the TSC glow curve is parallel with that of TL in contrast to the approximate calculation. It also shows a shift to higher temperatures winch is of the same magnitude as in the case of first-order kinetics

32 M Bohm and A Scharmann

[57]. Increasing retrapping leads to a shift of the TL maximum to higher temperatures and a reduction of intensity as is known from approximate solutions [57, 34].

In a special case the maxima of TL and TSC show no shift although retrapping occurs [57]. As a condition the density of thermally disconnected traps must be assumed as being large when compared with the traps H involved in the TL process (H/M> 1). This corresponds to a similar case in first-order kinetics. The other condition (!H/yM<: 1 must also be fulfilled.

As the computer simulation has shown, the anomaly in the TSC shape such as the failure of a peak in first-order kinetics (figure 2.5) or the slow decrease in second-order kinetics lies purely in the approximations (2.14) and (2.15), and is not contained in the original rate equations. The shift of the TSC maximum to higher temperatures compared with the TL maximum is also obtained by the exact simulation. It amounts to a few degrees. This value has the same magnitude as is found, for example, in alkali halides [47,48,61,62]. Furthermore it is seen that the shift gives neither information on the trap depth nor on the kinetic process. It depends above all on the absolute value of the recombination coefficient and the density of traps and has the same value in first- and second-order kinetics. Only if no shift is measured, i.e. when TL and TSC peaks exactly coincide, are two special cases possible, as the computer simulation has confirmed.

AH methods used to determine trapping parameters such as trap depths and frequency factors are derived from approximate solutions of the rate equations with the assump­tions relating to the density and charge of free carriers. Therefore they are only approxi­mately valid. The same kinetic model after exact simulation shows smaller maximum temperatures and half-widths [64]. Consequently the trap depths calculated so far with the usual methods are too small. For instance when, in the case of the method of different heating rates, a computer simulation is performed and the approximate formula (2.39) is applied a trap depth reduced to about 50% is determined. Parameters obtained from the initial rise method, which only uses the exponential increase at the beginning of the glow curve, are relatively independent of the kinetic reaction mechanism [34, 63].

2.9. Comparison with experiment

Considering the kinetic processes involving the change of charge carrier densities during thermal stimulation helps to explain the phenomena of TL and TSC. The experimental results can sometimes be analysed in simple kinetic models which allow the determina­tion of some trapping parameters of the solids. Moreover, the analysis should yield information on thermal relaxation processes and on the structure of defects. The useful­ness of such an approach, however, is questionable. In different kinetic models a reason­able fit to experimental results can also be obtained by suitable variation of the para­meters. This means that, unless the model which is applicable to the solid and the specific problem in question is known beforehand, any particular process can be analysed in a large variety of ways.

Therefore the atomic processes and structures must be studied first in greater detail. As well as TL and TSC other techniques must be performed to obtain information on optical, dielectric and magnetic properties. Only if the mechanism of charge and energy transfer and the defects are well understood can one try to construct a specific kinetic model. The following theoretical treatment consisting of an analysis of TL and TSC data

Theory 33

is performed not to obtain further information but only to confirm the existing atomic picture.

In the following example the electron paramagnetic resonance (EPR) technique is used in addition to TL and TSC [67]. The thermally stimulated processes of a specific glow peak in an x-rayed BaWC>4 crystal are analysed in the above-mentioned way. After x-irradiation at 80 K the EPR spectrum exhibits several lines due to the (WC^)3.- com­plex [65, 66]. From the small positiveg shift which is characteristic of orbitals which are more than half full a defect electron is assumed located in a completely occupied oxygen orbital. An isotropic hyperfine line originates by the interaction of the hole and 183W nuclei (natural abundance 14.3%). From the intensity of the lines a simultaneous interac­tion with two equivalent 183W nuclei must be assumed. This leads to the model of a 'self-trapped hole' spread over two neighbouring WO4 tetrahedra. Thus the centre is related to the Vk centre in the alkali halides.

Another line accompanied by two satellites of relative intensity 7.14% can be detected in the high-field region. The relative intensity corresponds exactly to that calculated for the interaction with the tungsten isotope 183W. Thus this centre is associated with a W5+ in a WO|~ complex.

The thermal stability of the centres is studied using the fractional heating technique. The x-irradiated crystal is heated up to a fixed temperature and the density of centres is then determined at 80 K again. The results are shown in figure 2.12. The hole centres disappear completely in the temperature region between 90 and 105 K. A corresponding decrease of the electron centre Ws+ is observed but the concentration only decreases to one-half of the initial value. Further heating gives rise to a complete decrease of the Ws+

centres. In the same temperature region the TL glow curve shows one maximum which can be

correlated with the decrease of centres. As well as the TL a glow curve of TSC is also

90 100 TEMPERATURE (K)

110

Figure 2.12. Relative concentrations of hole centres (a) and Ws+ centres (6) in x-rayed BaW04 crystals measured after annealing at the temperatures given on the abscissa.

34 M Bohm and A Scharmann

obtained after x­irradiation (figure 2.13). The shape of TSC peak is nearly identical with that of TL. The peak temperature of TSC shifts to a higher temperature only by about 0.6 K. The small half­width is another important feature of these peaks.

102 103 104 105 106 107 TEMPERATURE(K)

Figure 2.13. Thermoluminescence (TL) and thermally stimulated conductivity (TSC) of BaWO, crystals measured simultaneously after x­irradiation at 90 K.

It can be assumed that the migration of hole centres is due to warming the crystal above 80 K. This means that a thermally activated hole is recaptured at an adjacent site into an occupied t! orbital. This hopping process is to be considered as the diffusion of small polarons. The mechanism of recombination in TL can then be described by a transi­

tion of an electron from a W5+ complex to an adjacent hole centre. The assumption is based upon the fact that the thermal decay of hole centres can be correlated with that of W5+ centres.

The motion of polarons can be explained in a model involving thermally liberated electrons from traps. Thus, during thermal stimulation the kinetic processes are calculated in rate equations based on the simplest kinetic model with only a single trap and recombi­

nation level. As mentioned in §2.4 in the case of neglected retrapping the rate equations are

q dh/dT=-ah (2.60)

q dn/dT=ah - ynf (2.61)

with a = a0 exp(­E/kT) and a = transition probability of a polaron, E = thermal activa­

tion energy, h = density of hole centres, n = density of hopping hole centres,/= density of electron centres Ws+, and 7 = capture cross section for capturing a hole by a W5+ ion (recombination). ■ The exact computer simulation of TL calculated from equations (2.60) and (2.61) yields a glow curve which exhibits an asymmetric shape and a very small half­width just

Theory 35

like the experimental one (see §2.8). However, in the case of TSC the agreement between theory and experiment must be considered unsatisfactory. In the computer simulation the TSC peak is shifted by about 2-3 K to higher temperatures with respect to the TL peak whereas the measurement yields a shift of only AT = 0.6 K. Hence the kinetic model is to be extended by precisely considering all experimental data.

As a result from the EPR measurements and fractional heating technique the initial value of the density of recombination centres W5+ is diminished only by 2/3 to 1/2 during the thermal stimulation (figure 2.12). This result is considered in the kinetic model by the assumption that the density of recombination centres is to be approximately constant during the glow peak. This is the model described in §2.7 (see equation (2.42)). The calculation results in identical glow curves for TL and TSC. This means that no shift of the TSC peak is to be expected from this model.

As already mentioned this case is also discussed in a more common model with addi­tional thermally disconnected traps M. In this the density of additional traps is to be assumed as being large when compared with the density of traps involved in the TL process (H/MKl). The computer simulation also shows identical curves of TL and TSC [57]. The five-fold density of additional traps is sufficient to obtain equal shapes and maximum temperatures.

In the cases considered so far retrapping is neglected. By assuming such retrapping (j3/7>0) it is very difficult to obtain analytical expressions for TL and TSC. However, the computer simulation succeeds in representing the intensities of TL and TSC without the approximations commonly used. From the special case j3/y> l,H/M< 1 and fiH/yM< 1 the simulation shows equal behaviour of both thermally stimulated phenomena [57].

Hence, the glow peak in x-irradiated BaW04 at about 105 K may be approximately described in both models due to EPR, TL and TSC measurements. From hopping processes a retrapping may be assumed, as is proposed in the second model. However, this cannot be decided from kinetic considerations.

In a more precise analysis of this glow peak the small but finite shift of the TSC peak must be considered. This can be done in a qualitative manner [68,69]. Starting from both possible cases (5/y = 0 and 0/7>O, respectively, equations (2.12) and (2.13) are commonly valid. Combining these equations and eliminating the density of free carriers n leads directly to the relation.

en a = /. (2.62)

constant x jf

Let T^ be the temperature where the conductivity exhibits a maximum. Then one obtains the condition at this temperature:

3a 37 = 0. (2.63)

Now, if one takes the temperature derivative of equation (2.62) and evaluates it at the TSC peak 7^, where equation (2.63) is valid, one has

3/ / 3 / — = at T=1%. (2.64) 3 7 / 3 7

36 M Bohm and A Scharmann

The quantities JJL and y are assumed to be temperature-independent. Using the facts that the derivative of the density of recombination centres with respect to temperature is always negative due to the decrease (9//37'<0 at all temperatures) and the quantity // / is always positive one notes from equation (2.64) that

3 / / 3 r < 0 at T=T^. (2.65)

This expression indicates that the TL showing a negative slope is decreasing at the TSC peak. It means that the TSC peak is at higher temperatures than the TL peak. Thus, the smaller the slope of TL (dl/dT) is at T= T^, the smaller is the shift of the TSC peak. The small slope of the TL curve involves a slow decrease of recombination centres (df/dT). Consequently, in the case of constant density of recombination centres (df/dT = 0) the slope of the TL curve vanishes and no shift of the TSC peak occurs. Therefore the shift of the two glow curves depends on the rate of decrease of recombination centres.

In BaW04 crystals the density of recombination centres W5+ is not exactly constant during heating and thus identical glow curves of TL and TSC cannot be expected. How­ever, a complete decrease that gives rise to a shift of 2-3 K from computer calculations is not observed but the density of recombination centres decreases partially (figure 2.12). Hence, the TSC peak should be partially shifted to higher temperatures. The small measured shift of only 0.6 K leads to the idea of partially decreasing recombination centres. The good agreement between the experimental results and kinetic considerations confirms the assumption that Ws+ ions act as recombination centres for thermally activated holes.

2.10. Conclusions

In the literature many attempts have been made to calculate the related phenomena of TL and TSC from a kinetic model primarily in order to study defect properties and to have a tool for determining trapping parameters such as activation energies and frequency factors. Because of the changes in densities of electrons and holes during the thermal stimulation, the calculations are based on a simplified energy level scheme in the different phenomenological theories. The differential equations are usually solved in an approxi­mate way with the assumption that the density of the free carriers is small compared with that of trapped carriers during stimulation. However, even in the simplest model consisting of a single trap level and a recombination level, the analytical solutions seem to be reasonable only for TL. The shape of TSC has also been calculated in some cases, however, by the introduction and variation of additional parameters such as non-radiative transitions or additional thermally disconnected traps.

An exact solution has shown that neither of these additional assumptions are necessary. The simplest model is sufficient to explain the observed phenomena. As a consequence, the influence and magnitude of such additional effects cannot be concluded from TL and TSC measurements alone. Only if these measurements are related to studies on other opti­cal, electrical and magnetic properties of the sample will the experimental results provide a reasonable basis for a discussion of the electronic processes in solids. The present and probable future importance of the studies of TL and TSC depends on a strong inter­relation with other branches of solid-state physics.

Theory 37

References

1 Castner T G and Kanzig W 1957 7. Phys. Chem. Solids 3 178 2 Woodruff T O and Kanzig W 1958 7. Phys. Chem. Solids 5 268 3 Delbecq C J, Hayes W and Yuster P H 1961 Phys. Rev. 121 1043 4 Jones C D 1966 Phys. Rev. 150 539 5 Murray R B and Keller F J 1967 Phys. Rev. 153 993 6 Schulman J H and Compton W D 1962 Color Centers in Solids (New York: Macmillan) 7 Liity F 1968 Physics of Color Centers ed. W B Fowler (New York: Academic) pp 181-242 8 Kanzig W 19607. Phys. Chem. Solids 17 80 9 Pick H 1960 Z. Phys. 159 69 10 Seidel H and Wolf H C 1968 Physics of Color Centers ed. W B Fowler (New York: Academic) pp

537-624 11 Delbecq C J, Kolopus J L, Yasaitis E L and Yuster P H 1967 Phys. Rev. 154 866 12 Scharmann A, Grasser R and Bohm M 1977 7. Electrostatics 3 1 13 Adirowitsch E I 1953 Einige Fragen zur Theorie der Lumineszenz der Kristalle (Berlin: Akademie-

Verlag) 14 Randall J T and Wilkins M H F 1945 Proc. R. Soc. A 184 366, 390 15 Garlick G F J and Gibson A F 1948 Proc. Phys. Soc. 60 574 16 Schon M 1958 Tech. Wiss. Abhandl. Osram-Ges. 7 175 17 Halperin A, Leibovitz M and Schlesinger M 1962 Rev. Sci. Instrum. 33 1168 18 Kelly P J and Laubitz M J 1967 Can. J. Phys. 45 311 19 Urbach F 1946 Cornell Symp. (New York: Wiley) 20 Hoogenstraaten W 1958 Philips Res. Rep. 13 515 21 Nicholas K H and Woods J 1964 Br. J. Appl. Phys. 15 783 22 Booth A H 1954 Can. 7. Chem. 32 214 23 Bohun A Czech. 7. Phys. 4 91 24 Boiko I I, Rashba E I and Trofimenko A P 1960 Sov. Phys.-Solid St. 2 99 25 Grossweiner L I 1953 7. Appl, Phys. 24 1306 26 Lushik C B 1956 Sov. Phys.-JETP 3 390 27 Chen R 19697. Appl. Phys. 40 570 28 Fleming R J 1968 Can. 7. Phys. 46 1569 29 Land P L 19697. Phys. Chem. Solids 30 1683 30 Halperin A and Braner A A 1960 Phys. Rev. 117 408 31 Halperin A, Braner A A, Ben-Zvi A and Kristianpoller N 1960 Phys. Rev. 117 416 32 Townsend P D, Clark C P and Levy P W 1967 Phys. Rev. 155 908 33 Gobrecht H and Hofmann D 19667. Phys. Chem. Solids 27 509 34 Braunlich P 1967 7. Appl. Phys. 38 2516 35 Chemmy P J.Towsend P D and Levy P W 1967 Phys. Rev. 155 917 36 Mohan N S and Chen R 19707. Phys. D: Appl. Phys. 3 243 37 Satzhyamoorthy A, Bhalla K C and LuthraJ M 1976 7. Luminesc. 11 357 38 Mintoni C, Rucci A and Sepri A 1968 Ricerca Sci. 38 762 39 de Muer D 1970 Physica 48 1 40 Maxia V, Onnis S and Rucci A 19717. Luminesc. 3 378 41 Keating P N 1961 Proc. Phys. Soc. 78 1408 42 Shalgaonkar C S and Narlikar A V 19727. Mater. Sci. 7 1465 43 Chen R 1976 7. Mater. Sci. 11 1521 44 Kivits P and Hagebeuk H J L 1977 7. Luminesc. 15 1 45 Haering R R and Adams E N 1960 Phys. Rev. 117 451 46 Pickard P S and Davis M V 19707. Appl. Phys. 41 2636 47 Braunlich P and Scharmann A 1964 Z. Phys. Ill 320 48 Bohm M and Scharmann A 1969 Z. Phys. 225 44 49 Kanzig W 1960 7. Phys. Chem. Solids 17 88 50 Dussel G A and Bube R H 1967 Phys. Rev. 155 764 51 Kelly P and Braunlich B 1970 Phys. Rev. B 1 1587

38 M Bohm and A Scharmann

52 Bohm M and Scharmann A 1971 Phys. Stat. Solidi a 4 99 53 Braunlich P and Kelly P 1970 Phys. Rev. B 1 1596 54 Unger K 1962 Phys. Stat. Solidi 2 1279 55 Braunlich P 1963 Ann. Phys., Lpz. 12 263 56 Lamp P 1969 J. Phys. Chem. Solids 30 1693 57 Bohm M and Scharmann A 1971 Phys. Stat. Solidi a 5 563 58 Razdan K N, Brennan W D and Grossweiner L I 1970 J. Appl. Phys. 41 832 59 Razdan K N, Wiatrowski W G and Brennan W D 1973 J. Appl. Phys. 44 5483 60 May C E and Partridge J A 1964 J. Chem. Phys. 40 1401 61 Dutton D and Maurer R 1953 Phys. Rev. 90 126 62 Pipins P A and Grigas B P 1964 Opt. Spektrosk. 18 43 63 Saunders I J 1969 J. Phys. C: Solid St. Phys. 2 2181 64 Kelly P, Laubitz M J and Braunlich P 1971 Phys. Rev. B 4 1960 65 Zeldes H and Livingston R 1961 J. Chem. Phys. 34 247 66 Born G, Grasser R and Scharmann A 1968 Phys. Stat. Solidi 28 583 67 Bohm M, Cord B, Hofstaetter A, Scharmann A and Parot P 1975 J. Luminesc. 17 291 68 Fields D E and Moran P R 1974 Phys. Rev. B 9 1836 69 Gasiot J and Fillard J P 1975 Phys. Stat. Solidi a 32 K85

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

3 Instrumentation

H W JULIUS

3.1. Introduction

This chapter deals with the instrumentation used to evaluate thermoluminescence dosemeters. The basic phenomenon of thermoluminescence dosimetry, i.e. the stimula­tion of light by heating the phosphor after it has been exposed to radiation, takes place in the reader instrument.

The method of reading a TL detector is simple and straightforward. In a relatively short time (usually ranging from a few seconds to a few minutes) the TLD is heated from ambient temperature to some 200-300 °C, and the emitted light is measured quantita­tively. Thus the reader consists essentially of two basic parts:

(i) a heating device, and (ii) a light detecting system.

Constructing a reader instrument would therefore be easy if there were not so many pitfalls in the TLD method, such as:

(i) The method is a destructive one, i.e. the radiation energy absorbed in the detector (or at least most of it) is lost during the readout procedure. Care should therefore be taken not to lose information.

(ii) The behaviour of a TL detector (such as its sensitivity and fading characteristics) is substantially dependent on its thermal history. This means that — especially when no post-read (pre-irradiation) anneal is applied — the response of a TLD after re-use is influenced by the heat treatment during the previous readout.

(iii) Thermoluminescence dosimetry is a relative method, and therefore attention should be paid to high stability, combined with frequent calibration.

(iv) Because of surface-related phenomena, the light emitted tends to be contaminated with non-radiation-induced signals (chemiluminescence) which, when not suppressed, restrict the lower limit of detection.

The TLD method has a wide range of applications, many of which have special requirements as far as the evaluating instrument is concerned. Some fields of application are:

(a) personnel monitoring (requiring badges), (b) environmental dosimetry (requiring high sensitivity), (c) patient dosimetry in radiotherapy and x-ray diagnostics (requiring wide dose ranges), (d) age determination (requiring relatively large amounts of TL material), (e) military dosimetry (requiring special dosemeters and the use of portable instruments),

etc.

40 H W Julius

These subjects are dealt with in detail in the corresponding chapters in this book. In general, different requirements hold for instruments used for research purposes (i.e.

for the investigation of TL materials) and large-scale routine dosimetry. In the first case the recording of the light output as a function of temperature (or time) ('glow curves') will be made, requiring well-controlled temperature/time profiles, while speed is of no importance. In the second case the primary concern is high-speed operation (thus fast heating and preferably automatic loading and processing of detectors is needed), to provide a large throughput.

Some systems are specially designed to cover special applications, while others are less or more versatile. A large number of TLD systems have been developed in recent years, partly by industry, partly by laboratories and organisations for their own use (of which some are commercially available). Although not all of them can be mentioned, some systems are reviewed, after the principles of TLD instrumentation have been discussed.

3.2. The heating system

3.2.1. General

One of the essentials in heating a TL detector in the reader is that there should be optimal thermal contact between the heating medium and the detector. Insuffi­cient thermal contact tends to lead to irreproducible results. The way a TL detector follows the temperature of the heating medium depends on the thermal capacity and the thermal conductivity of the detector material. Loose powder, extruded ribbons or rods and sintered or compressed materials have, in general, good thermal conductivity. Materials of this kind usually have well-defined geometrical shapes, so that they can easily be brought into good contact with the heater. In contrast, detectors made of phosphors embedded in Teflon (PTFEI"), such as LiF-Teflon discs and rods, have low thermal conductivity. These materials tend to crinkle or curl up during heating. Good thermal contact is then more difficult to achieve, especially when they are heated in a tray, so that special precautions are needed (see §§3.2.2.1 and 4.2). This is even more important when glow curves are taken for research purposes, because large temperature gradients tend to blur out details in the light output as a function of temperature.

Often the total heat treatment of a TL detector is considered as being split up into three different steps:

(a) The pre-read heating, applied in order to remove completely the low-temperature peaks in the glow curve, i.e. to empty the low-energy traps (the release temperature of the electrons being at or below about 100 °C, and hence being subject to fading).

(b) The readout procedure itself. (c) The post-read (or pre-irradiation) anneal, applied either to remove all residual signals

from the dosemeter and/or to restore the trap distribution in the lattice in order to restore the original TLD properties (sensitivity and fading characteristics).

These three steps are frequently performed separately, using ovens (see §4.10) for (a) and (b). They can, however (and sometimes are), combined in one single read-anneal

f Polytetrafluoroethylene.

Instrumentation 41

cycle in the reader instrument, which therefore must be equipped to perform a flexible, programmable heat cycle, so that it can be adapted to various materials.

As far as the pure readout cycle is concerned, several philosophies and approaches have been developed.

In the early days a linear temperature rise was applied in most TLD readers used for both scientific purposes and routine measurements. This method (which is still in use) permits glow-curve analysis, the light output being plotted against time, and hence allowing the various peaks to be associated with temperature. In most cases the (variable) temperature ramp is followed by a plateau, the height and duration of which can be set on the reader panel. Response measurements can often be done by choosing a distinct temperature (or time) interval for the integration of the luminescent signal (figure 3.1). For large-scale routine measurements, this method tends to be too time-consuming. A rapid rise in temperature — usually not linear against time — is then preferred (figure 3.2).

-PLATEAU

-LIGHT INTEGRATION

ti t2 t 3

TIME Figure 3.1. Linear heating, combined with a plateau. The temperature (or time) interval during which the light output is measured can be varied. (I) temperature, (II) light output.

- IU sec Figure 3.2. Non-linear heating. Maximum temperature and measuring time are variable. (I) temperature, (II) light output.

42 HW Julius

As long as the characteristics of the dosemeter are not adversely influenced, there is only one requirement for the 7X0 function: that it should be highly reproducible.

Generally, it is not easy to select a special integration interval, as the luminescence begins too quickly. A pre-read heat treatment can, if necessary for a large number of dosemeters, be performed in an oven. The same holds, of course, for the post-read anneal.

Figure 3.3 shows the multiple plateau heating method, in which the three steps (a), (b) and (c), mentioned earlier, are combined.

LJ

3 I— < CE Ld Q.

UJ i—

PREHEAT

300

210-

L 100

I S

' I " I

<.oo°c I I

270°C j !

-150°C \l/\

-J\J

READ

I I I 1 -1 / w f

Vin *,

ANNEAL " 1

1 1 1

'/ I )

COOL

v \

\

3 Q-I—

o

X o

TIME Figure 3.3. Preheat, read and anneal are combined in one single programmable heat cycle. The various plateaux can be reached either linearly or non-linearly. (I) temperature, (II) light output.

As pointed out earlier (§3.1), the thermoluminescence response may be contaminated by spurious (non-radiation-induced) signals, due to excitation of impurities at the surface layer of the detector (chemiluminescence). This effect can be suppressed by heating the TL detector in an inert-gas atmosphere, for which purpose pure dry nitrogen gas (— 0.5-1 lmin~')is most commonly used (see also §4.3).

3.2.2. Heating methods

This subsection deals with various methods of heating thermoluminescence dosemeters. For the sake of simplicity they are discussed under different headings, although combina­tions are encountered in versatile instruments.

3.2.2.1. Controlled planchet heating The most common method of heating is still the one using a low-thermal-capacity (hence thin) metal planchet (tray), on which the dosemeter is placed (see also chapters 4 and 7). The heating power is generally provided by a low voltage-high current transformer. The planchet can be heated indirectly (see below) but it generally acts as the heater element itself, in which case it is connected across the transformer secondary winding, closing the circuit with some 10 mD. after it has been brought into the readout position. Therefore, the tray is placed in a drawer assembly by which it can be pulled out and pushed into the reader for (un)loading and reading, respectively (figures 3.4 and 3.5). The shape of

Instrumentation 43

TLD-

PLANCHET

SAMPLE DRAWER/

TEMPERATURE SENSOR

-LIGHT DETECTOR ( PM )

OPTICAL SYSTEM ( INCL. LENSES, MIRROR AND FILTERS )

LIGHT SOURCE

* - INERT GAS

TRANSFORMER HEATER CONTROL Figure 3.4. Schematic set-up of a conventional type of TLD reader with controlled planchet heating and sample drawer system. This principle is encountered in a number of commercial instruments.

Figure 3.5. Example of a sample drawer with planchet and TLD chip.

the planchet car. be adapted to the form and size of the TLD. Both solid forms and loose powder can be used.

Because of the low thermal capacity of the heater planchet, it is relatively simple to control its temperature versus time profile, just by controlling the generating voltage. A thermocouple is used to give a negative-feedback signal to the voltage control unit. A practical problem is to obtain good contact between the thermocouple junction and the heater element. Welding the thermocouple to the planchet itself gives adequate contact but makes the tray less easily interchangeable. When the thermocouple is simply mechanically pressed against the planchet, thermal contact is not reliable and hence heating is not reproducible. A solution for both problems has been found in heating the tray indirectly by means of a separate heater element which can be brought into close contact with the planchet. The thermocouple is then welded to the heater element.

44 HW Julius

Instead of thermocouples, infrared sensors, mounted underneath the planchet, have been used. Although thermal contact problems are avoided, this technique is not entirely satisfactory, because the sensitivity of the sensor can change, due to obstacles in the optical pathway (dust) or changes in the IR emitting property of the planchet.

The low-thermal-capacity heating method is very suitable for programmable heating cycles (see figures 3.1 and 3.3), so temperature ramps, plateaux, time intervals (ranging from about 20 s to over 1 min), etc can be set on the instrument, so that for each TL material the optimal readout procedure can be chosen (by trial and error). Although this seems to be an ideal situation, one should keep in mind that, if low residual dose and satisfactory fading characteristics can be achieved with a simple readout cycle — and they often can — reproducibility of the heat treatment is the most essential parameter to focus on, to obtain reliable results.

A substantial problem associated with the planchet method is to get sufficient and reproducible thermal contact between the heater and the detector. This is especially true for large PTFE disc-type dosemeters and even more so for thin ones. To hold the TLD tight against the tray, various techniques have been used, most of them applying some device on top of the dosemeter, so that the detector is 'sandwiched' between two layers. Springs, meshes, cruciform apertures, rings and quartz glass discs are used for this purpose. In some instruments these devices are made part of the heating system, so achieving lower temperature gradients. Obviously, these prevent a fraction of the emitted light from reaching the light detector. This is acceptable, provided this fraction is reproducible. An alternative solution is to hold the detector in place by suction.

Several TLD readers use a lens system to focus the TL light on to the cathode of the PM (see §3.3). Reproducible positioning of the detector is then of greater importance.

Care should be taken to keep the surface of the planchet clean, since changes in its reflectivity result in changes in effective light output of the TLD, in particular when transparent materials are used (see §7.2.1).

IR light emitted by the tray when reaching the maximum readout temperature can add a spurious signal to the emitted TL light. This will lead to high and usually irrepro-

ducible background signals, which of course limit the lower detection threshold.

3.2.2.2. The heater block method An analogous method for heating the TLD uses a metal block ('hot finger'), held at a constant sufficiently high temperature by a resistive heater element, which is brought into contact with the detector. Some advantages of this method are fast heating rates (hence short cycle times; some 10 s are typical) and simple and reliable construction. Because of the lack of flexibility, this technique is obviously not suitable for research purposes, but is quite popular in personnel monitoring systems. Since an appreciable amount of electric power is dissipated in the reader, cooling the photomultiplier tube is unavoidable in practice.

The way in which the TLD is brought to or held in its readout position depends on the design of the dosemeter holder. Sometimes the detectors are stuck on an adhesive Kapton tapef, while in other systems they are mounted in a metal plate, either sandwiched between two thin Teflon foils, or fitted by a clamping ring. An individual monitoring

■f Polyimide film with a pressure-sensitive silicone polymer adhesive on one side.

Instrumentation 45

system has been developed which uses dental-size TLD plaques (Teflon embedded with phosphors). Different areas of the dosemeter can be read by applying local heating with blocks (see also §4.2).

The hot finger method can also be used to read loose solid TLD and powder, in which cases thin platinum trays are used.

3.2.2.3. Hot gas heating In the late 1960s the idea was born of using nitrogen gas — applied in most TLD readers to reduce chemiluminescence — to heat the dosemeter (figure 3.6) [12]. The obvious advantage of this method is the almost perfect thermal contact between the detector and the heating medium, independent — within certain limits — of the form and shape of the dosemeter. The TLD, generally brought into the heating cavity by means of a vacuum needle, is heated by one or more hot nitrogen gas jets. In the first case only small TLD (up to 6 mm in diameter) can be read, while the latter can also read the larger disc­type dosemeters (12.7 mm diameter). Other advantages of hot nitrogen heating are a short heating cycle (about 10 s), low background signal, excellent reproducibility, easy loading procedures (which make automation a matter of course) and high versatility (ribbons, discs and rods can be read without interchanging instrument devices).

Disadvantages are similar to those of the hot finger system: the heating cycle does not lend itself to programming (the gas temperature rigorously being kept constant), and thus the method can be used only for routine measurements.

FRONT VIEW

1 - FURNACE 2 - THERMOCOUPLE 3 - BRASS TUBE t - P Y R E X GLASS READOUT CHAMBER 5 - EXTRUDED TLD LiF 6 - VERMICULITE INSULATION 7 - WATER-COOLED JACKET m^zj 8 - LISHT TRAP " ^ * 9 - REMOVABLE BRASS CARTRIDGE

10- ROTATING STAGE 11 - ESCAPEMENT 12 - ELEVATOR 13- VACUUM HOLDING PROBE U - MOTOR 15 - LENS SYSTEM 16- VACUUM LINE 17 - CAM FOLLOWER 16 - PM TUBE 19- MAIN DRIVE MOTOR 2 0 - FLOWMETER

SIDE VIEW

Figure 3.6. An example of a TLD reader, using hot nitrogen gas to heat the detector (after Petrock and Jones [12]). Variations on the theme have been utilised by others.

46 H W Julius

Instrument parts in the immediate vicinity of the heating cavity get warm, so that cooling (for instance, of the PM tube) is necessary. Obviously, powder cannot be handled unless it is sealed in small glass bulbs. In this case, there is no direct contact between the heating medium and the phosphor, so that hot air can be used instead of nitrogen.

3.2.2.4. RFheating A TLD system has been designed which uses RF heating [5]. The thermoluminescent material is sealed on to a graphite tray using silicone resin adhesive. The dosemeter is brought (by a drawer system) to above a flat, water-cooled RF coil (1 MHz), where it reaches its maximum temperature (300°C) in about 10 s. In principle, this heating method allows the collection of the light emitted from both sides of the dosemeter. Using a specially designed dosemeter and two photomultipliers, both surface and pene­trating doses can then be measured simultaneously. (The author is not aware of whether this has in fact been done.) Disadvantages of the RF method seem to be the large amount of RF power necessary, and the difficulties of controlling the heat cycle.

3.2.2.5. Optical heating Several years ago a projector bulb was used to heat the TLD [2], The method was abandoned because of problems associated with spurious light from the heater. Recently, new life has been given to the method. In a TLD system, specially designed for personnel

Figure 3.7. Examples of glass bulb TL doscmeters with built-in heater elements, (a) Phosphor directly bound to the heater, (ft) Phosphor fixed on to the outer surface of a hollow cylinder indirectly heated by a filament, (c) Extruded or hot-pressed phosphor in close contact with the heater element.

Instrumentation 47

dosimetry, detectors are used consisting of a thin layer of crystalline phosphor, bound to a polyimide film, which on the back side is coated with a light- and iR-absorbing carbon layer. By irradiating the latter with a pulsed IR beam, the detector can be heated in 0.6 s. The TL light is emitted and measured within 1.5 s (figures 3.27 and 3.28).

3.2.2.6. Built-in heater element Although this method has less to do with instrumentation than with the design of TL dosemeters, it will briefly be discussed here. Examples of dosemeters with built-in heater elements are usually in the form of a subminiature evacuated radio tube or a fuse-type glass bulb. Within the bulb the phosphor may either be directly bound to the resistive heater or fixed on to the outer surface of a hollow cylinder which is indirectly heated by a filament. A third option is to bring the solid TL detector into close contact with the

Figure 3.8. Drawer assembly for glass bulb TL dosemeter insertion.

Contact pi r .sv^

Figure 3.9. Glass bulb TL dosemeter adapter for dosemeter types (a) and (c) shown in figure 3.7.

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wires of a built-in heater element (figure 3.7). By introducing the dosemeter into the reader instrument, the contact pins (or caps) are connected to the heater voltage supply (figures 3.8 and 3.9).

Some advantages of these dosemeters are good thermal contact between the phosphor and the heater element and consequently good reproducibility, and sturdiness of the dosemeter assembly (which is of greater importance in military applications).

Some disadvantages are a low-energy cutoff due to the glass encapsulation, self-irradiation (which, however, can be minimised by using glass with low potassium content [11]), and high cost. Moreover, most types are fairly bulky.

3.3. The light detecting system

3.3.1. General

The technique of light detection is well known and not specific to TLD. The light measur­ing system does not affect the TL detector during readout — as the heating system does — and therefore this part of the reader has less to do with the pitfalls of thermoluminescence dosimetry. Nevertheless it should be given attention because of the large dose range to be covered (from a fraction of a millirad to several kilorad or even megarad) and the precision (and hence stability) required in radiation dosimetry.

The general task of the light detecting system is to convert the light emitted by the TL detector into an electronic signal (charge, current, voltage), which can be measured and used to activate output devices (meter, scaler, printer, chart recorder, computer, etc).

The light detecting system can be thought of as being divided into three parts:

(i) The light collecting system, (ii) The light detector and signal amplifier,

(iii) The signal conditioning system.

In figure 3.10 a schematic diagram of the light detecting system is shown.

CURRENT TO FREQUENCY C0NVERT0R

ELECTRO METER PHOTON COUNTING

HIGH VOLTAGE POWER SUPPLY

LENS I AND/OR MIRROR)

TLD OR LIGHT SOURCE HEATING SYSTEM

SIGNAL CONDITIONING SYSTEM

LIGHT DETECTING SYSTEM

LIGHT COLLECTING SYSTEM

INITIAL SIGNAL

Figure 3.10. Schematic diagram of the light detecting system of a TLD reader.

Instrumentation 49

3.3.2. The light collecting system

In order to achieve maximum detection efficiency, the first aim is to concentrate as much luminescence light as possible on the sensitive layer of the light detector. It would in principle be most efficient to bring the TLD into direct contact with the photocathode. However, because of the temperature sensitivity of the photocathode, thermal separation is necessary. To achieve this, several methods have been applied, using lens systems, heat filters, water layers, vacuum layers, light pipes, etc. Easy thermal separation is attained when the light detector is placed beside the heating system instead of above it in order to avoid heat transport by hot air (a mirror system can then be used).

The optical system is preferably designed in such a way that light collection is minimally affected by changes in the position of the dosemeter.

Optical filters are frequently applied in order to prevent infrared radiation — emitted by the heater elements or the TLD itself — reaching the light detector.

Sources of error in the light measuring system are introduced by obstacles (dust) in the light path or changes in reflectivity of the heater planchet or parts of the optical system.

3.3.3. The light detector and signal amplifier

The light emitted by a TL detector after being exposed to 1 mR is of the order of 10"I3lm. Up to now solid-state photodetectors are not sensitive enough to allow the measurement of such low levels and therefore only photomultiplier (PM) tubes are considered to be applicable in TLD. They combine high sensitivity and large dynamic range (l(T13-10~6im).

A PM tube is chosen for optimal cathode sensitivity, matching the wavelength typical of the TL material under consideration. As an example, LiF emits in the blue-green region, whereas Li2B407 emits in the red. For optimal results, these materials require different PM tubes. A low response to other wavelengths, especially IR, is preferred. Figure 3.11 gives an example of PM tubes with various spectral responses.

Sensitivity and signal-to-noise ratio are affected by a number of parameters. Some of them are now dealt with briefly.

Dark current is the signal which is generated by the PM while light is absent. The phenomenon is mainly due to thermal emission of electrons from the cathode layer. PM tubes, even of the same type, may show large variations in dark current, a reason why they are preferably selected for low noise. The remaining dark current is kept as low as possible by operating the tube at low temperature (Peltier cooling is generally used).

Temperature effects. Changes in temperaure affect the overall sensitivity of the PM (approximately 1%°C_1). The tube should therefore be kept at constant temperature.

Fatigue effect. If the PM is exposed to large light intensities, its sensitivity decreases, while the dark current increases. The effect of PM fatigue gradually vanishes over a period of hours. Errors may be introduced if reference light sources of high intensity are used.

Aging effects. After long use the gain of the PMT gradually decreases. In addition to long-term gain drift, a slow shift in spectral response may occur. If the sensitivity is cali­brated with a reference light source, the spectral emission of which deviates significantly from the wavelength of the luminescent light, errors can be introduced. (See also §3.3.5.)

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^ x ^ ^ ■ ^ \ ^

\

, -5 -20 rS-20

k 96S& \L-EXTENOED

L \ \ s

"20

\ V-s-io\ \ \

\\ Vs"\\ \

■ \ \ Is

" " \ \ \ \

0.5 0.6 0.7 0.8 WAVELENGTH MICRONS

Figure 3.11. Typical spectral response curves for PM photocathodes (EMI, 50 and 30 diameter tubes).

Magnetic fields may affect the PMT sensitivity due to changes in focusing. In this respect, magnetic fields generated by the heating power are often the culprit, especially when DC currents are used. These effects can be reduced by using a cylindrical mu­metal screen, surrounding the PM tube, which is kept at cathode potential.

Discharges, mostly resulting from too­high tube voltages, may cause charge bursts at the anode, due to light emission associated with the discharge. As a result the background signal is affected in an unpredictable way.

High tension variations have substantial influence on the PM sensitivity: 1% change in high tension may cause some 10% change in gain.

Current leakage which may occur at various locations in or at the surface of the PM tube may cause apparently high dark currents. Cleaning the connector pins may some­

times redress the problem.

3.3.4. The signal conditioning system

The signal conditioning system essentially serves to convert the PM signal into a quantity which can be measured quantitatively. The PM signal (which is typically at the picoamp level) generally needs further amplification. For the next step two methods can be applied.

3.3.4.1. Peak height measurement The height of the peak with maximum amplitude in the glow curve is taken as the relevant signal. It is measured by converting the PM current into a voltage by which a capacitor is charged. The (maximum) charge of the capacitor is then measured. Because the height of the glow curve main peak depends strongly on the heating rate of the dose­

Instrumentation 51

meter, this method requires extremely reproducible heat cycles (see figure 3.12). How­

ever, the improved signal­to­noise ratio is a real advantage in the case of low dose measurement. Moreover, the low­temperature peaks, which are subject to fading, are omitted. Nevertheless, the method is seldom used.

200

150

100

50 y

y\

■ /L / ~ ^ ~

/ /

/ *• INTEGRAL READING METHOD

x PEAK HEIGHT READING METHOD i i i I

z

<

CD

<

4 5 6 7 8 FILAMENT VOLTAGE

Figure 3.12. Peak height and integrated peak area as a function of the heating rate for CaF 2 : natural (after MBLE).

3.3.4.2. Peak area measurement This is the method most often applied in thermoluminescence dosimetry. The PM signal, having been digitised by a charge­to­pulse converter, is fed into a counter. The number of pulses at the end of the readout time is proportional to the integral light output of the dosemeter. A ratemeter, connected to the converter output, provides the signal used for plotting glow curves. As can be seen from figure 3.12, this method is less affected by changes in the rate at which the TLD is heated during readout.

As an alternative technique for measuring the integrated light sum, the method of photon counting has also been (and is still) used. Although one would expect a better signal­to­noise ratio, theoretical and experimental investigations [10] have shown that improvement by a factor of 1.4 is the maximum achievable.

3.3.5. The reference light source

The many sources of error discussed in the previous subsections make frequent sensitivity checks of the entire system a matter of necessity. The safest way to do this is with TL detectors, irradiated to a precisely known exposure (or dose). Both the light detecting system and the heat cycle are tested in this way.

For more frequent checks of the sensitivity, a reference constant light source is generally employed. It is placed at or near the normal position of the TLD (figure 3.4 shows an example). This can be done before each reading or at suitable intervals, either by the operator or as dictated by an automatic control system. If correctly applied, this method of system 'calibration' may trace changes in the optical pathway, changes in PM

52 H W Julius

sensitivity and malfunction of the electronic circuitry. Reference light source measure­ments have been used for automatic checks of the overall system sensitivity.

Light sources are usually made of long-lived radio-isotopes (14C or '"Sr) embedded in a scintillation material. Light emitted by the source should preferably have the same spec­tral composition as that of the TLD material in use. It should be noted that most light sources have outputs which are temperature-dependent. Errors may be introduced due to changes in the light source itself or variations in the spectral response of the photo-multiplier tube. The heating system remains unchecked. For more information on light sources and the errors encountered with them, see §§4.4, 7.2.1 and 11.4.

3.4. Special items

In the previous subsection only the two basic parts of a basic TLD readout instrument have been discussed. Modern instruments, however, are usually equipped with, or can be connected to, additional devices which extend their capabilities. Some of them are now briefly reviewed.

3.4.1. Glow-curve plotting

As has already been pointed out, the analysis of the light output as a function of time or temperature is essential if one wishes to investigate thermoluminescent phosphors. For this purpose many TLD readers are provided with a special output of the ratemeter signal, to which an X,Y or X,t recorder (plotter) can be connected. It is sometimes convenient to use a pulse height analyser to record the (pulsed) TL signal against time, because the digitised glow curve can more easily be prepared for (computer) calculations. The glow-curve signal can also be fed directly into a computer for pattern recognition, which has been suggested as being useful in routine dosimetry.

3.4.2. Output writing and response handling

Almost all TLD readers can be equipped with an output recorder, such as a simple printer, teletype or similar device. Printing calculators can also be used. Some readers have built-in electronic devices which can, for instance, subtract a predetermined background or can multiply the response by a calibration factor (in order to give the output in terms of dose or exposure). More sophisticated systems — in particular the automatic ones — sometimes have the capability of setting a lower and an upper level (by thumbwheels or keyboard) with which the response can be compared. The result of the comparison may cause an alarm signal or a prolonged heating time, or can be used to select automatically detectors with uniform sensitivity. Because such installations — which are intended for large-scale measurement — are generally computer-compatible, all data handling and instrument control is therefore preferably done by a minicomputer (or microprocessor).

3.4.3. Automation

After the method of thermoluminescence dosimetry proved to be successful in radiation dosimetry, its applications developed explosively. Consequently, there is a growing tendency towards automation.

Instrumentation 53

Two groups of automatic instruments can be distinguished:

(a) Systems particularly designed for personnel monitoring purposes. They are, in general, merely capable of processing some special kind of TLD badge and no other forms can be accepted. Badge handling includes identification decoding and reading the dosemeter(s) in the holder, which may be controlled by a microprocessor or large computer system for dose calculation and record-keeping of individual dose data. The variety in the degree of sophistication, possibilities and approaches is so enor­mous that they cannot be dealt with in any detail in the context of this chapter. Some rough information can be found in §3.5.

(b) Systems particularly designed for automatic readout of loose solid TL detectors. At the moment only a few systems of this kind exist (see §3.5). They all use a rotating disc type of sample changer to present the TLD to the reader. Transportation of the TLD to and from its readout position is done either by a built-in manipulator (such as is used in hot gas readers) or by a separate device. It should be noted that the con­ventional type of reader with a drawer mechanism can quite easily be transformed into a semi-automatic instrument: a vacuum needle, mounted at the end of the swinging arm, can take a TLD from a turntable and put it on the planchet (and vice versa). The drawer is then closed and opened by a suitable device, operating in con­junction with the loading system.

A TLD system has been developed which combines both reader types (a) and (b) in the same instrument. For each application the proper sampling device should be chosen.

It is generally stated that the use of automatic readers is only justifiable when very large numbers of dosemeters are to be evaluated. Although this might be true from the point of view of financial investment, it should be emphasised that automation usually improves accuracy considerably. Those who are familiar with TLD are aware of the many pitfalls of the method and the fact that most of them are associated with manual operation of the equipment and handling the dosemeters, which introduce irreproduci-bilities in the procedure. Automation can, to a large extent, overcome these problems.

As was said before, the heat treatment of a TLD has a decisive influence on its future behaviour. This holds true in particular for the post-read anneal. The greatest repro­ducibility is therefore obtained by performing no annealing whatsoever and using a single optimum heat treatment in an automatic reader.

3.5. TLD readers and systems

This chapter would not be complete without some review of available equipment. It is obvious, however, that a complete compilation cannot be given. While preparing the text, the author contacted all the TLD firms he knew of, in order to collect technical data.

Reactions were very divergent, ranging from no answer at all to abundant information. Both categories are represented in the following pages, where a short list of available TLD equipment is given together with some technical information which is only indicative. Performance data are strictly omitted. Further information should be obtained from the manufacturers. The address list is given in §3.6.

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3.5.1. Systems for the evaluation of loose TL detectors

3.5.1.]. Harshaw Model 2000 TL analyser The system consists of two components: the Model 2000-A TL Detector and the Model 2000-B Automatic Integrating Picoammeter (figure 3.13). The system uses heater planchets mounted in a sample drawer. The planchet temperature can be controlled, providing various temperature profiles, which are divided into three steps:

(a) a rapid pre-heat, ranging from 50 to 400 °C, followed by (b) a linear increase of temperature, adjustable from 4 to 25 °C s_1, for readout, followed

by (c) a maximum temperature plateau (50-400 °C).

Figure 3.13. Harshaw Model 2000 TL analyser.

The total integrating period can be chosen as either 10, 30, 60, 100 s or infinity. The TL signal is measured as the integrated area under the glow peak, represented by

the total charge delivered by the PM tube during the read cycle (digital counter with automatic ranging circuitry, showing three significant figures). The picoammeter has output connections to drive recorders for glow-curve plotting.

The system is capable of reading powder, ribbons, rods and other forms of solid detec­tors. A number of modifications (such as TL integration between two predetermined temperatures) are optional. Various accessories are available.

3.5.1.2. Teledyne Isotopes Model 7300 TLD reader The Model 7300 TLD reader (figure 3.14) is a manual instrument using the conventional sample drawer system to insert the detectors. The dosemeters are heated by means of interchangeable metal trays which follow a controlled temperature profile, which is

Instrumen tat ion 55

Figure 3.14. Telcdync Isotopes Model 7300 TLD reader.

divided into four steps:

(a) a pre-heat cycle of 7 s (150 °C), (b) a readout cycle of 12 s (linear ramp to a plateau), (c) an anneal cycle of 20 s (310 °C), (d) a cool period of 18 s (down to about 100 °C),

The user may either make a selection from three pre-fixed temperature programmes, each adapted to a special dosemeter (of the phosphor-Teflon type manufactured by Teledyne Isotopes), or use the manual mode (maximum readout temperature adjustable from 150 to 350 °C). Sensitivity, calibration, background (automatic subtraction), exposure range and nitrogen flow are set by manual controls (all protected by a door in the front panel to avoid inadvertent changes). A serial binary code decimal (BCD) output of the display on the front panel and a print command are provided.

3.5.1.3. Pitman Toledo Model 654 TLD reader The basic principle of the Toledo reader (figure 3.15(a)) is the same as in the two previous instruments. The dosemeter, when put in a metal tray, is shifted towards its readout position by means of a drawer system. In contrast to other readers, the tray is heated indirectly by a separate heater element which is brought into contact with the tray after the drawer has been closed.

Two interchangeable modules are available to control the heat cycle. The standard module has a preprogrammed three-step temperature profile (of which the pre-heat and anneal cycles can be switched off). With the research module the programme can be widely varied by the user (two to four steps - which include controlled cooling - and eight variables). The Toledo uses a feedback circuit to stabilise the sensitivity, using an internal reference light source. This system compensates automatically for HM gain drift, changes in the signal conditioning system and the light collecting system.

The reader is also equipped with systems for background subtraction and sensitivity adjustment, both controlled by thumbwheels.

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^SfWWW

(a) (b) Figure 3.15. (a) Pitman Toledo Model 654 TLD reader. (6) Pitman automatic sample changer attached to the Model 654 TLD reader (by courtesy of D A Pitman Ltd).

The manufacturer has various accessories available, among which is an automatic sample changer for up to 30 dosemeters (readout time 30-40 min) (figure 3.15(b)).

3.5.1.4. TNO hot gas automatic TLD reader This instrument (figure 3.16) uses three hot nitrogen gas jets to heat the dosemeters. Solid TLD (of any form) are brought into the heating cavity by a vacuum needle, mounted in a motor-driven manipulator (figure 3.17). The detectors can, for example, be presented to the reader by a turntable (maximum content 200 TLD), placed underneath the balcony, through which the vacuum needle protrudes. Evaluated TLD can be unloaded and new ones loaded during continuous operation. Readout time (normally 10 s) and (constant) gas temperature can be adjusted. Typical total cycle time is about 18 s. When operating in automatic mode, the reader checks all relevant parameters (gas temperature, nitrogen flow, sensitivity, vacuum) continuously and stops if one of them is outside the range. Responses can be given in terms of net dose or exposure by automatic background subtraction and multiplication by a calibration factor. The proper values are introduced by means of a keyboard. Values for lower and upper levels (also keyed in) may be used for special functions, such as automatic anneal and selection of dosemeters with uniform sensitivity (two-way ejection). Output connections are available for control by teletype, (micro)processor or other devices.

3.5.1.5. Harshaw automated TL analyser system (ATLAS) The ATLAS consists of a Model 2000-D TL Detector, a Model 2000-B Automatic Inte­grating Picoammeter, a vacuum pump and a printer. A heated laminar flow of nitrogen gas is used to heat the TLD elements (chips). A turntable disc, which can accommodate 50 elements, is inserted into the Model 2000-D unit and the cover closed. The TLD are brought into the readout position by a vacuum probe which protrudes through holes in the turntable and lifts the detectors into the reading chamber. One 50 element sequence

Instrumentation 57

. ^MWW******M*V

COOLING

(O)

TLD

— N 2 OUTLET

NEEDLE

PISTON

DRUM

CUBIC BOX

(EMERGENCY EXIT)

TLD-HOLDER

SAMPLE CHANGER

(6) Figure 3.16. Schematic diagram of the TNO automatic hot gas TLD reader, (a) TLD exposed to three hot nitrogen jets, (b) Manipulator, mirror and PM tube.

Figure 3.17. TNO automatic TLD reader with sampling mechanism for loose detectors with automatic chip loader.

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is completed in approximately 12 min. Key parameters are monitored continuously and will stop the system and prevent further operation if a fault is detected. The system sensitivity may be checked whenever the system is not processing dosemeters by using an internal reference light source. The instrument is shown in figure 3.18. The mechanical set-up is based on the one shown in figure 3.6.

Figure 3.18. Harshaw automated TL analyser system (ATLAS).

3.5.1.6. Therados TLD- 10 system This system (figure 3.19) is designed specifically for in vivo dosimetry at the therapeutic level. It uses probes made of 6LiF-Teflon enclosed in Teflon tubes of various lengths. For readout the tubes (a number of which can be connected up to a maximum length of 2 m) are put in a magazine, from which they are transported with constant speed towards the readout position. Heating takes place in two sequential steps (pre-read anneal and readout) by two separate ovens through which the tubes are led. The individual TLD are separated by steel balls which are sensed by an inductive element by means of which sequence numbers are generated. The system is equipped with a special calibration source (±175 mCi ^Sr) by which the tubiform dosemeters are irradiated.

3.5.1.7. Victoreen 2810 TLD reader This is a versatile manual instrument (see figure 3.20) capable of reading hot-pressed chips and rods, glass-encapsulated rods and glass bulbs (specially meant for environmental dosimetry). Readout cycles are built-in for both LiF and CaF2: Mn dosemeters. Cycles for other TL materials are programmed using auxiliary controls. The calcium fluoride glass bulb dosemeters are heated by constant current. The optical system has a conical reflector which collects 90% of the luminescent light. The instrument uses both peak height and integral readout methods. The dynamic range covers over seven decades, displayed on a 3 \ digit autoranging scaler.

Instrumentation 59

Figure 3.19. Therados TLD­10 system for therapeutic dosimetry (by courtesy of Instru­

ment AB Therados). ■. ' ; ' . . " '

Figure 3.20. Victoreen 2810 TLD reader (by courtesy of Victoreen, Inc).

3.5.2. Personnel dosimetry systems

At present there are a number of systems for personnel dosimetry, most of them being more or less automatic. Except for one, all of them are so­called closed systems, i.e. they are designed to handle only one special type of personal TLD badge (eventually in various configurations) as supplied by the manufacturer. From the point of view of individual monitoring, the heart of each system is the badge itself, a discussion of which, however, would go far beyond the subject of this chapterf. From an operational point of view, degree of automation, speed, versatility, etc ­ and hence the instrumental aspects ­ are of great importance. It will be clear that a careful description of individual dosimetry systems cannot be given without a detailed discussion of both personal dosemeters and the evaluation system, which would require a separate article (or several) for each of them. As a consequence, and because an incomplete description would not do any system justice, the author has decided to provide only a short list.

f For more information the reader is referred to chapter 10.

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3.5.2.1. Harshaw 2271 automated personnel monitoring TLD system (figure 3.21) Dosemeter

TLD card (3x4 cm2) with two chips, sandwiched in Teflon foils, BCD punched hole identification. The card is worn in a holder. Cards are stacked in a magazine for read­out or inserted by hand.

^TLD CARD LOADER

TLD CARD RECEIVER

Figure 3.21. Harshaw 2271 automated personnel monitoring system.

System components Detector/dosemeter identifier Automatic integrating picoammeter Card loader and cartridge Card receiver and cartridge Digital comparator

Heating method Hot finger Badges are stacked in a magazine.

3.5.2.2. Harshaw 2276 microprocessor-controlled TLD system (figure 3.22) Dosemeter

TLD card (3x4 cm2) with four chips, sandwiched in Teflon foil. Identification by bar code with Arabic numeral equivalent. Cards, worn in a holder, are stacked in a magazine for readout.

System components Transport module Logic module (microprocessor) Automatic integrating picoammeter

Heating method Hot finger

Instrumentation 61

Figure 3.22. Harshaw 2276 microprocessor-controlled TLD system.

3.5.2.3. Teledyne Isotopes 9100 automatic TLD reader (figure 3.23) Dosemeter

'Radi-guard' multi-area phosphor embedded in Teflon (3x4 cm2) with four readout areas and four backup areas. Identification by Arabic numbers printed on the detector surface, TLD plaque, wrapped in a plastic envelope, is worn in a holder with filters. Bare TLD are stacked in a holder for readout.

Figure 3.23. Teledyne Isotopes 9100 automatic TLD reader.

System components Badge sampler, decoder, readout system and electronics in one desk-type unit.

Heating method Three-stage heating by blocks.

3.5.2.4. Pitman 605 automatic TLD reader (figure 3.24) Dosemeter

Two elements (phosphor-embedded Teflon discs) in metal plate containing identifica­tion (punched holes). The dosemeter, wrapped in a plastic envelope, is worn in a

62 H W Julius

Figure 3.24. Pitman 605 automatic TLD reader.

plastic holder containing the filters. The plates are stacked in a magazine for automatic readout.

System components All in one cabinet.

Heating method Metal blocks.

3.5.2.5. Studsvik automatic TLD reader 1313B (figure 3.25) Dosemeter

Plastic holder with insert, containing identification (punched holes and corresponding Arabic number) and up to four pellets (4.5 mm diameter) or chips. This unit is worn in a plastic holder with clip. Dosemeters are loaded in a container for readout.

System components All in one unit.

Heating method Hot nitrogen gas.

3.5.2.6. TNO automatic TLD system (figure 3.26) Dosemeter

May, within technical restrictions, be made by the user. A badge is provided, consisting of a plastic holder with up to six TLD (at choice of the user, maximum 12 mm diameter). Identification by OCR-A figures. No holder needed. Dosemeters are loaded in two magazines or manually inserted for readout.

Instrumentation 63

m I f ;

Figure 3.25. Studsvik automatic TLD reader 1313B.

Figure 3.26. TNO automatic TLD system for personnel dosimetry.

System components Badge sampler and reader are separate devices.

Heating system Hot nitrogen gas.

3.5.2.7. National Panasonic automatic TLD system (figures 3.27 and 3.28) Dosemeter (see §3.2.2.5)

Four elements in a plastic holder, containing the identification (punched holes plus printed label).

System components All in one unit.

64 H W Julius

Figure 3.27. National automatic reader UD-710 AG TL badge system.

INFRA-RED RAY BEAM

OPTICAL GUIDE

PM-TUBE ^ i

THERMOLUMINESCENCE

DOSIMETER ELEMENT

UUL VOLTAGE APPLIED

TO LAMP

LAMP

SILICON FILTER

Figure 3.28. Schematic diagram of the optical heating system of National Model 710 AG automatic TLD reader.

Heating system Optical (IR) heating by a lamp in combination with photon counting.

3.6. Address list

Aloka Co., Ltd 6-22-1, Mure, Mitaka-shi 181 Japan

Central Research Institute for Physics of the Hungarian Academy of Sciences

Felelos kiadd: Feher Istvan Peldanyszam: 210 Torzsszam: 74-9876 Keszult a KFKI sokszorosito iizemeben Budapest, 1974 majus ho

Dai Nippon Tokyo Co., Ltd Phosphor Division Shin Tokyo Bldg, 3-3-1, Marunouchi Chiyoda-ku, Tokyo Japan

Desmarquest & CEC SA Division 'Oxydes Frittes' Zone Industrielle No. 1 F-27000 Evreux France

Instrumentation 65

D A Pitman Ltd Jessamy Road Weybridge, Surrey KT13 8LE England

Eberline Instrument Corporation PO Box 2108 SanteFe,NM 87501 USA

Instrument AB Therados Dalgatan 15 S-752 28 Uppsala Sweden

National Panasonic Matsushita Electric Trading Co., Ltd

Head Office PO Box 288 Osaka 541 Japan

POLON Sales Office Bielariska 1 00-086 Warszawa Poland

SAPHYMO-SRAT Services Commerciaux 51 Rue de l'Amiral Mouchez F-75-Paris-13e

France

Studsvik Energiteknik AB S-61182Nykoping Sweden

Teledyne Isotopes 50 Van Buren Avenue Westwood, NJ 07675 USA

The Harshaw Chemical Company Crystal & Electronic Products Department 6801 Cochran Road Solon, Ohio 44139 USA

VEB RFT Messelektronik Dresden Fetscherstrasse 70 DDR-8016 Dresden German Democratic Republic

Radiation Detection Company 385 Logue Avenue Mountain View, California USA

Victoreen Instrument Division 10101 Woodland Avenue Cleveland, Ohio 44104 USA

Radiological Service TNO Utrechtseweg310 N-Arnhem The Netherlands

V/O 'Licensintorg' 31 Kakhovka St Moscow, B-420 USSR

References and further reading

1 Becker K 1973 Solid state dosimetry (Cleveland: CRC Press) 2 Cameron J R, Suntharalingam N and Kenney G N 1968 Thermoluminescent dosimetry (Madison:

University of Wisconsin Press) 3 Frank M and Stolz W 1969 Festkorperdosimetrie ionisierender Strahlung (Leipzig: BSB B G

Teubner Verlagsgesellschaft) 4 Robertson M E A 1975 Identification and reduction of errors in thermoluminescence dosimetry

systems (Weybridge: D A Pitman)

66 HW Julius

5 Brunskill R T et al 1970 A sealed thermoluminescent dosemeter employing RF heating for routine individual monitoring Proc. IAEA Symp. on Advances in Physical and Biological Radiation Detec­tors, Vienna, November 1970 pp 67-76

6 Julius H W et al 1974 A versatile automatic TLD system under development Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, Poland, 27-31 August vol 2 (Krakow: Institute of Nuclear Physics) pp 675-89

7 Kartha M 1971 An automated thermoluminescence dosimeter reader Int. J. Appl. Radiat. Isotopes 22 131-4

8 Portal G et al 1977 A new automatic reader for big scale routine of TL personal dosimeters Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February 1977 (Giessen: I Physikalisches Institut of the Justus-Liebig Universita't) pp 257-61

9 Schlesinger T et al 1971 Photon counting as applied to thermoluminescence dosimetry Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, October 1971. Riso Rep. No. 249, pp 226-36

10 Spanne P 1974 Comparison of photon counting and charge integration as signal registration methods in low dose TL measurements Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, Poland, 27-31 August vol 2 (Krakow: Institute of Nuclear Physics)

11 Oberhofer M 1968 Inherent background reduction for the EG&G model TL-IL CaF2(Mn) thermo-luminescent-dosimeter Health Phys. 15 156-8

12 Petrock K F and Jones D E 1968 Hot nitrogen gas for heating thermoluminescence dosimeters Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF 680920, pp 652-69

Applied Thennoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

4 Accessory instrumentation

M OBERHOFER

4.1. Introduction

A complete TLD system consists, besides the basic readout instrument, of several addi­tional accessories, such as spare heating planchets, light sources, powder dispensers, tweezers, sieves, ultrasonic cleaners, annealing furnaces, annealing stands and irradiators. These accessories are dealt with in this chapter, which also contains some hints for their proper use.

Some items have already been mentioned earlier as part of the readout instrument, such as heating planchets (see §3.2.2.1) and light sources (§3.3.5). Some more remarks will be made with regard to them.

4.2. Heating planchets

As was mentioned in §3.2.2.1 the most common way of heating TL phosphors is the one using low-thermal-capacity metal planchets. Some of the planchets used with the Teledyne Isotopes TL readers Model 5100, 7100 and 7300 are reproduced in figure4.1(a).

Planchet type A is intended to be used with TL phosphors in powder form. The recessed circle helps keep the powders in place. For extruded chip dosemeters this planchet has four metal posts for keeping the chips securely centred (planchet type B). Micro-rod dosemeters are read in planchet type C. Here the rods are held in place between two metal rods. Planchet type D was designed for reading disc dosemeters measuring 9 mm or more in diameter and 0.4 mm thick. The discs are held firmly in place by four metal prongs. However, one prong would do in the case where the dosemeter extends into the centre of the planchet, thus simplifying planchet loading. To prevent thin (0.13 mm) or ultra-thin (20 £im) discs bending upwards or warping during the readout process planchet type E may be used with advantage. With the help of an insertion strip (F in figure 4.1(a)) the dosemeter discs are inerted under a metal mesh panel (figure 4.1(b)). In the case shown the mesh is fixed in the direction of the current. With some planchets of this type the mesh is point-soldered at right angles to the electric current direction. In this way current also flows through the mesh and consequently heats up the discs from above, giving a more homogeneous temperature increase throughout the dosemeter material. To reduce the fraction of light absorbed by the mesh, a hole may be punched into the mesh with a diameter that still allows the disc to be held down along its peri­meter. A 60% open area is normally chosen to obtain the best results. In all the mesh type planchets care has to be taken not to scratch the disc surfaces as this would certainly result in triboluminescence and a deterioration in accuracy.

68 M Oberhofer

(fl)

(b) Figure 4.1. (a) Some planchets used with the Teledyne Isotopes dosemeter system. A, planchet with recess for TL powder use; B, planchet for extruded chip dosemeters, measur­ing gin X | in X 0.035 in; C, planchet for micro-rod dosemeters; D, planchet for disc dose-meters measuring 9 mm or more in diameter and 0.4 mm thick; E, planchet for ultra-thin and 0.13 mm thick disc dosemeters; F, insertion strip, (b) Detail of a planchet for disc dose­meters showing insertion of TL disc dosemeter.

How the reading accuracy may be improved by proper planchet design has been impressively demonstrated by some authors. They describe a planchet version for a disc dosemeter, where part of the screened region is overlaid with a solid nichrome cover to reduce and/or prevent curling of the dosemeter during readout, which apparently was responsible for observed readout deviations of between 5 and 10% [1 J. With the improved planchet these variations are reduced by 0.16-2.8%, averaging around 1.11%.

Accessory instrumentation 69

As already indicated in §3.2.2.1 and shown in §7.2.1 (figure 7.4) the surface properties of the planchets substantially influence the TL signal and therefore the material from which they are made needs careful attention with regard to its light emissivity and reflectivity and the changes with continuous usage. From the very beginning a planchet type is desirable with a minimum emissivity in the visible and infrared. Both generally increase with temperature (incandescent light). Low planchet emissivity gives low 'zero readings' with­out a TL detector and consequently zero-reading variations which may have a negligible influence on the TL detector readings. This is of particular importance when low phosphor emissions from low radiation exposure have to be measured. In connection with the emissivity of planchets it should be remembered that some materials with low emissi­vity in the visible may have high emissivity in the infrared. Although the emissivity of a planchet should be low, its reflectivity ought to be large so as to reflect as much as possible of the light emitted by the TL detector during its heating up into the light collecting and detecting system. The light reflection should not vary with time or with repeated use of the planchet. Reflectivity changes are observed with materials whose surfaces easily oxidise with time or with successive heating cycles. Consequently non-oxidising planchets should be used, such as platinum. Inert-gas (nitrogen or argon) flow helps to reduce the oxidising effect. Where materials other than platinum are used — silver-plated heater planchets, for example — oxidising is likely to some degree even with inert-gas flushing, and cleaning of the planchets or replacing them by new ones may change their reflectivity with considerable TL signal changes as a consequence. Some attention is required here.

Trouble is also often caused by organic contamination of the planchet, such as finger­prints. The planchets therefore should not be touched with the fingers and should be cleaned frequently with tissue wetted with alcohol, acetone or trichloroethylene.

In connection with heating planchets another tip may help to obtain more reliable results. Often the cooling-down period in a heating cycle is too short to allow the planchet to be brought back to a sufficiently low temperature before starting a new reading cycle. This is particularly true for built-in heater elements, which sometimes cool down rather slowly because of their high thermal capacity if no forced cooling is provided. In this case temperature build-up often occurs, with reading trouble as a consequence. The optimum cooling-down period has to be determined experimentally for each instrument type. Several minutes (2-3) will do in most cases.

4.3. Gas flushing

According to §3.2.1 use is made of inert gas in most of the commercial TL reader systems to suppress spurious luminescence from oxygen effects and combustion phenomena. The gas should be turned on 15-20 min before taking the first readings. To obtain optimal quenching of spurious luminescence the gas flow needs to be adjusted properly by means of a pressure reduction valve attached to the gas bottle. The flow is usually indicated by a gas flow meter. The gas flow and its fluctuations are not too important, commonly between about 0.5 and 11 min-1, but it should, if possible, be kept at the same value after a certain flow has been decided upon. The gas should be pure and dry. Therefore the gas should be passed through a gas dryer bottle to remove moisture or through a purifier.

70 M Oberhofer

The background reduction with N2 or other gas flushing should always be verified before taking a series of measurements or at least when using a new gas bottle for the first time to avoid surprises.

4.4. Reference light sourcesf

Some instrument manufacturers supply reference light sources with different light intensi­ties (figure 4.2) and with different light colours matched to the light colour of the TL emission from the phosphors to be used (blue for LiF, orange for lithium borate and green for calcium fluoride and calcium sulphate).

Figure 4.2. Teledyne Isotopes standard light sources of 14C mixed with a luminescent phosphor for use in day-to-day checking of instrument stability.

In some instances for instrument stability checking and calibration the light sources are inserted into the drawer in place of the TL detector and brought under the photo-multiplier tube by pushing the drawer into the instrument. In this case it is important to make sure that the sources are all inserted in the same way, as the light source reading may sometimes depend on the source direction with respect to the PM tube position.

External light sources have the disadvantage that they are normally exposed to environmental light and in most cases show some additional light emission as a conse­quence of light excitation which, although it fades away exponentially in the dark, may be troublesome. It may take some minutes before the light-induced phosphorescence has decayed away to negligible intensity levels. In any case, before a check or calibration with an external light source one must ensure that the light source has been sufficiently long in its final position within the reader and that it yields average readings with an accept­ably low standard deviation, say less than ±2%. It should be borne in mind that the light source output is subject to internal source fluctuations of the order of ±0.4 up to ±0.6% and that the light output can be influenced by ambient temperature (see also §§7.2.1 and 11.4). In one case of a plastic scintillator light source —0.05% K_1 light intensity varia­tions were found and in a case of a Nal (TL) scintillator light source the value was — 0.3%K"1 [2]. Age may also affect the light output. So, for example, for some Na(Tl)

t See also §3.3.5.

Accessory instrumentation 71

scintillator light sources a time-dependent decrease in light intensity of —3% yr 1 was observed as a consequence of their 210Pb content, which decays with a half-life of 19.4 yr or with - 3 . 1 % yr"1 [2];

All these error sources were recently the subject of detailed studies [1]. According to these studies nitrogen affects plastic scintillator light sources, which may suffer reader response variations up to 5% during a working day. This is one reason for being careful with nitrogen gas flushing when using certain light sources for instrument checking and adjustment.

4.5. Powder dispensers

Where weighing of TL phosphor powder is too cumbersome — in routine dosimetry, for example — powder dispensers are employed.

In this case identical powder aliquots are prepared volumetrically. Figures 4.3 and 4.4 show schematically two commercially available powder dispensers. In both cases the metered volume is defined by a small hole (funnel) through a slide or a rotating metal plate. In its filling position the metered volume is just underneath a powder reservoir, from which it is filled. In its discharge position, reached in the first case by withdrawing the dispensing plunger from the device until a limit stopper prevents further movement and in the second case by rotating the metal plate around its axis towards a stop, which for reasons of simplicity is not shown in the figure, the powder is released through a discharge tube on to a planchet or into a pan or also into a dosemeter capsule. Repro­ducibility is improved considerably by shaking the dispenser with a vibrator, which may be timed. The dispenser is vibrated for a few seconds during the filling cf the metered volume and a few seconds during discharge. As a vibrator, either an electric engraving tool or a DC relay operated on alternating current, so as to produce a chattering motion, can be attached to the dispenser, if it does not already have a vibrator. In figures 4.5 and 4.6 the photographs of the powder dispensers shown schematically earlier are reproduced together with their vibrators. Figure 4.6(b) shows another version of the Harshaw

HANDLE

POWDER RESERVOIR

METERED VOLUME

DISPENSING PLUNGER

LIMIT STOPPER

DISCHARGE TUBE

PLANCHET

Figure 4.3. Tclcdyne Isotopes phosphor dispenser, schematically represented.

72 M Oberhofer

NUT FOR TIGHTENING OF DISPENSER ASSEMBLY

PLANCHET

Figure 4.4. Harshaw flat-ptfte powder dispenser, schematically simplified. In the actual device the handle, powder reservoir and discharge hole are not located in the same plane, as can be seen from figure 4.6.

"1

Figure 4.5. Teledyne Isotopes (formerly CON-RAD) phosphor dispenser with automatic vibrator feature.

dispenser with vibrator (here a DC motor-driven eccentric) manufactured by D A Pitman Ltd.

How reproducibility can be improved with a vibrator is demonstrated by an example. In table 1 data are compiled which were obtained with a vibrator produced by Eberline Instrument Corporation (Model PB-1 Powder Dispenser) together with LiF powder from The Harshaw Chemical Company, varying the time settings of the vibrator. The powder weight is the average of 10 aliquots.

Accessory instrumentation 73

(a) (b) Figure 4.6. (a) Harshaw powder dispenser with electrically (push-button) operated vibrator. (6) Model 624 vibrator powder dispenser of Pitman Instruments.

Table 4 .1 . Influence of vibrating times on the reproducibility of powder sample weight for samples prepared with a powder dispenser.

Vibrating time setting (s)

6 9

15 21

Average powder weight (mg)

31.916 32.610 33.311 34.125

Standard deviation (%)

±0.94 ±0.06 ±0.15 ±0.18

Maximum weight (mg)

32.01 33.02 33.52 34.97

Minimum weight (mg)

29.39 32.23 33.12 33.45

Maximum weight deviation (%)

8.5 2.4 1.2 4.4

From this table it is evident that the optimum vibrator time setting for the automatic vibrator used is 15 s which yields the lowest weight deviations between single samples and also a sufficiently low standard deviation (±0.15%) for the average weight.

Most phosphor dispensers are designed for 80-200 mesh phosphors, which include LiF and li2B407:Mn. The much finer CaS04 : Mn and CaF2 : Mn powders do not dispense readily. Weighing or a simple volume sampling should be used with such phosphors.

The following hints are given to avoid difficulties with dispensers of the types described here.

First of all, the dispensers must be clean, dry and free running. In a humid climate such as in the tropics, the transfer chambers of the slides and the charge and discharge tubes may be obstructed by humidity or humid corrosion products if the dispenser is not made out of non-corrosive material (high-quality stainless steel, for example). In this case cleaning of the disassembled parts of the dispenser should be done with acetone, possibly

74 M Oberhofer

making use of a pipe-cleaner, and the dispenser parts should be thoroughly dried after­wards with a hot air blower. To reduce malfunctioning of the dispenser it is recom­mended that it should always be kept in a dry atmosphere if not in use. In most cases it will be sufficient to keep it in a plastic bag with silica gel. From time to time the dispenser should be checked by weighing successive samplings and by determining their standard deviation, which should be the same as before. Of course, the phosphor must also be dry. If the powder shows a tendency to lump together, it should be stored in a desiccator. If the phosphor for one reason or another has become quite lumpy because of humidity, it must be dried at no more than 80 °C. For optimum precision, powder should be dispensed only when the reservoir contains appreciably more than one single metered volume of powder.

There may be some uncertainty as to the accuracy of the first aliquot from a newly filled dispenser. It is therefore advisable to put the first aliquot back into the hopper of the dispenser and begin measurements with the second aliquot.

Another hint may be useful in operating a dispenser successfully: care should be taken to prevent cross-contamination if the dispenser is used to dispense phosphor irradiated to various doses or phosphors of different radiation sensitivity.

Low-dose measurements can be greatly disturbed by contamination of the low-dose phosphor with a few grains of a material which has received high doses or has a much higher TL yield. To prevent this inconvenience, either the dispenser should be washed before re-use with, for example, acetone, or a separate dispenser for low-dose measure­ment only should be used. If the dispenser is washed, it must be dried carefully before it is used again.

Where more than one dispenser is used, it is necessary to check for differences in the metering volume. In an experiment the average weights of samples prepared with each vibrator should be compared and, if necessary, correction factors applied for each powder dispenser.

4.6. Mechanical tweezersf

Rods, chips and discs may be handled by stainless steel tweezers. Care has to be taken that in utilising them the dosemeters are not scratched, as scratching them normally leads to phosphor excitation and light emission upon heating with unpredictable readings. To avoid scratching the tweezer tips should be covered with some soft coating which can easily be cleaned and/or exchanged.

4.7. Vacuum tweezersf

For rapid and simple dosemeter handling without squeezing or abrading, vacuum tweezers are advantageous. Such tweezers may be acquired from Pitman Instruments, for example (see figure 4.7(a)), or made with a small vacuum or suction pump, some small diameter tube and a connecting rubber hose to the pump. Air is sucked into the tube with which the dosemeter is to be picked up. The dosemeter stays fixed to the tube end by means of the vacuum created at its surface. For discs the tube may have at its end a rubber or plastic sucker. Figure 4.7 demonstrates such a device schematically. Dosemeter release is t See also §7.26.

Accessory instrumentation 75

k- k, •£ - J .

t t *

(a) (6)

Figure 4.7. (a) Pitman vacuum tweezers for easy dosemeter handling (courtesy of D A Pitman Ltd), (b) Operating principle of vacuum tweezers, a Dosemeter pick-up, b dosemeter transfer, c dosemeter release.

effected by letting air enter into the vacuum within the most simple device just by stopping the pump or by releasing the finger tip from the tube top.

4.8. Sieves Where powder material is used for dosimetry it is necessary either to produce material with a grain size within a certain range, say from 80 to 200 mesh, or to make sure that the grain size corresponds to that specified or desired or to re-sieve it for cleaning. These sieves are available in chemists' shops in sets, allowing sieving for various average grain sizes and grain size spreads. For grain size spreads of 80-200 mesh no particular problems are encountered in sieving, pre-supposing that the material is dry.

Difficulties due to agglomeration may arise, for example for LiF, when trying to sieve powders ground to very small grain sizes.

4.9. Ultrasonic cleaners

If one wants to perform precise TLD measurements cleanliness is of the utmost importance, as dirt either within the reading system or on the dosemeter surface may cause spurious readings which give erratic results. Dirt also reduces the usable life of the dosemeter due to the burning into the surface of dirt and grease during the annealing process required before re-use, or even during the read cycle itself.

For cleaning the dosemeters ultrasonic cleaners are recommended. Ultrasonic cleaning may increase the life expectancy of the dosemeters considerably. For phosphor-Teflon dosemeters it was found that the introduction into the re-use cycle of a short ultrasonic cleaning period on average doubles the life expectancy, provided they are not exposed to doses in excess of 104 R, when radiation damage becomes important.

76 M Oberhofer

Ultrasonic cleaners are available in various sizes from different manufacturers. Normally the tank and generator are combined in a single unit. They are supplied with perforated stainless steel trays to hold the dosemeters and also with tank covers. The dosemeters are placed loosely into the perforated tray supplied with the cleaner and lowered into the cleaning solution for a period of 5-15 min with the instrument in operation.

The dosemeters are then removed, rinsed thoroughly in methanol, for example, and allowed to "dry on a lint-free surface before annealing. A solid tray is used instead of a perforated one if the dosemeters are micro-rods. For more information on dosemeter cleaning the reader is referred to §7.2.7.

4.10. Annealing furnaces

Annealing of the dosemeter material in many cases (LiF, for example) is performed in ovens. Often two ovens are used, one with a maximum temperature up to 400 °C, which is sufficient for all dosemeters in common use, and one up to 150°C. Emphasis should be put on good temperature control and constancy of the preselected temperatures by means of a precision temperature control regulator over extended periods of time. This latter feature is of importance in particular for ovens of low maximum temperature. LiF, for example, needs to be kept at 80 °C for over 24 h after high-temperature baking, if the standard annealing procedure is followed. If other procedures are followed with shorter anneal times the temperature constancy may be even more important (see also §7.2.5). Forced convection is desirable since there are then no temperature gradients within the oven and identical annealing temperatures are guaranteed for all dosemeters, independent of their position within the oven. It is convenient to have C02 cooling for controlled cooling-down of the dosemeter material.

In figure 4.8(a), (b) and (c) photographs of a number of 'conventional' ovens are reproduced, selected by Teledyne Isotopes as most suitable for TL dosemeter anneal. Any of the ovens should be brought up to the preselected temperature and allowed to stabilise well before dosemeter insertion. Balancing by adjusting the temperature frequently is difficult and is not recommended.

In order to avoid contamination of the annealing ovens with some material which might also contaminate the TL dosemeters later on the ovens should be set aside exclusively for dosemeter anneal.

Ultimately some other 'advanced' oven types came on to the market specially designed for dosemeter anneal. Their construction makes use of 'thermoplatesj which contain the heating elements and the associated thermocouple temperature sensors. One example (figure 4.8(G0) shows an oven unit offered by D A Pitman Ltd, which consists of a heated well lid, suitable for top loading of several special annealing blocks made of aluminium with holes to hold the dosemeters and a hole for inserting a stem thermometer. Thermal gradients through the system are extremely low due to the high thermal conductivity of aluminium.

Similar ovens with sample cassette plates are produced by Aloka Co, Ltd. This company also offers heater pans to be heated up by means of infrared light from a halogen lamp.

Accessory instrumentation 77

(a) (b)

<*.

^■**rxX.X^iyU£*Z.1?:!&V' i w ^ - ? ^ ^ ^ * - ^ ^ ^ ! ! . - - . - ^ : ^ - . - - . . ^ , .

(c) (d) Figure 4.8. (a) Annealing oven for high­temperature anneal. This compact muffle furnace has a temperature range (up to 1093°C) that makes it highly suitable for annealing phosphor powders. Designed for maximum efficiency, the control circuitry is exactly matched to the size of the heating chamber (courtesy of Teledyne Isotopes), (b) Annealing oven for medium­temperature anneal. This practical compact oven is ideally suited for annealing of CaS04 : Dy dosemeters. Amply insulated, the heating chamber has damper­

controlled induced air circulation and two adjustable shelves (courtesy of Teledyne Isotopes), (c) Annealing oven for medium­ and low­temperature anneal. This high­quality forced convection oven offers a temperature range (from 40 °C up to 300 °C) that makes it ideal for most annealing purposes. The forced convection design combines with a sophisti­

cated proportional temperature control system to assure easily controlled, constant temperatures (courtesy of Teledyne Isotopes), (d) D A Pitman Ltd programmed thermo­

plate annealing facility. Oven unit left, control unit right (courtesy of D A Pitman Ltd).

4.11. Annealing stands

Difficulties in obtaining identical thermal treatment of a whole batch of dosemeters may be overcome sufficiently well by the use of heater stands or other devices for the same purpose. Such a stand is shown in figure 4.9. where for example, disc dosemeters are placed between an aluminium or nickel plate of 3 mm thickness. To ensure good thermal

78 M Oberhofer

%

Figure 4.9. Doscmctcr annealing stand with phosphor-Teflon disc dosemeters. Top plate has been taken off to show how the dosemeters are arranged (courtesy of Tcledyne Isotopes).

contact the surface may be anodised black. For rod dosemeters plates with machined grooves are used which hold 100 micro-rods at one time. The depth of the grooves is such that the micro-rods project slightly and, by laying another flat plate over the top, the rods are held in good thermal contact with the surface along three lines. As many as five of these trays may be used at one time on the annealing stand, enabling 500 micro-rods to be annealed simultaneously. Four legs eliminate thermal gradients due to conduction of heat to or from the floor of the oven. The plate efficiently distributes the heat to the dosemeters. Another device, the cross section of which is given in figure 4.10, may be utilised with good results for annealing isothermally at least 50 disc dosemeters, if air exchange is possible by means of one or several holes. Severe coloration and consequently loss of sensitivity of the dosemeters is observed if air exchange is not possible [3].

END CAP TEFLON DISC DOSIMETERS

COMPRESSING SPRING

-TEFLON PLUGS

Figure 4.10. Cross-sectional view of an annealing tube for Teflon disc dosemeters.

Experience has been good with the annealing device shown in figure 4.11. This annealing stand consists of 13 plates interleaved with spacers. Each plate has 18 circular cut-outs, which will accept discs up to 12.7 mm diameter from ultra-thin up to 0.4 mm thick. The plates and spacers are made out of anodised aluminium and are stacked on a centre bolt so that a total of 234 discs can easily be assembled in a single unit of high packing density and good thermal conductivity. A stand-off base raises the main assembly approximately 2.5 cm above the oven floor so that thermal gradients due to conduction are minimized.

Accessory instrumentation 79

Figure 4.11. DA Pitman Ltd TLD annealing stand with 234 disc compartments.

4.12. Irradiators

For dosemeter calibration, portable irradiators are available, in one case (Teledyne Isotopes) being a device which contains '"Sr/^Y as the radiation source and which was designed for use with Teflon disc dosemeters. The device is offered for activities of 2, 10, 50 and 100 mCi with maximum dose rates of 5, 24, 120 and 240 rad min"1, respectively.

The discs are inserted into the irradiator by means of a drawer. A highly reproducible dose is also obtained with the Reference Dose Irradiator of Studsvik Instrument AB (figure 4.12), which can be varied within a wide range from 20 mrad well up into the kilorad range. The radioactive source here is also '"Sr/^Y (standard: 1 mCi, with options of 0.3, 3, 10 and 30mCi), placed in a heavy nickel-plated brass shield. The dosemeter to be exposed, which may have dimensions up to 30x40mm2 and a thickness of 4 mm maximum, is rotated within the irradiator by means of a synchronous motor which gives a well-defined speed, resulting in a constant dose per revolution (70 mrad for the standard

Figure 4.12. Studsvik reference dose irradiator 6527 B (courtesy of Studsvik).

80 M Oberhofer

version). The instrument can also be set in a way that the dosemeter remains stationary under the source. The time of irradiation in the stationary mode can be set from a few seconds up to 12 h, which corresponds to a dose of between about 50 mrad to 1 krad per mCiof90Sr/90Y.

Another irradiator version is shown in figure 4.13, which is produced by D A Pitman Ltd. This automatic irradiator is principally characterised by two opposed 1 mCi '"Sr/^Y sources (matched in activity to within 5%) with sliding shutter assemblies and a turntable with 30 apertures around its circumference into which thin Kapton trays with TL dose-meters (discs, chips, micro-rods) can be placed and which is aligned to carry the dose-meters between the sources. With the standard gearbox per revolution a dose in the order of 10 mrad (4-14 mrad according to speed) is administered to the dosemeters. The expo­sure reproducibility obtainable is better than 2% standard deviation.

Figure 4.13. D A Pitman Ltd Model 623 dosemeter irradiator (courtesy of D A Pitman Ltd).

In some instances the calibrators are part of the TL reader system, for example in the Therados TLD-10 system shown in figure 3.19 or in the Harshaw Model 2271 Detector and L Card Loader System.

4.13. Literature In the following some literature is listed which is exclusively dedicated to TL or TLD or where this subject is treated as a whole among other solid-state dosimetric methods.

1967 Luminescence Dosimetry. Proc. Int. Conf. on Luminescence Dosimetry, Stanford University, June 21-23. 1965. USAEC Division of Technical Information AEC 8, Symp. Ser.

Cameron J R, Suntharalingam N and Kenney G N 1968 Thermoluminescent Dosimetry (Madison: The University of Wisconsin Press)

1969 Solid State Dosimetry. Proc. NATO Summer School, Brussels, September 4-16, 1967 ed S. Amelinckx, B Blatz and R Strumane (New York: Gordon and Breach)

Accessory instrumentation 81

1969 Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. COW 680920

Frank M and Stolz W 1969 Festkorperdosimetrie Ionisierender Strahlung (Leipzig: B G Teubner Verlagsgesellschaft)

1971 Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC Research Establishment, Riso, October 11-14. Riso Rep. No 249, part I, II and III

Becker K 1973 Solid State Dosimetry (Cleveland: CRC Press)

Oberhofer M 1974 Termoluminescence Dosimetry, Lecture notes of a course given at the Bandung Reactor Centre, Bandung, Indonesia, June-October 1973 (Bandung, Indonesia: Pusat Reaktor Atom Bandung) PRAB: 335/HP-40/73

1974 Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August 27-31 Vol I, II and III (Krakow: Institute of Nuclear Physics)

Becker K and Scharmann A 1975 Einfuhrung in die Festkorperdosimetrie, Thiemig Taschenbuch vol 56 (Miinchen: Verlag Karl Thiemig)

1975 Technical Recommendations for the Use of Thermoluminescence of Dosimetry in Individual Monitoring for Photons and Electrons from External Sources. Radiological Protection 3 (Commission of the European Communities) EUR 5358 e

Robertson M E A 1975 Identification and reduction of errors in Thermoluminescence Dosimetry Systems (Weybridge: D A Pitman Ltd)

1977 Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February 14-17 (Giessen: I Physikalisches Institut of the Justus-Liebing Universitat)

Nambi K S V 1977 Thermoluminescence: Its Understanding and Applications (Sao Paulo: Centro de Protecao Radiologica e Dosimetria, Instituto de Energia Atomica) Informacao IEA 54.CPRD-AMD 1

1979 Thermoluminescence: Dosimetry and Applications, Bull. Radiat. Protection 2 No 4

1980 Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April 1-4. Nucl. Instrum. Meth. 175 No 1 (September)

References 1 Matthews R J and Stoebe T G 1980 Precision thermoluminescent dosimetry using CaSQ,: Dy

Teflon dosimeters Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April 1-4. Nucl. Instrum. Meth. 175 No 1 (September)

2 Burgkhardt B and Piesch E 1979 Systematical and statistical errors of the TLD-reader calibration with reference light-sources Report presented at 11th Meeting of the Working Group 'Assessment of Doses- caused by External Radiation Sources' of the Fachverband fur Strahlenschutz e.V., November 29-30, Karlsruhe

3 Verhagen H W, Petten ECN private communication

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

5 General characteristics OF TL materials

G BUSUOLI

5.1. Introduction

TL materials are at present widely used in many fields of dosimetry due to their properties that are now known from other classical dosimetry systems. This chapter deals with the general characteristics for TLD; special characteristics for certain applications (such as personal, environmental and chemical dosimetry) are discussed in the corresponding chapters.

5.2. Linearity

For many applications it is generally preferred to have a linear dose-TL signal relation­ship, i.e. proportionality between the TL signal and the dose, from D = 0.

The form of this function is determined by the TL yield YTh, defined as the quotient of the energy emitted as light ETL, the mass of the detector m and the absorbed dose D:

YTL = ETL/mD.

In the range where YJL is constant, the dose characteristic is linear and one can write:

QTL = aTYTLD

where at is a proportionality factor and QTL is the thermoluminescent signal from the phosphor.

Figure 5.1 shows a typical curve for the /th glow peak. On the y axis the thermo­luminescent signal is plotted (the zero dose signal being subtracted) while on the x axis the dose is plotted.

If a phosphor presents a glow curve with several maxima at different temperatures, either the integrated light emitted by every peak, or its height, or the whole of the light emitted by the phosphor as the sum of the contributions from the different peaks present a linear dose characteristic. Every curve will be characterised by a different light yield.

As can be seen from the figure, the dose characteristic is linear up to a maximum value Dm after which 'saturation' occurs or 'supralinearity' starts.

Table 5.1 gives the dose ranges within which detectors show a linear behaviour [1]. Both the lower and upper dose values are very different for each detector.

The data presented in the table demonstrate that for all of the phosphors shown the deviation from linearity is due to the presence of supralinearity. The origin of this

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W z o a. UJ /

/ /

/

/

/ LINEAR RANGE'

J\ Dm Ds DOSE

Figure 5.1. Example of a linearity curve

Table 5.1. Linearity ranges.

TL material Linearity range for 60Co gamma rays (order of magnitude) (rad)

LiF : Mg, Ti Li2B407 : Mn CaF2 : Mn CaF2 :Dy BeO

10"2-102

KTMO 2

10-MO 3

1CT5-103

l (T2- lu2

phenomenon is not yet well known. As an example, in figure 5.2 the dose characteristic for BeO (Thermalox 995) irradiated with ^Co gamma rays is given [2]. In this case, as is clearly shown in the figure, supralinearity is present for doses of the order of 50 rad.

For an explanation of the phenomenon, different hypotheses have been put forward, such as the creation of new traps as an effect of the irradiation [3] or an increase of the intrinsic TL efficiency, etc.

Even if the sensitivity of the TL detector increases in this region, the accuracy of the measurements made in the supralinearity range is lower than that for the linear region. This comes from the necessity of introducing correction factors that generate further errors.

The supralinearity can be a function of the linear energy transfer (LET) of the radiation and the dose limit at which supralinearity becomes evident is larger for high LET particles. Moreover, supralinearity influences the various TL peaks of the detectors in different ways. Figure 5.3 is an example of this behaviour; the figure gives the curves for the main peaks of LiF (called the first peak) and for the peak at about 280 °C (called the second peak) for irradiation using ^Co gamma rays (low LET) and using slow neutrons (high LET) [4]. The figure clearly shows the influences mentioned above.

Attention should be paid to the fact that if the supralinearity region is reached, the TL detector maintains the new sensitivity even after the readout process. This fact is a

General characteristics of TL materials 85

~ 1 0 A

u m 'o

3 Q. 3 O

10'

5 101

10'

10" 10"' 10c 10' 10'

1111 ' -,3 ,n4 10J 10"

EXPOSURE (R ) Figure5.2. BeO TL response plotted against 60Co gamma ray exposure.

=> 10 5

or <

CD

< b 10-D. I—

o

60r„ J • PEAK I \ . PEAK II

SLOW frPEAK I NEUTRONS I » PEAK II

10'

DOSE ( rod ) Figure 5.3. Total light output plotted against dose for the two main peaks of LiF for 60Co gamma rays and slow neutrons.

problem for those detectors that are re-used, as in the case of personnel dosimetry or, more likely for the dose levels present, in the case of clinical dosimetry. If a detector has been irradiated in the supralinearity range, in order to restore its previous sensitivity it will be necessary to perform a complete annealing cycle and not just a simple readout. Care must be taken on this point in order to avoid errors in the dose assessment.

Beyond the supralinearity range, due to the decrease in the number of available traps, the saturation effect of the phosphor becomes apparent. The saturation effect auto­matically determines the upper dose limit for each detector, which is usually taken as

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20% below the saturation value. Measurements made in the proximity of the saturation value are affected by larger errors and it is recommended that TL detectors not be used in the presence of such high doses. Moreover, attention must be paid to the fact that even a complete annealing of the phosphor does not completely restore the initial properties of the material due to the large amount of damage suffered by it.

Finally, another interesting point to discuss is the lower dose limit or threshold dose. For a given combination of TL material and readout device, the lower limit of detection is essentially determined by the variability of the signal obtained by the reading of non-irradiated detectors.

The detection threshold, which can be defined as the smallest dose that can be distin­guished significantly from a zero dose, can be taken as three times the standard deviation of the zero-dose reading, expressed in units of absorbed dose. It must be emphasised that the uncertainty of a dose, in the region of the detection threshold, may be of the order of 100%.

The zero-dose reading is calculated from the signal obtained when a non-irradiated detector is read. This signal may be due to triboluminescence and chemiluminescence, stimulation of the detector by visible and ultraviolet light, infrared emission of the heating element and dark current fluctuations of the photomultiplier tube and residual signals due to previous irradiations.

5.3. Response to photons

If a TL material is to be used for any dosimetric applications in the field of photon radia­tion, one of the main characteristics that must be known is its energy response. For the purpose of this subsection, energy response is defined as follows: the energy response is a measure of the energy absorbed in the TL material used in comparison to the energy absorbed in a material taken as the reference, when irradiated at the same exposure. (Normal reference materials in dosimetry are air and tissue.)

The energy response is therefore characteristic of each TL material and a direct measurement is obtained when the material is under electronic equilibrium conditions [5]. Energy response is not so easily measured, but its theoretical value is very helpful in selecting a particular TL material for any special applications.

The energy response can easily be calculated as the ratio between the mass energy absorption coefficients of the detector and of air respectively, in the energy range up to 3 MeV, i.e. where exposure is still defined. Calculations can also be performed for high energies but in practice they have no meaning as electronic equilibrium no longer exists [6].

Air is normally taken as the reference medium because a well-defined quantity, the exposure, can easily and accurately be measured for it and because the ratio between absorbed dose and exposure for air is a constant.

If S(E) is the energy response one has (where d stands for 'detector')

_ , „ . „ (Men/P)d

(^en/P)air

This formula is simply derived from the Bragg principle applied to large cavities.

General characteristics of TL materials 87

Since Ojen/p)d commonly refers to a compound or a mixture of different elements, the additivity rule must be used to calculate it at every energy value [7]:

- (Men/P)d = (Men/P)l ^ 1 + (Men/p)2 W2+... + ( / i e n /p) / Wt + . . .

where (penlp)i is the mass energy absorption coefficient of the ith element and Wi is its fraction by weight.

In a practical calculation dealing with phosphor doped with or containing various impurities, these must be considered on account of their high Z values.

The simple formula used for energy response calculations is valid under the following conditions [5]: (a) electronic equilibrium; (b) no self-absorption in the detector; (c) light yield per rad independent of the LET.

Moreover, the formula does not take into account the fluorescent x-radiation created in the material itself. Of course, more sophisticated calculations are possible.

In figure 5.4 some examples of energy response calculations are given [6].

1 - CaF2 : 3.0 7. Mn 2-CaSOi:0.217. Dy

- Co SOi :0.22V. Tm

0.1 1.0 10 100 ENERGY ( MeV )

1-METAPHOSPHATE GLASS 0.3 V. Mn

2 -A I 2 0 3

(a)

0.1 1.0 10 100 ENERGY ( M e V )

(b)

1.5

O Q_ 01

05

1

- 1

l-LiF(TLD-IOO] . 0 0.9V. ;Si 0.3 V. ; Mg 0 03V. ;P 0.025V.

2-Li2B407 : Mn 0.34V.

L 3-BeO - Thermalox 995

(c )

0.1 1.0 10 100 ENERGY ( MeV ) Figure 5.4. Examples of calculated energy

dependence for several TL detectors.

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For dosimetric purposes a material with an energy response which is as constant as possible over the energy range of interest is desirable. This can be achieved by choosing materials with a low effective atomic number Z. For special applications, however, high Z materials coupled with filters can be used. In this case the energy response of the detector is modified by the dosemeter design and must be determined experimentally. The factors to be used to correct for dosemeter response at a given energy level can be obtained from the ratio of the dosemeter reading at the energy in question and at some reference energy which is commonly taken to be that for "'Co gamma rays.

A particular practical example of calculated energy response compared with measured energy response is reported in figure 5.5 for BeO [2]. In this case the discrepancy between calculation and experimental data is not only due to the previously reported factors but also comes from something substantial and perhaps intrinsic to the material itself. For this special case a satisfactory explanation of the phenomenon has not yet been given; a hypothetical presence of high Z impurities has been demonstrated as being invalid from chemical analysis performed on the material.

2.0

o Q. if) W 1.5 a: ID > < 1.0 —I LU rr

_i i i_ _ i i '

10 100 1000 ENERGY ( keV )

Figure 5.5. Calculated energy dependence and measured energy response for BeO discs.

Sometimes, for special applications, a large energy response can be useful for deter­mining the energy components of the radiation field. In fact, by using several detectors with different filtrations, TL responses that are different in the presence of different radiation energies are obtained. Of course, if detailed information on the different energies is needed a large number of detectors should be used. In this case the dosemeter will be very complicated; moreover, it will be more likely to introduce large errors into the evaluation. Therefore the solution of using too many detectors in the same dosemeter is to be carefully considered.

5.4. Response to beta rays

TL materials are sensitive to beta rays and, theoretically, can also be used in the design of dosemeters for this radiation. However, due to the large dimensions of the grains, as in the case of a powder, or to the thickness of the sample, as in the case of a compacted

General characteristics of TL materials 89

detector, it will be impossible to satisfy cavity theory. Consequently any detector employed for beta measurements shows a response decreasing steeply in the low-energy range.

Figure 5.6. shows the relative response of LiF as a function of the mean beta energy of various sources [8]. Curve (a) refers to powder (0.15 mm average grain diameter) exposed at the surface of special beta sources; curve (b) is for 1 mm diameter LiF-Teflon rods at the surface of a container of the emitting solution; curve (c) is for LiF-Teflon immersed in a solution of the nuclide.

LU LO z o 0-LO LU on

LU

>

LU a: 0.02

0.02 0.1 1 MEAN BETA ENERGY (MeV)

Figure 5.6. Relative response of TL in LiF: Mg, Ti as a function of the mean beta energy of various emitters: (a) with powder (0.15 mm average grain diameter) exposed at the surface of a gelatine mould; ( i ) with 1 mm diameter LiF-Teflon rods, at the surface of a container of the emitting solution; and (c) immersed in a solution of the nuclide (after [8]).

Figure 5.7 is a further example for LiF chips and three different sized BeO discs [9]. From this figure it can be seen that in the range from 0.5 MeV up to ^Srj^Y, the dose in skin taken at a depth from 5 to 10 mg cm-2 can be determined with an uncertainty of a factor of 2. If greater accuracy is required, either the use of thinner detectors or the use of several detectors in order to obtain some information on the energy of the beta rays would be necessary.

The problem of beta dosimetry with TL materials is now being widely studied in various European laboratories and some comparisons are in progress as part of a programme by the Commission of the European Communities.

5.5. Response to neutrons

In many situations it is necessary to measure photon and electron doses in the presence of neutrons and it may be necessary to correct the results for the sensitivity of the detector to neutrons. On the other hand, for several applications and particularly for personal dosimetry, it is necessary to have detectors sensitive to neutrons in order to assess the dose equivalent of people working in the surroundings of nuclear power plants.

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Q LU or

o o

1X1

> 10'

LU or LU in z o 0_ to

_

J V

/ /

1

Jk

oLiF CHIPS .BeO (A) * BeO (B) * BeO (C)

i 0.1 5 1.0 5 10 Ep (max)(MeV)AT CALIBRATION POINT

Figure 5.7. Energy dependence for beta rays (after [9]).

An example of gamma detection in the presence of large amounts of neutron radiation is in the case of accident dosimetry. Measurements performed in an experimental reactor show that the biggest errors in gamma dose assessment come from the presence of thermal or slow neutrons and from the sensitivity of the TL detectors to these radiations [10].

The sensitivity to neutrons of TL phosphors depends on their isotopic composition and to some extent on the content of different impurities. For example, LiF and Li2B407can be manufactured without any difficulty with different abundances of 6Li and 10B, thus increasing markedly the slow neutron sensitivity (see also chapter 12).

Theoretical calculations and very accurate measurements in order to assess the sensitivity of different TL phosphors to slow and fast neutrons have recently been published [11-14].

Table 5.2 gives some published values for the response of a number of TL materials to a fluence of 1010 thermal neutrons cm-2 in terms of an equivalent exposure to ^Co gamma rays [1].

Table 5.2. Slow neutron sensitivity.

Material Response (1010cm 2)/Response (R of 60Co gamma rays)

LiF (TLD 100) Li2B,07 :Mn LiF (TLD 600) LiF (TLD 700) CaF2:Mn CaSO„: Dy BeO

350 310

1930 1.5 0.6 0.5 0.3

General characteristics of TL materials 91

The neutron response depends upon the sample thickness, the isotopic content of the material and the irradiation conditions. Moreover, the neutron sensitivity will also depend on the material surrounding the detectors. Therefore the values in table 5.2 can only be used as a rough indication of the thermal neutron sensitivity of the listed materials.

As can be seen from the table (apart from other considerations) one of the most useful materials for performing gamma measurements in a mixed neutron-gamma field seems to be BeO due to its low response. Conversely 7LiF shows a rather high response to neutrons and therefore corrections must be applied to this detector when used in a neutron dose-meter employing two detectors (6Li and 7Li) in order to discriminate gamma rays from neutrons.

Similar considerations apply to the sensitivity of TL phosphors to fast neutrons (see chapter 12). As an example table 5.3 shows the response of LiF and BeO to fast neutrons [15]. In this case the equivalent responses are rather low and if fast neutrons encountered in personal dosimetry have to be measured, it would be necessary to use some converters in front of the TL detectors in order to obtain greater sensitivities.

Table 5.3. Fast-neutron sensitivity.

Energy (MeV) Response

BeO

(10'° cm 2)/Rcsponse (R of 60Co)

LiF

0.5 1.0 2.7 4.2

15 Fission (critical assembly)

0.6 1.0 3.3 3.9

20 1.0

0.2 0.3 1.2 1.7

10 0.4

5.6. Fading

The release of electrons and holes from their traps and consequently their recombination is a statistical phenomenon, the probability of which is a function of temperature.

This probability is given by:

P = Sexp(-E/kT) with P = transition probability, S = vibrational factor characteristic of the centre, E = activation energy, k = Boltzmann's constant, and T = absolute temperature.

The half-life of the phenomenon is given by

T = 0.693 P'K

Table 5.4 gives the temperature of the different peaks and information on the stability of the carriers in the corresponding traps at 20°C for some phosphors [10]. When this information is not available in the table, an estimate of the loss of the signal during the period for a storage temperature of 20 °C is reported within parentheses. These values are only qualitative, as they are taken from different authors who have not necessarily used the same experimental parameters.

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Table5.4. Glow peak temperatures and half­lives for different TL phosphors.

TL material

LiF : Mg

CaF2 :Mn

CaF 2 :Dy

BeO

Li2B407 :Mn

Peak number

I II III IV V VI

I

I II III IV

I II III IV

I II III IV

Emission temperature (°C)

70 130 170 200 225 275

260

120 140 200 240/

70 160 180 ■» 2 2 0 /

50 90

200 1 2 2 0 /

Half­life

5 min 10 h 0.5 yr 7yr 80 yr ­

1% per day

25% per month

0% per 5 month

10% per 2 month

As is well known, the unintentional loss of the latent information is called fading. Temperature is normally responsible for this loss, but other quantities such as light can greatly influence the latent information in the TL material.

In table 5.5 the thermal fading measured for the most commonly used TL materials is given [1]. As can be seen, the majority of the listed materials do not have a large degree of fading; one exception is CaS04: Mn as it presents a peak at a relatively low tempera­

ture. Therefore this material cannot be used over such long periods of time as those normally accepted for routine personnel dosimetry.

Concerning thermal fading, the shallow traps will fade more rapidly than deep ones due to a larger transition probability. This fact can produce large errors in dose assess­

Table 5.5. Fading characteristics.

TL material

LiF : Mg Li2B407 : Mn CaF2 :Mn CaF2 :Dy BeO CaS0 4 : Mn

Thermal fading (25 °C)

~ 5 % in 1 year ­10% in 2 months

~ l % i n 1 day ~ 13% in 1 month

~ 8% in 3 months 50­85% in 3 days

Optical fading

Weak Weak ­Strong Strong ­

General characteristics of TL materials 93

ment and in order to avoid them the shallow traps can be emptied intentionally by a post-irradiation heat treatment either in a separate oven or in the reader as part of the readout process. This thermal treatment, when applicable, will increase the stability of the latent information even over very long periods.

Table 5.5 also gives some qualitative indications of optical fading, i.e. the modification due to artificial light or sunlight [1]. In fact, electron transitions may be stimulated by light (particularly near-uv light) giving rise to two effects that can take place simultaneously:

(a) the creation of a spurious signal, and (b) the loss of the latent dose information.

The two effects can be equally important; for example, for IiF-Teflon spurious doses of the order of several millirad have been observed for discs exposed to sunlight for several hours. At the same time irradiated LiF kept near a uv source for about one day had a reduction of the recorded dose of the order of 35%.

However, the sensitivity of different detectors to light is not a big disadvantage as it can be avoided by simply wrapping them in a light-tight envelope.

5.7. Annealing procedures

For each TL material used in dosimetric applications, it is extremely important to know the procedure for restoring its basic conditions after an irradiation. This procedure is called annealing and has two aims: the first is to empty the traps of the phosphor com­pletely after the irradiation and readout cycle; the second is to stabilise the electron traps in order to obtain, within narrow limits, the same glow curves even after repeated irradiations and thermal treatments. The annealing procedure is similar for every TL material and, in some cases such as LiF, it is very critical because if the procedure is not strictly the same, one can obtain significantly different results from repeated irradiations to the same exposure.

In table 5.6 are summarised the annealing procedures for several TL materials used in practice. As can be seen, the heat treatment needed to anneal LiF (TLD 100) and to stabilise its response is somewhat complex and it must be performed in a very rigorous and reproducible way.

The different annealings given in the table are necessary in order to empty all the traps (shallow and deep) of the phosphor. Many TL materials also have different peaks at low

Table 5.6. Annealing procedures.

TL material Annealing procedures Pre-read annealing

LiF (TLD 100) l h a t 4 0 0 ° C + 2 4 h a t 80°C 1 0 m i n a t l 0 0 ° C (or 2 h a t 100°C)

Li2B407: Mn 15 min at 300°C 10 min at 100°C BeO 15 min at 600 °C CaF2:Dy (TLD200) l h a t 4 0 0 ° C CaF2:Mn (TLD 100) l h a t 4 0 0 ° C LiF (PTL 700) 240-250 °C in the reader Performed in the reader

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temperature; these peaks are likely to be emptied even at room temperature (particularly peaks below 100°C) so in many cases it is necessary to perform a partial annealing before the reading in order to avoid a significant loss of information. This procedure is known as pre-read annealing and, in table 5.6, these treatments are also given.

Of course, the annealing procedure makes the evaluation time longer so that, in many cases, new materials have been studied in order to avoid a long heat treatment. This is the case, for example, for LiF (PTL 710) [16].

Finally, in order to perform a quick assessment of the dose recorded by the TL detec­tors, particular set-ups have been introduced in the readers for obtaining, in a simple and rapid way, the reading of TL light and, afterwards, the annealing of the detector, its reproducibility remaining the same. This procedure, however, can only be applied when the radiation dose is not too high and the residual signal is comparable to or lower than the background signal of the non-irradiated phosphor.

In the case of high irradiation it is therefore necessary to perform a high-temperature anneal of the TL detector.

5.8. Stability and reproducibility

The term stability here means physicochemical stability. A phosphor used for dosimetric purposes should not undergo any physicochemical changes during the repeated annealing processes and repeated exposures. This means that the glow curve, as well as the non-radiation-induced light emission and TL yield, must not change during extended storage of the material, repeated irradiation and reading. If these conditions are fulfilled the TL material can be used for dosimetric applications.

Several materials can be re-used many times without any noticeable changes in the material as long as it is not exposed to high doses.

As a consequence of the stability of the phosphor one can evaluate the reproducibility of each material to a certain dose level by calculating the standard deviation of a repeated set of measurements under the same exposure and reading conditions. As a practical example of reproducibility, table 5.7 gives some experimental data obtained with two different kinds of detectors irradiated at 1 R using ^Co gamma rays [9]. In this case the readings were performed in an automatic apparatus which uses hot nitrogen as the heating medium. At this dose level the readout cycle was sufficient to anneal the different phosphors.

Table 5.7. Reproducibility of four types

BeO (9.5 mm diameter)

Standard deviation 3.6% (over 6 d)

of detectors.

BeO (9.8 mm diameter)

9.8%

BeO (6.3 mm diameter)

1.0%

LiF (TLD 100)

1.1%

General characteristics of TL materials 95

5.9. Dose rate dependence

The dose rate dependence of LiF, Li2B407:Mn and BeO was studied by exposing these detectors to an x-ray beam which emits very short pulses of high intensity [17]. The pulse duration was 10~7s. The results obtained have shown that:

(a) the response of LiF is not modified up to an exposure of 1.5 x 1011 R s_1; (b) the response of BeO is not modified up to 5 x 1011 R s_1; (c) the response of Li2B407is independent of the dose rate up to 10 l2Rs_1.

The non-dependence on dose rate is an important requirement that demonstrates the possibility of using the TL detectors for measurements near installations delivering photons in very short pulses.

It has demonstrated that for radiological protection purposes the dose rate dependence of TL materials can be taken as insignificant.

5.10. Tribothermoluminescence (or triboluminescence)

Triboluminescence is a spurious signal that should be avoided, as otherwise it would be included in the measurement, thus increasing both the detection threshold and the error in the dose assessment.

The mechanism of this phenomenon is not well known: it is believed to be produced by the mutual friction of the crystals. The surface tensions so created release their energy as light during the heating process.

The triboluminescence depends on the physical state of the detector. Microcrystalline powder presents a larger triboluminescence in comparison with extruded or single-crystal detectors. For LiF, for example, the light emitted due to tribo-phenomena can be approximately equivalent to 1 rad or 20 mrad, depending on the physical state of the detector.

In order to eliminate this phenomenon, an experiment was carried out [18] which showed that it is sufficient to heat the sample in the absence of oxygen. Very good results can be obtained by putting the detector in an atmosphere of inert gas such as argon or nitrogen. No theoretical explanation has been given for this influence of the gas in eliminating triboluminescence.

Of course, the triboluminescence is more or less important depending on the dose received by the detectors. So in the case of dose measurements at therapy levels (doses of the order of several hundred rad) it is not important to use inert gas during the readout process. Conversely, in routine personnel dosimetry where the dose is not known a priori, it is essential to heat the detector in the presence of the inert gas (normally nitrogen).

References

1 1975 Technical Recommendations for the Use of Thermoluminescence for Dosimetry in Individual Monitoring for Photons and Electrons from External Sources. Commission of the European Com­munities Doc. EUR 5358

2 Busuoli G, Rimondi O and Vicini G 1973 Commission of the European Communities Doc. CEC(73)9

3 Cameron J R, Zimmerman D and Blond R 1967 Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAECDiv. Tech. Inf. AEC 8

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4 Busuoli G, Cavallini A, Fasso A and Rimondi O 1970 Phys. Med. Biol. 15 673 5 Attix F H 1968 Health Phys. 15 49 6 Bassi P, Busuoli G and Rimondi O 1976 Int. J. Appl. Rad. Isotopes 27 291 7 Morgan K Z and Turner J E 1967 Principles of Radiation Protection (New York: Wiley) 8 Becker K 1973 Solid State Dosimetry (Cleveland, Ohio: CRC Press) 9 Busuoli G and Julius H W 1977 Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, 14-17

February (Giessen: I Physikalisches Institut of the Justus-Liebig Universitat) 10 Portal G 1975 Rep. CEA-R-4697 11 Spumy F, Medioni R and Portal G 1975 Rep. CEA-R-4687,CEA-R-4688 12 Horowitz Y S, Freeman S and Dubi A 1979 Nucl. Instrum. Meth. 160 313 13 Horowitz Y Setal 1979 Nucl. Instrum. Meth. 160 317 14 Viragh E and Zsolnay E M 1980 Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, 1-4

April. Nucl. Instrum. Meth. 175 No 1 (September) 15 Tochilin E, Goldstein N and Miller W G 1969 Health Phys. 16 1 16 Portal G, Francois H and Blanchard P 1969 Rep. CEA-R-3476 17 Goldstein N 1972 Health Phys. 22 90 18 Schulman J H, Attix F H, West E J and Ginther R J 1960 Rev. Sci. Instrum. 31 1263

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

6 Preparation and properties of principal TL products

G PORTAL

6.1. Introduction

This chapter furnishes detailed information about the production, physical and dosi­metric properties of the most commonly used TL phosphors. The phosphors treated here are activated lithium fluoride (LiF), lithium borate (IJ2B4O7), magnesium borate (MgB407), beryllium oxide (BeO), calcium fluoride (CaF2), calcium sulphate (CaS04) and aluminium oxide (A1203).

6.2. Lithium fluoride

First studied by Daniels [18,19] who used crystals intended for optical applications, LiF was abandoned for other materials which do not present low-temperature traps. Studies were resumed by Cameron [11, 12] who conceived the systematic regeneration of LiF and encouraged the commercial production by The Harshaw Chemical Company (in the following just referred to as Harshaw) of products known as TLD 100, TLD 600 and TLD 700 depending on their preparation from natural lithium or lithium enriched with 6Li or 7Li (LiF TLD). A similar quality LiF, stabilised with sodium, was studied in France by Portal [45-47] and commercialised by Desmarquest & CEC, SA under the name of PTL 710, 716 and 717 (LiF PTL). These products do not necessitate systemic regeneration before each usage.

6.2.1. Preparation of LiF

The preparation of LiF is delicate. Only well equipped laboratories with trained staff can produce a material of good quality. It is generally more convenient to use commercial products. '

6.2.1.1. Preparation of LiF TLD powder Two methods were described in the Harshaw [33] patent.

6.2.1.1.1. Solidification method. The following materials are mixed in a graphite crucible:

Lithium fluoride 1000000 parts by weight Magnesium fluoride 400 parts by weight Lithium cryolite 200 parts by weight Lithium titanium fluoride 55 parts by weight

98 G Portal

After having obtained a homogeneous fusion of the mixture in vacuum the product is slowly cooled, pulverised and graded between 60 and 200 /im.

6.2.1.1.2. Single-crystal method. The mixture mentioned above is placed in a vacuum or inert-atmosphere oven in which a single crystal is drawn using the Czochralski method [17]. The graphite crucible is placed in a zone where the temperature is sufficiently high to obtain a homogeneous fusion mixture. It is then slowly moved to a zone of lower temperature which allows progressive solidification (about 15mmh_ 1). After cooling, this material is also pulverised and graded between 60 and 200 /mi.

This method, which takes more time to produce TLD grade l iF, allows preparation of more sensitive products than the solidification method described under § 6.2.1.1.1.

In both cases the TLD powders are annealed at 400 °C for some hours and at 80 °C for 48 h.

6.2.1.2. Preparation of compressed LiF TLD The LiF powder mixture is placed in a neutral-atmosphere press at a carefully chosen pressure and temperature, e.g. 3500 kg cm"2 and 700 °C. The piston of the press pushes the mixture through a hole which acts as a die. The bar so obtained is then sectioned to provide pellets of uniform thickness. The faces of the pellets are polished. The extruded dosemeters exhibit the same TL properties as the powder. The mixture undergoes a crystallisation, very likely due to the combined effect of temperature and pressure, for which the result is similar to that obtained by the Czochralski method.

6.2.1.3. Preparation of sodium-stabilised LiF PTL The following compounds are added to LiF powder [46]:

(i) 200 PPM magnesium fluoride, (ii) 1-2% by weight of sodium fluoride.

The powdered mixture is homogenised. It is then put in an aluminium oxide crucible and held at the crystallisation temperature for about 3 h in a nitrogen flow oven. The temperature is then progressively reduced to 60 °C in 45 min, the sample taken out of the oven and cooled rapidly. The product is finely pulverised, then subjected to an identical treatment. It is finally repulverised and graded between 60 and 200 /jm.

Annealing is carried out to encourage the creation of stable traps. An ordinary oven is made use of for this purpose with no nitrogen flow. 500 °C is used as the annealing temperature over 72 h. Cooling of the crystals is obtained by pouring them on to a cold metal plate. The final product has a sensitivity equivalent to the LiF TLD produced by the Czochralski method.

6.2.1.4. Preparation of LiF PTL pellets PTL TLD powder is finely sieved, compressed at about 5000 kg cm"2 in the desired form and then placed in a nitrogen gas oven. The chosen temperature is slighdy lower than the fusion temperature. Under these conditions a new fusion of microcrystallites is obtained. The pellets can be used after a simple annealing at 500 °C.

Preparation and properties of principal TL products 99

6.2.1.5. Preparation of LiF-Teflon dosemeters^ The production of LiF-Teflon (PTFE) dosemeters was first described by Bjarngard [4]. The commercial rights are protected by patents (e.g. British Patent Specification 1140028). Patent holders are Teledyne Isotopes Inc. The material is also produced, under licence, by D A Pitman Ltd.

The production technique is based on a thorough mixing of LiF powder of grain size less than about 80 /urn with fine Teflon powder; the average l iF grain size is about 10 im\. Both components need to be dry in order to avoid clogging. The optimum mixing time depends upon the design of the mixer and amount of mix. Too short a time results in an uneven phosphor distribution, whereas too long mixing times lead to the separating out of phosphor grains coated with fine Teflon powder. The mixture is either moulded into bars of, typically, 5-15 mm by compressing it into a mould, sintering at 365 C for about 1 h and slow cooling, or extruded, typically as a 1 mm rod. For moulding, the highest content of LiF is about 30% by weight. Higher concentrations would deteriorate the mechanical properties of the dosemeters. For extrusions, the maximum loading is about 10%, with 4% being the normal loading. The moulded or extruded material is cut into dosemeters of the desired shape. Discs are made from bars by parting off on a lathe or, for very (ultra) thin discs, on a microtome. Tape is fabricated by shaving the sides of a bar.

6.2.2. Physical properties of LiF

6.2.2.1. TL glow curve The TL glow curve of LiF is quite complicated because of its complex trap dynamics, as can be seen in figure 6.1.

The main peak normally used for dosimetric purposes, called the dosimetry peak, is peak V at a peak temperature of 225 °C. The corresponding trap level is very stable. The

C/1

z

LiF TLC

IV

III

II

- 1 0 0

V

z t— Z

I

1—

_!

I TEMPERATURE (°C) TEMPERATURE (°C ]

f This subsection has been inserted by the editors.

Figure 6 .1 . Glow curves of LiF TLD 100 (Haishaw [33]) and LiF PTL 710 (Des-marquest [22]), no particular annealing treatment.

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low-temperature peaks I, II and III are relatively unstable and must be suppressed by a thermal regeneration process.

In the case of LiF TLD this process consists of a heat treatment at 400 °C for two hours followed by a rapid cooling (figure 6.2). This thermal treatment restores the initial height of peak V and reduces the heights of peaks I, II and III. They are completely suppressed by a further heat treatment at 80 °C for 24 h. This treatment (standard anneal) is to be repeated before each exposure, since during the reading the effect of the thermal treatment is lost.

100 200 300 400 500 600 700 800 TEMPERATURE (°C)

Figure 6.2. Peak heights of LiF TLD 100 (peaks I-V) as a function of annealing tempera­ture for a 2 h anneal and rapid cooling.

For sodium-stabilised LiF PTL annealing is generally unnecessary with the reader operated in the proper mode (use of a preliminary heating plateau at about 160°C). As can be seen in figure 6.1, peaks I and II are feeble and peak III is practically non­existent. In these conditions the same sample can be re-used repeatedly without regenera­tion. Nevertheless, regeneration is necessary for heavily irradiated samples. Figure 6.3 shows that the best value for peak height V/peak height II is obtained with an annealing at 500 °C for half an hour and rapid cooling.

Figure 6.4 gives an idea of the stabilising effect of sodium on the TL glow curve. The glow curve for LiF without sodium is totally degraded after a thermal treatment at 210 °C over 15 s. This is the temperature up to which the phosphor is heated during one heating cycle. The height of peak II is increased considerably, reaching that of peak V at around 30 s.

The LiF PTL system is definitely more resistant. Figure 6.1 also shows that there is a peak at a temperature of 275 C corresponding to

a high-energy trap level in the phosphor. This peak shows up with relatively high absorbed doses (about 100 rad) and is more pronounced with exposure of LiF to high LET particles, such as the alpha particles produced by thermal neutrons in the 6Li(n, a)3H reaction. Some more high-energy trap levels with corresponding TL emission peaks exist up to 500 °C. They are not discussed here.

Preparation and properties of principal TL products 101

Figure 6 J . Peak heights of LiF PTL710 (peaks I­V) as a function of annealing tempera­

ture for a 3­h anneal followed by rapid cooling. 100 200 300 400 500 600

TEMPERATURE (°C)

~ 100 1/1

3 cL \—_ m or <

1 o HI I

I ■X-

<

90 ■■

80 ■■

70 •-

60 ■-

50 ■■

40 ■■

30 •■

20 ■-

10 \

0

LiF TLD 700 PEAK

J A ^ j N . i F TLD 700 PEAK I I

J - f -

15 +

LiF PTL 710 PEAK Y

LiF PTL 710 PEAK I I

+ 30 60 120

TIME OF HEAT-TREATMENT ( s )

Figure 6.4. Effect of a 210 °C heat treatment on the peak heights of LiF TLD 700 and LiF PTL 710.

6.2.2.2. Trap characteristics The emission maxima of LiF occur at the temperatures given in table 6.1. The half­lives of the corresponding traps at 20 °C are summarised in table 6.2.

One can state that, as a rule, the first two levels are unstable. They must not be used for long integration times. Level III only presents difficulties for integration times exceeding several weeks. Levels IV, V and VI are sufficiently stable for most dosimetric applications [49].

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Table 6.1. LiF peak temperatures.

Type of ph

LiF TLD

LiF PTL

Table 6.2.

Type of ph

LiF TLD

LiF PTL

osphor Peak

I II

III rv V

VI

I II V

VI

no. Peak temperature (°C) (maximum)

65 120 160 195 210 275

80 130 220 280

Half-lives of the LiF trap levels.

osphor Peak

I II

III IV V

I II V

VI

no. Half-life

5 min lOh 0.5 yr 7y r 80 yr

20 min 80h 1.3 XlO7 yr not measured

6.2.23. Emission characteristics The maximum emission wavelength of LiF is 400 nm for both LiF TLD [29] and LiF PTL [48]. All traps give an identical emission. The intrinsic TL efficiency (TL yield), defined as the ratio of energy emitted in the form of light per unit of phosphor mass and absorbed dose, is 0.04, a low value if compared with the majority of TL materials used. This explains the low sensitivity of LiF in general.

6.2.2.4. Model At present there is no completely satisfactory model. It is certain that magnesium plays an essential role in the creation of traps. Claffy [15] showed that, if LiF contains less than 1 PPM of magnesium, it is not thermoluminescent.

Magnesium has an atomic diameter (0.065 nm) close to that of lithium (0.060 nm) and can therefore easily substitute the latter. This has two consequences:

(i) It favours the creation of electron traps. With magnesium being divalent, a local excess positive charge is produced.

(ii) It favours the creation of electron holes, because in compensating the excess positive charge of the divalent magnesium ions substituting the monovalent ions of lithium, alkali ion valencies appear in the crystal lattice.

Preparation and properties of principal TL products 103

+ - + - (f±K- + - + - + vt±i> -- + - + - \+*) - + - + - '<*+/- +

* \ + - +"- vf+K- + -/|±j) - "©^ -\(±1) - + - + -\-») - (*/- + -\+f> DIPOLE DIMER TRIMER

n=1 n = 2 n = 3 Figure 6.5. Model for trap formation in LiF, dipoles and complexes (ffl = ion vacancy).

Grant and Cameron [30] showed, by dielectric measurements, that trap level II is due to the creation of Mg2+-n dipoles (figure 6.5,n = 1) and Harris and Jackson [32] showed that trap levels II and III are due to dipole coupling or dimers (figure 6.5, n = 2). Stoebe et al [61], following results of ionic conductivity measurements on samples annealed at different temperatures, provided the hypothesis that grouping of three dipoles (trimers) is responsible for trap levels IV and V (figure 6.5, n = 3). Claffy [15] explains the forma­tion of trap level VI by the regrouping of groups of dipoles. In some recent work Nink and Kos [43] show that F centres are involved in TL peak V. According to the authors the migration of Nig2* and Mg+ to F centres leads directly to the formation of so-called Z centres:

Mg2+ + FMg2+F = Z3

Mg+ + FMg2+F' = Z2.

The Z-centre concentration is enhanced when an increased concentration of isolated magnesium impurities is present (Z2-Z3 model). The creation of Z centres from F centres and isolated Mg ions and their reciprocal conversion leads to a probable explanation of Z2 centres to be responsible for TL peak V of LiF:Mg, Ti. According to Rossiter et al [53] the luminsecent centres are due to the presence of titanium and/or aluminium.

6.2.3. Dosimetric properties

LiF is the most commonly used TL material. It provides a good compromise between the desired dosimetric properties (see chapter 5). Its effective atomic number is sufficiently near to that of tissue (Zeff(LiF) = 8.14,Zeff(tissue) = 7.4) so as to provide a response which varies only slightly with photon energy: it varies by about 30% between 3.0 keV and 1.2 MeV (see figure 6.6 [48]) and one says that it is tissue equivalent.

Although its sensitivity is lower than certain other TL materials (see later subsections) it allows dose measurements from 10 mrad, which is a detection limit sufficient for most current dosimetric applications.

Figure 6.7 shows the dose characteristic of LiF. Supralinearity starts at 300 rad with more recent samples and at 100 rad if the reader is set in a way to eliminate the influence of peak VI of the glow curve. With earlier industrial LiF products, supra­linearity was observed starting at 800 rad. If LiF is pre-irradiated with a dose of 104 rad

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THEORETICAL CURVE ( Q ) 0"/. NQF (b) 2V. NQF

+ 3°/. NaF EXPERIMENTAL CURVE (LiF PTL-717)

10 1CT 10J

PHOTON ENERGY (keV) Figure 6.6. Energy dependence of the response of LiF.

10*

Figure 6.7. Dose characteristic of LiF. (a) Original material, (b) pre-irradiated (KVrad) material.

DOSE ( rod )

[24] its sensitivity increases by a factor of 2-3 and its linearity extends up to 104rad. Thermal regeneration reduces the effect of this treatment.

Samples which have received doses of 10srad and more since their fabrication cannot be regenerated any longer by thermal treatment. LiF containing 6Li is sensitive to thermal neutrons. Table 6.3 (from EUR 5358) summarises the results of LiF TLD dosemeter exposures to a thermal neutron radiation of 1010 n^cm"2. The material was standardised using a ^Co source. The data are expressed in roentgen-equivalents.

Preparation and properties of principal TL products 105

Table 63. Sensitivity of LiF TLD to thermal neutrons (from [68]).

Phosphor Sensitivity

TLD 100 TLD 600 TLD 700

328 1360

1.1

Table 6.4. Sensitivity of LiF PTL to thermal neutrons (from [48]).

Type of neutrons PTL 710 PTL 717

Thermal 2S2Cf Pu-Be 14.7 MeV

450 0.16 2.5 9.5

2.5 0.73 1.9 7.2

Table 6.4 [48] gives the sensitivity of LiF PTL to neutrons of different energies. Again the results are expressed in roentgen-equivalents.

Figure 6.8 [66] shows the relative response of LiF (peak V) for various LET radiations. This response decreases with increasing LET of the ionising particles (protons, alpha particles, etc). According to this figure the TL yield for a particles from the reaction 6li(n, a)3H, the average LET of which is 140keV/um_1, amounts to 0.22, a rather low sensitivity value. In contrast, trap level VI (peak VI) is particularly sensitive to a particles. For example, the ratio peak height Vl/peak height V is 0.015 for ^Co gamma rays and 0.175 for thermal neutrons. This difference in behaviour allows one to monitor the presence of thermal neutrons in a mixed radiation field (for more on neutron sensitivity of TL phosphors, see chapter 12).

w 1.0 o Q. i/i 0.8 111 CZ

UJ 0.6 > < 0.4 _ i LU a 0.2

ENERGY ( M e V ) 1000 200 50 20 10 5 3

T - £ r

T" T T

0.1 1.0

LET (keW/um ) Figure 6.8. Relative response of LiF as a function of LET and/or particle energy. A, protons of 720 MeVjO, a particles; *, a particles from 6Li(n,a) 3H reaction (from [66]).

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6.3. Lithium borate

6.3.1. Manganese-activated lithium borate

Schulman [59, 60] studied the preparation and properties of different borates activated with manganese. One of the most interesting borates turned out to be lithium tetraborate IJ2B4O7 +0.1% Mn. More recent work by Christensen (see [5]) resulted in materials which show improved air and tissue equivalence and higher stability with respect to air humidity.

6.3.1.1. Preparation The preparation of li2B407:Mn is relatively straightforward. Kirk et al [37] proposed a mixture of 32.5 g of lithium carbonate and 108.96 g boric acid. The mixture is added to a 5 ml aqueous solution of 75 mg MnCl2 • 4H20 which is heated at 100 °C for 12 h. The product is placed in a platinum crucible, heated up to 950 °C and then rapidly cooled. The crystalline mass thus obtained is ground and graded between 75 and 175 £im. Botter-Jensen and Christensen [5] added 0.25% by weight of silica to reduce the influence of humidity on the lithium borate which is very hygroscopic. The material can be sintered at 900 °C to prepare solid detectors or diffused into a plastic carrier such as Teflon or silicone rubber.

6.3.1.2. Physical properties The TL glow curve of Li2B407:Mn (figure 6.9) is composed of two groups of distinct peaks:

(i) one series of unstable peaks between 50 °C and 90 °C, (ii) a double peak, the first of which appears at 200 °C at low doses and the second of

which appears at 200 °C for doses in excess of 50 rad. Another peak appears with high LET radiation at 350 °C.

Only the double peak is suitable for dosimetric purposes due to its stability. However, this peak is less favourable than the dosimetry (210°C) peak of LiF because of its fading

20 l 100 50

TEMPERATURE C O

400

Figure 6.9. Glow curve of Li,B40,:Mn at 3.4, 8.6, 27.3, 69.2 and 960 min, respectively, after irradiation [59].

Preparation and properties of principal TL products 107

characterstics. In fact, Li2B407:Mn loses 10 and 37% of its information in 2 and 13 months, respectively, when kept at ambient temperature. Christensen et al [14] showed the effect of humidity on fading. The addition of Si02 (0.5%) to the phosphor reduces fading notedly.

In contrast to LiF the reading procedure does not change the distribution of traps in Li2B407:Mn. Therefore this phosphor can be used without regeneration. If regeneration is necessary, for zeroing the phosphor after high doses for example, it is sufficient to reheat the material at 300°C for 35 min.

The wavelength of the light emitted from Li2B407:Mn (main peak) is about 600 nm [29]. From this it follows that with the photomultipliers normally mounted in com­mercially available TL readers the sensitivity of Li2B407:Mn is low: 1/8 of that of LiF with an S 11 photocathode and 1/2 of that with an S20 photocathode and this in spite of the fact that the intrinsic sensitivity (TL yield) of Li2B407:Mn is a factor of 1.83 higher than that of LiF (0.04). Brunskill [8, 9] used an SbCsO photocathode. In this case the sensitivity reached a higher level. Christensen, on the other hand, used a photo-multiplier tube with an S photocathode. He found similar sensitivity for Li2B407:Mn and LiF. Attempts were made to modify the emission wavelength. Replacing manganese by silver, Moreno et al [41] obtained a uv emission which, though making the spectral distribution of the emitted light more favourable, enhanced its light sensitivity, an un­desirable feature.

6.3.1.3. Dosimetric properties The apparent sensitivity of Li2B407:Mn depends upon the characteristics of the reader used, as mentioned earlier.

In their preliminary work, Schulman et al [59] observed background noise, which limited the minimal detectable dose to lOOmrad. More recently reports by Christensen [13] and Brunskill [8] showed that it is possible to measure doses down to 10 mrad with sintered detectors. Christensen studied the influence of the manganese concentration on Li2B407:Mn properties. He concluded that samples prepared with 0.4% by weight of Mn presented the best compromise between sensitivity, energy dependence and humidity resistance. The samples he prepared had a linear response up to 300 rad and were supra-linear up to 3 x 106 rad (figure 6.10). Kirk et al [37] obtained a material linear from 1 to 10srad following a pre-irradiation of 10srad. This material is four times more sensitive but exhibits a background noise which limits the minimal detection of doses around lrad.

Li2B407:Mn is light-sensitive. This creates a background equivalent to about 15 mrad. Li2B407:Mn is the TL material which presents the least variable response with photon

energy. Jayachandran et al [34] showed that the energy dependence of the response of Li2B407: Mn is equal within ±3% to that of air and/or water with a 0.34 and/or 0.1% by weight content of manganese.

With the presence of 6Li and 10B the phosphor is very sensitive to thermal neutrons. Wallace and Ziemer [64] reported that a sample exposed to a dose equivalent of 1 rem yields a response to thermal neutrons equal to that of a sample exposed to 67 R y radia­tion. Christensen found 42 R rem-1 under slightly different conditions. Li2B407:Mn is therefore slightly more sensitive to thermal neutrons than is LiF TLD 100.

108 G Portal

10° 101 10* I03 10' I05

ABSORBED DOSE (rod)

Figure 6.10. Li2B„07:Mn TL output as a function of absorbed dose.

Wallace et al studied the response of Ii2B407:Mn to fast neutrons from a Pu-Be neutron source. They obtained 0.01 R rem-1.

Figure 6.11 demonstrates that the relative response of Ii2B407:Mn as a function of LET is quite different to that of LiF.

10 10° 101 102 10' LET (keV/pm)

Figure 6.11. Relative response of LiF, BeO and LijB407:Mn as a function of LET of charged particles [62].

6.3.2. Copper-activated lithium borate

Takenaga recently proposed a new sintered TL phosphor activated with copper.

6.3.2.1. Preparation To raw Li2B407 powder a solution of CuCl2 in acetone or alcohol is added and the mixture is then stirred and dried. The U2B4O7 powder containing the activator compound is heat-treated in air in a platinum boat for about 1 h at a temperature between 900 and 913 °C, which is just below the melting point (917 °C) of Li2B407.

By the heat treatment clear crystals, having an almost spherical form, are produced from the raw powder of Li2B407, which is porous and opaque. The heat treatment causes

Preparation and properties of principal TL products 109

diffusion of the activator into the Li2B407 crystal as well as recrystallisation and sintering due to diffusion, vaporisation and deposition.

6.3.2.2. Physical properties Li2B407:Cu phosphor exhibits two TL peaks. One is a small and rapidly decaying peak at about 120 °C and the other a dominant and stable peak at about 205 °C.

It has been found that the stoichiometric phosphor (lLi20-2B203) shows minimum absorption of humidity. In this case the loss in sensitivity due to the effect of humidity is 10 and 25% respectively after two and six months of storage.

6.3.23. Dosimetric properties The sensitivity to y rays is about 20 times higher than that of Li2B407:Mn produced by the Schulman method. One of the factors contributing to this relatively high sensitivity is the favourable spectral location of the TL emission which peaks at 368 nm.

Some other dosimetric characteristics of the stoichiometric phosphor were also in­vestigated, (i) Linearity: the TL output is linear with exposure to about 10s R, above which it is sublinear by colouring, (ii) Fading: the dominant peak at 205 °C fades less than 9% in intensity at 25 °C and 30% at 50 °C after 60 d in the dark, (iii) Light-induced fading: the TL output fades less than 10% after exposure to room lighting at 1000 lx for 3h .

63.3. Magnesium borate activated with dysprosium or thulium

This product was recently introduced by Prokic [51]. It contains a little amount of yttrium used as a sensitiser of the TL emission. The material is produced in the form of sintered dosemeters.

The wavelengths of the TL emission spectra for MgB407:Dy/Tm are typical of that for dysprosium or thulium atoms, which show emissions in the green-yellow band for Dy and emissions in the blue band for Tm. The influence of the co-activator reflects only on the relative emission efficiency of the basic activator, but the spectral distributions mainly stay unchanged for the Dy activator. However, the co-activator changes the emission in the Tm-activated material into green-yellow light.

MgB407:Dy/Tm has a sensitivity about seven times higher than LiF TLD 100. Its effective atomic number for photoelectric absorption is similar to the effective atomic number of LiF.

The material does not require annealing before irradiation and after read-out. Fading amounts to under 10% during the first 60 d after irradiation when the appropriate post-irradiation annealing is applied — similar to LiF — or after applying a corresponding read-out procedure with a preheating in the reader.

Magnesium borate TLD are not hygroscopic or light sensitive. With a Harshaw Model 2000 TL analyser this material detects exposures of 1 mR with 25 mg samples.

6.4. Beryllium oxide

BeO, like other oxides of calcium and magnesium, exhibits TL properties. It is the very small quantities of impurities always present in these materials which are responsible for

110 G Portal

the TL. Moore [40] was one of the first to study the TL properties of BeO. Jones et al [35], Tochilin et al [63] and Scarpa [54] studied its dosimetric properties.

6.4.1. Preparation

Only specialised laboratories are capable of producing BeO. It is commercially available in microcrystalline powder form. However, dispersed in air as a fine powder, this material is extremely toxic and therefore only specially equipped laboratories can use it in this form. Sintered pellets are also available which are used as transistor bases in the electronics industry. In this form the material presents no danger as long as abrasion of the surfaces and therefore production of powder and dust is avoided. These sintered materials contain sodium to increase their sensitivity.

6.4.2. Physical properties

The TL glow curve varies with the origin of the product. Scarpa [54] noted that the temperature of the main peak varies between 180°C and 280 °C and that some samples contain unstable traps.

The product chosen by Tochilin et al [63] showed two stable neighbouring peaks at 180°C and 220 °C, respectively. The second peak appears with relatively high doses and becomes preponderant about 1000 rad. Most samples show another peak at 350 °C. The stability of the main peak varies, according to different authors, from 0% in five months to 7% in two months. This variation probably depends upon the material chosen for study, the pre-reading treatment and the storage conditions.

Mandeville and Albrecht [39] observed that the TL light emission extends into the ultraviolet.

6.4.3. Dosimetric properties

With measurements carried out on a TL reader equipped with a standard photomultiplier the apparent sensitivity of BeO is low (about seven times less than that of LiF). However, with a reader equipped with a photomultiplier with a quartz window also permitting light with a short wavelength to pass, a sensitivity results which equals that of LiF. BeO containing sodium is very sensitive and allows the measurement of doses down to 1 mrad. However, the phosphor must be kept in the dark as a consequence of its high sensitivity to light. Fluorescent light accelerates fading by optical stimulation. Scarpa [55] noted that a sample placed 1 m from a fluorescent lamp of 80 W loses half of its information within 1 h.

The dose response curve is linear up to 50 rad. The following supralinear range extends up to doses of 5 x 105rad after which saturation occurs. These values are variable with LET.

BeO is a rather good tissue-equivalent material (Zef[ = 7.13). It is 60% more sensitive to 10-50 keV photon radiation compared with 60Co y rays. Agreement between experi­mental and theoretical energy dependence curves for the response is not too good. This anomaly seems to have to do with a linear energy transfer effect of the radiation on the detector response [63].

Preparation and properties of principal TL products 111

BeO is 2-3 times more sensitive to fast neutrons than is LiF, but it is much less sensitive to thermal neutrons. It is this property which sometimes makes BeO preferable to other phosphors. In fact, subjected to a radiation of 1010neutrons cm-2 BeO exhibits a response equivalent to 0.2 R of 60Co photon radiation, which is a response three times less than that of calcium fluoride. In contrast to LiF and Li2B407: Mn, BeO exhibits an increased response with increasing LET (figure 6.11). This is quite unusual and may be used for identifying the type of incident radiation.

The dosimetric properties of BeO make it interesting for its application in the field of personnel dosimetry.

6.5. Calcium fluoride

CaF2 exists in nature as fluorite. Its TL properties were studied by Wiedemann and Schmidt [65] and later by Kossel in 1954 and Houtermans in 1957.

Grogler et al [31] used it for dosimetry of ionising radiation. Ginther [25-27] studied the preparation of synthetic CaF2 activated with manganese (CaF2:Mn). Harshaw recently commercialised a CaF2 activated with dysprosium (CaF2:Dy). CaF2is not tissue-equivalent (Zeff = 16.57). It is a factor of 15 more sensitive to 30 keV photon radiation than to the photon radiation from 60Co. It is therefore necessary to use appropriate filters for energy compensation or reserve its use for the dosimetry of high-energy photon radiation.

CaF2 has two important advantages over LiF in certain applications:

(i) it is considerably more sensitive: the minimum detectable dose is about 10 times less;

(ii) it has a larger linear dose response range.

6.5.1. Natural calcium fluoride

Natural CaF2 has been used predominantly by the Belgian company Manufacture Beige de Lamps et de Material Electronique SA (MBLE). Its dosimetric properties were analysed in detail by Schayes [56, 57]. The most commonly found impurity activators in the natural products are the rare earths (Gd, Dy, Er, Tb) in bi- or trivalent states and trivalent uranium.

The TLD glow curve exhibits several peaks with a predominant stable peak at 250 °C (figure 6.12). The lower-temperature peaks are rather unstable. The peaks at higher temperatures corresponding to deeper trapping levels (not shown in figure 6.12) may be used for information storage from previous exposures and may be evaluated at a later date. By means of uv stimulation one can encourage trap-to-trap migration and refill the traps which correspond to the 250 C peak.

Adam and Katriel [1] analysed the trap depths of the MBLE material and obtained the results shown in table 6.5.

The half-lives of the trap levels are 3 months, 5 yr and 2 x 105yr, respectively, making the phosphor very suitable for long integration times, as long as the trapped electrons in the first trap level are released beforehand.

Increasing interest in natural CaF2 has been shown recently. This material is abundant in nature and production costs are low [42,44].

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100 200 300 TEMPERATURE C O

Figure 6.12. Glow curves of some TL products [23]. (A) CaSO„:Mn, (B) LiF:Mg TLD 100, (C) natural CaF2, (D) CaF2: Mn.

Table 6.5. Trap depths and frequency factors for natural CaF2.

Emission Trap depth Frequency temperature (°C) (activation energy) (eV) factor

110 175 263

1.2 1.65 1.71

4 X 1 0 ' S

6 X 1 0 " 4 X 1 0 "

20 100 200 250 300 400 500 TEMPERATURE C O

Figure 6.13. TL glow curves from different natural fluorites [58]. 1, 8, 9, S: sample designation.

Natural CaF2 phosphors exhibit different dosimetric characteristics depending upon their origin. Quite different glow curves are obtained. Figure 6.13 [58] gives an example of this variation. The TL emission of natural CaF2 is at a wavelength of 375 nm (see figure 6.14). It is therefore easy to distinguish between black-body emission and TL. This is advantageous for the measurement of low doses. With an appropriate TL reader the minimum detectable dose lies between 0.1 and 1.0 mrad. The sensitivity can be up to 50 times greater than that of LiF.

Preparation and properties of principal TL products 100

113

3500 4000 4500 5000 5500 6000 WAVELENGTH ( n m )

Figure 6.14. Light emission spectrum of some TL products [29]. (A) CaSO„:Mn, (B) LiF TLD 100, (C) natural CaF2 (260°C peak), (D) CaF2: Mn.

Linearity is observed up to 5 x 103rad, saturation between 104 and 10srad. The phosphor must be protected from light. If not, one may observe a diminution of

the TL signal or the appearance of a pre-dose due to the emptying of deeper traps. Sensitivity to both fast and thermal neutrons is low. The experimental results of

Tochilin et al [63] showed that CaF2 is half as sensitive to thermal neutrons as is LiF TLD 700. The sensitivity to 1010neutrons cm"2 with neutrons of 2.5 MeV and 14.5 MeV is 1 R and 4 R equivalents, respectively.

6.5.2. Manganese-activated calcium fluoride

Manganese-activated (3%) CaF2 is prepared by co-precipitation of CaF2 and MnF2 from a solution of CaQ2 and MnCl2 in ammonium fluoride (NH4)F. The precipitate is dried and heated in a neutral-atmosphere oven at 1200°C, powdered and graded. It may be pressed or sintered in the same way as described for LiF. It can also, like all TL microcrystallites, be diffused in a plastic material such as Teflon, as was already shown for LiF. The TLD glow curve apparently consists of a main peak at 260 °C. Actually this peak is a combined peak which results from several trap levels located very close to each other. High-temperature peaks also exist which turn out to cause evaluation difficulties, when the phosphor is exposed to high doses. A sample subjected to a high dose must be re-heated to empty the higher trap levels before being used for low-dose measurements.

An initial diminution of the signal of about 10% in the first few hours which decreases to 1% per day, was observed. Ginther [28] noted that this problem may be eliminated using slow heating rates (1 °C s"1).

The light emission at 500 nm is characteristic of the presence of manganese (figure 6.14).

The intrinsic sensitivity of CaF2:Mn is excellent: 0.44 compared with 0.04 for LiF. Synthetic manganese-activated CaF2, although less sensitive than the natural products,

is 3-5 times more sensitive than LiF. It allows measurements of doses in the order of some (1-2) millirad only. Certain commercial products, however, contain non-negligible amounts of radioactivity which make them unsuitable for low-dose assessment. The response of CaF2:Mn is linear up to 2 x 10s rad, saturation occurs at 106rad. This

114 G Portal

extraordinarily long linearity of response with dose is the main reason why CaF2:Mn is of so much interest. Sensitivity to neutrons is low. Blum et al [7] usedCaF2:Mn for measure­ments in mixed radiation fields. According to calculations a thermal neutron radiation of 1010 neutrons cm-2 yields a roentgen equivalent of 0.77 R. Due to the presence of im­purities in the phosphor an actual value of 1.05 R was measured by Prokic [51]. With 1010neutrons cm"2 of 14 MeV neutrons a roentgen equivalent of 3.54 is obtained. Com­pared to LiF the sensitivity of CaF2: Mn to 2-5 MeV a particles is a factor of two higher.

Based on Schulmann's work CaF2:Mn was produced commercially by the US company Edgerton, Germeshausen & Grier, Inc (EG&G), which had marketed a variety of dosemeter types.

6.5.3. Dysprosium-activated calcium fluoride

Harshaw markets, under the code TLD 200, a dysprosium-activated calcium fluoride (CaF2:Dy) which is considerably more sensitive than CaF2:Mn. Its properties were studied by Binder et al [2].

The glow curve of this phosphor actually consists of six peaks which are difficult to separate (figure 6.15). This holds true for the unstable peaks at 120 °C and 140°Caswell as for the stable peaks at 200 °C and 250 °C. Two higher-temperature peaks appear at 340 °C and 400 °C. CaF2: Dy is not a stable dosemeter material. It shows strong fading. A thermal treatment of 10 min at 80 °C has been proposed which reduces the fading from 25 to 13% per month [2].

10

9 -

8

7

6

5

U

3

2

1

i r CaF 2 : Dy, 1°C/sec

AFTER EXPOSURE

40 80 120 160 200 240 2 80 320 360 TEMPERATURE C O

Figure 6.15. Glow curve of CaF2:Dy [16].

The TL emission of CaF2:Dy shows three maxima at 460,483 and 576 nm. The latter maximum is detected with a variable efficiency depending upon the photomultiplier used.

With commercially available readers the sensitivity of CaF2:Dy is 15 times greater than that of LiF. With an S20 photocathode this factor is doubled.

Due to the presence of the numerous peaks in the glow curve, which develop following different laws, the dose response curve is complicated and only measurements based on light integration make sense.

Preparation and properties of principal TL products 115

Supralinearity may be eliminated by mepns of a thermal treatment at 600 °C for 2 h before exposure. The response is linear then up to 105rad with saturation at 106rad. This treatment, however, favours the second glow peak at 140 C which is relatively un­stable and diminishes the sensitivity of the phosphor by a factor of two.

The presence of dysprosium gives the material a non-negligible sensitivity to thermal neutrons.

CaF2:Dy serves mostly for the measurement of low doses but is increasingly being replaced by calcium sulphate activated with dysprosium which is more stable.

6.6. Calcium sulphate

Calcium sulphate (CaS04) was first used for dosimetry by Wiedemann et al in 1895. The first synthetic CaS04 prepared by Watanabe in 1951 was activated with manganese (CaS04: Mn). Since then a number of materials with different activators have been prepared and studied. CaS04is one of the most sensitive TLD products used in dosimetry.

6.6.1. Manganese-activated calcium sulphate

The glow curve of CaS04:Mn shows a single low-temperature peak at 90 C. For this reason the information stored in the phosphor is lost very soon and dosemeter readings need to be made shortly after exposure. Depending on the product and its fading characteristics, the TL readings for some standard exposure may vary from 15 to 60% in the first 10 h and from 40 to 85% in the first 3 d after exposure.

CaS04: Mn has been used by Bjarngard [3] for detection of doses as low as 20 /irad.

6.6.2. Samarium-activated calcium sulphate

CaS04:Sm [38] is about 2.5 times more sensitive than CaS04:Mn and has a glow peak at 200 °C with the wavelength of the emitted TL being at 600 nm. The material has low fading at ambient temperature but is very light-sensitive.

6.6.3. Dysprosium- or thulium-activated calcium sulphate

CaS04: Dy and CaS04: Tm are the latest and most interesting phosphors in the CaS04 phosphor series, since their response is considerably more stable. The phosphors were first prepared by Yamashita et al [67].

6.63.1. Preparation 75 mg of powdered dysprosium oxide (Dy203) or 80 mg thulium oxide (Tm203) are dissolved in 250 cm3 concentrated sulphuric acid (H2S04). The solution is poured into a crucible, heated up to 250°C and 34.4 g of calcium sulphate (CaS04-2H20) added. The solution is then evaporated at 300 °C until crystals of calcium sulphate are obtained. The crystals are pulverised and graded between 75 and 200 nm. Finally, the microcrystalline powder is heated at 750 °C for 2 h in an aluminium oxide crucible in air, followed by a final heat treatment at 400 °C for 15 min.

The US company, Teledyne, markets CaS04:Dy/Tm embedded in Teflon.

116 G Portal

6.6.3.2. Physical properties The TL glow curve of CaS04: Dy/Tm is similar to that of CaS04: Sm. The main peak (peak III) is found at 220 °C and two lower-temperature peaks (peaks I and II) at 80 °C and 120°C (see figure 6.16). These latter peaks are rather unstable. A higher-temperature peak (peak IV) appears at 250 °C for doses in excess of several hundred rad for CaS04: Tm and for doses of several thousand rad for CaS04: Dy. It is necessary to eliminate the first two peaks by a pre-heating procedure either in an oven or within the reader itself. The stability of the trap levels corresponding to the peaks III and VI varies according to dif­ferent authors. According to Yamashita et al [67] fading is 7% in 6 months, but according to Mejdahl it is 30%.

£ l

UJ

C0.SO4 :TmJlII\ 10 R CaSO^iDyf Uy

0 100 200 300 400 TEMPERATURE C O

Figure 6.16. TL glow curve of CaSO„:Dy and CaSO,:Tmat 10 R.

Peak no.

HI IV

Table 6.6. Trap level characteristics for CaS04 : Dy (from [50]).

Emission Trap depth Frequency temperature (°C) (activation energy) (eV) factor

220 1.4 2.07 X10'4

260 1.54 3.25 X1014

Half-life a t20°C(yr)

120 20300

By applying isothermal annealing, trap level characteristics were obtained for the trap levels III and IV of CaS04:Dy (table 6.6) [50].

CaS04:Dy and CaS04:Tm are light-sensitive. The background reading can be equiva­lent to as much as several tens of rad and the fading may be as much as 30% in 5 h in sunlight [67].

The luminescence spectrum of CaS04:Dy presents two peaks at 478 nm and 571 nm. That of the Tm-activated sulphate exhibits a main peak at 452 nm and less important structures at 360,470 and 520 nm.

6.6.3.3. Dosimetric properties CaS04: Dy and CaS04: Tm are equally more sensitive to photon radiation than is LiF. The lower detection limit is about 0.1 mrad.

The tribothermoluminescence noise is low.

Preparation and properties of principal TL products 117

The dose response of CaS04 : Tm is linear up to doses of 300 rad. Saturation starts at 104rad. The dose response of CaS04:Dy is linear up to doses of 3xl03rad and saturation occurs at 10srad. Regeneration is necessary for heavily irradiated phosphors and can be effected by annealing at 400 °C for 15 min.

The relative response of CaS04 as a function of photon energy is shown in figure 6.17 together with that of aluminium oxide (A1203). The effective atomic number of CaS04: Dy/Tm is 15.6, compared to 16.6 for CaF2. The response to 30 keV photon radiation is 11 times superior to that obtained with the photon radiation from 60Co (16 for CaF2).

—i i i 10'

Figure 6.17. Photon energy dependence of CaSO„ (O) and A1203 (A) response. , theoretical; , experimental.

ENERGY ( k e V )

Table 6.7. Neutron sensitivity of CaS04 : Dy/Tm (from [48]).

Neutron source and/or energy

Thermal 2"Cf Pu-Be 14.7 MeV

Roentgen equivalent per 1010 neutrons cm

0.6 0.3 1.1 6.87

The sensitivity to neutrons is given in table 6.7 with the sensitivity expressed as roentgen equivalent for a neutron radiation of 1010neutrons cm-2 [48].

Both phosphors are suitable for low-dose measurements. Blum e t al [6] used CaS04: Tm for 7 dosimetry in neutron fields.

118 G Portal

Recently Prokic [51] prepared sintered TLD from a mixture of CaS04:Tm powder and a few per cent of a particular component, which plays an essential role during the sintering process and simultaneously acts as sensitiser.

6.7. Aluminium oxide

Aluminium oxide (A1203) is interesting because it is available at low cost due to the large industrial production of ceramic and heat resistant materials made out of it.

A1203 has been used for a long time in radiation dosimetry. In 1954 Daniels and Rieman [20] used a personal dosemeter containing sapphire crystals fixed in a silver chloride base. However, in spite of all the research undertaken on sapphires and rubies, so far little use has been made of A1203. Kenney and Cameron [36] used a combination of Al203with LiF to measure the effective energy of some x-ray beams.

Rieke and Daniels [52] tried to explain the catalytic properties of alumina. They studied the influence of the crystalline lattice and its impurities on its TL properties and analysed samples baked at different temperatures representing different crystalline states.

Kelly examined the TL glow curve of pure and magnesium-activated aluminas. More recently Portal [46] used a commercial alumina (corindon) for dosimetry in areas with criticality risks. Various diamond spars with impurity ions are suitable as TL phosphors:

(i) red sapphire containing iron, (ii) blue sapphire containing iron, titanium and lithium,

(iii) gilt sapphire containing nickel and magnesium, (iv) ruby containing aluminium and chromium.

6.7.1. Physical properties

6.7.1.1. Glow curve Figure 6.18 shows the glow curve of A1203. It exhibits four peaks referred to as peak I, II, III and IV. The first three peaks are merged, whereas the fourth peak is well separated.

1 -

>■

\-

0.5

100 200 300 TEMPERATURE ( ° C )

Figure 6.18. Glow curve of A1 ;03.

Preparation and properties of principal TL products 119

Rieke and Daniels [52] observed TL peaks at the following temperatures (heating rate 25 °C mm-1): 103 °C, 123 °C, 164 °C and 236 °C.

The relative importance of the trap levels corresponding to these peaks varies con­siderably with the crystalline structure and impurity content. The first three glow peaks (I, II and III) seem to be correlated to imperfections in the crystalline lattice rather than to the presence of impurities. The 260 °C peak (IV) appears with bakes at temperatures greater than 1000°C when alumina is completely dehydrated. Its height is proportional to the sodium ion concentration. The sodium ions act as impurity.

Rieke and Daniels concluded that the 123°C peak corresponds to partially hydrated centres (Al(OH)2+), while the 164°C peak, which appears at higher baking temperatures, corresponds to an A10+-type centre. The 236 °C peak should be due to sodium ion trapping centres. Portal [46] measured the half-lives of trap levels I, II and III, which are 1 h, 140 h and 115 d, respectively. Trap level IV corresponding to peak IV is very stable (half-life at 20 °C = 1.8 x 107yr).

The TL glow curve of A1203 is not affected by the heating cycle during the read-out process. The material can be regenerated by annealing it at 500 °C for 1 h. Once irradiated, A1203 should be stored away from light.

6.7.1.2. Spectral emission of alumina Corindon exhibits a double emission which is the same for all four trap levels [48]: (i) a strong emission in the 650 nm region which is difficult to distinguish from black-

body radiation, (ii) a weaker emission between 390 and 550 nm.

It is the latter emission which is more interesting because it can be detected with an ordinary commercial TL reader. For suppressing emissions above the 450-500 nm region, a filter is needed. In practice the filters used for LiF are also suitable for alumina.

6.7.1.3. Dosimetric properties In figure 6.19 the glow curves of A1203 (curve a) and LiF (curve b) are reproduced for comparison reasons. The ratios between the heights of peaks III and IV of alumina and that of the main peak of LiF are 0.2 for peak III and 0.12 for peak IV.

b) Al203 (6 rad) c . , , „ _ c J Figure 6.19. Comparison of the glow curves of

7Y-, | , A l ^ (b) and LiF (a) (from [48]).

TEMPERATURE ( °C )

120 G Portal

If phosphor reading is effected shortly after exposure it is advantageous to perform light sum readings over peaks II, III and IV. In this way a sensitivity of about 30% of that of LiF is obtained and one is able to measure down to doses of about 50 mrad. If the reading is effected some time later after exposure it is preferable to analyse glow peak IV only avoiding decay corrections. The lowest detectable dose under these circum­stances is about 100 mrad as long as the readings are performed with N2 flushing. Without N2 flushing triboluminescence is responsible for an increase of the lower detection threshold to about lOrad. Figures 6.20 and 6.21 demonstrate that Al203has no linear dose characteristic for either glow peak (figure 6.20) or the total light sum (figure 6.21). Calibration curves must therefore be used. Saturation prevails at 106rad.

From figure 6.19 [48] it follows that alumina is less photon-energy-dependent than CaS04.

100 500 1000 ABSORBED DOSE ( r a d )

Figure 6.20. Dose characteristic of A1203.

Figure 6.21. Dose characteristic of A1203 (total light integration).

10J W 10= 10°

ABSORBED DOSE ( rad )

Preparation and properties of principal TL products 121

The sensitivity of alumina to neutrons is summarised in table 6.8. Alumina shows a very low sensitivity to thermal neutrons.

Table 6.8. Sensitivity of alumina to neutrons.

Neutron source Roentgen equivalent and/or energies per 1010neutrons cm"2

Thermal neutrons 0.5 J52Cf 0.38 Pu-Be 0.35 14.7 MeV 12.1

References

1 Adam G and Katriel J 1971 Luminescence Dosimetry. Riso Rep. 249, vol 1 (Riso, Roskilde: Danish AEC)

2 Binder W, Disterhoft S and Cameron J R 1969 Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAECRep. CONF 680920, p 43

3 Bjarngard B E 1962 Aktiebolaget Atomenergie: Stockholm AE-118 4 Bjarngard B E 1965 Proc. 1st Int. Conf. on Medical Physics, Harrogate 5 Botter-Jensen L and Christensen P 1972 Acta Radiol. Suppl. 313 247 6 Blum E, Bewley D K and Heather J D 1912 Phys. Med. Biol. 17 661 7 Blum E, Bewley D K and Heather J D 1973 Phys. Med. Biol. 18 226 8 Brunskill R T 1968 UKAEA-PG Rep. 837(W) 9 Brunskill R T 1970 Proc. 2nd Conf. IRPA, Brighton IRPA/2/P62 10 Busuoli G 1973 private communication 11 Cameron J R, Zimmerman D, Kenney G N, Buch R, Bland R and Grant R 1964 Health Phys.

10 25 12 Cameron J R, Daniels F, Johnson H and Kenney G N 1961 Science 134 333 13 Christensen P 1969 Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., Septem­

ber 1968. USAEC Rep. CONF 680920, p 90 14 Christensen P, Botter-Jensen L and Majborn B 1973 Regional Conf. on Radiation Protection,

Jerusalem 15 Claffy E W 1967 Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965.

USAECDiv. Tech. Inf. AEC 8, p 74 16 McCurdy D E, Schiager K J and Flack E D 1969 Miners, Health Phys. 17 415 17 Czochialski Z 1917 Z. Phys. Chem. 92 219 18 Daniels F 1950 Rep. 4th Symp. on Chemical Physics and Radiation Dosimetry, Edgewood,

Maryland p l48 19 Daniels F, Boyd C A and Saunders D F 1953 Science 117 343 20 Daniels F and Rieman W P 1954 University of Wisconsin, Final Rep. No 7 21 Dean P N and Larkins J H 1963 LAMS 3034, p205 22 Desmarquest & CEC Zone Industrielle no 1, 27000 Evreux, France 23 Fowler J L and Attix F H 1966 Radiation Dosimetry vol II, ed F H Attix and W C Roesch (New

York: Academic) p272 24 Frank M and Edelmann B U 1966 Kernenergie 7 228 25 Ginther R J 1954/ . Electrochem. Soc. 101 248 26 Ginther R J and Kirk R D 1956 USNRLProg. Rep. September, p i 2 27 Ginther R J and Kixk R D 1957 J. Electrochem. Soc. 104 365 28 Ginther R J 1965 CONF650637, p l l 8 29 Gorbies S G 1967 Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June

1965. USAEC Div. Tech. Inf. AEC 8, p 167

122 G Portal

30 Grant R M and Cameron J R 1966 J. Appl. Phys. 37 3791 31 Grogler N, Houtermans F G and Stauffer H \95SProc. 2nd Int. Conf. on Peaceful Uses of Atomic

Energy, Geneva 21 (New York: UN) p226 32 Harris A M and Jackson J H 1969 Br. J. Appl. Phys. 2 1667 33 Harshaw Chemie NV, Strijkiertel 95, De Meern, Holland 34 Jayachandran C A, West M and Shuttleworth F 1969 Proc. 2nd Int. Conf. on Luminescence Dosi­

metry, Gatlinburg, Tenn., September 1968. USAECRep. CONF-680920, p i 18 35 Jones D E, Petrock K F and Denham D H 1966 UCRL Prog. Rep. No 25 36 Kenney G N and Cameron J R \963 Prog. Rep. TID 19112 37 Kirk R D, Schulman H J, West E J and Nash A E 1967 Proc. Symp. on Solid-State Chemistry and

Radiation Dosimetry (Vienna: IAEA) p91 38 Krasnaya A R, Nosenko B M, Revzin L S and Yaskolko V 1961 Atomnayia Energyia 10 630 39 Mandeville C E and Albrecht H O 1954 Phys. Rev. 94 494 40 Moore L E 1957 J. Phys. Chem. 61 636 41 Moreno A, Archundia C and Salsbery L 1971 Proc. 3rd Int. Conf. on Luminescence Dosimetry.

Riso Rep. 249, vol 1 (Riso, Roskilde: Danish AEC) p305 42 Nambi K S V, Kathuria S P and Sunta C M 1969 Radiation Monitoring Thermoluminescent

Detector, Radiation Protection Monitoring (Vienna: IAEA) p321 43 Nink R and Kos H 1977 Phys. Stat. Solidi a 41 K157 44 Okuno E and Watanabe S 1971 UV-induced thermoluminescence in natural calcium fluorite

Luminescence Dosimetry. Riso Rep. 249, vol 2 (Riso, Roskilde: Danish AEC) p864 45 Portal G, Francois H and Blanchard Ph 1969 C.R. Congr. Europ. IRPA, Menton 9-11 October p79 46 Portal G 1971 French Patent 710 3757 (February 1971), Publ. 2 123 889 (March 1972) 47 Portal G, Bermann F, Blanchard Ph and Prigent R 1971 Proc. 3rd Int. Conf. on Luminescence

Dosimetry. Riso Rep. 249, vol 1 (Riso, Roskilde: Danish AEC) p410 48 Portal F 1975 CEA Rep. No R^1697 49 Portal G 1978 CEA Rep. No R-4943 50 Piaggio-Bonsi R, Lorrain S and Portal G 1976 Proc. 8th Congr. SFRP, Paris p566 51 Prokic M 1980 Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April 1-4, Nucl. Instrum.

Meth. 175 No 1 (September) 52 Rieke J K and Daniels F 1957/ . Phys. Chem. 61 629 53 Rossiter M J, Rees-Evans D B, Ellis S C and Griffiths J M 1971 J. Phys. D: Appl. Phys. 4 1245 54 Scarpa G 1970 Health Phys. 19 91 55 Scarpa G 1971 Proc. 3rd Int. Conf. on Luminescence Dosimetry. Riso Rep. 249 (Riso, Roskilde:

Danish AEC) p427 56 Schayes R and Brooke C 1963 Rev. MBLE 6 24 57 Schayes R, Brooke C, Kozlowitz I and Lheureux M 1967 Proc. 1st Int. Conf. on Luminescence

Dosimetry, Stanford University, June 1965. USAECDiv. Tech. Inf. AEC 8, p i38 58 Schayes R, Brooke C, Kozlowitz I and Lheureux M 1967 Luminescence Dosimetry. CONF-650637

p l38 59 Schulman J H, Kirk R D and West E J 1967 Proc. 1st Int. Conf. on Luminescence Dosimetry,

Stanford University, June 1965. USAEC Div. Tech. Inf. AEC 8, p l l 3 60 Schulman J H 1966 Proc. Solid State Chem. Radiat. Dos. Med. Biol., Vienna, 3-7 October (Vienna:

IAEA) IAEA STI/PUB/138, p3 61 Stoebe T G, Guilmet G M and Lee J K 1970 Radiat Effects 4 189 62 Tochilin E, Goldstein N and Lyman J T 1969 Proc. 2nd Inst. Conf. on Luminescence Dosimetry,

Gatlinburg, Tenn., September 1968. USAEC Rep. CONF 680920, p 424 63 Tochilin E, Goldstein N and Miller W G 1969 Health Phys. 16 1 64 Wallace R H and Ziemer P L 1969 Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg,

Tenn., September 1968. USAEC Rep. CONF 680920, p 140 65 Wiedemann E and Schmidt G C 1903 Ann. Phys. Chem. 54 604 66 Wingate C L, Tochilin E and Goldstein N 1965 USNRDL-TR 909 67 Yamashita Y, Nada N, Anichi H and Kitamura S 1971 Health Phys. 21 295 68 Technical Recommendations for the Use of Thermoluminescence for Dosimetry in Individual

Monitoring for Photons and Electrons from External Sources. Commission of the European Com­munities Doc. EUR 5358

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

Operational aspects

D F REGULLA

7.1. Introduction

The requirements of modern dosimetry for high local resolution of radiation fields, for ruggedness of the detector, an extended detection range, independence of detector response from dose rate, etc, can often not be fulfilled by conventional dosimetry tech­niques, such as ionisation chamber dosimetry. To a certain extent solid-state techniques, particularly thermoluminescence dosimetry (TLD), can cover these require­ments. The question is whether TLD can provide an accuracy of <10% (preferably <5%), and this with an uncertainty of 2-3% from the reference dosemeter already included. Considering the precision of data from the literature, one finds coefficients of variation as low as 1% or even less (figure 7.1), which indicates that high enough accuracy can, in principle, be provided for most applications.

However, it must be realised that these data from the literature have been obtained under laboratory conditions and by skilled personnel knowing about the various error sources. Therefore, both the newcomer to the field of TLD and the expert should

o <

Q cr < Q Z <

LU

10

1 1

\ \ \ \

o

-

1 1

1 r

uniformity

1 1

CaF2: Dy

within 10 detectors

precision ^ for one detecto

i i

o

r

1 1

O

1

_

"

-

0.1 1 10 102 103

EXPOSURE (m R) Figure 7.1. Precision for one detector and uniformity within ten detectors depending on exposure. Detector material is CaF2: Dy.

124 DFRegulla

anticipate higher variations under practical conditions. The following selected items may demonstrate the possible variability of error sources, a situation which will not change significantly in the near future if TLD technology is not seriously improved [1].

7.2. Parameters affecting precision

Any review of error sources in TLD must consider instability of the read-out instrument and of the influence quantities and performance parameters during irradiation, storage, reading and annealing (figure 7.2).

IRRADIATION

a) Temperature bl Incidental Direct

cISec.Particle Equilib

Pre Irr Storage : a) Temperature bl Light

J

\ Pre Irr. Annealing: a) High Annealing Temp. b)Cooling Rate c)Low Annealing Temp.

EVALUATION

a)Heating Rate b)Max. readout Temp.

dCooling Rate

Post Irr. Storage. a) Temperature b) Light

Post Irr Annealing: a) Temperature

b)Annealmg Time

Figure 7.2. Influences on-TLD precision within the measuring cycle.

7.2.1. Read-out instrument

The TLD read-out instrument should be operated at constant room temperature. A temperature increase was found to cause a reversible decay of the reference light source reading (figure 7.3) which in practice would erroneously be compensated for by in­creasing the voltage. For common TLD readers, the ambient temperature should not exceed 25 °C to avoid trouble from the electronics.

Unidentified errors from the electronics have been found due to spontaneous changes in light source readings up to ±2% on different days or due to effects of dry nitrogen, which is recommended as an inert gas (see later in this chapter) and which gave rise to an increase in PMT dark current by up to a facte, of 30 (see also §§3.3.5 and 4.4).

An important error arises from the fact that the TLD reader sensitivity is mainly con­trolled by means of a reference light source which is characterised by a certain emission spectrum. This spectrum is modified by the spectral transmission factor of filters and

Operational aspects 125

15 25 35 ROOM TEMPERATURE (°C)

Figure 7.3. Temperature dependence of reference light source reading. Curve A, Harshaw 2000 (Regulla, GSF Munich); curve B, Teledyne 7300 (Roenick, IRD Rio de Janeiro).

lenses in the optical path before it reaches the PMT. Since the spectral transmission factor is non-stable and will change in the course of the instrument's use due to surface contamination from dust or combustion products, the reference light source reading will decrease. Usually this is compensated for by increasing the high-voltage setting of the PMT. This procedure, however, will be correct only for those detectors with a TL emission spectrum comparable to that of the reference light source but may cause a major error for all detectors with a different emission spectrum. The reason is that the transmission factor does not change linearly over all the spectrum but selectively.

It should be noticed that the spectral response of most photomultipliers will shift with the change of voltage so that the calibration should be repeated. Moreover, the sensitivity check of the TL read-out instrument using a reference light source does not cover the heater planchet and its optical properties.

Influences on the TL signal due to the optical properties of the heater planchet are surface-related. Some tens of per cent of the TL light reaching the PMT photocathode may result from the planchet due to reflection. This figure can change with the change in reflectivity properties of the planchet surface during use (figure 7.4) (see also §§3.2.2.1 and 4.2). The error can be kept small by using permanent planchets, e.g. planchets made of noble metals such as platinum. To avoid this error, a different heating

(") (b) (c) (d) (e) (/) Figure 7.4. Appearance of silver-plated heater planchets in course of use (a-d) and of platinum planchets, (e) virgin and (/") frequently used.

126 DFRegulla

technique has to be used, e.g. hot gas. In this way, a geometrical error of up to 10% or more which arises from the positioning of the TL detector with respect to the planchet could also be kept small. All these figures were determined with a Harshaw Model 2000 TLD analyser.

As a precondition for low-level measurements, the read-out of the TL detectors is most often done under an atmosphere of inert gas. This provision reduces or eliminates effects of non-radiation-induced but thermally stimulated chemoluminescence at the surface of the detector or heater planchet. In addition it was found that the location of the glow curve when plotted versus the temperature (figure 7.5) as well as the achieved measuring precision in the case of multiple use (table 7.1) depend on the read-out atmosphere chosen. As a consequence, for example, an increased maximum read-out temperature would be necessary for read-out under dry argon gas if compared with dry nitrogen gas.

t, 6 -

o X

2 -

LU

-

-

r\^-^" a i r

f i ^ - d r y nitrogen

t lV" ^ a r 9 o n

f > f i 11 ll i l i

I i

J i

\ I

4 -

Figure

200 400 600 TEMPERATURE (°C)

7 J . Influence of the readout atmosphere on the glow curve of LiF detectors.

Table 7.1. Influence of the reading atmosphere on the precision of LiF : Mg,Ti extruded rods and on the PMT dark current. 24 uses in each atmosphere. Exposure 20 R.

Reading Standard Dark current Dark current atmosphere deviation (%) a t250°C(pA) a t300°C(pA)

Air Nitrogen Argon Oxygen

5.3 2.7 2.4 6.7

12 3

30 3

Operational aspects

7.2.2. Irradiation

127

Usually, irradiation of thermoluminescence dosemeters is performed at room tempera­ture. Relatively extreme temperatures will occur only in restricted areas, e.g. in reactor and space research. To clarify the effect of temperature during exposure on TL sensitivity, LiF : Mg,Ti detectors were irradiated in Perspex containers in liquid nitrogen (— 196 °C) and in water maintained at 0 °C, 20 °C, 50 °C and 90 °C after having reached temperature equilibrium with the ambient medium (figure 7.6).

-200 0 50 100 TEMPERATURE DURING EXPOSURE (°C)

Figure 7.6. TL output of LiF: Mg,Ti as a function of irradiation temperature. Curve A, Sunta and Watanabe 1976 Health Phys.; curve B, Regulla.

The measurements revealed that for irradiation at — 196°C TL sensitivity drops to about 80% when normalised to 20 °C. For irradiation between 0°C and 50 °C, TL sensi­tivity remained approximately constant. At 90 °C, however, TL sensitivity increased by about 14%. The elevated TL sensitivity remained constant over a period of 10 d while stored at room temperature.

To understand better the effect of irradiation temperature the glow curves of the detectors irradiated at the above-mentioned temperatures were recorded (figure 7.7). It is seen that the irradiation at 90 °C favours an additional electron trapping in a high-temperature peak around 260 °C besides the main TL peak at 210 °C. The high-tempera­ture peak does not appear in the case of low-temperature exposures at —196 °C.

A further error may arise from different irradiation geometries between calibration and measurement in an unknown radiation field, TL detectors show a non-negligible change of response depending on the direction of radiation incidence (figure 7.8). Evidently, the magnitude of this effect depends on detector geometry and material as well as on the radiation quality.

128 DFRegulla 300

100

~ 30

LiJ i n

/ / 1

1,

1 1

1

ll II 1

1 1 1

ll ll

ll 1 1

1 ll

1

1\ 1* H

1 I'­

1 * I ll

■ \

V y

/ / '

/ /

> / M /

Figure 7.7 Glow curves of LiF: Mg,Ti after irradiation at different temperatures: , 90°C; , 50°C; . . . , 20°C; , ­196°C; — . —, background. Heat rate 2 °C s"'.

100 200 300 TEMPERATURE ("CI

&>— 90° 90°/0° 1180°

0.5 1.0 1.5 HVLImm Al)

10 29 43 50 50 kVp

1Be 0.6 0 3AI

1 mm TOTAL FILTRATION

Figure 7.8. Directional dependence of TLD chips (3 X 3 X 0.9 mm3) for low photon energies. , CaF2:Dy; , LiF:Ti, Mg.

7.2.3. Storage

Between irradiation and read­out, the signal intensity can be influenced by the storage conditions and by light. The effect of storage temperature can, in general, be kept small by a proper annealing procedure at an elevated temperature before read­out. Even for storage for one week at 70 °C using CaF2 : Dy, which is subject to substantial fading even at room temperature, this fading can be almost totally eliminated by a proper pre­read­

out annealing procedure (figure 7.9), e.g. 15 min at 130°C (see also §7.2.4). The effect of light on the TL output is two­fold. Besides a reduction of the TL signal

depending on light intensity, wavelength (uv!) and duration of light exposure, a stimula­

tion of thermoluminescence was found (figure 7.10). The latter effect, simulating a ^Co

Operational aspects 129

Figure 7.9. Magnitude of fading of CaF2: Dy for different storage and pre­read­out annealing temperatures.

STORAGE TIME (min

100

< o LU

o 60

< o LU

20

LU - ♦ - '

■~~~^4o)

[b) / /

s* /

s

^ + ' •

•

r ■

^ . ■ -

10< uv EXPOSURE TIME (min)

< z o

iOO

8 £ 200 w

LU o: Z) (/) o CL X

10'

Figure 7.10. Effect of UV light (Philips mercury lamp HG 8; 9A; source distance 15 cm) on (a) irradiated and (ft) unirradiated LiF: Mg.Ti.

gamma exposure signal of up to several hundred milliroentgens for LiF : Mg,Ti or several roentgens in the case of CaF2 and CaS04, refers to a glow curve different from the one after ^Co gamma exposure and should be ascribed to surface physics (figure 7.11). In practice, a superposition of both light­induced fading and stimulation of the TL signal has to be expected.

In figure 7.12, results of a fading experiment at 70 °C storage temperature with the presence of daylight are reported for different types of TL materials. The decay be­

haviour of all detector materials was different from reference probes also stored at 70 °C but in the dark. Obviously, fading correction factors obtained from experiments in the dark should be applied very carefully in cases in which detectors might have been exposed to light during storage.

Thermoluminescence dosimetry is generally said to be independent of humidity. This well­known statement is, however, not true at least for lithium borate chips Li2B407:Mn,

130 DFRegulla

60

20

/ 1 / 1/

/]

/ / ^ Ugh

A j.

\

t exposu 30 mm

JOOr

\ irra

\ \

A 1

nR.Co60

lout pre diation -

s \ ^ ^

- and post-annealing

100 200 TEMPERATURE I t )

300

Figure 7.11. Comparison of LiF:Mg,Ti glow curves induced by 60Co 7-rays and ultraviolet light.

0.5h1h 3h 1d 3d 2w3w

STORAGE TIME (min) Figure 7.12. Comparison of fading behaviour of different TL materials after storage at 70 °C (a) in the dark and (6) with access of daylight (h = hour, d = day, w = week).

which in the course of a four week fading experiment at 50 °C and 70% relative humidity became unnaturally transparent and blew up during subsequent read-out (figure 7.13). As a consequence the detectors were destroyed. It is beyond doubt that any application of a TL detector material under conditions of increased humidity requires that the test pro­gramme should be carefully performed under these conditions.

7.2.4. Pre-read-out annealing

The purpose of pre-read-out annealing is to remove any existing low-temperature glow peaks > 100 °C that are thermally unstable even at room temperature and are responsible for spontaneous signal losses after irradiation (thermal fading). Low-temperature peaks

Operational aspects 131

Figure 7.13. Appearance of Li,B.,07: Mn chips. Left: new detectors; middle: after storage at 50 °C and 70% relative humidity; right: with subsequent read­out.

are typically present in CaF2; Dy, and in LiF: Mg,Ti which has not been annealed con­

ventionally (see figure 7.15). To optimise the pre­read­out annealing in the case of LiF: Mg,Ti, i.e. eliminating the

low­temperature peaks without significantly reducing sensitivity, the detectors were subjected to different pre­read­out annealing temperatures between 20 °C and 175°C for 10 min annealing time, respectively (figure 7.14). As a result, fading at room tempera­

ture was completely eliminated by about 15 min pre­read­out annealing at 100 °C, while the sensitivity loss was only about 15%. The glow curve so achieved is shown in figure 7.15.

To find the optimum duration of temperature treatment at 100 °C, the LiF detectors were subjected to pre­read­out annealing for different periods of time (figure 7.16). From these results it is obvious that sensitivity losses become significant only if the detectors

~ 1.00 > i— ■—i (/) z LU 1/1 LU >

LU

0.50

1 15V.

0 100 200 TEMPERATURE (°C)

Figure 7.14. Influence of the magnitude of temperature during pre­read­out annealing on the TL sensitivity of LiF : Mg.Ti. Annealing period 10 min.

132

120

80

40

DFRegulla

. / / / / /

/

*t ,'

I '

Re-usi

I

1 1 1 1 /

thK»°C/

without

1 1 A. 1 / \ /

V A — '

any ann

' ' A ' 1

ealing

100 200 TEMPERATURE l°CJ

300

Figure 7.15. Comparison of glow curves of LiF : Mg.Ti used after annealing at 400 "C with and without pre-read-out annealing (see §7.2.6).

are stored at 100 °C for extended periods, e.g. TL loss becomes about 50% after one day of storage at 100 °C. Additionally recorded glow curves revealed that after this extended heat treatment only the 210 °C glow peak is present. Johnson et al [2] observed that the pre-read-out annealing procedure can be applied before or after exposure without affecting the TL sensitivity. However, recent findings on this subject show that major research is still necessary to achieve the relevant information [3].

1.2

> £ 0.8

> 0.4

UJ cr

n l i i l l i 10 10' 10J

TIME Imin) Figure 7.16. Influence of pre-read-out annealing time at 100 °C on TL sensitivity of LiF : Mg.Ti normalised to the TL output after 5 min at 100 °C.

1 1 11

M M

1 1 i 1 1 1

1 Hour

, , i

i i

i i

i 1 1 1

i i n

lOoy

7.2.5. Read-out process

In most commercial TLD instruments the maximum read-out temperature is preset and controlled by a simple scale indication. If the temperature indication is incorrect for some reason, e.g. faulty contact between heating planchet and thermocouple or wrong reference temperature, then the preset read-out temperature may not be sufficient. The resulting errors were studied for LiF: Mg,Ti detectors by measuring the TL output for

Operational aspects 133

various maximum read-out temperatures at constant evaluation time (figure 7.17). If the temperature is lower than or equal to the main glow-peak temperature (210°C) the TL sensitivity decreases markedly, for instance to 60% for 200 °C if compared with com­plete read-out at > 250 °C. This is due to only partial release of the trapped electrons. For temperatures well above the main glow peak all traps are emptied which results in constant TL sensitivity of the detector.

P 0.8

z LU l/l LU > 0.4 < _ j LU cr

O D

150 250 TEMPERATURE l°C)

350

Figure 7.17. TL sensitivity of LiF : Mg.Ti as a function of maximum read-out temperature.

The TL output also depends on the holding time at maximum read-out temperature. This is shown in figure 7.18 for two different heating rates. The sensitivity drop at shorter holding times is due to the fact that there is no temperature equilibrium between heater planchet and detector. For a fast heating rate, e.g. 50°Cs_1 and 300°C maximum read­out temperature during 5 s holding time, 93% of the TL signal is integrated. If measured with a 250 °C maximum read-out temperature for the same holding time and heating rate, only 90% of the TL signal is collected. Therefore, a 250°C maximum read-out temperature, a heating rate of 50°Cs_1 and at least 15 s read-out time appears useful for fast routine evaluation.

1.00

0.90

0.80

300'cy/

7 250"C

o i. 15°C/se : SSO'C/se

c c

0 5 10 15 TIME AFTER REACHING THE PRESELECTED MAXIMUM TEMPERATURE (s)

Figure 7.18. TL sensitivity of LiF: Mg,Ti as a function of the holding time at maximum read­out temperature. Parameter: heating rate.

134 DFRegulla

In figure 7.19 the effect of heating rate during read-out on TL eutput of LiF: Mg,Ti is shown. This effect is different for different phosphors, even for the so-called integral measuring technique. Obviously, one should choose a different heating rate for the read­out of, for example, LiF: Mg,Ti or CaF2: Mn, in order to minimise the effect of heating rate fluctuations on TL output. These fluctuations probably arise from changes in the thermal contact between planchet and detector. As also shown in figure 7.19, the peak height measuring technique is significantly more dependent on the heating rate than the integral method and is therefore restricted to very special dosemeter assemblies.

0.8

0-.0.4

1 3 6 10 30 60102 300 600103

HEATING RATE |°C mm"')

Figure 7.19. Influence of heating rate during read-out on TL output for peak height and integral reading.

Studies of the influence of dry nitrogen gas flow on TL output revealed no effect of flow rates between 1 and 15 1 min-1. Hence, for economic reasons a flow rate of 1-2 1 min-1 seems to be sufficient even in cases of fast routine evaluation (see §4.3).

7.2.6. Post-read-out annealing

In order to erase the residual TL signals completely, the TL detectors must be annealed prior to re-use. This procedure is called pre-irradiation annealing or post-read-out anneal­ing. Cameron et al [4] proposed a standard annealing technique based on a high-temperature treatment at 400 °C for one hour followed by a long-term low-temperature treatment at 80 °C for 23 h in the case of LiF: Mg,Ti. While the aim of the high-temperature treatment is to erase all previous dose information from the detector, the aim of the low-temperature annealing is to restore the original shape of the glow curve and thus reproduce the TL sensitivity.

The following results concern the effect of the upper post-read-out annealing tempera­ture on the TL sensitivity. For this purpose two groups of LiF detectors were subjected to temperatures between 300 °C and 600 °C for periods of 15 min and l h prior to irradiation. These detectors were then exposed and read out. The results for both groups are shown in figure 7.20. For annealing at temperatures around 400 °C the TL sensitivity

Operational aspects

200

135

400 TEMPERATURE (°C)

600

Figure 7.20. Relative sensitivity changes of LiF:Mg,Ti due to non-constant high post-read-out annealing temperature. Parameter: annealing time. Normalisation to maximum sensitivity.

remains roughly constant. Annealing for more extended periods and at temperatures above 450°C reduces the sensitivity significantly, e.g. a one hour treatment at 600°C diminishes the sensitivity by about 30%. Hence, 15min annealing at 400 °C appears sufficient to reset the LiF detector sensitivity which, moreover, yields roughly the same sensitivity as a one hour treatment at 400 °C.

The reproduction of the upper post-read-out annealing temperature is of even greater importance for LiF of the French CEA. There, the prescribed 485 °C must be reproduced to within ± 5 °C in order to avoid serious changes of the TL sensitivity.

Besides the upper annealing temperature, the cooling rate during the subsequent cooling phase has a major influence on TL sensitivity and, hence, on precision, particu­larly for LiF: Mg,Ti. Varying the cooling rate from the 400 °C level down to room temperature revealed that a fast cooling rate produces a higher TL sensitivity because of low-temperature glow-peak formation. The difference in TL sensitivity between cooling rates of 3°Cmin_1 and 3000°Cmin~' were found to be up to a factor of four (figure 7.21). Therefore, for precise TL dosimetry with LiF:Mg,Ti, one should make every effort to reproduce the cooling rate as well as possible between calibration and measurements. It should be noted that for other TLD materials the cooling rate does not influence the TL sensitivity so much.

The low-temperature post-read-out annealing (usually 80 °C) subsequent to the high-temperature annealing, as mentioned at the beginning of §7.2.6, is intended to eliminate low-temperature glow peaks and therefore fading for LiF: Mg.Ti. However, this low-temperature annealing does not provide a simple cut-off of the glow curve, but changes the distribution of the glow curve due to a dynamic trap mechanism in LiF: Mg,Ti (figure 7.22). Subjecting LiF: Mg,Ti which has been annealed at 400 °C for 15 min to a low-temperature post-read-out annealing in the range 20-300 °C for a duration of one hour, the TL sensitivity is influenced as shown in figure 7.23. From the steep decay of the curve it becomes evident that good reproducibility of the low post-read-out annealing temperature is an important precondition for high-precision measurements with LiF: Mg,Ti.

136 DFRegulla

LQQ

200

n

I i l l

i i i i

I i l l

i I I I

I i l l

i I I I

i I I I

y^ 1

i I I I

1 10 102 103 10* COOLING RATE l°C mirf1)

Figure 7.21. TL sensitivity to LiF:Mg,Ti as a function of cooling rate.

Figure 7.22. Dynamics in the glow-curve structure of LiF: Mg,Ti as a function of post-read-out annealing time at the low-temperature level of 80°C [5]. Annealing time in hours.

Again it should be noted that the trap dynamics at low post-read-out annealing tempera­ture is particular to LiF : Mg,Ti and must not be considered for other TLD materials.

The above-described annealing technique proposed by Cameron et al [4] for LiF: Mg,Ti certainly reveals high precision but is inconvenient, because of the annealing periods needed of over 20 h. Therefore, a 'fast-annealing' procedure which is applicable even for LiF : Mg,Ti has been proposed by Regulla [6]. It consists of a short post-read-out anneal­ing (5 min) at a temperature of 400 °C and a 10 min pre-read-out annealing at 100°C to eliminate fading by suppressing the low-temperature glow peaks but without trying to restore the original glow-curve shape. The glow curve thus resulting for LiF: Mg,Ti is given in figure 7.15.

Operational aspects 137

100 200 TEMPERATURE |°C)

300

Figure 7.23. Influence of a non­constant low post­read­out annealing temperature on the TL sensitivity of LiF: Mg,Ti. Annealing period 60 min. Insets: glow curves taken after annealing at the indicated temperatures.

The precision attainable with the 'fast­annealing' technique [6] is strongly dependent on the accuracy with which the 100°C temperature level is controlled. Data on repro­

ducibility achieved by this method are given in figures 7.1 and 7.24. The measurements were made with LiF:Mg,Ti TLD 100 chips of dimension 3 x 3 x 0.9 mm3.

~ 2

3 0 < 1 > 1-2 LU

o u. z 0 o

5 >-2 (JJ Q

C

:— 4

— ■

b

</\

1 -

" ■ '

.____. 5 \ '--J J

y • S

!> c

1

s-

i

) " ^ ^

,! •

, — >'

k ?

\ ~~J J

|

( t>)

U 6 NUMBER OF USE

10

Figure 7.24. Reproducibility measurements with LiF: Mg.Ti using a 'fast­annealing' tech­

nique according to Regulla [6] . (a) O, single detector; • , corrected with group tendency. (b) Mean of group (10 detectors).

The furnace used in the present investigations was purpose­built and operated with an accuracy of (100 ± 2.5) °C. The reproducibility of results can still be improved further by using a precision temperature­controlled furnace. Meanwhile, coefficients of variation as low as 0.5% and better could be realised by an improvement in technology. These figures clearly demonstrate the usefulness and high practical value of the 'fast­annealing'

138 DFRegulla

procedure for laboratory and routine TLD application. At least 10 repeated measurements a day can be performed with the 'fast-annealing' technique compared with one measure­ment with the conventional 24 h annealing process.

In figure 7.24, the usefulness of a 'calibration detector group' is also shown. The detectors of this group are subjected to the same annealing programme as the detectors used for the measurement in the unknown radiation fields, but they are always exposed to a known exposure under calibration conditions. In this way, the influence of slight changes in the annealing procedure or of the read-out equipment and their effect on the precision of the measuring detectors can be avoided by evaluating a correction factor as the mean value from the results of the 'calibration detector group'. The scatter of readings from the 'measuring detectors' can thus be reduced.

7.2.7. Detector handling

The physical size of TL detectors is usually very small, which is an advantage. Tweezers must be used to handle them. It seems that frequently handled TL detectors lose sensi­tivity when handled with normal mechanical tweezers. This effect is due to microscopic scratches and other damage to the detector surfaces and edges. Figure 7.25 shows that the sensitivity loss may reach up to 20% after a 50-fold use in the case of LiF: Mg,Ti chips [7]. When 'vacuum tweezers' are used the sensitivity loss is almost negligible (see also figure 7.25). Therefore, handling with 'vacuum tweezers' appears to be strongly recommended in operational TLD.

In dosimetric practice, one cannot avoid crystals serving as TL detectors getting dirty in the course of their use. This is due to inorganic and organic detector contaminations baking on to and diffusing into the surface layer. As a result the TL sensitivity is reduced. This effect may amount to several tens of per cent. Careful handling of the detectors will help to limit the surface contamination. Once contaminated, most detectors can to a certain extent be cleaned again according to the washing procedure given by the manufacturer.

Instructions on cleaning procedures are available from, for example, Harshaw for extruded TL detectors: '(a) Between normal uses, the detectors should be rinsed with analytical grade methanol, (b) Detectors which seem extremely dirty or are touched with

40 60

Figure 7. tweezers

40 60 20 NUMBER OF USES

.25. TL sensitivity of LiF : Mg,Ti chips after repeated handling with (a) mechanical and (b) vacuum tweezers [7].

Operational aspects 139

the hands should be rinsed with warm trichlorethylene followed by the methanol rinse'. The application of an ultrasonic bath will increase the efficiency of the cleaning procedure. The ultrasonic treatment does not affect the TL signal if its duration does not exceed several minutes.

However, from experience, cleaning should usually be avoided. Regular cleaning, e.g. as part of the measuring cycle, affects the reproducibility and, hence, the precision of measurement markedly. This may be due to changes of the crystalline surface structure which can be observed under the microscope after the cleaning procedure. The changes in the detector surface are particularly pronounced if detergents (even in low concentra­tion) are used. While the cleaning effect may be remarkable (see figure 7.26), a permanent loss of TL sensitivity may be found at the same time. This apparently has to do with the partial dissolution of detector material in water which is strikingly demonstrated in figure 7.27. This figure compares the surfaces of two TLD 700 chips magnified by a factor of 1800 using an electron microscope. The chips had been deposited in double-distilled heavy water for 43 h.

Figure 7.26. Surfaces of two TLD 700 chips [11]. Left, virgin; right, after 43 h in double-distilled D,0 .

Figure 7.27. Microscopic view of the two chips shown in figure 7.26, magnification X 1800 [11]. Left, virgin; right, after 43 h in double-distilled D20.

140 DFRegulla

Obviously, it is the solubility which must be carefully considered when cleaning TL detectors over extended periods of time even in pure water. The solubility is also the reason why TL detectors should, in general, not be implanted into tissue without being wrapped in an appropriate cover. Dettmer et al [8, 9] report changes of surface properties and TL sensitivity of detectors which had been implanted uncovered; three weeks after implantation the detectors could not even be found. Obviously, the detectors had been dissolved by the inter-cellular fluid. Besides, tissue reactions were observed which lead one to consider also the biological toxicity of TL materials in cases of implantation.

For the operator of thermoluminescence dosimetry the question may arise of whether or not TL materials are toxic and should be handled with special care. Except for beryllium oxide, the considerable toxicity of which is recognised, there is no particular information in the literature. At least, Dettmer et al [10] conducted a pilot study using the drinking water of young rats saturated with LiF (solubility 0.27 g LiF per 100 cm3

cold water). From the results they strongly recommended that LiF should be handled with at least as much caution as other toxic fluorides.

Along with the toxicity of detector materials one should also consider a probable health risk from chemical solvents and detergents used for detector cleaning. Trichlor-ethylene, for instance, has an acute toxic effect on the human liver and central nervous system. It handling, if allowed at all, should be done with due care and using a hood.

7.3. Conclusion

A precondition for precise thermoluminescence dosimetry is the appropriate equipment, i.e. a short- and long-term stable read-out instrument and regulated annealing facilities. This requirement is not trivial, since such equipment is not at present fully available to the necessary quality. Hence, in the author's opinion, for high dosimetric reliability thermoluminescence dosimetry still requires a good deal of technological improvement. Besides, as long as the measuring cycle is not automated, any precision TLD will demand a 'precision operator' who can control the equipment and the different influence quantities and parameters while following the processing instructions exactly. This calls for long-term knowledge of the subject under consideration; book-learning is helpful but cannot replace operational experience. In the present situation, an overall accuracy of ± 10% should be achievable with TLD in practice, e.g. in radiation therapy. The question is whether this accuracy corresponds to what is needed.

References 1 Regulla D F 1980 Remarks on the present state of thermoluminescence dosimetry, Proc. 6th Int.

Conf. on Solid State Dosimetry, Toulouse, April 1-4 1980. Nucl. Instrum. Meth. 175 98 2 Johnson T L, Attix F H and Booth L F 1971 Health Phys. 21 22 3 de Planque G, Julius H W and Verhoef C W 1980 Effects of storage intervals on the sensitivity and

fading of LiF TLD Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April 1-4 1980. Nucl. Instrum. Meth. 175 177

4 Cameron J R, Zimmermann D W, Kenney G, Buch R, Bland R and Grant R 1964 Health Phys. 10 25

Operational aspects 141

5 Zimmermann D W, Rhyner C R and Cameron J R 1966 Health Phys. 12 525 6 Regulla D F 1971 Experience with the LiF TLD system and recommendations for its practical

application. GSF-Rep. S-124 7 Cox F M, Lucas A C and Kapsai B M 1976 The reusability of solid thermoluminescent dosemeters

and its relation to the maintenance of TL standards, Health Phys. 30 135 8 Dettmer C M and Galkin B M 1969 The toxicity of thermoluminescent phosphors, Proc. 2nd Int.

Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF 680920

9 Galkin B M, Dettmer C M and Suntharalingam N 1969 Sensitivity changes in solid thermo­luminescent dosemeters after subcutaneous implantation, Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF 680920

10 Dettmer C M, Galking B M and Hanna H J 1967 Phys. Med. Biol. 12 577 11 Guarducci D 1978 Studio della risposta dei dosimetri a TL alia radiaiione beta di bassa energia di

tritio. Thesis Universita degli Studi di Milano, Facolta di Scienze Fisiche

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

8 Precision and accuracy O/TLD measurements

G BUSUOLI

8.1. Introduction

This chapter describes the precision and accuracy with which TL measurements can be performed and those parameters that can influence precision and accuracy. As some confusion exists with the terms precision and accuracy, they must be defined in order that they should be used in the right way.

8.2. Definitions

8.2.1. Precision

Precision is a term related to the random uncertainties associated with the measurement, i.e. the uncertainties that have been derived by statistical methods from a number of repeated readings. In order to define the precision of a set of measurements, the standard deviation may be used. 'Low precision' means that random uncertainties are very high.

8.2.2. Accuracy

Accuracy is a statement of the closeness with which a measurement is expected to approach the true value. Accuracy includes the effect of both systematic and random uncertainties. The value of a quantity is understood to be considered as 'true' either by theoretical considerations or by comparison with a fundamental measurement. The indicated value is the value of a quantity as indicated by the relevant measuring device, sometimes also called 'reading' or measured value.

'High accuracy' means that the indicated and the actual values are nearly the same.

8.3. Assessment of random and systematic uncertainties

8.3.1. Random uncertainties

If the measured value of a quantity is represented by a parameter Y then, for a normal distribution, the probability of Y having a value lying between Y and Y + d7 is given by

P(Y)dY= exp - p - \ d Y oy/{2v) \ 2a2 /

where ju is a constant, equal to the value of Y at the maximum of the distribution curve,

144 G Busuoli

and a is a measure of the dispersion or width of the curve. The quantity a2 is called the variance of the distribution. The quantity a can be estimated from an analysis of the observations and this estimate, together with the number of degrees of freedom, are used to derive the random uncertainties.

If n measurements of the same quantity are performed, the best estimate of the constant \x of the distribution is given by the mean value Y:

_ 1 " n , = l

and the best estimate of the variance a2 of the distribution is given by the variance S2(Y):

1 S\Y) x Vi-ry

n - 1 ,fi The quantity S(Y) is called the standard deviation of the measurements.

Because any mean value Y comes from a limited number of measurements, repeated determination of Y will produce a series of different values. These, for a large n, will have a distribution close to normal, whatever the distribution of Y. The standard deviation of this distribution can be determined and it is called the standard error of the mean 5(F), given by

1 » _ , S\Y) S\Y) = X (V, - Y)2 = - ^ .

n(n - 1) ,= ! n

In many circumstances experiments consist of measurements that involve several quantities. Therefore, the value 7 of a physical quantity is linked to other separate physical quantities by the relationship Y=f(a, b, c,...) with the variances of the single quantities S2(a), S2(b),....

The estimated variance of Y is given by

S2(Y) = (bY/da)2S2(a) + (dY/bb)2S2(b)+(dY/bc)2S2(c) + ....

The same holds true for Y = f(a, b, c,...).

83.2. Systematic uncertainties

Whereas in the treatment of random uncertainties a straightforward statistical procedure can be applied, for systematic uncertainties this is not possible, since the probability distribution is not known.

If the value Y of a physical quantity is a function of a number of measurements a, b, c,... of separate physical quantities, i.e.

Y=f{a,b,c,...)

then, if the measurements are all independent, the systematic uncertainty (AY)a of Y, due to the systematic uncertainty Aa on a, is given by

(AY)a=\dY/da\Aa.

Precision and accuracy of TLD measurements 145

In practice there are two methods used to combine the different components in order to give the overall systematic uncertainty AY. The first is by a simple arithmetic addition:

AY=(AY)a+(AY)b + (AY)c + ...

ay da

Aa + dY db

Ab + dY

Ac +. dc

The second is to combine them in quadrature:

AY2 = (AY)* + (AY)2b + (AYfc +...

/bYf /dY\2 / ay \ 2

= — )Aa2 + — )Ab2+ — Ac2 + . \?>a/ \db/ \dc/

The first method probably overestimates the total systematic uncertainty, while the second tends to underestimate it. Therefore, in stating the systematic uncertainty of a physical quantity the component parts should be listed, together with the actual value of any constants and correction factors used; the method of summing the component parts should also be indicated.

8.4. Sources of errors in TLD

From the publication EUR 5358 'Technical recommendations for the use of thermo-luminescence for dosimetry in individual monitoring for photons and electrons from external sources', we can derive a list of commonly encountered sources of errors that affect the precision and accuracy in determining the dose under identical geometrical conditions.!

These errors will depend on the detectors, the reader and the evaluation procedure, the thermal history and the sensitivity of the detectors to neutrons.

8.4.1. Errors due to the detector

Variability of transparency and other optical properties of the detector.

Variability of the optical properties of any covering material, if this remains in position while the detector is read.

Temperature influences in excess of those taken into account by the calibration pro­cedures.

light effects.

Effects due to the energy and directional dependence of the dosemeter response.

Contamination of the thermoluminescent material (not radioactive contamination).

Ineffective and non-reproducible cleaning procedures applied to the detector.

Variability of the mass of the thermoluminescent material in the detector.

f Commission of the European Communities Document.

146 G Busuoli

Where the use of powder is concerned, the distribution of the powder in the tray of the reader.

Changes in the detector sensitivity due to radiation damage.

8.4.2. Errors due to the reader and evaluation procedure

Instability of the functions of the read-out device and the peripheral equipment.

Instability of the reference light source, due to intensity and spectral changes, with time and temperature.

Non-reproducibility and variability of the rate of inert-gas flow.

Non-reproducibility of the detector position in the reader and of the heat transfer between heater and detector.

Variations in the zero-dose reading.

Non-consistency of the thermal read-out cycle during calibration and measurement.

Changes in the optical properties of the read-out device, in particular due to variations in the reflectance of the heater element.

8.4.3. Errors due to the thermal treatment

Non-reproducibility of the pre-irradiation annealing procedure.

Non-reproducibility of the post-irradiation heat treatment.

Non-reproducibility of the thermal treatment during read-out.

The above list consists both of systematic and random errors and the loss of precision, introduced by these sources, can be minimised by carefully performing the whole measuring cycle with the TL dosemeters.

8.5. Precision of TL measurements

(a) Precision obtainable with one single dosemeter. If a single dosemeter is repeatedly irradiated with the same dose and is read keeping the different parameters constant, variations in the measured TL signal are observed.

These variations determine the precision with which a certain dose can be measured with the dosemeter. There are several sources of these variations and they are amongst those previously listed. Their relative weight on the precision of the results will depend, to some extent, on the dose value to be measured. One of the most important causes of variations comes from the detector's zero reading (or background signal), determined from repeated measurements on unirradiated dosemeters. This is particularly important a low doses, while with increasing doses the background and its variation become less and less important and finally can be neglected.

A further source of variation is due to the instrument instability determined through repeated readings of the light source normally placed inside the instrument. The contribu-

Precision and accuracy of TLD measurements 147

tion to standard deviation which originates from the instrument conditions normally does not amount to more than 0.3%.

Assuming that the dose is exactly the same in the different series of exposures, the total variance in a series of measurements at any dose level with one single dosemeter is given by the following equation:

Here crs is the percentage standard deviation of measurements when background effects are negligible, D is the dose, in millirad, and a^ is the variance of the read­out of un­

irradiated dosemeters in equivalent absorbed dose, here in millirad.. As an example, table 8.1 gives the standard deviations as measured in practice for

different TL detectors exposed to 0.1 R of 60Co gamma rays. The figures for the standard deviations reported in table 8.1 are representative of both the reproducibility of the detectors and the stability of the readers, whose setting remained unchanged during the entire experiment.

(b) Precision obtainable with several dosemeters of the same type. This is the normal situation encountered in practice; in fact the calibration curves are made with dosemeters which are not those used routinely, even if they belong to the same batch. In this case therefore, some more sensitivity variations among the dosemeters affect the precision obtainable. The variations of sensitivity are mainly due to the following reasons:

(a) variation in the amount of phosphor; (b) variation in the size of the compact dosemeters; (c) variation in the grain size; (d) variation in the optical density of some types of dosemeters due to the temperature

during the production process, as in the case of Teflon dosemeters.

These variations are normally reduced to a minimum by the producer himself either by putting the dosemeters into groups of equal sensitivity or by assigning to every dosemeter its sensitivity factor.

For the standard deviation of a group of dosemeters of the same type, the same formula holds as before:

Ui'M where, in this case, as is the percentage standard deviation of the group irradiated to the doseZ).

The percentage standard deviation is given by \2 / „ \2-|l/2

D (£)♦&)] which shows that the standard deviation depends on the dose to be measured and by increasing this dose it decreases to a constant value which depends on the variation in sensitivity.

-1^ oo

CTi

Table 8.1. Reproducibility test at the 100 mR exposure readings). o(%)

Participant

TLD material

1 2 repeated 3 exposures 4 5 100 mR 6 7 8 9

10

X(10 readings) <7(%)

e level for single detectors and 5 re-uses. X = relative standard deviation for X(10 detectors)

1

Li2B,07n c

X

0.99 1.00 1.01 1.01 1.00 1.01 1.03 0.98 1.00 0.97

1.00 1.7

<7(%)

4.0 5.2 1.2 2.1 3.9 0.9 1.1 6.9 4.8 3.7

2

LiFc

X

0.99 0.98 0.97 1.02 0.98 1.05 0.94 1.03 0.94 1.09

1.00 4.8

<r(%)

2.0 2.6 1.8 1.8 6.0 1.0 3.2 3.4 1.6 1.4

3a

LiFc

X

0.97 0.98 1.01 1.02 1.00 1.02 1.01 1.00 1.00 0.99

1.00 1.6

<7(%)

1.8 1.5 1.4 4.0 3.4 2.6 2.7 2.3 0.8 1.4

).

3b

CaF2c

X

0.98 1.01 1.01 1.01 1.00 0.97 0.99 1.01 1.01 1.01

1.00 1.5

o(%) X

1.8 1.0 0.9 1.4 1.8 0.8 0.9 1.0 0.9 1.2

4

LiF n c

o(%)

5

LiF n c

X

1.03 1.16 1.05 1.02 0.90 0.95 1.02 0.96 0.98 0.93

1.00 7.4

°(%)

16.5 10.9 11.8 10.4 9.3

10.1 23.2 10.5 10.7 15.2

= X(10 detectors)/.?(10 detectors and 5

6

LiFc

X

1.00 1.01 1.01 0.99 1.00 1.00 0.98 1.00 1.00 1.01

1.00 0.9

o(%)

6.0 5.3 3.8 3.6 3.8 3.3 1.7 4.2 3.5 5.4

7

LiFc

X

1.02 1.01 1.02 0.98 0.98 0.98 1.06 0.99 0.99 0.98

1.00 2.6

a(%)

1.4 1.4 3.5 3.3 1.7 1.9 2.2 1.0 3.1 2.3

8

BeO

X

1.04 0.86 1.07 1.04 1.01 0.92 0.93 1.02 1.06 1.05

1.00 7.0

o(%)

6.5 5.3 4.5 8.0 9.1 5.2 6.2 9.8 4.8

10.0

to K C Q j ^ .

c, commercial; nc, non-commercial.

Precision and accuracy of TLD measurements 149

Typical values for os are ± 2% and ± 3% for l iF powder (43 mg) and IiF-Teflon discs (0.4 mm thick) respectively. Corresponding background values in equivalent exposure of (8 ± 1) mR and (31 ± 3) mR are found.

From these data and the previous formula for the percentage total standard deviations, it is possible to calculate the data given in table 8.2 for the precision at different dose levels.

Table 8.2. Precision levels.

LiF powder LiF-Teflon discs

of TLD

10 mR

12% 30%

measurements

lOOmR

2.5% 4.0%

for different dose

1 R 2 R

2% 2% 3% 3%

If after use there is no change in sensitivity due to the thermal treatment or to the irradiation, a calibration factor or sensitivity factor can be assigned to each dosemeter and used to correct responses. In this case the precision obtainable can be of the order of 1% or even less.

8.6. Accuracy of TL measurements

As reported in the above definition, the accuracy of a particular dose measurement is defined by the difference between the measured value of the dose and the true dose with which the dosemeter was irradiated.

The most important variables that influence the accuracy are associated with the calibration of the dosimetric system and with the behaviour of the dosemeter when exposed to different kinds of radiation, i.e. energy dependence, dose rate dependence, etc.

To avoid, for example, inaccuracies due to the energy dependence, it would be desirable to perform the calibration of the dosemeters with the same radiation quality as the control. In many practical situations this is not achievable because the value of the energy is unknown a priori since the dosemeters have been exposed to a mixed x- and gamma ray field as in the case of routine dosimetry. In this case a systematic error is unavoidable even if very sophisticated techniques for energy correction have been applied. As an example, figure 8.1 gives a typical energy response curve for a practical dosemeter in which a filter is used so that a flat energy response within a large range can be obtained. If the energy varies from 80 keV to 60Co, the systematic error, introduced because the calibration is carried out at a well-defined energy value, will be of the order of about ± 5%. If, however, the radiation field contains mainly low-energy x-rays, as in the case for many practical situations, the error will be much higher.

Another example of systematic uncertainty comes from the inaccurate positioning of the dosemeter in the heating tray; this is particularly true for powder dosemeters if appropriate care is not taken.

150 G Busuoli

10

1 •

> < LU £ 0.1 o Q_

LU

cr

0.01 10

** «S *-

100 1000 ENERGY (keV)

Figure 8.1. Example of the systematic uncertainty due to energy response. • , CaF2: Mn (filtered); A, CaF2: Mn (unfiltered).

8.7. Accuracy in low-dose measurements

One of the recent applications of TL materials is for the measurement of low doses such as in the case of environmental monitoring around nuclear power plants.

For this application, the signal which is obtained is in many cases comparable with the background signal, even if using high-sensitivity materials. Therefore special attention must be given during calibration.

Reproducibility will be determined by giving one TLD repeated exposures equal to that resulting from an exposure rate of 10//Rh"1 during the field cycle. The responses will have a relative standard deviation of less than 3.0%.

Uniformity will be determined by giving TLD from the same batch an exposure equal to that resulting from an exposure rate of 10 /nR h"1 during the field cycle. The response obtained will have a relative standard deviation of less than 7.5%. If the level of uni­formity specified above is not obtained, a selection of TLD or an individual calibration will be necessary.

As for the accuracy, 95% of the final corrected values (after all appropriate corrections to the measured values are applied, including those for errors expected under field con­ditions) will differ from the true exposure value by less than 30% of the true exposure value.

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

Reference to other solid-state methods

E PITT AND A SCHARMANN

9.1. Introduction

The subject of this book is thermoluminescence dosimetry. Some other solid-state effects are also known which are useful in dosimetry. This chapter is designed to give a brief survey of them.

The basic absorption mechanisms of damaging radiation in a solid-state material are first summarised (figure 9.1).

INCOMING RADIATION

(a)

ABSORBING MATERIAL

(£>)

Figure 9.1. Scattering of ionising radiation, (a) Charged particles, e.g. c, p, a, etc; (b) neutral particles, e.g. n, 7, etc.

Two types of basic mechanisms must be distinguished:

(a) Directly ionising radiation consists of charged particles such as electrons, protons, positrons, a-particles, etc. The atoms of the absorbing material are directly ionised by Coulomb interaction.

(b) Indirectly ionising radiation consists of neutral particles such as neutrons, x-rays, 7 photons, etc, which primarily create charged particles by inelastic collisions or nuclear reactions. These charged particles ionise secondarily as described under (a). (The direct ionisation of neutral particles is negligible.)

The main effect on biological tissue is the ionisation and disturbance of molecules. Several effects are known which indicate ionisation products in solids. Only those effects

152 E Pitt and A Scharmann

which are of interest in dosimetry are considered. In every case it is required that ionisa-tion causes a reaction in the solid which is easily observed directly or indirectly and which can be used to measure the absorbed dose or dose rate. Linearity over several orders of magnitude is also required.

The most important effects for solid-state dosimetry are specified in table 9.1. Scintilla­tion dosemeters are used for dose ratemeters, whereas the other effects are used for the integration of the dose absorbed during a certain period.

Table 9.1. The most important effects used in solid-state dosimetry.

Effect

Radiophotoluminescence Coloration Photographic effect Exoelectron emission Track detection Neutron-induced defects Scintillation

Measured quantity

Luminescence Optical density Optical density Number of electrons Number of tracks Change of resistance Luminescence

Dosemeter material

Glass Crystal, glass Film Ceramics (BeO) Organic film Semiconductor (Si) Crystal

Radiation types measured

7 7, n 7, n 7, n, (3, a n n 7

The electronic processes which follow the ionisation are in general explained in the energy band diagram. A simplified model is illustrated in figure 9.2. During excitation electron-hole pairs are created. The holes move in the valence band and some of them can be trapped at recombination centres. The electrons diffuse in the conduction band. From there direct recombination with holes, sometimes by preference at special re­combination centres, is possible. A fraction leading to scintillations is counted by a photomultiplier. This mechanism is observed during and immediately after the excitation. It is therefore only useful for dose rate measurements. In the case of all other effects the second step after the excitation is the already mentioned trapping of electrons. The trap depth and the resulting emptying temperature are responsible for the stability of an integrating dosemeter. For routine dosemeters it is usually not less than 200 °C. The

CONDUCTION BAND

(o) 16) (c)

*-•-

'for/,. %'//X7/ %////%, '/,/. w /////, SURFACE -

VALENCE BAND

Figure 9.2. Mechanisms of solid-state dosemeters in the simplified energy band diagram, (a) Scintillation, (b) RPL, coloration, (c) TL, (d) TSEE.

Reference to other solid-state methods 153

electronic processes taking place in these integrating dosemeters are described in the following subsections.

9.2. Radiophotoluminescence (RPL)

Silver (Ag+)-doped phosphate glass shows the optical behaviour described by figure 9.3. Approached from short wavelengths, the lattice absorption decreases and vanishes at the band gap energy. At longer wavelengths the defect absorption band of Ag+ centres arises followed by the corresponding emission of light, the so-called photoluminescence, at still longer wavelengths. This electronic excitation and relaxation is observed in most crystalline solids at impurity centres.

arb units — absorption-

c o c/i m £ c o

o in

<

Ag* Ag* Ag° Ag°

Conduction Band

trap

Ag*

Ag° -J t

Valence Band

200 300 400 500 600 700 nm Wave Length

Figure 9.3. Optical behaviour of an RPL glass (left) and RPL mechanism (right).

After exposure to ionising radiation two additional bands are found, both situated on the long-wavelength side of the original Ag+ bands. A new radiation-induced photo-luminescence centre has been created by the recombination of Ag+ ions and electrons released to the conduction band. The concentration of these neutral Ag° centres (figure 9.3) is proportional over a wide range to the ionising radiation dose. Their luminescence, the radiophotoluminescence, is measured after selective excitation of the RPL absorption band only.

Figure 9.4 shows the block diagram of a commercial RPL reader. The light of a uv source is focused on the RPL glass. An optical filter selects the excitation wavelength of the RPL band. The emission is observed through a second filter by a photomultiplier. Trap centres, capturing some of the conduction electrons for a certain time after the absorption of ionising radiation, enhance the build-up of the RPL centres. This process can be accelerated by heat treatment for some minutes.

RPL exhibits a specific advantage over TLD. Dose read-out does not cause annealing of the radiation-induced effect. Therefore the loss of information by mistakes and errors

154 E Pitt and A Scharmann

UV source lens- optical C filters

RPL glass

photomultiplier

Figure 9.4. Principles of an RPL reader.

at any point of the reading procedure is avoided, whereas a repetition of the read-out is impossible for TLD. Another advantage of this effect is the possibility of registration of small doses over extended periods of time (~year) with a dosemeter for short duration monitoring (~day or month), which may be read several times without annealing.

The lower detection limit of about 100 mrad for commercial RPL systems is too high for personnel dosimetry. This limit is not due to the read-out system, but rather to the so-called pre-dose. This is the dose reading which is already monitored without excitation by ionising radiation. It stems from the luminescence of internal or surface defects and impurities of the glass rod. The luminescence mechanism has therefore been studied in detail. It was found that the luminescence decay may be described as a sum of three exponentials. One of them depends on the ionising radiation. Its decay time is about 3 /xs, whereas the two other components decay with a longer and a shorter time, respectively.

This behaviour leads to a modified RPL reader. The continuous light source is replaced by a nitrogen laser, which can be built easily and cheaply. Its emission wavelength of 337 nm corresponds to the excitation maximum of the RPL band. The pulse width of the laser flash is 8 ns. The intensity of this very simple laser varies over a range of 30%. Thus, normalising each measured luminescence value to the laser intensity is necessary.

Figure 9.5 shows the block diagram of the evaluation device. It exists only as a labora­tory model. The glass rod is'irradiated by the laser pulse. The luminescence is registered by a photomultiplier (RPL). A part of the laser intensity is deflected by a quartz slice to a second photomultiplier (laser), which measures the laser intensity for normalising. The two light intensities are integrated and the data analysed by a microcomputer. At the right-hand side of the figure the corresponding curves of the laser pulse, the luminescence pulse and the voltage pulse at the first dynode of the multiplier are plotted. 2 jus after the ultraviolet flash a large fraction of the pre-dose luminescence has decayed. Then during a period of 8 ids the ratio of radiation-induced to pre-dose luminescence shows a maximum, even for low x-ray doses. During this time the voltage pulse at the first dynode sensitises the photomultiplier. The registered luminescence is integrated and stored. Better accuracy is achieved by repetition of the procedure and calculation of the average value by a microcomputer based on a microprocessor unit, which also controls the measurements. In this way the lower detection limit of an RPL dosemeter is about 10 mrad. At higher doses a deviation of the linearity between dose and dose reading is observed. In this range the visible colouring of the glass is used as a measure for the dose and the absorption of the glass is registered by a third photomultiplier.

Reference to other solid-state methods

Quartz Slice RPL Glass

155

N ; Laser |~ ~ \ ^ v- PM Transmiss

Opt Filters

Integration Storage

Trigger

EH PM RPL

Integration Storage

Laser Pulse

Luminescence Pulse

Integration Storage

Microprocessor B

Voltage Pulse at the first Dynode

1 i

-^ r~ T

2

Printer

Figure 9.5. Block diagram of an RPL reader.

9.3. Colouring

As mentioned in the preceding subsection, a heavily irradiated glass exhibits an absorp­

tion in the visible range. This effect is well known from the investigations of colour centres in alkali halides (figure 9.6). Electrons lifted into the conduction band during the excitation process are trapped at anion defects in so­called F centres. The corresponding cation defects with trapped holes are denoted as Vj centres. Both are excited by visible light. This energy absorption gives rise to the colouring of the glass. Similar effects are known for nearly all transparent materials. They can be used for dosimetry.

00000 0 O,Q0 0 © 0v ■ )0 O © 0 © 0 ©

O©00© 0 ©> 0 0 ©0000 ©000©

F centre Figure 9.6. Colour centres.

V, centre

An absorption reader consists mainly of a light source and a detector for the trans­

mitted light. The transmission of the unirradiated dosemeter is only slightly decreased by ionising radiation doses, which are of interest in personnel dosimetry, since the concentra­

tion of the created colour centres is small. Thus the relative change of the light intensity is too small for sufficient sensitivity. Doses of 102rad up to 107rad and more are detectable by absorption dosemeters.

These basic considerations show that a dose reader for practical use should measure an effect which is absent in the unirradiated dosemeter and which is generated at high

156 E Pitt and A Scharmann

sensitivity by the ionising radiation. Otherwise a developing procedure is necessary which amplifies the effect as in film dosemeters.

9.4. Photographic process

Most dosemeter films consist of silver bromide crystals in a gelatin layer. These films were the most commonly used personnel dosemeters for a long time since their handling is easy and the processes are sufficiently well known from optical photography. The elementary processes are illustrated in figure 9.7.

conduction band

5r excitation

^

Ag+

Ag Ag * " ^ Ag+

valence band ; (a) (6) Figure 9.7. Elementary processes in an AgBr film.

By high-energy excitation an electron-hole pair is created and both particles are captured in deep traps (a). Then a rather mobile Ag+ ion migrates to a trapped electron and recombines to produce a neutral Ag atom (b). This represents a deep trap for another Ag+ ion. The Ag£ so formed is again a trap for an electron. By the alternate capture of electrons and Ag+ ions metallic silver grains grow in the gelatin layer and can be registered.

The film dosemeter no longer meets the standard required although it is still often used. The main reason is the lack of stability during storage in a warm and humid atmosphere. As a consequence one expects dosimetric and material changes, such as:

(a) changing sensitivity, (b) the gelatin becomes sticky and the film is destroyed, (c) the unirradiated film exhibits a fog, (d) destruction of the gelatin by microbiological reactions, (e) high fading up to 70% at 80% humidity.

But even if the film is used under optimal climatic conditions there are some dis­advantages compared with other personnel dosemeters:

(1) The relative energy response for 7-rays from 30 keV to 1 MeV covers a range of 1-30 as a result of the high AgBr concentration. Some efforts were made to compensate for this effect by using several filters and increased AgBr grain sizes together with special developing techniques.

(2) An error during the developing procedure or the storage destroys the stored information.

Reference to other solid-state methods 157

(3) The film is also sensitive to normal daylight.

Nowadays film dosemeters are commonly used together with several filters of different atomic number to determine the composition of the photon energy spectrum.

However, the film dosemeter is of some importance in neutron dosimetry. Filters are used to absorb 7-radiation which cannot be neglected in usual mixed fields. The neutron dose can be calculated by comparison of the filtered and the unfiltered parts of a film. In practice the error in this method is very high since the 7 sensitivity far exeeds the neutron sensitivity.

A comparison of an actual exposure with the read-out of several developing services has shown that, in the case of 7 dosimetry, the error of about half of all readings exceeded 50%.

Although the mentioned sources for errors in film dosimetry are well known, it is still in use because of its easy handling. The film, together with all necessary filters, can be placed in a thin badge. The identification mark is easily fixed to the film by applying pressure. Developing large numbers of films is feasible. The unexposed ones are separated by visual inspection.

9.5. Stimulated exoelectron emission

As is known from the TL theory, electrons are raised into the conduction band of an insulator by excitation with ionising radiation. They are then able to recombine directly with holes or to be captured in traps. A third mechanism is observed at the surface of some materials: low-energy electrons originating from the conduction band are emitted after excitation or during heat or light treatment (figure 9.2). They are called exo-electrons and occur at temperatures below thermionic emission. The exoelectron emission is a surface process which is competitive with the recombination process and which gives rise to light emission of spontaneous luminescence or thermoluminescence. Thus, for a schematic description of the mechanism a modified kinetic model may be used as is known from TL.

Exoelectron emission is observed not only at surfaces excited by ionising radiation, but can also be due to surface treatments such as

(i) mechanical deformation, (ii) desorption and adsorption reactions, (hi) chemical reactions.

These additional effects are a severe problem in exoelectron dosimetry: each of them is a source of erratic dose readings. Consequently the dosemeter must be kept clean and dry. The adsorption of water molecules very strongly influences the emission of most exoelectron dosemeters.

In general all laboratories use their own instrumentation for exoelectron dosimetry. Figure 9.8 shows a reader for thermally stimulated exoelectron emission based on a pro­portional gas flow counter. According to the diagram of figure 9.9 such a reader consists of a heater with the sample, in front of which is situated a cylindrical methane-filled gas tube. Emitted exoelectrons are accelerated in the electric field between the sample and the steel wire. They ionise gas molecules. The resulting electron current is collected at the wire. A corresponding voltage pulse at the exit of the gas tube is amplified and

158 E Pitt and A Scharmann

Figure 9.8. High-temperature TSEE gas flow proportional counter built at the Radiation Protection Division of the Joint Research Center of the European Communities, Ispra Establishment, showing water-cooled sample slide with oven withdrawn.

gas outlet gas outlet

high voltage amplifier supply

s sample

bridge spring rested

heater water cooling thermocouple

Figure 9.9. Diagram of a cylindrical TSEE gas flow proportional counter.

registered. Metal oxides, especially BeO, are used as a personnel dosemeter. BeO has a low effective atomic number and therefore exhibits an energy dependence nearly equal to that of human tissue. Its main peak is about 200 °C, so that it is stable at room temperature. A typical exoelectron curve of BeO is shown in Figure 9.10. Recently BeO thin-film detectors evaporated on graphite substrates have been developed. They are superior to the formerly used ceramic BeO with respect to an improved dose stability against mechanical and humidity influences.

The standard deviation of a certain exoelectron dosemeter can reach 20%, caused by the sensitivity to other surface treatments. Nevertheless exoelectron dosemeters are superior to other systems in some special cases. This is especially true in the low dose

Reference to other solid-state methods

t7) A

159

r r i -i—i m < >-i -i—i

LU

LU LU CO

400 200 250 300 350 TEMPERATURE (°C)

Figure 9.10. TSEE curve of BeO. 1500 A BeO on graphite heated in wet nitrogen at 1000°C.

range, where it is possible to detect microrad doses with a reasonable expenditure of effort. Although the surface character of exoelectron emission makes this method sensitive to disturbing mechanisms which cannot occur in the case of bulk effects such as TLD or RPL it offers some favourable properties too. Since the dose information originates only from a thin surface layer exoelectron emission enables a sensitive detection of nuclear particles which deposit their energy close to the surface, such as a-rays or low-energy /3-particles. For instance, radon concentrations of a few tenths of a picocourie per litre have been measured after an exposure time of only 1 d. After 1 h exposure insoluble tritium gas at a concentration of 2 x lO^jLtCimr1 can be detected. Because of the increasing number of nuclear power stations tritium dosimetry may become one of the main applications of the exoelectron method. In principle, the detection of fast neutrons is also possible if the dosemeter is covered by a polyethylene radiator which generates recoil protons.

Since exoelectron emission is an electronic process like TL emission the repeated use of the same dosemeter after annealing is possible.

9.6. Track detection

In the preceding chapters some dosemeter materials and methods have been described which are sensitive to 7-rays and sometimes additionally to neutrons. One of the main problems of personnel dosimetry is the detection of neutron doses in mixed 7-n fields because it is difficult to find a sensitive neutron detector which is also not sensitive to 7-radiation. Often a combination of two detectors is used, one monitoring the mixed-field dose and the other one monitoring the 7 dose. Nevertheless neutron dosimetry is an unsolved problem and in some nuclear research centres a neutron dose has never been registered with these personnel dosemeters.

Nuclear track detectors are selective fast-neutron detectors. They consist of organic films of high hydrogen content such as cellulose nitrate. The elastic energy transfer of

160 E Pitt and A Scharmann

fast neutrons penetrating into a solid depends on the atomic weight of the stopping nuclei. It exhibits a maximum for hydrogen atoms. These nuclei are released and also stopped in the solid. During this stopping process of neutrons and protons a latent track of destroyed material is generated. Some etchants are known whose etching rate is higher in the latent track than at the surface of the film. By this procedure the tracks become visible. KOH is a typical etchant proposed by the producer of cellulose nitrate detectors.

The dose reader counts the tracks generated. Computer-operated image analysing systems are used, which count the tracks automatically. This method is expensive, very complicated and the operating speed is rather slow. Under optimum conditions an area of 1 mm2, necessary for a lower detection limit of 0.5 rad, is investigated in 15 min. The sensitivity is limited by non-radiation-induced tracks which result from production defects, handling, etc. Evidently this system is not suitable for evaluations of large numbers of detectors.

For routine dosimetry a fast reader was developed based on spark counting (figure 9.11). The latent tracks of thin foils are etched through. The foil is placed as a dielectric between the electrodes of a capacitor. A ramp voltage is applied and the sparks occurring are counted. To avoid multiple counting of the same hole an aluminised polycarbonate foil is used as one of the electrodes. The aluminium in the environment of the hole is burned away. This reader is cheaper than the optical image analyser and the reading time is shorter. Using automatic devices reading times of about 1 min for one foil are feasible. When counting an area of 2.5 cm2 the sensitivity is 5 mrad for 14 MeV neutrons. For thermal neutrons n-a converters have to be used for the generation of the latent tracks.

HV +— H Z Z h R

COUNT

DISCHARGE = CAPACITOR C

•-PRESPARK c ^

mmmmrn 2 • COUNTER

i Figure 9.11. Block diagram of a spark counter. 1, high-voltage electrode; 2, detector foil, 3, aluminised polycarbonate film.

The track detection technique is applied not only in neutron dosimetry, but also for heavier particles, up to the heaviest atoms. In this case glass or crystal detectors of higher atomic numbers are used, corresponding to the higher weight of the incoming particles.

In recent years a new material called CR39 has been developed which shows an excellent contrast of the etched track. With this detector it may be possible to reach even lower dose ranges, especially at lower neutron energies. The energy response of CR 39 fits the rem curve fairly well.

Reference to other solid-state methods

9.7. Change of resistance in silicon diodes

161

One more neutron dosemeter of practical use is the silicon diode. Fast neutrons produce reversible defects in semiconductors; in the case of silicon, especially vacancies and Frenkel defects. These defects strongly decrease the conductivity by trapping carriers. The main advantage of this fast-neutron dosemeter is the small size, of the order of some cubic millimetres. But, as mentioned in the subsection concerning absorption dosemeters, low sensitivity is expected due to the reading procedure: a forward voltage is applied to the diode. The corresponding current decreases after irradiation. The reader shows the voltage change of the diode at a certain diode current. The relative change is small at small doses. Figure 9.12 shows typical characteristics of commercial diodes.

Figure 9.12. Forward current as a func­tion of forward voltage for unirradiated and neutron-irradiated silicon diodes [5].

A higher diode current seems to sensitise the system. The simultaneous warming-up of the diode during the reading procedure increases the annealing probability of the defects and thus the reader error increases.

Another disadvantage of the silicon diode detector is the low stability. Thermal annealing at room temperature leads to a fading of up to 10% during 10 d. Apart from this disadvantage, the dosimetric data of silicon diodes from a single production run vary over a wide range. Therefore the accuracy at 10 rad is about 10%. After individual adjustment a lower detection limit of 1 rad is achieved at reasonable accuracy with high-sensitivity diodes. The useful range is about three orders of magnitude. For less sensitive diodes the upper detection limit is 10000 rad. These properties show that the diode is interesting only for reactor, accidental or military dosimetry, but not for personnel monitoring.

Thermally stimulated 7: 10" s - l 0" rad exoelectron emission (TSEE) n: 10~4-104rad

10keV-30MeV > l M e V

High sensitivity, surface resolution Strong influence of chemical or mechanical treatment

Track detection n > 0.1 rad 1 keV-20 MeV Tissue equivalent, low sensitivity to 7, x, j3 background

Destruction by over-etching, sensitive to mechanical treatment

Photographic effect 50 mrad-103rad 40 keV-3 MeV Repetition of the reading is possible, no fading after developing

Destruction by wrong developing, strong fading, strong energy dependence

Coloration 7: 10M0 7 rad 15 keV-3 MeV Easy reading, important in high-dose dosimetry

Low sensitivity

Neutron-induced resistance n: 1-10" rad change

>0.4 MeV Simple reading, small volume Individual calibration necessary, strong fading, non-linear reading

to

h ]

Table 9.2. Properties of solid-state dosemeters.

Dose range

TLD

Energy range Advantages Disadvantages

7: 1 mrad-104 rad 10 keV-30 MeV

Radiophotoluminescence 7: 10 mrad- lCrad 15 keV-30 MeV

1% fading/month, low costs, storage unlimited, low energy dependence

Low fading, low costs, low energy dependence, storage unlimited, repeated monitoring possible

Annihilation of stored informa­tion by evaluation

Thick dosemeter badge

a a.

sh ft

s

Reference to other solid-state methods 163

9.8. Scintillation dosemeter

One of the main requirements mentioned for a dosemeter is the storage of the absorbed dose information. Here a differential dosemeter is briefly mentioned, which does not meet this requirement and which therefore can only monitor the dose rate.

The scintillation mechanism is as follows (figure 9.2): electron-hole pairs are created by exciting radiation. Both electrons and holes migrate separately or together to activator centres consisting, in general, of dopants. At these centres the luminescent recombination takes place. A photomultiplier registers the emitted light. This detection system is well known from nuclear physics. It is used only in laboratories and not in routine dosimetry because of its large size. Therefore only this brief description is given.

Several scintillators of practical interest are known, mainly inorganic crystals like alkali halides, organic crystals and plastics. By selection of the atomic number of the absorbing material it is possible to measure approximately air-equivalent or tissue-equivalent doses. Photon as well as particle radiation is detectable. Especially in clinical dosimetry the small size of the absorber itself is interesting. It can be placed far away from the light detector if connected by a light pipe.

9.9. Conclusions

Besides the well-known TLD some other dosemeter methods are interesting for practical use. Solid-state materials are principally qualified because information is easily stored for a longer time as a consequence of solid-state properties. This holds for radiophoto-luminescence, thermally stimulated exoelectron emission, film, track detector and coloration as well as for TLD. TLD is the best-developed system since much money was spent in its investigation. Nevertheless, each of the other systems has its advantage in special applications. The main features are compared in table 9.2. The limits given here are valid only for commercial systems. Some larger nuclear research centres have built their own systems with better results.

References and further reading 1 Attix F H, Roesch W C and Tochilin E Radiation Dosimetry (New York: Academic) 2 Frank M and Stolz W Festkorperdosimetrie (Weinheim: Verlag Chemie) 3 Becker K Filmdosimetrie (Berlin: Springer-Verlag) 4 Becker K and Scharmann A Einfiihrung in die Festkorperdosimetrie (Munchen: Verlag Karl

Thiemig) 5 Becker K Solid-State Dosimetry (Cleveland: CRC Press) 6 1976 Proc. 5th Int. Symp. on Exoelectron Emission and Dosimetry, Zvikov ed A Scharmann

(I. Physikalisches Institut, Universitat Giessen) 7 1977 Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo ed A Scharmann (I.

Physikalisches Institut, Universitat Giessen) 8 1979 Proc. 6th Int. Symp. on Exoelectron Emission and Applications (University of Rostock)

Part II: Applications

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

10 Application ofTLD to personnel dosimetry

E PIESCH

10.1. Introduction

Personnel monitoring is based on the international recommendations of the ICRPf. The primary objective of individual monitoring for external radiation is to assess, and thus limit, radiation doses to individual workers. Supplementary objectives are to provide information about the trends of these doses and about the conditions in places of work and to give information in the event of accidental exposure [1]. Depending on the kind of radiation hazard, the ICRP recommend maximum permissible dose (MPD) values. These are the maximum dose equivalent values which are not expected to cause appreciable body injury to a person during his lifetime. With respect to the various MPD values, the following quantities should be measured in personnel monitoring:

(a) skin dose or the surface absorbed dose to assess the dose equivalent to the basal layer of the epidermis at a depth of 5-10 mg cm-2, if only non-penetrating radiation has to be considered (x-rays < 15 keV, /3-rays);

(b) whole body dose or the dose equivalent at a depth of 400-1000 mg cm-2 below the surface of the body to assess or over-estimate the effective dose equivalent or the average dose equivalent in the critical organs for the case of penetrating radiation and whole body irradiation (x-rays > 15 keV, 7-rays, neutrons);

(c) extremity dose to assess the maximum value of the dose equivalent (skin dose) in tissue to any part of the hands, forearms, feet or ankles.

In contrast to film dosimetry, approximately tissue-equivalent TL detectors of small size and high precision in measurement are available which may serve as an ideal extremity dosemeter and as a basic dosemeter for the dose estimation of photons in the energy range of interest as well as for the detection of /3-rays and neutrons. In addition, a multi-detector badge offers practical possibilities for short-term and long-term monitoring periods, for separate indication of skin dose and body dose and finally for the estimation of radiation quality.

The role of TLD for an individual monitoring service is based on the following features:

(a) TL phosphors are available in solid form as chips, extruded ribbons or in a Teflon matrix; the dose reading of some materials is approximately tissue-equivalent and largely independent of the angle of radiation incidence;

(b) the dose reading is practically independent of dose rate up to 1011 rad s"1 and pro­portional to the dose up to several hundred rem;

f International Commission on Radiological Prolection.

168 EPiesch

(c) in some TL materials the fading at room temperature is so small, especially after a post-irradiation annealing, that they can be used for issue periods of up to 1 yr;

(d) TL detectors are convenient to wear, suitable for postal service, flexible in issue period, can be evaluated in less than 1 min and can be re-used, although a regenera­tion procedure prior to re-use is often necessary;

(e) TLD lends itself to automation; with an on-line computer the calibration factor for individual dosemeters can be stored and also the shape of the glow curve may be checked to verify the radiation-induced TL;

if) for the reassessment of the dose recorded, a redundant detector can be included in the badge design.

TLD, the most advanced and most intensively studied integrating dosemeter system, has now reached the stage at which it may replace or supplement film dosimetry. Primarily for applications in personnel monitoring, various suppliers offer a number of commercial TLD systems with manual or automatic evaluation systems (see chapter 3).

10.2. Performance specifications

According to the 'Technical Recommendations for Monitoring the Exposure of Individuals to External Radiation' [2], three kinds of dosemeters are discussed and recommended by the Commission of the European Communities:

(a) the non-discriminating basic dosemeter is mainly recommended for the group of low-risk persons to measure the dose without obtaining information on the radia­tion field;

{b) the discriminating basic dosemeter recommended for the group of high-risk persons should offer additional information on the radiation field;

(c) the extremity dosemeter is worn in addition to the basic dosemeter if the results from the basic dosemeter are not representative of doses received by the body extremities.

For the application of TLD systems in individual monitoring, Euratom has published technical recommendations [3] which should serve more or less as an indication of what is generally regarded as good practice and for assistance in avoiding the common sources of error. For the performance of the basic TL dosemeter only general figures are discussed (table 10.1).

Performance specifications and test procedures for the application of TLD systems in personnel monitoring are discussed in additional papers which are all still in the draft stage:

(a) the PTBf draft 'Requirements of the PTB for the type approval of TLD systems for radiation protection measurements' 1976;

(b) the ANSIf draft 'American National Standards Criteria for Testing Dosimetry Performance'N13.ll, 1978;

(c) the HPSSC§ WG/15 draft 'Proposed Standard Criteria for Testing Personnel Dosi­metry Performance' 1977;

f Physikalisch Technische Bundesanstalt. J American National Standards Institute Inc. § Health Physics Society Standards Committee.

Application of TLD to personnel dosimetry 169

Table 10.1. Recommended performance of basic thermoluminescent dosemeters after EUR 5358.

Basic property Non­discriminating Discriminating

Dose range (photons) ] „ m _ l Q 3 t Q lQ4nd

Dose range (electrons) J Energy range (photons) 0.01­50 MeV Energy range (electrons) 0.5­50 MeV Information on photon radiation Not required Necessary in the range

quality 10­200 keV Overall uncertainty For doses <50 rad: — 30%,+50% or ±50 mrad

(whichever is the greatest) >50rad: ­20%,+25%

Precision 2 a < 10% at 1 rad Photon energy dependence for body ­ 2 0 % + 40% ±15%

dose at 1 cm depth (over energy range given above)

Dependence on environmental conditions Insignificant Photon angular response ±30% [32] Fading <5% over the monitoring period at 25 °C

(see §6.2)

(d) the ISOf draft 'Personal and Environmental Thermoluminescence Dosimeters' 1979;

(e) the 'Standard Test Programme' recommended as a performance test to establish a solid­state dosemeter system in the lower dose range [9].

The PTB draft presents more detailed performance criteria. The requirements for the type approval fix the components of a TLD system, which consists mainly of one or several TL elements in a badge, of the TL read­out instrument (heating and indicating system) and of additional devices for calibration and annealing. For each quantity of interest a 'nominal minimum range' is given by definition. With respect to the provided application of TLD systems, the following minimum ranges and reference energies for photons are given:

10­60 keV, reference energy 30 keV 30­200 keV, reference energy 150 keV

100­1300 keV, reference energy 662 keV. The maximum permissible errors for the type approval of TLD systems are presented in table 10.2. Battelle­Northwest conducted a study to compare and evaluate the four performance standards existing in the USA and recommended the HPSSC standard for use in establishing performance criteria for personnel dosimetry [4].

The Naval Research Laboratory participated in a personnel performance testing pilot study on the basis of the new ANSI and HPS draft. Comments on the testing methods and procedures are given and recommendations are made for their improvement [27].

The standard test programme was organised from the Fachverband fur Strahlenschutz [9] as well as in the European Community to study TLD systems in the lower dose range [28]. The experimental procedure of the inter­laboratory test programme (see chapter 11, table 11.3) provides that measurement runs be carried out by each laboratory with a ■f International Organization for Standardization.

170 EPiesch

Table 10.2. Maximum permissible variations of influence quantities and dose reading according to the PTB requirements (second draft).

Quantity Reference value /max (%)

Photon energy range

Direction of radiation incidence Temperature (detector) Humidity (detector) Temperature (TL reader) Dose rate Light sensitivity Fading after exposure Operation voltage

Table 10.3. Some properties of thermoluminescent phosphors after EUR 5358.

Thermoluminescent Zeff Main Environmental Number of Dose range material glow maximum (nm) maxima (rem)

30keV 150keV 662 keV Of priority 20 °C 65% RA 20 °C Indication by manufacturer Indication by manufacturer 15 minat (20 t 2) °C Indication by manufacturer

+ 30 ±30 + 30 ±20 ±10 + 10 ±2 ±10 + 5 ±5 ±2

LiF: Mg, Ti Li2B„07:Mn, Li2B407:Mn Li :B,0 7 :Cu, CaF2:Dy CaF,:Mn CaF2: natural CaSO„:Mn CaSO<:Dy CaS04:Tm BeO A1203

Si

Ag

8.3 7.3 7.3 7.3

16.3 16.3 16.3 14.4 14.4 14.4

7.1 10.2

230

220 185 215 260 260 100

180 300

400

665 368 500 500 380 500

350 699

6

2 2 3 1 6 1

1 2

10 2-104

lO 'MO 6

10"2-106

10 s -10 6

10" MO" 10 MO 6

io-5-io* 10"4-10s

io-"-io5

10-M0 5

l O ' - l O "

batch of 10 dosemeters taking 10 readings for each dosemeter or repeating the measure­ment cycle 10 times. The dose range has been related to a multiple of the lowest detectable dose Z)ldl defined here as three times the standard deviation (3a value) of the zero-dose reading au of unirradiated dosemeters after subtraction of the dark current reading ao- According to the recommended test procedure every laboratory tests their own dosemeter systems with respect to zero-dose reading, the lower detection limit, the standard deviation as a function of exposure, long-term stability of the system and temperature effects during storage at 70 °C.

10.3. Detector materials and specific requirements!

10.3.1. TL materials and detectors

From the TL materials listed in table 10.3 mainly LiF, U2B4O7, CaS04, BeO and A1203 doped with appropriate activators are in use which are commercially available as single f For general requirements, see Chapter 5.

Application of TLD to personnel dosimetry 171

crystals, as hot-pressed or extruded elements, as powder embedded into a Teflon matrix or in glass capillaries or fixed in thin layers of about 20 mg cm"2 on a metal or plastic foil. Generally, most of the TL phosphors are suitable for personnel monitoring but preferably tissue-equivalent detectors are used. For LiF or U2B4O7 encapsulated in glass, one has to consider the loss in response in the low photon energy range. For automatic read-out, the detector chips are encapsulated between two Teflon foils in dosemeter cards.

Besides the most commonly used LiF: Mg,Ti from Harshaw, the new commercially available French LiF: Na,Ti (PTL 717) reduces the low-temperature glow peaks, resulting in a better fading characteristic [6]. Nink [7] published data for an experimental pre-doped LiF (instead of a Ti02 activator, Ti-doped LiF was directly used for adding) and a Ti-doped LiF. Both materials enhance the main peak, evidently showing excellent fading and reproducibility results [8]. National produces II2B407: Ag,Cu detectors of20mgcm"2

thickness for automatic read-out [25]. Further TL materials and thin detectors developed for P dosimetry are described in §5.1.

Sintered MgB407: Dy(Ti) was recently four. J to be a new highly promising TL material which, in the future, may replace LiF: Mg,Ti in personnel monitoring [46]. Compared to LiF : Mg,Ti this material shows only a single glow peak, higher sensitivity and precision (<1%) and a similar energy dependence and fading characteristic; compared to IJ2B4O7 no effect of light sensitivity and hygroscopy has been found.

For the choice of TL materials different aspects and properties with respect to routine monitoring should be considered:

(a) the dosemeter properties of the detector including dose range, energy response, reproducibility, uniformity within a batch, energy response and fading;

(b) the requirements for a monitoring service including performance specifications and type approval, the necessary precision and accuracy of the TLD system, the com­plexity of the annealing procedure, the type of service, i.e. national or decentralised, and the degree of automation.

The accuracy of dose measurement is significantly limited by the uniformity in response within the group of detectors of the same type. The group of detectors should have the same sensitivity according to the PTB requirements within a highest permissible variation of 10%, according to Euratom with a 2a value of < 10%. Experimental results of the variation in sensitivity within a detector group are presented in figure 10.1 for different TL systems [9]. On the basis of calibration exposures individual calibration factors can be applied to each dosemeter, for example, by an on-line computer in a fully automated system.

10.3.2. Energy dependence

Since most of the TL detectors are not tissue-equivalent, the response of TL materials is a function of photon energy. The energy dependence of the response depends on the detector's thickness and encapsulation. The lowest dependence on energy was found for Li2B407 detectors (figure 10.2). The most commonly applied LiF:Mg,Ti is approxi­mately tissue-equivalent, resulting in an over-estimation of + 40% for low-energy photons in the 30 keV range and in an under-estimation of — 40% for high-energy photons in the

172 E Piesch

"

•

■

3 : 1 6

7

' 5 ; i \ ! ' ■

28

36

35

33

2U

17

15

12

8

7

« 3

1 0 VALUE OF 10 DETECTORS

1—1 TLD ra RPL

jy 31

24

22~1

13

23

21 19

11

27

20

18

K

5

2

30

26

25

11

16

10

1 -hi . Figure 10.1. a batch.

Variation in sensitivity within

BATCH UNIFORMITY 0 VALUE IN •!.

PHOTON ENERGV IN keV

Figure 10.2. Energy dependence of LiF:Mg,Ti and Li2B407: Mn.Si dosemeter systems compared to the FD­1 phosphate glass in the spherical capsule.

10MeV range. BeO detectors show an unexpectedly high over­estimation of about + 60% at lOOkeV [19].

High­Z materials such as CaF2 and CaS04with an over­estimation of about a factor of 12 at 50 keV are therefore used in personnel monitoring mainly as a supplement to nearly tissue­equivalent detectors in order to estimate radiation quality.

The relatively low directional dependence of the dose reading of tissue­equivalent detectors becomes worse, especially for low­energy photons, if the detector is worn inside a plastic encapsulation.

Additional changes in the energy dependence arise due to back­scattering and absorption of the radiation in the body of the wearer as shown in figure 10.3 for LiF. A further description of the energy dependence is given in chapter 5.

10.3.3. Fading

According to Euratom's recommendations, the uncertainties introduced into the results due to the application of corrections for instability of latent information (e.g. fading) should not exceed ± 5% during the regular monitoring period. The fading characteristics

Application of TLD to personnel dosimetry 173

° >-2.0

> 1.0 -

t/>

LJ 0.5 -CC 1/1

10 _L

LiF DOSIMETER HARSHAW RIBBONS TLD 100

0.125'x 0.125"x 0.035 i J_ ' ■ i

20 1000 50 100 200 500 QUANTUM ENERGY ( keV )

Figure 10.3. Energy dependence of the dosemeter reading of an LiF dosemeter irradiated in free air (a) and on the front side of an Alderson human dummy (b), without secondary electron equilibrium for 1.2 MeV.

of some TL materials are presented in table 10.4 and figure 10.4 as a function of storage period. Most of the TL materials show a fading lower than 5% for monitoring periods up to 3 months. No corrections are therefore necessary to fulfil the recommendation. A preheating before evaluation may significantly reduce the fading for some TL materials [6, 26] (see also chapter 11).

Table 10.4. Fading characteristics of some common thermoluminescent materials after Eur 5358.

Thermoluminescent Thermal fadingf (25 °C) Optical fadingt material

LiF:Mg Li2B<07:Mn CaF2:Mn CaF2:Dy CaS04:Mn CaSO„:Dy CaSO„:Tm BeO A1203

Very weak ~ 5% in 1 yr Weak Weak ~ 10% in 2 months Weak Average ~ 1% in 1 d ­

Weak ~ 13% in 1 month Strong Strong­50­85% in 3 d Weak ~ 1­2% in 1 month Weak Weak ~ 1­2% in 1 month Weak Weak ~ 8 % in 3 months Strong Very weak Strong

f These data are taken from the literature and should be regarded as an indication of the magnitude of the effect. Fading can be minimised by applying appropriate low­temperature annealing procedures. t Due to sunlight: at the present time information of a more specific nature is not available.

10.3.4. Reproducibility and re-use

The reproducibility of a TLD system depends on the detector type, the zero­dose reading, the batch uniformity in response and zero­dose reading, the read­out technique and the individual reader quality. Reproducibility curves of various TLD systems presented in figure 10.5 result in a characteristic shape which decreases for low doses as a function of exposure and reaches a constant value for higher doses. The standard deviation may vary significantly for TLD systems in use [9, 28]. No significant difference has been found

174 E Piesch

STORAGE TEMPERATURE 25 °C

—— LiF Mg.Ti «—» CaF2:Dy «—» CaS0< Dy TEFLON — • L i ^ O , Mn TEFLON

F A D I N G OF T L D S Y S T E M S PREHEATING TOO X j 20mm

Figure

10 100 1000

STORAGE PERIOD IN DAYS

10.4. Fading results of different TL materials for storage at 25 °C.

100

o 5

Si 0.1

CnSIUm LiF Moji Li,BA:Mn LiF:Mg.Ti

0.1

LijB.O^Mn :CaS0l:Tm/(JoF2:Dy ! LiF:Mg;Ti

1 10J 10' 10 10' EXPOSURE IN mR

Figure 10.5. Reproducibility of dose measurement plotted against exposure for different TLD systems [28]. Is value of 10 dosemeters.

between manual and automatic read-out systems. The relative standard deviation versus exposure curve of TLD systems can be described by a two-parameter fit:

s = (l/D)(2s2uDl+s?y/2

taking into account the actual exposure D, the zero-dose reading Du of unirradiated dose-meters, the s values of the zero-dose reading (su) and of the reference dose reading (sT). For more details see § 11.4.2 and [29].

For re-use of TL detectors the constancy and reproducibility of the dosimetric properties is of general importance. As the response and the zero-dose reading often change with the thermal and irradiation history of the detector the user can adopt one of the following techniques:

(a) Prior to re-use, a special thermal treatment (pre-irradiation treatment) for each detector is recommended from the manufacturer or can be found experimentally by optimising the temperature-time characteristic of annealing to obtain repro­ducible results. For LiF: Mg.Ti, for instance, an annealing at 400°C/lh and 100°C/3 his applied.

Application of TLD to personnel dosimetry 175

{b) External annealing procedure in an oven may be replaced by an extended thermal treatment during or immediately after the read-out cycle. Here the detector is measured once at, or slightly above, the maximum read-out temperature for a period of some tens of seconds.

Automatic read-out systems mostly apply an internal annealing procedure in the reader below 400 °C. Detectors embedded in or covered with Teflon can be annealed only up to 300°C, resulting in a significant zero-dose reading after high pre-exposure. Detectors previously exposed to high doses should therefore be separated from those exposed to low doses only. This should be taken into account for TLD systems with fully automatic read-out procedures where the detectors are mostly covered by Teflon foils.

The PTB requirements for TLD systems allow maximum variations in the response of ± 5% from the mean value of the batch, if the same detector has been exposed to a reference dose and re-used 20 times. Experimental results of a test programme with different TLD systems are presented in figure 10.6, showing the standard deviation of the response for 10 dosemeters which have been exposed and re-used 10 times without recali-bration of the reader [9].

6

2

1

20

36

26

21

21

19

12

6

3

35

33

17

6

7

32

31

29

11

1—1 TLD r » i RPL

27

9

7

5

3

2 < | B | | | | 25 | |,e

REL STANDARD DEVIATION 1 I a VALUE ) IN V.

Figure 10.6. Variation in response of TLD systems after 10 times of re-use.

10.3.5. Residual reading

In the milliroentgen dose range the zero-dose reading of TL detectors defines the lower detection limit as well as the reproducibility of the dose readings. The dose readings of unirradiated detectors are caused by:

(a) triboluminescence, visible and ultraviolet light stimulation; (b) the infrared emission of the heating element; (c) the dark current fluctuation of the PM tube; (d) residual signals due to the annealing and irradiation history of the dosemeter.

If optimal annealing procedures are not applied, previous irradiations are mainly responsible for an uncontrolled build-up or a time-dependent change of the residual reading. Test procedures which normally apply a repeated programme of exposure and read-out cannot correctly simulate the actual exposure conditions because the exposure

176 E Piesch

frequency and the dose received may vary significantly in practice. The residual reading of an automatic system is presented in figure 10.7 for repeated exposures to doses of 250 mR and a standard annealing at 380°C/lmin resulting in an increase of the residual reading up to approximately 3 mR [10].

_ 2 -as

tu — 00 LU

9 o 00 °r LU X Q- LU ' ' ■ I J I ' I ■ I

STANDARD ANNEALING j 380°Clmin _^

i ■

100 200 500 I 2 5 10 20 50 EXPOSURE READING (CYCLE)

Figure 10.7. Residual dose per exposure as a function of exposure-reading cycles; exposure for each cycle is at 250 mR [10].

the

The residual dose reading mainly depends on the TL material and the irradiation history of the individual detector and has been found to be of the order of 0.03% up to 6% of the pre-exposure without taking into account the reader-dependent fraction of the zero-dose reading (figure 10.8) [11].

8 I Sio

:

§

10

U^O,-. Mn.Si

10 10" 10 10" 10' GAMMA EXPOSURE IN R

Figure 10.8. Relative dose reading at second evaluation

After high exposure a second read-out of the dosemeter may be applied to re-assess the dose. A uv photo-transfer technique was recently developed for the re-assessment of the exposure in LiF [32] which results in an increase of the intrinsic response by a factor of five. The uv re-estimation is independent of y or n radiation [33].

Application of TLD to personnel dosimetry 177

10.4. Personnel dosemeter systems!

10.4.1. General aspects

In mixed beta-gamma radiation fields, it is sufficient to estimate the dose equivalent to the skin and the whole body [2]. To meet these requirements, such a dosemeter should contain two tissue-equivalent TL elements with different shieldings, for instance, covered by a tissue-equivalent material of about 7 mg cm"2 and 500 or 1000 mg cm"2, respec­tively. In most cases, it is known that the major part of the dose received will always be due to penetrating radiation (x-rays, 7-rays) or to non-penetrating radiation (low-energy 7-rays or x-rays), so it may be sufficient to use a basic dosemeter containing only a single tissue-equivalent TL element. In practice, however, the response curves of LiF and Li2B407 are only approximately tissue-equivalent, as can be seen in figure 10.2 [5].

The discriminating basic dosemeter recommended by Euratom should provide additional information on the radiation quality and, if possible, an indication as to whether the dose has been received predominantly from in front of or behind the wearer. TLD does not lend itself to the provision of information on the radiation field. Informa­tion on the quality of photons can be obtained in the energy range 10-200 keV by using special techniques and additional dosemeters, for example:

(a) at least two elements and a filter system similar to the film badge design which, however, would lead to a marked angular response; or

(b) two different TL materials, one tissue-equivalent and one with a high-energy response characteristic, for instance LiF and CaS04.

10.4.2. Practical dosemeter systems

After early applications in nuclear research centres and at nuclear power reactor stations, mainly in the United States, and also until now centralised services in Austria, Denmark, Sweden, the Netherlands and recently in France, the United Kingdom, Germany and Fin­land automatic read-out systems in personnel monitoring are already being used or are in preparation [23, 24, 30]. On the other hand, a number of smaller monitoring services with some thousands of dosemeter evaluations per month are still using non-automatic instruments, besides personnel monitoring mainly for application in extremity and environmental dosimetry.

The most important aspect for the large-scale use of TLD systems in personnel monitoring is the application of an automatic TLD system. The main properties of some automatic readers are presented in table 10.5. The different automatic read-out systems now commercially available make use of multi-element dosemeters with two, three or four detectors or detector fields within a badge with at least one unfiltered detector (detection of skin dose) and one filtered (detection of whole body dose).

Some of these systems are mentioned in chapter 3 together with the types of personal dosemeters used with them. Figure 10.9(a)-(e) shows five commercially available personal dosemeter badges. Figure 10.9(/) shows some multi-element dosemeter cards produced by Harshaw, figure 10.9(g) the multi-element dosemeter cards produced by the Institute of Radiation Protection, Helsinki, and figure 10.9(h) the multi-element badge of National Panasonic. t See also §3.5.2.

178 E Piesch

Table 10.5. Properties of automatic TLD readers [12].

Heating principle Detector

Number of detectors Reading (s)/detector Badges per hour Working days for

10 000 badges

Harshaw 2271

Hot finger LiF: Mg, Ti chip

2 35

100 12

National UD510

Hot air BeO: Na thin layer CaS04:Tm glass capillary

3 33

110 11

Studsvik 1313A

Hot nitrogen Li 2B 40, : disc

4 40 90 14

Mn, Si

Teledyne 9100

Hot element LiF : Mg.Ti Teflon matrix

4 47 75 16

French CEA

Hot plate LiF: Na, Mg disc

4 8

450 3

The TLD detectors used in the multi-element dosemeters for automatic read-out are:

(a) LiF : Mg,Ti or CaF2: Dy chips covered on both sides by Teflon foils inside the dosemeter card produced by Harshaw (figure 10.9(f));

(b) LiF: Mg,Ti and Li2B407: Mn,Si chips not fixed and movable inside the holder, made by Studsvik (figure 10.9(A));

(c) LiF and CaS04 embedded in Teflon (Teledyne Isotopes) and sealed in a black plastic foil to avoid light exposure and contamination (figure 10.9(c));

(d) U2B4O7: Cu,Ag and CaS04:Tm in a layer of about 20 mg cm-2 cemented on a plastic foil and coated on both sides by thin foils of 10 mg cm-2 inside the dose-meter card of National Panasonic (figure 10.9(h)).

Instead of Teflon protection foils two Teflon rings are also used from both sides to fit the rectangular chips on the edges inside the dosemeter card [31] (figure 10.9(f)).

Because of the interest in automatic read-out instruments, results of a standard test programme are presented in table 10.6 for seven TLD systems using automatic evaluation [9]. By definition, the dose at the lower detection limit Dm was found experimentally as the 3 a value of the zero-dose reading of unexposed dosemeters. The long-term variation of the dark current of the zero-dose reading and of the results for repeated exposures to lOOD^ were investigated over a monitoring period of 10 d. The reproduci­bility of representative TLD systems is presented in figure 10.5 as a function of exposure. The s values are high for low doses caused by the au value of the system but decrease with increasing dose and result in a constant value for doses above 100Z>idi- For a longer period or other systems the reproducibility may vary (figure 10.10) depending on the quality of the detector batch, of the individual reader and the read-out technique applied [29]. This is demonstrated in figure 10.11 taking TLD 700 and TLD 600 systems as examples. On the basis of experimental standard deviation versus dose curves the different error sources of TLD systems can be interpreted qualitatively. The reproducibility of dose reading is improved in the low dose range by using a quick, instead of a slow, read-out technique for peak no 5 + 6 (factor 2.6) or only an evaluation of peak no 5 instead of peak no 5 +6 (factor 2). On the other hand, higher s(D) values have been found for new TLD 700 batches compared to old ones manufactured in 1974 (a factor of 2.5 for low

Application of TLD to personnel dosimetry 179

ia)

Si (e)

E5780Q9t n 115148780267 |

150780866

(?) (/O Figure 10.9. (a) Thermoluminescence dosemeter of the Austrian Atomic Energy Research Organization Ltd. (b) Studsvik TL dosemeter type 6541A. (c) TLD badge developed and used by the Swiss Public Health Department, Section of Radiation Protection (Swissbadge). Detector: Teledyne Isotopes Radi-Guard multi-area dosemeter. (d) Nuclear station badge produced and distributed by R S Landauer Jr & Co, Division of Technical Operations, Inc (by courtesy of R S Landauer Jr & Co), (e) TLD badge with coded insert designed by the National Radiological Protection Board (NRPB) Harwell with plastic dome-shaped filter (700 mg cm -2), produced by D A Pitman Ltd (courtesy of D A Pitman Ltd). (/) Harshaw multi-element TLD cards, a Two-element card with binary code decimal (BCD) identification (see also figure 9(c)). b Three- and four-element cards with bar code (Codabar ). Left card used with Model 2276 microprocessor-controlled TL dosimetry system. Right card used with Model 2271L automated TLD system, (g) Multi-element TLD cards without protection layers, pro­duced by the Institute of Radiation Protection, Helsinki. (/;) National Panasonic composite TL dosemeter.

180 E Piesch

Table 10.6. Performance of automatic TLD read-out systems.

Reader type Harshaw 2271 National UD 510 Studsvik 1313A

TLdetector TLD 100 TLD 100 Li2B407: Ag,Cu CaSO„:Tm TLD 700 Li2B40,:Mn, Si Annealing No No Internal Internal No No

T L D 2 0 0 400° C 1.5 h

Z>ldi (mR)t 1.2

Dark current:): a0 (mR) 0.7 o(%) ±26 Zero dose % au (mR) o (%)

Reproduci­bility

l a value (%) lOODidi 1.7 30 mR 3.9

0.9

1.3 ±2

2.6 ±40.5

7.9 6.3

5

2 ±62.5

4 ±44.2

2.6 16.8

2

--

3 ±14.6

1.1 3.0

2.0

0.55 ±8.1

2.4 ±24.5

1.3 4.1

1.2

3.5 ±6.5

1.8 ±29.3

1.3 2.3

0.08

0.06 ±8

0.1 ±23

2.2 1.3

t Z>idl dose at the lower detection limit equal to 3 a value of zero dose a u of unirradiated dosemeters. $ Long-term stability during 10 d, variation of mean value for 10 readings per day.

doses), between TLD 700 and TLD 600 (a factor of 5 for low doses) as well as after annealing 60 times at 400 °C (a factor of 3 for high doses).

The application of automatic TLD systems in personnel monitoring requires micro­computer control of the automatic read-out functions and the data processing and recording. This technique is described in detail by Duftschmid [35] for an automatic computerised TLD personal monitoring service.

The properties of different commercial TLD systems were discussed under the aspect of data security and service availability with respect to a large-scale or a decentralised monitoring service [12, 39]. Laboratories with large-scale TLD monitoring also apply an individual calibration of each chip and the loss of dosemeters has been found to be of the order of 2% yr"1 [39]. As a supplement of the performance test of TLD systems by means of the standard test programme [9] the report of an inquiry about the experience with read-out systems [36] discusses the reliability of the reader, mechanical, electronic and automatic dosemeter identification as well as the malfunctions and service experience of 38 different read-out systems in 24 different laboratories. Compared to readers with manual evaluation, additional malfunctions have been found from the automatic trans­port of the dosemeter card. But there are more administrative and economic reasons, as well as the lack of international recommendations for the layout of detectors or reading equipment, which are responsible for the fact that the application of TLD systems as a film replacement will come somewhat later than expected some years ago [13].

Application of TLD to personnel dosimetry 181

BATCH

TLD 6 0 0 ^

FACTOR 5

QUALITY IN ZERO DOSE READING

TLD 700

. x a

» 10 10 10 EXPOSURE IN mR

100

10

1

\

FA

READ-OUT TECHNIQUE

>v TLD 700

\ » \ P E A K No 5.6

PEAK N o s \ ^ ^ N .

CTOR 5 b

Vf 103

EXPOSURE IN mR

(a) (b)

ANNEALING

\

FACTOR 3

HISTORY

TLD 700

ANNEAL 6 0 -

d

(c) EXPOSURE IN mR

(d)

BATCH UNIFORMITY IN RESPONSE

10' 10' EXPOSURE IN mR

Figure 10.10. Reproducibility of TLD 600 and TLD 700 dosemeters as a function of exposure repre­senting the following error parameters: (a) batch quality in zero-dose reading, (b) read-out technique, (c) annealing history, (d) batch uniformity in response.

w 10- — •

t 2

7

5

£

2

1

29

20

15

10

7

6 3

6

36

35

26

17

12

8 1

f—1 TLD

T^\ RPL

3

33

32

19

11 5

28

24 9 27 1 18 1 16 | 25 1

REL.STANDARD DEVIATION ( 2o VALUE ) IN V. Figure 10.11. Frequency distribution of reproducibility for different TLD and RPL systems exposed to 100 times the lowest detectable dose D\$\. Number of system according to the standard test programme [9].

182 E Piesch

10.5. Special applications

10.5.1. ^-radiation and extremity dosimetry

High-energy /3-rays may contribute significantly to the depth dose in tissue. In such /3-radiation fields it is therefore recommended to measure both skin dose and whole body dose by using two separate tissue-equivalent dosemeters. For |3 energies below 1 MeV the response changes significantly with the energy distribution dependent upon the thickness of detector and shielding. For the dose measurement of non-penetrating radiation, ideally the skin dose should be measured with a tissue-equivalent detector of 5 mg cm"2

thickness covered by a layer of tissue-equivalent material 5 mg cm"2 thick. Usually some­what thicker detectors are used which under-estimate the dose of low-energy /3-rays by up to 50%. For a detector with an actual thickness of about 150 mg cm"2 or 200 mg cm"2, the energy threshold achieved in practice is about 0.6 MeV or 0.8 MeV (figure 10.12).

100

50

M

o a. if) UJ t r _ i i—

?n

10

b

d c

I I I I

Al DISC

76 mm ) m )

CaF2 : Mn CHIP (0.75mm) CaF2 : Mn ROD ( 1 mm) _iJ i i l _

200 500 1000 2000

MAXIMUM BETA RAY ENERGY ( keV ) Figure 10.12. Sensitivity to beta radiation [15].

5000

To improve the sensitivity for low-energy 0-ray emitters, different techniques have been adopted:

(a) a thinner detector as well as a 'window' is more suitable; (b) non-transparent detectors are suitable to discriminate the TL light output from the

surface and deeper layers; (c) a multi-element dosemeter may provide information on j3- or 7-ray energy so that

corrections for self-absorption can be applied.

Thin UF-Teflon detectors of 0.2-0.02 mm thickness show high j3 sensitivity but are expensive, fragile and difficult to incorporate in a dosemeter. For non-transparent graphite-mixed l iF and JJ2B4O7 investigated at Riso [16], the energy threshold of 0-rays was found to be in the range of 0.2 MeV for detectors having a content of 5-10%

Application of TLD to personnel dosimetry 183

LIF WITH AND WITHOUT GRAPHITE ^^/f

_

10V. /

ov.

■ / /

/ /' — * // —

//,

y^s//f > < / / / / 's //' ^ ///

/'/ / O

7 .

FRONT SIDE BACK SIDE

'

L12B4O7 Mn WITH AND WITHOUT GRAPHITE

0.02 0 05 0.1 0.2 0.5 1.0 AVERAGE BETA ENERGY ( MeV)

D.02 0.05 01 02 05 AVERAGE BETA ENERGY (MeV)

(a) (b) Figure 10.13. (a) Energy response curves of LiF dosemeters with 0, 5, 10 and 15% content of graphite exposed to beta irradiation [16]. (b) Energy response curves of Li2Bj07:Mn dosemeters with 0, 0.5, 1, 2, 4 and 8% content of graphite exposed to beta irradiation [16].

graphite (figure 10.13). Similar TLD 100 Harshaw chips containing graphite show a response of the order of 30% for 204T1 (£"max = 0.23 MeV), a sensitvity of 0.2 compared to transparent LiF and a zero dose of the order of 0.1 R.

A new approach at the Berkeley Nuclear Laboratories uses ultra-thin bonded discs (UTB) composed of 6 mgem"2 (UT) LiF-Teflon or CaS04:Dy-Teflon discs which have been thermally bonded to thick Teflon bases [17]. A limitation of UTB is their light sensitivity which gives rise to a high zero dose of 500 mR. Thin CaS04:Dy layers of about lOmgcm"2 fixed to an aluminium disc also show a high /3-sensitivity up to 0.2 MeV (figure 10.14) [15]. For application in mixed /3-y fields a three-element dose-

200 500 1000 2000 5000 MAXIMUM BETA RAY ENERGY (KeV)

cc E cc UJ Q. UJ in z 2 1/1 UJ cc

(b)

. - . C a S 0 4

-^K

. I -

Dy

,JS

^ o L i F ( l ) ^ ^ 0 ^ _ c v

—■-J-""

V / ^ < ^ . L i F ( 1 ) / L i F ( 2 )

, ,i i rTTT-rrn-o

- • - 08

50 100 500 1000

EFFECTIVE PHOTON ENERGY ( k e V )

cc E en UJ

a. UJ i/> z o D. I/) UJ

0.4 cc

1.6

1.2

(a) Figure 10.14. Beta (a) and gamma (b) sensitivity of the three-element finger ring dosemeter [15] which consists of an entrance window ( l m g c m - 1 ) and a thin CaS04:Dy detector (~10 mg cm"2) on an Al disc behind the window. Behind an additional Al shield (1.5 mm) and a Pb shield (0.5 mm) two detector elements LiF (1) and LiF (2), respectively, are applied to correct for energy dependence of photons.

meter uses the CaS04 detector on the top (skin dose) and two different shielded LiF elements below the skin dosemeter. If low-energy gamma radiation is excluded, the dose-

meter is suitable for simultaneous measurement of beta and gamma doses.

184 E Piesch

The new dosemeter from National uses a Li2B407: Ag,Cu layer of 20mgcnf2 coated on both sides by thin foils of about 10 mg cm"2 and is heated by an ultra-red lamp. The detector element represents an optimum arrangement with respect to tissue equivalence and dose measurement of low-energy /3-rays and photons [25].

The sensitivity to /3-rays may be increased by using a window of 1 mg cm-2 in front of the TL detector [38] or a boron-diffused surface layer in UF [37] which creates a new glow peak, also resulting in a better discrimination of photons.

Tissue-equivalent layers in front of a TLD 700 dosemeter of 0.9 mm thickness change the 0-ray response significantly as can be seen in figure 10.15 for different (3-ray emitters. The ratio between skin dose (7 mg cm-2) and depth dose (500mgcirT2) is dependent upon the /3-ray energy and the contribution of photons and in practice has been found to be about 3 for 106Ru or up to a factor of 30 in mixed fi-y fields.

In personnel monitoring the measurement of skin dose is required above all for extremity dosimetry. For the purpose of measuring finger and hand doses different kinds of dosemeters have been developed which use IiF-Teflon discs in a black light-proof polyethylene pouch (figures 10.16(a) and (b)) or LiF, Li2B407 or BeO chips up to 0.9 mm thickness inside a steel ring, plastic ring or a bracelet with 'windows' in optimal cases of 1-7 mgcnT2 thickness (figures 10.16(c) and (d)). A new design of a finger ring and/or bracelet dosemeter (figure 10.16(e)) is applicable for semi-automatic handling and reading [40].

Because of the difference in the maximum permissible dose values for skin dose (30 rem yr_1) and depth dose (5 rem yr"1) the measurement of skin dose is required only in ]3-radiation fields or in mixed (3-y radiation fields if a ratio of//(skin)///(depth) > 1 is expected.

BETA DOSE READING VS. TISSUE DEPTH

TLD 700/0.9mm thick

R/rd tor 7mg/cm 2

,06Ru 1.0

»°Sr/9°Y 0.92 "Tl

"Pm

0.2

"0.02

I 10 100 1000

ABSORBER IN mg / cm2

Figure 10.15. Beta dose reading of a 0.9 mm thick TLD 700 detector as a function of shielding.

10.5.2. Low-energy photons

The under-estimation of low-energy photons below 20 keV is dependent upon the thick­ness and shape of the detector and the self-absorption in the dosemeter badge which may

Application of TLD to personnel dosimetry 185

(c) (d)

(«)

Figure 10.16. (a) Teledyne Isotopes TL finger and hand dosemeter. The dosemeter consists of an LiF-Teflon disc in a light-proof (black) polyethylene pouch adhered to a band-aid type tape, (b) Tele-dyne Isotopes finger and hand dosemeter applied for assessment of the dose to the finger basal layer, (c) Examples of TL finger ring dosemeters. a Ring developed by the Gesellschaft fur Strahlen- und Umweltforschung Miinchen mbH and produced by the Physikalische Werksta'tten Dr Pychlau KG, Freiburg. This ring uses two LiF micro-rods (1 mm dia X 6 mm), one in a Cu filter screw for p and j discrimination and photon energy estimation, b Ring produced by Heist KG, Germany, allows TL phosphor discs and chips to be inserted. Mechanical attachment to the ring and protection is achieved by means of shrink foils, (d) TNO TLD badge (by courtesy of Dr H W Julius), (e) TLD extremity dosemeter developed by H W Julius (TNO) and G Busuoli (CNEN) (by courtesy of Dr H W Julius).

186 E Piesch

be improved significantly by using thin detectors (see §10.5.1). The type approval draft of the PTB recommends three different energy ranges and a maximum error of 30% for the energy dependence and also for low-energy photons (see table 10.2).

The dose reading of liF is not exactly tissue-equivalent for photons in the x-ray energy range of lOkeV (see figure 10.2). The over-estimation of about 40% at about 30 keV may be reduced by an adequate filter which, however, causes an under-estimation of low-energy photons in the energy range of 10-20 keV. The energy dependence of LiF is the main limiting factor for the overall uncertainty of dose measurements in the lower energy range and may be changed by applying a suitable filter in front of the detector (figure 10.17).

IJ2B4O7 is a TL material with the best tissue equivalence, showing, however, the same disadvantages of self-absorption if no thin detector elements are applied. The energy dependence of BeO, on the other hand, results in an over-response of 75% in the energy range of 100 keV [19]. TL materials with a high effective atomic number Z are not recommended for the low-energy range.

1

V

LiF TLD-100 RibbonsTNObadge Response relative to 60Co(tin)

i

/ A / / / / y^x I I /s/ / / / / ' 0PEN WIND0W

/ i l l / / / / 2 1 0 m m PLASTIC / / / / 6 / / / / J 1 Smm ALUMINIUM

/ / / / I n / I / I 2 Omm ALUMINIUM / / / / I '/ I f S 0.1mm COPPER

/ / / / / 1 / ' s °Imm c O p p E R

If/ / / / / 7. O.Jmm COPPER / / / / / a / / ' ' 0 m m T I N

1 // // jf / 8 10mm TIN

, ^ r * * ^ . 1 , 1

-C.5

5 10 2 5 100 2 5 1000 2

EefflkeV)

Figure 10.17. Energy response curve for LiF TLD 100 chips (Harshaw: 1/8X1/8X0.035 in3) in the TNO badge, covered by various filters [14].

10.5.3. High-energy photons

The response of TL detectors decreases with photon energies higher than 1 MeV, resulting in an under-estimation of the dose reading which is dependent on the thickness and shape of the detector and above all on the thickness of the badge shielding.

At reactor sites, for example, 6 MeV 7-rays occur due to the reaction , 60(n, 7)17N in the water used for neutron moderation and core cooling. In table 10.7 results of a 9 MeV calibration of different TL detectors are presented [18]. According to the detector thickness, LiF and ^ B ^ v chips show a relative response of 0.24-0.63 compared to that at 1.3 MeV. A better response of 0.78 was found for bulb dosemeters and 1.12 for CaF2:Dy in the perforated tin sphere. As can be seen from the depth dose results in figure 10.18, the application of a tissue-equivalent filter with a thickness of 3 mg cm"2 or

Application of TLD to personnel dosimetry 187

Table 10.7. Relative response of dosemeters to 9 MeV photons.

Detector Manufacturer Shielding (mg cm 2) Relative response]-

LiF: Mg, Ti in Teflon 6 mm dia X 0.4 mm Teledyne Isotopes TLD 700 ribbons 3 X3 X 0.9 mm3 Harshaw

TLD 700 bulb Harshaw CaF2:Dy

TLD 200 ribbons 3 x 3 X 0.9 mm3 Harshaw

Phosphate glass 8X8X4 .7mm 3

Ionisation chamber PHY-SEQ 6 LB 14862

Schott u. Gen.

7 7

500

0.27 0.40 0.57 0.78

7 0.37 Perforated tin sphere 1.12

7 0.72 Perforated tin sphere 1.28

La Physiotechnie Pocket dosemeter 0.58 Berthold Pocket dosemeter 0.74

f Response per rad at 9 MeV corresponding to the maximum value at 3 g cm 2 tissue depth compared to the response at 1.2 MeV.

— 100 -O

<

if) O Q

I d CC

50

•

-

-

-

* ' v PHOSPHATE GLASS DOS. 2\ IN GLASS / A PHOSPHATE GLASS DOS. 8J 8 x 8xA.7mm3

/ •L iF:Mg,Ti 1 o IONIZATION CHAMBER J I N P H A N T 0 M

, i . i , i , i ,

Figure 10. measured

1 2 3 U

DEPTH IN PHANTOM OR GLASS IN g /cm 2

.18. Comparison of depth dose distribution for 9 MeV photons in beam direction in the phantom and in a single block of phosphate glass.

Al filter of 1 cm is sufficient for a correct dose reading of tissue-equivalent detectors. In the high proton energy range non-tissue-equivalent detector material or filters may also be applied.

10.5.4. High dose range

In the high dose range (see also chapter 17) most of the TL materials show non-linearity in the dose reading versus exposure curve. Supralinearity of the dose reading starts at doses of about 100 R and the response at the maximum varies between 1.6 and 3.8 compared to that at 10 R for the TL phosphors investigated (table 10.8). Due to damage

188 E Piesch

Table 10.8. High­dose properties of TLD materials [20].

Detector type Starting Supra­ Relative Radiation Residual dose (R) point (R) linearity response damage

exposure at at 100 R 1000 R maximum maximum (R)

7LiF:Mg,Ti T L D 7 0 0

7LiF: Na, Mg PTL717

Li2B407:Mn,Si pellets

Li2B407:Mn T L D 8 0 0

CaSO„:Tm UD­100 MG

CaSO„:Dy Teflon

CaF2:Mn Teflon

CaF3:Dy T L D 2 0 0

100

100

200

2X103

200

100

200

200

5X104

3X10"

105

4X10"

5X104

5X10"

5X10"

5X10"

3.83

3.52

3.76

2.08

3.58

3.06

1.6

2.62

2X10 3R

2X10 3R

No

Damage

Sensitisation

­

­

No damage

1

0.03

0.036

2.2

0.6

2.2

6.0

4.8

17

0.3

0.25

2.3

12

44

95

65

effects, mainly LiF shows a reduction in sensitivity after exposures to doses of more than 1000 R (figure 10.19) [20]. The residual dose reading which is found directly after the evaluation of pre­exposed dosemeters was found to be proportional to the pre­exposure dependent upon the annealing treatment and the irradiation history of the detector (see §10.3.3). Annealing procedures before re­use are necessary to anneal the residual dose reading. If no external pre­irradiation annealing can be applied, as for LiF in Teflon or

CaSC. :Tm

Li,B,0,:Mn.Si •■ * Li,BiCVMn —

CaFjtDy . -

LiF:Mg.Ti~ LiF:Na.Mg _ .

GAMMA PRE-EXPOSURE IN R

Figure 10.19. Change in response after pre­exposure.

Application of TLD to personnel dosimetry 189

chips packaged in Teflon cards for automatic read-out, a selection and exchange of highly exposed dosemeters is recommended to avoid measurement errors in the low dose range (see also §10.3.3).

10.5.5. Neutrons^

In the presence of neutrons it may be necessary to correct the gamma dose results for the sensitivity of the detector to neutrons. The thermal neutron response depends upon the thickness and isotopic content of the nuclides and the encapsulation within the badge. The response to intermediate neutrons is lower by several orders of magnitude.

However, due to the effect of back-scattered neutrons, the detector worn on a body will receive twice the radiation for incident thermal neutrons and still some percentage for incident fast neutrons (albedo factor 0.8 for thermal and 0.1 for 1 MeV neutrons). The response to thermal neutrons therefore only gives a rough indication for the correc­tion of neutron sensitivity.

Because of the high thermal-neutron sensitivity of LiF, it is recommended to use TLD 700 detectors for the dose measurement of photons in mixed radiation fields. In practice, a glow-curve analysis may be helpful in detecting any contribution from thermal neutrons if no additional neutron detector has been used. In the low dose range below 10 rem, the peak height ratio P2/Pi found for photons (0.04) and neutrons (0.4) differs by a factor of about 10 (figure 10.20) [20].

GAMMA EXPOSURE IN R

GAMMA EQUIVALENT NEUTRON EXPOSURE IN R

Figure 10.20. Peak height ratio P2/Pt for LiF. (a) y exposure (TLD 600 and 700), (b) neutron exposure (for TLD 600 only).

An albedo neutron dosemeter uses a cadmium or boron shield facing the source to eliminate incident thermal neutrons and to yield a dosemeter response which is mainly caused by thermal neutrons back-scattered from the body. Commercially available dose-

f This topic is treated in more detail in chapter 12.

190 E Piesch

NEUTRON ENERGY IN eV

Figure 10.21. Response of the Karlsruhe single-sphere albedo dosemeter | 221

<=3 BORON LOADED PLASTICS CD PLASTICS

LiF- DETECTOR

(c) Figure 10.22. (a) Constructional details of the Karlsruhe albedo neutron dosemeter. (b) Close-up view of the Karlsruhe albedo dosemeter (courtesy of Heist KG), (c) Karlsruhe albedo dosemeter belt (courtesy of Heist KG).

Application of TLD to personnel dosimetry 191

meter types developed by Harvey et al [47], Hankins [41], Hoy [42], Piesch and Burgkhardt [21] and Brunskill and Way Wang [43] are presented in detail in chapter 12. The main properties of albedo dosimetry are the neutron detection without any energy threshold and, on the other hand, a significant decrease in response above 10 keV, as can be seen for the Karlsruhe albedo dosemeter in figure 10.21 [22], constructional details of which are presented in figure 10.22(a) and a close-up view in figure 10.22(A). Figure 10.22(c) shows the same dosemeter on a belt.

Albedo neutron dosemeters of the discriminating analyser type use two or three different 6LiF/7liF detectors which are positioned, for example, inside a boron-loaded plastic badge in such a way that incident thermal neutrons (detector a) or incident/back-scattered intermediate neutrons (detector m) can be indicated separately from the albedo reading (detector i) in order to correct over-sensitivity to thermal and intermediate neutron leakage. The application of two albedo dosemeters with a dosemeter belt results in an approximately directional independence of the dose reading.

In neutron monitoring today the application of albedo dosemeters is based on field calibrations in the environment of each neutron facility which may be performed, for instance, by using the rem meter (figure 10.21) with the TLD detector c in the centre as a phantom for two albedo dosemeters. Figure 10.23 shows examples for field calibrations at the Heidelberg Compact Cyclotron and at the Karlsruhe Research Reactor FR2. The effective response /?eff(i) of the albedo dosemeter presented as a function of the reading ratio i/a may vary by one order of magnitude around one facility caused by local changes of the thermal radiation and/or the moderation of fast neutrons. The calibration curve allows the correction of energy dependence for each individual dosemeter reading in personnel monitoring [44]. The evaluation technique applied in routine monitoring allows the interpretation of neutron stray radiation fields by the direct use of the graph presented in figure 10.23 for two neutron facilities, for example, or an on-line computer

FIELD CALIBRATION

REL NEUTRON DOSE EQUIVALENT RATIO Hy , / H n IN 7.

100 20 10 5 2 1 &5 01

CORRECTION OF THE LOCAL CHANGE IN

0 1 L THE NEUTRON SPECTRUM

DETECTOR REAOINC RATIO a h l / a ( a )

Figure 10.23. Response of the Karlsruhe albedo dosemeter as a function of reading ratio i/a on the basis of field calibrations [44].

192 EPiesch

program for data processing and recording to interpret neutron stray radiation fields in terms of neutron dose equivalent separated for the energy groups 0.4 eV, 0.4eV-10keV and lOkeV-lOMeV and £"eff for fast neutrons [22]. In contrast to the multi-sphere technique the single-sphere albedo technique offers the advantages of a similar field analysis and the direct measurement of the actual albedo dosemeter response at the location of interest. This standard technique has been applied for field analysis at different linear accelerators, neutron therapy facilities, research and power reactors [45].

*

10.5.6. Accident dosimetry

The extended dose range of TL dosemeters applied in routine personnel monitoring also makes it possible to fulfil the requirements for an accident dosemeter. In the high dose range the following properties of TL detectors should be taken into account: (a) after irradiation to high doses the supralinearity above 100 R should be corrected; (b) after the first read-out the residual dose reading may be used for a second dose

evaluation; (c) the glow-curve analysis gives information about the contribution of other types of

radiation, above all neutrons; (d) after high neutron exposure of LiF the intrinsic effect of the neutron-induced

tritium content results in a storage time-dependent increase of the zero-dose reading which, in addition to the first read-out, may be used to separate photon and neutron exposures and to estimate the exposure to thermal neutrons [48].

In mixed gamma-neutron radiation fields the gamma dosemeter reading has to be corrected with respect to the neutron sensitivity of the dosemeter in the neutron energy range for «th, n-t and «f. The application of albedo dosemeters is limited because a simple albedo detector does not give information about neutron dose and because of the relatively high neutron sensitivity. For a moderated fission spectrum linearity is expected up to a dose equivalent of about 300 rem. Higher neutron exposures result in a gamma-equivalent dose reading of more than 1000 R and thus in radiation damage of the detector response. In accident dosimetry albedo dosemeters of the analyser type like the Karlsruhe albedo dosemeter give sufficient results for the gamma and neutron dose, an interpretation of the neutron spectrum and the orientation of the person in the radiation field.

10.6. Future trends

Although most of the users are interested in the application of automatic TLD read-out systems in personnel monitoring, there is obviously a discrepancy between the properties of commercial TLD systems and the requirements of highly centralised large-scale monitoring services. Future improvements of commercial equipment are necessary to reduce the malfunction rate of the read-out and to perfect the data security, the availability of tissue-equivalent TLD materials and the service situation.

In personnel monitoring the following future trends can be seen:

(a) because of data storage and service, smaller monitoring services rather than centralised ones will apply automatic read-out systems;

Application of TLD to personnel dosimetry 193

(b) automatic read-out systems will also be applied for use with extremity dosemeters and albedo neutron dosemeters;

(c) thin detectors will be produced which will be suitable for 0 and extremity dosimetry.

The dose reading of tissue-equivalent detectors is practically independent of the direction of the incident radiation, at least for photon energies above 30 keV. TLD, however, does not lend itself to information about the radiation quality and the direction of the incident radiation. For the estimation of radiation quality, two detectors with different energy responses must be applied. The dose reading ratio of the detector behind the window and the filtered detector only gives rough information on the radiation quality, depending on the homogeneity of the radiation field. The accuracy of dose reading is not only based on the properties of the TLD system adopted but also on the carefully applied technique of handling, evaluation and annealing treatment. Workshops or standard test programmes may serve to establish high accuracy in the laboratory and may stimulate improvements in the read-out cycle.

References

1 1970 Protection against ionizing radiation from external sources ICRPPubl. 15 2 1975 Technical recommendations for monitoring the exposure of individuals to external radiation

CECRep. EUR5287e 3 1975 Technical recommendations for the use of thermoluminescence for dosimetry in individual

monitoring for photons and electrons from external sources CEC Rep. EUR 5358e 4 Nichols L L 1977 A test for the performance of personnel dosimeters Rep. BNWL - 2159 5 Christensen P 1971 Li2B407 and LiF TL dosimeter for routine personnel monitoring Proc. IAEA

Symp. on Advances in Physics and Biological Radiation Detection plOl 6 Burgkhardt B, Herrera R and Piesch E 1977 Long-term fading experiment with different TLD

systems Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p75

7 Nink R and KosM-11980 An improved LiF material for thermoluminescence dosimetry J. Physique to be published

8 Hahn D and Nink R 1977 Situation and development of solid state dosimetry, from the PTB-point of view Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p l 7 5

9 Piesch E and Burgkhardt B 1978 RTL Systeme im Bereich kleiner Dosen: Vorstellung eines Test-programmes und Ergebnisse an 39 Systemen Rep. Fachverband fur Strahlenschutz FS-78-17 AKD or KfK-2626

10 Miyagawa K et al 1977 An access control system using TLD for radiation monitoring in atomic power plant Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p281

11 Piesch E, Burgkhardt B and Singh D 1977 Properties of TL dosimeters after high gamma irradiation Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalishes Institut, Universitat Giessen), p94

12 Regulla D F and Drexler G 1977 Results and discussion of laboratory experiences with different automated TLD readers for personnel monitoring Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p262

13 Becker K 1976 Status and trends in personnel monitoring — a worldwide survey Kerntechnik 18 345

14 Julius H W 1976 Introduction of an automated TLD personnel monitoring system and centralized dose record keeping in the Netherlands Proc. 9th Jahrestagung Fachverband fur Strahlenschutz, Alpbach Fs-75-12-T, p i25

194 EPiesch

15 Benco L et al 1977 Thermoluminescent beta dosimetry for routine personnel monitoring Proc. IRPA Congr., Paris vol 4, pi 261

16 Koczinsky A et al 1974 Graphite-mixed non-transparent LiF and Li ;B407 :Mn TL dosimeters com­bined with a two side reading system for beta-gamma dosimetry Proc. 4th Int. Conf. on Lumi­nescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), vol 2, p641

17 Charles M W 1977 The development of a practical 5 mg/cm2 skin dosimeter Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p313

18 Burgkhardt B, Piesch E and Schmitt A 1977 Depth dose distribution of 9 MeV photons in a single phosphate glass compared to phantom results Nucl. Instrum. Meth. 141 141

19 Busuoli G and Julius H W 1977 Possible use of BeO in beta-gamma personnel dosimetry Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p225

20 Piesch E, Burgkhardt B and Sayed A M 1974 Supralinearity and re-evaluation of TLD 600 and TLD 700 in mixed neutron and gamma fields Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), pl201

21 Piesch E and Burgkhardt B 1974 An LiF albedo neutron dosimeter for personnel monitoring in mixed radiation fields Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), vol 3, p l l 2 3 and 1918Proc. IAEA Symp. on Advances in Protection Monitoring, Stockholm

22 Piesch E and Burgkhardt B 1980 Application of the TLD albedo technique for monitoring and interpretation of neutron stray radiation field Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

23 Niewiadomski T (ed) 1974 Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics)

24 Scharmann A (ed) 1977 Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen)

25 Technical Note Development of a new type thermoluminescence dosimeter (Matsushita Electric Industrial Co Ltd)

26 Burgkhardt B and Piesch E 1978 Nucl. Instrum. Meth. 155 293, 299 27 Luersen R B and Johurson T L 1979 Results of the Naval Research Laboratory's participation in a

personnel dosimetry performance test in pilot study NRL Memorandum Rep. 4048 28 Burgkhardt B, Piesch E and Seguin H 1980 Some results of a European interlaboratory test pro­

gramme for integrating dosimeter systems for environmental monitoring Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

29 Burgkhardt B and Piesch E 1980 Reproducibility of TLD systems - a comprehensive analysis of experimental results Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

30 Portal G (ed) 1980 Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

31 Toivonen M 1979 Individual TL detector characteristic in automated processing of personnel dosi­meters Rep. STL-A27

32 McKinlay et al 1980 Photo-transferred thermoluminescence technique and its application to the routine re-assessment of absorbed dose in the NRPB automated personal dosimetry system Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (Sep­tember)

33 Douglas J A et al 1980 The effect of LET on the efficiency of dose re-estimation in LiF using UV photo-transfer Proc. 6th Int. Conf. on Solid Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

34 Regulla D F 1980 Remarks on the present state of thermoluminescence Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

35 Duftschmid K E 1980 The automatic computerized TLD personal monitoring system in Austria Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

36 Burgkhardt B, Moos W and Piesch E 1979 Ergebnisse einer Umfrage iiber Erfahrungen mit Thermo-lumineszenz- und Phosphatglas-Auswertegeraten Rep. FS 79-19-AKD SI7

Application of TLD to personnel dosimetry 195

37 Christensen P and Majborn B 1980 Boron diffused thermoluminescent surface layer in LiF TLD's for skin dose assessment Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. lustrum. Meth. 175 No 1 (September)

38 Uchrin G 1980 A new type of extremity dosimeter Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Inst rum. Meth. 175 No 1 (September)

39 Grogan D, Bradley R P and Ashmore J P 1980 Some problems associated with large scale TLD monitoring and their solutions Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. lustrum. Meth. 175 No 1 (September)

40 Julius H W and Busuoli G 1980 An extremity (finger ring) dosimeter based on TLD Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

41 Hankins D E 1973 A small inexpensive albedo-neutron dosimeter Los Alamos Rep. LA-5261 42 Hoy J E 1972 Personnel albedo neutron dosimeter with thermoluminescent 6LiF and 7LiF

Savanah River Lab. Rep. DP-2377 43 Brunskill R T and Way Wang F S 1980 A personal thermoluminescence dosimeter for the measure­

ment of 0y, y and neutron dose Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

44 Piesch E and Burgkhardt B 1978 The role of an analyser albedo dosimeter in routine monitoring and the current situation for the calibration technique Proc. 7th DOE Workshop on Personnel Neutron Dosimetry, Rep. PNL-2807 p25

45 Piesch E and Burgkhardt B 1980 A new technique for neutron monitoring in stray radiation fields Proc. 5th Int. IRPA Congr., Jerusalem vol III, p 121

46 Prokic M 1980 Development of high sensitive CaS04: Dy/(Tm) and MgB„07: Dy/(Tm) sintered thermoluminescent dosimeters Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

47 Harvey J R, Hudd W H R and Townsend S 1973 Neutron Monitoring for Radiation Protection Purposes, Symp. Vienna STI/PUB/318, ppl99-218

48 Piesch E, Burgkhardt B and Sayed AM 1978 Activation and damage effects in TLD 600 after neutron irradiation Nucl. Instrum. Meth. 147 178-84

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

11 Application ofTLD systems for environmental monitoring

E PIESCH

11.1. Introduction

A significant aspect of environmental monitoring is the limitation of radiation exposure values in man from the nuclear power fuel cycle. In the case of gaseous radionuclides released to the environment from a nuclear installation, the acceptable limit of exposure is based on an annual dose equivalent of 30 mrem, a value about a third that from natural terrestrial and cosmic radiation. An increasing amount of public interest in environ­mental monitoring programmes is being focused on the environmental impact of radiation arising from nuclear power operations and the corresponding detection of slight variations in the natural radiation background.

TLD systems are widely applied to environmental monitoring programmes near nuclear installations, TLD systems with high reproducibility in the milliroentgen dose range are required in order to measure exposures equal to that resulting from an exposure rate of 10 /iR h"1 during field periods of from several days up to a year.

For the application of TLD systems in environmental monitoring, therefore, special performance criteria and techniques for selecting, testing, calibrating and using a TLD system have been established:

(a) The ANSI N545-1975 standard [1] specifies the minimum acceptable performance of TLD and outlines test methods for compliance. To meet these requirements, each laboratory has to carry out tests for determining their own limits of error.

(b) On the basis of a standard test programme [41] and an inter-laboratory comparison, the performance and quality of a broad spectrum of TLD systems can be compared with respect to variations in the properties of reader, TL material, read-out method and annealing technique adopted in the laboratories.

(c) Annual control exposures as well as inter-comparison experiments organised by the ERDAf serve as a valuable way to compare one's own results with the international level [3].

This chapter reviews the properties of TLD systems and the common techniques used in the application of TLD in environmental monitoring, taking into account the results of a test programme and recent investigations of the long-term fading as well as the calibration technique and the interpretation of experimental results.

f United States Energy Research and Development Administration.

198 EPiesch

11.2. Performance specifications

In contrast to the application in personal monitoring, TLD systems for environmental monitoring have to fulfil high requirements, such as

(a) good precision and reproducibility of measurement over the exposure range of interest (10-100 mrem);

(b) low fading over the field exposure period (3-12 months); (c) insensitivity to environmental parameters, i.e. temperature, moisture, humidity,

light; (d) approximate tissue equivalence in dose reading; (e) low self-irradiation due to natural radionuclides in the TLD phosphor or holder; (/) encapsulation in a plastic holder to provide secondary electronic equilibrium,

shielding against |3-rays and light as well as water tightness; Q?) calibration techniques for each field cycle to guarantee the highest precision for the

conversion to exposure and to correct for fading, transit exposure and zero-dose reading.

Up to now only the American National Standard [1] gives performance criteria for TLD systems used for the measurement of environmental exposure levels. This standard specifies minimum acceptable performance of TLD used for environmental measurements, outlines methods to test TLD systems and provides procedures for calibration, field application and reporting. The performance criteria and specifications for TLD systems are discussed here on the basis of this standard until such time as other technical recom­mendations [45] or recommendations of the Commission of the European Communities are available.

In order to meet requirements for environmental monitoring, tests to determine error parameters should be carried out at least once a year if a particular TLD system is adopted. As can be seen from table 11.1, the general testing procedures of ANSI N545-1975 provide specific tests for uniformity of batch sensitivity, reproducibility, depend­ence of exposure interpretation on the period of the field cycle, energy and directional dependence, light and moisture dependence and self-irradiation. The results of tests should be within the limits given for the errors at the 95% confidence level.

The performance of TLD systems should be determined under laboratory conditions. Taking into account additional corrections applied to compensate for the errors under field conditions, the overall error of measurement under field conditions should be less than 30% at 95% confidence level.

11.3. Properties of commercial TLD systems

11.3.1. TL materials and systems

Typical TL phosphors suitable for monitoring environmental radiation are presented in table 11.2. The materials indicated in this table have been investigated experimentally in the past and employed in measurement programmes with varying success. Most of the TL materials (CaF2, CaS04) have been chosen because of their high sensitivity which allows exposure periods of 1 week to 3 months. Due to the energy dependence of the

Application of TLD systems for environmental monitoring 199

Table 11.1. TLD performance criteria for environmental application (ANSI N545­1975).

Testing procedures Uniformity within field batchf Reproducibility for one TLDf Field cycle interpretation}: ratio 7(f)/2/(r/2) Energy dependence (30 keV­3 MeV)

for > 80 keV for <80keV

Direction dependence during rotation through two perpendicular planes

Light dependence} Moisture dependence} Self­irradiation}

±15% ±5% <0.9

<20% Factor of 2

±10% <10% <10% «10MRrr '

Performance specifications Test exposure under laboratory conditions! ±10% Correction for field conditions determined from the

testing procedures Factor of 2 Overall measurement error under field conditions ±30%

■f For an exposure rate of 10 MR h~' during field cycle and at the 95% confidence level. } For a period t equal to the field cycle.

Table 11.2. Characteristics of thermoluminescence phosphors suitable for environmental radiation measurements [4].

Phosphor

LiF (TLD 700)t Li2B407:Mn CaF,:Mn CaF,: natural CaF2 :Dy (TLD200)f

CaSO„: Dy

Zeff

8.2 7.4

16.3 16.3 16.3

15.5

Room temperature fading (%/month)

Negligible <10 5 Negligible (After pre­read­

out anneal) < 2

Self­irradiation (MRh"1)

Negligible None reported 7­22 9­13.8 Negligible

None reported

Reported lower exposure limit } (mR)

0.85 ­1.1

<1 0.5

0.5

Refer­

ences

47 48,49 50,51 52.53 54

55

tTLD 100, 200 and 700 are Harshaw Chemical Co designations, but are widely used. TLD 100 (natural LiF composition) and TLD 700 (enriched in 7Li) have about the same 7­ray responses. } Lower limits highly dependent on photomultiplier tube used.

reading, these detectors are only suitable for environmental measurements if holders are used which provide for energy compensation and thus avoid errors up to a factor of three. Because of the effect of light sensitivity and fading most of these TL materials

200 EPiesch

should be rejected for long-term monitoring. For environmental application, therefore, mainly chips or high-sensitivity bulb dosemeters of OF, CaF2: Dy and CaS04 activated with Dy and Mn are widely used. To complete this review, more-insensitive RPL glass dosemeters are also applied in environmental measurements. In addition to short read-out cycles, the low fading characteristic of glasses allows long-term exposure periods of up to 6 yr.

The dosemeter properties of TLD systems may differ significantly due to the individual properties of the reader, the TL material and the evaluation technique used in the laboratory. Differences in the dosimetric properties of TLD systems are mainly due to:

(a) the type of reader and the individual properties of the photomultiplier component in the reader;

(b) the dosemeter material, the activator, the matrix material, the form, thickness and mounting of the detector, as well as the history of the batch;

(c) the read-out procedure, in particular the heating cycle and the maximum heating temperature which have been selected;

(d) the thermal treatment of the detector prior to measurement (pre-heating) or the thermal treatment used prior to re-use (annealing);

(e) the history of the TLD batch, i.e. number of annealings and the amount of pre­exposure.

Although dosemeter systems with high accuracy may now be applied, the properties of different TLD readers, even of one type, are not comparable because of variations either in the PM dark current or in the reproducibility, which may vary by more than a factor of 100 and a factor of 4, respectively [2]. Results from other laboratories with the same type of reader or the application of the read-out technique which may be found in the current literature cannot replace an extended performance test with each reader or dosemeter system. Compared to the results of an inter-laboratory test programme, single data reported in the literature or the recommendations in the NCRP Report no 50 [4] cannot reflect the present technology or the state of the art.

For TLD systems applied in environmental monitoring programmes, therefore, exten­sive performance tests are required to establish high accuracy, i.e. small random and systematic measuring errors in the low dose range.

The state of the art may be found in the results of inter-laboratory test programmes which have recently been organised by the 'Workshop Dosimetry' of the Fachverband fur Strahlenschutz [2] and the European Communities [41]. Here the properties of 43 and 45 different dosemeter systems, respectively, have been investigated. According to the recommended test procedure the laboratories tested their dosemeter systems with respect to zero-dose reading, the lower detection limit, the standard deviation as a function of exposure, long-term stability of the system and temperature effects during storage at 70 C. The experimental procedure of the inter-laboratory test programme (table 11.3) provides measurement runs to be carried out by each laboratory with a batch of 10 dosemeters taking 10 readings for each dosemeter or repeating the measurement cycle 10 times. The dose range had been related to a multiple of the lowest detectable dose £>idi defined as three times the standard deviation (3 s value) of the zero-dose reading au of unirradiated dosemeters after subtraction of the dark current reading a0.

Application of TLD systems for environmental monitoring 201

Table 11.3. Experimental procedure for the performance of the standard test programme of TLD systems.

Run Characteristic quantity no

Number of Test instruction dosemeters (measure­ments)

Zero reading Dark current Immediately

70°C/16h

(10X) Read-out without dosemeter 10 Irradiate new or annealed dosemeters to

300 mR; annealing, read-out 10 Irradiate to 300 mR, annealing, store for

16 h/70°C, read-out

4

5 6 7

8

Reproducibility 1-Oldl

30 mR 5D I di to lOOOOidi Individual sensitivity

Long-term stability

Light source Dark current Reference dose Zero reading Zero reading 70 °C/16h

Influence of temperature at 70°C

10

10 10 10

(10X) (10X) 10 10 10

10

In each case anneal and irradiate to 3s value of the zero reading immedi­ately (=Z3ldl) to 30 mR to 5, 10, 20, 50, 100 and 1000£>idi to lOOOOidi

Over 10 d per 10 dosemeters/measure-ments without additional calibration of instruments Measure 10 X Adjust and measure 10 X Annealing, irradiate to 1000Z)]d[, read-out Annealing, read-out Store at 70°C/16 h, read-out

Irradiate two dosemeters prior to storage at 70°C for 10, 6, 2, 1 d and two reference dosemeters (storage 2 h/25 °C) to 1000£>idi, read-out together

11.3.2. Batch uniformity

For TLD of the same batch, the sensitivity of the individual detector may scatter over a range of the order of ± 10%. High reproducibility therefore requires an individual calibra­tion of each TLD in a batch or a selection of TLD by the manufacturer or user, resulting in a high batch uniformity. The relationship between the deviation in response within a batch and the resulting reproducibility is presented in figure 11.1 for different dosemeter systems. By definition, both quantities are identical for a batch calibration showing devia­tions in the range 3-9% (relative standard deviation), for selected batches also lower than 1%. Individual calibrations of the TLD, which should be considered for an environmental TLD system, improve reproducibility by more than 3% to better than 1.9% (la value).

202 E Piesch

m s

INDIVIDUAL CALIBRATION

BATCH CALIBRATION

1 2 3 4 5 6

REPRODUCIBILITY 0 IN 7 .

Figure 11.1. Batch uniformity against reproducibility of dose measurement [2].

11.3.3. Reproducibility

The reproducibility of measurement is a function of detector type, size and sensitivity, of batch uniformity and reader quality. For each TLD system, a reproducibility test should be performed in the dose range of interest, starting with an exposure equal to the lower detection limit D\&\, which may be defined as the 3s value of the zero-dose reading to an exposure of lOOOZJjdi. Experimental results of the standard deviation as a function of exposure are presented in figure 11.2 for different TLD systems. The typical shape of the reproducibility against exposure curve is caused by two parameters, the variation in the zero-dose reading of the system and its reproducibility in the high dose range which is explained in more detail with respect to the error of measurement in §11.4.

A constant reproducibility is expected for exposures of about lOCD^, which is equal to 30 mR and 100 mR, respectively, for most of the LiF systems independent of reader type and the application of an automatic read-out technique, 0.2 mR and 10 mR for CaF2 systems, 5 mR and 100 mR for CaS04 and 10 mR and 100 mR for Li2B407. The reproducibility (Is value) achievable in practice is found to be 1-4% for LiF, 2-5% for LiaE^Ov, 2-6% for CaF2 and 2-3% for CaS04. To optimise TLD systems for an environ­mental monitoring programme, field exposure periods of the order of 100D]^ are recom­mended. The reproducibility for an exposure of 30 mR expected after a field period of approximately six months is presented in figure 11.3 on the basis of 10 dosemeter readings per batch. Only one-third of the TLD systems investigated showed relative s values of about 2%.

11.3.4. Zero-dose reading

The zero-dose reading of a TLD system depends on the PM characteristic, the properties of the TL material, the annealing treatment and the annealing and irradiation history of

Application O/TLD systems for environmental monitoring 203

(a)

LijB.O, NATIONAL

aoi

L'lBiO, STUDSVIK

Caf} STUDSVIK

1 10 DOSE IN mR

1000

10' 10" 10' 10 103 10' 105

EXPOSURE IN mR

Figure 11.2. (a) Relative standard deviation ( l a value of 10 dosemeters) against exposure for different TLD systems, LiF systems excluded [2]. (b) Relative standard deviation ( l a value of 10 dosemeters) against exposure for LiF systems [2]. (c) Relative standard deviation ( l a value for 10 dosemeters) against exposure for LiF automatic systems [2].

204 E Piesch

DOSIMETER SYSTEMS

Figure 11.3. Reproducibility of TLD systems at 30 mR [2].

the batch. The zero-dose reading is presented as the sum of components, the read-out without dosemeter a0 and the read-out of unexposed dosemeters au. For highly sensitive TLD readers, the zero dose au may be much higher than the dark current OQ.

In the lower dose range, the mean value of the zero-dose reading (a0 + au) (figure 11.4) has to be subtracted from the dosemeter reading. By definition, the lower detection limit Adi is given by the 3s deviation of the zero-dose reading. The results in figure 11.4 demonstrate that the mean value of the zero-dose reading has been found to be smaller than Dldi and, for most of the TLD systems, to be in the range 1 -10 mR.

1—1 MEAN VALUE | O^'0,u > r^ JBVALUE ID,™,]

Li2B t07 CoF2 CQSOJ

u u K ' i a z o T t f t i z i K i i s B Q i T M e k O B n i ] E H a n x > » u X B X U

DOSIMETER SYSTEMS

Figure 11.4. Zero-dose reading of different TLD systems [2].

11.3.5. Long-term stability

With respect to the standard test programme, the stability of the system is investigated over a period of at least 10 d, taking into account a daily check of the dark current, the

Application of TLD systems for environmental monitoring 205

zero­dose reading of unexposed dosemeters after repeated measurements/annealings and the reproducibility of dose reading. For example, for the long­term stability of the reader, the zero­dose reading shown in figure 11.5 may vary between a factor of two and a factor of ten, especially for systems not applying an annealing treatment. On the other hand, there are also TLD systems in use showing a low and constant zero­dose reading. The change in sensitivity of the order of 5% can be corrected by an additional reader calibration taking into account control dosemeters from the same batch and annealing run.

1.

"Jfto, as*

In

B MEAN AND MAXIMUM VALUES - \

V BEADING BEFORE IEST R REGENERATED BEFORE REUSE

DOSIMETER SYSTEMS

Figure 11.5. Long­term stability (over 10 d period) of zero­

dose reading [2].

11.3.6. Energy and directional dependence

The reading of LiF and TJ2B4O7 dosemeters has been found to be sufficiently tissue­

equivalent (figure 11.6). If a non­tissue­equivalent TL material is adopted, the energy dependence of the dose reading can be improved by using suitable energy compensation filters (figures 11.7 and 11.8). Perforated tin spheres, for example, are in use to reduce

■

— / ■— / /

THER

/ >S£L'F

V1

/DOS I

■

HOLUMINESC 3.3*Q9mnT> B

Mg.Ti

ENT DOSIMETER HIND 50mg/on> FOR

HWmg/em1 FOR

It Or Mn.Si

-4 1 -1

PHOSPHATE PERFORAT

|

GLASS DOSIMETER

1

<0 6M«V >0 6M,V —

-OOS2

»

1

-—

PHOTON ENERGY IN ktV

Figure 11.6. Energy dependence of LiF:Mg,Ti and Li2B407:Mn, Si dosemeter systems.

206 EPiesch

Figure 11.7. Environmental TL dosemeter with 3 mm copper (brass) compensation shield (filter) for TL phosphor Teflon discs, designed by the Health Physics Institute of the Gesell-schaft fiir Strahlen- und Umweltforschung Miinchen mbH, produced by the Institute of Radiation Protection and Dosimetry of CNEN, Brazil.

Figure 11.8. Bulb-type TL dosemeters with energy compensation shields, (a) Harshaw Bulb TL dosemeter ( O m i n : 0.1 mR for the Dy-activated CaF, phosphor dosemeter), (b) MBLE CaF2 TL dosemeter type PNP291 {Dmm: 0.1 mR), (c) Victoreen CaF2:Mn dose­meter Model TL-35 (£>min: 0-1 m R if read with Model 2600 reader).

the over-sensitivity of CaF2:Dy in the energy range below 100 keV (figure 11.9). In stray radiation fields around nuclear plants, the compensation filter avoids measurement errors of the order of a factor of three. The internal directional dependence of the dose reading should be small in the main direction of the radiation incidence and may be spoiled by the suspension of the dosemeters on dense objects such as trees, resulting in directional anomalies or shieldings of the radiation field.

11.3.7. Fading

In environmental monitoring, dosemeters are freely exposed in air up to monitoring periods of one year during which time the influence of sunshine may cause a relatively high temperature. The temperature-dependent fading gives rise to a significant loss of

Application O/TLD systems for environmental monitoring 207

CaF2:Dy- DOSIMETER HARSHAW RIBBONS TLD 200 0.125x0.125x0.035"

o 3mm LUCITE • SPHERICAL CAPSULE

2mm AU2mm Sn,PERFORATED

1000 QUANTUM ENERGY IN keV

Figure 11.9. Energy dependence of the dosemeter reading of a CaF2: Dy dosemeter with spherical encapsulation (produced by Heist GmbH, Glottertal, BRD) (open circles) and without an energy compensation filter (full circles).

signal during exposure and storage before evaluation. Laboratory experiments may result in the estimation of the fading rate as a function of temperature. Because of changes in the ambient temperature profile and in the exposure rate during the monitoring period, the actual fading during each field period should be checked by using pre-exposed control dosemeters at a representative location.

• A long-term fading experiment was recently performed [6, 7] in the temperature range 5-100 °C up to storage periods of one year. The fading effects of pre-exposed TL materials are presented in figures 11.10-11.12 as a function of the storage period. The fading

10 too 1000

STORAGE PERIOD IN DAYS

Figure 11.10. Fading of CaS04:Dy Teflon against storage period related to 1 d storage at 25 °C and a post-irradiation treatment at 100°C for 20 min [7].

208 E Piesch

i 1000 10 too

STORAGE PERIOD IN DAYS

Figure 11.11. Fading of CaF2:Dy Teflon against storage period related to 1 d storage at 25°C and a post­irradiation treatment at 100°C for 20 min [7].

100

90

80

j . 70

? 60 O z 50 a 2 40

a. 20

10

0

'

"

. i

LiRMg.Ti RIBBONS TLD 700 100 °C/20min

» -

• v ^

i 1 •■

~ ^ "

— - 25 °C

— * • 50°C

_ ^ ^ v 70 °C

- 100 °C

1 1000 10 100

STORAGE PERIOD IN DAYS Figure 11.12. Fading of LiF:Mg, Ti Teflon against storage period related to 1 d storage at 25 °C and a post­irradiation treatment at 100 °C for 20 min [7].

results are based on a post­irradiation annealing at 100 °C/20 min and on reference dose­

meters of the same set that were stored under laboratory conditions and exposed one day before evaluation.

LiF: Mg, Ti, the TL material most frequently used, shows fading effects of 5­30% after more than one year of storage at 25 °C and 50 °C. For highly sensitive CaF2: Dy ribbons applied in environmental monitoring mainly for short exposure periods, the dose reading after one year of storage at 25 °C was still found to be of the order of 90%. This is the

Application of TLD systems for environmental monitoring 209

result of the post-irradiation annealing at 100°C/20min. Without applying any pre­heating, a similar fading is already found after a storage period of 1 d. The annual fading at 50 °C was found to be 45% compared with 30% for LiF : Mg,Ti.

Further improvements of the fading characteristic may be found by pre-heating at temperatures higher than 100°C or by longer annealing periods in the range of 100 °C, both of which may simulate longer annealing periods at ambient temperature [8]. For instance, pre-heating at 120°C/20 min is sufficient for LiF: Na,Mg to reduce the fading to practically zero even for storage at 50°C/50 d. For LiF: Mg,Ti, on the other hand, pre-heating at 100°C/15 h reduces the fading more effectively than pre-heating at 140 °C. A further increase of temperature does not improve fading results.

The reduction of fading as a function of the pre-heating temperature is presented in figures 11.13 and 11.14 for LiF:Mg,Ti and CaF2:Dy after 20 d storage at 50 °C. In contrast to pre-heating in the reader, an extended pre-heating in the oven may reduce the fading from 17% and 60% without pre-heating to values of the order of 5% at 130 °C/ 20 min. An extended pre-heating of CaF2: Dy, however, reduces sensitivity in both cases of the order of 20%.

PREHEATING PERIOD AT 100 *C IN MINUTES PREHEATING PERIOD AT KO*C IN MINUTES

Figure 11.13. Fading reduction by applying a pre-heating treatment at 100°C [8] for LiF:Mg,Ti (left) and CaF2:Dy (right) after 20 d storage at 50 °C.

t0 60 80 100 120 UC

POST-IRRADIATION TREATMENT AT °C

40

O

5 30 < u. g 20

10

20 tO 60 80 100 120

POST-IRRADIATION TREATMENT AT °C

Figure 11.14. Fading reduction by applying a pre-heating treatment at higher temperatures for LiF:Mg,Ti (left) and CaF2:Dy (right) after 20 d storage at 50 °C.

210 E Piesch

On the basis of post-irradiation annealing treatment at 100 C/20 min, the long-term fading at 20 C is sufficient for most of the TL materials investigated. For an application in environmental monitoring, however, extended pre-heating in the oven but not in the reader may improve the fading characteristic (see table 11.4).

Table 11.4. Application of pre-heating for fading reduction.

Material Pre-heating treatment

f Fading after 20 d of storage at 50 °C.

Relative response

Fadingf (%)

LiF:Mg,Ti

LiF:Na,Mg Li ;B407:Mn, Si CaF2:Dy

100°C/5h 135°C/20 min 120 "C/20 min 100°C/2h 100°C/16h 130°C/20min

0.8 0.62 0.9 0.78 0.3 0.2

3 3 1

20 2 5

The standard test programme makes use of a temperature treatment at 70 °C over a period of 10 d in order to simulate environmental fading effects and to check, more or less, the post-irradiation annealing technique applied to the system. The high scatter of the fading results presented in figure 11.15 mainly resulted from variations in the annealing treatment applied in the laboratories. Discrepancies in published fading data [21, 38] may be attributed to different TLD systems which may vary in the TL phosphor, the kind of activator and, above all, the annealing and read-out technique.

For the TLD systems adopted for an environmental monitoring programme the applica­tion of supplementary techniques for post-irradiation annealing, read-out and calibration should be provided.

Further fading corrections can be achieved by measuring the fading at the field loca­tion itself. A good approximation of the time-dependent fading process in the environ­ment can be found by the Randall-Wilkins theory [9]. In table 11.5, formulae of interest are presented which can be applied to the different kinds of field and/or control exposures before or during the field exposure period. In estimating the real fading effect, uncertainties arise from:

(a) the radiation field, which shows fluctuations of the exposure rate or several short-term exposures;

(b) the ambient temperature profile during the field exposure; (c) the knowledge of the fading rate, which is inconstant in Tand t due to several glow

peaks with different functions of \(T) and dependent on the annealing treatment before read-out;

(d) a constant self-irradiation due to trace radioactivity in materials used for the encapsulation of the TL phosphor;

(e) the fact that, under practical conditions, no exponential fading function has been found after longer periods of storage.

Application of TLD systems for environmental monitoring 211

2 ( 6 8

STORAGE PERIOD IN DAYS

2 4 6 8 10

STORAGE PERIOD IN DAYS

z o o Si K

Ul Q :

100

80

fin

40

20

n

^ Li2 B407

\ \ \

\v\ V v \ w. — - ^ ® =3 ®

®

0 2 4 6 8 10 0 2 4 6 8 10 STORAGE PERIOD IN DAYS S T 0 R A S E p E R | 0 D | N „„

Figure 11.15. Fading of various TLD systems after storage at 70 °C [2].

Table 11.5. Formulae for fading correction.

Initial exposure X0 at time t = 0:

It = aX0 exp(-A.f) =/„ exp(-Xf).

Constant exposure rate X during field exposure:

It = a(X/X)[l - e x p ( - X f ) ] .

Pre-exposure before or during the field exposure period for subgroups of dosemeters:

It = I0 exp[ - \ ( f - f,-)] + {aX/\)[\ - e x p ( - \ f ) ] .

Actual single exposures X, at time t( during the field exposure period:

It = 2 / , -exp[- \ (f - r ( ) ] + ( a i / \ ) [ l - e x p ( - W ) ] -i

where It = light intensity at time t, a = conversion factor, X = \(T) thermoluminescence fading rate dependent on ambient temperature profile, X = exposure rate during field period.

212 E Piesch

11.3.8. Environmental effects

Environmental agents such as light, friction, moisture and humidity may affect the TLD signal and the zero-dose reading of unexposed dosemeters in an uncontrolled way. The creation of spurious TLD signals and the simultaneous loss of the latent dose information may occur due to:

(a) contamination of the TL phosphor; (b) tribothermoluminescence; (c) stimulation of the TLD by visible or uv light; (d) oxygen atmosphere during read-out; (e) humidity and moisture.

From the TLD materials of interest, it is mainly LiF which shows the lowest influence and the best insensitivity to these effects. For example, the TL dirt present in the environ­ment of Chalk River simulates an apparent exposure to TLD 100 as illustrated in table 11.6. A few micrograms of dirt may contaminate the TLD and could give rise to signals

Table 11.6. Influence of contaminants on the TL read-out [33].

Contaminant Thermoluminescence (apparent R mg"1)

Sandy Sub-soil Top soil Sand from ash tray Road sand

0.6-0.7 0.14-0.22

0.2-0.4 2.7-5.2

<

£ 1.0

100 200 100 200 300 too 300 "0 TEMPERATURE C O

Figure 11.16. (Left) Dosemeter response to friction (rubbed with sharp tweezers three times); and (right) glow curves of different dosemeters for 5 mR exposure taken with Corning 4303 filter and argon flow [10].

Application of TLD systems for environmental monitoring 213

comparable with those measured in environmental dosimetry [33]. The contamination effects call for rigorous cleaning before read-out or a dosemeter package which effectively excludes dirt (see also §§4.9 and 7.2.7).

The effect of tribothermoluminescence caused by rubbing the detector with PVC foils or by sharp tweezers during preparation and handling shows glow peaks similar to that of radiation-induced TL. Careless handling of the tweezers may increase the background reading of magnesium silicate and calcium sulphate by a factor of 2-12, arid for LiF by less than a factor of two (figure 11.16) which may simulate a significant exposure compared to the flow curves presented for 5 mR [10] (see §§4.6, 4.7 and 7.2.6).

The effect of visible or uv light exposure increases the zero-dose reading of unexposed TLD (figures 11.17 and 11.19) but may also reduce the signal effectively as is shown for white plastic shielded CaF2 : Dy during a three-month field monitoring period (figure 11.18). For environmental application, only a light-tight package (opaque paper and black polyethylene foil) may protect the TLD against long-term light exposure.

100 200 300 TEMPERATURE C O

Figure 11.17. Glow curves of the dose-meters exposed to 1 h long fluorescent light (2000 lx) [10].

100

0 20 10 60 TIME FROM EXPOSURE (d )

Figure 11.18. Fading of environmental monitoring TLD (the symbols in parentheses in the key refer to the differ­ent filters: PI, plastic;Cw, clear plastic, white paper).

214 EPiesch

100

50-

* Mg2Si04Tb CaS04:Dy LiF.Mg.Ti LiF-7

10°

^

10

EXPOSURE TIME IN min Figure 11.19. Increase in the dosemeter background (lower) and fading of 0.1 rad response (upper) owing to fluorescent light (2000 lx) [10].

In most of the TLD phosphors no significant effects of moisture and humidity were found for chips and Teflon detectors. The reproducibility of measurement, however, is highly affected by humidity, resulting in a high scatter [7]. In Li2B407:Mn in Teflon the fading effect .is superimposed by a damage effect which occurs after long-term storage at high humidities and is strongly influenced by temperature (figure 11.20). An inert-gas atmosphere is of high importance to quench spurious luminescent signals during low-dose read-outs. Nitrogen and argon, having an oxygen content of less than 0.1%, are used resulting in a reduction of zero-dose reading by a factor of 8-16 (figure 11.21) [10].

11.4. Calibration technique for dosemeter batch and reader

Since the relative sensitivity of TLD or the TL response in milliroentgens is a system-dependent parameter, any variations in the reader conditions, the annealing treatment or the read-out parameters may cause a significant change in the relative sensitivity. For the measurement with TL dosemeters, therefore, the reader and each batch of field dose-meters have to be calibrated individually by additional control dosemeters.

For the use of TLD in environmental monitoring, extensive calibration techniques have to be applied for sufficiently qualified systems to reach a precision of about 10%. The procedure for measurement and calibration is illustrated in figure 11.22. In addition to

Application of TLD systems for environmental monitoring 215

423 DAYS

180 DAYS

275 DAYS

423 DAYS

STORAGE AT

o o 50 "t

• . 40 °C

AFTER

/so l»

DAYS

DAYS

MASON ET AL

""" .

1974

^ * TEFLON

POWDER

20 40 60

STORAGE TEMPERATURE IN °C

20 80 100

Figure meters

40 60

REL.HUMIDITY IN V.

11.20. Li2B407:Mn fading due to humidity (left) and damage in Li ;B407:Mn Teflon dose-due to humidity (right) [7].

E

< > D a

D O

tr

< CD

•AMg2Si04:Tb

• LiF:Mg ,Ti

TLD-100 CaS04:Dy

20 40 60 80 ARGON FLOW RATE IN l/h

Figure 11.21. Influence of argon flow rate on the dosemeter background investigated with the integrating reader. Mg2Si04 : Tb, LiF : Mg,Ti and TLD 100 with the BG-12 filter, reading temperature 250°C; CaS04: Dy without filter, reading temperature 270°C [10].

field dosemeters (BF), two or more groups of control dosemeters are needed to calibrate the system (Bc), to correct for the fading during the field exposure (Bf) and to separate exposures which the dosemeter batch may receive during transit (Bt) or storage (B0) in the laboratory. The control dosemeters are shielded in lead at the laboratory over the whole period (Bc, B0) or after transit to the site during field exposure (Bt). Both

216 E Piesch

Dosemeter batch B

Pre-irradiation annealing

Encapsulate

Field dosemeters Control dosemeters Bf B, Bc B0

fading transit calibr. zero dose

Travel to site

Field exposure B F Bf

Shield exposure Bt

Shield exposure B c B„

Travel back Calibration

Post-irradiation annealing

Read-out

Figure 11.22. Procedure for TLD calibration and measurement.

environmental and control dosemeters should be pre-annealed, post-annealed and evaluated together.

For statistical reasons, at least 10 control dosemeters for both Bc and B0 are needed for calibration. Control dosemeters are exposed in the range of 1 R, or 1000 times the dose of the lower detection limit D ld]. The unirradiated control dosemeters B0are used to correct the zero-dose reading which includes the PM dark current, the zero-dose reading of unirradiated TLD as well as the dose absorbed from cosmic rays and any internal radio­activity. Additional information about the exposure rate in the shield has to be considered to enable the estimation of the zero-dose reading and the exposure received during the storage separately.

The field exposure may be calculated from each TLD reading /,• by using the following equation:

Ci Ii-Io (11.1)

where D = field exposure in mR, // = read-out of field dosemeter (BF),/ t = mean read-out of control dosemeters (Bt) for transit exposure corrected for exposure in the shield, /j = mean read-out of control dosemeters (Bc) in the shield exposed to Dx (calibration dosemeters), 70 = mean read-out of control dosemeters (B0) in the shield during field

Application of TLD systems for environmental monitoring 217

exposure, c,- = calibration factor of the single TLD, C1 = mean calibration factor of control dosemeters (Bc).

Here the control dosemeters for the calibration of the batch (ct =I>i(/i — I0)~l) are

measured together with the field dosemeters, whereby the individual calibration of each TLD (Cj = D2(l2~A))-1) was found before or after the field cycle by an additional calibration exposure D2 of the batch.

Individual dosemeter calibration is advantageous if

(a) a permanent identification of individual dosemeters is feasible; (b) the variation in response within a batch is higher than the variation in response of

a single dosemeter after repeated use (system-dependent parameter); (c) the variation of response (batch uniformity) is higher than ±3%; (d) only a small number of TLD is involved.

Table 11.7. Properties of light sources in TLD readers.

No. Light source Internal Relative Activity Reading radioactive light (nCi) time

Reader Scintillator nuclide intensity (s)f (ncds -1)

1 u i, m n n * Nal (TL) Ra.o: 47 , . . 2 Harshaw2000A N a J ( T L ) ^ Q % ^ 100 3 Toledo Plastic "C.p 38.5 5000 § 0.45

■f Light source reading time to achieve a standard deviation of 0.5% (s value from 10 readings). t Data from the manufacturer Pitman Ltd, UK. § Calculated on the basis of light source 3 as a reference and a statistical error comparison.

For the internal calibration of TLD readers, reference light sources are mainly applied (table 11.7) which may show significant variations in light intensity and reproducibility [42, 43] (see also §§3.3.5, 4.4 and 7.2.1):

(a) on the basis of a daily check with internal and external light sources the day-to-day response of the Harshaw 2000 A + B reader was found to overlap the seasonal fluctuations by about 1% and the Toledo 651 reader of the order of 6% (figure 11.23);

(b) due to nitrogen effects the plastic scintillator in the Toledo reader shows changes of the reader response of up to 5% during a working day;

(c) the temperature dependence of the light intensity was found to be — 0.05% °C_1

for the plastic scintillator and — 0.3% "C1 for the Nal (TL) scintillator (figure 11.24); id) by using the internal light sources, a standard deviation of 0.5% was found after a

reading time of 0.45 s for the Toledo reader and 100 s for the Harshaw reader (figure 11.25);

(e) due to the 210Pb content in the Nal (TL) scintillator a time-dependent decrease in light intensity of 3% yr"1 was found for the internal light source in the Harshaw reader.

218 E Piesch

TOLEDO 661 READER

«>v^^/ V v 4

^ HARSHAW 2000 A READER

TEMPERATURE IN HARSHAW 2000 A READER

20 VV^v>A f / \^vA^w/vVVW^

T 1977 1978 1979

Figure 11.23. Long­term fluctuation of external light source check [43].

106

10.

102

j . 100 z

g 98 a < UJ °l 96 UJ or

91

> S T

1

_ i

i

• —•

K. ■ — . ^

'

1

T |

r-v ii '

1 NaI(TI| ."c Nal (TI|.™Ro IN KARS

PLASTIC

HAW READER

SCINT. "C IN TOLEDO READER

20 30 (0

LIGHT SOURCE TEMPERATURE IN t

Figure 11.24. Relative reading of light sources as a function of light source temperature immediately before read­out [43].

The light source calibration of TLD readers may be significantly improved by applying light sources with a low temperature characteristic, a low standard deviation of the light intensity and insensitivity to nitrogen gas flow.

Application of TLD systems for environmental monitoring 219

NaI[Tl) . Rn (•—•!

Nal(TI).uC(<>—o|

SO 100

Figure 11.25. time [43].

5 10 20

READING TIME IN SECONDS

Relative standard deviation of the light source reading as a function of reading

11.5. Reproducibility and overall uncertainty of measurement

For a quantitative interpretation of the standard deviation as a function of exposure it is useful to distinguish between reader- and batch-dependent error sources.

(a) Reader parameters are, for instance, the photomultiplier dark current, the quality of the heating planchet and deviations in the maximum heating temperature during the evaluation.

(b) Dosemeter batch parameters are mainly the batch uniformity and the zero-dose reading dependent on the batch quality and the irradiation/annealing history of the batch.

A quantitative explanation of the standard deviation against exposure curve was found by a two-parameter fit given by the following formula:

\ l /2 (11.2) 1

s(D) = - [2(s20e + sld)(D'u - D 0 ) 2 + ( 4 + s2

d) D2)l/2 (A1 \"

where s0e = relative standard deviation for the dark current a0, sod = relative standard deviation for the zero-dose reading of unirradiated dosemeters au = au — a0, srd = relative standard deviation for an exposure to 1000£>idl (batch uniformity and history), sre = rela­tive standard deviation for an exposure to 1000 D\di (reader properties),/) = exposure in mR, Du = exposure equal to zero-dose reading au of the unirradiated dosemeter batch after subtraction of a0 with Du = D'u — D0, B = relative standard deviation at high doses, A = absolute standard deviation at very low doses.

Due to the subtraction of the zero-dose reading the standard deviation is found to be high for low exposures. The s value decreases as a function of exposure, reaching a

220 E Piesch

z

z g

£

100

0.1

V^s„,Dui

l ( D ) - 1 0 0 ^MJ^T

IV. ]

' for 0-100 DLDL

s(D)depends on a,b,d

LOWER DETECTION LIMIT Duii = 3 "VBu 1

a ZERO DOSE READING b READ-OUT TECHNIQUE c BATCH UNIFORMITY/CALIBRATION d ANNEALING/ IRRADIATION HISTORY

- J & 2 2

for D-100DlDl s(D)depends on b.c.d

io-' 10° 10' & IO3 10

EXPOSURE D IN mR Figure 11.26. Analysis of the standard deviation against exposure curve [44]; factor of -Jl only if subtraction of Du is based on a single zero-dose reading.

constant value at higher exposures. The exposure range with a constant reproducibility depends above all on the reader type as well as on the quality of the individual reader.

The test experiment for reproducibility, previously described as a performance criterion for TLD systems, provides an experimental estimation of the standard deviation in the dose range of interest (figure 11.26, see also figure 11.2).

In spite of the high uncertainty and scatter in the s values for exposures at the lower detection limit Djdi the experimental results are sufficiently represented by the theoretical curve [2, 41, 44].

The uncertainty of measurement can be estimated directly from the statistical error of the system during the read-out period taking into account the results of field measure­ments and the read-out of calibration dosemeters. Results of the control dosemeters, unexposed and exposed to D\, are used to calculate the standard deviations su of the zero-dose reading and sr of the reference exposure Di which should preferably be an exposure to 1000 D\A\.

Instead of using equation (11.2), additional control dosemeters may be directly exposed to a dose equal or similar to Dt which is expected from the natural radiation background at the site during the field exposure period.

The overall uncertainty of measurement is given by statistical and systematic errors [31, 34-37] which may be estimated for the dosemeter batch and for each monitoring or read-out period by means of calibration dosemeters. Table 11.8 presents a summary of error parameters resulting from the performance specifications of dosemeters for environ­mental monitoring [1, 46] and expected for optimal systems with respect to the measure of the annual natural background exposure of 70 mR.

Taking into account the experimental results of the standard test programme [2, 45] the calculated overall uncertainty results in values of 15% or 10 mR related to 70 mR for the best dosemeter systems (figure 11.27). For the use of n dosemeters at the same location the statistical error of measurement is reduced by a factor of n _1 /2

11.6. Interpretation of field exposures

TLD systems are applied to measure time and space variations of the natural radiation background and to monitor an increase in the background level due to additional

Application of TLD systems for environmental monitoring

Table 11.8. Uncertainty of solid-state dosemeter systems at 70 mR [45].

221

Parameter

Statistical errorst Dose reading (single dosemeter with

subtraction of zero dose) Calibration

Single dosemeter Dosemeter system Non-linearity

Fading correction Instability reader/cleaning for glass Transport dose

Relative error

Permissible valuef

±5

±3 ±5

(±3) ±3 ±2 ±5

(%)

Optimal system

TLD

±3

±2 ±1

±3 ±1 ±3

RPL

±5

±2

±3 ±3

Systematic errors % Energy dependence Direction dependence Calibration Fading Light and humidity influence Long-term stability Transport dose, zero dose, etc

Overall uncertainty, Ug5% Statistical ( /R Systematic (/g

±20 tlO ±5 ±5 tlO ±5 ±3

±20 ±29.2

±35.3

±5 ±3 ±5 ±5 ±3 ±3 ±3

±12.9 ±12

±17.6

±5 ±3 ±5 ±5 ±3 ±5 ±3

±15.7 ±12.8

±20.2

f Requirements for dosemeter systems for environmental monitoring related to a measure­ment with one dosemeter including calibration. t Relative standard deviation in per cent. § Maximum measuring errors in per cent.

radiation around nuclear plants or to gaseous effluents released from nuclear power stations. Two different environmental programmes are generally applied:

(a) monitoring of time and local dependent variations in exposure by using TLD during short-term field periods of 2 or 4 weeks;

(b) monitoring of local variations in the total annual exposure during field periods of half a year or a year.

Fluctuations in the background radiation with both time and location are principally due to changes in soil moisture and surplus water which dilute and shield natural sources in the ground and have been found to be of the order of ±0.7 JLIR h_1 or ±20% of the terrestrial gamma radiation [11]. Supplementary techniques of calibration and data correction are applied to eliminate the background component. For the interpretation of

222 E Piesch

30

20

10-

TOTAL UNCERTAINTY U R » U S

STATISTICAL UNCERT. UR

LiF:Mg,Ti

H| | H | 1 P | |HA||HA

Li2B407

SAMNA

CaF2

S A M H M H B

CaS04

N B L L L

RPL

[Mil

P PITMAN S STUDSVIK N NATIONAL T TELEDYNE

A AUTOMAT B BULB DOSIMETER

35 I 3 1 9 17 IB 26 27 30 28 32 33

DOSIMETER SYSTEM No

Figure 11.27. Calculated overall uncertainty of TLD systems at 70 mR on the basis of experimental results [2] and data from table 11.8 (after [45]).

TLD data, various approaches [11-15] may be used in order to isolate time-varying exposures from both components (see table 11.9).

To analyse source fluctuations in the range of 0.1 /iR If1, a qualitative estimate of the exposure is needed, taking into account effluent release, wind frequency direction data and the stack location distance. For a continuous emission of a noble radioactive gas mixture from a 100 m stack, the exposure rate decreases approximately with the inverse of the radius [11].

The assumption of a time- and space-invariant background is not strictly valid because both the background exposure and its variation may differ significantly from one site to another and may fluctuate during the seasons. Climatological data have been used with

Table 11.9. Interpretation of environmental measurements.

Source fluctuations Effluent-wind frequency model

Background fluctuations

Invariance of background with location

Invariance of background with time

Climatic exposure model

Correlation of local exposure to qualitative estimate using a wind frequency-weighted 1/R model

Exposure changes at reference location equal to nearby locations Measure of the total background exposure prior to or during reactor shutdown Semi-empirical correction for soil water and surplus water calculated from local climatological data

Application of TLD systems for environmental monitoring 223

considerable success to allow for the seasonal fluctuations of the terrestrial gamma radiation [1,12,13,39].

Long-term field exposure periods of the background radiation result in a mean annual exposure which significantly compensates for short-term and seasonal fluctuations of the local exposure.

To determine dose contributions due to the emission of nuclear power stations, the local individual background dose must be taken into account. For different measuring stations an average background dose can also form the basis which, by definition, may be the local dose rate averaged over the area prior to operation of the reactor or a measure­ment of the local dose at a reference location. Difficulties of interpretation arise, how­ever, if local fluctuations in the natural background radiation and statistical measuring errors from both measurements result in calculated negative exposure values.

The measuring uncertainty increases due to subtracting the background dose. The lowest detectable dose which is found for a difference measurement is thus given by the amount of the background dose and the quality of the dosemeter system. Figure 11.28 shows the lowest detectable dose which can just be interpreted as a contribution of a nuclear plant as a function of the difference (Z)2 —/),) where D2 and D^ are the annual accumulated dose values at the location of interest and the corresponding background dose, respectively. For an application in environmental monitoring, therefore, a relatively high measuring accuracy and reproducibility in the dose range of 10 mrem is required.

Groups of five, three or two dosemeters are used to improve the reproducibility of measurement at a single field location. The precision of measurements for short-term exposure periods was found to be better than 3% for 3 mR with LiF by using the average over five dosemeter results [11]. This permits a quantitative assessment of variations as small as a few milliroentgens per year in the environmental gamma radiation field.

PERMISSIBLE OVERALL UNCERTAINTY

OPTIMIZED DOSIMETER SYSTEMS

DIFFERENCE Dj -D, IN mR

Figure 11.28. Overall uncertainty for a significant increase in the natural background level of 70 mR.

224 EPiesch

11.7. Practical application

For environmental monitoring, different TLD systems are in use. Generally, there is no need to differentiate between the soft and penetrating components of the environmental radiation, if approximate tissue-equivalent dosemeters such as LiF are shielded by 500 mg cm"2 of plastic. On the other hand, the additional beta sensitivity of unshielded LiF has to be considered, resulting in an increase of dose reading due to the natural back­ground radiation, which was found to be (14 + 4)% for a shielding of 50 mg cm"2 of plastic and an exposure 2 m above ground.

The energy spectrum from environmental natural 7-rays shows a soft component of 27% of the total photon flux density in the energy range below 100 keV. Therefore, the dependence of the dosemeter response on energy cannot be neglected if CaF2 or CaS04 detectors are used [16, 19, 22-27, 30, 32]. At Savannah River Plant, CaF2 dosemeters are positioned behind a silver and a plastic filter to detect the presence of photons with energies below 100 keV. For natural background exposure, the average response of the dosemeters behind the plastic shield was (30 ± 8)% higher than that for the one behind the silver shield [29]. At Karlsruhe, stray radiation and 'skyshine' from the waste disposal site near the fence lead to a three-fold over-estimate of unshielded CaF2 compared to LiF or CaF2 shielded by a perforated tin filter [18].

Shielded CaF2 dosemeters have been used to measure the mean exposure level during the flight time in different types of aircraft [36]. According to the altitude of the flight, the exposure level for a DC-10, Boeing 707 or 747 is about five times higher than for a Boeing 737. The transit exposure of detectors mailed to and from the field site may vary from between lmR and 100 mR due to contributions from cosmic radiation and/or additional exposures from radioactive sources during transit [3].

Up to now, different solid-state dosemeters have been used for the monitoring of the environment of nuclear faculties. At Karlsruhe Nuclear Research Centre, dosemeters were distributed at more than 250 field sites along the fence and in circles of 1, 2 and

Table 11.10. Measuring accuracy for background measurements in the environ­ment of Kf Z Karlsruhe.

Dosemeterf

Glass

LiF:Mg,Ti CaF2:Dy Dose rate meter

Field period

6 month 6 yr 6 month 4 weeks Direct reading

Mean value of annual dose (m

58±12 64±4 70±5.5 59±4|| 60 ±6

iR)t 2CJ value (%)§

20 6 9 7

10

+ Glass and CaF2: Dy in spherical capsule, LiF: Mg, Ti covered by 50 mg cm"2. t Average of about 80 measuring points along the fence of KfZ with 2a standard deviation. § Statistical measuring error determined from the deviation of the measured value of double dosemeters or double measurement at each measuring point. II Without 41Ar emission from FR 2.

Application of TLD systems for environmental monitoring 225

3 km diameter. For the interpretation of the dose profile measured at the fence, we have to consider the emission of 41Ar from the FR2 reactor in the main direction of the 41Ar exhaust air plume, 'skyshine' from the waste storage site and direct radiation from the Institutes.

The reproducibility found for the measurement of the natural background for the different dosemeters used is presented in table 11.10. In addition to the TLD, phosphate glass dosemeters have been in use since 1966 for exposure periods up to 6 yr. Due to the 41 Ar exhaust, an increase of the natural background level has been found to be of the order of 5 mR yr-1 at the area boundary and, as expected by calculation, a maximum of about 20 mRyr"1 at the northeast corner of the fence. CaF2:Dy dosemeters are applied for short-term monitoring of the fence near the radioactive waste storage site [14, 17, 18].

The reproducibility of dose measurement can be estimated for each individual field cycle by using the frequency distribution found for the difference in dose readings of two dosemeters exposed at the same field site.

A comparison of LiF and ionisation chamber results [28, 35] shows that the average agreement for the total period indicated is within 0.5% and the mean difference for a 4 week measurement period is 2.7%. It is believed that the overall accuracy for these types of terrestrial 7-ray plus cosmic-ray measurements is about ± 5% (standard deviation).

The method for analysing TLD data collected fortnightly or monthly makes use of the frequency plot and log normal probability plot for each location (figure 11.29) [40].

20 30 U) 50 60 SAMPLIN6 PERIOD

2 20 1.0 60 80 98 99 CUMULATIVE PERCENT

Figure 11.29. Frequency and log normal probability plot for a location in the Hanford environs [40].

226 EPiesch

From the log normal plots the geometric mean (xg:50% intercept) and geometric standard deviation or slope (ag: ratio of 84% to 50% intercept) were determined. The 95% probability intercept of the plotted data means that 95% of all measurements are expected to be less than this value.

A semi-empirical method for the estimate of the time-varying background exposure expected at a particular location is based on detailed soil moisture considerations [12,13]. The climatological model takes into account local meteorological and hydrological data. Variations in the surplus water (difference between precipitation and evaporation of the water content of the soil) mainly reflects natural background changes. The model agrees with the TLD results (accuracy 3.5%) within 5% in 75% of the cases, 10% in 94% of the cases and 15% in all cases [13].

References

1 American National Standard 1975 Performance, testing and procedural specifications for thermo-luminescence dosimetry (environmental applications) ANSI N545-1975

2 Piesch E and Burgkhardt B 1978 RTL und RPL systeme im Bereich kleiner Dosen: Vorstellung eines Testprogrammes und Ergebnisse an 43 Systemen Rep. KfK 2626

3 Burke G de P, Gesell T F and Becker K 1977 Second international intercomparison of environ­mental dosimeters under field and laboratory conditions Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p365

4 1975 Environmental Radiation Measurement. NCRP Rep. No 50 5 Piesch E 1974 Solid state dosimetry - 10 years of routine use and development Kerntechnik 16 71 6 Burgkhardt B, Herrera R and Piesch E 1976 Fading characteristics of different thermoluminescent

dosimeters Nucl. Instrum. Meth. 137 41 7 Burgkhardt B, Herrera R and Piesch E 1977 Long-term fading experiment with different TLD

systems Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p75

8 Burgkhardt B and Piesch E 1978 The effect of post-irradiation annealing Nucl. Instrum. Meth. 155 293-9

9 Randall I T and Wilkins M E F 1945 Phosphorescence and electron traps Proc. R. Soc. A 184 366 10 Niewiadowski T 1976 Comparative investigations of characteristics of various TL dosimeters, part

II, Low dose measurementsNucleonika 21 1097 11 Burke G de P 1972 Thermoluminescent dosimeter measurements of perturbations of the natural

radiation environment Proc. 2nd Int. Symp. on Natural Radiation in the Environment p305 12 Burke G de P and McLaughlin J E 1974 Performance criteria for environmental radiation monitor­

ing with TLD systems IEEE Trans. Nucl. Sci. NS-21 444 13 Burke G de P and Marcin D G 1973 Interpretability of TLD measurements made in environs of a

nuclear power reactor Trans. Am. Nucl. Soc. November, p537 14 Piesch E 1977 Long-term dosimetry with solid state dosimeters for personnel and environmental

monitoring Kerntechnik 19 27 15 Lindeken C L et al 1972 Geographical variations in environmental radiation background in the

United States Proc. 2nd Int. Symp. on Natural Radiation in the Environment p317 16 Burke G de P and Shambon A 1972 Investigation of thermoluminescent dosimeters for environ­

mental monitoring Rep. HASL-265 17 Burgkhardt B and Piesch E 1972 Use of CaF2 thermoluminescent dosimeters for measuring the

natural background radiation Kerntechnik 14 128 18 Burgkhardt B, Piesch E and Winter M 1973 Long-term use of various solid state dosimeters for

environmental monitoring of nuclear plants - experience and results Proc. 3rd Int. Congr. IRPA, Washington p394

19 Shambon A 1974 CaSO„:Dy TLD for low level personnel monitoring Rep. HASL-285

Application of TLD systems for environmental monitoring 227

20 Konig LA, Piesch E and Winter M 1974 Die 7-Strahlenbelastung der Umgebung des Kern-forschungszentrums Karlsruhe Proc. Jahrestagung des Fachverbandes fur Strahlenschutz, Helgoland p615

21 Becker K 1974 Stability of film and thermoluminescence dosimeters in warm and humid climates Atomkernenergie 23 267

22 Becker K 1974 Integrating dosimeters for environmental radiation assessment Rep. Conf-741219-1 23 Duftschmid K E1975 Evaluation of CaF2:Dy bulb thermoluminescent dosimeter for environmental

radiation monitoring around nuclear facilities Rep. SGAE Ber. no 2462, St-45/75 24 Duftschmid K E and Strachotinsky Ch 1974 Entwicklung und Erprobung hochenergetischer Dosi-

metersysteme in Hinblick auf die Erfassung der radioactiven Umgebungsstrahlung in Osterreich Rep. SGAE Ber. no 2543, ST-49/75

25 Szabo P P 1975 Investigation of properties of CaS04:Dy thermoluminescent dosimeters Rep. KFKI-75-1

26 Budd T 1976 The properties of CaF ; : Dy used as a thermoluminescent dosimeter at low doses Rep. AERE-R8385

27 Gwiazdowski et al 1974 The comparison of long-term parameter stability of various RTL detectors used for measurements of natural background radiation Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), p 989

28 Lowder W M 1977 Environmental radiation dosimetry with ionizing chambers and thermo­luminescence dosimeters Proc. 4th Int. Congr. IRPA, Paris vol 1, p59

29 Lorrain S, Piaggio-Bonsi R and Portal G 1977 Stabilite de divers sulfates de calcium RTL destines aux mesures d'environnement Proc. 4th Int. Congr. IRPA, Paris vol 4, pl249

30 Vana N, Aiginger M and Erath W 1977 Measurement of doses in the 1 mrad range by means of LiF, CaF2 and CaS04 dosimeters Proc. 4th Int. Congr. IRPA, Paris vol 4, p 1253

31 Piesch E and Burgkhardt B 1977 TLD and RPL dosimeter performance criteria for environmental monitoring based on type test and long-term experience Proc. 4th Int. Congr. IRPA, Paris vol 4, p i 245

32 Toombs G L and Paris R D 1977 Comparative response of thermoluminescent dosimeters in environmental monitoring situations Proc. 4th Int. Congr. IRPA, Paris vol 2, p525

33 Jones A R 1977 The application of an automatic thermoluminescence dosimetry system to environmental gamma dosimetry Rep. AECL-T8 35

34 Burke G de P and Gesell T F 1976 Error analysis of environmental radiation measurements made with integrating detectors Proc. NBS SP456, p l87

35 McLaughlin J E 1976 Environmental radiation measurements Proc. NBS SP456, p233 36 Kramer R, Regulla D F and Drexler G 1977 TLD environmental radiation monitoring: processing,

experiences and data interpretation Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p298

37 Piesch E and Burgkhardt B 1977 Properties of TLD and RPL systems for environmental monitor­ing Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p335

38 Christensen P, Botter-Jensen L and Majborn B 1973 Influence of ambient humidity on TL dosi­meters for personnel monitoring Proc. Regional Conf. on Radiation Protection, Jerusalem

39 de Planque G 1980 TLD measurements and model calculations of environmental radiation exposure rates Proc. 3rd Symp. on Natural Radiation in the Environment CONF 780422, vol 2, p987

40 Fix J J and Blumer P J 1977 Thermoluminescent dosimeters (CaF2:Dy) measurements of the Hanford environs, 1971-1975 Rep. BNWL-2140

41 Burgkhardt B, Piesch E and Seguin H 1980 Some results of a European interlaboratory test pro­gramme of integrating dosimeter systems for environmental monitoring Proc. 5th Int. Conf. IRPA, Jerusalem and Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April. Nucl. Instrum. Meth. 175 No 1 (September)

42 Nollmann C, Burgkhardt B and Piesch E 1979 Parameters effecting the overall calibration accuracy in TLD 700 thermoluminescence dosimetry Nucl. Instrum. Meth. 161 449-58

43 Burgkhardt B and Piesch E 1981 Systematical and statistical errors of the TLD reader calibration with reference light sources Health Phys. 40 549

228 EPiesch

44 Burgkhardt B and Piesch E 1980 Reproducibility of TLD-systems - a comprehensive analysis of experimental results Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse, April, Nucl. Instrum. Meth. 175 No 1 (September)

45 Piesch E 1979 Anforderungen an Festkorperdosimeter zur Messung der integralen Gammadosis in der Umgebungsuberwachung Proc. Fachgesprdch Uberwachung der Umweltradioakrivitat p84

46 'Technische Empfehlungen fur Festkorperdosimeter zur Umbegungsuberwachung'desArbeitskreises 'Dosismessung externer Strahlung' des Fachverbandes fur Strahlenschutz, in Vorbereitung

47 Shambon A 1972 Some implications of a laboratory study of LiF dosimeters for environmental radiation measurements Rep. HASL-251 (New York: USAEC)

48 Binder W, Disterhoft S and Cameron J R 1969 Dosimetric properties of CaF2Dy Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF-680920, p43

49 Becker K 1973 Solid State Dosimetry (Cleveland, Ohio: CRC Press) 50 Burke G de P 1972 Investigations of CaF2:Mn thermoluminescent dosimetry system for environ­

mental monitoring Rep. HASL-252 (New York: USAEC) 51 Brinck W, Gross K, Gells G and Partridge J 1975 Special field study at the Vermont Yankee

nuclear power station, Personal communication (US Environmental Protection Agency, Cincinnati, Ohio)

52 Aitken M J 1969 Low-level environmental radiation measurements using natural calcium fluoride Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF-680920,p281

53 Schulman J H 1967 Survey of luminescence dosimetry Luminescence Dosimetry, Symp. Ser. 8 ed. F H Attix (Washington: USAEC), p3

54 Denham D H, Kathren R L and Corley J P 1972 A CaF2: Dy thermoluminescent dosimeter for environmental monitoring Rep. BNWL-SA-4191 (Battelle Northwest Laboratories, Richland, WA)

55 Yamashita T, Nada N, Onishi H and Kitamura S 1971 Calcium sulfate activated thulium or dysprosium for thermoluminescence dosimetry Health Phys. 21 295

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

12 Applications ofTL materials in neutron dosimetry

J A DOUGLAS

12.1. Introduction

Because of the complex nature of neutron dosimetry some background information is provided. The two-stage process involved in transferring energy from neutrons to matter is discussed and the reactions categorised. The choice of appropriate parameters to monitor the effect of interest is then presented before the terms used in neutron dosi­metry are defined.

The response of TL materials to neutrons depends greatly on the neutron energy so the dosimetry of thermal neutrons is dealt with separately from that of intermediate and fast neutrons. Possible methods of increasing the low response to fast neutrons are surveyed together with possible applications of these techniques. Finally some of the possible future developments for fast neutron dosimetry are discussed.

12.2. Neutrons and dosimetry

The dosimetry of neutron radiations is not as simple or as precise as the dosimetry of gamma radiations because of the numerous energy transfer processes involved and the variation of the reaction cross sections with energy. The transfer of energy from neutrons to a medium is a two-stage process involving the production of ionising particles or radiation either in the medium or in the material adjacent to it. These secondary radiations dissipate the transferred energy by undergoing electronic and nuclear collisions in the medium.

12.2.1. Neutron reactions

Neutron reactions producing ionising secondaries can be divided into the following categories:

(1) Scattering (a) Elastic: the momentum and kinetic energy are conserved. (b) Inelastic: the neutron is re-emitted with a reduced energy, leaving the target nucleus

in an excited state. The nucleus returns to the ground state by emitting a gamma-ray.

230 J A Douglas

(2) Capture (a) Radiative: the excess energy resulting from the absorption of the neutron is

released almost instantaneously by the nucleus as a capture gamma-ray. (b) Particle emission: if the neutron energy is high enough a transmutation will occur

with the ejection of a proton or alpha particle, e.g. 6Ii(n, a)3H and 10B(n, a)7 l i . (c) Fission: the neutron is absorbed and the nucleus splits into two fragments plus

neutrons. (d) Spallation: the nucleus is fragmented ejecting several particles and nuclear frag­

ments. Only significant above 20 MeV.

These reactions can occur either in a TL detector or its environment and will affect its response. Similarly they can occur in the material being monitored by the dosemeter.

Most chemical and biological effects, for a given type of radiation, are directly related to the energy absorbed. However, the microscopic spatial distribution of this absorbed energy is also important, especially in biological material where a high rate of energy deposition along the track of an ionising particle is more effective at killing a cell than is a low rate. Thus, in addition to the absorbed energy, the energy spectrum of the incident neutron is important in chemical and biological systems since this determines the rate of energy deposition by the recoil particles.

The damage caused by neutrons to materials with a lattice structure is closely related to the number of atomic displacements resulting from collisions, which in turn is related to the number of incident fast neutrons. The incident neutron fluence and energy spectrum are therefore good parameters to specify the damage in this case.

12.2.2. Terminology

The following definitions are mainly taken from ICRUf 26 [44].

(1) Fluence, <$>, of particles is the quotient of dW by da, where djV is the number of particles which enter a sphere of cross-sectional area da.

(2) Fluence rate, 0, is the quotient of d<£ by dr, where d<I> is the increment of particle fluence in the time interval dr.

(3) Energy fluence, ^ , of particles is the quotient of d£fl by da, where d£fl is the sum of the energies, exclusive of rest energies, of all particles which enter a sphere of cross-sectional area da.

(4) Absorbed dose, D, is the quotient of de by dm, where de is the mean energy im­parted by ionising radiation to the matter in a volume element, and dm is the mass of the matter in that element. (The unit is the rad =100 erg g"1. The SI unit is the gray, 1 Gy = 1 J kg-1 = 100 rad.)

(5) Kerma dose, K, is the quotient of dEti by dm, where dEn is the sum of the initial kinetic energies of all the charged particles liberated by indirectly ionising particles in a volume element and dm is the mass of the matter in that element. The units are the same as absorbed dose. Note that kerma is independent of the complexities of the energy transported by charged particles. It also has a defined value for a

f International Commission on Radiation Units and Measurements.

Applications of TL materials in neutron dosimetry 231

sample of vanishingly small size which is embedded in some other material or in free space. This is not the case with absorbed dose. It is therefore often convenient to refer to a value of kerma for a specified material in free space or at a point in a different material. This is the value which would be obtained if a small mass of the specified material were placed at the point of interest.

Because of the finite range of secondaries, the two steps of energy transfer — energy absorbed from the secondaries (absorbed dose) and the energy transferred to the secondaries (kerma) — take place at different points in the medium. Under charged particle equilibrium, kerma and absorbed dose are essentially the same.

(6) Dose equivalent, DE, is the risk-related quantity recommended by national and international bodies for the control of personnel exposed to neutrons. Dose equivalent is the product of absorbed dose, D, the quality factor, QF, and other modifying factors. For external radiation the modifying factors are unity, except for the eyes. The QF is also unity for x- and gamma-rays but is related to the linear energy transfer of the more densely ionising particles and takes into account the different biological effectiveness. Fluence to dose-equivalent conversion factors at specific energies and depths in soft tissue have been published [4, 43, 83]. Figure 12.1 is taken from the data in ICRP 21 [42] in which the dose equivalent per n cm"2 was based on the maximum dose equivalent in a tissue-equivalent sphere 30 cm diameter with a unidirectional broad beam of monoenergetic neutrons. The ideal personnel neutron dosemeter would have a fluence response of that shape.

1-7 —

N

E < j

E 01 i _

"c CD a > 'u cr LU a> c/l o o

IU

10"8

10"9,

--

•r ' I i i i i i i i il

o

■ i

0

1 1 1 11

001 10 0-1 1 Neutron Energy (MeV)

Figure 12.1. Fluence to dose equivalent conversion factors [42]. o, Snyder [84].

(7) Linear energy transfer, LET, of charged particles in a medium is the quotient of dE by d/ where dE is the energy lost by the particle while traversing a distance d/. The unit is usually keV jum-1, or alternatively MeV cm2 g"1.

(8) Lethargy, this is an alternative way of representing neutron energy often used by reactor physicists. The lethargy C/is defined by the equation

dU=-dQnE) = -dE/E.

232 J A Douglas

Therefore

U=\n(E0/E\

E0 is usually taken as 10 MeV. Lethargy increases as neutron energy decreases, e.g.

Energy Lethargy

10 MeV Thermal (0.025 eV)

0 ln(4xl0 8 )=19.8

Fluence can be defined per unit lethargy interval, but it is often plotted against In (energy interval). Figures 12.2(a) and (b) show a \\E spectrum plotted as (a) flux per unit energy and (ft) flux per unit lethargy.

0001 001 0-1 1 ,u 0 . 0 0 1 0 .01 0.: T 10

Energy (MeV) Energy (MeV)

Figure 12.2. 1/E spectrum plotted in terms of (a) fluence per unit energy interval, (b) fluence per unit lethargy interval.

(9) Energy groups. It is sometimes convenient to subdivide neutrons into the following compartments according to their energy:

(a)

(b) (c) (d)

Thermal: Neutrons which are in thermal equilibrium and having a Maxwellian distribution of velocity with a mean at 0.025 eV. Generally neutrons with energy < 0.5 eV are called thermal. Intermediate: 0.5 eV-l 0 keV. Recoil protons < 10 keV are not ionising. Fast: 10 keV-10 MeV. Mainly elastic scattering in tissue. Relativistic: > 10 MeV. Inelastic scattering and capture.

12.3. Thermal neutron detectors

Neutrons are usually accompanied by gamma radiation. Two detectors of widely differing neutron and gamma-ray sensitivities are used to separate them. Table 12.1, which was compiled by Ayyangar et al [6], is a list of thermal neutron responses of various TL materials as quoted in the literature. There are many factors which can affect the reading from a phosphor and may account for some of the apparent discrepancies in the table. Some of these are listed below:

Applications O/TL materials in neutron dosimetry 233

(a) The read-out system. The glow curves resulting from gamma and neutron irradiations are not necessarily the same [20]. The relative response thus depends on the basis of the comparison. Phosphors can be heated either using ramp heating (temperature raised at a constant rate) or plateau heating (temperature raised quickly to a pre-set value and then held for a pre-set time at that temperature, and then possibly repeated at one or two higher temperatures). In the case of ramp heating different results can be obtained by using peak height or the integrated light output. In the case of plateau heating the use of a pre-read-out heating phase and its temperature will affect the apparent response. The type of photomultiplier could also affect the results since the spectrum emitted by a phosphor may depend on the type of incident radiation (see § 12.3.1).

(b) Supralinearity. Deviations from linearity in the light output occur at different doses for gamma and thermal neutron radiations. How this is allowed for will affect the quoted response.

(c) Detector self-shielding. Some thermal neutron-sensitive phosphors contain atoms with a very high cross section (e.g. 945 b for 6Li). In these cases most of the neutrons will be absorbed within a short distance of the surface and the rest of a thick phosphor will be inaccessible. (0.1 mm of TLD 600 absorbs 50% of incident thermal neutrons.) The thickness of a phosphor thus affects its apparent response. So the neutron sensitivity of TL material should always be quoted for a 'thin' detector. The neutron response of a detector will depend on its orientation, unless it is a sphere, since it is the cross-sectional area with respect to the neutrons, not the volume, which is important.

(d) Batch-to-batch variation. Small changes in the concentration of impurities with high cross sections (such as the amount of 6Li in TLD 700) will have a significant effect on the response to thermal neutrons. In spite of the apparent variations in the reported responses, it is possible to choose from table 12.1 phosphors of high and low thermal neutron responses which might be suitable to measure the components in a mixed field.

The following have acceptably high responses to thermal neutrons: 6LiF either as TLD 600 or TLD 100 Li2B407:Mn CaS04(Mn,5Li).

There are many with a low response. The ideal detector or combination of detectors should have the following

characteristics:

(a) the response to neutron and photon radiations should be independent of energy; (6) the response should be linear with dose; (c) the fading should be small; (d) the sensitivity should be adequate to enable doses to be determined for the separate

components of a mixed field to the required precision; (e) the phosphors should be easy to use and re-use; (/) they should be easy to manufacture and cheap.

The characteristics of the three phosphors with the highest thermal neutron responses will now be examined.

134 J A Douglas

Table 12.1. Comparison of the thermal neutron responses of various TL materials as quoted in the literature (compiled in [6]).

TL phosphor Thermal neutron response (R/10 , o ncnr 2 )

Mg2Si04:Tb CaSO„ : Dy (Harshaw) CaS04:Dy (DRP) CaS04 :Tm(DRP) CaSO„:(Dy, 6Li) (DRP) CaS04 : (Mn, Li) CaS04:(Mn, "Li) CaS04 :Tm CaF2:Mn (Harshaw)

CaF2:Mn (EG & G) CaF2:Mn (Conrad) CaF2 (MBLE) CaF2 (fluorite) CaF 2 :MnTLD08 (Yugoslavia)

CaF2:Mn (Philips) CaF2:Dy TLD 200 (Harshaw)

LiF TLD 700 (Harshaw)

LiF 7 (Conrad) LiF TLD 100 (Harshaw)

LiF (Conrad) LiF TLD 600 (Harshaw)

LiF 6 (Conrad) BeO

Li2B407:Mn (Harshaw)

0.21 [6] 0.52 [6] 0.38 [6] 0.21 [6] 6.2 [6]

100 [46] 1050(46)

0.23 [9] 0.6 [6] 0.1-0.13 [76] 0.58 [94] 0.2 [76] 3.5 [1] 0.16(78] 0.77 (theoretical [73]) 1.05±0.08 (experimental [73]) 0.07±0.01 [75] 0.59 [6] 0.5-0.65(76] 1.1(6] 0.7(81] 0.87-0.96 [76] 1.0(78] 2.5(25] 1.3 [60]

23 [25] 330 [6] 200(100] 200[99] 220 [81] 535 [76] 310(96]

65(78] 490(25] 360[60] 165 [81]

1520 [6] 870[23]

2190(96] 1200-1700(76] 2650 [25]

0.45 [6] 0.13±0.08[79] 0.20(94] 0.17-0.29(79] 0.2(78] 0.5±0.5[51]

390(6] 230[76]

Applications of Th materials in neutron dosimetry 235

TL phosphor Thermal neutron response (R/lO^ncm"1)

Li2B407:Mn(DRP) Li2B„07:Mn (Wallace) LijB„07:Mn (Christensen)

Li,B407:Mn (Brunskill)

390 [6] 670[96] 300 [60] 420[22] 400­500 [17]

12.3.1. Lithium fluoride

Lithium has two naturally occurring stable isotopes, 6Li and 7Li, and lithium fluoride is available with three different isotopic compositions, namely:

natural LiF enriched 6IJF enriched 7 l iF

(Harshaw TLD 100, 7.4% 6Li) (Harshaw TLD 600, 95.6% 6Ii) (Harshaw TLD 700, ~ 0.01­0.04% 6 l i )

The main contribution to the thermoluminescence after thermal neutron irradiations comes from the a­particles and tritons produced from the 6Li(n, a)3H reaction. Because of the very high capture cross section of 6LiF for thermal neutrons (945 b compared with 0.033 b for 7Ii(n, 7)8Li) it has the highest thermal neutron sensitivity of any phosphor. Figure 12.3 [91] shows that the 6Li capture cross section is inversely pro­

portional to the neutron velocity, V, apart from a resonance at 0.255 MeV. The cross

Energy (MeV)

105

10'

£ 103

o m

c £ 1 0

2

u 0) I

2 10 o

4 1

10'

: ' : ■

r._

-*\

j . ^

■

= .

L.

' "

^

10"8 10"

7

"'I I ' '

A

>,

1 11

^ ^ ^

^-

."I I ■ .

10"6

• ) ■ '

/ •

• •

10"5

11 I I I I | r

1 ~~~~ 1 |l I

1

l] 1

M 1 1 I | 1 1 1 1 | 1

| 1

■s^l

1 ^ ^ V

\

.!>

10" 1 1 l l l l l |

"3Cd(n

6L i ( n

197. , Auln

~-

L —TH r-rr

10

Tf'cd! a) 3

H -rf^Aui

, , ,

-

_ " ■

_ : -

: -'. -

: ;

LLU

10"' 10"J

1 10 10 2 10"

2 10"1

Energy I MeV)

Figure 12.3. Neutron capture cross sections of 6Li, "3Cd and ' "Au (after [91]).

236 J A Douglas

sections of cadmium and gold, which are shown for comparison, are also proportional to 1/V for low energies. It can be seen that the energy of the cadmium cut-off is not the same as the gold resonance and that there is no pronounced cut-off for 6Li. Although cadmium shields and gold activation foils are both used to measure thermal neutrons they do not measure precisely the same spectrum. Some intermediate-energy neutrons are included in measurements made with gold foils and 6Li detectors. The cadmium cut-off is usually taken as 0.5 eV.

The a-particles (2.07 MeV) and tritons (2.74 MeV) from the 6Ii(n, a)3H reaction have a higher LET than x- and gamma radiations, so the emission spectra, supralinearity and glow curves are different in the two cases.

Figure 12.4 shows that the optical spectra emitted by 6LiF after irradiation of a-

particles contains more red light than after beta and gamma irradiations. Since the TL effect from thermal neutrons is due to a-particles and tritons, the emission spectrum after neutron irradiation will also contain longer wavelengths than after beta or gamma irradiations.

100-

(/) c CD

c .c en

_ i _ i 1-0) > _ . a CD tr

80

60

40

20

/ / " "V ft \ .

/ ' > / / II

/ / / ' / / / / / / / /

/ / • • ' ••■.

/ ' / ' ••' / / .' / / / / / / //.•'

/ / . ■ •

11/ II:

/ ' ■

II: II:

//: 1 1

V \\ \\ \\ V a/ v • \ \ . ■ • ' ' •

\ \ "" Y\

ft\ \ ^ \ \

X. X ^ o

i i i \ - 1

300 350 400 450 500 550 600 650 Wavelength (nm)

Figure 12.4. Optical emission spectrum of the main TL peak of LiF:Mg,Ti after exposure to different types of radiation.

The departures from linearity of the responses of 6LiF and 7LiF to gamma and neutron radiations are shown in figure 12.5 [70]. The onset of supralinearity in the response to gamma radiations is different for 6LiF and 7LiF. The neutron response of 6LiF remains constant to a much higher light output. Because the density of ionisations is high along the tracks of a-particles and tritons, the 6LiF is effectively operating in the supralinear region even for very low doses. There is a danger of ambiguity when expressing the neutron response in terms of apparent γ-ray dose. Care must be taken to make it clear whether it is the actual dose of 7-rays which would produce the same

Applications OF TL materials in neutron dosimetry

Thermal Neutron Fluence in n/cm

237

10c 10s 1010 1011 1012 10 ,13 10 ,14

1000

100

lihli|""l ihlili|""l i l i l i l f " ! ! lihli'i""1, ,\,\Mf'"\ !hI'IMI11"1! ,Wilji 10' 10J 10" 103 10° 10' Gamma Equivalent Neutron Exposure in R

0 ' 0 0 1 I—i i i mill i i i mill I i i i null i I i I 10' 10z 10J 104 103 10° 10' 10°

137Cs Gamma Exposure in R Figure 12.5. The variation with exposure of the relative responses of TLD 600 and TLD 700 to neutron and gamma irradiations. • , neutron response TLD 600; o, gamma response TLD 600; A, gamma response TLD 700 (after [70]).

luminescence or whether it is the dose based on the 7-ray response in the linear region that is used.

Figure 12.6 [58] shows the differences in the glow curves after neutron and photon irradiations. The positions of the '210 °C peaks are slightly different but the main differ­ence is that the ratio of the height of the '285 °C peak to that of the '210°C peak is greater after neutron irradiations than after gamma irradiations. This effect has been noted by many other workers [5, 20, 35, 59]. Attix [3] was the first to suggest the application of this phenomenon to personnel neutron dosimetry and further studies in this direction have been made by Nash and Johnson [64] and Budd etal [19]. Nash and Johnson use linear heating and record peak heights whereas Budd et al use plateau heating and record the integrated light output. Both systems produce two simultaneous equations which can be solved for the neutron and 7-ray doses.

Budd et al have also noted that the height of the '285 °C' peak relative to the '210 °C peak varies with photon energy. The ratio for 48 keV x-rays is approximately twice that for radium 7-rays. Uncertainty of the photon energy in a mixed field will increase the uncertainty of the neutron dose. However, the error in the total neutron plus 7-ray dose is acceptable for personnel dosimetry.

Fading of the higher temperature peaks is likely to be small. However, departures from linearity begin at lower doses (at ~30 rad for gamma radiation and ~300 rad equivalent for thermal neutron irradiation [20] compared with ~300 and ~2000 rad respectively, for the '210 °C peak [70]). These disadvantages are outweighed by the gain in using only

238 J A Douglas

T i m e [ s e e s ) Temperature I C)

Figure 12.6. Glow curves from TLD 600 after gamma and neutron irradiations, (a) Linear heating (2°C s"' ramp), (6) plateau heating [58).

one detector and avoiding the possibility of confusing two detectors of identical appear­ance. The effect of fast neutrons on the '285 °C peak will be briefly mentioned in § 12.4.

There has been some confusion in the literature as to whether the effects of neutron and gamma irradiations are additive. Wallace et al [97] found that vapours were released in some of the phosphor containers which induced fading and created the illusion of non-additivity.

The damage to 6LiF caused by neutrons is cumulative since each 6Li(n,a)3H reaction reduces the number of 6Li nuclei available for subsequent neutron reactions. In addition, there is damage along the tracks of a-particles and tritons due to the high ionisation. This damage will affect future luminescence only when a new track crosses an old one. The probability of this happening is small until the phosphor has received a large cumulative dose(~1010ncnf2).

12.3.2. Lithium borate

Lithium tetraborate (manganese-activated) was one of the earliest low atomic number alternatives to LiF [49, 80]. It is simpler to handle as it does not require annealing after reading and it is more nearly tissue-equivalent for photon irradiation than LiF. How­ever, it has the disadvantage of being hygroscopic with a consequent high rate of fading. Christensen et al [22a] report 30% fading after storage in moist air at 25 °C for two months compared with no observed fading after storage in dry air for the same time and temperature.

Lithium borate can also be activated by silver. Thompson and Ziemer [93] found that the most favourable energy response was obtained at a concentration of about 0.1%. This is less hygroscopic than the manganese-activated phosphor and has good stability when stored in the dark but fades rapidly when exposed to uv light [65, 93].

Applications of TL materials in neutron dosimetry 239

The light emitted from silver­activated borate has maxima at 290 and 365 nm, while the manganese­activated phosphor has a broad emission spectrum with a maximum at 600 nm, which makes it difficult to distinguish between the phosphor luminescence and the infrared black­body radiation. It should be possible with the appropriate photo­

multiplier to detect lower levels of radiation with Li2B407: Ag than with Li2B407:Mn. The responses of both silver and manganese­activated versions are linear for gamma

radiations up to about 102rad and for thermal fluences up to about 10" n cm"2 [52, 93]. The glow curves after gamma and neutron irradiations are the same in Li2B407:Mn, a simple peak at ~190°C [52], but in the silver­activated form there is an extra pronounced peak at 350 °C following exposure to neutrons [87]. The usefulness of this peak for dosimetry in mixed fields could be beneficially pursued.

The response of activated natural lithium borate is due to the presence of 6Li and 10B, both of which capture neutrons and emit α­particles. R T Brunskill (private communica­

tion) has now produced isotopically enriched 7Li211B407:Mn in powder form so that matched neutron­sensitive and neutron­insensitive lithium borate detectors are available.

12.3.3. Calcium sulphate

The three main activators which have been used with CaS04 are manganese, dysprosium and thulium. The manganese­activated phosphor fades very rapidly (~30% in 10 h and 40% in 1 d [14, 32, 54]) and is thus only suitable for short laboratory exposures. The glow curves for dysprosium­ and thulium­activated phosphors are similar with a main peak at 220 °C, which is consistent with the reported low fading rates of 1­2% in 1 month at 25 °C [88, 89, 101]. The thermal neutron responses of CaS04:Dy and CaS04:Tm are both low (0.38 and 0.23 equivalent rad per 1010n cm"2, respectively, for 0.05% activator) [6, 7]. This is because the main TL process is neutron capture by the activators, which are present in low concentrations. The capture cross sections are 930 and 130 b, respectively. The response of CaS04:Tm to 7­rays and charged particles is ~35 times that of LiF [9] and that of CaS04:Dy is slightly higher [101]. This high sensitivity plus the low response to thermal neutrons makes them good detectors of the 7­ray component in mixed neutron­gamma fields.

The high sensitivities of CaS04 phosphors to charged particles encouraged workers to mix them with compounds having high thermal neutron capture cross sections to obtain phosphors with high thermal neutron sensitivities. 6Li salts are an obvious choice, and they have been tried with CaS04:Mn [46], CaS04:Dy [6] and CaS04:Tm [9, 45, 89]. Intimate mixtures are essential, so finely powdered phosphors and non­luminous lithium salts are used. The 7­ray sensitivity of such mixtures is essentially proportional to the concentration of the phosphor. The slight deviation is due to differences in effective atomic number. The neutron response is a maximum when the proportions are roughly 1:1. Figure 12.7 [89] shows the responses of varying mixtures of CaS04:Tm and 6LiF.

Mixtures with CaS04:Mn can have a thermal neutron sensitivity about 10 times that of 6LiF but the fading rate is still as high as for CaS04:Mn [46].

Ayyangar et al [6] have reported that mixing 6Li2C03 with CaS04: Dy transfers energy from the 210 °C glow peak to that at 130 °C (figure 12.8). This has the unfortunate effect of increasing the fading to ~60% in a month. Mixtures of CaS04:Tm with Li2S04 [9] and with 6LiF [89] do not appear to increase the rate of fading. The response of a 40%

240

6LiF 20 £0 60 80 CaSCVTm Content,wt %

CaS04:Tm

Figure 12.7. TL response of CaSO„:Tm, 6LiF mixtures to thermal neutrons (o) and 7-rays (•). , calculated (after [89]).

210°C 130°C 130°C 130°C 130°C

0% 0 0 1 % 002% 004% 0-3% Figure 12.8. Glow-curve changes in CaSO„:(Dy, *Li) as a function of the weight per cent of the 6Li2C03 content.

Li2S04 mixture is linear, without phosphor damage, up to a neutron fluence of ~2.5 x 1014 n cm"2 and has a sensitivity two-thirds that of TLD 600. By using 6Li2S04 Beach and Huang hope to reach a sensitivity approximately seven times that of TLD 600. The response of a 50% 6LiF mixture is linear up to a fluence of only 10'° n cm-2 before phos­phor damage occurs. The sensitivity of this mixture is approximately six times that of TLD 600. By using a pair of CaS04:Tm (6LiF) and CaS04:Tm (7LiF) detectors Takenaga was able to detect 1 mrem of thermal neutrons with a standard deviation of 4.1% if no gamma radiation is present rising to 30% in the presence of 1 rad of gamma radiation.

It is possible to adjust the concentrations of these mixtures to obtain the same dose-equivalent sensitivities to thermal neutron and gamma radiations [6, 7]. More develop­ment of the CaS04: Tm (6Li2S04) system should be fruitful.

Applications of TL materials in neutron dosimetry 241

0> o c 3

o

10

10"° r

(N

- 5 icr =-E

Q. 3 01 O S a: a >

* 10

LU o a

10

-8

"9

-10

-11

-12

--

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1 1 IIMIj 1 1 1 l l l l l j 1 1 1 l l l l l j 1

l ^ W - ^ N

/ x

\ i

A /o / s <p / *~~ ' / o

/ <b° •

' 1 1 1 * 1 ■ ' ' 1 1 ■

1 1 Mil l

H

-_

-

: ~ ~

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---

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— -\ -

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-7

E o c o (_ -*-

ai c - 1 0 " ' -

­10 -8

1 0_" r

^10 -9

O) a> a E 0)

10 i-2 10" 1 10 10' Neutron Energy (MeV)

Figure 12.9. The variation with neutron energy of the responses of CaS04: Tm and BeO:Na. o, CaS04(Tm) (expt) [90]; , CaSO„(Tm) (calc) [90]; •, BeO(Na) (expt) [90]; ,BeO(Na)(calc) [90]; A, BeO(Na) (expt) [94]; v, BeO(Na) (expt) [34].

12.4. Intermediate and fast neutron dosemeters

In general, TL phosphors have lower responses to fast neutrons than to equal doses of beta and gamma radiations because there is enhanced recombination and saturation along the tracks of high LET particles such as recoil nuclei. In addition, phosphors are non­

hydrogenous so that the absorbed dose is less than in tissue, for the same fluence. Because nuclear collisions are involved in the energy transfer processes, the luminescence will usually increase with energy. Figure 12.9 compares the experimentally determined responses for CaS04:Tm [90] and BeO:Na [34, 94] with the values calculated by Tanaka and Furuta [90]. The responses of 6LiF and 7LiF are shown in figure 12.10 [92]. Below about 100 keV the 6Li(n,a)3H reaction becomes dominant with its 1/F cross section (see figure 12.3). The sensitivity to 1 MeV neutrons is much smaller than to thermal neutrons (~2.5 and ~2000 equivalent rad/1010ncm~2, respectively). The neutron response of the 285 °C peak in 7LiF, which has a higher neutron sensitivity relative to the 7­ray sensitivity than the 210°C peak, also increases with energy as shown in figure 12.11 [26].

Various techniques have been tried to increase the neutron response of the main dosi­

metry peaks such as (i) using a hydrogenous material to provide recoil protons, (ii) using a foil of fissionable material and detecting the fragments and (iii) using a moderator to thermalise the neutrons. These techniques are not unique to TL phosphors and have also been used, sometimes to greater advantage, with nuclear emulsions, etched foils and scintillation counters.

12.4.1. Proton radiators

These can be subdivided into two classes as follows: intimate mixtures and sandwiches.

242 J A Douglas

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10"H =-

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: 1 I I I I IH | 1 1 1 ■ I ■ 111 1 1 1 1 11 ll| 1 1 1 l l l l l | 1 1

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^ W «•*> / 0°

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: '.

-

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--; -

-

-

10"5 ___

'E u

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o 1_ "5 <D C

10"7 _-

'o>

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a E I_ CD

10-9 *

10"12

10"3 10~2 10"1 1 10 102

Neutron Energy (MeV) Figure 12.10. The variation of the neutron responses of 6LiF and 7LiF with neutron energy (after [92]).

uu

50 T3 a

0) a. 0) w 30 o Q. or 20

S?

10

n

-

-

y*252

I

Cf

1

/ A m Be

i i i

^ ^ ^ J 5 M e V

i i

0 2 k 6 8 10 12 U 16 Neutron Energy (MeV)

Figure 12.11. Neutron energy response of the 285 °C peak in TLD 100 [26].

12.4.1.1. Intimate mixtures. The hydrogenous compounds which are used must either be able to withstand the high temperatures of read­out and anneal or be easily separated from the phosphor after irradiation. Readily volatile liquids such as alcohol and water are natural choices. The difference in response between phosphors exposed wet and dry indicates the enhancement due to recoil protons.

Applications of TL materials in neutron dosimetry 243

After contact between liquids and phosphors there is a build-up of thermoluminescence [74, 85], which is probably due to the etching of the surface defects in the phosphor. Puite delayed exposures for ~100h after mixing CaF2:Mn with alcohol to allow equi­librium to be reached. In the case of LiF in water there is the competing process of induced rapid fading [85].

Karzmark et al [47] and Wingate et al [99] found that suspending LiF in alcohol increased its response to 14 MeV neutrons by a factor of 2.5 or 4 depending on whether the mixture was shaken or not during the irradiation. However, the response is still very energy-dependent. At 2.9 and 14.9 MeV the responses are 0.17 and 0.66 equivalent 7-ray kerma per neutron kerma in tissue respectively [12]. The variation is due to the relationship between neutron energy, recoil proton range and the grain diameter of a phosphor. Reducing the grain diameter will increase the surface area per unit volume and hence increase the TL from short-range recoil protons. The variation of the response with neutron energy would thus be reduced. However, the inherent radiation sensitivity of LiF falls for very fine grains [48].

Alcohol has also been used to boost the fast neutron response of CaF2: Mn [74] and CaS04:Dy [98] which have much higher radiation sensitivities than LiF. It is possible to detect a fluence of 5 x 105 n cm-2 from a Pu-Be source with a pair of dry and alcohol-suspended CaS04:Dy dosemeters. Allowance has to be made for the effect of alcohol on the 7-ray sensitivity. Unruh et al [95] were unable to find a non-hydrogenous liquid which would produce the same effect on the 7-ray sensitivity of the 'dry' dosemeter.

The big disadvantage of liquid radiators is that stirring or shaking is necessary to maintain uniform mixing and constant response. Solid matrices, which do not require such treatment, have been developed incorporating CaS04:Dy [10, 11] and CaS04:Tm [15]. In the latter case Blum et al encapsulated CaS04:Tm powder in one PTFE tube and a 3:1 mixture with glucose in another. The glucose was washed out and the phosphor dried prior to read-out. After irradiation in a cyclotron beam, the TL outputs were in the ratio of about 4:1 which is sufficient to give good discrimination between the neutron and 7-ray components in the range 10 mrad to 50 rad. The neutron response changes by less than 10% when the tube is rotated 90° in the beam.

Becker et al [10, 11 ] have developed a solid matrix which will withstand the necessary read-out temperatures, thus greatly simplifying the technique. An intimate 1:1 mixture of finely powdered CaS04:Dy (~4jum diameter grains) and p-sexiphenyl, which has a melting point of 450 °C, is hot-pressed into a pellet 6 mm diameter x 1 mm thick. The non-hydrogenous counterpart has a PTFE matrix. Table 12.2 shows that a ten-fold enhancement is achieved at 14 MeV but less at lower energies. This technique could become as simple to employ as 6LiF/7LiF pairs and will be more precise and less subjective than nuclear emulsions which are presently employed for personnel fast neutron dosimetry.

Table 12.2. Fast neutron response of CaSO„:Dy embedded in p-sexiphenyl.

Radiation source p-sexiphenyl response/Teflon response Neutron dose (rad)/ (normalised for equal gamma-ray response) gamma-ray dose (rad)

HPRR Reactor 3 7 14 MeV beam 10 25

244 J A Douglas

12.4.1.2. Sandwiches. A simpler type of proton radiator is a sheet of hydrogenous plastic in contact with a phosphor. Blum et al [16], Sunta et al [87], Spurny et al [86] and Mohammadi et al [62] have investigated such systems with CaF2:Mn in Teflon, TLD 100, CaS04:Tm, Li2B407: Mn, Li2B407: Ag, natural CaF2 and A1203. Table 12.3 shows that the enhancement is proportional to the hydrogen present in the radiator and that a larger enhancement is obtainable with A1203 than with LiF. However, there is a very pro­nounced variation with neutron energy and angle of incidence because the range of proton recoils decreases both with neutron energy and also with increasing angle of incidence. This severely limits the use of the technique. However, when calibrated at a particular energy Blum et al [16] found that the discs of CaF2:Mn embedded in Teflon with covers of polyethylene and lead were suitable for clinical use in beams down to doses of a few rad.

Table 12.3. Effect of proton radiators on the relative response of some thermoluminescent sintered discs to 14.7 MeV neutrons.

Material Response relative to 60Co 7-rays with respect to energy absorbed in wet tissue

Without radiator (under Al)

PMMA (9.1% H)

Polyethylene (14.3% H)

n a t L j F 7LiF A1203

0.12±0.02 0.12+0.02 0.17±0.03

0.29±0.03 0.30±0.02 0.65 ±0.02

0.39 ±0.04 0.38 ±0.04 0.96±0.08

In addition to acting as a dosemeter, a stack of phosphor-loaded hydrogenous discs can also be a crude spectrometer for neutron beams. The range of a recoil proton is a function of its energy, so the maximum range is related to the energy of the incident neutron. Figure 12.12 is a histogram of readings from a stack of epoxy resin discs loaded

<

B 300 0

0 200

READ

ING

PE

8

U MeV NEUTRONS

-

0 500 1000 1500 2000 2500 DEPTH INTO STACK (pm)

Figure 12.12. Histogram of readings from a stack of epoxy resin discs loaded with CaS04 : Sm after exposure to 14 MeV neutrons.

Applications of TL materials in neutron dosimetry 245

with CaS04:Sm which had been exposed to 14 MeV neutrons [30]. At this energy the TL increases up to a depth of ~2000 fjm when equilibrium is reached, showing that this is the maximum range of the protons. The magnitude of the luminescence at equilibrium is proportional to both the hydrogen concentration in the matrix and also to the incident neutron fluence. The depth at which equilibrium occurs indicates the neutron energy. The proton range varies very rapidly with energy which means that the method is sensitive, but ultra-thin discs will be needed at low energies for good resolution. For 1 MeV neutrons a disc thickness of about 4 /um is required. The use of such a device is naturally limited to beams.

12.4.2. Fission foils

Ettinger et al [29] have proposed substituting a foil of fissionable material in place of a proton radiator. This would increase the energy deposited per neutron interaction. Ettinger used natural CaF2 powder glued to an aluminium or acetate backing held in contact with a 238U foil 0.4 mm thick. The competing (n,p) and (n,a) reactions of ^Ca and 42Ca deposit many times less energy than the fission fragments. The range of fission fragments and the a particles from the decay of 238U are very similar (~15 /xm in CaF2) and cannot be separated. To obtain maximum discrimination the fission foil should there­fore be separated from the phosphor after irradiation to reduce the effect of the natural decay. A neutron fluence rate of ~5 x 105n cm"2s_1 produces the same TL response as the natural decay of 238U, which indicates a lower detection limit. Fluence rates of ~107

n cm"2 s"1 were easily detectable without the need to cool the photomultiplier. This technique may be useful for calibrating fluences. The performance would be

improved by using a foil of 237Np instead of 238U because it has a lower spontaneous fission rate, lower specific activity and lower energy threshold [13].

12.4.3. Moderators

An alternative method of increasing the energy transferred from fast neutrons to phos­phors is to thermalise the neutrons and use a phosphor with a high thermal neutron cross section. Such systems can be sub-divided into (i) moderator surrounding the detector and (ii) moderator behind the detector. In the latter category the detector responds to back-scattered or albedo neutrons and will be dealt with in § 12.4.4.

Distenfeld et al [23] and Engelke [27] in collaboration with Piltingsrad [72] and with Israel [28] have developed dosemeters using pairs of 6LiF and 7LiF phosphors at the centre of polyethylene spheres. Figure 12.13 [53] shows the effect of sphere diameter on the energy dependence. No diameter gives a flat response from 1 to 106eV. Barothoux et al [8] suggested that a good rad response could be obtained by subtracting 5% of the response of a 4.2 in sphere from that of a 12 in sphere. More recently Apt and Schiager [2] have shown that a 10 in diameter sphere has an almost dose-equivalent response for neutrons up to 7 MeV.

Pairs of 6LiF/7LiF detectors placed along a diameter give information about the incident spectrum. Figure 12.14 shows that the distance from the surface to the position of maximum response increases with neutron energy. The ratio of the response at the centre to that at the surface also increases with energy. Both of these effects can be used

246 J A Douglas

10"1-

CD l/l C o Q . l/l dl

cr

10 -2

10' i-3

■3 - - 4 10

,­5 10'

10"'

A 8 1 ' N \ \ \

\ \

10"' 10° 101 10

2 103 10

4 105 10

6 107 10

8

Energy (eV) Figure 12.13. Calculated responses of TL detectors in polyethylene spheres of various diameters (from [53]).

1 2 3 4 5 6 7 8 9 Distance from Surface of

the Sphere (ins)

10

Figure 12.14. TL output of detectors placed at different depths in a polyethylene sphere, 1 8 in diameter. Normalised to 2 X 107 neutron cm"'.

to indicate the effective neutron energy. The latter is used in the 10 in diameter poly­

ethylene sphere of Engelke and Israel, which has a pair of 6LiF/7LiF phosphors at the centre and at 26 points on the surface. The ratio of surface to centre readings enables the effective energy to be deduced and hence the appropriate calibration factor to apply to the centre readings to obtain the dose equivalent. In the case of non­uniform fields a computer is employed to solve the simultaneous equations. The minimum detectable dose equivalents are 30 mrem of neutrons and 10 mrem of gamma radiation.

Applications of TL materials in neutron dosimetry 247

Pairs of detectors have also been positioned along the axes of cylinders to act as spectrometers and dosemeters for beams of neutrons. Longworth [55] proposed such a scheme using a cylinder 41cm diameter x 54 cm long which had an internal thermal neutron shield. The incident spectrum was unfolded by a computer program. It had a resolution of ~12% at lower energies improving to 5% at 1 MeV, provided that the exposure was long enough to allow the fluence to be determined with a precision of 2%.

Singh et al [82] have also developed a spectrometer and dosemeter using pairs of 6LiF and 7LiF detectors at 0.8 cm intervals along the axis of a polyethylene cylinder 25 cm diameter x 25 cm long. The dose equivalent measured at a depth of 10 cm is almost inde­pendent of energy. The ratio of the reading at 10 cm to the maximum reading gives the effective energy. The depth at which the maximum occurs multiplied by the ratio of maximum reading to the surface reading indicates the thermal neutron content of the beam.

Spectrometer dosemeters are conceptually good dosemeters because they provide spectral information in addition to fluence or dose equivalent. Cylindrical versions, however, can only be used with beams of known direction.

12.4.4. Personal albedo

The human body contains hydrogenous material which moderates and scatters neutrons. The back-scattered neutrons which re-emerge from the body with lower energy are called albedo neutrons. Many personnel neutron dosemeters have been designed based on the detection of the thermal and epithermal albedo neutrons. Unfortunately both the fraction of neutrons re-emerging with thermal energy and also the neutron fluence to dose-equivalent conversion factor vary with energy (figures 12.15 and 12.1) in such a way that the overall response of simple albedo dosemeters is highly energy-dependent.

Various attempts have been made to improve the response by (a) incorporating a moderator in the dosemeters so that the thermal and epithermal neutron detector will

Energy of Incident Neutrons(eV) Figure 12.15. Thermal neutron albedo from the body as a function of incident neutron energy [40].

248 J A Douglas

respond to moderated incident and albedo neutrons and (b) incorporating extra detectors to provide information about the incident spectrum so that a correction factor can be applied.

Figure 12.16 [71] shows schematically some of the types of dosemeter which have been designed. Types (a) [31] and (b) (Burger, private communication) respond to both incident and albedo thermal and epithermal neutrons, whereas (c) (Preston, private communication, [37]) responds to neither incident nor albedo thermal neutrons but only to epithermal plus internally moderated neutrons. Types (d), (e) and (/) [40] have thermal neutron shields to cut out the direct action of incident thermal neutrons. Types (g) [18], (h) [41], 0) [21], (k) [50] and (/) [36] distinguish between incident and albedo response. Type (m) [69] has an additional component which responds to epithermal and fast neutrons. This allows corrections to be applied to the responses of the other two pairs of detectors for their responses to fast and epithermal neutrons.

No discrimination between incident and albedo neutrons

Discr iminat ion against incident thermal neutrons

Separation of incident and albedo neutrons

Separation of incident, thermal and epithermal neutrons

6 7

Cd

6 7 CA eirn La TT?:

C d 6 7

fe&

Cd

Cd

^J!3^ 6 7

Figure 12.16. Schematic diagram of types of albedo dosemeters (by courtesy of Piesch [71]).

The response of some of the European dosemeters to monoenergetic neutrons is shown in figure 12.17 [24]. It can be seen that, apart from the thermal neutron response, all the curves are the same shape. The findings are in agreement with those of Hankins [37], who reviewed some American dosemeters, namely:

(1) adding polyethylene to an albedo dosemeter does not change the shape of its neutron energy response;

(2) including polyethylene increases the dosemeter sensitivity; (3) dosemeters with no thermal neutron shield are more sensitive to neutrons of all

energies than those which are completely surrounded by one; (4) variation in neutron energy or spectrum greatly alters the sensitivity of albedo

systems thereby limiting their use.

Some other conclusions of Hankins were:

(5) the sensitivity increases slightly as the body-to-dosemeter distance is increased but not by much if there is no polyethylene or it has a complete thermal neutron shield;

Applications ofn materials in neutron dosimetry 249 100p­

o Burger O Piesch Albedo x Harvey Albedo A Preston ♦ Brunskill Albedo

(Evaluated by Brunskill I

0-5 1 0 1-5 Neutron Energy IMeV)

2 0

Figure 12.17. Responses of various dosemeters plotted against neutron energy [24].

(6) as the moderator thickness is increased the sensitivity increases rapidly at first and then remains fairly constant;

(7) most of the response is from the neutrons re­emerging from the body; (8) the thickness of the shield has little effect on the response unless it is on the back

of the dosemeter; (9) decreasing the dosemeter diameter greatly increases its response to thermal neutrons

but not to fast neutrons; (10) the thermal neutron­sensitive and ­insensitive TL detectors must be located at the

same or equivalent positions in the dosemeter. 7LiF probably cannot be used to determine the 7­ray dose.

If a dosemeter is worn on the back as well as one on the front, then the ratio of the response on the back to that on the front increases with neutron energy. As the neutron energy increases neutrons penetrate further before becoming thermalised, thus moving the effective source of albedo neutrons away from the front dosemeter towards the back one.

Figure 12.18 shows that for the two multi­component dosemeters investigated by Douglas and Marshall [24] the ratio of incident to albedo response is independent of energy, apart from the responses to thermal neutrons. In the case of irradiation by a neutron spectrum, this ratio would indicate the thermal neutron content of the incident radiation.

12.4.4.1. Choice and use of an albedo dosemeter. The albedo response of all dosemeters changes by a factor of approximately 15 in the energy range 0.1­1.7 MeV. Therefore a simple albedo dosemeter cannot be used to measure incident neutron dose without extra information to enable a correction factor to be applied. Three types of working environment will now be considered: (a) where the neutron spectrum is constant, (b) where there is one neutron source but varying degrees of moderation and (c) where there is more than one neutron source and/or type of shielding.

250 J A Douglas

Q) if) C o Q. I/) <D

en ~c Q)

•D O C

tt-5, O A

Piesch Brunskil

0 0-5 10 1-5 Neutron Energy (MeV)

2 0

Figure 12.18. Ratio of incident to albedo responses at different neutron energies.

(a) Constant spectrum. In an environment where the spectrum is constant a simple albedo dosemeter can be used provided either it is directly calibrated in the area or simple spectrometry is performed in the area to determined the correction factor [40].

(b) Varying degrees of moderation. As a spectrum of neutrons undergoes increasing moderation by hydrogenous material the mean energy will decrease and the fluence of thermal neutrons will increase. There will thus be a direct relationship between the fraction of thermal neutrons present and the mean effective energy. The ratio of incident to albedo responses of a multi-element dosemeter could thus be used to allow for the degree of moderation. Piesch [69] and R T Brunskill (private communication) have shown this to be the case. However, for multi-element dosemeters to work with mixtures of different degrees of moderation of the same spectrum then the correction factor must be proportional to the fraction of thermal neutrons in the incident spectrum.

(c) Different neutron sources and/or shields. There will not necessarily be any relation­ship between the fraction of incident neutrons which are thermal and the mean energy for different initial neutron spectra. Nor will there be a direct relationship in the case of different shields around the same neutron source. A metal shield with resonances, e.g. cadmium or iron, will obviously upset the thermal to mean energy relationship. Therefore in these cases a multi-component albedo dosemeter is not a solution to the problem. An alternative approach is to use a single-component albedo dosemeter together with another dosemeter having a different energy response such as a nuclear emulsion dosemeter on a fission foil.

12.4.4.2. Effects of angle of incidence. Figure 12.19 shows the variation of response with angle of incidence for the Harvey dosemeter. For any energy of incident neutrons, the effect of the angle of incidence on the response is a product of two terms. The first

Applications of TL materials in neutron dosimetry 251

Figure 12.19. Effect of the angle of incidence on the response of Harvey dosemeters. Elliptical phantom (20X30 cm2): X, lOOkeV neutrons; o, 700 keV neutrons; □, 1.7 MeV neutrons. Cylindrical phantom (20 cm diameter), v, 25:Cf source.

Angle of Incidence

relates to the fluence of thermalised neutrons produced behind the dosemeter. This fluence is proportional to the number of neutrons incident per unit area, which in turn is proportional to the cosine of the angle of incidence. The second term allows for the variation with angle of incidence of the distance between this source of thermalised neutrons and the dosemeter on the surface of the phantom. This distance decreases as the angle of incidence increases and also as the energy of the incident neutrons decreases.

For low energies of incident neutrons, where the source of thermal neutrons is near the surface, the predominant term is that of the variation of incident fluence, i.e. cosine of the angle of incidence. For higher energies the variation with angle of the position of the source of thermalised neutrons becomes more important and maxima appear in the response curves.

Most of the energy from incident neutrons is deposited close to the point of entry into the body. The dose is thus proportional to the number of neutrons per unit area of body surface, which in turn is proportional to the cosine of the angle of incidence. Hence a dosemeter, which basically has a cosine response, is acceptable for personnel dosimetry. The increased response for higher energies at some angles is an extra safety factor.

12.4.4.3. Conclusions. It has been shown that the shape of the energy response curve is the same for all albedo dosemeters in the range 0.1­1.7 MeV. Because the response at 0.1 MeV is approximately 15 times that at 1.7 MeV, extra information is required to correct for the effective energy of incident neutrons. Additional elements in multi­

component dosemeters do not provide enough information, except in the restricted case of varying moderation of one type of neutron source. A single­component dosemeter can be used in environments with a constant neutron spectrum provided the correction factor is determined by calibration. A multi­element dosemeter (e.g. Piesch) is useful to make corrections to the calibration for variations in spectrum due to local variations in hydro­

genous moderation or back­scatter.

252 J A Douglas

12.4.5. Activation

The elements of some TL phosphors form radioactive isotopes after neutron capture. The radiation subsequently emitted by them doses the phosphor internally. Annealing the phosphor immediately after exposure removes the direct effect. The thermoluminescence after a suitable storage period will be proportional to the induced activity which in turn is proportional to the neutron fluence. This technique has been used by Mayhugh et al [61] to measure thermal neutron doses using CaS04:Dy, CaF2:Dy and natural CaF2. By suitably selecting phosphors containing elements with different energy thresholds for neutron capture a dosemeter can be obtained which provides information on the energy spectrum and has complete discrimination against gamma radiation. A system containing several different phosphors has been extensively investigated by Pearson and Moran [67]. Table 12.4 shows the available neutron reactions in some phosphors.

Table 12.4. Neutron activation reactions in some TL phosphors.

Phosphor

TLD700 (LiF:Mg,Ti)

TLD200 (CaF2:Dy)

CaF2:Mn

CaS04:Dy

ZnO:Tm

Mg2SiO„:Tb

Reaction

"F(n, 2n)'8F

"F(n,2n)1 8F

Threshold

12MeV

12MeV 164Dy(n,T)165Dy Thermal

"F(n,2n) , 8 F S5Mn(n,7)56Mn 32S(n, p)32P

12MeV Thermal

2.5 MeV 164Dy(n, 7 ) ' "Dy Thermal 64Zn(n, p)MCu 68Zn(n,7)69Zn 24Mg(n,p)24Na

4MeV Thermal

7 MeV

Capture cross section

50mba t 14 MeV

50 mb at 14 MeV 2.8 kb

50mba t 14 MeV 13b

250mbat 14 MeV 2.8 kb

200mbat 14 MeV l b

200 mb at 14 MeV

Decay

r

r P~

r r P~ F ECB* <r f

' mode

0.64 MeV

0.64 MeV 1.3 MeV

0.64 MeV 2.8 MeV

1.7 MeV 1.3 MeV

0.6 MeV 0.9 MeV

2.8 MeV

Half-life

109 min

109 min 140 min

109 min 2.5 h

14d 140 min

12.8h 57 min

15h

Some of the criteria for suitable phosphors are as follows: (1) The half-lives of the neutron-induced isotopes in a phosphor must be sufficiently

different for the separation of the activities to be possible. (2) The half-lives must not be very short otherwise most of the induced activity would

decay before annealing is complete (e.g. the t1/2 of 16N and 190 are 7.2 and 29 s, respectively, which are too short).

(3) The half-lives must not be very long otherwise the dose rate will not be much greater than background.

(4) The exposure time must be short compared with the half-life for the induced activity at the end of the exposure to be proportional to the fluence. (When the exposure is one tenth or less of the half-life then the activity is proportional to the fluence within 7%.)

(5) Fading rate must be low.

Applications of TL materials in neutron dosimetry 253

It can therefore be seen from table 12.4 that fluences of thermal neutrons and those of energy >12 MeV cannot be separated with either CaF2:Dy or CaF2:Mn. However, using CaS04:Dy, thermal neutrons can be separated from those of energy >2.5 MeV by reading after 1 d and again after 14 d. Using ZnO:Tm, Mg2Si04:Tb and TLD 700 the neutron fluences above 4, 7 and 12 MeV, respectively, can also be measured, thus providing reasonable spectral information.

Because the prompt response of the phosphors is 2-3 orders of magnitude greater than the delayed response, it is essential that the prompt response is completely and quickly removed. Pearson achieved this by placing the phosphors in holes in an aluminium block which was heated in an air furnace at 500 °C for 5 min followed by rapid cooling in a lead block for 2 min. This unfortunately rendered them sensitive to uv light.

The low TL output means that photon counting has to be used in the reader to obtain reasonable statistics. The thermoluminescence is limited by the number of activations produced. Therefore the response can be increased by (a) increasing the mass of the phosphor, (b) using a reaction with a large cross section or (c) using a reaction where the induced isotope emits beta particles of higher energy (the delayed dose measures the energy deposited not the number of beta rays).

This is a useful technique for short exposures to relatively high fluence rates such as in neutron radiation therapy or nuclear accident dosimetry.

12.5. Possible future developments

12.5.1. Use of deeper traps

Lucas and Kapsar [56] are developing a system using the 150°C and 240 °C peaks in CaF2:Tm. The 240 °C peak is mainly sensitive to fast neutrons in contrast to the main dosimetry peak (150°C) which has a low response to neutrons. At this stage it is not clear whether there is sufficient discrimination against gamma radiation for this to be useful for personnel monitoring.

12.5.2. Creation of a phosphor by ion implantation

A dosemeter consisting of an intimate mixture of two powders of fine particle size was investigated at Harwell by Pells and Hughes [68]. One would be a host for luminescence and the other an activator. Initially the host would be very pure, containing no activator. Collisions between fast neutrons and activator atoms near the host would implant activator ions into the host, thus creating a phosphor. The extent of the implantation would be proportional to the neutron dose. The newly created phosphor could then be exposed to a known dose of x-rays or electrons and the thermoluminescent output measured. This would be proportional to the number of injected activator ions and hence to the neutron dose. It has been shown theoretically [68] that an ideal ion-injection dosemeter with an ideal reader would have millirad sensitivity. However, there are many problems in realising this:

(1) fabricating a large interfacial area per unit volume with complete mixing of the powders;

254 J A Douglas

(2) the dosemeter volume is limited by loss due to self-absorption and scatter. The light loss perhaps could be reduced if the refractive indices of host and activator were matched.

More ideas are needed to solve the problems.

12.5.3. Internal proton radiator

Morato et al [63] have diffused hydrogen ions into CaF2:Dy and so produced a phosphor with an internal proton radiator. By this method they claim to have increased the response to neutrons relative to gamma radiation by a factor of 80. Further work is required to increase the ion concentration by at least another order of magnitude for the system to be feasible for routine personnel monitoring.

Although there is no one satisfactory fast neutron dosemeter there is hope for the future.

References and further reading

1 Amperex Electronic Corporation 1969 Report on Thermoluminescence Dosimetry 2 Apt K E and Schiager K J 1975 A passive environmental dosimeter Health Phys. 28 474-6 3 Attix F H 1968 Thermoluminescent dosimeter US Patent No 3484 605 4 Auxier J A, Snyder W S and Jones T D 1968 Neutron interactions and penetration in tissue Radia­

tion Dosimetry vol 1, ed F H Attix, W C Roesch and E Tochilin, chap 6, pp 296-315 5 Ayyangar K, Reddy A R and Brownell G L 1969 Some studies in thermoluminescence from

lithium fluoride and other materials exposed to neutrons and other radiations Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF. 680920, p525

6 Ayyangar K, Chandra B and Lakshmanan A R 1974 Mixed field dosimetry with CaSO„:Dy Phys. Med. Biol. 19 656-64

7 Ayyangar K, Lakshmanan A R, Chandra B and Ramadas K 1974 A comparison of thermal neutron and gamma ray sensitivities of common TL materials Phys. Med. Biol. 19 665-76

8 Barothoux A, Benezcch G and Zeborowski H 1964 Dosimetric des neutrons technique multisphere Conf. on External Source Dosimetry, Paris

9 Beach J L and Huang C-Y 1976 Mixed field dosimetry with CaSO„(Tm) and CaSO„(Tm): Li Health Phys. 31 452-5

10 Becker, K, Tham T D and Haywood F F 1973 Compounds of TLDs and high-melting organics for fast neutron personnel dosimetry Proc. 3rd Int. Congr. of International Radiation Protection Associations, Washington

1 i Becker K H, Haywood F F, Perdue P T and Thorngate J H 1975 Fast neutron solid-state dosimetry US Patent No 3896 306

12 Becker K 1975 Solid State Dosimetry (Cleveland, Ohio: CRC Press) 13 Bird T V and Davics B L 1974 An evaluation of the fission track method as a personal neutron

dosemeter Rep. NRPB-R30 14 Bjarngard B 1967 Use of manganese and samarium activated calcium sulphate in thermo­

luminescence dosimetry Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF. 650637, p 195

15 Blum E, Bewley D K and Heather J D 1972 The use of CaS04 :Tm powder for fast neutron dosi­metry Phys. Med. Biol. 17 661-2

16 Blum E, Bewley D K and Heather J D 1973 Thermoluminescent dosimetry for fast neutron beams using CzF2:MnPhys. Med. Biol. 18 116-234

17 Brunskill R T 1970 The development and use of lithium borate as a thermoluminescent phosphor for radiation Proc. 2nd Int. Congr. of International Radiation Protection Associations, Brighton Paper 62

Applications of TL materials in neutron dosimetry 255

18 Brunskill R T 1977 Albedo-type neutron dosemeter Health Phys. 32 455 19 Budd T, Marshall M, Peaple L H J and Douglas J A 1979 The low and high temperature response of

lithium fluoride dosemeters to X-rays Phys. Med. Biol. 24 71-80 20 Busuoli G, Cavalini A, Fasso A and Rimondi O 1970 Mixed radiation dosimetry with LiF (TLD-

100) Phys. Med. Biol. 15 673-81 21 Busuoli G, Cavalini A, Civolani O and Lembo L 1976 Dosimetri ad albedo per la dosimetria

personate neutronica Energia Nuclcarc 23 no 10 22 Christensen P 1967 Manganese activated lithium borate crystals, glasses and sintered pellets as

thermoluminescence dosimeters Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF. 650637, pp 90-117

22a Christensen P, Botter-Jensen L and Majborn B 1973 Influence of ambient humidity on TL-dosimeters for personal monitoring Proc. Regional Conf. on Radiation Protection, Yavne, Israel vol l , pp 194-206

23 Distenfeld C, Bishop W and Colvett D 1967 Thermoluminescent neutron-dosimetry system Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF 650637, pp457-66

24 Douglas J A and Marshall M 1978 The responses of some TL albedo neutron dosimeters Health Phys. 35 315-24

25 Dua SK, Boulenger R, Ghoos L and Mertens E 1971 Mixed neutron-gamma dosimetry Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, October. Riso Rep. 249, pp 1074-88

26 Endres G W R and Lucas A C 1974 The use of a high temperature trap in 7LiF for fast neutron dosimetry Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), pp 1141-54

27 Engelke M J 1969 Neutron measurements using thermoluminescent dosimeters Los Alamos Rep. LA^1335

28 Engelke M J and Israel H I 1974 A method for establishing effective neutron energy and fluence in mixed radiation fields Health Phys. 27 173-9

29 Ettinger K V, Durrani SA and Christodoulides C 1970 Observation of thermoluminescence induced by fission fragments Radiat. Effects 5 99-102

30 Facey R A 1968 Proton range as a neutron energy indicator in thermoluminescence dosimetry Trans. A m. Nucl. Soc. 11 No 1

31 Falk R B 1971 A personnel neutron dosimeter using lithium fluoride thermoluminescent dosi­meters Dow Chemical Co. RFP-1581

32 Fowler J R and Attix F H 1966 Solid-state integrating dosimeters Radiation Dosimetry vol 2 (New York: Academic Press)

33 Gay ton F M, Harvey J R and Jackson J H 1972 Thermoluminescence and its application in reactor environments J. Br. Nucl. Energy Soc. 11 125-40

34 Goldstein N, Miller W G and Rago P F 1970 Additivity of neutron and gamma exposures for TLD dosimeters Health Phys. 18 157-8

35 Gorbics S G and Attix F H 1968 LiF and CaF2:Mn thermoluminescent dosimeters in tandem Int. J. Appl. Radiat. Isotopes 81-9

36 Griffith R V 1973 The use of ,0B-loaded plastic in personnel neutron dosimetry Proc. IAEA Symp. on Neutron Monitoring for Radiation Protection Purposes, Vienna (Vienna: IAEA) pp 237^19

37 Hankins D E 1972 Factors affecting the design of albedo-neutron dosimeters containing lithium fluoride thermoluminescent dosimeters Los Alamos Rep. LA-4832

38 Hankins D E 1973 Progress in personnel albedo dosimetry Los A lamos Rep. LA-UR-73-1262 39 Hankins D E 1973 A small inexpensive albedo-neutron dosimeter Los Alamos Rep. LA-5261 40 Harvey J R, Hudd W H R and Townsend S 1973 Personal dosimeter for measuring the dose from

thermal and intermediate energy neutrons and from gamma and beta radiations Proc. IAEA Conf. on Neutron Monitoring for Radiation Protection Purposes, Vienna (Vienna: IAEA) ppl99-216

41 Hoy J E 1972 Personnel albedo neutron dosimeter with thermoluminescent 6LiF and 7LiF Savannah River Lab. DP-1277

42 ICRP Publication 21 1973 Data for protection against ionising radiation from external sources Supplement to ICRPPubl. 15 (Oxford: Pergamon)

43 ICRU Report No 19 1971 Radiation Quantities and Units (Washington: ICRU)

256 J A Douglas

44 ICRU Report No 26 1977 Neutron Dosimetry for Biology and Medicine (Washington: ICRU) 45 Iga, K, Yamashita T, Takenaga M, Yasuno Y, Oonishi H and Ikedo M 1977 Composite TLD based

on CaS04:Tm for 7-ray, X-rays, (3-rays and thermal neutrons Health Phys. 33 605-10 46 Ikeya M, Ishibashi M and Itoh N 1971 Thermoluminescent response of the mixture of CaS04(Mn)

and Li2S04 to thermal neutron and 7-ray fields Health Phys. 21 429-33 47 Karzmark C J, White J and Fowler J F 1964 Lithium fluoride thermoluminescence dosimetry Phys.

Med. Biol. 9 273 48 King S D 1974 Studies of the properties of lithium fluoride powders of different particle size in

relation to thermoluminescence dosimetry Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), pp873-95

49 Kirk R D, Schulman J H, West E J and Nash A E 1967 Studies on thermoluminescent lithium borate for dosimetry Proc. Symp. on Solid-State Chemical Radiation Dosimetry, Vienna p91

50 Kocher L F, Nichols L L, Endres, G W R , Shipler DB and Haverfield A J 1973 The Hanford thermoluminescent multi-purpose dosimeter Health Phys. 25 567-73

51 Lakosi L, Szabo P P and Makra S 1975 BeO as a thermoluminescent dosimeter Central Research Institute for Physics, Budapest KFKI-75-10

52 Lakshmanan A R, Rajendran K V, Ayyangar K and Madhvanath U 1976 Thermal neutron and gamma ray mixed field dosimetry with Li ;B407 :Mn Health Phys. 30 489-91

53 Lazanoff A S and McLaughlin J E 1969 Feasibility study of using LiF detectors in a neutron flux integrator United States Atomic Energy Commission HASL-206

54 Lippert J and Mejdahl V 1967 Thermoluminescence readout instrument for measurement of small doses Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF 650637 p204

55 Longworth J P 1970 A neutron flux spectrometer with nearly constant sensitivity over the energy range thermal to 14 MeV Berkeley Nuclear Lab. RD/B/N/1416

56 Lucas A C and Kapsar BM 1977 The thermoluminescence of thulium doped calcium fluoride Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen), p 131

57 Lucas A C, Moss R H and Kapsar B M 1977 Thermoluminescent CaF2(Tm) and method for its use US Patent No 4039 834

58.Marshall M, Douglas J A, Budd T and Churchill W L 1977 A two temperature readout of thermo­luminescent LiF, its properties and uses for personnel dosimetry Proc. 4th Int. Congr. of Inter­national Radiation Protection Associations, Paris, April Communication N307

59 Mason E W 1970 The effect of thermal neutron irradiation on the thermoluminescent response of CON-RAD type-7 lithium fluoride Phys. Med. Biol. 15 79-90

60 Majborn B, Botter-Jenson P and Christensen P 1972 Thermoluminescence dosimetry applied to areas with mixed neutron and gamma radiation fields Proc. Symp. on Dosimetry in Agriculture, Industry, Biology and Medicine, Vienna (Vienna: IAEA) IAEA-SM-160/25, pl69

61 Mayhugh M R, Watanabe S and Muccillo R 1971 Thermal neutron dosimetry by phosphor activa­tion Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, October. Riso Rep. 249, pp 1040-50

62 Mohammadi H and Ziemer P L 1978 Thermoluminescence dosimetry of fast neutrons using activated lithium borate phosphors Nucl. Instrum. Meth. 155 503-6

63 Morato S P, Rzyski B M and Nambi K S V 1977 Development of hydrogen doped crystals for fast neutron detection Int. Conf. on Defects in Insulating Crystals, Gatlinburg, Tenn., October

64 Nash A E and Johnson T L 1977 LiF(TLD-600) thermoluminescence detectors for mixed thermal neutron and gamma dosimetry Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February (I Physikalisches Institut, Universitat Giessen)

65 Odor D L and Ziemer P L 1973 Photon induced fading of lithium borate thermoluminescent dosi­meters Proc. 18th Annual Meeting of Health Physics Societies, Miami Paper 14

66 Otterberg J E and Cipolla S J 1977 Neutron dose monitoring using a multiple lithium fluoride TLD badge Health Phys. 33 256-8

67 Pearson D W and Moran P R 1975 Fast neutron activation dosimetry with TLDs Wisconsin Univer­sity Rep. COO-1105-227

68 Pells G P and Hughes A E 1976 An attempt to fabricate an ion injection type fast neutron dosi­meter Harwell Rep. AERE-R 8554

Applications of TL materials in neutron dosimetry 257

69 Piesch E and Burgkhardt B 1974 An LiF albedo neutron dosimeter for personnel monitoring in mixed radiation fields Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), pp 1123-40

70 Piesch E, Burgkhardt B and Kabadjova S 1975 Supralinearity and re-evaluation of different LiF dosimeter types Nucl. Instrum. Meth. 126 563-72

71 Piesch E 1977 Progress in albedo neutron dosimetry Nucl. Instrum. Meth. 145 613-9 72 Piltingsrad H V and Engelke MJ 1973 A passive broad-energy-response neutron spectrometer-

dosimeter Proc. Symp. on Neutron Monitoring IAEA-SM-167/75 vol 1 73 Prokic M 1971 Determination of the sensitivity of the CaF3:Mn thermoluminescent dosimeter to

neutrons Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, October. Riso Rep. 249, pp1051-62

74 Puite K J 1969 Thermoluminescent response of CaF2:Mn mixed with organic liquids in thermal and fast neutron fields Health Phys. 17 661-7

75 Puite K J 1971 Thermoluminescent sensitivity of CaF ;:Mn in a mixed neutron-gamma field Health Phys. 20 437

76 Reddy A R, Ayyangar K and Brownell G L 1969 Thermoluminescence response of LiF to reactor neutrons Radiat. Res. 40 552-62

77 Rogers D W O, Walsh M L, Orr B H and Teekman N 1977 Albedo dosimeter response to mono-energetic neutrons Health Phys. 33 251-4

78 Scarpa G 1970 A study of the dosimetric properties of beryllium oxide Proc. 2nd Int. Congr. of International Radiation Protection Associations, Brighton (Abstract in Health Phys. 19 91)

79 Scarpa G, Benincasa G and Ceravolo L 1971 Further studies on the dosimetric use of BeO as a thermoluminescent material Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, October. Riso Rep. 249, pp427-41

80 Schulman J H, Kirk R D and West E J 1967 Use of lithium borate for thermoluminescent dosi­metry Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF 650637, p 113

81 Simpson R E 1967 Response of lithium fluotide to reactor neutrons Proc. 1st Int. Conf. on Lumin­escence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF 650637, pp444-56

82 Singh D, Burgkhardt B and Piesch E 1977 A passive neutron spectrometer and dosimeter using LiF: Mg, Ti thermoluminescent detectors Nucl. Instrum. Meth. 142 409-15

83 Snyder W S and Neufeld J 1957 Protection against neutron radiation up to 30 MeV NBS Hand­book p63

84 Snyder W S 1971 Dose distributions in a cylindrical phantom for neutron energies up to 14 MeV Protection against neutron radiation. NCRP Rep. No 38, pp46-84

85 Spumy Z, Novotny J and Hedvicakova L 1971 Thermoluminescent dosimetry using lithium fluoride in aqueous suspension Phys. Med. Biol. 16 295-301

86 Spumy F, Kralik M, Medioni R and Portal G 1976 A new thermoluminescent dosimeter for 14.7 MeV neutrons Nucl. Instrum. Meth. 137 593-4

87 Sunta C M, Nambi K S V and Bapat V N 1973 Fast neutron response of thermoluminescent detec­tors with the proton radiator technique Proc. Symp. on Neutron Monitoring for Radiation Protec­tion Purposes, Vienna IAEA-SM-167/10

88 Szabo PP 1975 Investigation of properties of CaS04:Dy thermoluminescent dosimeters Central Research Institute for Physics, Budapest KFKI-75-1

89 Takenaga M 1977 Thermoluminescent response to thermal neutrons of mixture of CaS04:Tm and non-luminous "LiF 7. Nucl. Sci. Tech. 14 292-9

90 Tanaka S and Furuta Y 1974 Neutron responses of thermoluminescent dosimeters, BeO(Na), CaS04(Tm) and its mixture with 6LiF and 7LiF Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, August (Krakow: Institute of Nuclear Physics), pp 1213-28

91 Tanaka S and Furuta Y 1976 Usage of a thermoluminescence dosimeter as a thermal neutron detector with high sensitivity Nucl. Instrum. Meth. 133 495-9

92 Tanaka S and Furuta Y 1977 Revised energy responses of 6LiF and 7LiF thermoluminescence dosimeters to neutrons Nucl. Instrum. Meth. 140 395-6

93 Thompson J J and Ziemer P L 1973 Thermoluminescent properties of lithium borate activated by silver Health Phys. 25 435-41

94 Tochilin E, Goldstein N and Miller W G 1969 Beryllium oxide as a thermoluminescent dosimeter

258 J A Douglas

Health Phys. 16 1-7 95 Unruh C M, Baumgartner W V, Kocher L F, Brackenbush L W and Endres C W R 1967 Personnel

neutron dosimeter developments Proc. Symp. on Neutron Monitoring for Radiological Purposes, Vienna p433

96 Wallace R H and Ziemer P L 1969 Studies on the thermoluminescence of manganese activated lithium borate Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAECRep. CONF 680920, pp 140-7

97 Wallace R H, Ziemer P L, Kastner J and Oltman B G 1971 The relationship between encapsulation and apparent fast neutron induced fading in TLD Health Phys. 20 221

98 Weng P S and Chen K M 1974 Response of CaSO„(Dy) phosphor to neutrons Nucl. Instrum. Meth. 117 89-92

99 Wingate C L, Tochilin E and Goldstein N 1967 Response of lithium fluoride to neutrons and charged particles Proc. Jst Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAECRep. CONF 650637, pp421-34

100 Woodley R G and Johnson N M 1967 Thermoluminescence induced by low-energy alpha particles Proc. 1st Int. Conf. on Luminescence Dosimetry, Stanford University, June 1965. USAEC Rep. CONF 650637, pp502-6

101 Yamashita, T, Nada N, Onishi H and Kitamura S 1970 Calcium sulphate activated by thulium or dysprosium for thermoluminescence dosimetry Health Phys. 21 295-300

Applied Thermoluininescence Dosimetry. Eds M Oberhofer and A Schannann © 1981 ECSC, EEC. EAEC. Brussels and Luxembourg

13 Glow-curve analysis

A c LUCAS

13.1. Introduction

Since the beginning of research in thermoluminescence dosimetry, the shape of the glow curve obtained in heating thermoluminescent materials has been regarded as important in determining the nature of the material itself and the key to its acting as a suitable dosemeter. In recent years, the role of thermoluminescence dosimetry in personnel dosimetry has increased to the point that a large share of film dosemeters have been replaced. A characteristic of film dosemeters, often lamented as missing in thermo­luminescence dosemeters, is the fingerprint which may be present after the initial readout. It may be used in studying the conditions of the exposure of the film. Such a fingerprint is also present in thermoluminescence dosemeters in the form of the glow curve. In recent years, the glow curve has been shown to be of importance in a variety of ways as a support measurement in dosimetry. The. discussions that follow provide a beginning to analysis of some of the methods which can be used to analyse the glow curve for dosimetric purposes.

13.2. Recording of glow curves

Glow curves may be recorded in a variety of ways. Each may have advantages or dis­advantages, depending upon the particular program requirements. The simplest method, sketched in figure 13.1, is to generate linear glow curves on a strip chart recorder which is connected to an electrometer amplifier. The method requires range-changing, either automatic or manual, in order to meet the usual demands for recording data over

ELECTROMETER

Figure 13.1. Simple method for generating linear glow curves.

260 A C Lucas

a wide range of dosemeter exposures. Interpretation of recordings involving range changes requires some skill and training.

Range changes may be eliminated in strip chart recordings by incorporating logarithmic electronics in the system. The resulting glow curve may be interpreted some­what more easily, since the apparent shape is independent of the value of the exposure. Such systems can be made to operate in the range from 1 pA to 100 /iA without signifi­cant non-linearity. In such a system, sketched in figure 13.2, faithful recording of glow curves is possible, over exposures ranging from 1 mR to 500 R.

STRIP CHART RECORDER

Figure 13.2. Method for generating logarithmic glow curves.

Systems which digitise the anode current from the photomultiplier directly require an alternative method for driving a strip chart recorder. Figure 13.3 shows a block diagram of one such method. The pulses from the digitiser are fed to a counting ratemeter circuit which is in turn connected to the strip chart recorder. The method has a disadvantage in speed of response, since smoothing must be used to avoid the display of single pulses at low counting rates.

PMT

CHARGE DIGITIZER

STRIP CHART RECORDER

Figure 13.3. Method for generating logarithmic glow curves from a charge digitiser.

An alternative system which may be used in conjunction with systems which digitise the current directly is shown diagrammatically in figure 13.4(a). The system uses computer memory to store the counts as a function of time during the generation of the glow curve. The data may then be scaled and summed as desired after accumulation. Further, the data may be stored compactly using magnetic recording devices, for rapid recall at a later date. Such a device is shown in figure 13.4(6).

Glow-curve analysis 261

PMT

CHARGE DIGITIZER

MICRO­PROCESSOR

MAIN COMPUTER

DISC MEMORY

(a)

Figure 13.4. (a) Sketch of a system for storing digitised glow curves, (b) Harshaw Model 2080 digitised glow-curve displayer.

13.3. Measurement of neutron dose equivalent

Deep traps have been shown to exist in both LiF : Mg,Ti and CaF2: Tm, which are some­what more sensitive to neutrons than are the normally instrumented traps. Figure 13.5 shows a sketch of the glow curves obtained for LiF irradiated alternately with neutrons and gamma rays. While the total amount of light available from the deep-trap measure­ment is not great, the gamma-ray rejection obtainable using this method is very good. Care must be taken in arriving at a model for calculating the neutron and gamma-ray exposures when using such materials in a practical system. Consideration of several models has led to the following method for analysing LiF data in practical situations.

First, define the variables:

(i) R i is the integral over the main peaks, (ii) R2 is the integral over the deep trap,

(iii) Bi is the integral over the main peaks for no exposure, (iv) B2 is the integral over the deep trap for no exposure, (v) g is the response of the deep trap to neutrons divided by its response to gamma

rays, and

262 A C Lucas

100 200 T.°C

Figure 13.5. Sketch of the glow curves obtained for LiF irradiated alternately with neutrons and gamma rays.

(vi) / is the response of the deep trap to gamma rays divided by the response of the main peak to gamma rays.

Now, let the system be calibrated so that it is direct-reading for gamma-ray exposures and readout of the main peak. Then for the deep trap

(R2-B2)=fgDn+f(Rl-Bl) and for the main peak

(R1-B1)=Dy + kDn

where h — g/10. Now, solving (13.1) for£)n gives

(R2-B2)-f(Rl-Bl) D„

fg Substituting (13.3) into (13.2) and letting h = g/10, we obtain

(R2-B2) Dy=l.l(Rl-Bl)- 10/

Typically,/has the value 0.005, so that

(R2 -B2)- 0.005(R{-Bi) D„

and

Dy=l.l(R1-B1)

0.005^

0.05

(13.1)

(13.2)

(13.3)

(13.4)

(13.5)

(13.6)

Glow-curve analysis 263

100 200 T,°C Figure 13.6. Sketch of the glow curve for CaF2: Tm.

Figure 13.6 shows a sketch of the glow curve for CaF2:Tm. Because of numerical differences in the neutron sensitivity and in the amount of light normally emitted in the readout process, a slightly different approach is used in analysing the readout. Define the terms Ru R2, B1 and B2 as before. In this case, the precision of the method is strongly dependent upon establishing sensitivities for the two species of trap separately instead of relying on a common ratio/. The solution is best derived by defining:

(i) S1 as the response of the first peak to gamma rays, and (ii) S2 as the response of the second peak to gamma rays.

Further, since the neutron response is relatively small and of the same order of magni­tude for the two traps, it is as well to treat them separately in the derivation, for clarity.

For the deep trap

(R2-B2) = S2(gDn + DJ

and for the lower-temperature trap

(Rl-Bl) = Sl(hDn+D1).

Combining equations (13.7) and (13.8) yields

1 / Dn=

(x-h)\

R-2 -»2 -*M -"1

" ) •

(13.7)

(13.8)

(13.9)

In practice, the quantity (g - h) is measured directly in neutron calibration using this equation and S1 is near unity as a result of gamma-ray calibration. Further, since the neutron response for the first peak is quite small, we have

0 , R1-B1

Si (13.10)

264 A C Lucas

Typical values for the response g for TLD 600 deep traps is shown in figure 13.7. For these data, the dosemeter was operated as an albedo dosemeter on a water phantom. Further, typical values for the response g for TLD 700 are shown in figure 13.8. In this case, the dosemeter material was placed in a simple polypropylene holder for exposure, since there is no thermal-neutron sensitivity of consequence.

The response of CaF2:Tm, shown in figure 13.9, is principally due to fast neutrons and has a threshold near 3 MeV. No significant thermal-neutron response was noted.

(TL/RAD)n (TL/RADJA 1000

100-

TLD-600 WITH 10cm CH2

Figure 13.7. Energy dependence for the deep traps in TLD 600 operated as a neutron albedo dosemeter.

10

1.0

TLD-700

En,MeV 0.1 10

Figure 13.8. Energy dependence for the deep traps in TLD 700 irradiated with neutrons.

'

-

CALCIUM FLUORIDErTHULIUM

X /d .d

\ /

En,MeV Figure 13.9. Energy dependence for the deep traps in CaF2:Tm irradiated with neutrons.

Glow-curve analysis 265

However, care must be taken to avoid exposure to low-energy x-rays which produce an effect similar to that of fast neutrons. Further, care should be taken to avoid the effects of fading of the lower-temperature traps.

13.4. Beta-ray measurement

Laskey and Moran [1] have shown that boron may be diffused into LiF:Mg,Ti in such a way as to induce thermoluminescence, producing a glow curve at a temperature well above that at which the normal peaks appear. Figure 13.10 shows their comparison of glow curves obtained for alternate gamma-ray and beta-ray exposures of a dosemeter. The data demonstrate that, in principle, it becomes possible to observe the effects of gamma rays and beta rays independently in a single dosemeter. The method depends for its operation not only on the separation of the two glow curves but also on the fact that low-energy beta rays do not penetrate LiF to a great depth.

(O t -2 3 >-tr < IT 1-m a. <

A

A f B / \ / ^ / \ / ' i

/ '

/ / /

/ /

/

300 0 100 200 TEMPERATURE CO

Figure 13.10. Single-crystal TLD 100 with boron diffused into the first 2.0 Mm. Curve A is response to beta rays from tritium; curve B is response to gamma rays from caesium.

Development of the method for practical dosimetry requires that consideration be given to the relative contributions to the two separate glow-curve structures by gamma rays and beta rays. The two separate regions in the dosemeter may be considered to be two similar dosemeters from the point of view of the gamma rays but thin in one case and thick in the other when considered from the point of view of the beta rays. A typical energy dependence for such a case is shown in figure 13.11.

Mathematically, then, the measurement involves the solution to the familiar two-detector problem. For the thick part

and for the thin part

R2 = pG + mB.

(13.11)

(13.12)

266 A C Lucas

BETA RAY ENERGY. MeV

Figure 13.11. Sketch of the energy dependence which would be expected for very thin and very thick dosemeters irradiated with beta rays.

If p is small, then the beta-ray dose is

B=R2/m

and the gamma-ray dose is

G = Rt -kR2/m.

(13.13)

(13.14)

While this method has been shown to be feasible, it has not been applied in personnel dosimetry.

13.5. Fading correction

While many dosimetric materials exhibit little or no fading in routine applications, the most-used materials, LiF:Mg,Ti and CaF2:Dy may exhibit fading under some conditions of use. In particular, LiF:Mg,Ti is often used in the thermally quenched state in personnel dosimetry as a matter of convenience. In this state, the lower-temperature

QUENCHED

ANNEALED

100 200 T,°C

Figure 13.12. Comparison of the glow curves for iJF:Mg,Ti used alternately in the annealed and quenched states.

Glow-curve analysis 267

trapping centres exist in greater numbers than when the material has been subjected to a rigorous annealing procedure. Figure 13.12 shows a comparison of the glow curves for the two states of the material.

The LiF: Mg,Ti glow curve has been studied in rigour by a number of authors. It is generally found to consist of a combination of six discrete trapping centres, some of which interact with each other. Grant et al [2] have been able to reconstruct the glow curve analytically by considering each trapping centre to act mathematically:

y = / 0 p e x p ( - f p d r i (13.15)

where

p=Se-E/kT (13.16)

p is the probability that charge will be untrapped, S is a constant characteristic (s_1) of the trapping centre, E is another constant characteristic (eV) of the centre, k is Boltzmann's constant, 1.38 x 10~16 erg K_1, and Tis the absolute temperature (K).

In that work, the trapping centres were found to be described by constants having the values in table 13.1. Numerical evaluation of the probability of untrapping for reasonable ambient temperatures shows that traps 1, 2 and 3 are most affected.

Table 13.1.

Trap S(s"') E (eV)

1 2 3 4 5

5 .00X10" 2 .00X10" 5.00X10'2

1.20X10'2

2.20X1013

0.82 1.08 1.13 1.15 1.36

In practice, it is usually best to take advantage of experimental fading data to form a set of corrections. One can show that, if the logarithm of the thermoluminescence from one peak is plotted as a function of the logarithm of the thermoluminescence from another peak, a straight line will result. It seems reasonable, then, to adopt that way of plotting as a standard method to achieve independence of temperature in applying the data. One such example is illustrated below.

Figure 13.13 shows a sketch of the LiF glow curve with peak 2 shaded. In this example, peak 2 is used as an indicator of fading and the integral under the curve (the shaded portion) is accumulated separately and in addition to the integral over the whole glow curve. In order to produce the fading effect for this example, dosemeters were held at temperatures ranging from 50 to 100 °C for 10 min. Figure 13.14 shows the value of the complete integral plotted as a function of the integral over peak 2. The data form a relation which can be used to correct other experimental data where the fading is unknown.

268 A C Lucas

T,°C

Figure 13.13. Glow-curve analysis used in fading correction example.

1.0

0.8

0.6

0.4

0.2

o P/S

Figure 13.14. Value of the glow-curve integral plotted as a function of the integral over peak 2 (the shaded peak) of figure 13.13.

13.6. Determination of time from exposure

Occasionally, where personnel dosemeter data are not clearly the result of known occupa­tional exposures, it becomes useful to determine the time between exposure and readout of the dosemeter. Algebraically, the time (in seconds) is

In (///„) -Se-ElkT (13.17)

where J is the measured luminescence for a given low-temperature peak, e.g. peak 2 or 3 for LiF, J0 is the value of J if no time had elapsed, S, E and k are as defined above and T is the absolute temperature (K).

The principal uncertainty in this method derives from uncertainty in estimating T. A single degree error in its estimate results in error of the order of 10% in time estima­tion. Where high precision is required in the estimate, the temperature may be deter­mined by measuring the fading for a dosemeter which has been exposed and held under controlled conditions for periods comparable to the subject exposure. This method has been successfully applied in several investigations with good results.

Glow-curve analysis 269

13.7. Verification of data

Any given TL measuring system has, in principle, its own set of most probable failure modes. Experience with systems having continuously operated strip chart recorders has shown that operators may be trained to recognise faults in th shape of the glow curve which may then be used to invalidate incorrect data. One such system which has been operated in such a way is the Harshaw Model 4000 TL analyser.

In principle, it is further possible to effect automatic recognition of faults where glow curves are accumulated in computer memory. Some specific faults which are detectable are:

(1) lack of a glow peak, (2) failure to heat, (3) electrical interference, (4) glow peak at wrong temperature, (5) saturation of readout, and (6) high dark current.

References

1 Laskey J B and Moran PR 1977 TLD-100 diffused with boron: a new 'surface sensitive' TL phosphor Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo, February 14-17 (Giessen: I Physikalisches Institut of the Justus-Liebig Universitat)

2 Grant R M Jr, Stowe W S and CorellJ 1968 Computer assisted theoretical and experimental analysis of the thermoluminescence of LiF:Mg Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAECRep. CONF 680920

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann ©1981 ECSC, EEC, EAEC, Brussels and Luxembourg

14 Application of TLD in medicine

A F McKINLAY

There are two important areas of absorbed dose measurement in medicine:

(i) absorbed dose measurement in radiotherapy, and (ii) absorbed dose measurement in diagnostic radiology.

14.1. Radiotherapy measurements

The difficulties of accurately predicting absorbed doses in radiotherapy by calculation have in the past led to the development of in vivo measurement techniques. While entrance and exit absorbed doses could be measured using films and conventional ionisa-tion chambers, intracavitary measurements were limited by the minimum size of available ionisation chambers [1] (typically 20 mm x 5 mm diam.).

When Daniels first developed thermoluminescence as a practical method of assessing ionising radiation exposure, it was realised that the technique could be applied in the field of clinical measurement. Brucer used some of Daniels' single crystals of Harshaw LiF to make internal in vivo measurements in cancer patients injected with radioactive material [2].

The arrangement of radiotherapy treatment fields is conventionally carried out using a combination of calculations involving standardised geometries together with depth dose and transverse dose measurements in phantoms. The final check on the absorbed dose delivered to the patient can be carried out by in vivo dosimetry. Similarly, absorbed doses to organs not involved in the treatment, which should be kept to the minimum, can be measured.

Thermoluminescence dosimetry has proved a useful technique for a variety of purposes in radiotherapy, including measurements of therapy machine output, beam uniformity checks and the measurement of absorbed dose in phantoms and in vivo for both internally and externally applied fields. Thermoluminescent dosemeters have a high precision, provide rapid retrieval of information (using on-site readers), have good environmental stability, have good water or tissue equivalence and have a wide range of sensitivities. The last characteristic is particularly important for in vivo measurements of absorbed dose. Because of their small size, thermoluminescent dosemeters also give good spatial resolution. This is of particular value in many radiotherapy techniques where the absorbed dose has a rapid spatial variation. Thermoluminescent dosemeters may also be used to measure the absorbed dose to experimental animals.

14.2 Diagnostic radiology measurements

The collective dose from medical exposure has been estimated to represent the largest single man-made contribution to both the somatic and the genetically significant dose

272 A FMcKinlay

equivalent to the population of the United Kingdom as illustrated in figure 14.1 — representing some 31% of the total somatic dose and 9% of the total genetic dose, and 95% and 85% respectively of the man-made contributions [3]. In other developed countries, the estimated figures are similar. By far the largest contribution is from diagnostic radiology, estimated as 10 times the sum of the contributions from nuclear medicine and radiotherapy.

rQk\

FALL-OUT, 0 .6% /U^IISCELLANEOUS SOURCES, 0.4%

^^OCCUPATIONAL EXPOSURE,0.4% RADIOACTIVE WASTE DISPOSAL,0.2%

Figure 14.1. Annual genetically significant dose equivalent to the UK population. (Taylor and Webb [3], reprinted with the permission of the National Radiological Protection Board, Chilton.)

New radiodiagnostic techniques have been introduced, some involving the use of complex machines such as the head and body scanners (computerised axial tomography, CAT). Thermoluminescence dosimetry has proved to be a useful method in the comparison of patient absorbed dose from these new techniques as well as from the more traditional ones [4-6].

Diagnostic absorbed dose measurements are important for:

(i) improving the design of equipment to reduce patient absorbed dose, and (ii) improving radiographers' techniques in the use of equipment to reduce patient

absorbed dose, and for providing a measurement database for epidemiological analysis of population radiation absorbed dose from diagnostic radiology.

Thermoluminescent phosphors such as LiF:Mg,Ti and particularly the more tissue-equivalent Li2B407:Mn have been used for such measurements. Thermoluminescent dosemeters have three main advantages over ionisation chambers for this type of measurement:

(a) they are small and unobtrusive, (b) they are radio-transparent to most x-radiation, and (c) they do not require connecting leads and are easily attached to the patient.

Application of TLD in medicine 273

14.3. Factors in the choice of dosemeters for clinical use

A number of factors have to be considered in the choice of the material and form of the dosemeters for clinical dosimetry. The most important factors are:

(i) estimated absorbed dose range of the intended measurements, (ii) estimated equivalent photon energy or linear energy transfer (LET) of the radiation, (iii) immediate environmental conditions around the dosemeter, and spatial resolution.

14.3.1. Absorbed dose range

The sensitivity of any thermoluminescent dosemeter is proportional to the mass of active phosphor present, within limits imposed by geometrical and thermal considerations relevant to the readout system. Dosemeters which contain only thermoluminescent material, e.g. powder, extruded ribbons and rods, have a much higher sensitivity than dosemeters consisting of a phosphor held in a matrix of binder material, e.g. PTFE-based discs, tape and micro-rods.

The range of absorbed doses encountered in clinical and biological irradiations is very large. In diagnostic radiology, absorbed doses may range from less than 10/zGy to lOOmGy to the gonads and from approximately lOOjuGy to about 100mGy to the skin. Hence, at the lower end of the diagnostic radiology range, there is a need for a sensitive form of dosemeter, e.g. powder or extruded forms. However, in therapy dosimetry, a single treatment fraction of absorbed dose may be several grays, and, in many animal or cell irradiations, tens of grays may be required. Less-sensitive forms of dosemeter can be used, especially those incorporating the phosphor powder in PTFE as discs, tape and micro-rods. The measurement of high absorbed doses will often necessitate using the dosemeter above the linear thermoluminescence absorbed dose response region. Under these conditions, the'degree of supralinearity needs to be determined. Often, in radio-therapy, simultaneous measurements of absorbed dose in the treatment region and the much smaller absorbed dose in the shadow of shielding may be required. In such cases, two types of dosemeter, such as extruded rods and PTFE-based rods, may be used in parallel. This particular combination of dosemeters, if appropriately oriented with respect to the absorbed dose gradient, should provide good spatial resolution.

14.3.2. Photon energy range and LET of radiation

Modern routine radiotherapy and radiodiagnostic techniques use a wide range of photon energies from conventional x-ray machines of approximately 10 keV to 200 keV, from 137Cs (0.67 MeV) and 60Co (1.25 MeV) teletherapy units and from accelerators producing high-energy electrons and megavoltage photons.

The total light emitted by an irradiated phosphor is proportional to the total radiation energy absorbed by it. In tissue, the absorption of Compton-scattered electrons is the most important absorption process in the photon energy range from approximately 20keV to 10 MeV. For elements such as lithium, boron, oxygen, fluorine, etc, of low atomic number, and for photon energies up to about 15 keV, the photoelectric effect is dominant. Thereafter, up to 10 MeV, Compton scattering is most important. For elements of high atomic number, such as those used as dopants (e.g. Ii, Mg, Mn, etc), the photoelectric effect is dominant up to several hundred kiloelectronvolts.

274 A FMcKinlay

The advantage of using materials consisting mainly of atoms of low atomic number with only a few dopant atoms of higher atomic number is obvious because of their good approximation to tissue and air. This is particularly true of phosphors based on lithium borate (figure 14.2).

0.0

1.0

-

-

I ' ' '

A

"" B ~^X

c \ \

D \ / \ S . —-N~C*^»-

E

I i ' '

1 '_

-

-

-

^^^ -

10 10" 10' 103

PHOTON ENERGY (keV)

Figure 14.2. Photon energy responses of a number of phosphors relative to that of air.

Curve

A B C D E

Phosphor

CaF2

CaSO„ A1203

LiF Li2B40,

Photon effective atomic number

16.3 15.3 10.2 8.2 7.4

In order to evaluate the absorbed dose to a phantom or patient using thermolumi­nescent dosemeters, it is essential to know the relative energy responses of the dosemeters throughout the range of energies used. The primary response calibration of dosemeters is usually carried out using a 60Co source (1.25 MeV) and the responses at all other energies and for all other radiations are expressed as multiples or fractions of this. For clinical applications, the response is most usefully expressed as the light emitted per unit absorbed dose in tissue or water. This will be a function of radiation energy and of the physical form of the dosemeter.

The beam quality has to be known. Photon-beam quality determination, although in principle relatively straightforward to determine in free air, is difficult to determine uniquely in water or solid phantoms because of two effects:

(1) the contribution from lower-energy scattered radiation from the phantom and external shielding material, e.g. the applicator cone at short focus-to-skin distances (also at low photon energies the effects of dosemeter orientation and self-shielding become increasingly important (figure 14.3)), and

Appl

110

£ 100

£ 90 an

> BO <.

i

Q= 70

60

cation of TLD in medicine

i i _ _ — i '

^^PERPENDICULAR

_ ' ^~~ ^ /PARALLEL

; / :

1 1 1 1

275

0 1 2 3 4 5 HALF-VALUE THICKNESS (mm AD

Figure 14.3. The measured responses of LiF extruded ribbon dosemeters, exposed with their square faces parallel and perpendicular to the axis of the incident radiation. (Morgan and Bateman [7], reprinted with the permission of Pergamon Press Ltd.)

(2) the effective 'hardening' of the beam with increasing depth in the water or solid phantom.

The thermoluminescence absorbed dose (water and polystyrene) response of LiF:Mg,Ti to high-energy radiations, of which those of principal interest in clinical applications are megavoltage photons and high-energy electrons, has been widely investi­gated and reported in the literature. The results have often been inconsistent. Some investigators have measured approximately 10% reduction in response to high-energy radiations compared with that to ^Co gamma radiation, and others have not found any reduction. Much discussion has revolved around the application of various generalised cavity theories to attempt to explain the observed effects and to reconcile the differences. For useful reviews and entry into the literature of this subject the reader is recommended to read the papers of Paliwal and Almond [8] and Ruden and Bengtsson [9]. The relative energy responses of a number of different physical forms of LiF:Mg,Ti, powdered Li2B407:Mn and CaF2:Mn dosemeters for a range of photon and electron energies of particular relevance to clinical use are presented in table 14.1.

14.3.3. Environmental factors

Absorbed dose range and radiation energy considerations apart, environmental factors such as temperature, humidity, contact with body fluids, insertion into catheters, sterilisation, etc, influence the choice of dosemeter form and packaging. If dosemeters are not protected from their environment, the result is often low precision and sometimes gross error in absorbed dose measurement.

14.3.4. Temperature and humidity

During exposure under clinical conditions, dosemeters may come into contact with heat (human body core temperature is 37 C) and/or high-humidity environments. If implanted or introduced into body cavities, they can come into contact with body fluids. Some phosphors (especially in powder form) have been shown [14] to be affected by humidity (figure 14.4) as well as by storage at elevated temperatures which induces fading.

S'

Table 14.1.

^ \ ^ ^ Dosemeter ^ ^ ^

Radiation ^ \ ^

Photons 10 keV 17keV 23keV 37keV 51 keV 81 keV 97keV

6°Co 1.25 MeV 6 MV

18.5 MV 22 MV 35 MV 42 MV 50 MV 65 MV

Measured thermolu:

0.1 mm LiF-PTFE discs3

1.2 1.4 1.45 1.43 1.36 1.15 1.12

1.00 0.96 ---0.96 _ _

minescence per gray in water for thermoluminescent dosemeters relative to '

0.4-0.5 mm LiF-PTFE discs3

0.95 1.31 1.38 1.42 1.35 1.15 1.12

1.00 0.94 ---0.93 _ _

LiF extruded ribbons3

0.78 1.36 1.43 1.45 1.38 1.17 1.13

1.00 0.97 ---0.96 _ _

LiF-PTFE rods3

_ -_ ----1.00 0.97 ---0.97 _ _

LiF rods3

_ ------1.00 0.97 --0.90b

_ 0.91 b

0.91b

LiF powd

_ ------1.00 -0.94 0.93 -_ _ _

'"Co.

erc CaF2 powderc

_ --— ---1.00 -0.96 0.96 --_ _

Li2B„07:Mn powderc

_ --_ ---1.00 --0.98 --_ _

Electrons 4.3 McV 5 MeV 6 MeV 7.4 MeV 9 MeV 9.8 MeV 10.0 MeV 11.6 MeV 12 MeV 14.3 MeV 15 MeV 18 MeV 19.4 MeV 20 MeV 28.2 MeV 30 MeV 35 MeV 39.2 MeV 40 MeV

0.93 --0.93 -0.93 -0.93 -0.94 --0.96 -0.96 _ _ 0.98 _

0.90 0.90 0.92 0.92 - - -0.95d _ _ _ _ _

- 0.94 0.91 0.91 0.94 0.94 - _ ^ - - - - - 0.94 0.97 § 0.91 0.91 0.94 0.94 - - £r

0.90d - - - - - » 0.91 0.91 0.94 0.94 - - - § ' - - - - 0.93 0.98 0.96 o 0.91 0.92 0.95 0.95 - - - "^ - - - - 0.92 0.97 0.96 £ - - - - 0.89 0.97 0.97 s -0.92 0.93 0.96 0.96 - - - a

0.92d - - - - _ | 0.92 0.94 0.96 0.96 - - - S'

0.94d _ _ _ - S' 0.95d _ _ _ _ _

0.93 0.95 0.97 0.97 --,d 0.92"

a Ruden [10]. Turner and Anderson [11].

c Almond and McCray [12]. Bistrovic et al [13], lucite phantom.

- J

278 A FMcKinlay

80 LU

z o ft 60

LU > 40 t—

< i LU

20

-

"

" "

^\^

■

-

■ i i i 20 40 60 80

RELATIVE HUMIDITY (%) 100

Figure 14.4. Effects of humidity on the response of Li2B407:Mn 38 days pre­irradiation storage at 40 °C. (Mason et al [14].)

14.3.5. Other agents

If dosemeters in solid form are attached directly on to the skin using adhesive tape, care should be taken to remove all traces of adhesive from the dosemeters before readout. Adhesives often exhibit thermoluminescence following exposure to visible light and/or ultraviolet radiation. The simplest way of avoiding these effects is to seal the dosemeters inside protective envelopes (e.g. polythene).

14.3.6. Sterilising of dosemeters

Occasionally, a clinician or biologist will require dosemeters to be sterilised. The three common methods of sterilising, i.e. autoclaving, chemical sterilising and irradiation with 254 nm ultraviolet radiation, can all have a gross effect on the inherent sensitivity of the dosemeter or may induce spurious luminescence. In general, provided the phosphor is effectively sealed in a protective envelope or catheter which is opaque to the sterilising ultraviolet radiation, either chemical or ultraviolet sterilising at normal ambient tempera­

ture is recommended. Normal ambient temperature is emphasised, as the effects of elevated temperatures on the normal sensitivity of phosphors, especially LiF:Mg,Ti, can be significant. The effects of autoclaving can be particularly severe.

14.3.7. Spatial resolution

Good spatial resolution of absorbed dose measurement is generally useful, and is essential in the determination of high absorbed dose gradients. Thermoluminescent dosemeters are available in many shapes. Powder acts like a fluid and will adopt the shape of its container. The micro­rod and extruded ribbon (and hot pressed) dosemeters are so small, 1 x 1 x 6 mm3 and 3.2 x 3.2 x 0.9 mm3, respectively, that the effective size of the dose­

meter is often limited in practice by the requirement to have adequate build­up to ensure electronic equilibrium.

Application OF TLDin medicine 279

14.4. Radiotherapy absorbed dose measurements

14.4.1. Simple geometry phantoms

In radiotherapy, the specification of the complete absorbed dose distribution within the radiation beam in a phantom is a prerequisite to ensuring that the prescribed absorbed dose is delivered to the target volume in the patient. A common method is to employ published depth dose data and an isodose chart. An example of such a chart is illustrated in figure 14.5. This chart refers to a section containing the beam axis parallel to one side

5

_ i _ »

x o i— | 10 Q_ 2

X 1—

15

20

|

'-11^: ■ t \ \

J' K)°4

20%

—1—100%

- _ _ 90% - -

^ - _ _ e o % _ _ _ _ - ^ '

~- 70% - ^

~-___60%___ ^

•~^_ 50% - '

- 40% - -

30% ^

. -

l¥ yl \\ \ : / 1

y '\

20%/

1 1 —

1 1 —

10%

—

_ -

—

-

Figure 14.5. Radiotherapy isodose chart for a 60Co teletherapy unit. Field size lOXlOcm2

source­to­skin distance 80 cm.

of a 60Co 10x10 cm2 therapy beam for a fixed source­to­skin distance (SSD) of 80 cm. The lines mapped on the chart link points of equal absorbed dose expressed as a percentage of the peak absorbed dose. In the case of 60Co radiation, the peak absorbed dose occurs at the optimum 'build­up depth' in water, 5 mm. For x­ray beams produced with generating potentials of less than 400 kV, the depth doses are conventionally expressed as a percentage of the surface absorbed dose. Similar charts are used for fixed source­axis distance (SAD) beams. However, in these, the isodose values are expressed as a percentage of the absorbed dose at the target deep within the phantom, SAD isodose charts are used when the target volume is located on the axis of rotation of the tele­

therapy machine. The selection of an appropriate isodose chart can be difficult, as the absorbed dose distribution in the phantom depends on the beam dimensions, SSD or SAD, the radiation quality, the source size, the geometry of the beam and the positioning of the beam collimators. The International Commission on Radiological Units (ICRU) [15] therefore recommends the use of isodose charts which are exactly specified for the

280 A FMcKinlay

particular equipment being used. This criterion can be established, as 1CRU recommend, by a series of single measurements using an ion chamber or thermoluminescent dosemeters.

In their simplest form, the measurements consist of:

(1) measuring the depth dose distribution along the central axis of the beam in a water or water-equivalent phantom, and

(2) choosing one particular phantom depth (which in the case of 150 keV to 10 MeV and l37Cs and 60Co teletherapy beams, ICRU recommends as 5 cm) and measuring the radiation absorbed dose profile across the beam at this depth.

After normalisation of the depth dose measurements at 5 cm depth, the published dose data which one intends to use can be compared with them and corrected accordingly. Similarly, the measured beam profile can be compared with that obtained from the published isodose chart.

Since 1968, the International Atomic Energy Agency (IAEA) and the World Health Organisation (WHO) have been running a programme of intercomparison of ^Co tele­therapy units in the various radiotherapy centres throughout the world. This resulted from investigations carried out in 1965 which revealed that there was no suitably calibrated radiation measuring instrument in use in about 30% of the radiotherapy centres investigated [16]. A simple test procedure based on thermoluminescence dosimetry is used to assess the accuracy of delivered absorbed doses in the centres.

LiF:Mg,Ti powder dosemeters contained in PTFE capsules are sent to radiotherapy centres. The measurement technique used in this study illustrates:

(i) the practical use of thermoluminescent dosemeters for radiotherapy depth dose measurements in a simple phantom, and

(ii) methods to eliminate effects of fading and other variable environmental factors.

The procedure used is illustrated in figure 14.6.

LiF POWDER

STANCARD ANNEAL

IRRADIATE CONTROL

IRRADIATE REFERENCE

A

B

C

D

R

-^ TEST 2Gy'

TEST 2 mm

^ READOUT

Figure 14.6. Method employed by IAEA and WHO for the intercomparison of delivered absorbed doses from 60Co teletherapy units in various radiotherapy centres throughout the world. A and B are test measurement dosemeters. C and D are irradiated and unirradiated control dosemeters. R are calibration reference dosemeters. (Eisenlohr and Jayaraman [16].)

Application O/TLD in medicine 281

Participant centres are sent four sets of dosemeters. They are requested to irradiate one test set (A) with an absorbed dose of 2 Gy in water at 5 cm depth on the central axis of a ^Co 10x10 cm2 therapy beam with an 80 cm SSD. Another test set (B) is to be irradiated under similar conditions for 2 min. A control set (C) which has been given a known absorbed dose by IAEA and a control set (D) which is unexposed accompany sets A and B at all times except during irradiation. Sets C and D provide information about any environmental or spurious effects, such as thermal fading, unintentional irradiation, etc, which might adversely affect the test dosemeters. In addition, reference sets (R) are irradiated by IAEA in a standardised 10x10 cm2 60Co beam (SSD 80 cm) at a depth of 5 cm in water. The absorbed dose rate expressed in grays per minute in water is obtained from a measurement of the exposure rate in free air using a calibrated ionisation chamber. All dosemeters are then read out together, eliminating possible calibration errors due to fading and effectively standardising the readout procedure for all dosemeters.

14.4.2. In vivo measurements

While the measurement of complete absorbed dose distribution in a phantom is essential in planning the treatment of a patient, the ultimate check on the absorbed dose delivered to the patient can only be made by in vivo absorbed dose measurements. Thermo­luminescent dosemeters have proved to be particularly useful for this purpose.

The relevance of in vivo dosimetry is illustrated by the flowchart shown in figure 14.7. This flowchart is a simplified form of that used by ICRU [15] to illustrate a systems approach to radiotherapy. In vivo measurements verify that the absorbed dose prescribed by the clinician, and calculated and set up by the physicist and the radiographer, has been delivered. Further, it may be used to monitor any change in field uniformity caused by changes in the many treatment parameters.

In vivo measurements can be divided into four classes.

Admini -stration

Examine and

diagnose

Decide on

treatment

Measure patient

Set up patient

Start trea ment

-— Make

casts etc. for patient

Verify dose in vivo

Verify and

accept

Monitor further

treatment in vivo

Com jute dose

Assess res j i ts

Figure 14.7. Flowchart illustrating the relevance of in vivo dosimetry in radiotherapy planning and treatment.

282 A FMcKinlay

14.4.2.1. Class 1 -entrance absorbed dose measurements. These are used mainly to check the machine output, the absorbed dose distribution profile across the patient, particularly in the penumbra of shielding, and the positioning of shielding in relation to the patient. If the measured values are at variance with those prescribed and calculated, the cause can be investigated and appropriate corrective action taken. The spatial resolution afforded by thermoluminescent dosemeters is particularly useful in these measurements.

14.4.2.2. Class 2 - exit absorbed dose measurements. These are used mainly to check the absorbed dose delivered to points deep within the body. The measurements should agree with calculations for exit absorbed doses. For such measurements, the dosemeters should be provided with sufficient backscatter material. Again, good spatial resolution may be important.

14.4.2.3. Class 3 - intracavitary absorbed dose measurements. The absorbed dose within a body cavity, e.g. the mouth, nasopharynx, oesophagus, vagina, rectum, etc, can be measured using dosemeters sealed inside a catheter as shown in figure 14.8. The position of the dosemeters may be checked using radio-opaque markers and exposing an x-ray film. The increase in scattered radiation resulting from the presence of radio-opaque markers of high atomic number can cause uncertainties of a few per cent in the absorbed dose to the dosemeter. This can be measured and allowed for.

Figure 14.8. Solid forms of thermoluminescent phosphor. Left, TLD 700 extruded ribbon dosemeters; right, LiF-PTFE micro-rod dosemeters; centre, micro-rod dosemeters in catheter tubing. (Photograph courtesy of the National Radiological Protection Board, Chilton.)

14.4.2.4. Class 4 - individual spared organ absorbed dose measurements. The absorbed dose to spared (shielded) organs can be measured, but often no build-up can be used as this would in itself result in an increased absorbed dose to the organ.

Application of TLD in medicine 283

14.5. Examples of the use of TL dosemeters in radiotherapy

Thermoluminescent dosemeters are routinely used for a number of different types of in vivo radiotherapy measurements in centres throughout the world. The following examples illustrate some of the principles of their use.

14.5.1. Measurement of absorbed dose during 'mantle therapy' for Hodgkin's disease

The radiotherapy treatment of Hodgkin's disease involves the irradiation of a large area of the body. The treatment field is designed to deliver a therapeutic or prophylactic absorbed dose to the axillary, cervical and mediastinal lymph nodes. Many different treatment fields have been used, but one commonly used configuration consists of anterior and posterior parallel and opposed fields as illustrated in figure 14.9. There is

Figure 14.9. Mantle therapy treatment for Hodgkin's disease. Shielding is shown as a projection on to body surface.

also the need to shield presumed healthy organs, e.g. lips, eyes, lungs, kidneys, bone joints, etc, and for posterior irradiation the spinal cord needs to be shielded. To avoid excessive exposure of the skin (skin sparing), the treatment is carried out using either ^Co or megavoltage x-ray photons and the shielding is positioned some distance (typically 20 to 50 cm) above the entrance surface of the body. This is achieved using 'individually tailored' moulds of polystyrene, one anterior and one posterior, with appropriately cut out channels containing lead-shot shielding. Alternatively, appropriately shaped lead absorbers, placed on a Perspex plate and positioned above the body, have been used. The technique is termed the 'Gothic arch' or 'mantle' technique. A typical four-week course of treatment involves a total prescribed absorbed dose of between 30 and 40 Gy delivered in 20 fractions.

Thermoluminescent dosemeters, usually LiF extruded ribbons or rods or PTFE-based discs or micro-rods sealed in thin protective polythene sachets, are attached to the body under moulded blocks of wax or in small Perspex containers. The dimensions of the wax or Perspex are chosen to provide build-up appropriate to the photon energy of the beam and to ensure electronic equilibrium. With the use of high-energy photon radiation, e.g. 42 MV x-rays, Ruden [10] recommends the use of a maximum build-up of 15 mm Perspex and an experimentally derived factor to correct the apparent absorbed dose.

14.5.2. Intracavitary absorbed dose measurement

The small size and shape of the extruded and PTFE-based micro-rod dosemeters have enabled in vivo measurements of absorbed dose inside body cavities which hitherto were

284 A FMcKinlay

often difficult and sometimes impossible. As illustrated in figure 14.8, these dosemeter!, can easily be inserted and sealed in catheters.

A very good example of such applications is the in vivo measurement of absorbed dose distribution in the pelvis during intracavitary 226Ra and external beam therapy for carcinoma of the uterine cervix [17-19].

The dosemeters are introduced into the pelvis via the external femoral veins. In one technique [17], sterile PTFE catheters are first inserted in the veins. This process can be monitored using x-ray fluoroscopy and television. Inner tubes containing gold radio-opaque markers are then introduced into the catheter to assess accurately the intended positions of the TL dosemeters. The radium is then applied and its position in relation to the dosemeter markers can be assessed. The marker catheter tubes are then removed and replaced by two others, each containing 15 micro-rod dosemeters spaced some 16 mm apart. Using this technique of outer and inner catheters, there is no need to sterilise the inner dosemeter catheters.

14.6. Diagnostic radiology absorbed dose measurements

In the diagnostic range of photon energies, LiF phosphor can over-respond by as much as 40% compared with tissue. However, Li2B407:Mn is an extremely good match for tissue over this range of photon energies and, by adjustment of the fractional amount of manganese present, the response can be 'trimmed' to match more closely that of air, water or tissue; e.g. Jayachandran [20] suggested 0.34%w/w for air equivalence and Christensen [21] 0.45% w/w. Langmead and Wall [22] found that, using Li2B407:Mn powder containing 0.15% w/w of manganese, they could measure absorbed dose in tissue, from x-rays of unknown quality, with a predicted error associated with photon energy of not greater than ±5% and an overall uncertainty of not more than ± 15%. This is an' important characteristic of Li2B407:Mn phosphor because, while the effective energy of the primary beam can be assessed, at least in air, by half-value layer measurements, the quality of the lower-energy scattered radiation and the magnitude of its contribution to absorbed dose are difficult to assess. li2B407:Mn is therefore particularly suited for such measurements. While sensitive solid forms of li2B407:Mn, such as extruded ribbons, are commercially available, they are relatively expensive and this tends to exclude them from large-scale measurement programmes. Loose powder is probably most suitable for this application at present.

14.6.1. Human phantom measurements

In contrast with simple homogeneous phantoms, a most useful phantom for absorbed dose measurements in radiotherapy and diagnostic radiology is one which is designed, as far as is practicable, to simulate the structure of the human body. The torso and head and neck of such a phantom are shown in figure 14.10. In proportion, it is equivalent to an 'average man' 1.75 m tall and weighing 73.5 kg. It is made from tissue-equivalent synthetic rubber and contains a complete human skeleton, lung-equivalent material and airways corresponding to the maxillary sinuses, nasopharynx, trachea, etc. It is composed of a number of 25 mm thick transverse sections each containing a matrix of 5 mm diameter holes spaced 3 cm apart. Each hole can accommodate a dosemeter holder/

Application of TLD in medicine 285

Figure 14.10. Human-like phantom made from tissue-equivalent synthetic rubber. (Photograph courtesy of the National Radiological Protection Board, Chilton.)

Figure 14.11. Dosemeter capsules, each containing 30 mg of phosphor, being inserted into a transverse section of phantom. (Photograph courtesy of the National Radiological Protection Board, Chilton.)

capsule or a solid plug of tissue-equivalent material. The complete phantom contains over 3000 holes and additional ones can be drilled if required. Suitable Perspex or polythene capsules can each contain approximately 35 mg of powdered phosphor.

Human-like phantoms are extremely useful for absorbed dose measurements in diagnostic radiology where their human form allows not only precise and realistic positioning of 'the patient' in the beam but also positioning of dosemeters to measure the absorbed dose to specific organs, including the gonads. Combined with the use of thermo­luminescent dosemeters, they have proved especially useful in the assessment of'patient' absorbed dose imparted by computerised axial tomography (CAT) (e.g. [4, 5]). The recent measurements by Wall were performed using Li2B407:Mn powder dosemeters contained in plastic containers and inserted in the phantom slices as illustrated in figure 14.11. In

286 A FMcKinlay

these examinations, regions of dosimetric interest included not only the section of the patient (phantom) undergoing radiological examination at a particular instant in time but also the adjacent sections which are irradiated as a result of the divergence and scatter of the primary beam. Wall et al [5] also included measurements of absorbed dose to the lens of the eye, thyroid, gonads and skin. For these they used Li2B407:Mn powder in polythene sachets.

14.6.2. In vivo measurements

While measurements using human-like phantoms are extremely useful in the assessment of absorbed dose in diagnostic radiology, the measurements are not performed under entirely realistic conditions. The information gained from these measurements takes no account of the skill and experience of the radiographer, the quality and suitability of equipment and differences in shape and size of patients. In vivo measurements on patients undergoing routine radiological examinations in hospitals provide a much more realistic assessment under everyday practical conditions. Li2B407:Mn dosemeters are especially useful for these measurements. They are tissue-equivalent and radio-transparent except on high quality mammograms. In general they do not interfere with the diagnostic quality of the image and cause little inconvenience to patient, radiographer and radiologist.

Langmead et al [23] used Li2B407:Mn powder dosemeters, which were the same as those shown in figure 14.12, for the measurement of absorbed doses to patients under­going various forms of radiological examinations including cardiac catheterisation, barium enemas, intravenous pyelography and mammography. Maximum skin and gonad absorbed doses were measured. The measurement positions of the l x l cm2 dosemeters for mammography are shown in figure 14.13.

This series of measurements constituted a pilot survey of absorbed doses to patients, and recently this work has been extended to include other radiological techniques.

Figure 14.12. Black polythene sachets containing Li2B407:Mn powder being attached to the skin of a patient. (Photograph courtesy of the National Radiological Protection Board, Chilton.)

Application of TLD in medicine 287

4 cm

Figure 14.13. Arrangement of Li2B40,:Mn dosemeters (shown in figure 14.12) for mammography measurements. (Langmead et a! [23])

References 1 Seivert R M 1934 Acta Radiol. 15 193 (in German) 2 Daniels F, Boyd C A and Saunders D F 1953 Science 117 343 3 Taylor F E and Webb G A M 1978 Radiation exposure of the UK population, NRPB Rep.

NRPB-R77 (Harwell: National Radiological Protection Board) 4 Horsley R J and Peters V G 1976 Br. J. Radiol. 49 810 5 Wall B F, Green D A C and Veerappan R 1979 Br. J. Radiol. 52 189 6 Wall B F, Fisher E S, Paynter R, Hudson A and Bird P D 1979 Br. J. Radiol. 52 727 7 Morgan T J and Bateman L 1977 Health Phys. 33 339 8 Paliwal B R and Almond P R 1975 Phys. Med. Biol. 20 547 9 Ruden B I and Bengtsson L G 1977 Acta Radiol. Ther. Phys. Biol. 16 157 10 Ruden B I 1976 Acta Radiol. Ther. Phys. Biol. 15 447 11 Turner A F and Anderson D W 1973 Phys. Med. Biol. 18 46 12 Almond P R and McCray K 1910Phys. Med. Biol. 15 335 13 Bistrovic M, Matricic Z, Greenfield M A, Breyer B, Dvornik I, Slaus I and Tomas P 1976 Phys.

Med. Biol. 21 414 14 Mason E W, McKinlay A F, Clark I and Saunders D 1974 Proc. 4th Int. Conf. on Luminescence

Dosimetry, Krakow, August ed T Niewiadomski (Krakow: Institute of Nuclear Physics ) p 219 15 ICRU 1976 Determination of absorbed dose in a patient irradiated by beams of x or gamma rays

in radiotherapy procedures, ICRU Rep. 24 (Washington, D.C.: ICRU) 16 Eisenlohr H H and Jayaraman S 1977 Phys. Med. Biol. 22 18 17 Johansson J M, Lindskoug B A A and Nystrom C E 1969 Acta Radiol. Ther. Phys. Biol. 8 360 18 Joelsson I, Ruden B I, Costa A, Dutreix A and Rosenwald J C 1972 Acta Radiol. Ther. Phys. Biol.

11 289 19 Joelsson I and Backstrom A 1970 Acta Radiol. Ther. Phys. Biol. 9 233 20 Jayachandran C A 1970 Phys. Med. Biol. 15 325 21 Christensen P 1967 Manganese-activated lithium borate as a thermoluminescent dosimetry

material, Riso Rep. 161 (Riso: Danish Atomic Energy Commission Research Establishment) 22 Langmead W A and Wall B F 1976 Phys. Med. Biol. 21 39 23 Langmead W A, Wall B F and Palmer K E 1976 Br. J. Radiol. 49 956

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann ©1981 ECSC, EEC, EAEC, Brussels and Luxembourg

15 Application ofTLD in biology and related fields

MOBERHOFER

15.1. Introduction

A number of scientists working in the field of biology and related topics have found TLD techniques a very useful and satisfying way of solving some of their problems. The diversity of applications allows room to present here only a few representative examples of the whole spectrum. Short reviews of applications in the following fields are given:

(a) animal experiments, (b) bone dosimetry, (c) photon radiation quality measurements, (d) toxicity determinations, (e) general biology and biochemistry, (/) ecology, and (g) animal habit studies.

15.2. Animal experiments

15.2.1. Example one

As early as 1963, H N Kriegel and coworkers [1] applied TLD for internal body dosimetry after ^Srj^Y incorporation in an animal experiment. Investigations were made into the biological behaviour of radio-strontium in pregnant animals. Whereas the deposition of radio-strontium in the foetus by placental transfer was known for a variety of experi­mental conditions, there were doubts about the dose occupancy of the embryo during the total gestation period arising from the radio-strontium deposited within the female skeleton. CaF2:Mn dosemeters, prepared by pouring CaF2:Mn powder into plastic tubes 10 cm long and 1.5 mm in diameter and soldering the tubes at the ends, were implanted close to the left uterus horns below the ovaries of a number of rats. After the animals had recovered from the implantation operation, they received injections of practically carrier-free solutions of '"'Sr/^Y with different activities (25 to 100/iCi) into their tail veins. Seven days post-injection the animals were killed, the dosemeters were recovered and the amount of ^Sr/^Y as a percentage of the initially administered activity was determined by ashing of the killed animals at 800 °C.

Dosemeters which had been implanted into animals which did not receive radio­active injections and non-implanted dosemeters were used as controls. The distribution and retention in the animal organism of '"Sr/^Y within the range of applied activities is assumed to be independent of the initially deposited amount of radionuclide mixture.

290 M Oberhofer

If this is true, the TL of the phosphor used should be proportional to the injected activity and/or proportional to the activity measured in the animal after seven days.

This was indeed the case, as could be shown. The dose was determined by putting TL dosemeters identical to the ones used in the animal experiment into ^Sr/^Y solutions of different activity concentrations in 250 ml bottles for seven days. The dosemeters were exposed in the middle of the bottles so that the dose in rads could be calculated from

D&= 51.2 x 103x W0xCxt (15.1)

where Wp is the mean beta-ray energy in megaelectronvolts, t is the exposure time, and C is the concentration of the '"Sr/90 Y solution in millicuries per millilitre. For 1 mCi ml"1

the expression yields 405 rad. Now correlating the TL output measured with the TLD system available to the

absorbed dose received by the dosemeters, a calibration factor of 2.2 x 10"2 rad/relative unit of TL output was calculated. With this factor, the external dose to the foetus of the 145 g rat, which had received 75 /iCi 90Sr/90Y and retained 85% of that activity after seven days, was determined as 32 rad within this period.

Experience shows that this dose, and consequently an incorporated '"Sr/^'Y activity of 75 /iCi is sufficient to cause osteosarcomata with a high probability.

15.2.2. Example two

In another experiment, LiF TLD were applied to measure gamma-ray doses in vivo in male sheep ingesting 137Cs to demonstrate that designation of the whole body as a critical organ will adequately reflect the dose to the gonads in the case of a relatively uniformly distributed gamma-ray emitting radionuclide. The dosemeters, encapsulated in 4 mm thick Teflon capsules for beta shielding, were surgically implanted and exposed in vivo for 15 days to measure gamma dose rates of 20 to 60mradd~1 in sheep with body burdens of approximately 300 /dCi of 137Cs. In a second group of sheep, dosemeters were implanted prior to feeding with 50 /nCi of 137Cs per day. These animals were killed at intervals so that the accumulation of gamma-ray doses in various organs during a month of 137Cs feeding could be studied. The study resulted in the finding that the gamma dose to the gonads is less than that to the midpoint of the whole body and that the use of the whole body as the critical organ for relatively uniform distribution of gamma-emitting radionuclides will adequately reflect the gamma-ray dose to the gonads.

15.3. Bone dosimetry

TLD has been used to measure the mean bone marrow dose in specimens of human vertebrae irradiated with x-rays and gamma rays [2] following a proposal of F W Spiers. For this purpose, 3 mm thick transverse sections of lumbar vertebrae with different trabecular structures were cut and washed clean of marrow. The marrow spaces and the trabecular thickness were measured and the specimens were filled with finely (< 10/mi) ground TLD-grade LiF before they were covered with cortical bone slices and exposed. After irradiation, the LiF was recovered and the mean marrow dose determined by means of the TL of the LiF powder.

Application of TLD in biology and related fields 291

Higher mean marrow doses were obtained for specimens with smaller cavities and the results agree reasonably well with calculations by Spiers based on the concept of a distribution of pathlengths in trabecular bone.

Similarly, the mean marrow doses for trabecular bone containing ^Sr/^Y were deduced by applying tire TLD method to sections of femur of a beagle dog and/or a miniature swine which had been raised on a diet containing ^Sr/^Y [3]. In this case, the concentration of ^Sr/^Y in the sections also had to be determined.

The experiments were repeated with specimens of human vertebrae artificially loaded with (3-particle emitting radio-isotopes (e.g. 131I).

The works described show that TLD can be applied successfully to problems of inter­face dosimetry. This was also demonstrated by R J Schulz [4] who used ultra-thin (5-10 /im) LiF-Teflon discs to study the changes in absorbed dose near tissue-bone interfaces.

Later, dose and photon-energy measurements for bone marrow were performed in human (wax) phantoms utilising TL phosphors to obtain rad/R curves for bone marrow (figure 15.1) to be used in the design of a personal dosemeter following the variations of absorbed dose in bone marrow with photon energy [5].

" 0 8

V * — 100 Kvp X-RAYS

2 4 ' A m GAMMAS

20 30 50 70 100 200 300 500 1,000 PHOTON ENERGY (KEV EFFECTIVE)

Figure 15.1. Mean dose to bone marrow (rad/R) for a human phantom with rotational irradia­tion obtained with LiF dose-meters. (After R A Facey [5].)

15.4 Photon radiation quality measurements

In a number of radiobiological experiments, variations of the x-ray spectra with depth in tissue or bone, for example, may considerably influence the results. This is a reason for checking the spectral composition of the radiation field at various points. For determina­tion of the effective x-ray energy, two TL dosemeters with differing energy responses can be used [6]: LiF, with a rather flat energy dependence of its sensitivity, and CaF2:Mn or CaS04:Dy(Sm, Tm), with a steep slope of the sensitivity curves in the energy region from 45 to 150 keV, to mention two possible phosphors (see figure 5.4 in chapter 5).

If a sample of each phosphor is exposed simultaneously to the same radiation, the quality of which is unknown, then the ratio of their TL responses uniquely determines this quality. This occurs in locations normally not accessible by other techniques (for example, ionisation chambers in vivo) or where the presence of another detector or some detectors would influence the radiation field.

'Tandem' dosemeters have been built [7] which make it possible to distinguish different radiation qualities in one reading. If a small hot-pressed CaF2:Mn dosemeter is

292 M Oberhofer

bonded to the top surface of an extruded LiF dosemeter, for example, and the system heated out after exposure to an unknown 7-radiation field, a glow curve results with the glow peaks from LiF and CaF2:Mn, the main peak height of which immediately indicates the effective energy of the photon radiation. This is shown in figure 15.2. The peak-height ratio curve derived from it is given in figure 15.3.

100

80

S 60 <

40 -

x o 20 -

-

-

-

LiF TLD-700 ,

\ 38 keV / \L 70 /

f\ /&. i v 1 1 8 / / ifwjr'169/ /

/ ' 1250 \ \ ^ /

— 1 ^===r

CaF2 :Mn

J~^ 38 keV

70 \

118 \

1250 ] - = = = .

15 30 45 60

HEATING TIME (SEC) Figure 15.2. Glow curves obtained with a LiF-CaF2: Mn 'tandem' dosemeter for different quality photon radiation. (From Gorbics and Attix [7].)

o fee

3.0

20

1.0

0.6

03 0.2

0.1

--— -

0 \

1 , , 1,

V - CALCULATED

\ CURVE \ NORMALIZED

V _ 1

30 60 100 300 600 1000 2000

EFFECTIVE ENERGY (keV)

Figure 15.3. Peak-height ratio of the glow curves obtained with the LiF-CaF2:Mn 'tandem' dosemeter as a function of effective photon energy, derived from figure 15.3. (From Gorbics and Attix [7].)

15.5. Toxicity determinations

Recently, W Kriegseis and A Scharmann described a novel potential application of TL for the determination of the fibrogenic properties of quartz and coalmine dust, which cause pneumoconiosis and/or silicosis if inhaled for a long time [8].

Irradiated dust samples of natural quartz exhibit characteristic TL maxima at 165 K and 250 K. This TL is considered to be surface-specific and can be used to estimate

Application of TLD in biology and related fields 293

the amount of uncovered quartz surface, which was shown to take part in pathogenic interactions. The TL emission is attributed to defects at the boundary between the quartz surface and absorbed species containing OH groups, for instance water (methanol in the case of carbon).

For toxicity determinations, the absolute TL intensity of the 165 K peak of the dust samples, the relative increase of this intensity after water interaction and the TL curve shape are taken as criteria.

Cytotoxicity results of biological in vitro cell tests confirmed the findings.

15.6. General biology and biochemistry

K S V Nambi in a summary on TLD [9] reports on various applications of TL techniques in the study of biological and biochemical systems, mostly performed in India. Hydroxy-benzoic and aminobenzoic acids, urea, nucleic acids, proteins, plant leaves, algae and bacteria were investigated under different conditions for their TL characteristics [10]. The inter- and intramolecular transfer of radiation damage in nucleic acids, proteins and their constituents could be correlated with their TL behaviour [11]. The photosynthetic electron transport routes in the Z-diagram were correlated with TL and additional routes determined [12], and the interaction between salts and proteins could be understood from the TL patterns [13].

15.7. Ecology

15.7.1. Plant nutrition studies

TLD has also been applied in some ecological studies, with good results. A group of US research workers [14] used LiF powder micro-dosemeters to study

the dose to tree trunks in relation to the dose measured in the soil around the tree roots. For this purpose, micro-rods were placed about 2 to 3 cm beyond the bark into the trunks of some white oak trees around Argonne National Laboratory, others were buried into the soil 5 ft away from the trees and others were located above the soil around the trees to monitor the soil surface and ground for gamma and beta radiation.

The dosemeters within the tree trunks were removed in successive intervals of two weeks. The experiment permitted a comparison of inter- and intraspecies variability insofar as the selected trees were representative.

From the TL plots, seasonal changes in vegetation, marked first by the growth of grass and secondly by the coming into leaf of the trees, could be seen, arising from alterations in the localisation of soluble nutrients including '"'K, and other ions such as 137Cs in the micro-environment of the trees. The accumulated dose curve for the trees (figure 15.4) was characterised by a sharp rise that coincided with bud break and early leafing. The positioning of the micro-dosemeters just at the outer surface of the woody cylinder of the tree trunk provided maximum exposure to the radionuclides being carried in the sap stream. Movement of mineral elements in the trunk is greatly increased during the phases of bud break and leaf and twig growth.

Further results from the experiment are as follows. The environmental radioactivity in the trees at the time of the experiment could be related to cycling of natural 40K radionuclide and possibly of 137Cs remaining from earlier fall-out. The concentration of

294 M Oberhofer

radon and of its accompanying daughters has also been shown to be related to meteorological variables. The location of the micro-dosemeters just under the bark greatly accentuates a response to differential movement of radionuclides in the sap. Development of foliage by the grass and trees raises the level of K and Cs in the above-ground environment.

UJ in z o a. 10 UJ or

I _ i i—

Ul

—

C

-

-

)

T , -

"I 1 1 1

MARCH

5.5 m R / w e e k

» , _ T /

/ / T

\ \ ]/

0.9 mR/week V

i A

' -"

^

[ l / T \J FULL -U4-""T LEAF I 1 BUD BREAK

, , i T i , , i '1 U APRIL 8 MAY 12

TIME (WEEKS)

or E ^ Ul (/) o o Q UJ \— < _ l Z>

Z>

o <

Figure 15.4. Accumulated doses of the dosemeters placed in the tree trunks. Broken lines: approximate seasonal dose rates. (After W C Ashby and others [14].)

15.7.2. Sedimentation studies

In the Joint Research Centre, Ispra, micro-TLD have also been used for the measurement of phosphate diffusion in sediments [15], which is of interest with regard to the eutrophication of lakes, for example. The study required knowledge of the concentration gradient produced by the diffusion of phosphates from the lake water into the sediment. Tliis concentration was determined with the help of a small P04 diffusion test facility using a sediment core from the bottom of the lake of interest and sea water which received a mixture of natural and radioactive phosphor (in the form of disodium phosphate) in an amount corresponding to the actual phosphate content of the lake water. After a certain sedimentation period, the 32P concentration along the sediment core was determined with micro-TLD (CaS04:Dy) in a stringer which was inserted into the core.

It turned out that phosphates concentrate near the surface of the sediment and that there is little diffusion into the sediment. This also explains why phosphates are released so easily from the sediment into the sea water under anoxic conditions during the hot summer months.

Application of TLD in biology and related fields 295

15.8. Animal habit studies

Other scientists have reported applications of miniature TLD for measuring the dose to small rodents moving around above the soil in a radioactive ecological site. Glass tubes filled with powdered CaF2:Mn and LiF and placed in polyethylene tubing were fastened around the necks of the rodents used in the experiment. From the dose measured, two groups of animals could be distinguished: one group representing animals mostly dormant underground and the other representing animals normally active, tire exposure to animal per surface area being smaller in the former case.

This example shows nicely how TLD may be used to examine the habits of animals and to assist in predicting precisely the exposures to an entire population, for example insects.

References 1 Kriegel H, Hiring N and Neumann G 1963 Strahlentherapie 122 41 2 Zanelli G D 1968 Proc. Symp. on Microdosimetry, Ispra, Italy, 1967. Rep. EAEC-EUR 3747 d-e-f,

p527 3 Zanelli G D and Spiers F W 1969 Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg,

Tenn., September 1968. USAECPubl. Conf. 680920 4 Schulz R J 1966 Phys. Med. Biol. 11 623 (abstract) 5 Facey R A 1968 Health Phys. 14 557-568 6 Cameron J R and Kenney G N 1963 Radiat. Res. 19 199 (abstract) 7 Gorbics S G and Attix F H 1968 Int. J. Appl. Radiat. hot. 19 81 8 Kriegseis W and Scharmann A 1980 Proc. 6th Int. Conf. on Solid State Dosimetry, Toulouse,

France, 1-4 April 1980. Nucl. Instrum. Meth. 175 No 1 239^10 9 Nambi K S V 1977 TL: its understanding and applications Rep. INF. IEA 54, CPRD-AMD 1,

MAIO (Sao Paulo, Brazil: Instituto de Energie Atomica) 10 Aramu F, Maxia V, Serra M and Spano G 1972 J. Lumin. 5 439-448 11 Tatake V G 1975 Proc. Nat. Symp. on TL and its Applications... Kalpakkam, Madras, India,

12-15 February 1975. Bhabha Atomic Research Centre Rep. CONF-750294 12 Sane P V 1975 Proc. Nat. Symp. on TL and its Applications... Kalpakkam, Madras, India,

12-15 February 1975. Bhabha Atomic Research Centre Rep. CONF-750294 13 AltekarW, Tatake V G and Sane P V 1975 Proc. Nat. Symp. on TL and its Applications...

Kalpakkam, Madras, India, 12-15 February 1975. Bhabha Atomic Research Centre Rep. CONF 750294

14 Ashby W C, James N B, Kastner J, Oltmann B G and Moses H 1966 Thermoluminescent dosimetry and environmental radiation studies ANL Rep. ANL-7220, pp 115-19

15 Oberhofer M 1977 Atomkernenergie 30 164

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

16 High-level photon dosimetry with TLD materials

M OBERHOFER

16.1. Introduction

For very high dose levels in the 104 to 108rad range, for example in radiation chemistry and technology (polymerisation, vulcanisation of rubber, cracking of hydrocarbons), food processing, radiation sterilisation, material testing, etc, chemical dose measuring techniques such as the Fricke (Fe2+, Fe3+) system, the ceric-sulphate system, the ferric-cupric system, the oxalic system or organic dyes in liquid solution are being used successfully together with calorimetric methods.

The former techniques have been reviewed by Fricke and Hart [1], by Holm and Berry [2] and by Stolz [3] and also in a publication by the IAEA [4]. Most of the methods require much effort or, at least, careful attention in order to obtain good results and are too expensive for routine measurements, where hundreds or even thousands of single measurements have to be performed. A less expensive method may be found in the application of the thermally stimulated emission of luminescence light (thermolumi­nescence, TL) of certain materials following exposure to radiation (radiothermo-luminescence, RTL) or in TL-related phenomena.

Some years ago, Gorbics et al [5] compared several such thermoluminescence dosi­metry (TLD) systems for the 103-106R gamma exposure range. This investigation was performed with the aim of finding the system with the most convenient handling characteristics, for a special application, and also low cost for single throw-away use, relatively small divergence from linearity, lack of dose-rate dependence, good reproduci­bility of response from one dosemeter to another without the necessity for annealing, weighing or individual calibration and similarity of atomic number between the TL material (phosphor) and the dose reference material, here silicon. Additional desirable features would be no signal loss with time, possibly no sensitivity to light, small size and also the possibility of detecting exposure inhomogeneities within the irradiation volume. The phosphors investigated by Gorbics and coworkers were calcium fluoride activated with manganese (CaF2:Mn), lithium fluoride activated with manganese and titanium (LiF:Mg,Ti) and lithium borate activated with manganese (Li2B407:Mn). All of these phosphors have dynamic ranges extending to at least 105R and do not exhibit excessive TL signal fading during storage at room temperature. This is of importance if the exposure has to be performed at a rather low exposure rate over an extended period of time or if the phosphor can only be evaluated (read) at a later date.

In some cases, high dose level measurements have to be performed at elevated process temperatures, requiring phosphors which do not lose their information during exposure at these temperatures. From their comparative studies, Gorbics and coworkers concluded that, for their purpose, a 0.4 mm thick, 5% loaded CaF2: Mn-Teflon disc was the choice

298 M Oberhofer

for routine measurements of absorbed dose (the TL reading could be performed with a 3% standard deviation at 103R, not corrected for phosphor mass variations) in transistor (silicon) material, which at the time of the investigation (1973) was commercially available at a price of $0.30 each.

For other purposes, the other two TL phosphors may be more suitable, depending on the materials to be exposed to high doses. So LiF:Mg,Ti with an effective atomic number Z of 7.4 may be the better phosphor for reference materials with similar Z. Some of the results of Gorbics and coworkers will be presented below and will be supplemented by further relevant data obtained by other authors. Besides LiF:Mg,Ti, Li2B407:Mn and CaF2:Mn, some other phosphors (currently used mainly for personal dose assessment and environmental dose control) will be considered for their suitability for high dose level measurements. The review will conclude with a table of useful data on phosphors applicable in dosimetry at high exposures.

16.2. Lithium fluoride

The TL phosphor LiF:Mg,Ti is available commercially in several forms, as a powder, as hot-pressed chips and also incorporated in Teflon as rods, ribbons and discs.

Its glow curve, that is the TL light output plotted versus the heating temperature, shows several peaks with the normally used 'dosimetry' peak at about 200 °C. Either the peak height or the light sum under the peak may be used for dosimetry. As the peak height depends upon the heating rate (Randall-Wilkins shift) and its use for quantitative exposure or dose measurements requires reproducible heating rates, the light sum is normally used for most applications.

If the light sum response is plotted as a function of exposure, one observes that, beginning with a certain exposure, the TL output increases more than proportionally with exposure. This phenomenon is called 'supralinearity' and is observed in most TL phosphors. The extent to which supralinearity exists is best seen if the relative TL response of the phosphor per exposure unit, here per roentgen, is plotted over the exposure. To be independent of the type of reader, the TL output per roentgen may be normalised to an exposure, for example of 103R.

This has been done in figure 16.1 for the 200 °C peak of various LiF:Mg,Ti dosemeter types manufactured by The Harshaw Chemical Co. (powder and chips) and/or Teledyne Isotopes Inc. (the Teflon disc dosemeters). The curves were obtained with a Harshaw Model 2000 TLD reader with the maximum heater temperature set to maintain the light sum under the 200 °C peak. The horizontal part of the curve indicates exposure pro­portionality or linearity with exposure up to 1000R. Above 1000R, LiF:Mg,Ti is supralinear with a maximum response per roentgen just below 10SR. From here on, towards still higher doses, the response per roentgen decreases again. This is in good agreement with the findings of other authors [6, 7], some of whom [8, 9] also find that the supralinearity depends on the grain size of the phosphor.

In a very recent publication, Piesch et al [10] found a response that was higher by a factor of 3.4 to 4.1 (137Cs irradiation), instead of 2.3 to 2.6 (60Co irradiation) according to Gorbics et al, at the maximum compared with the response at 10 R. The authors also found that supralinearity starts at 100 R and has a different form for LiF TLD 600 and TLD 700 —a reason to be careful in assuming that the characteristics of LiF:Mg,Ti

High-level photon dosimetry with TLD materials 299

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60Co y-RAY EXPOSURE(R) Figure 16.1. 200"C TL peak light sum response per roentgen for LiF (TLD 700) exposed to 60Co gamma radiation, normalised to 1.00 at 103 R. (After Gorbics et al [5].)

dosemeters are the same even if material is used with identical activator composition and preliminary treatment. Changes do occur from batch to batch and the dose response curves need to be checked!

The upper dose limit is governed by saturation effects in the phosphor crystals and is normally set 20% below the saturation value. The useful dosimetry range was found to extend to about 10SR for all types of dosemeters which were checked. Supralinearity is a drawback insofar as the response curve has to be known for dosemeter evaluation.

In the case of complete linearity, only the TL response per roentgen needs to be known in order to arrive at the actual exposure in roentgens or dose in rads. According to La Riviere [11], less supralinearity at high exposure levels up to 104R occurs if the LiF phosphor has been pre-exposed to a high dose and if the deep traps in the phosphor have not been annealed. Practically no supralinearity up to 5 x l 0 5 R is observed if the LiF:Mg,Ti has been modified by diffusion of hydroxide ions into the lattice. The chemical treatment is described by De Werd and Stoebe [12]. So far the method has not found any practical application.

Concerning the reproducibility with which exposures may be determined at high doses utilising commercial LiF:Mg,Ti dosemeters, the following standard deviations for sample-to-sample reproducibility determined by Gorbics et al will be mentioned here. At 103R they obtained:

±2.6% for 10 mg powder samples (home-dispensed), ±0.9% for hot-pressed chips,

(a) (b) (c) ±5.1% for 0.4 mm thick Teflon discs, and (d) ±4.5% for 0.13 mm thick Teflon discs.

The values were determined with the virgin dosemeters as received from the manufacturer, without any heat treatment (annealing) before or after irradiation. No correction for mass variations was applied.

300 M Oberhofer

As can be seen, the best results were for chips, with a standard deviation of about ±1%. For exposures above 103R, no similar data for sample-to-sample reproducibility could be found in the literature.

In studying the reproducibility of different dosemeters of the same type, one should always recall that there may be considerable changes from batch to batch, as was shown by Regulla [13]. Not only is it apparently very difficult, if not impossible, for the manu­facturer of the TL material to reproduce the average detector sensitivity from one batch to another, but it also seems to be difficult to obtain the same sensitivity distribution within a single batch. This can be seen from figure 16.2, where the distribution of the sensitivity of two batches of LiF:Mg,Ti TLD 100 detectors delivered together is given. For micro-rods exposed to 10R, Regulla finds sensitivity variations within one batch as high as a factor of 3 and typical relative standard deviation values of around 15%. Within four years the TL sensitivity of one dosemeter type from the same producer decreased by a factor of 5! From this, it may be concluded that, also with regard to dosemeter reproducibility, one should be very careful in assuming that the dosemeter-to-dosemeter reproducibility stays the same from batch to batch, particularly over extended periods of time. Like the dose characteristic, the reproducibility needs to be checked repeatedly.

Earlier it was mentioned that a good phosphor should have no excessive signal fading. At ambient temperature (25 °C), this fading may be rather low for LiF:Mg,Ti, about 5% in one year if thermally stabilised (for example, by a heat treatment at 100°C over lOmin after exposure just before reading). One should take care when considering the fading characteristic, as large variations from the value given above can be found depending on the environmental temperature and humidity. Thus, instead of 5% per year, a signal loss of 10% is found in six weeks if the phosphor is kept at 32 °C instead of 25 °C [14].

LiF:Mg,Ti can also be used for exposure measurements beyond 105R if, instead of the 'dosimetry peak' at 200 °C, some other glow peaks which appear at high doses are used.

at uJ

BATCH X-504-S (3-4) 755 DETECTORS

i DETECTOR SENSITIVITY (ARBITRARY UNITS) Figure 16.2. Distribution of the sensitivity of LiF:Mg,Ti TLD 100 micro-rods from the same source, exposed to 10 R. (From Regulla [13].)

High-level photon dosimetry with TLD materials 301

Starting with 4 x 104 to 6 x 104 R, a peak appears which centres at a temperature of around 280 °C and saturates at about 2 x l0 6 R [15]. The evaluation of the peak is some­what difficult owing to the presence of the 200 °C peak, which in spite of increasing exposure is decreasing. As the total light output does not change too much with dose in the temperature range of the peak, use is made of the ratio of the peak heights which varies strongly with exposure. This method can be used for exposure measurements up to 107R, independent of sample weight. In figure 16.3 a 60Co calibration curve for LiF:Mg,Ti TLD700 (Harshaw) is reproduced [15], which is based on the ratio of the 200 °C peak height to that at 280 °C. According to Kitahara et al, undoped LiF may also be used with success for high dose measurements [16]. The authors had prepared LiF powder from purified Li2C03, reagent grade HN03 and NH4F, which contained only some natural impurities. The glow curves were characterised by two glow maxima, one at 150 °C and one at 335 C. The response curves of the material exposed to 60Co gamma radiation are reproduced in figure 16.4. This figure indicates that the response to ^Co radiation is linear from 102R up to about 5x l0 4 R utilising the light sum under the 150 °C peak. No linearity is obtained for the light sum under the 335 C peak. The

10 t-

5 1

i i i i i 11 I 0.01 10 0.1 1.0

DOSE, Mrad Figure 16.3. LiF TLD 700 calibration curve, (peak height at 200 °C) -e- (peak height at 280°C) plotted against dose. (After Jones and Martin [15].)

6 0Co GAMMA RAY IRRADIATION

O 150 °C PEAK

a 335°C PEAK

EXPOSURE, R

Figure 16.4. TL response for the two LiF glow peaks of undoped LiF powder for 60Co radiation. (After Kitahara et al [16].)

302 M Oberhofer

latter peak may be used up to an exposure of 108R with proper calibration of the material.

Goldstein et al [17] find yet another glow peak between 425 and 475 °C, not for TL-grade but for optical-grade LiF manufactured by Isomet Corp. (Palisades, New Jersey, USA). This material, Isomet 1, was chosen because of the lack of dominant glow peaks below 350 C. All the crystals (ground and sieved to a size between 100 and 200 mesh) which were checked showed a measurable TL for megarad exposure. The high reading temperature (450 °C) made it necessary to modify the heating and timing circuits in a commercial reader, which was a Controls of Radiation Inc. TLD reader. As the infrared light emission from the sample holder and the sample itself at 457 C is rather disturbing, an infrared-light-absorbing filter and a blue filter were needed to minimize interference by blackbody radiation. Figure 16.5 shows the relative light output of the 450°C glow peak for the Isomet material for exposures up to 109R starting from 10s R. Values could be read from glow curves for exposures as low as 104R. As can be seen, the response is nearly linear up to 5 x 107R where the material starts to become saturated. If, instead of Isomet 1, another LiF material from the same manufacturer, called Isomet 2, is exposed, even higher values (2 x 108 R) for the beginning of saturation are noted.

10' 10° EXPOSURE (R)

Figure 16.5. Relative TL output of the 450 °C peak for two optical-grade LiF powder materials from Isomet Corp. as a function of exposure. (From Goldstein et al [17].)

The curves are still useful beyond the saturation starting point if one takes advantage of the changes in the shape of the glow curves above and below the saturation regions, together with a progressive shift of the glow peaks towards higher temperatures with higher exposures. Figure 16.6 shows what was obtained with 16 mg samples of Isomet 1 LiF powder exposed to ^Co radiation of indicated exposures. In the case of Isomet 1, exposures higher than 108R and, in the case of Isomet 2, from 5 x 108 to 109R, may be measured in this way.

Where a commercial TLD reader cannot be modified for enabling high-temperature peaks also to be evaluated or where a high-temperature TLD reader cannot be built, yet

High-level photon dosimetry with TLD materials 303

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TIME (s) Figure 16.6. Glow curves for Isomet 1 LiF powder sample exposed to (A) 8X10SR, (B) 107R and (C) 2 X 10" R 6°Co radiation. (From Goldstein et al [17].)

another possibility exists to measure elevated exposures up to 2x 108Rand even higher, beginning at about 106R, by performing optical density measurements of absorption bands. Such absorption bands are induced in optical-grade l iF (Isomet 1, for example, or similar material from E Leitz, Wetzlar, Germany) by radiation, if the exposure is-sufficiently high. This method, of course, no longer has anything to do with TL, but it is an interesting alternative and will be outlined here in some detail.

In figure 16.7 the optical absorption spectrum of optical- (standard uv) grade window chemical LiF from Isomet Corp. is reproduced [18], which was obtained 24 h (to allow for peak growth) after exposure to 2x l0 7 R. For evaluation, the band at 450 nm (M band) is preferably used, which according to Claffy and coworkers [19] is thermally more stable than others (R and N2 band).

Figure 16.8 gives the increase of optical density at the absorption peak as a function of exposure for the three peaks. In these measurements, cleaved crystal plates were used. The optical density was obtained before and after irradiation with a Cary Model II spectrophotometer. Prior to exposure, the crystals were annealed at 550 °C for 20 min. Exposures beyond 2 x 108R could still be measured, probably up to 109R, by making use of the N2 absorption band. The method has the advantage that the crystals can be thermally bleached and re-used. The re-use produces the same optical density versus exposure curve and reproducibility remains unchanged. The variations in optical density measurements at a given exposure for different crystals was found to be less than 5%. The drawback of the method described is the relatively high cost of the single crystals, causing Claffy et al [19] to check whether the LiF optical absorption method could be performed with LiF samples in powder form. Leitz uv-grade LiF was ground, thoroughly mixed and then sieved through 100-200 mesh. The 'apparent' optical density (AOD) of

304 M Oberhofer

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Figure 16.7. Absorption peaks of Isomet optical-grade LiF, 24 h after exposure to 2 X107R. (From Vaugham and Miller [18].)

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.8. Optical density of the three different absorption bands of Isomet optical-versus exposure. (From Vaugham and Miller [18].)

30 mg samples was determined with a diffuse reflectance apparatus described in detail there [19].

Two typical absorption spectra are shown in figure 16.9, which compare with the spectra obtained by Vaugham and Miller (see figure 16.7). With increasing dose and the subsequent changes in optical absorption (AOD) of the M band (X = 450nm) and the R2 band (X = 377 nm) in Leitz LiF, it follows that the method may be used for the measurement of doses between IO6 and IO7 rad or between IO6 and 10s rad, respectively. Contrary to Vaugham and Miller's results for cleaved crystals, there is no linear response of the M band utilising the powder material. The AOD exposure curves of the M and R2 bands are not linear, and therefore calibration curves are necessary.

Claffy and coworkers also showed that there was no detectable difference in TL output of the LiF material in the 400-475 °C region with or without a prior 60 min exposure to light in the diffuse reflectance apparatus. This means that dose determination by AOD

High-level photon dosimetry with TLD materials 305 ^ 0 . 9 Q ° 0 8

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Figure 16.9. Absorption spectra of Leitz LiF powder exposed to "Co gamma radia­tion. (After daffy et al [19].)

measurement can therefore safely precede standard TL readout procedures, as described by Goldstein et al [17], and thus replication of dose information be achieved. This may be useful where repetition of exposure would not be feasible.

At room temperature, the stability of both the M and R2 bands, as functions of time, after exposure is adequate for most practical applications.

Also, the radiophotoluminescence (RPL) of TL­grade LiF [20] as well as optical­grade LiF [19] may be utilised for high dose level measurements, which between 10 and 106R is nearly a linear function of exposure. This is shown in figure 16.10. Excitation should be performed with 450­455 nm light.

The maximum RPL occurs at an exposure of more than 10 times the saturation value of TL measurements (see figure 16.10). The RPL also turns out to be very stable at normal

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DOSE [R] Figure 16.10. Radiophotoluminescence (full curve, excitation with 450 nm light) and thermoluminescence (broken curve, total light sum) of TLD­grade LiF:Mn,Ti (TLD 100) as a function of exposure. (From Regulla [20].)

306 M Oberhofer

ambient temperatures and in the dark (uv light causes fading) after thermal stabilisation at 100 C for 5 min. No fading could be observed within one month after the end of irradiation. Instead of TLD-grade LiF (LiF 100, for example), non-TLD-grade LiF in powder form can also be used for RPL measurements. For Leitz uv-grade LiF, Claffy and coworkers [19] obtained optimum excitation with light at 455 nm. If the RPL is plotted over the exposure of the material, one obtains the curves shown in figure 16.11. There is apparently linearity only from 2 x 10s to 2 x 106rad for the 520 nm band.

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From what has been mentioned so far, it is evident that there are a number of ways to apply TL-grade and also optical-grade LiF in high-level dosimetry, by taking advantage not only of its TL emission but also of the emission of radiophotoluminescence light and by performing absorption measurements. In any case, exposure to high doses causes changes within the dosemeter materials, which remain even after a high-temperature annealing and in most cases excludes re-use of the material.

Heating LiF:Mg,Ti at 400-700 °C for several hours brings the material back to zero signal, but at the same time reduces its sensitivity to, for example, 33% of its original value after exposure to 2 x 10s rad. In this case, the material is thrown away, which may be expensive considering that the cost of TL-grade LiF is rather high. The high cost of the dosemeter material thus often excludes its application in high-level dosimetry.

16.3. Lithium borate

The next TL phosphor to be considered for the measurement of high doses is li^O^.Ma., which like LiF: Mg,Ti is commercially available in several forms. It can be purchased as a powder (Harshaw), as hot-pressed chips (Harshaw) and incorporated in Teflon (0.13 mm and 0.4 mm thick discs, Teledyne Isotopes).

The light sum response per roentgen as a function of ^Co gamma-ray exposure is shown in figure 16.12 for virgin dosemeters of various forms. The curves in the figures were

High-level photon dosimetry with TLD materials 307

60CoY-RAY EXPOSURE (R) Figure 16.12. 200-250°C TL peak light sum response per roenten for Li2B407:Mn as a function of exposure (60Co gamma radiation), normalised to 1.00 at 103R. (After Gorbics etal [5].)

obtained with the same instrument under the same conditions as the corresponding curves for LiF: Mg,Ti in figure 16.1.

Compared with those curves, one also notes that here supralinearity starts around 103R but extends up to higher doses. The hot-pressed chips are nearly linear up to 5 x 104 R and then demonstrate relatively little supralinearity up to 106 R. Linearity may be significantly improved (up to 10s rad) by pre-exposure of the phosphor as was shown in chapter 6 (§6.3.1.3). All forms of Li2B407:Mn dosemeters can be applied for exposure measurements up to 106R.

Unfortunately, the reproducibility of Li2B407:Mn is rather poor in the dose range from 10s to 106R, which might exclude the use of the material in this dose range for routine application. For the powder and hot-pressed chips, the standard deviation of the TL readings at 103 R was determined to be about ±5%, for the discs of 0.4 mm thickness the standard deviation was ±6.3% and for the 0.12 mm thick ones ± 12.7% was obtained at the same dose.

The advantage of Li2B407:Mn compared with LiF:Mg,Ti is its simple glow curve with one main peak, the temperature position of which depends to some extent on the exposure level. Another advantage is that the material does not exhibit complicated trap dynamics and thus does not require complicated annealing procedures to obtain good reproducibility. Zeroing of the phosphor only requires heating at 300 °C for 15 min; this may not be of interest to the user if the phosphor is used in powder form and thrown away after use. This is to be recommended, as the material can be produced cheaply and simply in large quantities in any laboratory.

From the point of view of tissue equivalence, Li2B407:Mn is an ideal phosphor, but it also has some important drawbacks, namely that it is highly soluble in water and very hygroscopic. This latter disadvantage can be improved by adding 0.25% Si02 to the basic material. The phosphor may also show undesirably high fading from 0% within three months at 25 °C up to 37% per year at the same temperature, depending on its production and composition.

308 M Oberhofer

16.4. Calcium fluoride

A high-Z TL phosphor with a much higher energy dependence compared with LiF:Mg,Ti and Li2B407:Mn is CaF2:Mn, which also offers possibilities for high dose level measure­ments with a reported linearity at response levels up to about 10s R, which is also about the maximum measurable dose. According to measurements by Gorbics and coworkers [5], this is not quite the case, as shown by the curves in figure 16.13, which were obtained in the same way as the curves in figures 16.1 and 16.12. Apparently, supra-linearity increases with decreasing phosphor thickness, as can be seen.

m u . TEFLON y'^s

Figure 16.13. TL light sum response per roentgen as a function of ' t o ex­posure for CaF2:Mn-Teflon discs (Teledyne Isotopes), 10 mg powder (Harshaw) and 0.9 mm thick chips (Haishaw), normalised to 1.00 at 103R. (After Gorbics era/ [5].)

6 0Coy -RAY EXPOSURE (R)

Looking at the standard deviations of the TL light sum readings at 103R without correction for mass variations, CaF2:Mn is the next best to LiF:Mg,Ti with ±2% for the 10 mg powder samples, ±5.4% for the hot-pressed chips and ±3.1% and/or ±4.8% for the Teflon discs of 0.4 mm and 0.13 mm thickness, respectively. Fading, which for many applications is the limiting factor, is not negligible with CaF2:Mn, and was found to be 4% after 1 h, 8% after 10 h and 12% after 100 h. The same holds for calcium fluoride activated with dysprosium (CaF2: Dy) (Harshaw TLD 200). Calcium fluoride activated with thulium (CaF2:Tm) (Harshaw TLD 300) has a slightly better fading characteristic.

16.5. Other TLD phosphors

16.5.1. Metaphosphateglass

There are some more phosphors which are potentially suitable for high dose level measurements, such as, for example, manganese-activated low-Z metaphosphate glass, which is utilised as a photoluminescence reference light source in phosphate glass (radio-photoluminescence, RPL) dosimetry. This material exhibits strong TL upon exposure and is of interest because of the absence of any 'trap dynamics', that is the absence of shifting of the trap distribution, which in other TLD phosphors such as LiF:Mg,Ti often considerably impairs the precision of dose measurement.

Besides the possibility of obtaining higher precision with phosphate glasses, they are easy to use, fast to re-use owing to the absence of complicated annealing programmes and exhibit nearly no thermal fading.

High-level photon dosimetry with TLD materials 309

If, according to Regulla [13], manganese-doped (1.0%Mn) phosphate glass (identical with the so-called radiation-resistant reference glass in RPL dosimetry) is exposed to radiation and heated up to about 350 °C, a TL glow curve is obtained, the light sum of which is proportional to exposure from about lOmR to some kiloroentgens and then slightly supralinear up to 106R, where saturation is noted, as shown in figure 16.14. The curve was obtained with 6x6x1 .5 mm3 plane plates and a Harshaw Model 2000 TL analyser. As can be seen from the figure, the upper limit of exposure detection is about 106R, which may be extended to 107R by measuring the decrease of photoluminescence and to even higher exposures by using absorption measurements.

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The precision obtained with the material is particularly high, characterised by a relative standard deviation better than ±2% when individual detectors are re-used. There is no observable thermal fading after proper post-irradiation annealing. The detectors are mechanically and also chemically very stable, being neither toxic nor soluble in water. The glass can also be made in the form of small rods and thus would be ideally suited for a number of high dose level measuring problems, where small dosemeters are desirable.

16.5.2. Beryllium oxide

Beryllium oxide, BeO, is suitable for some high dose level measurements, because it is rather energy-independent, has an effective atomic number Z of 7.5, which is comparable with that of LiF:Mg,Ti and Li2B407:Mn, and in particular is known to be a material of high chemical, mechanical and thermal inertness if used in its non-toxic ceramic form (Thermalox 995 from Brush Beryllium Co., for example). As a TL phosphor, it may be used up to exposures of 10s R and, with care, up to 106 R.

310 MOberhofer

Again BeO shows supralinearity, this time starting at 10-50 R, dose values which are rather low and requiring calibration curves for high exposures. Reproducibility does not seem to be too good. Crase and Gammage [21] report an intergroup standard deviation from the mean response above a few milliroentgens in the range 8-11%. This may be better at still higher exposures. Fading is accelerated by visible light, against which the material has to be protected. Thermal fading is about 10% in three months at 30 °C and 90% relative humidity. The material is not cheap, which does not favour its wide applica­tion in routine high-level dosimetry.

16.5.3. Calcium sulphate

There are two calcium sulphate TL-grade types which are activated with different rare earths, CaS04:Dy and CaS04:Tm. Those phosphors are of interest because, like Li2B407:Mn, they may easily be produced in large quantities in any laboratory and, as a consequence, are very economic TL materials.

According to Yamashita and coworkers [22], the dose response curve of CaS04:Dy is linear up to an exposure of 3x l0 3 R, supralinear between 3 x l 0 3 and 104R and reaches the saturation level at around 10s R. This is also demonstrated in figure 16.15 (curve (A)), which is taken from reference [22]. With CaS04:Tm (curve (B)), supra­linearity already starts at 300 R, as can be seen from the same figure. There is only a little fading for both materials (1-2% in one month, 24 h after exposure).

S K

s. in

(B) CaSOi, Tm^

(A) CaSOi Dy

,0-3 ,o-2 ,0-1 10° » ' K)2

EXPOSURE, R K)3 K)4 K)5 K)6

Figure 16.15. CaS04 TL dosemeter response versus Yamashita et al [22].)

l7Cs gamma-ray exposure. (After

16.5.4. Aluminium oxide

Another TL emitter with a fairly linear response up to 105rad [23] is A1203 (ruby), which is cheap and has a better energy dependence than CaF2:Mn and the phosphors CaS04:Dy and/or CaS04:Tm. There is still a strong uv fading which necessitates keeping the phosphor in the dark during application and evaluation.

16.6. Final remarks

Performing a review like the one presented here reveals that many of the TL phosphors used today for low-level dose assessment in radiation dosimetry (mostly in personal

Table 16.1. Some data on TL phosphors which have potential for use in high-level dosimetry (doses given in rad)

TL material

LiF:Mg,Ti

LiF, powder, undoped LiF, optical-grade crystal

LiF, optical-grade powder

Li2B407:Mn chips

CaF2:Mn

zeff

8.3

Isomet 1

Isomet 2

7.3

16.3

Type of measurement

TL TL RPL

TL

TL optical density TL optical density RPL

TL

TL

For 60Co radiation linear range

10"2-103

-10-106

2X10 2 -5X10" --10"-5X107

10--2X10"

2X10 ! -2X10 6

1 0 ' M O 3

10"2-5X104

10- -10 3 (10 5 )

Minimum dose

10"2

5X10" 10

2X10 2

2X10 5

10" 10 ' 104

106

2X10S

10"2

lO"2

1 0 -

Maximum dose

10s (200 °C peak) 107 (280 °C peak) 106

105(150°Cpeak) 108(335°C peak)

10' (450 °C peak) 2 X 1 0 M 0 9

2 X 10" (450 °C peak) 107-10' 2X10"

106

106

10s

Fading at 25 °C

5%/yr

0%/month

variable from 0%/3 up to 37%/yr

4%/l h

month

£? §

a »*-. "a o

a. o Co

is-

to S *-* 3. 5.

Glass: Mn

BeO

CaSO„:Dy CaS04 :Tm

12.0

7.1

16.3 14.4

TL

TL

TL TL

10~2-10~3

10"2-10

1 0 - - 3 X 1 0 3

1 0 - - 3 X 1 0 2

10"2

10"2

i o -i o -

106

10 MO 6

104-105

1 0 M 0 5

8%/10h 12%/100h

0%/month

10%/3 months, 30°C, 90% rel. hum.

1-2%/month

10.2 TL io-'-io2 10" 105

312 MOberhofer

dosimetry) can to some extent also be utilised for high-level dosimetry depending on quite a number of factors such as dose level, irradiation and storage temperature, atomic number of reference material, ease of handling, sample-to-sample reproducibility and, last but not least, phosphor price, if large numbers of dosemeters must be used. If price be the governing factor, it seems that in many applications Li2B407:Mn is the best phosphor, which, if home-produced, can be used as a throw-away material.

Table 16.1 concludes the review, summarising some relevant high-level dosimetry data on the phosphors discussed here.

References 1 Fricke H and Hart E J 1966 Radiation Dosimetry vol. 2 Chemical Dosimetry (New York:

Academic Press) p 167 2 Holm N W and Berry J, eds 1970 Manual on Radiation Dosimetry (New York: Marcel Dekker) 3 Stolz W 1972 Strahlensterilisation, Grundlagen und Anwendungen in Medizin und Pharmazie

(Leipzig: J A Barth) 4 IAEA 1972 Dosimetry Techniques Applied to Agriculture, Industry, Biology and Medicine

(Vienna: IAEA) 5 Gorbics S C, Attix F H and Kerris K 1973 Thermoluminescent dosimeters for high dose application

Health Phys. 25 499-506 6 Sunta C M, Bapat V N and Kathuria S P 1971 Effect of deep traps on supralinearity, sensitisation

and optical TL in Lif TLD Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, Roskilde. Riso Rep. 249, vol. 1. p 146

7 Eggermont G, Jacobs R, Janssens A, Segaert O and Thielens G 1971 Dose relationship, energy response and rate dependence of LiF-100, LiF-7 and CaS04:Mn from 8 keV to 30 MeV Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso, Roskilde. Riso Rep. 249, part II, p444

8 Shiragai A 1970 Effect of gTain size and initial trap density on supralinearity of LiF-TLD Health Phys. 18 728

9 Zanelli G D 1972 Particle size and supralinearity in LiF Phys. Med. Biol. 17 99 10 Piesch E, Burgkhardt B and Kabadjova S 1975 Supralinearity and re-evaluation of different LiF

dosimeter types Nucl. Instrum. Meth. 126 563-572 11 La Riviere P D 1969 A unique throwaway LiF dosimeter Proc. 2nd Int. Symp. on Luminescence

Dosimetry, Gatlinburg, Tenn., September 1968. USAECRep. CONF 680920, p 78 12 De Werd L A and Stoebe T G 1971 The influence of hydroxide impurities on TL in LiF Proc.

3rd Int. Symp. on Luminescence Dosimetry, Danish AEC, Riso, Roskilde. Riso Rep. 249, part I, p 78

13 Regulla D F 1972 Radiothermoluminescence of Mn-activated metaphosphate glass: its application to low and high-level photon dosimetry Proc. on Dosimetry in Agriculture, Industry, Biology and Medicine (Vienna: IAEA) paper SM/160/8, pp 215-227

14 Becker K 1974 Stability of film and thermoluminescence dosimeters in warm and humid climates Atomkernenergie 23 267

15 Jones R J L and Martin J A 1968 Use of LiF (TLD-700) for doses greater than 0.1 Mrad Health Phys. 14 521-522

16 Kitahara A, Saitoh M and Harasawa S 1976 Analysis of the TL-response of LiF-powder to thermal neutron and gamma ray exposures Health Phys. 31 4-46

17 Goldstein N, Tochilin E and Miller W G 1968 Millirad and megarad dosimetry with LiF Health Phys. 14 159-162

18 Vaugham W J and Miller L O 1970 Dosimetry using optical density changes in LiF Health Phys. 18 578

High-level photon dosimetry with TLD materials 313

19 Claffy E W, Gorbics S G and Attix F H 1971 Radiation induced optical absorption and photo-luminescence of LiF powder for high level dosimetry Proc. 3rd Int. Conf. on Luminescence Dosi­metry, Danish AEC, Riso, Roskilde. Riso Rep. 249, part II, p 756

20 Regulla D F 1972 Lithium fluoride dosimetry based on radiophotoluminescence Health Phys. 22 491-496

21 Crase K W and Gammage R B 1975 Improvements in the use of ceramic BeO for TLD Health Phys. 29 739-746

22 YamashitaT, Nada N, Onishi H and Kitamura S 1971 CaS04 activated by Tm or Dy for TLD Health Phys. 21 295-300

23 PhilbrickCR, BuckmanWG and Underwood N 1967 Ruby as TL irradiation dosimeter Health Phys. 13 798

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann ©1981 ECSC. EEC. EAEC. Brussels and Luxembourg

17 Application ofTLD in reactor engineering

J R A LAKEY

17.1. Introduction

The reactor engineer must resort to in-pile measurements and shield radiation surveys to check and monitor his design predictions and to monitor reactor plant performance. His design codes usually give the flux of primary radiation, and it is possible to evaluate the secondary radiation and to compute reliable values of kerma, absorbed dose and, where appropriate, effective dose equivalent. Many assumptions must be made to reduce the task to economically acceptable proportions, and benchmark experiments are used to provide experimental data, so that a reliable dosemeter is essential.

The thermoluminescent dosemeter (TLD) has many advantages for the reactor engineer, although it can rarely be used without calibration corrections using cavity ionisation theory and subtraction of neutron'reactions. The TLD can be designed to yield the desired response for gamma radiation and can be an effective 'gamma dose equivalent' meter. The neutron response is more complex, and it is most useful for thermal-neutron flux measurement provided that a self-absorption correction is applied. The reactor engineer can take advantage of the passive response and linearity of the TLD, its possi­bility of mass readout and its reasonable freedom from fading. The use of the same TLD in occupational radiation surveys provides a link in the dosimetric study which eases problems of interpretation.

The detector geometry is important in deriving its response function, although thin TLD are exempt from some factors. In this chapter, reference is made to computer codes used for evaluation of spectral and geometrical factors both for radiation sources and detectors. Less sophisticated methods may be satisfactory, and the chapter summarises in appendix 17.1 the useful point kernel/build-up factor equations which can be applied to TLD.

Practical examples are described and include assessment of reactor coolant activity, accumulated activity transfer and measurement of half-life.

Reactor shields have to be tested and the test data have an important influence on future design, so that accuracy and precision requirements are important. Frequently, the detector must occupy a hostile environment — temperature cycling, time elapsed from exposure to readout, location restricted due to shield layout, limited locations inside shield, spectral and angular variation in air and the influence of mixed radiation fields are considered. The use of spectrometry and computer code predictions and bench­mark tests are essential features. Solution to these problems involves the application of 'cavity theory', which is described in appendix 17.2.

The use of TLD in hostile environments demands the use of a suitable protective container, correction for fading, precautions due to activation (i.e. build-up of tritium) and contamination.

316 JRALakey

The control of external radiation exposure of radiation workers requires reliable knowledge of the radiation environment, and TLD can be used both to survey the en­

vironment and to measure the exposure subsequently received. The advantage of TLD is that the same detector can be installed in an anatomical phantom so that calibration factors are common throughout the investigation. For example, this can be applied in connection with the dosimetry of gamma photons from 16N.

17.2. A survey of the applications of TL in reactor environments

Gay ton et al [1] review general applications of TL and stress the value of the device in reactor environments because the dosemeter is passive and not subject to serious fading. This is important when the experiments must fit within a commissioning programme. Gamma­ray measurements within the pressure vessel of a shutdown reactor yield evidence on which the radiation damage to closed­circuit TV equipment can be anticipated. It is particularly useful for fine­structure measurements at the shield surface but is of course influenced by beta contamination. Gayton illustrated fuel structure measurements (figure 17.1) and showed an application of TLD to spectrometry (figure 17.2).

17.2.1. The application of TLD to gamma dosimetry

The measurement of gamma dose in reactor shields is a valuable application of TLD and the simple methods of calculation using build­up factor methods, summarised in appendix 17.1, are effective and reasonably accurate provided that the dosemeter has the same radiation properties as the shield and that corrections are made to the build­up factors to allow for multilayer shield effects. When measurements are made close to the core, in particular for gamma heating, evaluation corrections must be made for the neutron

9 13 17 21 25 29 33 37 41 45 - ■ - ' - - •-■ 1261 3 2 / 3 6 / 4 0 / 4 4 / 4 B

IRON SHIELD Figure 17.1. Section through end shield and cooling fins of a CEGB fuel flask with associated surface exposure rate. (From Gayton et al [1].)

Application of TLD in reactor engineering 317

SJ E E;

H'.r :r :r :c ~ j ~ V/////A

HZ& •7^7^r,^n

EZ

0 0

POLYTHENE : LOW DENSITY BORON CARBIDE IN ARALDITE

ALUMINIUM TIE RODS

ALUMINIUM FRONT AND BACK PLATES

£ | N7N7N7N7N7N II £1 ^iruinnjui/uuuuinnnjuuinjijuirinmnjinnf ~ f ruuuiAfuuuuuuuui/uinnjwniuuuuuuuui

1 7N7N7N7N7N7

I .1 l?cm I 30 cm

SCHEMATIC SECTION THROUGH DELTA SPECTR. SCHEMATIC SECTION THROUGH CENTRE CORE

Figure 17.2. Construction of the Delta spectrometer. (From Gayton et al [1].)

response, and the influence of the gamma/neutron response, the gamma spectrum and the detector environment must be evaluated. Table 17.1 summarises the work in this field and appendix 17.2 discusses the use of 'cavity' theory for this correction. In this chapter, several attempts to measure gamma heating are described including a method of evaluating gamma heating without knowledge of the gamma spectrum. The most sophisticated methods apply Monte Carlo codes to correct the detector response to yield a reliable measurement of energy deposition. Future developments using very thin dosemeters promise to improve the methods since the correction factors can be reduced.

17.2.2. Measurement of gamma build-up factors

De Franceschi et al [15] took advantage of the TL of concrete itself to measure build-up factors for concrete shields with a ^Co source. Concrete has low sensitivity, about 30 rad being the minimum, and is linear to 2xl04rad. The authors did not apply a cavity correction factor although the decision was intuitive. Results agreed well with calculations using the empirical Taylor fit to dose build-up factors.

Burke and Becks [16] measured dose build-up factors from 0.662 MeV gamma rays on multilayered aluminium and lead shields and compared the results with the DP-1 code and with the build-up factor from the semi-empirical Kalos method. Excellent agreement was obtained with the slightly modified Kalos formula.

The original formulation by Kalos is [17, 18]

B(X,Y) = Bb(Y)+[Bb(X+Y)-Bb(X)]k(X)C(Y) (17.1)

where B(X, Y) represents the dose build-up factor in air for normally incident gamma rays penetrating X mean free paths (MFP) of material 'a' followed by 7 MFP of material V , Bb(Y) is the single-slab build-up factor for Y MFP of material 'b', and Bb(X + Y) is the single-slab build-up factor for (X + Y) MFP of material 'b'. Also

k(X) = B3(X) - 1 Bb(X) - 1

(17.2)

is recognised as the ratio of the scattered dose component transmitted through X MFP of material 'a' relative to that transmitted through X MFP of material 'b', and C is a

oo

Table 17.1. Previous use of TLD in reactor dosimetry.

<5 Author Source of radiation Dosemeter type Comparison of results Comments

Beyer [2]

Leonard and Bransford [3]

Simons et al [4]

Reilly et al [5]

Gayton et al [ 1 ]

Tappendorf [6]

Water-moderated critical assembly

TLD 600,700 powder and PTFE rods

Water-moderated AFFRI- TLD 700 rods TRIGA reactor

Reflector and core of fast TLD 100, 700 rods reactor assembly in steel tubes

In-pile environments TLD 700 rods and PTFE associated with lead shields discs and adjacent voids

Various CEGB power reactors

ATR critical facility

Simons and Yule [7] Zero-power fast reactor

TLD rods and powder

CaF2:Mn rods 1 X 1 X 6 mm3

TLD 700, CaF2 rods and PTFE discs

23SJJ

"7Au (Cd covered) activation foils

Paired tissue-equivalent and graphite ionisation chambers

Two-dimensional discrete ordinate transport code

ANISN computer code

Variation of 6Li content in different batches of TLD 700 allowed for

Gamma heating measurement. Errors of 10-20% due to neglect of fast and thermal neutron response of TLD 700

The need to make cavity corrections before comparing expt and calc. is emphasised -no results given

Gamma heating measurements. Bragg-Gray cavity correction factors successfully used for PTFE discs. Mentions 65 M™ PTFE discs but no results presented

Review article referencing gamma heat measurement and thermal neutron flux monitoring experiments

Gamma heating measurements. Uses CaF7 in preference to LiF because of its negligible response to thermal neutrons. Simple cavity correction applied

Gamma heating measurements. Mainly con­cerned with influence on response of dosemeter size and surrounding material

Muir and La Bauve [8] Core reflector and shield of TLD 700 chips in stainless a fast reactor assembly steel

Knipe [9]

Gomaae/a/ [12]

Boulette et al [13]

Lowe [14]

Zero-power fast reactor assembly

Tanaka et al [10] 60Co source

Maerker et al [11] Tower shielding facility at ORNL

Vertical channel of 2 MW research reactor, water-moderated, reflected 10% enriched uranium fuel

Fast reactor mock-up assembly

Zero energy reactor

TLD 700 rods

TLD 700 rods

TLD 700

TLD 700 chips and PTFE discs, LijB40, tablets

TLD 700 rods in stainless-steel capsules

TLD 700 in PES

Computer code

Computer code and ionisation chamber

Point kernel calculation

Ionisation chamber and computer code

Intercomparison of results from glass, sand, LiF and Li2B40, dosemeters

Discrete ordinate transport theory

" 5 U foil

Absolute dose measurements. Quite good agreement with code. Differences attributed to code approximations. Cavity correction factors applied

Gamma heating measurements. Assumed TL response of TLD 700 to thermal neutrons is negligible. Cavity correction factors applied. Agrees within 10% with ion chamber measurements

Gamma ray heating. General approximation derived for TLD is calibrated in iron. Suggests that neutron contribution to TLD 700 response is negligible

Radiation heating in stainless steel and sodium systems. Results compare well with ion chamber. Deviations from calculated results are due to neutron effects

Recommends TLD 700, sand and glass for gamma heating measurements, and Li2B40, for thermal neutron measurements. However, agreement only to within a factor of 2

Absolute dose measurement. Agreement to within 30%. Discrepancies attributed to calculational errors

Agreement within 5.0% of calculated gamma flux in the reflector region

5 El <-+

o

a'

I'

320 JRALakey

correction factor from the fit to the Monte Carlo calculations. Kalos postulated that

C(Y) = 1 for lead followed by water (17 3)

C ( r ) = exp(-1.7F) + ( a / f c ) [ l - exp( - r ) ] for water followed by lead

where

a ~ (P Compton/MtotaOa ^ (^Compton/A'totaOb

i.e. the ratio of Compton attenuation coefficients to the total attenuation coefficients in the respective media. Shimizu discusses the rationale for this form of equation, in particular the fact that k{X) C(Y)-+a for large values of Y.

The authors compared the resulting inferred build-up factors with calculated and experimental data for aluminium-lead slabs and with limited comparisons made for other slab combinations and source energies, and decided to modify only the C{Y) term in the Kalos formula. Let

C(y) = e x p ( - 7 r ) + / 3 [ l - e x p ( - y ) ] (17.4)

where /3 = 1.5 for any high-atomic-number (Z) material followed by any low-atomic-number material, @ = a/k(X) for any low-Z material followed by any high-Z material, where a is the same as above, and

McomptonOower-Z material) 7 = .

McomPton(higher-Z material)

Furthermore, when applying the formula to a multilayered slab of n layers, they treat the first (n — 1) layers as a single slab of the (n — l)th material for the purpose of calculating a and 7. The build-up factor for this (n — l)th slab, however, is computed by applying the formula to a double-layered slab of (n - 2) layers followed by the (n - l)th layer. For example, the build-up factor for a slab consisting of X MFP of lead, followed by Y MFP of aluminium, followed by Z MFP of lead is calculated first by applying the above formula to X MFP of lead followed by Y MFP of aluminium. Using this build-up factor and a for aluminium followed by lead the build-up factor for the triple-layered slab is computed.

17.2.3. High-energy gamma rays

Bishop et al [19] utilise reactor coolant water on the LIDO reactor to create a uniform disc source mainly of 16N and shielded spectra calculated using the MCNID Monte Carlo code. A similar rig is installed at the URR Risley giving 6-7 R h"1 at the source surface. The energy of photons is 6.13 MeV and a convenient calibration source can be achieved using 19F(p, a7)160 with a proton energy of 340 keV.

Nitrogen-16 has a half-life of 7.4 s and emits gamma rays of 7.1 MeV and 6.1 MeV in 6% and 76% of its disintegrations. Dose rate measurements in a simple geometry have been used to evaluate the intensity of this source in the coolant of water-moderated reactors. Avery et al [20] made calculations of steam pipe dose rates using the MORSE code. The presence of 1SC activity (ratio 12.0 ± 2, 16N to 15C) and uncertainties in the delay time from leaving the core (~4 s) and the partition coefficient of 16N between

Application of TLD in reactor engineering 321

steam and combined steam and water (~0.69) had to be considered to gain a value for 16N flux which produces 0.9 ± 2% of the dose rate (between 100 and 500 mR h"1 at full power at distances up to 100 cm from 14.45 cm radius steam pipe).

D Faddy and G Thompson (private communication) have used TLD LiF powder calibrated at 0.66 MeV gamma-ray energy to evaluate coolant activity of a pressurised water reactor (PWR) by measurements on coolant pipes. Locations were selected in contact with lagging at the centre of long pipes and in locations well shielded from sources other than the adjacent pipe. Gamma exposure rates were calculated with a point kernel build-up factor code.

17.2.4. Fast reactor core measurements

Simons et al [4] used TLD for dose mappings, single-cell heterogeneity, blanket-reflector and reflector-core interface studies in the EBR-11 series of ZPR-3 critical assemblies. TLD rods loaded in 0.125 inch diameter and 0.035 inch wall thickness 304 stainless-steel tubes were irradiated for approximately 100 W h. Analysis utilised the two-dimensional discrete ordinate transport code DOT in R-Z geometry, and the gamma calculations were S ^ approximations. Some problems were experienced in correlating the gamma-ray energy deposition with TLD responses.

Simons and Yule [7] reviewed earlier gamma heating measurements in which cavity theory has been applied to TLD. With large cavities, the electron spectrum is important, and these authors averaged over the three components of the primary electron spectrum produced by the more energetic gamma ray. Tabulated values of mass stopping power were used but, although the method works, it has not been tested over a range of gamma energies.

Figure 17.3 illustrates how the correction for the energy absorbed per unit mass of the dosemeter material varies with the energy absorbed per unit mass of the wall when the three levels of cavity are considered — the escape of scattered radiation is not taken into account. The dosemeters were applied to heating measurements in a range of gamma-ray spectra typical of the fast reactor.

17.2.5. Reactor gamma heating measurements

Haack and Majborn [21] compared the use of TLD against calorimeters in an experimental tube of the Danish Research Reactor DR3. Bismuth shielding was used to vary the ratio of thermal neutrons to gamma rays (around 1012 neutrons/cm2 s and 106rad gamma/h). Li2B407:Mn was used because it had high sensitivity to thermal neutrons compared with LiF TLD 700 which is 7 Li-enriched and therefore relatively insensitive to thermal neutrons -responses were reported at 320R and 1.4R respectively for the ^Co equivalence to 1010 neutrons/cm2. Dosemeters were mounted in 6 mm diameter holes in aluminium rods and the dose due to 28A1 decay was corrected, but this was only possible for LiF TLD 700 and corresponded to about 5% of the measured 7-ray dose. Fast-neutron flux was about 4 x 1010 neutrons/cm2 s, and its contribution was negligible in this experiment. Figure 17.4 gives the relationship between calorimeter and TL measurements and shows excellent agreement, and it was concluded that TLD have the advantage of small probe size,

322 J R A Lakey 50

0.5 1.0 5 GAMMA-RAY ENERGY ( MeV )

Figure 17.3. Variation of \/f(Ty) as a function of energy for a 3 X 3 X 0.8 mm'' (g = 0.243 g cm - 2) 7LiF dosemeter surrounded by iron, based on large-cavity, general-cavity and small-cavity ionisation theories.

TJ a

3 -

UJ

O a

o a- o

i -

x CALORIMETER MEASUREMENTS o THERMOLUMINESCENCE

MEASUREMENTS

WITHOUT Bi

.WITH Bi

0 20 40 60 80 100 HEIGHT ABOVE CORE CENTRAL PLANE (cm)

Figure 17.4. 7-ray dose rate versus height above core central plane as measured with the calorimeter and with thermoluminescence dosemeters (referring to a reactor power of 0.97 MW).

measurements can be made simultaneously in many positions and thermal flux can be deduced.

Experiments were performed by Reilly et al [5] to simulate in-pile environments associated with lead shields and adjacent voids. No correction was made for self-shielding

Application of TLD in reactor engineering 323

or the cavity effect but a simple application of the mass stopping power ratio in water regions close to lead gave the correct response as predicted by ANISN (Engle [22]). Gomaa et al [12] used TLD and other devices to measure gamma heating of the EY-RR-1 reactor. The TL materials were 7LiF-Teflon 0.5 mm x 8 mm diameter and 0.4 mm x 10 mm diameter and a 7LiF crystal 3 x 3 x 1 mm3. These were calibrated with ^Co gamma rays. Radiation effects were observable at 10srad in 7LiF-Teflon which became coloured but these were usable up to 10s rad.

17.2.6. Fast test reactor mock-up shield tests

Radiation heating studies in a stainless-steel and sodium shield were reported by Boulette et al [13]. Lithium fluoride rods were enclosed in stainless steel and irradiated for lOOOWh. Uncertainties in the TLD dose interpretation include sample non-uniformity, calibration, spectrum variations and fast-neutron effects. Experimental biases were estimated to be +30% out of core and — 10% in the core.

Analysis was performed using discrete ordinates transport theory (S8P3) for gamma flux and three-dimensional diffusion theory for the source calculations (3 DB code). The illustrated distribution, which does not include the biases (figure 17.5), extends along the centreline of the EMC, traversing the axial shields and reflectors as well as the core region. This distribution records the heating in steel. Estimated one standard deviation limits are shown but the experimental bias would increase the discrepancy between theory and experiment.

Muir and La Bauve [8] reported gamma measurements in this facility compared with calculated data using a flux-dose conversion factor without correction for cavity ionisa-

tion theory. Correction for cavity theory used the method of Simons and Yule [7] and fast-neutron effects were calculated using the work of Wingate et al [23].

Benchmark measurements were established by Maerker et al [11] in a stainless-steel and sodium system using TLD and ion chambers. Calculations applied cross-section data from ENDF/B-IV in the two-dimensional discrete ordinates code DOT starting with an absolute disc source produced at the Tower Shielding Facility at ORNL. The gamma-ray spectral effect of replacing TLD material with iron and the neutron response of the TLD were estimated. All dosemeters were in excellent agreement but showed increasing underprediction of the gamma-ray heating with penetration.

50 100 150 DISTANCE ( c m )

Figure 17.5. Analytical/experimental com-

■ parison of gamma heating in stainless steel in FTR/EMC along axial centreline.

324 JRALakey

17.2.7. Gamma heating measurements in an unknown spectrum

Tanaka et al [10] derived a method for estimating gamma heating from TLD measure­ments in the absence of knowledge of the gamma spectrum. The absorbed dose DM (Ey) at a point in a material 'M' for gamma rays of energy Ey is given as

DM(Ey) = CEy^(Ey) mMenC^M (17.5)

where Ey is the gamma-ray energy (MeV), C is the conversion constant from mega-electronvolts to ergs (erg/MeV), *p(Ey) is the gamma-ray fluence of the energy Ey (cm-2), and ml^en(^y)M is the mass energy absorption coefficient of the material 'M' (cm2g-1).

Assuming the gamma-ray spectrum is not influenced by the existence of a TLD, the absorbed dose BTLD(Ey)M

m TLD at the same position in the material 'M' is represented as follows

^TLDC^TOM = ^ T L D ( £ V ) K^TOTLD-M

= CEy^(Ey)mtien(Ey)TLD £(£7)TLD-M (17.6)

where KTLD(Ey) is the kerma of TLD for gamma rays of energy Ey (erg g"1), K ^ T L D M is the absorbed dose correction factor to obtain the absorbed dose of TLD placed in the material 'M' from the kerma, and mMen^^LD is the mass energy absorption coefficient of TLD (cm2g_1).

The observed value by TLD is the thermoluminescence induced by energy absorption of gamma rays. Therefore, the relation between the thermoluminescence and the absorbed dose of the TLD must be defined. Furuta and Tanaka [24] have obtained a general relation of the thermoluminescence and the adsorbed dose for any TLD and radiations, and showed that the integral thermoluminescence of TLD is given using a conversion efficiency by

^TLDt^M = V(Ey)TLD ^ T L D ^ J M (17.7)

where Gjho(Ey)M is the integral thermoluminescence of TLD exposed in material 'M' (light unit), and v(Ey)TLD is the mean (conversion) efficiency to obtain the integral thermoluminescence from the absorbed dose (light unit/erg g_1).

From equations (17.5)-(17.7) the relationship of the absorbed dose of material 'M' and the integral thermoluminescence of TLD which is exposed by gamma rays at the relevant point in material 'M' becomes

r, (F \ - l 1 m/*en(gT)M „ , „ . uU\p-i)-T7^r\ rTTTT 77TT ^ T L D ( ^ 7 ) M

?V£7-lTLD-M VifiyJTLD mMenl/^/TLD = /7(£ '7)T L D .M GjLD(Ey)M. (17.8)

Consequently, the conversion factor /7(£"7)TLD-M m u s t be defined for estimating the absorbed dose of gamma rays in the material by TLD.

17.2.7.1. Absorbed dose correction factor. The absorbed dose correction factor is dependent upon the size and the kind of TLD, the surrounding material and gamma-ray energy. This factor [7, 25] is close to unity in the energy region below 2 MeV in any combinations of TLD and material. In contrast, this factor has values differing from unity

Application of TLD in reactor engineering 325

in the energy region higher than 2 MeV. It has been suggested that the conversion factor becomes nearly constant in the region higher than 60Co gamma-ray energy.

17.2.7.2. Mean (conversion) efficiency. It has been considered that the mean efficiency for gamma rays is independent in the gamma-ray energy range from 0.1 keV to 10 MeV, where the integral thermoluminescence is proportional to the amount of irradiation. Thus, the mean efficiency can be given by a constant, say I?TLD- If the absorbed dose of TLD is known for a gamma-ray field, the mean efficiency can be calculated from equation (17.7). It is difficult, however, to know accurately the absorbed dose of TLD, since it is not always easy to evaluate the electronic equilibrium or to know the absorbed dose correction factor. Fortunately, the absorbed dose correction factor is close to unity for 60Co gamma rays. Then, when the TLD is calibrated by a known field of 60Co gamma rays in air, the mean efficiency may be represented as

J?TLD = rK^CohLD = I / ^ T L D C 6 0 ^ ) * . (17.9)

In practice, when the TLD is exposed by 60Co gamma rays of 1 R and the roentgen (R) ^Co equivalence unit is used as a light unit of the integral thermoluminescence, the value of the mean efficiency may be defined as

7?TLD = - ^ m / i e " L C ° ) a i r (R "Co equivalence/erg g"1). (17.10) 86.9 m/ien(6°Co)TLD

From these considerations, the conversion factor is approximated using the mean efficiency and the mass energy absorption coefficient, as follows:

/ 7 (£ 7 )TLD-M = — m M e"ffM • (17.11) 7?TLD m^en(^7)TLD

17.2.7.3. The interpolation method. The most important factor in the conversion factor is the ratio of the mass energy absorption coefficients of the material and TLD. AS an example, figure 17.6 shows the ratio of the mass energy absorption coefficient of iron to those of various TLD normalised at 60Co gamma-ray energy, and the effective atomic number of these TLD is shown. Though the deviation of the ratio from unity is fairly large in the region below 1 MeV, it is noticed that the deviation decreases when the atomic number of TLD is close to that of iron, which varies smoothly with the effective atomic number of the TLD. Thus, the ratio becomes smaller than unity for the TLD with an effective atomic number larger than that of iron, but it becomes larger than unity for the TLD with an effective atomic number smaller than that of iron. The same characteristics of the mass energy absorption coefficient may be considered to be held between any TLD and materials. This is a fundamental and important fact to be used for this method.

By exposing several kinds of TLD whose atomic number is different from that of the material considered and using the value of the conversion factor for ^Co gamma rays to estimate the absorbed dose of the material, it is found that

/ 7 r C o W M ^ ^fcC°)M ■ (17,2)

VTL.D mMenl ^°hhD

The results will be underestimated for a TLD having a smaller atomic number than that of the material, or overestimated for the opposite case.

326 JRALakey

Q _ _l >

~2

UJ in CM

C i-1

*-t; E <

^ S ^ N UJ - I v - <

c 2 a-o E z

I U U

10

1.0

0.1

: Fe/7UF (Mg)

="'N Fe/Na2SCUDy) ~ \ Fe/CaSCMTm) ^ % \ Fe/SrS04(Dy) "- N \ Fe/BaSCM Dy) : n " \ \

\ \ \

?^^^^ ^^^ •* -\ " ^ ' A /' : v.^ ^/

i > i<"i i ' i i

2 8.2 11.6 15.2 29.2 45.5

^ ^

■ 11 I I

0.03 10 0.1 1.0 PHOTON ENERGY ( MeV)

Figure 17.6. The ratio of the mass energy absorption coefficient of iron to those of various TLD normalised at "Co gamma-ray energy (1.25 MeV).

If the conversion factor defined by equation (17.12) is used over the whole energy range, the absorbed dose estimated by various TLD increases with the effective atomic number of the TLD. Thus, the absorbed dose in the material can be estimated by interpolating the values expressed as a function of the atomic number, which are measured by using several kinds of TLD.

This useful method is limited in accuracy by the unknown shape of the relationship between response and Z and by the need to place the two detectors precisely in the same position.

The experiments in the Zebra Reactor carried out by Knipe [9] used a zirconium-

walled solid-state cavity dosemeter (SSCD) incorporating 7LiF thermoluminescent dosemeters (TLD). The TLD used 7LiF (0.007% 6Li and 99.993% 7Li) manufactured by Harshaw in rods 1 x 1 x 6 mmJ. Post-irradiation annealing used was for 1 h at 400 °C, 16 h at 80 °C and readout by Conrad Model 7100. ^Co was used for calibration using an equilibrium wall of TLD during exposure and supralinearity appeared at 200 rad to the LiF. Irradiations were held within this limit at a core temperature of 50 °C and readout took place seven days after irradiation to allow for fading.

Neutron effects were corrected using the method of Wingate et al [23] and were less than 0.1% in the core due to 6Li(n, CL)T reactions. The major contribution was due to 7Li and F recoils.

The TLD 'cavity' was corrected for energy deposition relative to the 'wall' using the Monte Carlo tracking program PROCEED [26] and the measurements were compared to Monte Carlo calculations using the MCNID program [27] in one-dimensional plane

Application of TLD in reactor engineering 327

geometry. The dose at points in the core was dominated by the gamma rays which originated in the immediate vicinity and so splitting for variance reduction was limited to regions close to the source and standard deviations of 4% were obtained. Dose­rate conversion factors [28] for zirconium were obtained by interpolation and were used to obtain energy deposition rates. Corrections were applied for neutron interactions (15%) and the cavity correction for the zirconium wall was approximately 13%; figure 17.7

1500r

£ 1000 f .

□ w 500h 2 z o u cc

UI i -< cc

o a a UJ m cc o in m <

100

50

10

: t ••• • • .

+ +•! r,

i -

-

'. . -

■ j QJ ^ on

r^ — U

, I . I . 1

•• ft.

K

UJ UJ 1- cc Z 5 0 O U

i

• MONTE CARLO CALCULATION ( STANDARD DEVIATION U'U)

+ TLD MEASUREMENTS

•

*#

.+ • +

cc

LE

TE

D

BR

EE

DE

•

Q_ f ^ UJ O Q 3

1 , 1 .

* • •

(_) _) UJ UJ —1 UJ u. 1- ui I/) cc

1

PH

ITE

/ +

L

EC

TO

R

< l i -CC UI o cc

. 1

UI

1-

9; ° < < ^ ^ o in

, i . 0 200 400 600 800 1000 1200 K00 1600 1800

DISTANCE FROM CORE CENTRE ( m m ) Figure 17.7. Comparison of Monte Carlo calculations of absorbed dose rate and TLD measurements in Zebra Assembly 12.

gives the comparison with the calculations. The TLD measurements were clearly responding to the fine structure in the core and were systematically lower by an average 10%. The TLD measurements were also lower in the steel­aluminium reflector but were in good agreement within the axial breeder. These discrepancies were thought to be due to shortcomings in the calculations, probably source data, rather than to errors of experi­

mental technique.

17.3. Application to neutron dosimetry

The use of TLD for thermal­neutron dosimetry gives the possibility of an absolute response due to the use of known capture cross sections and reactions. Surprisingly the literature, summarised in table 17.2, shows considerable variation and it is now evident that self­absorption corrections are of great importance (the method developed by Horowitz et al [53] is indicated in appendix 17.3). However, the response also depends on the readout cycle used, the isotopic composition of the TL material and the neutron

Table 17.2. Summary of TL response of LiF to thermal-neutron experiments. oo

LiF responsej

Author TLD 100 TLD 600 TLD 700 Dosemeter type Readout cycle Comments

Cameron et al [29]

Wingateef al [30]

Simpson [31]

Distenfeld et al [32]

370

200

220

200 Woodley and Johnson [33]

Ayyangarera/ [34, 35] 95

Wallace and Ziemer [ 3 6 ] 310

Reddyeffl/[37] 535

Tochilin et al [38, 39] 160-250

0.7

880

550 0.9

2190

1200-1700

Scarpa [40] 65

0.87-0.96

1.1-1.6

1.0

Powder in polythene cylinders 0.108 cm X 0.5 cm

Powder

Powder in stainless-steel capsules

Powder in polythene capsules

Powder in gelatin capsules

Powder

Powder in gelatin capsules

400 °C integrated Gives cadmium ratio showing variation of counts response with neutron temperature.

Recommends small dosemeter to minimise self-shielding. Elementary self-shielding factor calculation

- Contains consideration of the variation in response of TLD 700 to thermal neutrons. Fission foil and tissue-equivalent chamber calibration

Peak height response Plutonium-beryllium used as neutron source

Thermal column of graphite research reactor used

400 °C integrated readout

Achieved proportionality between thermal neutron flux and TL response for TLD 600 up to 1012n/cmJ. Emphasises need to allow for response of TLD 700 to thermal neutrons

Integrated TL response Primarily a LijB407 study

Integrated TL Emphasises need for small dosemeter to response up to 380 °C minimise self-shielding

- Pu-Be source + graphite moderator, cali­brated with Au foil. Gamma dose determined by shielding TLD 100 from thermal neutrons

- Values quoted in a study of BeO TLD

Jahnert [41,42]

Dua e t al [43]

Majborn [44]

Haack and Majborn [45]

Ayyangar et al [46]

McKlveen and Schwenk [47]

Tanaka and Furuta [48]

Rossitei el al [49]

Horowitz et al [50]

Schumacher and Krauss [51]

Bartlett and Edwards [52]

490

360

2650

330

0.35

2.5

1.3

1.4

TLD 700 powder

TLD rodsj

TLD chips §

TLD chips

Integrated response up to 350°C

300 °C

1520

1350

893

-

2400

2180

-

1.1

-

-

1.62

0.19

1.26

0.23

Powder in nylon capsules

Chips

Powder in PTFE capsules and chips

Powder in PTFE capsules and chips

Chips

PTFE-LiFi discs 6 mm diameter, 4 mm thick

Chips

140°C preheat followed by 270°C readout

Integrated response

—

-

-

Integrated count

a particle response survey, a particle response to energy 2 MeV. Response to 10 , on/cm2

Linearity to 10 , 2n/cm2 for TLD 600. Variation of 6Li in different batches of TLD 700 noticed

Thin TLD are used to show effect of self-shielding though response of chips only is given

Feasibility study of Danish Research Reactor. Study compares favourably with calori-metry measurements

Found thermal neutron response to decrease with sample thickness because of self-shielding

Linearity of TL response up to 10'2n/cm2

with those from BF3 detector

Parallel field, self-shielding correction applied

Measurements made to calibrate medical therapy dosimetry system

Response to thermal neutrons calculated from TL response 2.5 MeV a particles

ix =§

Q

a

3.

t 101 0n/cm2= 1 R '"Co gamma radiation. § TLD chips - sintered blocks of LiF, 0.3 X 0.3 X 0.089 mm3

t TLD rods - sintered blocks of LiF, 1 X 1 X 6 mm3. " PTFE discs - discs containing 30% LiF, diameter 4 mm, thickness 0.4 mm. to

330 JRALakey

temperature in which the device is exposed. Some attempts have been made to use activation of the phosphor to yield a measurement free of gamma effects.

The application to fast-neutron dosimetry is more difficult and does not readily permit correction for the gamma field. However, various spectrometer systems have been attempted using TLD in neutron moderators. The use of TL for neutron spectrometry has been attempted using the Delta spectrometer (figure 17.2) in which TLD are installed in a 31 cm long x 9 cm radius polythene cylinder. A removable polythene core 10 cm in diameter is separated by 1.5 cm boron-loaded Araldite and the TLD capsules containing natural LiF and 7LiF are placed at 1 cm intervals along the face. The threshold sensitivity is 106n/cm2 and the theoretical resolution is 10%. An equivalent gamma spectrometer contains lead- and aluminium-shielded TLD in nine separate units from which nine energy groups can be resolved.

A simpler device using a polythene cylinder containing pairs of TLD 600 and TLD 700 along the axis supplemented by a 10 inch polythene sphere containing one pair of detectors is reported by Singh [54]. The author concludes that this is a useful device for monitoring in shielded areas and can yield an indication of neutron energy by comparison of the reading at 10 cm depth and at the maximum provided that the gamma radiation field is negligible.

17.3.1. TLD 700 response to reactor neutrons

Leonard and Bransford [55] reported measurements in six fields in the exposure room of the AFRRI-TRIGA reactor:

(1) in free air from the bare TRIGA core, (2) behind 2,4 and 6 inches of lead, and (3) through 3.54 and 7.08 inches of water.

The authors reported errors as large as 10-20% using TLD 700 to determine the gamma dose in tissue-equivalent material at a depth of 18 cm if the response of the dosemeter to fast and thermal neutrons was ignored. This response was determined by calibration against tissue-equivalent and graphite ionisation chambers. However, the 'quench effect' reported by Oltman [56] due to fast neutrons was avoided by pre-irradiation of the TLD 700 using "'Co to gain a 50-100 rad dose.

Reactor flux measurement using TLD in a research reactor (McKlveen and Schwenk [47]) gave erratic results at thermal-neutron fluxes less than 104n/cm2s but these problems were mainly due to fission-product-decay gamma rays from a previous high-power operation, and at fluxes above lO^n/cm2 s supralinearity was apparent and reports were made of extra sensitivity. At fluxes exceeding 1012 to 1013 TLD ribbons were badly discoloured and at 1012 significant activation products were present. The fluence used was around 1013n/cm2 for low flux measurements, although the authors do not clearly specify irradiation times.

Mayhugh et al [57] use CaS04:Dy, CaF2:Dy and natural CaF2 for thermal-neutron dosimetry. After neutron irradiation, the phosphor is stored and the decay of 165Dy (2.3 h) gives the lowest detectable limit of 5 x 107n/cm2; 4sCa (165 d) with a limit of 6xl01 0n/cm2 could also be used. The procedure involves annealing immediately after irradiation to remove the TL induced during irradiation.

Application of TLD in reactor engineering 331

Similar results were reported by Wang et al [58], but the authors' lower detection limit for dysprosium activation was 2 x 108 n/cm2.

17.3.2. Response of6LiF and nLiF TLD to fast neutrons

Furuta and Tanaka [59] examined the response using monoenergetic fast neutrons from a 2 MV Van de Graaff accelerator. The devices were calibrated experimentally and changes in glow curve were recorded and expressed by Gaussian analysis. The authors conclude that neutron kerma efficiencies were

T?(X)6LiF(n) = 4.17xlO - 3

i7(X)7LiF(n) = 4.34xl0"3

compared to y efficiencies

i?(X)6LiF(6OCo) = 0.0112

T?(X)7LiF(60Co) = 0.0117

R ^Co equivalences/erg g *

R ^Co equivalences/erg g_1

R/erg g"1

R/erg g_1.

The energy response of integral thermoluminescence is given for 6 l iF in figure 17.8 and for 7liF in figure 17.9.

o a. 3 i±j

icr8

O LU

S n! io-"

-. 10-6 pi

-CALCULATION (WITHOUT RECOILED FLUORINE) (WITH RECOILED FLUORINE)

EXPERIMENT

ul_ ' !

° C

-10" o a.

10

0.01

5 ^ or — U T-

■8 °- oi < 5 O

^ > _ 100 0.1 1.0 10

NEUTRON ENERGY ( MeV ) Figure 17.8. Energy response of integral thermoluminescence of 6LiF TLD to neutrons.

17.4. Environmental monitoring

The use of TLD measurements to evaluate population exposure due to the noble-gas fission product releases from Three Mile Island emphasised the advantage of a passive system for emergency use. Although the natural radiation background varies, it can be adequately predicted and with suitable sensitivity exposures as low as about 0.1 mR can be estimated.

332 JRALakey

o

z ­«

z E

10"

10' ■10

<_> _ l

o ^ s it io-'

2

Mxl/10)

_i i 0.01

CALCULATION (WITHOUT REC. FLUORINE J CALCULAT. (WITH RECOILED FLUORINE) EXPERIMENT (THIS WORK )

(C.L. WINGATE ET AL ) D .. ( N. GOLDSTEIN ET AL )'

Z

10'

10

10"

-8 z o a.

3 Z

or "~ UJ Y

<

100 0.1 1.0 10 NEUTRON ENERGY ( MeV)

Figure 17.9. Energy response of integral thermoluminescence of 7IiF TLD to neutrons.

Burke and Marcus [60] reported the problem of interpreting environmental radiation levels in terms of the gaseous release from a boiling water reactor (BWR) power station. Fluctuations caused by cosmic radiation and terrestial radiation have to be identified ­

the latter appears to be most important and is dependent on soil moisture changes and surplus ground water which dilute and shield the sources. This variation has been modelled and compared with monthly TLD measurements. After correction for reactor source and meteorological variations (assumed constant across the site), an improved correlation was obtained.

Brinck et al [61] describe the Environmental Protection Agency field study to assess TLD and other dosemeters for the detection of external radiation exposure due to dis­

charged radioactive gases from a BWR. The TLD system used a hot­pressed CaF2:Mn cylinder bonded to a heater element and installed with an aluminium­lead­tin shield to eliminate over­response to gamma radiation below 100 keV. Calibration used a radium source and fading was found to occur mainly in the first 5 h after exposure with no measurable fade after 24 h. Thus monthly readings are not affected by fade if all readings are delayed until 5 h after the end of the exposure period.

Internal background due to dosemeter activity was around 1.98±0.09/jRh­1. This yields a minimum detectable exposure over 1 month of 3 x 720 x 0.09 /JR or 0.19 mR provided that natural environmental background variation can be eliminated. Surveys of natural radiation background have been made by Powers et al [62] and showed back­

ground variation by 30% over 4 week periods. The lower level of annual dose due to normal reactor operation which could be measured with these dosemeters was 5 mR.

TLD were used by Weng et al [63] to record fall­out during the period 1971­1975 produced by nuclear tests at Lop Nor, China. The detectors were CaS04:Dy and CaS04:Tm manufactured by the authors and three samples were installed in each measuring location at 1 m above ground. These detectors appeared to be affected by exposure rate and read low for rates of 0.1 to 2 mR min­1.

Application of TLD in reactor engineering 333

17.5. Miscellaneous applications

The use of TLD for activity assessment is an excellent supplement to more complicated procedures and TLD can frequently be used in hostile situations where other radiation monitors would be useless.

Activated corrosion products (crud) are a major source of occupational radiation exposure during maintenance operations of water-moderated power reactors [64, 65] and typically contribute up to 88% of the total dose rate. Reliable spectral analysis is useful to evaluate the activity distribution [66] although the spectrometer can be calibrated on sections of the reactor coolant pipework, TLD measurements can be used to obtain the absolute level of dose rate.

17.5.1. Accumulated activity transfer

TLD can be used to evaluate integrated activity transfer or mean flow rate by analogy with methods developed using other radiation detectors as shown by Lakey [66]. When activity is transferred along a pipe or conveyor belt, the dose (in rad) received by a collimated detector viewing a section of the pipe is

cna D = —

v

where C is the total activity transferred (Ci), n is the dose rate per unit activity (rad s-1

Ci_1), a is the aperture width (cm), and v is the source velocity (cms -1), n can be obtained by calibration with a point source of the appropriate nuclide. A known quantity of activity held in the flow as a tracer can be used to assess the mean flow rate.

17.5.2. Measurement of half-life of131I in thyroid

Malone and Cullen [67] applied four I JF-PTFE disc dosemeters, set in a dressing, to the surface of the patient's neck. Dosemeters were removed and returned to the hospital for readout, thus reducing the number of visits required to be made to the hospital. A simple neck phantom constructed from lucite (water-filled) was used for measurements with 131I. It was necessary to use a more effective tissue-equivalent fluid for 12SI measurements and the composition of this fluid is given in the paper.

Appendix 17.1. Calculation of gamma photon absorbed dose

The most important quantity characterising the penetration and diffusion of gamma radiation in extended media is the linear attenuation coefficient n, whose magnitude depends on the photon energy E and the atomic number Z of the medium, /u may be defined as the probability per unit path length that a photon will interact with the medium.

In 'good' geometry (or narrow-beam) conditions, it can be assumed that every scattering and absorption event leads to permanent removal of photons from the beam. The reduction in intensity of an incident beam I0 (photons/cm2 s) to Ix (photons/cm2 s) having passed through a thickness x cm of material with linear attenuation coefficient /i

334 JRALakey

is given by

4= / 0 exp( -Mx) .

For situations more complex than a collimated beam, the attenuation is still exponential but is modified by two additional factors:

(1) a geometry factor, depending essentially on source geometry, and (2) a 'build-up' factor which takes into account secondary photons produced in the

absorber (mainly as the result of Compton scattered photons which reach the detector).

The dose (or flux) at the detector will be the sum of the uncollided flux 70exp(— jux) plus the radiation which undergoes scattering to reach the detector. Allowance for this scattered radiation is made by the introduction of a 'build-up' factor B whose magnitude is a function of energy, n and x. Thus for a broad beam of x-rays

I = I0 exp ( - fix) B (E, jix).

Selection of build-up factor depends upon certain 'rules of thumb' based upon qualitative application of theory. For heterogeneous shields comprising alternate layers of a heavy and light element, approximate methods for the determination of build-up factor have to be used.

(a) High-Z material followed by low-Z material. An approximate procedure is to take B as the product of the separate build-up factors for the two materials. In high-Z materials, scattered photons are only slightly degraded in energy and only scattered through small angles, and therefore the uncolhded source term incident at the second absorber is simply multiplied by the build-up in the high-Z material.

(b) Low-Z material followed by high-Z material. The build-up factor for this system corresponds roughly to that of the heavy element whose thickness (in mean free paths) corresponds to that of the composite system.

A17.1.1. Empirical relationships for build-up factors

(a) Linear build-up factor:

B{E,ixx)= 1 +a(E)nx.

(b) Taylor form:

B(E,nx)=A(E)exp[-al(E)nx] + (1 -A)exp[-a2(E)lix]

where A, ai and a2 are 'fitting' parameters. This expression may be approximated to

B(E, fix) = apx.

This will lead to an underestimation at low values of/JJC.

(c) Capo's form: a four-term polynomial which covers a wide range of energies (0.5-10 MeV) and is accurate over 10-20 mean free paths:

B(E, nx) = fa(E) + 0i(£)M* + fo{EWxf + P3(.E)(Mxf-

Application of TLD in reactor engineering 335

A17.1.2. Calculation of flux from extended sources

Calculation of flux enables the exposure rate to be estimated by the expression:

X= 1.7x10~6(pE Rh"1.

Use Q for source strength: Qo point source (photons/s), Qx line source (photons/cm s), Q2 plane source (photons/cm2 s), Q3 volume source (photons/cm3 s).

Extended sources may be treated as arrays of point sources and by application of the expressions in § A17.1.2.1 the flux due to extended sources may be derived.

A17.1.2.1. Flux from point source. For a point source located a distance r from the point of measurement:

Qo 4717

2

With attenuation:

Qo 4irr

With build­up:

Qo 4nr

4> = —-exp(-/jtx).

<p = -^—2 exp(-nx)B(E, fix).

A17.1.2.2. Flux from a line source. For a line source, distance Z, without any shielding, half­angle i//:

<P = — . 2TTZ

For an infinitely long source, i// = -njl:

♦ - * . 4Z

A17.1.2.3. Flux from a shielded line source, distance Z, neglecting build-up

A Q l ■< ^ 4>=—— sec i(jit, \p).

2TTZ

336 JRALakey

A17.1.2.4. Flux from a shielded line source, distance Z, with linear build-up

Q\ Q^ixt rii 4> = — sec Hjit, \jj) + [exp(— /ir sec0)] sec 6 dd.

2TTZ 2ITZ J 0

A17.1.2.5. Flux from a plane source of radius A, unshielded at a perpendicular distance Z from its centre

0 = 7 l o 8 e ( 1 + ^ l

A17.1.2.6. Flux from a plane source of radius A, shielded, neglecting build-up

A17.1.2.7. Volume source. Flux in a cloud of radioactive or airborne contamination (where /i is small). At the centre, flux due to volume element is

4irr2dr 4-nr2,

and therefore

, = 2 3 f dr = Jo

Q3A.

A17.1.2.8. Flux in an absorbing medium. For example, a solid or liquid where n is significant:

4irr2dr 47T7-2

and therefore integrating gives

d<p = Qi ——— exp(-/zr)

4> = Q3\ exp(-/ir)dr = —[1 -exp(- ju4)] . Jo M

A17.1.2.9. Flux in an infinite absorbing medium. A large compared to l//i, / i / > 3 . Therefore

exp(-/L4)->-0

and hence

0 = Q3ln-

Application of TLD in reactor engineering

A17.1.3. Absorbed dose in infinite absorbing radioactive medium

This can be calculated on the basis of:

Energy absorbed/cm3 = Energy released/cm3

337

Energy released

Absorbed dose rate

Q3E MeVcnTV

Q3E 1.602x10" 100

3600 rad h"

A17.1.4. Calculation of coolant activity

If the residence time of the coolant in the reactor core is T (s) and the external circuit time is t (s), the activity N0 at the exit from the core in dis g_1 s"1 is

N0 = 20 [1 -exp(Xr)]

1 - e x p [ - X ( r + r ) ]

where £ is the cross section per gram, <j> is the average core flux, and X is the decay constant for activity.

Appendix 17.2. Cavity ionisation theory

Measurement of the absorbed dose in a medium exposed to ionising radiation necessitates the introduction of a radiation-sensitive device into that medium. Usually, this device will differ from the medium in both atomic number and density and will constitute a discontinuity which was considered to be a cavity in the original Bragg-Gray theory [68].

To derive a more general treatment, it is necessary to consider the boundary or interface between the medium and the detector.

Figure 17.10 depicts the dose distribution commonly encountered at an interface between two dissimilar materials when placed in a photon field. The photons passing through the wall medium Z\ generate an electron spectrum characteristic of that medium and the incident photon. In medium Z2, representing the detector, the photons generate an electron spectrum characteristic of medium Z2.

Now, take the case of the 'cavity' in the dissimilar medium. The absorbed dose required is the dose in the wall medium but the dose measured is actually the dose in the

WALL

Figure 17.10. Typical dose distribution at gamma-iiradiated interface.

338 JR A Lakey

'cavity' in the wall medium. A cavity correction factor has to be applied to convert the latter to the former. The size of the cavity with respect to the range of maximum energy electrons in the wall determines the cavity theory to be applied.

If the cavity is small (figure 17.11(a)), then it can be assumed that no electrons are generated by photons in the cavity and the electron spectrum characteristic of the wall remains undisturbed. In this case, the ratio of energy deposited in wall medium to that deposited in the cavity is just the ratio of mass stopping powers averaged over the electron spectrum as assumed in the Bragg-Gray theory. If the cavity is large (figure 17.11(c)), then inside the cavity the electron characteristic of the cavity material is

[a) (£>) (c) Figure 17.11. Absorbed dose distribution through cavities of varying size: (a) very small cavity; (b) intermediate sized cavity; (c) very large cavity.

established and the ratio of absorbed dose in the wall to that in the cavity is equal to the ratio of mass energy absorption coefficients of the two media, i.e. the interface effects are negligible when averaged over the whole cavity. It is the intermediate case (figure 17.11(b)) that is most difficult because the electron spectrum in the cavity is a mixture of the wall and cavity electron spectra. This is usually just the situation when TLD are placed in radiation fields. Burlin [69] has formulated a theory to predict the ratio of absorbed dose in cavity to surrounding medium and table 17.3 presents values of these correction factors calculated for Harshaw type LiF TLD chips in different energy photon fields in different wall media.

The Burlin theory states that:

cavity correction factor = / = absorbed dose in cavity medium absorbed dose in wall medium

= dmS^+(l-d) (P-en/p), cavity

(Pen/P> Wall

Application of TLD in reactor engineering 339

Table 17.3. Ratio of absorbed dose in TLD chip to absorbed dose in wall medium for different photon energies.

Initial photon energy (MeV)

Wall media

Al Air Cu Pb

0.1

0.586 0.973 0.082 0.019

0.5

0.936 0.935 0.951 0.387

1.0

0.988 0.939 1.049 0.969

where m5wa!ity is the ratio of mass stopping powers for cavity and wall, pen/p is the mass energy absorption coefficient,

_ l _ e x p ( - l s ) Bx

B is the effective mass attenuation coefficient of the electrons, and x is the mean electron path in the cavity.

Table 17.3 reveals the importance of the correction factor whenever measurements are made using TLD in media of different radiation attenuation properties. The effects are most noticeable when cavity and wall medium differ in effective Z by more than a factor 2.

Apparent changes in response of TLD can be produced by backscattering from objects to which the dosemeter is attached. Johns [70] has recently made the point that the reported differences in 'response' of a LiF TLD on a phantom compared with that in free air can be shown to correspond roughly to the changed intensity which one would expect due to backscattering. Thus, if response is defined properly, there is little difference between the response on the phantom and in free air.

The energy-dependent response of lithium fluoride was studied by Van Prooyen and Johnson [71] using Burlin cavity theory. The ratio of cavity dose to medium dose was evaluated experimentally with the dosemeter embedded in various thicknesses of the media of interest. Results were given for lucite and copper and were shown to fit Burlin's theory.

The advantage of thin TLD is illustrated in table 17.4 which gives Lowe's [72] calcula­tions of the cavity correction factor for a polyether sulphone (PES) LiF dosemeter with thickness 5 mg cm-2 exposed in aluminium to a wide range of gamma energies.

Table 17.5 presents experimental results of TLD chip response in different media when exposed to the same source at the same distance for the same length of time — the results have been normalised to the response in lead to facilitate comparison.

Using the Burlin cavity theory, these measured doses can be converted to the dose in air at the same point — these values are of course equal for each photon source assuming negligible attenuation of photons by the wall medium.

If we assume that the 1 MeV cavity correction factor is valid for 60Co gamma photons (~ 1.25 MeV), we can interpret the experimental data in table 17.5 using the data from

340 JRALakey

Table 17.4.

Cavity correction factor Initial electron Gamma source (3 mg cm"2 PES LiF in AI) energy (MeV)

«°Co 1.13,1.33 MeV 1.070 1.038 , J 7 Cs

0.66 MeV 1.077 0.477

X-ray 100 keV 1.082 0.100

Table 17.5. Absorbed dose in TLD chip in different surrounding media for three photon energies.

131Cs 6°Co "N

table 17.3:

For lead

dose in air

Wall medium

Al Fe

0.662 0.672 0.761 0.762 0.592 0.680

_-°chip (^en/P)chip

/ G-Ien/P)pb

1.00 0.027 = x

Pb

1.000 1.000 1.000

0.969 0,038

= 0.73

which is very close to measurements in aluminium which gives the correct dose.

For aluminium

0.761 0.027 dose in air = x = 0.77.

0.988 0.027

Appendix 17.3. The intrinsic TL response per absorbed neutron

Horowitz et al [53] used the Kandi-II diffractometer to derive the TL response in TLD 600 taking great care to correct for neutron self-shielding and derived a response which showed that the neutron-induced kerma is 0.135 ± 0.01 times as effective as the ^Co-induced kerma in producing TL in TLD 600. Table 17.6 gives the absorption parameters

Application of TLD in reactor engineering 341

Table 17.6. Absorption parameters of TLD 100, TLD 600 and TLD 700 the thermal neutrons (nth).

njh cross section TLD 100, TLD 600, TLD 700, Reaction (10""cm2) E(cnT') S(cnT') E(cmM)

6 Li (n , a ) r 6Li(n,7) 7Li(n, 7 ) " F ( n , 7 )

942.4f 40 X 1CT3

36 X 1 0 ' 3

9X 10"3

4.33 1.8 X lO"" 2.1 X10"3

5.5 X10-*

57.14 2.43 X 1(T3

1.01 X10" 4

5.72 X lO"4

4.03 X 10"3

1.71 X10"7

2.2 X 10"3

5.5 X10" 4

f Vlasov et al [73] quote a confidence level of 0.5%. Below 10 keV, follows the 1/v law to within 1%.

of TLD materials — TLD 100 and TLD 600 are dominated by the large cross section and large positive Q value of 6Ii(n, a)r reaction which yields a 2.06 MeV a particle and 2.72 MeV triton for every absorbed neutron. In TLD 700 the 7Li(n, 7)8Li and 19F(n,7)20F reactions are of comparable cross section. Becker [74] has tabulated the expected values of the impurity content of Harshaw LiF as follows:

Al(20ppm), Ca(6ppm), Mg (300 ppm), Si(40ppm), Ti(5ppm); Dy(1.5xl0"3ppm), Eu (7x 10_sppm), Mn (1.5 xl0_2ppm).

These imply a negligible contribution to thermal-neutron absorption of TLD 700 but the response to thermal neutron kerma is still very complex due to the two reactions. In TLD 600 and TLD 100 the only thermal-neutron reaction of importance is 6Li(n, a)r.

In view of the large discrepancies reported in the literature, the authors carried out neutron sensitivity measurements in an experimental configuration where flux depression effects are negligible and self-shielding can easily be determined. A 6LiF shield 1 cm thick was used to screen out thermal neutrons and the attenuation of ^Co photons used for the gamma calibration was 2.5%. The results of TLD 600 and TLD 700 irradiation are given in Table 17.7. Doses did not exceed 100 R, thus avoiding a supralinear response and no significant fading was observed.

Table 17.7. Thermal neutron sensitivity of TLD 600 and TLD 700f.

TLD 600 sensitivity in R (6°Co).

En = 13.8 MeV

£•0 = 81.0 MeV

S"

370 ± 45

327 ± 20

S' S n

4020 ±480 2630 ±315 0.14, ±0.018

1470 ± 9 0 2330 ±145 0.132± 0.008

h

0.135± 0.011

TLD 700 sensitivity in R (60Co).

£•„ = 81.0 MeV

S" = S'

0.12 ± 0.007

S

0.19 ±0.011

h

0.149

f S" is the experimentally measured sensitivity, S' is the sensitivity corrected for neutron self-shielding, and S is the response translated to a Maxwellian distribution of thermal neutrons at T= 293.6 K.

342 JRALakey

The authors obtained a value of «(TLD600) = 0.135± 0.011 and stress that the response of TLD 600 is very strongly dependent on geometry. «(TLD 700) = 0.149, but this neglects the kerma produced by decay of 6Li and 20F which contributes 50% of the TL sensitivity (see table 17.8). Attix calculated «(TLD 100) = 0.37 using a track interaction model but this should be regarded as speculative. The authors conclude that the TL sensitivity of high LET kerma is lower than currently predicted. The Horowitz method [75, 76] applied by Furuta and Tanaka [77] gave excellent agreement with the value of « = 0.91xl07n/cm2R.

Table 17.8. Induced kerma in TLD 700.

Reaction

6 Li(n,a)r

"Li - ^ "Be "Be—>-2a 3°F -^* J°Ne

Charged particle species

a T

e" a

e"

Kerma (MeV/absorbed

2.02 2.76

~0.6 ~ 3

- 0 . 7

nth) (Kerma) £ (MeV cm"')

8.14 X 10"3

11.12 X 10"3

- 1 . 3 X 10"3

- 7 X 10"3

- 0 . 4 X 10 3

A17.3.1. The calculation of intrinsic TL response per absorbed neutron: Correction for self-shielding

Horowitz et al [53] calculate the self-shielding factor provided that sufficient details of the experimental configuration have been reported (e.g. isotropic fluence, cylindrical samples with given radius and height). For powdered samples an average density of 1.25 g cm-3 is assumed. The absorption probability x has been calculated via the Monte Carlo technique and is sampled over a Maxwellian spectrum at T= 293.6 K. They have presented these results in a separate publication for a large variety of cylindrical geometries. The self-shielding factor can be deduced from the relation

X<M/4 = 20F or 0 /0=^x /42F (A17.1)

where A is the surface area of the TL sample exposed to the thermal neutrons, V is its volume, 0 is the unperturbed impinging neutron fluence (neutrons/cm2), 0 is the average neutron fluence within the TL sample and 0/0 is the self-shielding factor.f Equation (A17.1) is to be applied only for the case of an isotropic fluence. The ratio n: can thus be calculated from the experimentally measured sensitivity 5,- to thermal neutrons in the following manner:

n{ = kerma in TL sample due to 5,- R(60Co)/o: and triton kerma in TL sample due to an impinging neutron fluence of 1010n/cm2

= Sip KcyS0>(4.78)(l .6 x 10-6)

= ^,-5I-CI/a7v'ocj,-0(0/0)(4.78)(1.6 x 10 -6). (A17.2) f The macroscopic cross section E is calculated from the formula o(2200)N0pu>j/A. For a Maxwellian energy distribution the cross section at 293.6 K is 12.9% smaller. The density p equals 2.64 g cm"3. When powder is used the density is 1.25 g cm"3 and the cross sections in Table 17.6 should be reduced accordingly.

Application of TLD in reactor engineering 343

Table 17.9. Data for LiF phosphorsf.

A C

t Attix [78].

TLD 100

25.94 80.7

0.074

TLD 600

25.06 83.5

0.956

TLD 700

26.02 80.5

0.000 07

It is important to note that the geometry of the flux-dosemeter sample enters only through the self-shielding factor. In equation (A17.2), i = 1, 6, 7 representing TLD 100, 600 and 700, respectively, 5,- is the experimentally measured TL sensitivity in (60Co) roentgens required to produce the same integral TL signal as an impinging neutron fluence of 1010n/cm2, Ai is the atomic number, Q is the conversion factor from 60Co roentgens to ergs/g, 4.78(1.6 x 10"6) is the energy in ergs liberated per absorbed thermal neutron, a is the 6Ii(n, a)r microscopic cross section at En = 25.3 meV expressed in cm2, N0 is Avogadro's number, and CJ,- is the 6Li isotopic content. Table 17.9 shows the accepted numerical values for Ah Ct and co,-. Equation (A17.2) for TLD 700 (neglecting, for the moment, the kerma contributions from the 7Li(n, y) and 19F(n, 7) reactions) reduces to

6.9xlO9(S/0) n7= = (A17.3)

(0/0) In the case of TLD 700, (0/0) is almost exactly one. It is thus immediately obvious that the values of S ranging from 0.7 to 2.5 result in values of nn ranging from 0.48 to 1.7.

Equation (Al 7.2) for TLD 100 and TLD 600 reduces to

5.04 xlO5 (5/0) 6.43xlO6(S/0) n6 = = and n, = . (A17.4)

(0/0) (0/0) A table of Monte Carlo calculated self-shielding factors for most standard sized LiF TLD is given by Horowitz et a! [79].

References

1 Gay ton F M, Harvey J R and Jackson J H 1972 Thermoluminescence and its application in reactor environments/ Br. Nucl. Eng. Soc. 11 (2) 125

2 Beyer R F 1969 The measurement of gamma levels with LiT TL material in a mixed 7-n field Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, Tenn., September 1968. USAEC Rep. CONF. 680920

3 Leonard B E and Bransford C L 1970 TLD 700 response to reactor neutrons Trans. Am. Nucl. Soc. 13 (2) 885

4 Simons G G, Stenstrom D G, Meneghetti D and Keeny W P 1970 Gamma ray dose evaluations for the ZPR3/EBR-11 critical assemblies Trans. Am. Nucl. Soc. 13 (2) 880

5 Reilly H J, Robinson R A and Peters L E Jr 1971 TLD measurements of gamma heating of heavy elements Trans. Am. Nucl. Soc. 14 (2) 910

6 Tappendorf T C 1972 Reactor gamma heat measurements with TL phosphors Aerojet Nuclear Company, Idaho, Rep. ANCR-1057, UC-80

7 Simons G G and Yule T J 1974 Gamma ray heating measurement in zero-power fast reactor with thermoluminescent dosimeters Nucl. Set Eng. 53 162

344 JRALakey

8 Muii D W and La Bauve R J 1974 Analysis of ZPPR/FTR-2 TLD measurements using ENDF/B-111 data Trans. Am. Nucl. Soc. 18 June 395

9 Knipe A D 1975 TLD gamma ray energy deposition measurements in the zero energy fast reactor Zebra Proc. 1st ASTM-Euratom Symp. on Reactor Dosimetry, Pet ten, 22-26 September 1975, Paper UKI

10 Tanaka S, Takeuchi K and Furuta Y 1975 Application of thermoluminescence dosimeters for nuclear heating measurements of gamma rays and neutrons Proc. 1st ASTM-Euratom Symp. on Reactor Dosimetry, Petten, 22-26 September 1975, p 699

11 Maerker R E, Muckenthaler F J and Clifford C E 1977 Radiation heating studies in a stainless steel and sodium shield Trans. Am. Nucl. Soc. 27 772

12 Gomaa M A, Sayed A M, Eid A L and El-Kolaly A M 1975 Reactor dosimetry using TLD and Rh-activation techniques Proc. 1st ASTM-Euratom Symp. on Reactor Dosimetry, Petten, 22-26 September 1975, p 623

13 Boulette E T, Marr D R and Bunch W L 1979 Analysis of TLD measurements in the FTR/EMC Trans Am. Nucl. Soc. 16 338

14 Lowe D A 1979 private communications 15 De Franceschi L, Gentili A and Sabbatini V 1973 Thermoluminescence in concrete measure­

ments for determining gamma ray transmission curves and buildup factors Nucl. Instrum. Meth. 109 25-27

16 Burke G de P and Becks H L 1974 Calculated and measured dose buildup factors for gamma rays penetrating multilayered slabs Nucl. Sci. Eng. 53 (1) 109-12

17 Taylor J J 1954 USAECRep. WARD-RM217, Westinghouse Electron Corp., January 18 Kalos M H 1959 Unpublished in U Fano, L U Spencer and M J Berger (eds) Encyclopedia of

Physics, vol. XXXVIII (2) (Berlin: Springer Verlag) 19 Bishop G B, Smitton C and Packwood A1972 Benchmark data for high energy gamma penetrations

Proc. 4th Int. Conf. on Reactor Shielding, Paris, 1972 paper E8 20 Avery A F, Carter M D, Chestnutt M M and Miller P C 1972 Dose rates in the vicinity of the

Winfrith SGHWR Proc. 4th Int. Conf. on Reactor Shielding, Paris, 1972 21 Haack K and Majborn B 1973 Reactor gamma heat measurements with calorimeters and thermo­

luminescence dosimeters Nucl. Instrum. Meth. I l l 283-286 22 Engel W W Jr 1967 A users manual for ANISN: a one dimensional discrete ordinates transport code

with anisotropic scattering Rep. K-1693, Union Carbide Corp. 23 Wingate C L, Tochilin E and Goldstein W 1965 Response of lithium fluoride to neutrons and

charged particles Proc. Conf. on Luminescence Dosimetry, Stanford, 1965. USAEC CONF 650637 24 Furuta Y and Tanaka S 1974 The relation between light conversion efficiency and stopping power

of charged particles in thermoluminescence dosimeter Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow, vol 1 (Krakow: Institute of Nuclear Physics) p 97

25 Knipe A D 1974 A gamma ray energy deposition benchmark experiment Rep. EACRP-L-101, p 11 26 Knipe A D 1976 Gamma ray energy deposition in fast nuclear reactors Thesis, London University,

1976 27 Bendall D E 1966 McNID - a Monte Carlo program for calculating the penetration of neutrons in

systems with cylindrical symmetry Rep. AEEW-R308 28 Hubbel J H 1969 Photon cross sections, attenuation coefficients and energy absorption coefficients

from 10 keV to 100 GeV Rep. NSRDS-NBS 29 29 Cameron J R, Zimmerman D, Kenney G, Buck R, Bland R and Grant R 1974 Thermoluminescent

radiation dosimetry utilizing LiF Health Phys. 10 25 30 Wingate C L, Tochilin E and Goldstein N 1967 Response of LiF to neutrons and charged particles

Proc. Int. Conf. on Luminescence Dosimetry, Stanford 1965. USAEC CONF 650637, p 421 31 Simpson R E 1967 Response of LiF to reactor neutrons Proc. Int. Conf. on Luminescence Dosi­

metry, Stanford, 1965. USAEC CONF 650637, p 444 32 Distenfeld C, Bishop W and Colvett D 1967 Thermoluminescent neutron dosimetry system Proc.

Int. Conf. on Luminescence Dosimetry, Stanford, 1965. USAEC CONF 650637, p 457 33 Woodley R G and Johnson N M 1967 Thermoluminescence induced by low-energy particles

Proc. Int. Conf. on Luminescence Dosimetry, Stanford, 1965. USAEC CONF 650637, p 502 34 Ayyangar, K, Lakshmanan A R, Chandra B and Ramadas K 1974 A comparison of thermal neutron

and gamma ray sensitivities of common TLD materials Phys. Med. Biol. 19 665

Application of TLD in reactor engineering 3 4 5

35 Ayyangai K, Reddy A R and Biownell G L 1969 Some studies on TL from LiF and other materials exposed to neutrons and other radiations Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, 1968. USAEC CONF 680920

36 Wallace R H and Ziemer P L 1969 Studies on the TL of manganese activated lithium borate Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, 1968. USAEC CONF 680920

37 Reddy A R, Ayyangar K and Brownell G L 1969 TL response of LiF to reactor neutrons Radiat. Res. 40 552

38 Tochilin E, Goldstein N and Lyman J T 1969 The quality and LET dependence of three TL dosimeters and their potential use as secondary standards Proc. 2nd Int. Conf. on Luminescence Dosimetry, Gatlinburg, 1968. USAEC CONF 680920, p 424

39 Tochilin E, Goldstein N and Miller W G 1969 Beryllium oxide as a TLD Health Phys. 16 1 40 Scarpa G 1970 A study on the dosimetric properties of BeO Health Phys. 18 91 41 Jahnert B 1972 The response of TLD-7000 TLDs to protons and a-particles Health Phys. 23 112 42 Jahnert B 1971 TL research of protons and a-particles with LiF (TLD 700) Proc. 3rd Int. Conf. on

Luminescence Dosimetry, Danish AEC, Riso. Riso Rep. 249, p 1031 43 Dua S K, Boulenger R, Ghoos L and Martens E 1971 Mixed neutron gamma dosimetry Proc. 3rd

Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso. Riso Rep. 249, p 1074 44 Majborn B, Jensen L B and Christensen P 1972 Proc. Symp. on Dosimetry in Agriculture, Industry,

Biology and Medicine, Vienna. IAEA SM-160/25, p 169 45 Haack K and Majborn D 1973 Reactor gamma heat measurements with calorimeters and TLDs

Nucl. Instrum. Meth. I l l 283 46 Ayyangar K, Lakshmanan A R, Chandra B and Ramadas K 1974 A comparison of thermal neutron

and gamma ray sensitivities on common TLD materials Phys. Med. Biol. 19 (5) 665 47 McKlveen J W and Schwenk M 1976 Reactor flux measurements using TLD Nucl. Technol. 31 257 48 Tanaka S and Furuta Y 1976 Usage of a TLD as a thermal neutron detector with high sensitivity

Nucl. Instrum. Meth. 133 495 49 Rossiter M J, Lewis V E and Wood J W 1977 The response of TLDs to fast and thermal neutrons

Phys. Med. Biol. 22 4 50 Horowitz Y S, Benshahar B, Mordechai S and Dubi A 1977 TL in LiF induced by mono-

energetic, parallel beam, 13.8 and 81.0 MeV diffracted neutrons Radiat. Res. 69 402 51 Schuhmacher Hand Krauss 0 1 9 7 8 T L D o f a mixed neutron and gamma ray field Proc. 3rd Symp.

on Neutron Dosimetry in Biology and Medicine, eds G Burger and H G Ebert. Rep. EUR 5848, DE/EN/FR,p713

52 Barlett D T and Edwards A A 1979 The light conversion efficiency of TLD 700 for a-particles relative to 6°Co gamma radiation. Private communication

53 Horowitz Y S, Benshahar B, Mordechai S, Dubi A and Pinto H 1977 Thermoluminescence in LiF induced by monoenergectic parallel beam, 13.8 and 81.0 MeV diffracted neutrons Radiat. Res. 69 402

54 Singh D, Burgkhart B and Piesch E 1977 A passive neutron spectrometer and dosimeter using LiF: Mg/Tl thermoluminescent detectors Nucl. Instrum. Meth. 142 409-415

55 Leonard B E and Bransford C L 1970 TLD 700 CLiF) response to reactor neutrons Trans. Am Nucl. Soc. 13 (2) 885

56 Oltman B G, Kartner J, Tedeschi P and Beggs J N 1967 The effect of fast neutron exposure on the 7LiF thermoluminescent response to gamma rays Health Phys. 13 918

57 Mayhugh M R, Watanabe S and Muccillo R 1971 Thermal neutron dosimetry by phosphor activa­tion Proc. 3rd Int. Conf. on Luminescence Dosimetry, Danish AEC, Riso. Riso Rep. 249

58 Wang T K, Weng P S and Hsu P G 1978 Measurement of reactor thermal neutrons with dysprosium activated calcium sulphate TLD J. Nucl. ScL Technol. 15 (1) 72-75

59 Furuta Y and Tanaka S 1972 Response of 6LiF and 7LiF thermoluminescence dosimeters to fast neutrons Nucl. Instrum. Meth. 104 365-374

60 Burke G de P and Marcus D G 1973 Interpretability of TLD measurements in the environs of a nuclear reactor Trans. Am. Nucl. Soc. 17 537

61 Brinck W, Gross K, Gels G and Partridge J 1977 Special external radiation field study at the Vermont Yankee Nuclear Power Station Health Phys. 32 221-230

62 Powers R P, Watson J E, Phelps S R and Fong S W 1978 Survey of external environmental radiation levels in N. Carolina Health Phys. 34 722-727

346 JRALakey

63 Weng P S, Chu T C, Hsu C N and Su S J 1977 Detection of hot fall-out in Taiwan in the period 1971-1975 Health Phys. 33 241-246

64 Spiers E W and Lakey J R A 1972 In situ analysis of shielded gamma active material Proc. 4th Int. Conf. on Reactor Shielding, Paris, October 1972, paper C2-1

65 Garrett P M 1978 Current trends in occupational radiation exposure at U.S. commercial power plants Nucl. Eng. Int. 23 (April) 51

66 Lakey J R A An improved method of, and apparatus for, examining radioactive samples British Patent 42783/59

67 Malone J F and Cullen M J 1975 The influence of thyroid geometry on the response of LiF and CaS04 TLDs to I131 and I , 2S irradiation Br. J. Radiol. 48 762

68 Gray L H 1929 Proc. R. Soc, A122 647 69 Burlin T E 1966 Br. J. Radiol. 39 727 70 Johns T F 1976 Health Phys. 31 185 71 Van Prooyen and Johnson W R 1974 Energy dependent response of lithium fluoride TLDs Trans.

Am. Nucl. Soc. 19 476 72 Lowe D private communication 73 Vlasov M F, Dunford C L, Schmidt J J and Lemmel H D 1972 Status of neutron cross section data

for reactor radiation measurements. Part I: Reactions of high priority IAEA Document INDC (NDS) - 47/L

74 Becker K 1973 Solid state dosimetry (Cleveland, Ohio: CRC Press) 75 Horowitz Y S and Dubi A 1977 Comment on the use of thermoluminescence dosimeter as thermal

neutron detectors Nucl. Instrum. Meth. 146 455 76 Horowitz Y S, Dubi A and Benshahar B 1976 Self shielding factors for TLD-100 and TLD-600 in

an isotropic flux of thermal neutrons Phys. Med. Biol. 21 976 77 Furuta Y and Tanaka S 1976 Response of 6LiF and 7LiF thermoluminescence dosimeters to

thermal neutrons Nucl. Instrum. Meth. 133 495 78 Attix F H 1969 Isotope effect in LiF thermoluminescent dosimeters Phys. Med. Biol. 14 147-148 79 Horowitz Y S, Freeman S and Dubi A 1979 Monte Carlo calculated absorption probabilities for

cylindrical and rectangular TLD probes in isotropic thermal neutron fluences Nucl. Instrum. Meth. 160 313

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

18 Application ofTLDfor dating: a review

G A WAGNER

18.1. Introduction

In nature, many materials occur which are TL phosphors. These materials may be of geological, archaeological, biological and even extraterrestrial origin and comprise different objects such as rocks, ceramics, slags, bones and meteorites. They are exposed to natural radiation and, therefore, may acquire significant levels of natural TL. By measuring this, one can determine the radiation dose that these objects have received during the past. In other words, these materials can be used as natural TL dosemeters for their natural radiation environment.

Apart from direct dosimetry applications, such as recording the radiation dose in space by studying meteorites, the natural dose is especially useful for determining age. Since the natural dose steadily accumulates with time, the total dose is directly related to the radiation exposure, that is the 'age' of an object. Another, however less important, application of natural TL is its use for thermometry. In this approach, one determines from the degree of natural TL fading the thermal history of an object. A further TL application which is due to the TL characteristics rather than natural TL is the source identification of objects.

The purpose of this chapter is to summarise the present state of the methodical development and applications of TL dating. The other aspects which are connected with TL in natural objects are covered by the literature and references given in the bibliography.

18.2. Dating method

18.2.1. Basic principle

TL dating of geological and archaeological samples essentially consists of two basic steps, the determination of the natural dose Dn which a sample has absorbed during its past and the determination of the corresponding natural dose rate Dn. From these two values, the TL age t can be calculated according to

t=Dn/Dn.

For the determination of the natural dose from the naturally occurring TL, one needs to know the TL sensitivity which varies from sample to sample. The sensitivity has to be measured for each sample and is derived from the artificial TL which is induced by laboratory irradiation with known doses. This necessitates calibrated radiation sources. The natural dose rate mainly originates from the radioactive elements, uranium, thorium, potassium and rubidium. To a lesser degree, cosmic radiation is also a contributing factor.

348 GA Wagner

Various methods exist for the evaluation of the natural dose rate. They will be discussed in more detail later.

Assuming no partial fading of natural TL over the sample's history, the age determined in this way dates either the formation of the sample or the time of previous heating which was sufficiently high for complete erasure of any previous TL.

18.2.2. Complications

Unfortunately, in practice, TL dating is more complex than this simple principle might suggest. Several difficulties have to be taken into account. These difficulties can be divided into those which are caused by the TL phenomenon itself, those which are caused by the artificial irradiation and those which are introduced when evaluating the natural dose rate.

The major problems caused by the TL phenomenon are lower TL efficiency for a-radiation than for /3- or 7-radiation, non-linear growth of TL with increasing dose (supralinearity, saturation), change of TL sensitivity by heating (especially the pre-dose effect), spurious (i.e. non-radiation-induced) TL, and thermal and anomalous fading of TL.

The lower TL efficiency of a-radiation with respect to either 0- or 7-radiation is the consequence of the high ionisation density along the a-particle tracks with the associated saturation of the available electron traps. The ratio of a-radiation TL efficiency to that of 0- or 7-radiation is usually expressed as the fc-value (Tite [78], D W Zimmerman [27]), or the a-value (Aitken and Bowman [31]). The different TL efficiencies are manifested in a modification to the general age equation

kDa+Dp+Dy+Dc

where Dp is the artificial 0 dose ('equivalent dose') which produces a TL signal equivalent to the naturally occurring TL. Da, Dp, Dy and Dc are the fractional natural a-, /3-, 7- and cosmic-ray dose rates, respectively.

Supralinearity is the increase of TL sensitivity during the first few hundred rad of a radiation dose (Tite [78], Aitken [1]). In the case of supralinearity, a correction ('intercept correction', Fleming [12]) has to be applied for dating. At higher doses, around 10s rad, the TL growth curve becomes sublinear due to saturation of the electron traps. Saturation generally restricts TL dating to the last million years.

One often observes a change in TL sensitivity after a sample has been heated in order to drain its TL. This difference in sensitivity between first and second glow may be due in part to the changed transparency of the sample and in part to the pre-dose effect. The pre-dose effect is the increase of sensitivity from first to second glow, which depends on the magnitude of the radiation dose that the sample has absorbed before heating (J. Zimmerman [29]). Regardless of its causes, the consequence of the sensitivity change for dating is that natural TL and the equivalent dose cannot be measured on the same sample material. Instead, one needs in principle at least two aliquots. From the first aliquot one derives the natural TL. The second aliquot receives a known artificial dose in addition to the natural ones before one measures its TL. In this way, one constructs a TL

Application of TLD for dating: a review 349

growth on unheated sample material. In practice, more than two aliquots are needed in order to check the identity of the aliquots and the linearity of the TL growth curve.

Spurious TL may have several causes such as pressure, friction, light exposure and chemical reactions. Since a spurious TL component would increase the apparent age, one suppresses it by measuring the TL glow under an extremely pure nitrogen or argon atmosphere (Aitken et al [5]). On the other hand, fading of the natural TL would lower the apparent age. Thermal fading, as well as spurious TL, can be recognised by the plateau test (Aitken et al [5a], McKeever [17]). Another, not yet well understood, kind of fading is the so-called 'anomalous fading' which is commonly observed in feldspars and zircons (Wintle [24]). It may be detected by storing artificially irradiated sample aliquots for several weeks and comparing their TL signal with freshly irradiated aliquots (Wintle [26]).

In order to determine the ^-factor (or a-factor) and the equivalent dose, one needs well calibrated radioactive sources. Usually 241Am or 242Cm are used as a sources and ^Sr/^Y as 0 sources. The energy dosimetry of such sources is an extremely difficult task since it depends on many parameters such as type, size and activity of the source, the radiation geometry, the type and thickness of the target and others. Large systematic errors in dating may and do result from inadequately known sources. Lately, major efforts on an interlaboratory basis have been in the more accurate calibration of sources. Literature pertinent to this aspect is given in the references.

When evaluating the dose rate, major problems are caused by the heterogeneous, spatial distribution of radioactivity, by possible disequilibrium in the radioactive decay chains and by the absorption of radiation by the water content in the sample.

When the radioactivity is heterogeneously distributed on a scale which is comparable with or larger than the range of the radiogenic particles, detailed microdosimetry con­siderations have to be taken into account for the dose rate evaluation. This is best explained by using, as an example, pottery which contains quartz grains of lOOjum diameter in a fine-grained matrix of less than 10 /im grain size. All the radioactivity is contained in the matrix. The 0- and 7-particles from the matrix with their average ranges of a few millimetres and 30 cm, respectively, would penetrate the quartz grains and the matrix equally. However, the a-particles with their average range of 25 fim would only reach the outer shell of the quartz grain leaving an a-sheltered core. Therefore the a dose received by the quartz grains would be less than that received by the matrix. This illustrates that dose rates can be evaluated only for defined grain sizes. This necessitates grain size fractionations for the dating of samples with heterogeneously distributed radioactivity (Fleming [11]).

Radioactive disequilibrium within the decay chains is a serious setback for TL dating. For correct evaluation, it would be necessary to know at which member within the chain disequilibrium occurs (Meakins et al [51]). One of the members most susceptible to dis­equilibrium is 222Rn in the 238U chain. As a rare gas with a half-life of 3.8 days, radon-222 may easily diffuse from a sample (Desai and Aitken [49]). There are several approaches to discover radon loss (Aitken [43], Carriveau and Harbottle [47]). Other candidates for disequilibrium are 230Th and 226Ra (Pernicka [57]).

Finally, any water content in the sample and the surrounding soil may absorb radiation. In order to make appropriate corrections, the natural water content has to be determined (J. Zimmerman [27]). Apart from the present moisture content, its seasonal and long-term climatic changes must also be considered.

350 G A Wagner

18.2.3. Dating techniques

Because of the complications outlined in the previous section, several dating techniques have been developed.

18.2.3.1. Quartz inclusion technique. The quartz inclusion technique was developed for the large (compared to a range) quartz grains in pottery (Fleming [12]). The quartz grains free of radioactivity are separated from the crushed pottery and their a dose shell is etched away with hydrofluoric acid. The remaining cores of the quartz grains which are used for TL measurement have only received the /3 and y contribution of dose from the matrix. Since the |3 dose is also to some degree attenuated in coarse quartz grains, a correction must be applied (Mejdahl [53], Bell [45]). However, new investigations have somewhat modified this simple picture. The quartz grains themselves may be radioactive (Sutton and Zimmerman [59]) and their etching rate can vary considerably among the grains (Bell and Zimmerman [8]). The main advantages of this dating technique are the elimination of the difficult a dosimetry and the resistance of quartz against anomalous fading.

18.2.3.2. Fine-grain technique. On the other hand, the fine-grain technique uses only the matrix of the pottery below 10 jLim grain size (D W Zimmerman [27]). This fraction received the full a, 0 and 7 contributions of the natural dose rate. For artificial irradiation, one needs both a and /3 sources. Because the fine-grain matrix from archaeological ceramics commonly contains feldspar (Singhvi and Zimmerman [19]), the TL of fine-grain fractions is susceptible to anomalous fading.

Both the quartz inclusion and the fine-grain techniques are the standard TL dating techniques for archaeological ceramics.

18.2.3.3. Subtraction dating technique. Both techniques are combined to the subtraction dating technique (Fleming and Stoneham [15]). In this case, only the a dose of the fine-grain matrix is used by subtraction of the 0 and 7 dose recorded in the coarse quartz grains from the total dose recorded by the matrix. The advantage of this technique is the elimination of the environmental 7 dose. This technique, therefore, is suitable for the dating of objects which have been removed from their natural environment, such as museum objects. Its dating accuracy, however, is lower than for the two standard techniques.

18.2.3.4. Zircon inclusion technique. The zircon inclusion technique (Sutton and Zimmerman [20], D W Zimmerman [28]) uses the highly radioactive, coarse zircon grains. The lOOjum large grains are separated from the pottery and their TL is measured. It is possible to use single grains for dating. Because the zircon grains are large compared to the a range, but small compared to the j3 and 7 ranges, they absorb practically only their own a. dose. Therefore, zircon inclusion ages of ceramics are independent of the moisture content and the environmental radioactivity. Serious obstacles, however, are the zoned uranium inhomogeneity within most grains and the anomalous fading of TL in zircon. This technique is of current interest mainly for authenticity testing.

Application of TLD for dating: a review 351

18.2.3.5. Pre-dose dating technique. Glow curves of quartz show a large peak in the 110 °C region. The peak-height for a given test dose depends strongly on the pre-dose that the quartzes have received before heating to about 500 °C. This sensitisation behaviour is the basis for the pre-dose dating technique for quartz (Fleming [13,14a], Aitken [3]). The sensitisation from the natural pre-dose is compared to that induced by a known laboratory dose. This technique is especially suited for young, quartz-bearing ceramics. Owing to saturation it is restricted to roughly the last thousand years.

18.2.3.6. Photo transferred TL technique. Recently, efforts have been made to use photo-transferred thermoluminescence (PTTL) for dating, PTTL describes the phenomenon of the re-excited TL by uv illumination after annealing. It is explained by the uv-induced trans­fer of electrons from deeper traps into shallower traps. Feasibility studies for PTTL dating of zircon, apatite (Bailiff [6]) and of quartz (Bowman [9], Sasidharan et al [18]) have been carried out. The PTTL technique looks promising, since many of the difficulties caused by the TL phenomenon can be circumvented.

18.2.4. Natural dose rate

The natural dose rate, which an archaeological or geological sample receives, originates in part internally, i.e. inside the sample, and in part externally, i.e. in its environment. Therefore, dose rate evaluation has to take into account both the sample and its surroundings. The degree to which the dose rate is internal and the degree to which it is external depends on the size of the sample in comparison to the range of the various radiations. For instance, a brick from inside a thick, homogeneous wall would receive virtually all its dose internally with the exception of a small external cosmic-ray contribu­tion. On the other hand, a brick fragment lying in soil would receive essentially only its a and (3 doses internally and its y dose externally. These examples illustrate that sampling for TL dating must be done thoughtfully with radiation dosimetry in mind.

There are several techniques by which the natural dose rate can be determined: the direct analysis of the radioactive elements, a counting (or uranium and thorium) and TLD techniques. Since all these techniques have advantages and disadvantages a combina-ticn of different techniques is best (Aitken [42]).

For direct analysis, the radioactive elements, uranium, thorium, potassium and rubidium, are determined in the sample and surroundings. For U, Th and Rb, neutron activation may be used, and for K and Rb, atomic absorption or flame photometry may be used. Fission track analysis of U and Th (Wagner [60]) has the advantage of revealing the distribution of these elements on the microscopic scale. The dose rates are calculated from the elemental contents (Bell [44, 46], Carriveau and Troka [48]). For the evaluation of dose rate from U and Th decay chains, one needs to know about the degree of radio­active equilibrium.

Direct a counting of the sample has the advantage of taking into account the dis­equilibrium. This technique is well suited for the determination of the a dose rate. The powdered sample is put on a ZnS screen and the scintillations are counted (Tite and Waine [21]). In order to evaluate the contribution from U and Th to the (3 and 7 dose rate, however, one needs to know the Th/U ratio and the degree of radioactive equilibrium.

TLD can also be used for measuring external dose rate (Aitken [41], Mejdahl [54]) as well as the internal dose rate of pottery (Mejdahl [53, 73]). CaF2 and CaS04 are used as

352 G A Wagner

phosphors. The external dose rate is recorded by putting capsules in the position from which the samples have been removed for dating. The capsules are left for one year in order to make allowance for seasonal variations of the dose rate. The use of TLD for dose rate evaluation has the advantage of eliminating the calibration of radioactive sources (Aitken [40]). TLD also takes into account radioactive disequilibrium and radiation attenuation by the moisture content. Some problems still exist with the energy depend­ence, the TL stability and the radiation geometry of the phosphors.

18.3. Dating applications

18.3.1. Archaeological

18.3.1.1. Ceramics. Already, 20 years ago, in the early stage of TL dating, ceramic materials had attracted the interest of those working with this method. Ceramics are in the category of the most common and most important archaeological objects. They are produced by firing clay. The temperatures of firing are sufficiently high to reset the TL clock of the clay. It has been shown that firing at 400 °C for 30 min eliminates the previous TL (Carriveau [64]). Therefore, the TL age determines a well defined event. Actually, the essential methodical progress in TL dating was achieved during the 1960s when studying ceramics. As a consequence, most dating techniques were developed first for ceramics and later applied also to other materials. The research papers listed in the references reflect this development.

There are numerous applications of TL dating to ceramic materials, mostly to pot­sherds, but also to bricks, fired clay and soil and to furnaces. The measured TL ages range from the earliest fired clay artifacts, such as the palaeolithic figurines from Dolni Vestonice (Zimmerman and Huxtable [82]) to recent samples. The appended literature gives a cross section through these widespread activities.

As an example, in figure 18.1, glow curves of the fine-grain and the coarse quartz fractions from a fired clay are shown. The fired clay was found on the inner wall of a mediaeval potter's furnace at Liibeck. TL dating established that this furnace was abandoned in 1240 AD + 60 years (1 a) (Wagner [79]).

A byproduct of ceramic dating is authenticity testing of ceramic works of art (Fleming [66]). Although in this application there is much less necessity for age accuracy, one has to restrict the amount of material because of great value of many art pieces. TL authen­ticity testing can be carried out on less than 50 mg drilling powder. Also, bronze objects which still contain sand or clay in their core are suitable for TL authenticity testing as was demonstrated on the famous bronze horse from the Metropolitan Museum, New York (Zimmerman et al [83]). The techniques of zircon grain, quartz inclusion and fine-grain dating proved the authenticity of this much debated art object.

18.3.1.2. Heated rocks. In archaeological excavations, rocks are occasionally found which appear to have been heated by prehistoric man. During the heating process, temperatures may have been reached which were sufficiently high to erase the previous geological TL in these rocks. The TL age of such rocks dates the heating. Because geological ages are generally some orders of magnitude larger than archaeological ages, one must ensure that no residual TL survived the heating. Such residual TL may be recognized from the shape of glow curves and through the plateau test.

Application of TLDfor dating: a review

FINE GRAIN

353

170 rod eau

~[ 1 -

200 300 400 TEMPERATURE C O

Figure 18.1. Glow curves for particles from a fired clay.

As an example, the quartz inclusion and fine-grain techniques have been applied to burnt sandstones which probably served as cooking stones. Reasonable TL ages of the first millenium BC have been found (Huxtable et al [88]).

Another heated stone material is flint. Its heating probably served the purpose of improving its properties for tool making. The flint samples must not be crushed due to triboluminescence. Instead TL is measured on polished slices of flint (Goksu and Fremlin [86]). The complicated dosimetry problems when irradiating thick slices of flint have been overcome by Aitken and Wintle [84]. TL dates up to 50 000 years have been reported by Goksu et al [87]. The work on the Terra Amata site has shown that flint dating might be applicable at least up to 250 000 years (Wintle and Aitken [93]).

18.3.1.3. Slags. Metallurgical slags are produced, analogous to ceramics, by an event which involves heat. It should therefore be possible to date ancient smelting activities with TL. Such data would be highly welcomed by metallurgists. Unfortunately, very little work has been done on slags. Carriveau [99], using slag material which was ground to a fine powder, reported few ages on silver slags and iron slags which seem to be in reasonable agreement with independently known ages. In the case of copper and bronze slags, which did not have nearly as many silicate inclusions, Carriveau found TL ages consistently much higher than they should be. However, in order to date ancient metal­lurgical activities, one may also apply TL dating to heated soil adjacent to slags, crucibles, tuyeres and smelting furnaces.

18.3.1.4. Bones and shells. Bones and shells have also been considered for TL dating. Pre­liminary experiments have revealed high levels of spurious TL, namely triboluminescence

354 G A Wagner

and chemiluminescence (Jasinska and Niedwiadomsky [98], Christodoulides and Fremlin [96]). In order to lower this spurious component, Driver [97] used thin slices instead of powdered material and extracted the organic matter. Before judging if these biological materials are suitable for TL dating, further studies on their TL characteristics are necessary.

18.3.2. Geological

18.3.2.1. Volcanic events. Volcanic rocks form at high temperatures and cool down rapidly to low temperatures. From this point of view, they seem to be well suited for TL dating. Volcanic rocks from the Quaternary Period are difficult to date by other existing methods. Therefore the potential of TL dating for young volcanic rocks has been realised for some time. However, the TL characteristics of volcanic rocks are not known well enough yet. Also difficulties with sample preparation and natural dose rate evaluation still exist. However, some direct applications of TL dating to volcanic rocks have been reported. May [113] dated plagioclases from Hawaiian alkalic basalts. The method seems to work for ages up to 250 000 years. Another example is the dating of tuff layers with human footprints in Anatolia, for which ages between 26 000 and 65 000 years were reported (Goksu [109]). In order to circumvent anomalous fading, commonly observed in plagioclase, Valladas [92] used high temperature TL between 500 and 700 °C.

Apart from direct applications to volcanic rocks, it is also possible to use adjacent rocks which were sufficiently heated. This approach was applied to the inversely magnetised lava flows of Laschamp and Olby in the Chaines de Puys (Gillot et al [108]).

Quartz separated from a granitic inclusion within the Laschamp flow gave 35 000 ± 3000 years and quartz pebbles from baked palaeo-soil undernearth the Olby flow gave 38 000 ±6000 years.

18.3.2.2. Calcareous deposits. In the early stages of TL dating, most activities were concentrated on limestone age determination. However, many difficulties had been encountered (see e.g. Zeller and Ronca [119]). Recently, interest has been revived in TL dating of calcareous deposits, essentially of stalagmites. Such age data would be of great interest to Quaternary geology and Palaeolithic prehistory.

In a systematic study Win tie [118] investigated the TL characteristics of stalagmites. In that study some problems such as spurious TL, changing natural dose rate due to initial disequilibrium in the uranium decay chain and others have been overcome. A major problem still seems to be the heterogeneous distribution of uranium.

18.3.2.3. Loess, till and ocean deposits. In the past few years, many TL ages in Quaternary sediments such as loess and glacial till have been reported by Soviet and Chinese workers (see e.g. Hiitt etal [120]). Obviously there must be some process connected with sedimentation which erases the previous TL, resetting the clock. The bleaching of TL is most probably caused by exposure to light. This was recently shown experimentally by Wintle and Huntley [122] who exposed ocean sediment to simulated sunlight. By using the fine-grain technique, these authors obtained, for an ocean sediment core, TL ages between 9000 and 140 000 years which are in agreement with independently known ages.

Application of TLD for dating: a review 355

18.4. Conclusion

Methodically, TL dating is by no means easy. Owing to its complex nature it is still a long way from becoming a routine dating method. Even when dating pottery, methodical difficulties still arise. Therefore, most laboratories engaged in TL dating combine its practical application with methodical studies.

Reliable dating techniques now mainly exist for archaeological ceramics. However, quite commonly even such material must be rejected due to unsatisfactory TL character­istics. For archaeological ceramics which have been thoughtfully collected (Aitken [61]) and which have satisfactory TL characteristics, age accuracies of ±5% to ±10% can be obtained.

At the moment, interest is increasing in the application of TL dating to Palaeolithic and Quaternary samples such as heated soils and stones, stalagmites, volcanic rocks and young sediments.

References and selected reading

Dating techniques

1 Aitken M J 1968 TL-dating in archaeology, in Thermoluminescence of Geological Materials ed D J McDougall (New York: Academic Press) p 369

2 Aitken M J 1976 TL age evaluation and assessment of error limits: revised system Archaeometry 18 233

3 Aitken M J 1978 Pre-dose dating: prediction from the model PA CT J. 2/3 319 4 Aitken M J and Fleming S J 1972 TL dosimetry in archaeological dating, in Topics in Radiation

Dosimetry Supplement I, ed F H Attix (New York: Academic Press) p 1 5 Aitken M J, Fleming S J, Reid J and Tite M S 1968 Elimination of spurious TL, in Thermo­

luminescence of Geological Materials ed D J McDougall (New York: Academic Press) p 133 5a Aitken M J, Tite M S and Reid J 1963 Thermoluminescence dating: progress report Archaeometry

6 65 6 Bailiff I K 1976 Use of phototransfer for the anomalous fading ofTL Nature 264 531 7 Bailiff I K, Bowman S G E, Mobbs F S and Aitken M J 1977 The phototransfer technique and its

use in TL-dating J. Electrostatics 3 269 8 Bell W T and Zimmerman D W 1978 The effect of HF acid etching on the morphology of quartz

inclusions for TL-dating Archaeometry 20 63 9 Bowman S G E 1978 Phototransferred TL in quartz and its potential use in dating PACT J. 2/3

381 10 Charalambous S and Michael C 1976 A new method of dating pottery by TL Nucl. Instrum. Meth.

137 565 11 Fleming S J 1966 Study of TL of crystalline extracts from pottery Archaeometry 9 170 12 Fleming S J 1970 TL-dating: refinement of the quartz inclusion method Archaeometry 12 133 13 Fleming S J 1973 The pre-dose technique: a new TL dating method Archaeometry 15 13 14 Fleming S J 1975 Supralinearity corrections in fine-grain TL-dating: a re-appraisal Archaeometry

17 122 14a Fleming S J 1978 The pre-dose method: basic elements PA CT J. 2/3 315 15 Fleming S J and Stoneham D 1973 The subtraction technique of TL-dating Archaeometry 15 229 16 Huxtable J 1978 Fine-grain dating, PACT J. 2/3 7 17 McKeever S W S 1979 A note on the plateau-test in TL-dating Ancient TL 6 13 18 Sasidharan R, Sunta C M and Nambi K S V 1978 Phototransfer method of determining archaeo­

logical dose of pottery sherds PACT J. 2/3 401 19 Singhvi A K and Zimmerman D W 1978 The luminescent minerals in fine-grain samples from

archaeological ceramics PA CTJ. 2[3 12

356 GA Wagner

20 Sutton S R and Zimmerman D W 1976 TL-dating using zircon grains from archaeological ceramics Archaeometry 18 125

21 Tite M S and Waine J 1962 TL-dating: a re-appraisal Archaeometry 5 53 22 Valladas G, Gillot P V and Guerin G 1978 Dating plagioklase? PACT J. 2/3 251 23 Valladas G and Valladas H 1978 High temperature TLArchaeo-Physica 10 506 24 Wintle A G 1973 Anomalous fading of TL in mineral samples Nature 245 143 25 Wintle A G 1975 Effects of sample preparation of the TL characteristics of calcite Mod. Geol. 5

165 26 Wintle A G 1978 Anomalous fading PACT J. 2/3 240 27 Zimmerman D W 1971 TL-dating using fine grains from pottery Archaeometry 13 29 28 Zimmerman D W 1978 TL-dating using zircon grains PACTJ. 2/3 458 29 Zimmerman J 1971 The radiation-induced increase of the 100"C thermoluminescence sensitivity

of fired quartz / . Phys. C: Solid St. Phys. 4 3265

Artificial irradiation, radiation sources

30 Aitken M J 1978 Interlaboratory calibration of alpha and beta sources PA CT J. 2/3 443 31 Aitken M J and Bowman S G E 1975 TL-dating: assessment of alpha particle contribution

Archaeometry 17 132 32 Aitken M J and Wintle A G 1977 TL-dating of calcite and burnt flint: the age relation for slices

Archaeometry 19 100 33 Murray A S and Wintle A G 1978 Beta source calibration PACT J. 2/3 419 34 Pernicka E and Wagner G A 1979 Primary and interlaboratory calibration of beta sources using

quartz as thermoluminescent phosphor Ancient TL 6 2 35 Stadler A and Wagner G A 1978 TL-dating of ceramics: laboratory simulation of the natural radia­

tion dose PACT J. 2/3 448 36 Varma M N and Carriveau G W 1978 A precise alpha source calibration for TL-dating PACT J. 2/3

414 37 Wintle A G and Aitken M J 1977 Absorbed dose from a beta source as shown by TL dosimetry Int.

J. Appl. Radiat. hot. 28 625 38 Wintle A G and Murray A S 1977 TL-dating: reassessment of the fine-grain dose rate Archaeometry

19 97 39 Zimmerman D W 1972 Relative TL effects of alpha- and beta-radiation Radiat. Effects 14 81

Radiation dose rate

40 Aitken M J 1968 Evaluation of effective radioactive content by TL dosimetry, in Thermo­luminescence of Geological Materials ed D J McDougall (New York: Academic Press) p 463

41 Aitken M J 1969 Thermoluminescent dosimetry of environmental radiation on archaeological sites Archaeometry 11 109

42 Aitken M J 1978 Dose-rate evaluation PACT J. 2/3 18 43 Aitken M J 1978 Radon loss evaluation by alpha counting PACT J. 2/3 104 44 Bell W T 1976 The assessment of the radiation dose-rate for thermoluminescence dating Archaeo­

metry 18 107 45 Bell WT 1979 Attenuation factors for the observed radiation dose in quartz inclusions for TL-

dating Ancient TL 8 2 46 Bell W T 1979 Thermoluminescence dating: radiation dose-rate data Archaeometry 21 243 47 Carriveau G W and Harbottle G 1978 Direct measurement of the fraction of radon loss in ceramics

by gamma-ray-spectroscopy Archaeo-Physika 10 423 48 Carriveau G W and Troka W 1978 Annual dose rate calculations for TL-dating Archaeo-Physika 10

406 49 Desai V S and Aitken M J 1974 Radon escape from pottery: effects of wetness Archaeometry 16

95 50 Malik S R, Durrani S A and Fremlin J H 1973 A comparative study of the spatial distribution of

uranium and of TL-producing minerals in archaeological materials Archaeometry 15 249

Application of TLD for dating: a review 357

51 Meakins R L, Dickson B L and Kelly J 1978 The effect on TL-dating of disequilibrium in the uranium decay chain PACT J. 2/3 97

52 Meakins R L, Dickson B L and Kelly J 1979 Gamma-ray analysis of K, U and Th for dose-rate estimation in TL-dating Archaeometry 21 79

53 Mejdahl V 1978 TL-dating: a TL technique for beta-ray dosimetry PACT J. 2/3 35 54 Mejdahl V 1978 Measurement of environmental radiation at archaeological sites by means of TL

dosimeters PA CTJ. 2/3 70 55 Mejdahl V 1979 TL-dating: beta-dose attenuation in quartz grains Archaeometry 21 61 56 Murray A S, Bowman S G E and Aitken M J 1978 Evaluation of the gamma dose-rate contribu­

tion PACT J. 2/3 84 57 Pernicka E 1979 Radiometric detection of deviations from secular equilibrium in the uranium

decay chain 19th Int. Symp. on Archaeometry, London 58 Sasidharan R, Sunta C M and Nambi K S V 1979 Error implications in case of underdetermined

U-Th concentration ratio in pottery samples Ancient TL 2 8 59 Sutton S R and Zimmerman D W 1978 TL-dating: radioactivity in quartz Archaeometry 20 67 60 Wagner G A 1976 Dose rate evaluation for TL-dating by fission track counting Proc. Symp. on

Archaeometry and Archaeological Prospection, Edinburgh

Archaeological applications

(a) Ceramics 61 Aitken M J 1977 TL-dating and the archaeologist Antiquity 51 11 62 Aitken M J, Zimmerman D W and Fleming S J 1968 Thermoluminescence dating of ancient pottery

Nature 219 442 63 Becker K and Moreno A 1974 Some applications of thermoluminescence measurements in ancient

ceramics Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow (Krakow: Institute of Nuclear Physics) p 1023

64 Carriveau G W 1974 Annealing threshold and TL-dating of ceramic material MASCA Newsletter 10 3

65 Fleming S J 1968 Thermoluminescence age studies on mineral inclusions separated from ancient pottery, in Thermoluminescence of Geological Materials ed D J McDougall (New York: Academic Press) p 431

66 Fleming S J 1970 Authenticity testing of art ceramics by the thermoluminescence-method - some important examples, in Application of Science in Examination of Works of Art ed W J Young (Boston: Museum of Fine Arts) p 206

67 Fleming S J 1972 Thermoluminescence authenticity testing of ancient ceramics using radiation-sensitivity changes in quartz Naturwissenschaften 59 145

68 Gorier J 1965 Die Thermolumineszenz und ihre Anwendung zur Alterbestimmung von Keramik-scherben Archaeo-Physika (Koln: Bohlau Verlag) p. 109

69 Grogler N, Houtermans F G and Stauffer H 1960 Uber die Datierung von Keramik und Ziegel durch Thermolumineszenz C. R. Reunion Soc. Suisse Phys. 33 595

70 Huxtable J, Aitken M J and Weber J C 1972 Thermoluminescence dating of baked clay balls of the poverty point culture Archaeometry 14 269

71 Ichikawa Y, Nagatomo T and Hagihara N 1978 Thermoluminescent dating of Jomon pattern pottery from Taishaku Valley Archaeometry 20 171

72 Mazess B and Zimmerman D W 1966 Pottery dating from thermoluminescence Science 152 347 73 Mejdahl V 1969 Thermoluminescence dating of ancient Danish ceramics Archaeometry 11 99 74 Mejdahl V 1972 Progress in TL-dating at Riso Archaeometry 14 245 75 Mejdahl V and Winther-Nielsen M 1978 Dating of Danish ceramics by means of the quartz inclusion

technique PA CTJ. 2/3 131 76 Pernicka E 1979 Beitrage zur Thermolumineszenz-Datierung urzeitlicher und fruhgeschichtlicher

Keramikfunde Anz. Osterr. Akad. Wiss. Math.-Naturwiss. Kl. 1979 1 77 Ralph E K and Han M C 1966 Dating of pottery by thermoluminescence Nature 210 245 78 Tite M S 1966 TL-dating of ancient ceramics: a reassessment Archaeometry 9 155

358 G A Wagner

79 Wagner G A 1979 TL-Datierung am Topferofen Koberg 15 in Liibeck Liibecker Schriften Archaol. Vorgeschichte 3 83

80 Wagner G A and Bischof H 1977 Echtheitstests mittels Thermolumineszenz an altchinesischen Keramikplastiken Archaol. Naturwiss. 1 20

81 Whittle E H and Arnand J M 1975 Thermoluminescent dating of neolithic and chalcolithic pottery from sites in Central Portugal Archaeometry 17 5

82 Zimmerman D W and Huxtable J 1971 Thermoluminescent dating of Upper Palaeolithic fired clay from Dolni Vestonice Archaeometry 13 53

83 Zimmerman D W, Yuhas M P and Meyers P (1974) TL authenticity measurements on core material from the bronze horse of the New York Metropolitan Museum of Art Archaeometry 16 19

(b) Heated rocks

84 Aitken M J and Wintle A G 1977 TL-dating of calcite and burnt flint: the age relation for slices Archaeometry 19 100

85 Bowman S G E and Seely M A 1978 The British Museum flint dating project PACT J. 2/3 151 86 Goksu H Y and Fremlin J H 1972 TL from unirradiated flints: regeneration JL Archaeometry 14

127 87 Goksu H Y, Fremlin J H, Irwin H T and Fryxell R 1974 Age determination of burnt flint by

thermoluminescent method Science 183 651 88 Huxtable J, Aitken M J, Hedges JW and Renfrew AC 1976 Dating a settlement pattern by

thermoluminescence: the burnt mounds of Orkney Archaeometry 18 5 89 Ichikawa Y and Nagatomo T 1978 Thermoluminescence dating of burnt sandstones from Senpkuji

CwePACTJ. 2/3 174 90 Melcher C J and Zimmerman D W 1977 Thermoluminescent determination of prehistoric heat

treatment of chert artifacts Science 197 1359 91 Rowlett R M, Mandeville M D and Zeller E J 1974 The interpretation and dating of humanly

worked silicous materials by thermoluminescent analysis Proc. Prehist. Soc. 40 37 92 Valladas G 1978 Thermoluminescence dating of burnt stones from a prehistoric site PACT J. 2/i

180 93 Wintle A G and Aitken M J 1976 Thermoluminescence dating of burnt flint: application to a Lower

Palaeolithic site, Terra Amata Archaeometry 19 111 94 Wintle A G and Oakley K P 1972 Thermoluminescent dating of fired rock-crystal from Bellan

Bandi Palassa, Ceylon Archaeometry 14 277 95 Wright D A 1979 A Swedish vitrified fort: dating by conventional TL Ancient TL 8 13

(c) Biological materials (bones, shells)

96 Christodoulides C and Fremlin J H 1971 Thermoluminescence of biological materials Nature 232 257

97 Driver H S T 1978 The preparation of thin slices of bone and shell for TL PACT J. 2/3 290 98 Jasinska M and Niewiadomski T 1970 Thermoluminescence of biological materials Nature 227

1159

(d) Metallurgical slags

99 Carriveau G W 1974 Dating of 'Phoenician' slag from Iberia using thermoluminescence tech­niques MASCA Newsletter 10

100 Carriveau G W 1974 Application of thermoluminescent dating to prehistoric metallurgy Preprint

(e) Identification

101 Afordakos G, Alexopoulos K and Miliotis D 1974 Using artificial thermoluminescence to reassemble statues from fragments Nature 250 47

Application of TLD for dating: a review 359

102 Carriveau GW and Nievens M 1978 Guatamalan Obsidian source characterization by thermo­luminescence/VlCr/ 2/3 506

103 Goksu H Y and Tiiretken N 1978 Source identification of Obsidian tools by thermoluminescence PACT J. 2/3 356

104 Huntley D J and Bailey D C 1978 Obsidian source identification by thermoluminescence Archaeometry 20 159

105 Leach B F and Fankhauser B 1978 The characterization of New Zealand Obsidian sources using thermoluminescence Preprint

Geological applications

(a) Volcanic events

106 Aitken M J, Fleming S J, Doell R R and Tanguy J C 1968 Thermoluminescent study of lavas from Mt Etna and other historic flows: preliminary results, in Thermoluminescence of Geological Materials ed D J McDougall (New York: Academic Press) p 359

107 Bechtel F, Schvoerer M, Rouanet J F and Gallois B 1978 Extension a la prehistoire, a l'ocean-ographie et a la volcanologie de la methode de datation par thermoluminescence PACT J. 2/3 481

108 Gillot P Y, Labeyrie J, Laj C, Valladas G, Guerin G, Poupeau G and Delibrias 1979 Age of the Laschamp Paleomagnetic excursion revisited Earth Planet. Sci. Lett. 42 444

109 Goksu H Y 1978 The TL age determination of fossil human footprints Archaeo-Physika 10 455 110 Huxtable J and Aitken M J 1977 Thermoluminescent dating of Lake Mungo geomagnetic polarity

excursion Nature 265 40 111 Huxtable J, Aitken M J and Bonhommet N 1978 Thermoluminescence dating of sediment baked

lava flows of the Chaine de Puys Nature 275 207 112 Hwang F S W 1970 Thermoluminescence dating applied to volcanic lava Nature 227 940 113 May R M 1977 Thermoluminescence dating of Hawaiian alkalic basal ts / Geophys. Res. 82 3023

(b) Calcareous materials

114 Bangert J and Hennig G J 1978 Effects of sample preparation and the influence of clay impurities on the TL-dating of calcitic cave deposits PACT J. 2/3 281

115 Gallois B, Nguyen P H, Bechtel F and Schvoerer M 1978 Datation par thermoluminescence de coraux fossiles des Caraibes PACT J. 2/3 493

116 Johnson N M and Blanchard R L 1967 Radiation dosimetry from the natural thermoluminescence of fossil shells Am. Mineral. 52 1297

117 Nambi K S V and Mitra S 1978 Thermoluminescence investigations of old carbonate sedimentary rocks Neues Jahrb. Mineral. Abh. 133 210

118 Wintle AG 1978 A thermoluminescence dating study of some quaternary calcite: potential and problems Can. J. Earth Sci. 15 1977

119 Zeller E J and Ronca L B 1963 Reversible and irreversible thermal effects on TL in limestone Earth Science and Meteorites compiled by J Geiss and E D Goldberg (Amsterdam: North-Holland) p 281

(c) Loess, glacial till, ocean sediments

120 Hiitt G, Smirnov A and Tale I 1978 The application of thermoluminescence of natural quartz to the study of geochronology of sedimentary deposits PACT J. 2/3 363

121 Wintle A G and Huntley D J 1978 TL dating of sediments PACT J. 2/3 374 122 Wintle A G and Huntley D J 1979 Thermoluminescence dating of deep-sea sediment core Nature

279 710

360 GA Wagner

Extraterrestrial applications

123 Brito U, Lalou C, Valladas G, Ceva T and Visocekas R 1973 Thermoluminescence of lunar fines (Apollo 12, 14 and 15) and lunar rock Geochim. Cosmochim. Acta 3 (Suppl. 4) 2453

124 Crozaz G, Walker R and Zimmerman D 1973 Fossil track and thermoluminescent studies of Luna 20 material Geochim. Cosmochim. Acta 37 825

125 Durrani S A 1971 Thermoluminescence in meteorites and tektites Mod. Geol. 2 247 126 Durrani S A 1977 Temperature and duration of the shadow of a recently-arrived lunar boulder

Nature 266 411 127 Houtermans F G and Liener A 1966 Thermoluminescence of meteorites J. Geophys. Res. 71

3387 128 Liener A and Geiss J 1968 Thermoluminescence measurements on chondritic meteorites, in

Thermoluminescence of Geological Materials ed D J McDougall (New York: Academic Press) p559

129 McKeever S W S and Sears D W 1978 Thermoluminescence and terrestrial age of the Estacado Meteorite Nature 275 629

130 McKeever S W S and Sears D W 1979 Meteorites and thermoluminescence Metecritics 14 29 131 Melcher C L and Sears D W 1979 Thermal stability of thermoluminescence in chondrites

Meteoritics 14 249 132 Sears D W 1978 The dating of meteorites PACT J. 2/3 231 133 Vaz J E and Sears D W 1977 Artificially-induced thermoluminescence gradients in stony

meteorites Meteoritics 12 47

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann ©1981 ECSC, EEC, EAEC. Brussels and Luxembourg

19 TL dating: techniques and problems

M J AITKEN

19.1. Introduction

The basic notion of TL dating is that the firing of pottery by ancient man resets the 'TL clock' to zero and that, during the subsequent centuries, the trapped electron popula­tion builds up at a uniform rate due to the weak flux of ionising radiation provided by radioactive impurities (a few parts per million of uranium, thorium and potassium-40) in the clay itself and in soil in which the pottery was buried after the archaeological site fell into disuse. Suitable apparatus for detecting the consequent 'natural' TL is shown in figure 19.1. Because the minerals in pottery have a rather low TL sensitivity, and because the accumulated exposure during antiquity is only of the order of a thousand roentgen, the natural TL is usually rather dim. Not only is a fast heating rate required, together with a high-sensitivity low-noise photomultiplier (e.g. EMI Type 9635) and a solid angle of light collection approaching 7r steradians, but also it is important to have severe discrimination against thermal radiation by means of carefully selected colour filters and to suppress any non-radiation-induced TL. This latter, usually referred to as 'spurious' TL, is a surface effect induced during sample preparation; for most samples it is adequately suppressed by making the measurement in an atmosphere of dry oxygen-free nitrogen or argon.

Sample Heater curreni of = 100A to give healing rale of 20 °C per second

Figure 19.1. Apparatus for TL dating measurement.

362 M J Aitken

The 'glow curve' for natural TL — a typical one is shown in the top right-hand corner of figure 19.1 - h a s no TL below about 250°C. This is because the traps which are emptied below that temperature are too shallow to retain their electrons without serious loss during the centuries of burial. For dating purposes, we are concerned with the TL emitted above 350 C; in practice, the useful range does not usually extend beyond 450 °C because above that temperature thermal radiation becomes excessive despite colour discrimination, and in any case the TL intensity usually falls off due to increased thermal quenching (de-excitation of the luminescent centre by emission of a phonon instead of a photon). For a single trapping level, the glow curve would be a single peak about 50 C wide; that of figure 19.2 is evidently composed of a number of overlapping peaks and it is to be inferred that there are several types of trap present, either within the same mineral or in several different minerals. The presence of several types of trap has the advantage that it permits the 'plateau test' (see figure 19.2) to be used to check whether the traps associated with a given temperature range are deep enough for stable retention of electrons during antiquity. Further reading about the technique will be found in the references and in particular chapter 3 of reference [1].

50

c 30-

10

500

-1000

- 500-

200 300 400 500 Temperature (°C)

Figure 19.2. The plateau test. The upper part shows the glow curves corresponding to the natural TL and to the artificial TL following 1000 rad of beta irradiation. The lower part shows the equivalent dose, which here is taken to be equal to

natural TL

artificial TL X 1000 rad

as function of temperature. The onset of the plateau is indicative that a sufficiently high glow curve temperature has been reached for the TL to be associated with traps that are deep enough to retain their electrons with negligible leakage during archaeological times. The onset of the plateau is usually around 350°C.

TL dating: techniques and problems 363

19.1.1. The age relation

In its simplest form, this is

natural TL age = . (19.1)

(TL per unit dose) x (dose per year) TL per unit dose is the sensitivity of the sample for acquiring TL; it is measured by exposing the sample to a known dose of nuclear radiation from an artificial radio-isotope source. The resulting glow curve is referred to as 'artificial TL'. Dose per year (D) is evaluated from radioactive and chemical analysis of the sample and its surrounding soil.

The natural dosage received by the pottery (see table 19.1) is a mixture of alpha, beta and gamma radiation plus a contribution of a few per cent from cosmic rays. On account of the high ionisation density, they produce alpha particles substantially less efficient than beta particles and gamma rays. Hence it is necessary to rewrite (19.1) in the form

C M t = : ^—. — (19.2)

Xafla + XpCD/J + Ay+A:)

where t is the age in years, GN is the natural TL, Xa and Xp a r e T L Per rad for alpha and beta particles respectively, and Da, Dp, Dy and Dc are the respective annual dose rates for alpha, beta, gamma and cosmic radiation.

Experimentally, it is convenient to define

* = Xalxp and denoting the equivalent dose by

Dp = GN/X/3

Table 19.1. Component dose ratef for typical pottery and soil.

Effective a p 7 Total total*

191

190

177

558

f Dose rates are given in milliiads per year and correspond to pottery and soil having 2.8 ppm of uranium, 10 ppm of thorium and 2% potassium (measured as K,0). The corresponding alpha count rate would be 10 per kilosecond for a thick sample of area 13.8 cm2 and a counting efficiency of 85%; the uranium and thorium contributions to the count rate would be approximately equal. % Assuming £ = 0.15.

Thorium-232 series: Radio-isotopes before thoron Thoron and later products

Uranium-238 series: Radio-isotopes before radon Radon and later products

Potassium-40

Totals

Effective dose rates

X}™

Sj™ -

1517

227 (41%)

iEH SJ)»»

sH 4s" 136.4 41

206 124

206 124 (37%) ' (22%)

818

852

177

1848

-

364 M J Aitken

we have

t=— ^-E—. - . (19.3) kDa+DB+Dy+Dc

So far it has been assumed that the acquisition of TL is linear with dose and that, as is implicit in the caption to figure 19.2, there is no change of TL sensitivity when the sample is heated to 500 °C in the course of measuring the natural TL. For most samples these assumptions are upset by two effects, supralinearity and sensitisation, which will be discussed in §19.3.

19.1.2. Sample preparation

For accurate dating, the technique of sample preparation is intrinsic to the principles on which the age is calculated. This is because of the heterogeneous nature of pottery fabric — it is a baked clay matrix in which there are mineral inclusions, sometimes ranging up to a millimetre in diameter. These inclusions, usually of quartz or feldspar, were either present in the raw clay or were added by the potter in order to improve refractory qualities. The TL sensitivity of these inclusions is much higher than that of the clay matrix but in general (potassium feldspars and zircons are notable exceptions) the radioactive impurities are carried in the matrix. The average range of the alpha particles from uranium and thorium, which provide a substantial part of the natural radiation dosage, is only about 25 fun and as a consequence the average dose received by an inclusion decreases with size once its diameter exceeds a few microns. Since the grain-size distribution varies between fragments, for accurate results it is necessary to separate out grains of a given size range and to use these for measurement of both natural and artificial TL.

19.1.2.1. Fine-grain technique. There are two basic approaches. In the fine-grain technique developed by Zimmerman [2], the material extracted consists of grains that are small enough for the attenuation of the alpha dosage to be negligible. The pottery frag­ment is crushed by slowly squeezing in a vice; grains in the size range 1-8 /im are separated in acetone and utilising the fact that the settling time is determined by the diameter. After separation, the selected grains are resuspended in acetone and deposited on aluminium discs (usually 10 mm in diameter and 0.5 mm thick) in a thin layer of a few microns. About a dozen such discs are prepared from each sample and if all goes well the TL reproducibility is about ± 5%.

19.1.2.2. Quartz inclusion technique. In the quartz inclusion technique developed by Fleming [3], the grains selected are large enough for there to be severe attenuation of the alpha dosage, but not so large that there is serious attenuation of the beta dosage. By etching these grains in hydrofluoric acid, the outer skin that has received alpha dosage is removed and in evaluating the age is put equal to zero. The size range of the grains is usually 90-150 /nm, a small correction being made for the attenuation of the beta dosage. Weighed portions of these grains (typically 5 to 10 mg) form the samples on which the TL measurements are made.

TL dating: techniques and problems 365

As just indicated, the quartz technique is based on the assumption that the grains are devoid of radioactivity. However, recent measurements by Sutton and Zimmerman [4] indicate that in some grains there is a small but just significant content of uranium. Another recent quartz investigation, by Bell and Zimmerman [5] using an electron micro­scope, categorises grains into 'shiny' and 'frosty'. With the latter, the etching by hydrofluoric acid proceeds in an irregular way that does not correspond to the orderly removal of the outer skin assumed above. In passing, one may note two other recent publications connected with quartz. In dating the sandy clay of the burnt floor and walls of a kiln, Valladas [6] found it possible to eliminate intrusive unheated grains by virtue of their lower magnetic susceptibility — the heated grains were presumed to have acquired iron impurity from the clay matrix during firing. Courtois et al [7] in dating Amazonian pottery noted a parasitic TL component that they ascribed to the deliberate addition of siliceous wood by the potter.

19.2. Application

19.2.1. Archaeological dating

Had radiocarbon dating not been well established, thermoluminescence would have had the task of establishing the chronological framework of prehistoric archaeology. Leaving until later the question of TL potentiality prior to the 50 000 year limit of radiocarbon's range, let us consider what TL has to offer in a complementary role [5]. First and fore­most, it gives a direct date for an object of archaeological significance. The changing technique and style of pottery often form the basis of archaeological chronology, whereas the association of a radiocarbon sample with an archaeological phase may be open to question. Also, the event dated by TL is the actual firing of the pottery whereas in the case of radiocarbon dating of wood or charcoal the event may be several centuries earlier than the archaeological association. Secondly, there are some sites prolific in pottery on which suitable radiocarbon samples do not occur.

The accuracy obtainable with TL in good circumstances is at present somewhat better than ±10% of the age. It is unlikely that it will ever be improved beyond ±5% because the stored information in the pottery is not recorded to any better accuracy — for instance, owing to uncertainty about the average water content during burial (which affects the dose rate). This is somewhat poorer than is usually quoted for radiocarbon though for some periods it is no worse than indicated by a realistic assessment of accuracy having regard to the short-term fluctuations discussed in § 19.4.

The 'good circumstances' required for high TL accuracy refer on the one hand to pottery having satisfactory TL characteristics and on the other to whether or not the actual burial situation permits a reliable estimate of the gamma dose rate. The latter is determined by the soil or rock within 0.3 m of the sample and unless the surround is homogeneous it is difficult to make a reliable estimate. These considerations mean that samples should be collected specifically with TL dating in mind and preferably by the TL specialist — who in any case should visit the site either to bury TLD capsules or to make measurements with a scintillation spectrometer. Table 19.2 lists typical collection requirements. Pottery may display unsatisfactory TL characteristics in a variety of ways.

366 MJAitken

Table 19.2. Notes on collection of potsherds for TL dating.

The sherds 1 Number: For each level we need a set of between 6 and 12 sherds.

2 Size: At least 6 mm thick, at least 30 mm across. Bigger sherds are preferred.

3 Type: We like a variety of fabric types, if available. Surface decoration or glaze does not matter.

4 Context:

(i) Only sherds that have been buried to a depth of 30 cm (1 ft) or more for at least two-thirds of their burial time are acceptable. This means that pits and ditches that have been filled up fairly quickly (either by silting or by ancient man) are ideal sources.

(ii) The sherds should be at least 30 cm from any boundary (e.g. edge of pit, change of soil type, wall, floor rock surface),

(iii) The best situation is one of sparsely occurring sherds in a uniform soil which is relatively free of other materials (e.g. rock, building debris, shell or bone). A small scatter of stone does not matter as long as none of the sherds selected were close against a large one; the bigger the stone the more serious will be the effect.

Treatment 5 Avoid prolonged exposure to direct sunlight. (With flint samples try and avoid any

exposure to sunlight at all, and put them in an opaque bag.) 6 Avoid excessive heating of the sherds. Their temperature should not exceed the boiling

point of water (100°C, 212°F).

7 Avoid exposure to ultraviolet, infrared, x-rays, beta-rays or gamma-rays.

8 (a) The sherds should not be washed but put directly in a plastic bag plus any lumps of earth attached, within a few minutes of removal from the soil and tied up tightly. This bag should be put inside a second outer plastic bag, which should also be tied tightly. This will allow us to measure the water content of the sherds as found in the ground. (The procedure is not necessary for flint samples.)

(b) If washing is necessary to confirm the identity of the sherds, then this consideration should take priority. However, if the sherds have to be washed, then detergents or other additives must not be used for washing. Do not bother to dry the sherds. Please tell use whether or not washing has been done.

9 We require a small handful of soil that is typical of that in which the sherds are buried. The soil should be tightly double-bagged as for the sherds.

10 Exposure of the soil to sunlight, ultraviolet, etc, does NOT matter.

11 We require also a sample of each type of material occurring in large proportions within 30 cm of the sherd. In the case of a scatter of small stones in the soil, these should be included in the soil sample in correct proportion.

General 12 Please, above all, avoid sherds whose inclusion in a set is in any way doubtful. The

method gives the date at which the pot was made; consequently we do not want residual pottery from earlier periods. A sherd which has been burnt at some later period is of no use because the date obtained may be intermediate between the firing and the burning.

13 Information about burial conditions is essential; this should include a sketch section of the context (and, if possible, photographs) showing very roughly the points from which the sherds were taken and the deposits for at least 30 cm around.

TL dating: techniques and problems 367

Table 19.2-Continued

14 Please try and give a rough estimate of how the average water content of the soil relates to that of the sample supplied. It is also useful to know how the water content of the soil varies between contexts and with respect to surface conditions (e.g. 'though bone dry at the surface in these hot climatic conditions, by a depth of 2 m the soil was pretty well saturated'). Obviously it is also important to know if the water table is (or has been) anywhere near to the contexts concerned. If there is any seasonal or long-term information about variations in rainfall, we would like to know it.

15 The sherds are destroyed in the course of measurement.

Acceptance of samples (by Oxford Research Laboratory for Archaeology) 16 The present policy is to undertake a limited number of specific projects rather than to

accept samples on a piecemeal basis. Primary considerations are:

(i) whether the present accuracy (between ±5% and ± 10% of the age) is good enough for the problem concerned,

(ii) suitability of site and material, for the technique, (iii) archaeological importance, and (iv) lack of, or ambiguity in, other dating evidence.

17 Since some types of clay do not have suitable TL characteristics, it is a good idea to send a variety of samples for preliminary testing before going to the trouble of proper collection. These test samples do not need burial soil, nor do they need to fulfil the collection requirements; they are needed solely as examples of the types of fabric available on the site.

The TL may be too dim, the growth with dose may be unmanageably non-linear, it may exhibit anomalous fading (see §19.3) despite passing the plateau test, or it may be largely composed of spurious TL. For one reason or another, a substantial proportion of archaeological sites cannot be dated by TL.

What are possible materials for TL dating prior to the 50 000 year limit of radio­carbon? Baked clay in the form of pottery does not extend beyond about 10 000 years ago, though occasionally earlier in the form of figurines: in Czechoslovakia, for instance, there are baked clay fragments which have been TL dated by Zimmerman and Huxtable [8]. However, fire was used by ancient man as a means of cracking flint for use in tools and weapons, and some such flints are sufficiently heated for use in dating [9-11]. Heated stones associated with fireplaces are another possibility [12].

All discussion so far has been concerned with samples that have been heated. There is also the possibility that deposited carbonate (e.g. stalagmites and travertine associated with cave dwelling) can be dated by TL. The vital question is whether or not the crystals start off with a trapped electron population of zero on formation. Besides this, with semi-transparent materials such as flint and calcite there is the possibility that ambient light has induced a significant TL signal in the sample; exposure to light can be controlled during excavation and in the laboratory but not during antiquity. Promising TL measure­ments have been made on Foraminifera from ocean cores and extension to shells associated with human habitation, although beset with problems, would be a very worthwhile development.

368 MJAitken

19.2.2. Recent geology

The carbonate materials just mentioned are more directly relevant to Quaternary geology than to archaeology. In this context, the obvious material for TL dating would seem to be volcanic lava and, in particular, lava carrying reversed magnetisation such as the Laschamp flow in the French Chaine-des-Puys [13]. Unfortunately, it seems to be a fairly general rule that the feldspar minerals carried in volcanic lava suffer from anomalous fading. This phenomenon does not appear to afflict quartz (or calcite), but unfortunately the amount of quartz in lava flows of interest is rather meagre. However, when clay or rock having satisfactory TL characteristics has been heated by the lava flow, there is a possibility of obtaining a date indirectly.

19.2.3. Authenticity testing

It is here that TL has had as revolutionary an impact as that of radiocarbon in archaeology. For testing authenticity it is usually a question of deciding between an age of less than a century and one of upwards of a thousand. Uncertainty about gamma dose rate is then unimportant and it does not matter that the 'burial circumstances' are unknown. The result of a TL test may alter the value of an object from an astronomical figure to a negligible amount. Consequently, although the answer does not need to be accurate, it does need to be reliable and so tests need to be conducted with meticulous care. Reputable art dealers have doubtful pieces tested as a matter of routine, the usual fee for such a service being £50-£100. A sample of about 50 mg is sufficient in most cases and this is obtained by drilling a small hole in an unobtrusive location.

There are some styles of ceramic ware for which art historians have had doubts as a whole, and application of TL in some such cases has led to agonising reappraisals. It has had academic impact in the sense that previously the art historian may have been studying the forger's view of man's cultural and artistic development rather than actuality. An example [14] of this is the so-called Hacilar ware anthropomorphic vessels and figurines said to be 7000 years old and to have come from the renowned site of that name in south-west Turkey. Because their supposed origin was from a cultural phase following soon after the first appearance of pottery in that part of the world, the fineness of technique and beauty of form attracted particular interest. Of the seven most important pieces tested — the magnificent double-spouted vessels, each spout being in the form of a head with obsidian eyes — only one was found to be genuine. Out of a total of 66 pieces tested there were 48 modern forgeries. In another application — to Chinese Hui Hsien ware — there were no genuine examples among the pieces available for testing.

Can a modern forgery be irradiated so that it will exhibit TL appropriate to a genuine object? An inexperienced operator could well be misled by even a clumsy attempt. To do the job properly, the forger would need to employ a physicist having some training in TL and with appropriate experimental facilities available. Even then it is difficult to see how artificial irradiation could fool a technique such as zircon dating (see §19.3). On account of their high uranium content, zircons carry a very much higher equivalent dose than other constituent minerals in pottery and it is not possible to achieve this situation by external irradiation. Another possibility is by reconstitution, i.e. the fake object is made from ground-up ancient brick by means of some chemical cementing agent. If no heat is used in the process, then the TL age will be that corresponding to the ancient

TL dating: techniques and problems 369

brick. Of course, there is much more to it than this; it is likely that the chemical agent will reveal itself by excessive spurious TL and in any case there are other techniques that can be applied, e.g. thermogravimetric analysis and archaeomagnetism.

19.3. Recent research and outstanding problems

19.3.1. Supralinearity and sensitisation

Evaluation of TL sensitivity as indicated in figure 19.2 by measurements made on a sample from which the natural TL has been drained in the course of the first glow curve is not, in general, a valid procedure. A substantial change in sensitivity may occur when a sample is heated; part may be due to changed transparency and reflectivity and part may be due to the radiation dose that the sample has received previous to heating, the latter being referred to as the pre-dose effect. Whatever the details of the mechanisms involved, this sensitisation between 'first glow' and 'second glow' presents a very serious complica­tion in TL dating. The effect itself can be circumvented by using the additive method of equivalent dose evaluation (see figure 19.3). This has the procedural disadvantage of

500 1000 Addilive dose (rads)

1500

Figure 19.3. Additive method of evaluating equivalent dose (ED). Of three equal samples, the first is used to measure N, the natural TL, the second to measure (N + 10), the natural plus artificial TL after a dose of p rad, and the third (N + 2/3); in the above, 0 = 700. The supralinearity correction A is determined by means of a growth curve such as figure 19.4 using samples from which N has been removed. The sum of ED and A equals AD, the archaeological dose.

requiring precise interrelation of the TL from different portions and the more serious objection that, although the additive method checks for linearity in the acquisition of TL in the dose region above the level of the natural dose, it can say nothing about linearity at lower levels of dose. In fact, a common characteristic of many TL minerals (and of some artificial phosphors, lithium fluoride for example) is a lower sensitivity during the first few hundred rads of dosage than subsequently; this supralinearity is illustrated in figure 19.4.

An empirical way of correcting for supralinearity is to use already glowed portions to determine the second-glow growth characteristic, and to assume that despite a change

370

600 700 300 400 500 Radiation dose (rad)

Figure 19.4. Typical supralineai dependence of TL on radiation dose for pottery samples irradiated with beta or gamma radiation. The degree of supralinearity is measured by the intercept A.

in sensitivity on heating there is no change in the intercept A. The justification for this procedure is that it gives improved agreement in tests with known age samples. It must be admitted that it is a rather bland assumption and indeed with some samples a pre-dose effect shows itself in the supralinearity [15, 16]. Studies of the supralinearity character­istics of individual minerals are only of help as a general guide because the quantitative behaviour depends on the annealing conditions that the particular sample has experienced. Also, in the fine-grain technique, one is dealing with a mixture of minerals of unknown identity. An advantage of the quartz inclusion technique is that if attention is restricted to the 'benign' peak at 375 °C, the sensitivity change on heating is often less than 5%, giving grounds for belief that the supralinearity intercept will be free from pre-dose effects too. Unfortunately, the 'malign' 325 C peak commonly dominates the TL from quartz.

It is not difficult to suggest mechanisms that might give rise to supralinearity and pre-dose effects — though far from easy to establish which is operative in any particular mineral. The simplest explanation is that additional traps are being created in the early stages of the irradiation, thereby increasing the sensitivity. An alternative is the 'competition model' in which it is assumed that there is a second set of traps (which do not give rise to TL) competing for electrons. These are assumed to saturate earlier than the TL traps and so, as the competing traps gradually approach saturation, the competition is suppressed and more electrons are available for the TL traps. Another explanation is in terms of an enhancement in the probability that an electron freed from a trap will produce luminescence — such enhancement could arise because of an increase with dose of the number of activated luminescent centres, for instance.

With increasing application to Palaeolithic samples, low-dose non-linearity becomes unimportant but allowance for the effect of approaching saturation becomes critical. Failure to make sufficient allowance can lead to substantial overestimate of age. There is not much work to hand on the question of whether in this dose region any reliance can

TL dating: techniques and problems 371

be placed on the second-glow growth characteristic, and non-linearity in the additive first-glow growth characteristic is an insensitive indicator of approaching saturation. The situation is further complicated by the likelihood that, at these high dose levels, trap creation becomes important so that the TL growth characteristic continues to rise beyond the level corresponding to saturation of existing traps.

19.3.2. Pre-dose dating

In the case of the 110 °C peak in quartz the pre-dose effect is so strong that it is the basis of an accurate dating technique particularly applicable to samples of the last thousand years [17, 18]. There is no sensitisation until the sample has been heated to 400-500 °C and detailed investigation by Zimmerman [19] has established a model (see figure 19.5) in which the essential mechanism is thermal transfer of holes from 'reservoir' traps to luminescent centres (which are thereby activated). This is the only well established model that we have in TL dating at present.

This 110 °C peak is not present in the natural TL, its half-life being only about 2 h. It is, however, a highly sensitive peak, and exposure to a dose (the test dose) of around 1-10 rad is usually sufficient to induce an accurately measurable TL. The increase in sensitivity (as indicated by the response to the test dose) consequent on briefly heating the sample to 400 or 500 °C is proportional to the total radiation dose (the pre-dose) that the sample has received subsequent to its firing in antiquity. Thus although the traps responsible for the 110°C peak carry an insignificant number of electrons, there is nevertheless a memory of the radiation dose that has been received, a memory that is unlocked by the brief heating to 400 or 500 °C (such as the sample experiences in the course of a normal glow curve).

The sequence of measurements is essentially as follows:

(i) Give test dose and measure response, S0. (ii) Heat to 500 °C (the activating temperature),

(hi) Give test dose and measure response, S^. (iv) Give laboratory calibrating dose, LD (usually several hundred rads).

(iv)(a) Give test dose and measure S^. (v) Heat to 500 °C.

(vi) Give test dose and measure response, 5N + ( 3 .

Since (5N — S0) is proportional to the dose received during antiquity, AD, and (5N + J 3 —5N) is proportional to LD, we have

Jw — Sn AD = — 7xLD. (19.4)

It is of course necessary to check, for each sample, that the proportionality of the sensitivity increase to dose is valid for the range of dose concerned. The effect usually saturates in the region of 500 rad.

The pre-dose phenomenon can usually be observed in any fine-grain or whole-grain sample containing quartz, but for accurate results it is best to extract the quartz as in the inclusion technique — though it is important to omit the etching with hydrofluoric acid

372 MJAitken

since for some reason this upsets the pre-dose behaviour. For a 10 mg sample of separated quartz, a typical sensitivity change is in the range 2-20% per rad of pre-dose and so an equivalent dose of as little as 10 rad can be measured accurately. Not all pottery contains quartz with satisfactory pre-dose characteristics, but when the technique is applicable it is a very powerful method, particularly for authenticity testing. It can often give an answer in cases when the conventional method is inapplicable because of too low TL intensity in the high-temperature region of the glow curve or because it is upset by spurious TL or held in doubt because of the possibility that abnormal fading or reheating has occurred.

From studies of the associated radioluminescence, exoelectron emission and the effect of ultraviolet irradiation, Zimmerman [19] has established that the sensitivity enhance­ment is due to an increased availability of charged luminescence centres and has proposed the model shown in figure 19.5. In this Tt and T2 are electron traps and L and K are hole traps. Ti is the shallow trap which, with the luminescent centre L, is responsible for the 110°C TL peak. Trap T2 is presumed to be deep enough not to be emptied by heating to 500 °C and it is introduced into the model so that charge balance can be maintained. The TL process consists of the thermal release of electrons from the traps Tt and the capture of some of these into the luminescent centres L. Such capture only occurs for centres which are charged with a hole and so the TL sensitivity is proportional to the number of centres which are so charged.

Conduction band

- O -

-o-R

Figure 19.5. Energy-level model for the pre-dose effect. (After Zimmerman [19].) ' Valence band

Firing of the pottery in antiquity is hypothesised to empty nearly all traps and consequently the sensitivity S0 after firing is low. Subsequently, the natural radiation dose rate produces electrons in the conduction band and holes in the valence band; the electrons are trapped in T2 (because Tx is too shallow to retain them) and the holes are captured at K (rather than L because of the former's presumed higher capture cross section). Thus during antiquity the hole population in K gradually builds up.

In step (i) of the pre-dose procedure, the test dose charges the traps Tt with a small number of electrons and puts a small number of holes into A". On heating through 100°C the electrons are released and those that find a charged luminescent centre give rise to TL. Because there has been no change in the number of holes trapped at the centres of

TL dating: techniques and problems 373

type L, the observed sensitivity S0 is the same as if the measurement had been made immediately after firing.

The heating to 500 °C in step (ii) causes thermal release of holes from K and these are then captured at L (because any recaptured at K are immediately re-ejected). Hence, the sensitivity S^ measured in step (iii) is proportional to the number of holes that had been accumulated in K during antiquity.

19.3.3. Transfer dating

In the context of thermoluminescent dosimetry, it is well known that, with natural calcium fluoride, electrons can be transferred from deep to shallow traps by illumination with ultraviolet light. By observing the TL from the shallow traps only, a measure of the deep-trap population can be obtained without heating the sample to the high temperature that would be necessary for direct observation of its deep trap TL. Currently, the effect in this and other phosphors is under development for ultraviolet dosimetry.

The effect has been studied in quartz by Schlesinger [20]; for instance, by means of 250-350 nm light, electrons can be transferred from a peak at about 500 K to peaks in the range 150-300 K, the sample being held at liquid-nitrogen temperature. At that author's suggestion, preliminary studies have been made by Bowman and Bailiff [21-24] with a view to utilisation in dating. With the above discussion of non-linearity and pre-dose difficulties in mind, there is obvious advantage in being able both to measure the deep-trap population (by transfer) and to empty the deep traps (by prolonged bleaching) without having to heat to 400 or 500 °C, for heating is certainly responsible for some of the sensitivity changes that occur between first glow and second glow. Of course, the bleaching with ultraviolet may give rise to sensitivity changes for other reasons, but the work of Bowman [22] suggests that with careful selection of wavelength this is not necessarily so.

There are also advantages simply because the TL is observed at a lower temperature. Interference by thermal radiation and spurious TL is avoided; this is particularly relevant when the TL is green or of longer wavelength, as in the case of calcite, and it is difficult to discriminate by means of colour filters. Also, thermal quenching of the luminescent centres is less.

In principle, the technique gives greater selectivity of the traps from which electrons are released and thus it may be possible to avoid traps having malign properties such as anomalous fading.

19.3.4. Anomalous fading

The expected lifetime of an electron in a trap of depth E and frequency factor s is given by

T - „-i s'1 exp(E/kT) (19.5)

where k is Boltzmann's constant and T is the storage temperature. Determinations of E and x by means of kinetic studies lead to lifetimes that are of the

order of a million years or more for peaks occurring at glow-curve temperatures of 350 °C or higher. However, grossly anomalous behaviour has been observed by Wintle [25] in

374 MJAitken

the behaviour of feldspar minerals. For volcanic lavas of known age, the observed TL was significantly too low sometimes by an order of magnitude, although the expected ages were no greater than 50 000 years and the TL utilised was in the 400-500 °C region of the glow curve.

The results of short-term stability tests were more anomalous still: losses of 10-40% of the 350-500 C TL in periods varying between a few hours and a few days were observed for some samples of sanidine, fluorapatite, labradorite, andesine, zircon and bytownite. Not all samples of these minerals showed the effect, and for quartz and calcite the fading was insignificant over months of storage. The phenomenon has also been observed in lunar samples and it has been suggested by Garlick and Robinson [26] that it is due to the subsidiary escape of electrons from traps due to overlap of wavefunctions — a wave-mechanical 'tunnelling' process.

Apart from their discouraging implications for geological lava dating, these observations call into question the basic validity of pottery dating using fine-grain samples or other forms of sample in which there is a substantial contribution from feldspar minerals (unfortunately the TL from feldspars is in general much brighter than that from quartz). However, the satisfactory results obtained with samples of known age indicate that this behaviour is not common among the mineral constituents of pottery. Also, the short-term stability tests that are now made routinely on fine-grain samples show that for most sites it is only the occasional sample which exhibits measurable loss (5-10%) for a storage period of up to a month. However, there are some sites for which the majority of samples are afflicted [27, 28].

The rate of percentage loss is initially rapid [29], getting progressively slower as the time elapsed since irradiation increases. To account for the initial rapid loss in terms of equation (19.5) (and to explain the observation that there is substantial fading even for storage at 77 K) requires £"~0.5eV and x~10 3 s _ 1 , and it is difficult to accept the thermal release model on which (19.5) is based as valid for such values. The tunnelling explanation [29] is more credible, and in terms of this the progressive slowing down of the fading is seen as the using up of nearby centres, tunnelling to these being much more probable than to distant ones. From the dating point of view, the crucial questions are, first, whether for a sample which shows no anomalous fading over a period of months it can be reliably assumed that there has been none over thousands of years and, secondly, whether study of short-term fading can be used to make a quantitative estimate of long-term fading. Comparison with radiocarbon can give some degree of answer to the first question. But beyond the limit of radiocarbon, in the middle and lower Palaeolithic ages where there is most need for TL, the only empirical approach is through intercomparison of TL dates obtained on different materials — if there is fading, the amount is likely to be different. However, until satisfactory calcite dating is achieved, the only generally available material is burnt flint (and that is rather sparse). Although short-term tests and comparisons with radiocarbon may give no indication of fading in flint, the utilisation [10] of this material in dating sites that are approaching an order of magnitude older than the limit of radiocarbon raises the question as to whether the tunnelling probability can indeed be sufficiently low, and gives emphasis to the need for a quantitative understanding of the process.

An experimental study by Zimmerman [30], following the work of Wintle [29] suggests that there is a correlation between anomalous fading and the short-term delayed

TL dating: techniques and problems 375

luminescence that follows irradiation; this may provide a much needed tool for investigating the affliction.

Although the tunnelling process is only weakly dependent on temperature, the deeper the trap the lower should be the probability. Hence there should be advantage in utilising traps beyond the usual 500 °C limit of the TL glow curve. Access to these is possible by means of the ultraviolet transfer technique and Bailiff [23] has obtained encouraging indications that anomalous fading can thereby be reduced and perhaps eliminated.

19.3.5. Trap depth determination

One of the most widely used methods for determination of trap depths is by study of the temperature dependence of the TL during the initial rise of the glow curve peak. The portion of glow curve used needs to be sufficiently below the peak temperature for there to have been no significant emptying of the traps. The TL intensity is then proportional to exp{—E/kT}, and a plot of the logarithm of the intensity versus {kT)_1 yields a straight line of slope E. Using this method the mean lifetime for the 325 C peak in quartz has been reported as 3000 years for geological alpha quartz [31] and as 200 years for quartz extracted from Romano-British pottery [32]. Both of these values conflict strongly with the observation that for this pottery sample the TL age based on the 325 °C peak is within a few per cent of the TL age based on the 375 C peak (for which the predicted lifetime is 40 million years). There is even greater conflict with the observation that for burnt sand having a radiocarbon age of 30 000 years the TL age based on the lower peak is within 10% of that based on the upper one [32].

Investigation by Win tie [32] indicates that the reason for this discordance is that as the glow-curve temperature increases, efficiency of the luminescent centres decreases because of increasing de-excitation by emission of phonons rather than photons (i.e. thermal quenching). Thus the TL intensity rises less rapidly than in the absence of the effect and an erroneously low value of E is obtained. Study of the prompt luminescence confirmed that the luminescence efficiency was in fact strongly temperature-dependent. Use of alternative methods for determining trap depth (by isothermal decay and by peak shift with heating rate) predicted a lifetime of about 30 million years, a value that is consistent with the TL ages mentioned earlier.

For the 230 °C peak in the archaeological quartz, the peak shift method predicted 40000 years instead of 2 years according to the initial rise method. For the 110°C peak in quartz and for the 275 °C peak in calcite, thermal quenching effects are not significant; the lifetime of the latter is calculated to be 100 million years from trap depth measurements [33].

19.3.6. Influence of temperature of irradiation

For some phosphors, the TL peak-height response is slightly dependent on the tempera­ture of irradiation [34]. For natural calcium fluorite and for CaS04:Tm, the response is lower by 5-10% if the irradiation is carried out at 100 °C instead of 20 °C; for LiF (TLD 100) the response is about 10% higher and for CaF2: Mn it is the same to within 0.5%. For fluorapatite and burnt limestone, the response was the same to within 5%, but for the 325 C peak of an archaeological sample of quartz, the response was lower by 17%.

376 MJAitken

The possibility that the decreases were due to thermal untrapping of electrons was excluded. In the case of the quartz sample, the effect was still present when the dose rate was reduced by a factor of 3000 but it was absent for a saturating dose of 40 krad. A possible explanation is that the effect is due to temperature dependence of the trapping cross section. However, on the basis that alpha-particle tracks are microscopic regions of TL saturation, this explanation is not consistent with the observation that the effect occurs as strongly for alpha irradiation as for beta.

On the basis of the above results, the effect is not significant for dating. Additionally, in one sample of archaeological quartz the effect was present in the 325 °C peak but not in the 365 C peak and it is the latter on which reliance is usually placed for dating. On the other hand, Khazal et al [35] have reported a strong temperature dependence for the 375 C peak in a sample of natural quartz, though using a heavy dose of gamma radiation. Relative to irradiation at room temperature, they found a response of x 0.65 at 0°C, of x 0.018 at - 25 °C, of x 0.0054 at - 78 °C and of x 0.0017 at - 95.4 °C.

Strong temperature dependence has also been reported for the efficiency of ultraviolet transfer in natural calcium fluorite [23, 36]. The efficiency is of the order of 50% greater at 10 C than at room temperature, the effect getting stronger as the wavelength of illuminating light is increased. It has been suggested that the effect is associated with one or two excited levels of the donor trap, these levels lying a few tenths of an electronvolt below the conduction band.

19.3.7. Assessment of dose rate

The established system for fine-grain dating is to use thick-source alpha counting for evaluation of the uranium and thorium contributions and some form of chemical analysis (e.g. flame photometry, x-ray fluorescence, atomic absorption) for the potassium. The use of a disposable zinc sulphide screen remains supreme over other more sophisticated techniques on account both of cheapness and of low background. Using the a-value system [37], the conversion of the alpha count rate into effective alpha dose rate can be made without knowledge of the stopping power of the sample or indeed of the decay chain details. However, both of these are needed for evaluation of beta and gamma dose rates and a reassessment of the energy release per parent nuclide disintegration has recently been made by Bell [38, 39]. This reassessment is based on published nuclear data tables and, owing to the complexity of the decay schemes, it is no mean task. The revised values [39] for the alpha, beta and gamma contributions (in mrad per year per ppm by weight of parent) are 73.8, 2.86 and 5.14 respectively for the thorium series, and 278, 14.6 and 11.5 for the uranium series. The beta and gamma dose rates from 1% of natural K20 are 68.2 and 20.5.

For quartz inclusion dating the preferred method of dose rate evaluation is by TLD and, incidental to developing such a system for beta dose rate evaluation, Bailiff [23] has made an experimental check of the value derived by Bell from the nuclear data tables; there is agreement to within the 5% limit of experimental error. Other systems for dose rate evaluation by TLD have been reported by Mejdahl [40-42] and by Aitken [43].

For some pottery and soils, the degree of escape of the radioactive gases 220Rn and 2"Rn is sufficient to affect the dose rates substantially. The earlier assumption that such escape does not occur on wet sites has been shown [44] to be invalid. It has also

TL dating: techniques and problems

'O'ring seal

377

Highly polished inner surface

Black wax seal

Sample tray

36 mm

Figure 19.6. Gas cell for evaluation of radon emanation.

been shown [45] that the established method of alpha counting in which the sample is sealed in a container can give rise to a gross overestimate of the activity if there is substantial radon emanation in the sample. A more satisfactory technique is to measure the alpha activity with the sample unsealed and to evaluate the gas escape in a separate experiment in which the sample is sealed in a gas cell (see figure 19.6) such that the only particles which can reach the scintillator screen are from emanated radon. Confirmatory checks that laboratory assessment does give a reliable indication of the situation during burial can be made by radiochemical measurement of polonium-210; this is supported by 21-year lead-210 which is subsequent to radon in the uranium decay chain.

19.3.8. Environmental uncertainties

However accurate the laboratory techniques and however well behaved the TL minerals, there remain several environmental causes of uncertainties. One of these is the change of radioactivity in a sample due to leaching or deposition by groundwater; in geology, uranium is well known for its mobility. Another is the attenuation of the dose rate by the water content of sample and soil. Pottery commonly has a saturation water content of around 10%, and soil 20%. The average water content during burial depends on the climatic history of the site and perhaps on water table variations, but even if these latter are known they are not easily translated into water contents — except when the site is known to have been excessively wet or excessively dry throughout. Even in the former case there remains doubt as to the extent to which water content affects the alpha dose rate — the pore structure of pottery is not necessarily fine enough for water to be interposed in the path of the majority of the alpha-particle tracks.

Specifically with regard to gamma dose rate, it has to be assumed that the sample has spent the major part of its burial time with the same surroundings as found by the excavator. Quite apart from later erosion or other disturbance, there is sometimes doubt as to how rapidly the sample became buried. There is also the question of how strictly the collector has kept to the requirement of a homogeneous surround of soil to a distance of

378 MJAitken

0.3 m, a requirement that is often restrictive and irksome particularly if none of the samples on a site fulfil it.

Hence there is strong reason to develop subtraction methods by which the gamma dose rate is eliminated and at the same time to attempt to use impervious materials to avoid uncertainty due to water content. With any subtraction technique, there is an inevitable increase in the size of the error limits, and with most samples, until evaluation of equivalent dose is more precise, what is gained on the swings is lost on the roundabouts. However, for pottery samples in which the alpha particles contribute at least a third of the fine-grain TL, Fleming and Stoneham [46,47] have demonstrated that dates accurate to ±12% can be obtained by subtracting the equivalent dose obtained with quartz inclusions from the equivalent dose obtained with fine grains. This removes dependence on beta and gamma dose rates and, besides allowing dating of pottery from burial contexts which do not meet the usual requirements, it makes it possible to date objects from museum shelves, as demonstrated by application to four Renaissance terracottas [47]. However, besides the need for a strong alpha contribution, there is also the requirement for a large enough sample (at least several cubic centimetres) to provide sufficient quartz.

Several other subtraction methods have been proposed. Poupeau et al [12] suggest that for dating heated rocks from ancient fireplaces it is feasible to eliminate the gamma dose rate by using two rocks of very different radioactivity, one high and one low; also, rocks have the advantage of low water content and low radon emanation. In application [48] to the Palaeolithic site of Pincevent in France, the archaeological dose in quartz grains in a piece of granite was found to be 5600 rad whereas that in pieces of sandstone was very low (<0.2 ppm U, <0.6 ppm Th, <0.1% K20). Its quartz was effectively acting as a monitor of the environmental dose (gamma plus cosmic) so that the net dose obtained by subtraction (5600 — 1500) arose from only the internal radioactivity of the granite. In the 'quartz attenuation' technique proposed [49], but not yet demonstrated, by Mejdahl and McKerrell, the net quantity utilised is the difference in dose between inner and outer parts of large quartz grains (about 0.5 mm across) due to attenuation of beta radiation. The same authors also propose quartz-feldspar subtraction which utilises the difference in dose between large grains of potassium feldspar and large grains of quartz due to the beta contribution from internal potassium in the feldspar. Anomalous fading in the latter is likely to be a difficulty here.

There is complete elimination of the need to make direct measurement of any radio­activity in the method DATE (difference d'attenuation temporelle des emissions) proposed by Langouet et al [50]. In this the dose rate is evaluated by determining the equivalent dose indicated by a TL peak having too short a lifetime for linear accumula­tion. The ratio of this to the true equivalent dose (as indicated by a peak having a sufficiently long lifetime) can be used as a measure of the dose rate as long as the too-short lifetime is known. Effectively it is determined by measuring the equivalent dose ratio for a series of similar samples of known age. The basis of this method is most easily appreciated by recalling a simple version of it that was used by Johnson [51] in dating contact-baked limestone. In this, the lifetime of the lower peak was short compared to the age so that the peak was in thermal equilibrium at a level determined by

XlD = G1lr1 (19.6)

TL dating: techniques and problems 379

where Xi is the TL per rad for this peak, G] is the observed peak-height, 7i is its lifetime, and D is the dose rate.

For a higher peak of long lifetime,

X2Dt = G2 (19.7)

where t is the age. Hence if T\ is known and the ratio X1/X2 determined by artificial irradiation, t can be found. According to the preliminary results obtained by Langouet et al, the 325 °C peak in quartz has a sufficiently short lifetime for their purpose. This conflicts with the earlier discussion in §19.3.5; presumably different types of quartz are involved.

19.3.9. Zircon dating

Zircon grains carry such a high concentration of uranium (typically several hundred parts per million) that the TL contribution from radioactivity in the pottery matrix in which they are embedded, and from external gamma rays, is barely significant. This powerful form of radioactive inclusion dating was first proposed by Zimmerman [52] and is under development at the Laboratory for Space Physics of Washington University, St Louis, Missouri, USA. Its validity for archaeological dating has been tested by Sutton and Zimmerman [53] with zircons extracted from half-a-dozen pottery fragments of known age and its utility in authenticity testing has been demonstrated by application to the ceramic core of the famous bronze horse of the New York Metropolitan Museum of Art [54].

In its original form, this technique involved measurement of TL sensitivity and alpha dose rate as in conventional TL dating. Single grains were dated individually, alpha radiation being used for sensitivity measurements and induced fission tracks for the uranium and thorium content. However, although for some grains the age obtained was correct, for others it was substantially too low. On investigation this was found to be due to anticorrelation between TL sensitivity and radioactivity, the spatial distribution of the former being mapped by means of cathodoluminescence and of the latter by means of induced fission tracks. The TL sensitivity measured by artificial irradiation is dominated by the regions of high sensitivity and since these are remote from the uranium and thorium the value obtained is not relevant to age calculation. For obvious reasons the mapping cannot be done until all TL measurements on a grain are complete and hence the occurrence of zoning in a substantial proportion of grains means that there is a great deal of wasted effort. To circumvent this, the so-called 'natural method' is being developed: after measurement of the archaeological TL, GN, for a grain, or a group of grains, the sample is stored for a time t' of the order of six months after which the re-accumulated TL, GR, is measured again; the age t is given by

GN . t = -rf. (19.8)

In concept the method could not be simpler but the experimental difficulties are severe. Apart from the need for high detection sensitivity in order to obtain a statistically meaningful number of counts, say a thousand, in the measurement of GR, it is vital to eliminate spurious TL which without extreme precaution is liable to mask GR. Quite apart

380 M J Aitken

from measurement problems, there is the ever-present difficulty that some grains exhibit anomalous fading. Nevertheless, zircon dating is potentially of high importance just because it avoids uncertainties arising from water content and environmental radiation. Also, in the natural method it avoids the assumption that TL sensitivity is independent of dose rate.

In an attempt to reduce the storage time required for measurement of GR, Mobbs [55] investigated the possibility of storage at liquid-nitrogen temperature so that lower-temperature peaks, which are an order of magnitude more sensitive, could be utilised. The regrowth of the higher-temperature 'dating' TL would then be inferred by measuring the ratio of high-temperature TL to low-temperature TL following artificial irradiation. Unfortunately, this ratio was found to be different from grain to grain and consequently it seems likely that it will be different between different sensitivity zones within a grain, in which case the approach is invalid.

References and further reading 1 Aitken M J 1974 Physics and Archaeology 2nd edn (Oxford: Clarendon Press) 2 Zimmerman D W 1971 Archaeometry 13 29-52 3 Fleming S J 1970 Archaeometry 12 135-46 4 Sutton S and Zimmerman D W 1978 Archaeometry 20 67-9 5 Bell W T and Zimmerman D W 1978 Archaeometry 20 63-6 6 Valladas H 1977 Archaeometry 19 88-95 7 Courtois L et al Proc. 5th Int. Conf. on Luminescence Dosimetry, Sao Paulo (Giessen:

I Physikalisches Institut of the Justus-Liebig Universitat) p 459 8 Zimmerman D W and Huxtable J 1971 Archaeometry 13 53-7 9 Goksu H Y, Fremlin J H, Irwin H T and Frysell R 1974 Science 183 651-4 10 Wintle A G and Aitken M J 1977 Archaeometry 19 111 11 Aitken M J and Wintle A G 1977 Archaeometry 19 100 12 Poupeau G, Sutton S, Walker R and Zimmerman D 1977 Thermoluminescent dating of heated

rocks 1976 Symp. on Archaeometry and Archaeological Prospection, Edinburgh ed H McKerrell (London: Her Majesty's Stationery Office)

13 Gillot P Y, Valladas J, Laj C, Valladas G, Guerin G, Poupeau G and Delibrias G 1979 Earth Planet. Sci. Lett. 42 444-50

14 Aitken M J, Moorey P R S and Ucko P J 1971 Archaeometry 13 89-141 15 Fleming S J 1975 Archaeometry 17 122-9 16 Bowman S G E 1975 Archaeometry 17 129-32 17 Flemings J 1973 Archaeometry 15 13-30 18 Aitken M J and Murray A S 1976 The pre-dose technique: radiation quenching 7976 Symp. on

Archaeometry and Archaeological Prospection, Edinburgh ed H McKerrell (London: Her Majesty's Stationery Office)

19 Zimmerman J 1971 / . Phys. C: Solid St. Phys. 4 3265-76 20 Schlesinger M 1965 J. Phys. Chem. Solids 26 1761 21 Bailiff I K 1976 MSc Thesis Oxford University (unpublished) 22 Bowman S G E 1975 DPMI Thesis Oxford University (unpublished) 23 Bailiff I K 1976 Nature 264 531-3 24 Aitken M J, Bailiff I K, Bowman S G E and Mobbs S F 1976 The phototransfer technique in TL

dating 1976 Symp. on Archaeometry and Archaeological Prospection, Edinburgh ed H McKerrell (London: Her Majesty's Stationery Office)

25 Wintle A G 1973 Nature 244 143-4 26 Garlick G F C and Robinson I 1972 in The Moon eds S K Runcorn and H Urey: International

Astronomy Union pp 324-9 27 Whittle E H and Arnaud J M 1975 Archaeometry 17 5-24

TL dating: techniques and problems 381

28 Whittle E H \915 Archaeometry 17 119-22 29 Wintle A G 1977 / . Lumin. 15 385-93 30 Zimmerman D \V 1977 Radiative recombination and anomalous fading 1977 Symp. on Archaeo­

metry and Archaeological Prospection, Philadelphia Abstracts, p 42 31 Aitken M J and Fleming S J 1972 Topics in Radiation Dosimetry, Supplement I, ed F H Attix

(New York: Academic Press) pp 1-78 32 Wintle A G 1975 Geophys. J. R. Astron. Soc. 41 107 33 Wintle A G 1976 Basic problems in TL dating the formation of calcite 1976 Symp. on Archaeo­

metry and Archaeological Prospection, Edinburgh ed H McKerrell (London: Her Majesty's Stationery Office)

34 Aitken M J, Huxtable J, Wintle A G and Bowman S G E 1974 Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow (Krakow: Institute of Nuclear Physics) p 1005

35 Khazal K A R, Hwang F S W and Durrani S A 1975 The effect of the temperature of irradiation on the TL sensitivity of quartz 1975 Symp. on Archaeometry and Archaeological Prospection, Oxford Abstracts, p 8

36 Sunta C M and Watanabe S 1977 Proc. 4th Int. Conf. on Luminescence Dosimetry, Krakow (Krakow: Institute of Nuclear Physics)

37 Aitken M J and Bowman S G E 1975 Archaeometry 17 132 38 Bell W T 1976 Archaeometry 18 107 39 Bell W T 1977 Archaeometry 19 99 40 Mejdahl V \91Q Archaeometry 12 147-71 41 Mejdahl V 1972 Archaeometry 14 245-56 42 Mejdahl V 1972 Dosimetry techniques in thermoluminescent dating Danish Atomic Energy

Commission, Riso Rep. 261 43 Aitken M J 1969 Archaeometry 11 109-14 44 Desai V and Aitken M J 1974 Archaeometry 16 95 45 Desai V 1974 MSc Thesis Oxford University (unpublished) 46 Fleming S J and Stoneham D 1973 Archaeometry 15 229 47 Fleming S J and Stoneham D 1973 Archaeometry 15 239 48 Poupeau G, Sutton S, Walker R M and Zimmerman D W 1976 Thermoluminescent dating of fired

rocks: application to site of Pincevent Ninth Congr. Union Int. Sci. Prehist. Protohist., Nice in press

49 Mejdahl V and McKerrell H 1976 Progress and problems with automated TL dating 1976 Symp. on Archaeometry and Archaeological Prospection, Edinburgh ed H McKerrell (London: Her Majesty's Stationery Office)

50 Langouet L, Roman A and Gonzales R 1976 Datation de poteries anciennes par la me'thode DATE 1976 Symp. on Archaeometry and Archaeological Prospection, Edinburgh ed H McKerrell (London: Her Majesty's Stationery Office)

51 Johnson N 1965 / . Geophys. Res. 70 4653 52 Zimmerman D W 1971 Science 174 818 53 Sutton S and Zimmerman D W 1976 Archaeometry 18 125 54 Zimmerman D W, Yuhas M P and Meyers P 197'4 Archaeometry 16 19 55 Mobbs S F and Aitken M J 1977 Thermoluminescence and phototransfer at low temperatures

1977 Symp. on Archaeometry and Archaeological Prospection, Philadelphia Abstracts, p 41 56 Aitken M J 1977 Antiquity LI 11 57 Sanders H P 1973 Archaeometry 15 159 58 Morariu, V V, Bogdan M and Ardelean I 1977 Archaeometry 19 185 59 Aitken M J, Zimmerman D W, Fleming S J and Huxtable J 1970 Thermoluminescent dating of

pottery Proc. 12th Nobel Symp., Uppsala, August 1969, in I Olsson (ed) Radiocarbon Variations and Absolute Chronology (Stockholm: Almqvist and Wiksell) pp 129-40

60 Bronson B and Han M C 1972 Antiquity XLVI 322-6 61 Caton Thompson G and Whittle E H 1975 Antiquity XLIX 89-97 62 Fagg B E B and Fleming S J 1970 Archaeometry 12 53-5 63 FlemingSJ 1971 British School at Athens Supplementary Volume No. 7, pp 343-4. 64 Fleming S J and Fagg B E B 1977 Archaeometry 19 86-87 65 Huxtable J, Aitken M J and Weber J C \912 Archaeometry 14 269-75

Applied Tliermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann ©1981 ECSC, EEC, EAEC, Brussels and Luxembourg

20 Application ofTL dosemeters in dose standardisation and inter comparison

G SCARPA

20.1. Introduction

As is universally accepted for all physical quantities and units, radiation quantities and units, such as exposure, kerma and absorbed dose, should be standardised on an inter­national basis, which means that the methods and criteria for their instrumental and theoretical evaluation should be the same all over the world.

The International Commission on Radiation Units and Measurements (ICRU) is the institution charged with the task of formulating the recommendations on methods for the establishment of radiation quantities and units under any practical conditions.

The advantages of such standardisation are obvious in all the fields of application of radiation. Investigations in radiation therapy and radiation biology have demonstrated that differences of 10% in absorbed dose can produce clearly observable variations in biological response [1]. On the other hand, scientists carrying out studies on dose-effect relationships must be able to repeat their experiments and to compare their results with those of other centres. In radiation protection, health physicists have to comply with national and international regulations. In industrial radiation processing also there are strict rules on minimum and, sometimes, maximum doses.

The methods of achieving such a standardisation (or uniformity of evaluation) of absorbed doses can be grouped into two classes:

(1) the indirect classical methods of 'dissemination of standards', and (2) the direct intercomparison methods.

20.2. Dissemination of standards

The dissemination of standards is a very well established technique, developed mainly in the UK [2] and based on the instrumental hierarchy illustrated in figure 20.1. Every country should have:

(a) one national 'primary' standards laboratory, with primary standard dosemeters, such as free-air chambers and graphite chambers;

(b) a limited number of 'secondary' standards dosimetry centres, with secondary standard dosemeters, i.e. very stable and very well maintained dosemeters, which are periodically checked with the national primary standards; and

(c) an unlimited number of so-called 'field' dosemeters, which are simply the instru­ments used to carry out routine dosimetry in every institute; the field instruments are periodically sent to secondary standard dosimetry centres and checked with the secondary standard dosemeters.

384 G Scarpa

Country A

Primary standard

Secondary standard

periodica] checks

Secondary standard

Country B

Primary standard

Secondary standard

Secondary standard

Figure 20.1. Diagrammatic illustration of instrumental hierarchy used in dissemination of standards. F = field instrument.

The primary instruments belonging to different countries are periodically intercompared by using special 'transfer' standards. In this way, indirectly, all the field dosemeters of all countries should be kept at the same response level and, therefore, should measure doses in a uniform way.

20,3. Direct intercomparison methods

The main feature of these systems is the active participation of many (sometimes hundreds of) institutes belonging to the same country (interregional or national inter-comparisons) or, more often, to different countries (international intercomparisons). All participating organisations are linked in that they are concerned with the same sort of activity, e.g. radiobiological experiments on late effects, 60Co radiation therapy, neutron radiation therapy, environmental dosimetry, and so on.

As illustrated in figure 20.2, the peripheral institutes do not interact directly, but through a central laboratory, which is seldom a primary standards laboratory. According to the procedure used, a distinction can be made between (a) 'instrument inter­comparisons' and (b) 'dose intercomparisons', the subject of the first being the dosimetric devices by which the laboratories evaluate doses, while the second kind of inter-comparison deals with the results of such evaluations.

In the intercomparisons of instruments (see figure 20.3) each participant takes (or sends) his own instrument (the one used for routine measurements) to a central institution where a very stable irradiation facility is available to give an unspecified, but constant, dose. The instruments are then taken (or sent) back and read by participants, and results are eventually compared.

In the intercomparisons of dose (see figure 20.4) a central institution takes (or sends) dosimetric integrating devices (transfer devices) to each participant. The participant irradiates them to an agreed dose using the same irradiation facility as for routine experi­ments, calibrated by means of the local field instrument. The transfer devices are then taken (or sent) back to the central laboratory, where they are read and results are compared.

Application of TL dosemeters in dose standardisation and intercomparison 385

Country A I Country B

Institute ^ ^ ^

Institute ^ ^

1 1

Central laboratory

1 1

^ ^ Institute

Institute

Country C | Country D

Figure 20.2. Diagrammatic illustration of direct intercomparison method.

Preparation of dosemeters by peripheral institutes

Taking or sending to central lab.

Reading by peripheral institutes

Taking or sending back of dosemeters

Irradiation to an unspecified but constant dose

Figure 20.3. Diagrammatic illustration of a dosemeter intercomparison.

Preparation of transfer devices by central lab.

Reading of transfer devices by central lab.

Taking or sending to peripheral institutes

Taking or sending back to central lab.

■ ■

Irradiation to a specified dose

Figure 20.4. Diagrammatic illustration of a dose intercomparison.

From the practical point of view of having uniform dose units in every institute carrying out irradiations (for clinical, biological or industrial purposes), method (b) is more direct and so more reliable [6], as it also takes into account the technique, right or wrong, by which local instruments are actually used. In other words, two institutes may have fully comparable dosimetric instruments but still irradiate animals or patients to significantly different doses, just because the instruments are used in a different way. This concept does not apply, of course, to radiation protection, both as personal and environ­

mental dosimetry, where doses are not 'administered' but 'received' and people are only interested in knowing whether or not the dosemeters they use give a correct reading. In this case, instrument intercomparisons should be undertaken.

3 86 G Scarpa

Both field dosemeters used in method (a) and transfer devices used in method (b) can be classified into two main categories:

(1) classical electronic dosemeters, such as ionisation chambers, scintillation dosemeters, etc, and

(2) solid-state dosemeters (also referred to as 'passive' dosemeters), mainly represented by TL detectors.

Instruments of the first category are large, delicate and expensive so that they usually need to be carried personally to and from the irradiation site. Solid-state devices, in contrast, are so small, rugged and inexpensive that they can be sent by mail without any complications and at a far lower cost. This is the reason why the great majority of inter-comparisons are now performed by means of mailed TL phosphors (so-called 'mailed intercomparisons,).

20.4. Characteristics of TL dosemeters used for mailed intercomparisons

The most important features of such solid-state dosemeters are their long-term stability, their reproducibility, their sensitivity and their energy response.

The need for a good long-term stability arises from the fact that in every inter-comparison the time elapsing between initial preparation of dosemeters and final readout is rarely shorter than a couple of weeks, and sometimes as long as three months (environ­mental dosemeter intercomparisons). During this period, dosemeters undergo two opposed phenomena: on the one hand an increase of the zero-dose signal (background) in the form of non-radiation-induced (NRI) peaks, and on the other a decrease of the radiation-induced signal (fading). Both phenomena are more or less influenced by ambient temperature and humidity. From this point of view, excellent TL materials seem to be LiF and BeO.

The reproducibility of measurements should be within ±2 or 3% for a single detector, falling to less than 1% if groups of 8-10 detectors are employed.

As to sensitivity, it should be adequate at the dose level of interest, e.g. the order of a gray in the field of radiotherapy and radiobiology and the order of a fraction of a milligray in intercomparisons of environmental detectors. CaF2 and CaS04 seem to be the best in the latter case.

A more or less pronounced energy dependence of the response is a property shared by all known TL materials. Calcium-based TL detectors, such as CaF2 and CaS04, are the worst from this viewpoint, even though the use of appropriate metal filters can overcome this drawback, at least to some extent. LiF and BeO have, in contrast, the flattest energy response curve, due to their tissue equivalence.

20.5. Practical examples of mailed intercomparisons

20.5.1. IAEA-WHO 60Co dose intercomparison

The first large-scale postal intercomparison by means of TL detectors was that started in 1966 by the IAEA in connection, later, with the WHO, aiming to check the accuracy of dosimetric evaluations performed in a number of radiotherapy institutes throughout the

Application of TL dosemeters in dose standardisation and intercomparison 387

world, using ^Co facilities. More than 400 experimental data have been collected in a recently published survey of results [3].

Transfer dosemeters were LiF-filled capsules, in groups of three, which the participants were asked to irradiate within a water phantom, at 5 cm depth, to a dose of 2 Gy, as evaluated by each institute. Two control capsules, one exposed to a known dose and the other not exposed, were sent together with the test capsules to detect possible environmental influences during transport and storage.

About one-third of the participants obtained results which were outside the range ±5% of the correct dose. Improvements were observed in subsequent intercomparisons of the same kind.

20.5.2. EULEP dose intercomparison

Another very important intercomparison project was the one organised in 1971-1976 by the European Late Effect Project Group (EULEP) among 18 European radiobiological and biomedical laboratories working in the field of late effects of x-rays in mammals. The project was divided into three successive sessions, each composed of two or three experimental runs.

Excluding the first run of the first session, in every experiment the participants were asked to expose a mouse-sized test phantom containing three LiF-filled capsules, one central and two peripheral. Apart from the test phantom, additional mouse-phantoms had to be placed around, in order to simulate the real irradiation of a group of mice. The central dosemeter had to receive an absorbed dose of 2 or 3 Gy, in condition of maximum backscatter and using a minimum focus-to-phantom distance of 60 cm. The participants were invited to use an HVL between 1 and 3 mm Cu and to quote the actual HVL employed.

The knowledge of HVL was necessary to evaluate, at least roughly, the average energy of the x-ray beam actually used by each participant. Based on this energy, an appropriate correction factor could be applied in order to allow for the energy response of LiF, ranging between 1.26 and 1.15 in this HVL interval [1,4, 5].

The results of the three intercomparison sessions are summarised in table 20.1. A considerable improvement can be seen in some cases: in the third session (1976) only one institute was outside the limit of ±5% of the standard dose.

The following recommendations were formulated [1].

(a) The accuracy of the dosimetry is considered to be satisfactory when the mean value of the results from a laboratory differs by less than 5% from the standard value.

(b) When a difference between 5 and 10% exists, a small discrepancy in the dosimetry in indicated.

(c) If the difference is more than 10%, a recalibration of the dosimetry system is recommended.

20.5.3. International intercomparisons of environmental dosemeters

Three international intercomparison studies of environmental dosemeters have been organised in 1974, 1976 and 1977 by Gesell and Burke, with 56, 128 and about 150 participants, respectively. A fourth session is now in progress. The aim of these

388 G Scarpa

Table 20.1. Mean relative absorbed dose and corresponding standard deviation for 1971, 1973 and 1976 EULEP dosimetry intercompaiisons. (From Broerse et al [1].)

Participants

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Mean

1.169 1.045 1.049 1.017 0.996 0.953 0.830 1.125 1.010 1.090 0.977 ----1.041 1.123 -

1971

Stand. dev. (%)

1.5 0.7 1.9 1.5 2.1 2.0 13.3 5.9 0.4 2.5 0.6 ----0.3 11.2 -

Mean

0.929 1.027 1.078 0.985 1.019 0.959 0.850 0.992 1.011 1.028 0.968 1.070 0.979 1.013 ----

1973

Stand. dev. (%)

1.8 2.7 4.5 0.9 2.3 5.9 6.7 3.4 0.9 7.2 2.0 0.7 0.1 1.3 ----

Mean

1.011 1.079 1.019 0.969 0.982 0.956 0.952 0.995 0.974 0.951 -0.960 -0.960 0.956 0.976 1.033 0.993

1976

Stand. dev. (%)

1.6 3.5 1.6 1.7 2.7 0.7 4.5 6.0 0.7 3.5 -4.1 -2.1 1.9 5.0 5.0 1.6

procedures is to verify the accuracy and reliability of small and inexpensive passive detectors (mainly TL phosphors) commonly used today to measure environmental doses, instead of scintillation or gas ionisation devices.

Each participant was requested to send by airmail a set of six dosemeters of each type that he wished to intercompare: two 'field' dosemeters, two 'laboratory'dosemeters and two 'control' dosemeters. Upon receipt, the dosemeters were transferred to a low-background shielded area, whose exposure rate had previously been measured. After a few days the field dosemeters were separated from the others and deployed at the field site, where a portable recording ion chamber was also located to estimate radiation exposure during the experiment. The exposure of field detectors lasted 90 days; in the mid-part of this period the laboratory dosemeters were irradiated to an unrevealed extent by a 226Ra source. At the end of the third month, the field dosemeters were recovered, reunited with the laboratory and control detectors and returned by airmail to the participants, together with a response form.

On this form, the participants were asked to estimate the actual field and laboratory exposures (Xp and X]_) from the data measured with the three groups of dosemeters, using the following formulae:

Xp = Xp - Xc + X§

ATL = X^ - XQ

where X' are the exposures measured with field (F), laboratory (L) and control (C) dosemeters and Xs is the exposure received from control dosemeters in the storage site.

Application of TL dosemeters in dose standardisation and intercomparison 389

The participants were also requested to state their error estimates (two standard deviations), including all the known errors, such as those arising from the detector imprecision, calibration errors, etc. The results of the first two intercomparisons have already been published by Gesell and Burke [7,8].

Referring to the second intercomparison study, figures 20.5 and 20.6 illustrate the distribution of the results of the 'laboratory' and 'field' dosemeters exposed to 21.3 and 17 mR, respectively. If results exceeding 3a are eliminated as outliers, the resulting averages are 18.8 and 16.4 mR and 71% and 65% of the data fall within ±20% of these

15 -

<-) 10 z UJ

o

I I I I I I I I I I I I I I II ' I I I I I I I I I I ' I I I ' I '

RJ

^ESTIMATED LABORATORY EXPOSURE

. . n .n l lnn . . . . n . 10 15 20 25 30 35 40

EXPOSURE ( mR) Figure 20.5. Second international intercomparison of environmental dosemeters: results of 'laboratory' dosemeters. (From Burke and Gesell [8].)

15 -

6 io

a UJ

i 11 i 11 i i i i 11 1 1 1 11 i i i 11 i i 1 1 1 1 1 ■ ■ 1 1 1 1

IONIZATION CHAMBER MEASUREMENT

in n/

n...n v. J_I_ 1 1 JL -5 0 5 10 15 20 25 30

EXPOSURE ( m R ) Figure 20.6. Second international intercomparison of environmental dosemeters: results of 'field' dosemeters. (From Burke and Gesell [8].)

390 G Scarpa

average values. As regards the types of TL detectors used in this study, 36% were LiF:Mg,Ti, 18% CaF2:Dy, 17% CaS04:Dy and 10% CaF2:Mn.

20.6. Conclusions The direct intercomparison method, both for dosemeter intercomparison and dose inter-comparison, is becoming more and more widely used for dose standardisation purposes.

In the last two decades a number of intercomparisons have been undertaken on a national, as well as an international, scale. Most of them have been based upon the use of small, inexpensive TL dosemeters, which may very easily be sent by ordinary mail. The ever-increasing number of intercomparisons is justified by the valuable data that can be easily collected by this method, at a very low cost.

It must be emphasised, however, that the intercomparison method is normally not a substitute to the classical method of dissemination of standards, but merely a comple­mentary technique, useful to spot possible failures of the latter. It must be added that sometimes a direct intercomparison is the only system that can be used to standardise doses, e.g. among countries in which neither primary nor secondary standards are yet available.

References 1 Broerse J J. Zoetelief J and Puite K J 1978 Dosimetry intercomparisons for evaluation of late

effects of ionizing radiation A eta Radiol. Oncol. 17 225 2 Kemp L A W 1977 Dissemination of the roentgen unit for radiotherapy purposes, in Ionizing

Radiation Metrology (Bologna: Editrice Compositori) pp 227-249 3 Eisenlohr H H and Jayaraman S 1977 Phys. Med. Biol. 22 18 4 Puite K J and Crebolder D L J M 1974 Energy dependence of TL dosimeters for x-ray dose and

dose distribution measurements in a mouse phantom Phys. Med. Biol. 19 341 5 Puite K J 1976 Phys. Med. Biol. 21216 6 Broerse J J and Puite K J 1974 The usefulness of intercomparison studies for the improvements of

x-ray dosimetry Phys. Med. Biol. 19 732 7 Gesell T F, de Planque Burke G and Becker K 1975 An international intercomparison of environ­

mental dosemeters under field and laboratory conditions Rep. ORNL-TM-4887, April 8 de Planque Burke G and Gesell T F 1979 Second international intercomparison of environmental

dosemeters under field and laboratory conditions Health Phys. 36 (3) 221

Applied Thermoluminescence Dosimetry. Eds M Oberhofer and A Scharmann © 1981 ECSC, EEC, EAEC, Brussels and Luxembourg

APPENDIX The new radiological (SI) units and their conversion to the units previously used

At the 15th and the 16th General Conferences of Weights and Measures, special names were adopted for some units of the Systeme International d'Unites (si) used in the field of ionising radiation dosimetry.

The gray, symbol Gy, has been adopted as the special name for the si unit of absorbed dose and of other quantities in the field of ionising radiations, which can be expressed in joules per kilogram (e.g. absorbed dose index, kerma and specific energy imparted). One gray is equal to one joule per kilogram:

1 Gy = 1 J kg"1.

The sievert, symbol Sv, has been adopted as the special name for the si unit of dose equivalent. One sievert is equal to one joule per kilogram:

1 Sv = 1 J kg-1.

The becquerel, symbol Bq, has been adopted as the special name for the si unit of radioactivity. One becquerel is equal to one second to the power of minus one:

1 Bq= 1 s"1.

Unit multiples are expressed by adding the following symbols of prefixes to the unit symbols:

Name Letter Multiple

tera giga mega kilo milli micro nano

T G M k m M n

1012

10' 10' 103

10"3

icr6

10"'

To convert from one set of units to the other, the following relationships hold:

1 rad(rd) = 10"2Jkg"1=10"2Gy

1 Gy = 100 rad

l rem=10" 2 Jkg" 1 = 10~2Sv

1 Sv = 100rem

392 Appendix

1 Ci (curie) = 3.7 x 1010s_1 = 3.7 x 1010Bq (exactly)

1 Bq = x 10"10Ci~ 2.703 xlO"nCi. 3.7

No special name for the unit of exposure, up to now called the roentgen (R), was adopted. The unit for exposure is 1 coulomb per kilogram:

1 R = 2.58xKr 4Ckg _ 1

1 Ckg_1 = x l0 4 R. 2.58

Thus we can summarise this in the following:

Absorbed dose l r a d = 1 0 " 2 G y l G y = 100rad Kerma l r ad = l(T3Gy l G y = 1 0 0 r a d Dose equivalent l r e m = 1 0 " 2 S v l S v = 1 0 0 r e m Activity 1 C i = 3 . 7 X l 0 ' ° B q 1 Bq = 2.703 X 1 0 ' " Crf Exposure 1 R = 2.58 X 10""C kg"1 1 C kg" '= 3.876 X 103Rf

| These values are rounded

Index

Absorbed dose, 230, 231, 392 calculation of, 333, 337 correction factor, 324 index, 391 measurement in medicine, 271, 284-6

Absorption bands, 303 of ionising radiation, 153,172 mechanisms of radiation absorption, 151 reader, 155 self-, 87 ,182,183,184

Accessory instrumentation, see Instrumentation

Accidental exposure, see Exposure Accident dosimetry, see Dosimetry Accumulated activity transfer, see Activity

transfer Accuracy, 123, 149-50, 171, 193

definition, 143 improvement, 68

in supralinearity range, 84 in low dose measurements, 150 of TL-measurements, 149

Acetone, 69 Activation, 252-3

determination, 24-5 energy, 24, 29, 91

Activator, 239, 253 Activity, 392

transfer, accumulated, 333 Actual fading, see Fading Additivity rule, 87 Adhesives, TL of, 278 Address list of instrument manufacturers, 64-5 Afterglow, 4 AgBr, see Silver bromide Ag° centres, 153 Ag* centres, 153 Age determination, see Dating Age equation, 347-8, 363-4 Ageing effect, influence on PM gain, 49 Aggregates, 13 Air, 126

hot, 49 reference medium, 86

Air-crafts, mean exposure levels in, 224 measurements in, 224

Albedo dosemeter, see Dosemeter calibration, 190 choice and use, 249-50 effective response, 190 response, 249 schematic diagram of types, 248 single sphere, 189-90

Albedo dosimetry, 190 Albedo factor, 189 Alcohol, 69, 242, 243 Alkali halides, 12 ,14 ,155 ,163 A1,03, see Aluminium oxide Alpha converter, 160 Alpha particles, 151, 235, 236 Alumina, 118

sensitivity to neutrons, 121 special emission, 119

Aluminium oxide ,118-21,311 dose characteristic, 120 dosimetric properties, 119-20 dose range, 170 effective atomic number, 170, 274 energy dependence, 87, 117 fading, 173 half-lives of peaks (trap levels), 119 high photon dose measurements, 310 glow-curve, 118 history, 118 main glow, 170 neutron dosimetry with, 244 peak temperature, 119 physical properties, 118, 170 effect of proton radiator, 244 regeneration, 119 sandwiches, 244 triboluminescence, 120

AlokaCo.Ltd ,64 ,76

394 Index

Alpha-factor, 349 Alpha-particles, 236,238 Ambient temperature profile, 210 American National Standards Institute Inc.

(ANSI), 168,197,198,199 Amplifier, signal, 49-50 Analysis of TL and TSC data, 32-6

phenomenological, 18-21 Angle of incidence, 244, 250, 251 Angular response, 169, 177 Animal habit studies, TLD-application for, 295 Animal experiments, 289-90 Anneal, automatic, 56, 76-8, 93 Annealing, 169,175,193

fast, 136-7 furnaces, see Furnace isothermally, 78 of Teflon dosemeters, 175 post-irradiation, 168,208,209,210 pre-irradiation,40,134,174,188 post-read (out), 40,134-8 pre-read(out), 40, 42, 94, 130-1 procedures, 93-4 repeated, 205 standard technique, 100,134, 136 stands, 77-9 temperature, 135 treatment, 188

Anomalous fading, see Fading ANS-standard for environmental TLD-systems,

198 ANSI, see American National Standards Institute

Inc. Applications of TLD, 182-92 Archaeological dating, see Dating Argon, 126, 213

flow-rate, 215 Argon-41, 225 Arrhenius equation, 16 ATLAS, 56,58 Atomic number, effective, 88 Auger recombination, 17 Austrian Atomic Energy Research Organisation

Ltd, TL-dosemeter, 179 Authenticity testing, 368 Automatic anneal, see Anneal

control system, 51 instruments, 53 ,178,192

residual reading, 176 performance, 180

Automation, 52-3 , 59, 168, 171 Automated reader, see Reader Automatic control system, 51 Automatic system, residual reading, 176 Average dose equivalent, 167 Average man, 284

Background fluctuations, 221,222 increase, 212,214 component, elimination, 221 dose, local individual, 223 measurements, at Karlsruhe, 224 radiation, 197, 220,223,224, 225 energy spectrum, 224 reduction, 70 signal, 44 ,50 , 146

Backscatter, 172 ,189,245,247,251 Badge, 59,169, 184,189,190

boron loaded plastic, 190 handling, 53 multi-detector, 167 shielding, 186

Band band-band recombination, 18 gap,18,153 model, 18-21, 152

Bar-code, 179 Basal layer, 185 Basic dosemeter, see Dosemeter Batch-to-batch variation of TL-response to

thermal neutrons, 233 Batch history, 219

sensitivity variation within, 172 uniformity, 173 ,198,199,201,202, 217,

219 BCD, see Binary code decimal

identification, 179 punched hole identification, 60

Barium tungstate, TL of, 33-6 BaWO„, see Barium tungstate Beam hardening, 275

uniformity checks, 271 Belt dosemeter, see Dosemeter BeO, see Beryllium oxide BeO : Na, see Beryllium oxide Beryllium oxide

BeO, 8,109-11, 152,234,241 annealing procedure, 93 ceramic, 158 see also Thermalox chips, 184 discs, beta-ray energy response, 89 dose range, 170 dose rate dependence, 95 dosimetric properties, 110, 311 effective atomic number, 110, 158, 170 emission temperature, 92 energy response, 87, 88, 172, 184 fading, 92, 110,173 glow-curve, 110 high photon dose measurements with,

309-10 history, 110

Index 395

Beryllium oxide (continued) LET response, 111 light emission, 110 light sensitivity, 110 linearity range, 84 main glow, 170 neutron sensitivity, 90-1, 111, 234 peak half-lives, 92 preparation, 110 properties, 170

physical, 110 relative response as a function of LET of

charged particles, 108 reproducibility, 94 response curve, 110 sensitivity to fast neutrons, 91 supralinearity ,110,310 thermally stimulated exoelectron emission,

158 thermal neutron response, 234 tissue equivalence, 110 toxicity, 139 TSEE curve, 159 BeO:Na, 110

neutron response, 241 Becquerel, Henri, 3

unit (Bq), 391 Beta-radiation dosimetry, 88-9 ,182-4 ,186 ,

193,265 Beta-rays, 167,182,184,253

response to, 88 Biochemistry, TL-application in, 293 Bimolecular kinetics, sec Second-order

kinetics Binary code decimal (BCD), 179

identification, 179 Biological effects, 230 Biology, application of TL in, 289-95 Black-body radiation, 239 Block-heating, 61, 62 Body-dose, see Dose Boltzmann factor (constant), 16, 91 Boron-loaded plastic badge, see Badge Boron shield, 189 Boyle, Robert, 3 Bones, dating of, 353 Bone dosimetry, 290-1 Bq, see Becquerel, unit Bracelet dosemeter, see Dosemeter Bragg-Gray principle, 86, 337-8 Bragg reflections, 18 Built-in heater elements, sec Heater elements Build-up, 175

factors, measurement of, 317, 320 Bulb dosemeter, see Dosemeter Burlin theory, 338-9

Cadmium shield, 189, 236, 250 Calcareous deposits, dating of, 354 CaF,,CaF2:nat.,CaF2:Mn,CaF,:Dy,CaF2:Tm,

see Calcium fluoride Calcium fluoride, CaF2, 4, 7, 70 ,111-5 ,170 ,

254 dosemeters, shielded, 224 history, 6 effective atomic number, 117,170 energy dependence, 274

(response), 87,172 fading, optical, 129

systems reproducibility, 202 thermal neutron response, 234

Calcium fluoride, CaF,: Dy (TLD 200), 8 ,188, 199,252

annealing procedure, 93 characteristics, 199 directional dependence, 128 dosemeters, 186, 206 dose range, 170 dosimetric properties, 114-5 emission temperatures, 92 energy dependence, 207 fading, 92 ,114 ,128 ,129 ,173 ,199 , 208,

211,266-7 fading reduction, 209,210 glow-curve, 114 high-dose properties, 188 linearity range, 84 lower exposure limit, 199 main flow, 170 multi-element dosemeter, 178 neutron activation reactions, 252 neutron sensitivity, 115, 234 neutron separation, 253 peak half-lives, 92 over-sensitivity reduction, 205 physical properties, 114 precision, 123 reproducibility, 174 relative response at maximum, 188 response to 9 MeV photons, 187 self-irradiation, 199 supralinearity, 115, 188 Teflon, fading, 208 radiation damage, 188 residual dose, 188 response change, 188 TLD-200,188,199,252 sensitising, 114 thermal neutron response, 234 thermal neutron dosimetry with, 252 uniformity, 123 wavelength of TL, 114

396 Index

Calcium fluoride, CaF,: Mn (TLD-400), 4, 113-4 ,243 '

annealing procedure, 93 beta sensitivity, 182 characteristics, 199 dosemeter, 206 dose range, 170 dosimetric properties, 114, 311 emission temperature, 92 energy response, 172 fading, 92, 113, 173, 199 fast neutron response, 243 glow-curve, 112, 113 heat-rate influences, 134 high photon dosimetry with, 308 linearity, 84, 113, 114 lower exposure limit, 199 low temperature peaks, 131, 132 main glow, 170 neutron activation reactions, 252 neutron response, 114, 243 neutron separation, 253 peak half-life, 92 powder dispensing, 73 preparation, 113 properties, 170

physical, 113 relative energy response, 275, 276 relative response at maximum, 188 reproducibility, 308 residual dose, 188 saturation, 113 self-irradiation, 199 sensitivity, 113 supralinearity, 188, 308 Teflon dosemeters, 113, 244

high dose properties, 188 neutron dosimetry with, 244

Teflon sandwiches, 244 thermal neutron sensitivity, 20, 234 wavelength of TL, 113

Calcium fluoride, natural, CaF2: nat., 3, 4, 6, 111-3,245

characteristics, 199 close range, 170 dosimetric characteristics, 112 emission temperatures, 112 fading, 199 frequency factors, 112 glow curves, 111-2 half-lives of trap levels, 111 history, 111 impurity activators, 111 light sensitivity, 112 linearity, 112 lower exposure limit, 112, 199

main glow, 170 neutron dosimetry, 244, 252 neutron sensitivity, 113, 234 peak stability, 111 properties, 170 sandwiches, 244 self-irradiation, 199 sensitivity, 112 thermal neutron response, 234 trap depths, 111-2 UV stimulation, 111 wavelength of TL, 112

Calcium fluoride, CaF,: Tm (TLD-300), 253 fast neutron response, 264 glow-curve, 203 traps, 261

Calcium sulphate, CaS04, 6, 115-8, 239-40 colour of TL light, 70 effective atomic number, 117, 170, 274 energy dependence (response), 117, 172 fading, 211

optical, 129 history, 6, 115 main glow, 170 multi-element dosemeter, 178 neutron dosimetry with, 239-40 neutron sensitivity, 117 systems, reproducibility, 202 thermal neutron response, 234

Calcium sulphate, CaSO„: Dy, 8, 115, 243 background, 214, 215 activation energies, 116 annealing, 77 beta-sensitivity, 182 characteristics, 199 dosimetric properties, 116-8, 311 embedded in p-sexiphenyl, fast neutron

response, 243 emission temperatures, 116 energy response, 87 fading, 116, 173, 174, 199, 207, 211, 239 fast neutron response, 243 frequency factor, 116 friction, 213 glow-curve, 116,213,240 half-lives of traps (peaks), 116 high photon dose measurements with, 310 intermediate and fast neutron dosimetry,

243 light influence, 213, 214

sensitivity, 116 linearity, 117 lower exposure limit, 199 neutron activation reactions, 252 neutron dosimetry with, 252 neutron separation, 253

Index 397

Calcium sulphate, CaS04 : Dy (continued) preparation, 115 properties, 170

physical, 116 regeneration, 117 relative response at maximum, 188 residual dose, 188 self-irradiation, 199 sensitivity to 7-rays and charged particles,

239 supralinearity, 188, 310 Teflon dosemeters, 115

fading, 207 high dose properties, 188

thermal neutron response, 90, 234, 239 trap depths, 116 wavelength of TL, 116

Calcium sulphate, CaS04 : Mn, 6, 7, 115 basic work on, 6 fading, 92, 115, 173,239 glow curve, 112, 115 mixtures with compounds of high thermal

neutron capture cross section, 239-40

powder dispensing, 73 properties, 115, 170 quantitative measurement of UV radiation, 6 sensitivity, 115 thermal neutron response, 233, 234, 239

Calcium sulphate, CaS04 : Sm glow curve, 116 glow peak, 115 fading, 115 light sensitivity, 115 neutron spectrometer, 244, 245 sensitivity, 115 wavelength of TL, 115

Calcium sulphate, CaS04 : Tm, 115 dosimetric properties, 116-8, 311 energy response, 87 fading, 173, 174,239 glow curve, 116 high dose properties, 188 high photon dose measurements with, 310 light sensitivity, 116 linearity, 117 neutron dosimetry with, 117, 243, 244 neutron energy dependence, 241 multi-element dosemeter, 178 physical properties, 115 preparation, 115 properties, 170 radiation damage, 188 regeneration, 117 relative response at maximum, 188 reproducibility, 174

residual dose, 188 response change, 188 sandwiches, 244 sensitivity to 7-rays and charged particles,

239 supralinearity, 188, 310 Teflon dosemeters, 115 thermal neutron response, 234, 239, 240 wavelength of TL, 116

Calibration, 39, 51 , 58, 70, 79, 120, 149, 150, 169, 171, 180,186, 190, 198, 201, 210,213-8,214,215

Calibration detector (dosemeter), 138, 220 Calibration, individual dosemeter, 217

factor, 56, 149, 171,217 field, 190, 192 internal, 217 source, 58, 168 techniques, 198

Calibrators, 79-80 Capacity, thermal, 40 Capture, 230, 239, 252 Capture cross section, 235 Cascades, 15 Cathodoluminescence, 4 Cathode sensitivity, optimal, 49 Cavity theory, 86, 89, 275, 337-40 CEA, see Commissariat a l'Energie Atomique CEC recommendations for personnel TLD

systems, 168 Cellulose nitrate, 159

detector, 160 Central Research Institute for Physics, 64 Ceramics, 152

dating of, 352 Charge, 48

carriers, lifetime, 27 digitiser, see Digitiser neutrality, condition of, 20

Charge-to-pulse converter, 51 Chemical effects, 230 Chemi(o)luminescence, 4, 39, 42, 86, 126

reduction, 45,126 Chips, 167, 171, 180,200,212

embedded in Teflon, 178 extruded, 67 handling, 74 hot pressed, 58, 171

Ci, see Curie Cleaners, ultrasonic, 75-6 Cleaning, 212

procedures, 138-9 Clinical measurements, 271-87 Coalmine dust, determination of fibrogenic

properties, 292-3 C02-cooling, 76

398 Index

Codabar®, see Bar code Coefficients of variation, 123 Collective dose, 271 Coloration, 152, 162

during anneal, 78 Colour centres, 155 Colouring, 155-6 Comitato Nazionale Energia Nucleare (CNEN),

185 Commissariat a l'Energie Atomique, 178 Commission of the European Communities,

168, 169,198,200 Compressed LiF TLDs, preparation, 98 Compton effect, 15, 273 Computer memory, 260

on-line, 168, 171, 190,261 simulation, 30-2

Conduction band, 15, 16, 18, 152, 153, 157 Conductivity, thermal, 40 Contamination of phosphor surfaces with dirt,

125,138 organic, 69 influence on TL read-out, 212

Control dosemeters, see Dosemeters system, automatic, 51

Conversion factors, 131, 231 of SI units, 391

Converter output, 51 Cooling, 46 Cooling-down period, 69 Combustion phenomena, 69 Computer, 52-3, 180,269 CON-RAD, 72 Converters, 50 Cooling-rate, 135 Corindon, 118, 119 Corrections, 172, 192, 199 Coulomb interaction, 151 Counter, 51 CNEN, see Comitato Nazionale Energia

Nucleare Compensation shield, 206 CR-39,160 Critical organ, 167 Cross-contamination, 74 Cross section, 16 Crystal, 171 Curie, 392 Current, 48 Current leakage, influence on dark current of

photomultiplier, 50 Czochralski method, 98

Dai Nippon Tokyo Co. Ltd, 64 Damage, see Radiation damage Damage, effect, 212

Dark current, 49, 50, 170, 200, 204 fluctuations, 175 long term variation, 178 standard deviation, 219

Data analysis, 32-6, 225, 261 processing, 180, 190 recording, 180 security, 192 verification, 269

Dating, 39,347-81 accuracy, 365 apparatus, 361 applications, 352-4 archaeological, 352-4, 365-7 basic principle, 347 complications, 348-9, 361-81 environmental uncertainties, 379-9 geological, 354 geology, recent, 368 method, 347-52 of bones, 353

calcareous deposits, 354 ceramics, 352 loess, 354 ocean deposits, 354 pottery, 365 rocks, heated, 352-3

volcanic, 354 shells, 353-4 slags, 353 till, 354 volcanic events, 354 zircon, 379-80

radiocarbon, 365 recent geology, 368 techniques, 350-1 fine grain technique, 350, 364 phototransferred TL technique, 351 pre-dose technique, 351, 371-2 subtraction technique, 350, 378 quartz inclusion technique, 350, 364-5,

377 transfer technique, 373 zircon inclusion technique, 350-1, 379-80

D A Pitman Ltd, see Pitman Defect creation, mechanism, 14-6 Defects

electronic, 11 ionic, 12 in crystals, 16 surface, 154

Density, optical, 152 Depth-dose, 182, 184

distribution, 187, 280 in tissue, 182

Desmarquest & CEC SA, 64, 97

Index 399

Detection limit, lower, 44, 175 threshold, 86

Detector material specific requirements, 170 handling, 138-9 self-shielding, 233 tissue-equivalence, 187

Detectors, loose, systems for evaluation, 54-9

Diagnostic radiology, measurements, 271-2, 284-6

Diamond spars, 118 Dielectric measurements, 103 Digitiser, 51,260, 261 Dimers, 103 Dipole, 103

coupling, see Dimers Directional dependence, 199, 205 Direction of incidence

see Radiation incidence Directly ionising radiation

see Radiation Dirt, influence on TL-readings, 75, 212 Disc, handling, 74

memory, see Memory Dispenser, 71-4

flat-plate, 72 handling, 73-4 malfunctioning, 74

Dissemination of standards, 383 Discharges, influence on background signal,

50 Directional dependence of TL-phosphors, 128 Donor-acceptor recombination, 19 Dose

body-, 167 calculation, 53 depth, 184 equivalent, 167, 177, 190, 231, 245, 246,

247,392 extremity-, 167,182-4 finger-, 184 hand-, 184 profile, measurements of, 225 range, 169 reassessment, 168 residual, 44, 191 skin-, 167, 177, 182 surface absorbed, 167 threshold, 86 whole body-, 167, 177, 182 zero-, 86

Dose equivalent, 167, 231, 391 Dose limit

lower, 86, 170 upper, 85-6

Dosemeter albedo (neutron), see also Albedo dose-

meter, 189-91, 192, 193, 247, 251, 264

badges, commercial, 177-8 basic, 167, 177 belt, 190 bracelet, 184 bulb, 46, 47, 58, 186, 200, 206 calibration, see Calibration card, 171, 180 control, 205, 207, 213, 215, 216, 220 curling, prevention of, 68 discriminating, basic, 168, 177 extremity, 167,168, 185,193 field, 215,216, 383 identification, automatic, 180 intercomparison, see Intercomparison film, 5,157 finger, 183,184,185 hand,185 loss, 180 multi-element, 177-8, 182 non-discriminating, basic, 168 ring, 183,184,185 scintillation, 152, 163 solid state, 8, 162 systems, personnel, 177-81

practical, 177-81 re-assessment, 176 tandem, 291-2 three-element, 183

Dosemeters (TL-) advantages over ionisation chambers, 272 choice for clinical use, 273 in medicine, 273 properties, 170 recommended performance, 169 types of, 168, 179 thin, 184 tissue equivalent, 124, 182

Dose rate, 170 assessment of for dating, 376-7 natural, 351-2

Dose-rate dependence, 95 Dosimetry

accident, 90, 191-2,253 beta, 182, 193 see also Beta-radiation dosimetry environmental, 177 extremity, 177, 182-4, 193 film,167, 168 integrating, 3 military, 39 patient, 39 peak of LiF, 99

400 Index

Dosimetry (continued) personnel, 111, 167-95, 237, 247, 248,

249,250,251 application of TLD in, 167, 195

photon, 184 requirements, 123 solid state, 152 routine, 71 ,95 , 149 therapy, 58, 253 thermoluminescence, 5, 123

Drawer assembly (system), 42, 43 for glass-bulb TL-dosemeters, 47 mechanism, 53

Dust, 49

Eberline Instrument Corporation, 65, 72 Ecology, TL application in, 293-4 Edgerton, Germeshausen & Grier, Inc., 114 Effective atomic number, 88 Effective dose equivalent, 167 Effects used in solid state dosimetry, 152 EG&G, see Edgerton, Elastic collisions, 14-5 Electrical conductivity, 20 Electroluminescence, 4 Electrometer amplifier, 259 Electron, 151 Electron-hole

pairs, 15, 152 holes, 102 traps, 102 equilibrium, 86-7, 278

Electron paramagnetic resonance (EPR), 33 Electronic processes, 14-5 Electronic circuitry, malfunction of, 52 Electronics, logarithmic, 260 Emission, infrared, 175

spectra, 236, 239 Emissivity of planchets, 69 Encapsulation, 198 Energy band

diagram, 152 model, see Band model compensation (filter shields), 198, 205, 206 components in radiation field determina­

tion, 88 dependence, 87, 162, 171-2, 199, 205, 224 fluence, 230 gap,18 level diagram, 19 levels, 18 range, 169, 170 response, see also Response, 86-7, 169

calculation, 86-7 examples of, 87 of TL materials to photons, 86

modification by filters, 88 spectrum, neutron, 230 states, 18 traps, see Traps

Entrance absorbed dose, 271 measurements, 282

Environmental TL dosemeters, 206 dosimetry, 39, 58 intercomparison, 387-8 effects on TLD signal, 212-3 factors (parameters), 169, 198, 275 measurements, interpretation, 222 monitoring, see also Monitoring, 197-227,

331-2 programmes, 221

Epoxy resin, 244 EPR, see Electron paramagnetic resonance Epidermis, basal layer, 167 ERDA, see United States Energy Research and

Development Administration Error, parameters, 198, 220

overall, measurement, 198, 199 sources, 124-41,145-6, 218

Errors avoiding of, 168 due to detector, 145-6 due to presence of neutrons, 90 due to reader and evaluation procedure, 146 due to thermal treatment, 146

Escape coefficient, 19 probability, 16

Etchants, 160 Etching rate, 160 EURATOM, 168, 171, 172,177 European Community, see Commission of the

European Communities Evaluation

second, 176 manual, 180

Exit absorbed dose, 271 measurement, 282

Excitation (thermal), 11, 16-8, 152 Excitons, 15 Exoelectrons, 157

emission, 152, 157 Expected lifetime, 373 Exposure, 392

accidental, 167 interpretation, 198

Extremity dose, dosimetry, see Dose, Dosi­metry

Extruded TL materials, 171 ribbons, see Ribbons

Fachverband fur Strahlenschutz e.V., 169, 200

Index 401

Fading, 40, 44, 91-2, 128-30, 162,168, 169, 170, 172-3,174, 198,199,200, 205-11,214,349

actual, 207 anomalous, 349, 373 at70°C, 211 characteristics of some TL materials, 173 correction, 210-11, 266-8

factors, 129 due to humidity, 215 experiment, 207 film-, 156 improvement of fading characteristics, 210 light induced, see also Fading, optical,

128-9 optical, 93, 128-9 rate, 210, 211 real effect, 210 reduction, 209, 210

Fast annealing procedure, 136 Fast neutron detection, see Neutrons Fast reactor core, measurements, 321 Fast test reactor mock-up, shield tests, 323 Fatigue effect, influence on PM sensitivity, 49 Fault recognition, 269 F-centre, 13, 103, 155 F'-centre, 13 Fy^-centre, 13 Field application, 198

calibration, see Calibration cycle, 198 dosemeter, see Dosemeters exposure, 211

calculation, 216 interpretation, 220-1 period, 198

location, 210 Filament, 46 Film, 152,156

badge, 177 dosimetry, 167, 168 dosemeter, 5, 157 response to 7-rays, 156

Filters, 49, 88, 124, 177, 184, 186,187, 205, 224

Fine grain dating technique, see Dating Finger dose, see Dose

ring dosemeter, see Dosemeter First-order kinetics, 21 Fission foil, 245 Fluence, 230, 241, 245, 247, 251, 252

rate, 230, 232 to dose equivalent conversion factor, 231

Fluorescence,4 Fluorescent foils, 5

light, 213, 214

effect on TL-phosphor, 213, 214 Fluorescent x-rays, 87 Fluorite, see Calcium fluoride, natural

Wblzendorfer, 7 Franck-Condon principle, 17 Frenkel defect, 14 Frequency factor, 17, 24, 29, 373

determination, 24-6 temperature dependence, 26 plot for TLD data analysis, 225

Friction, effect of on TL-response, 212, 213 Furnace, 40, 76-7, 137 Future trends

in personnel dosimetry, 192-3 in neutron dosimetry, 253-4

Gamma, build-up factors, see Build-up factors radiation detection in presence of neutrons,

see also mixed fields, 90 dosimetry in reactor-shields, 316-7 heating, measurements in an unknown

spectrum, 324 equivalent dose reading, 192 photons, 151

Gamma-ray rejection, 261 Gas dryer bottle, 69

flow meter, 69 rate, 134

flushing, 42, 69-70, 126 influshing, influence on background, 215 heating, 45 inert, 42, 212

Geological dating, see Dating Geology, recent, dating, see Dating GesellschaftfurStrahlen-undUmweltforschung

mbH Munchen (GFS), 185, 206 Glass bulbs, 46

bulb dosemeter, see Dosemeter dosemeter, dosimetric properties, see

Phosphate glass Glow curve, 11, 17 ,40 ,50 ,83 ,213 ,233 ,236 ,

237 analyses, 41 , 189, 191,259-69 dependence on read-out atmosphere, 126 digitised, 52, 261 for natural TL, 362 interpretation, 260 linear, 259 logarithmic, 260 measurement, 7 plotting, 52, 51, 54 recording of, 259-60 shape, 168 storage, 260-1 TL and TSC, calculated, 29-30

402 Index Glow, peak, 83

transient, 3 Glucose, 243 Gold foil, 236 Grain size, 243, 253 Gray, unit, 230, 391 S-shift, 33 Gy, sec Gray

Half-intensity, 26 Half-life, 252

of fading, 91 for different phosphors, 92

Half-width, 26, 32 Halide ions, 12 Hand dose, see Dose

dosemeter, sec Dosemeter Handling of detectors, 138-40, 193, 212 Harshaw, 65, 72, 80, 97, 111, 114, 138, 171,

177, 178, 179,199,206,217,261 automated TL analyser system, 56, 60, 126,

180 performance, 180

automated personnel monitoring TLD system, 60

microprocessor controlled TLD system, 60-1

digitised glow-curve displayer, 261 TL analyser, 54, 269

H-centre, 14, 27 Health Physics Society Standards Committee

(HPSSC), 168 Health risk, 140 Heat cycle, 41 , 51, 55

testing, 51 Heater

built-in heater element, 47-8 block method, 44-5, 61 device, 39 element, 43, 49, 175

built-in, 47, 69 infrared emission, 86

hot gas, 45 optical, 46 planchct, see Planchet RF-heating, 46

Heating, fractional, 33 linear, 41,238 medium, 40, 45 method, 42, 44 multiple plateau, 42 non-linear, 41 optica], 46-7, 64, 184 planchets, see Planchcts plateau, 233, 238

power, 42 programmable, 42 ramp, 233 rate, 21,134, 298

effect on TL output, 134 influence on peak position, 50-1 fluctuations, 134 hyperbolic, 22

RF-, 46 system, 40-8 technique, fractional, 33

Heist KG, 185 HFS, see Hyperfine structure High dose properties of TL-phosphors, 188 High dose range, 187, 191 High-energy radiation, 275, 320-1 High-exposure, 176 High-level photon dosimetry with TLD-

materials, 297-313 High temperature anneal, 94 High-tension variations influence on PM

sensitivity, 50 History of TL, 3-9 Hodgkin's disease, 283 Hole centres, 33 Hopping process, 34 Host, 253 Hot finger, 44 ,45 ,60 , 178 Hot gas TLD reader, 53, 56, 94 Hot gas heating, 45, 178 Hotplate, 178 Hot pressed TL-materials, 171 HPSSC, see Health Physics Society Standards

Committee standard, 169

Human phantom measurements, 284-6 Humidity, 129

detector, 170 effect on TL read-out, 212, 275-6

Hyperfine structure, 13

Identification, automatic dosimeter, 180 1CRP, see International Commission on

Radiation Protection ICRU, see International Commission on

Radiation Units and Measurements Imperfections, 11 Implantation of TL dosemeters, 139, 275 Impurities, 11 Impurity ions, 16 Incandescent light, 69 Incidence of radiation, 167, 170, 205 Indirectly ionising radiation, 151 Individual spared organ absorbed dose measure­

ments, 282

Index 403

Induced activity, 252 Inert gas atmosphere, 42

flushing, 69, 95 Inflection temperature, 26 Influence quantities, 170 Infrared emission (radiation), 49, 175

emissivity of planchets, 69 heating, see Heating, optical sensor, 44

Initial rise method, 26, 32 Institute of Radiation Protection and

Dosimetry, IRO, CNEN, Brasil, 206 Institute of Radiation Protection, Helsinki,

177 multi-element cards, 179

Instrumentation, 39-66 Instrumentation accessories, 67-81 Instrument AB Therados, 65 Instruments

automatic, 53 stability checking, 20

Integral method, 134 Integration interval, 42

of TL-signal, 41 Intercept correction, 348 Intercomparison, 383-91

direct methods, 384 experiments, 197 international, 389 mailed, 386 of 60Co-teletherapy units, 280

Interlaboratory comparison, 197, 200 test programme, 169, 200

International Commission on Radiation Protection (ICRP), 167

International Commission on Radiation Units and Measurements (ICRU), 383

International Organisation for Standardisation, 169

Intermediate and fast neutron dosemeters, 241-53

Interpolation method, 325-7 Intracavitary absorbed dose measurements,

271,282,283-4 Interstitials, 12, 14 Intrinsic response, 176

per absorbed neutron, 340-3 In-vivo dosimetry, 58, 271, 281-2, 286, 290 Ionic conductivity measurements, 103 Ion implantation, 253 Ionisation, 152, 153

chamber, 225 Ionising radiation,

directly and indirectly, 151 detection by TL, 4

IR-heating, see Heating, optical

Irradiation geometry, effect of, 127 history, 176, 188 of phosphors, temperature effects, 127

Irradiator, 79-80 automatic, 80 reference dose-, 79

ISO, see International Organisation for Standardisation

Isodose chart, 279 Isomet, optical grade LiF, 302-3

Jahn-Teller effect, 13 J-131 in thyroid, half-life measurement, 333

Kapton (tape), 44 trays, 80

Kerma, 230, 231,391,392 Kinetic balance, 20 KOH, see Potassium hydroxide Ar-value (factor), 348-9

Landauer, R S Jr & Co., nuclear station film badge, 179

Latent dose information, loss of, see also Fading, 93, 212

Latent track, 160 Law of detailed balancing, 16 Lens systems, 44, 125 LET, see Linear energy transfer

influence on supralinearity, 84-5 Lethargy, 231-2 Liberation rate, 20 LijB407: Ag, see Lithium borate Li;B407: Ag, Cu, see Lithium borate LLB40,: Cu, see Lithium borate LL,B407: Mn, see Lithium borate Li2B407: Mn, Si, see Lithium borate LiF, see Lithium fluoride LiF : Mg,Ti, see Lithium fluoride LiF : Na,Mg, see Lithium fluoride Lifetime, 21,29 Light collecting system, 49 Light dependence, 198, 199, 212

detecting system, 39, 48-52 detector, 49 effects, 212 emission, non-radiation induced, 94 sensitivity, 170, 200

Light source, 49, 51-2, 55, 70-1, 124-5, 146, 217

check,219 emission spectrum, 124-5 errors due to, 125 external, 70-1

long term fluctuations, 218

404 Index Light source (continued)

internal, 58,217 and external, daily check with, 21 7

plastic scintillator, 70, 217 temperature dependence of light

intensity, 217 properties, 217 reading, relative standard deviation, 219 temperature effects, 218

Light stimulation, 175, 212 Light sum (integrated), 6, 51 , 120, 298 Light yield, 87 Linear energy transfer (LET), 231, 233, 236,

241 Linearity, 83

ranges of, 84 Linearity curve,

example of, 84 Literature on TL and/or TLD, 80-1 Lithium borate, graphite mixed, 182

tissue-equivalence, 184, 205 systems, reproducibility, 202

Lithium borate, LLB.,0,: Ag,Cu, 170, 171,178, 184,238,239

Lithium borate, L i ^ O , : Ag, 107, 238-9 sandwiches, 244

Lithium borate, Li,B40,: Cu, 108-9 dosimetric properties, 108-9 physical properties, 109 preparation, 109

Lithium borate, Li,B407: Mn, i'TLD-800), 8, 106-9, 1*70, 188,238-9

annealing procedures, 93 characteristics, 199 chips, 184, 186 colour of TL-light, 49, 70 diagnostic measurements with, 284 disadvantages, 307 dosemeters, energy response, 107, 183, 206 dose range, 170 dose-rate dependence, 95 dosimetric properties, 107, 311 effective atomic number, 170 emission temperatures, 92 energy dependence, 87, 107, 183, 171-2 fading, 92, 106-7, 173, 174,211,212,215,

238 reduction, 210

glass dosemeters, 171 glow curve, 106, 239, 307 high photon dose measurements with,

306-7 history, 106 humidity influences, 129-30 in-vivo measurements with, 286 isotropically enriched, 239

light sensitivity, 107 linear range, 84, 239 lower exposure limit, 107, 199 main glow, 170 minimal detectable dose, 107, 199 medical application, 272 phosphors

neutron dosimetry with, 238-9, 244 neutron sensitivity, 90, 107-8, 234-5

peak half-lives, 92 physical properties, 106, 170 powder dispensing, 73 preparation, 106 properties, 106, 170 regeneration, 107 relative response as a function of LET of

charged particles, 106, 108 relative response at maximum, 188 response to high LET radiation, 106 response change, 188 reproducibility, 174, 307 residual dose, 188 sandwiches, 244 self-irradiation, 199 silicon rubber dosemeters, 106 supralinearity, 107, 188, 307 Teflon dosemeters, fading, 212, 215 thermal neutron sensitivity, 90, 233,234-5 tissue-equivalence, 184, 205, 238, 274, 307 TLD-800, high-dose properties, 188 TL-yield, 102 wavelength of emitted TL light, 107, 239

Lithium borate, L L , B 4 0 , : Mn,Si dosemeter system, energy dependence, 206 multi-element dosemeters, 178 properties, 170 radiation damage, 188 relative response at maximum, 188 residual dose, 188 response change, 188 supralinearity, 188

Lithium carbonate, LijC03, 239, 240 Lithium fluoride, LiF : Cu,Ag,

properties, 170 Lithium fluoride, LiF : Mg,Ti (TLD-100), 8,

97,97-105, 171, 183, 186, 194, 212 ,215,235,235-8

annealing procedure, 93, 174 background, 214, 215 beta-dosimetry with, 265 beta-sensitivity, 89, 182, 224 capture cross section, 235 characteristic, 199 chips, 184, 186 colour of TL light, 49, 70 compressed, 98

Index 405

Lithium fluoride, LiF : Mg.Ti (TLD-100) (continued) cooling-rate influence, 136 damage by neutrons, 238 data analysis, 261 depth dose distribution, 187 directional dependence, 128 discovery, 6 dose characteristic, 103-4 dosemeters, energy dependence, 87, 183, 206

with moderators, 245 dose range, 170 dose-rate dependence, 95 dosimetric properties, 103-5, 311 effective atomic number, 103, 170, 199,274 emission characteristics, 102

temperatures, 92, 102 wavelengths, 49, 102, 236

energy response, 87, 105, 171-2, 173, 184, 186,206

fading, 92, 171, 174, 199, 208, 209, 211, 266-7

optical, 129 reduction, 209,210

fast neutron response, 243 first applications, 6, 97 friction, 213 glass dosemeters, 171 glow curve, 30, 99-101, 112, 136,213,

238,261,262,298 after irradiation at different

temperatures, 128 effect of low post-read-out annealing

time, 136 explanation, 27

graphite mixed, 182 heat-rate influences, 134 high-dose properties, 188 high-energy radiation, 275 high LET radiation exposure, 100 high photon dose measurements with,

298-306 history, 8, 97 light influence, 213, 214 linearity range, 84, 236 lower exposure limit, 199 low temperature peaks, 131, 132 main glow peak, 170 medical application, 272 microrods, 185 model, 102-3 multi-element dosemeter, 178 neutron activation reactions, 252 neutron dose dependence, 236 neutron energy dependence (response),

241,242

neutron dosimetry with, 235-8 neutron sensitivity, 105-6, 189, 234 neutron spectrometer, 245 optical emission spectrum, 49, 236 optical-grade crystal, dosimetric properties,

311 optical-grade powder, dosimetric properties,

311 optical-grade, TL properties, 302 peak half-lives, 92, 102 peak height ratio, 189 peak temperatures, 92, 102 thermal neutron response, 234 physical properties, 99-103 powder, 97-8

dispensing, 73 non-TL-grade dosimetric properties, 311

preparation, 97-9 pre-read-out annealing, 131-2 properties, 99-105, 170 proton radiator effect, 244 PTL, 97 PTL-717, 171 PTL pellets, preparation, 98 PTL, sodium stabilised,

annealing, 100 preparation, 98

radiation damage, 188 reassessment of dose, 176 regeneration, 104 relative energy response, 275, 276 relative response as a function of LET and/

or particle energy, 105 relative response as a function of LET of

charged particles, 108 relative response at maximum, 188 relative response to beta rays, 89 reproducibility, 94, 137, 171, 174, 299-300 residual dose, 188 response change, 188 response to 9 MeV photons, 187 response to fast neutrons, 91 response to slow neutrons, 90, 105 RPL of, 305 reproducibility, 94, 137, 171, 174, 299-300 self-irradiation, 199 sensitivity, 103, 133, 135, 136, 137 sodium-stabilised LiF PTL, 98 supralinearity, 103-4, 188, 236-7, 298-9 systems

relative standard deviation, 203 reproducibility, 202

Teflon dosemeters, 99, 182, 188 discs, 183, 184,185 fading, 208 preparation, 99

406 Index

Lithium fluoride, LiF : Mg,Ti (TLD-100) (continued)

sensitivity to sunlight, 93 therapeutic use, 58

thermal neutron response, 234, 239, 240 tissue equivalence, 103, 205 TLD, 97

compressed, preparation, 98 powder, preparation, 97-8

TL yield, 102 toxicity, 139 trap characteristics, 101-2 trap formation, 103 trap level, half-lives, 102 trap mechanism, 135 trapping centres, 261, 267 triboluminescence, 95 UV influence, 130

sensitivity, 93 variation of response with neutron energy, 242 wavelength of TL, 49, 102,236

Lithium fluoride, LiF : Na,Mg, 178 fading, 209

reduction, 210 high dose properties, 188 radiation damage, 188 relative response at maximum, 188 residual dose, 188 response change, 188

Lithium fluoride, LiF : Tl,Ti, 178 Lithium sulphate, LijS04, 239, 240 Lithium tetraborate, see Lithium borate Loess, dating of, see Dating Log normal probability plot for TLD data

analysis, 225 Long term stability, 204-5 Low dose measurements, 51

accuracy, 150 with TL powders, 74

Low energy photons, see Photons Lower detection limit, 170, 175, 178, 199,

180 ,202 ,204 ,216 ,220 Lowest detectable dose, 223

definition, 200 Low level measurements, 126 Low temperature peaks, elimination, 51 Luminescence

common features, 152 signal, 41 spurious, 69

Luminescent signal, 41

Magnesium, 102 Magnesium borate, MgB407, 109 Magnesium borate, MgB407: Dy(Ti) sintered,

171

Magnesium borate, MgB407: Dy/Tm, 109 annealing, 109 effective atomic number, 109 emission spectra, 109 fading, 109 sensitivity, 109

Magnesium borate, MgB407: Tm, 109 Magnesium orthosilicate, see Magnesium

silicate Magnesium silicate, Mg;Si04 : Tb, 213, 234,

252,253 background, 214, 215 friction, 213 light influence, 213, 214 thermal neutron response, 234

Magnetic recording devices, 260 Manipulator, 56 Mass energy absorption coefficient, 86 Magnetic fields, influence on PM sensitivity, 50 Mailed intercomparison, see Intercomparison Malfunctions of read-out system, 180 Malfunction rate, 192 Mammography, measurements, 287 Manganese sulphate, MnS04, first applications,

6 Mantle therapy, 283 Manual evaluation, 181 Manufacture Beige de Lamps et de Material

Electronique, SA (MBLE), 111, 206 Manufacturers of TLD-equipment, 64-5 Markers, radio-opaque, 282 Maximum permissible dose (MPD), 167, 184,

197 Maximum temperature, 24, 32 MBLE, see Manufacture Beige . . . Measurements, repeated, 205 Measuring techniques, pitfalls, 39 Measuring time, 41 Medicine, dosimetric applications of TLDs in,

271-87 M-centre, 13 Mean (conversion) efficiency, 325 Mean lifetime, 18 Memory, 261 Metal block, 44 Metaphosphate glass, see Phosphate glass Meteorites, TL from, 7

thermal history, 7 Methanol rinse, 76, 139 MgB407, see Magnesium borate Mg2Si04: Tb, see Magnesium (ortho) silicate Microcomputer control, 180 Microprocessor, 52-3, 261 Micro-rod, 67, 78

cleaning, 76 Military dosimetry, see Dosimetry

Index 407

Minicomputer, 52-3, 180 Mirror systems, 49 Mixed beta-gamma radiation fields, dosimetry,

177,183 Mixed gamma-neutron radiation fields, 189,

192 Mixtures of TL phosphors with hydrogenous

components, 242-3 MnSQ,, see Manganese sulphate Mobility, 20 Models, kinetic, 21-4 Moderation, 249, 250 Moderator, 241, 245, 247, 249 Moisture dependence, see also Humidity, 198,

199 effect on TL signal, 212

Monitoring, environmental, 150, 197-227, 205, 210,

214,220,223 performance criteria, and technique, 197 individual, objective, 167 long term, 200 neutron, 190 personnel, see also Dosimetry, personnel,

39 ,53 , 167, 168, 171,177, 184, 191,192

performance specifications, 198 periods, 207 routine, with TLDs, requirements, 171 service, role of, 167, 177, 180, 192

requirements, 171 automatic computerised, 180

systems, personnel, 44 Monomolecular kinetics, see First order kinetics MPD, see Maximum permissible dose Mu-metal screen, 50 Multidetector badge, see Badge Multi-element dosemeter, see Dosemeter Multi-element TLD card, 129 Multiple plateau heating, see Heating Multiplier, see Photomultiplier Multisphere technique, 190

NaCl, see Sodium chloride National Panasonic Matsushita Electric Trading

Co. Ltd, 65, 177, 178, 180, 184 National Panasonic automatic TLD reader

system, 63-4 National Panasonic composite TL dosemeter,

179 National Radioprotection Board (NRPB) TLD

badge, 179 Natural calcium fluoride, see Calcium fluoride Natural dose rate, see Dose rate Natural radiation and TL, 7 N-centre, 13

Neptunium, 245 Neutron, 15, 151,167, 189

albedo, see also Albedo, 245, 247, 248, 249, 250,251

beams, 244, 245, 247 capture, cross sections, 235 detection, 190 detectors, characteristics, 161, 233 dose equivalent, 190

measurement of, 261 doses in mixed y-n fields, 159 dosimetry, 157, 159, 160, 161, 189-90,

327-331 application of TL materials in, 229-57

energy, 243, 244 energy groups, 232 fast, 229, 232, 241, 245, 248, 249, 253,

254 induced defects, 152 induced resistance change, 162 intermediate, 232, 241 monitoring, see Monitoring reaction, 229-30 response of TL materials, 89-91 response, influencing factors, 91 fast neutron detection, 159 spectrum, 249, 252

interpretation, 192 stray radiation fields, 190 thermal (see also slow), 232, 235, 237, 239,

240, 241, 245, 248, 249, 250, 251, 252 Nitrogen, 42, 45, 56, 62, 69, 70, 95, 124, 126,

134,213,217,218 effect on plastic scintillator light sources,

217 Noise, of PM tubes, see PM tubes Non-radiation-induced (signal) light emission,

see Light emission and signal Non-stationary processes, 18 Nuclear facilities, environment monitoring of,

224 Nuclear track detectors, 159

etching technique, 160

Ocean deposits, dating of, 354 OCR-A figure identification, 62 On-line computer, see Computer Operational aspects, 123-141 Operation voltage, see Voltage Operator, 140 Optical density, 152

measurement, 303 fading, see Fading heating, see Heating system, 49

Optimal systems, 221

408 Index

Organic crystals, 163 Orientation in radiation fields, 192 Output devices, 48

writing, 52 Oven, annealing, see Furnace Oxygen effects, 69, 212

Pair production, 15 Panasonic, see National Panasonic . . . Pathway, optical, 51 Patient dosimetry, see Dosimetry Peak area measurement, 51 Peak height, 83

influence of temperature of irradiation, 375-6

Peak height ratio for photons and neutrons, 189

Peak height measurement, 50-1, 134 Peak position, 25 Peltier cooling, 49 Penetrating radiation, see Radiation Penetration depth, 15 Per cent standard deviation, 147 Performance specifications (criteria), 168, 169,

171, 198,199,220 Performance testing pilot study, 169 Performance test, 200

of TLD systems, 181 Personnel dosimeter systems, 59-64, 177-81 Personnel dosimetry, see Dosimetry

monitoring, see Monitoring Person, orientation, 192 Phantom, 190

human, 291 measurements, 271, 284-5 simple geometry, 279-81

Phenomenological analysis, see Analysis Phosphate glass, 6, 153, 225, 308-9

depth dose distribution, 187 dosimetric properties, 311 energy response, 87 fading, 200 response to 9 MeV photons, 187 silver activated, 6

Phosphor characteristics, 199 dispenser, see Dispenser high dose properties, 188 general properties, 170

Phosphorescence, 3 Photocathode, see also Cathode, 49 Photodetector, solid state, 49 Photoelectric effect, 15, 273 Photographic emulsion

properties, 5 plates, sensitivity to radiation, 4 process (effect), 152, 156-7, 162

Photoluminescence, 4 Photomultiplier (tube), 44,49, 107, 110,125,

202,239,260 aging effects, 49 background signal (dark current), 50 current, 50 dark current, 49, 50, 124, 200, 216, 219

fluctuation, 86,175 discharges, 50 dynamic range, 49 fatigue effect, 49 gain, 50 gain drift, 49, 50 high tension variation influences, 50 magnetic field influences, 50 noise, 49 overall sensitivity, 49 response to IR,49 sensitivity, 50

changes, 50 signal, 51 special response, 49, 50, 52 temperature effects, 49

Photon counting, 51 , 253 beam quality determination, 274 dosimetry, see Dosimetry

high level, 297-313 energy spectrum (radiation quality),

determination, 92, 157, 169, 291 interactions, 18

Photons, 15 energy response of TL materials to, 86-8 high energy, 186-7 low energy, 184-6

Phototransferred TL technique, 351 Physicochemical changes, 94 Physikalische Technische Werkstatten Dr

Pychlau KG (PTW), 185 Physikalisch Technische Bundesanstalt (PTB),

168,184 requirements, 168,169, 170, 171,175

Pitman automatic TLD reader, 61-2 Pitman, D A, Ltd, 65, 72, 73, 75, 79, 80, 99,

179,217 Pitman TLD reader, 55, 56 Planchets, 42 ,43 ,49 , 67-8, 125-6, 219

cleaning, 69 emissivity, 69 heating, 42-4 infrared (IR) emission, 44 method, 44 optical properties, influence on TL signal,

125 organic contamination, 69 problems with, 44 reflectivity, 44, 69

Index 409

Plant nutrition studies, 293-4 Plastic scintillator light source, see Light source Plastics, 163 Plateau heating, see Heating

test, 349, 362 Plotter, 52 PM, see Photomultiplier (tube) Polaions, motions of, 34 POLON,65 Polyethylene, 244, 245, 246, 247, 248

pouch, 184,185 Polyimide film, see Kapton Polytetrafluorefhylene, see Teflon® Polyvinylchlorine foils, 212 Positron, 151 Postal service, 168 Post-irradiation annealing, see Annealing Post-irradiation heat treatment, see also

Annealing, 93 Post-read-(out) anneal(ing), see Annealing Potassium hydroxide, 160 Potsherds, collection of for TL dating, 366-7 Pottery dating, see Dating Powder aliquots, 71

dispensers, see Dispenser grains, 88 embedded in Teflon, 171 manipulation, 71, 74 sieving, 75

Precision, 123, 143-50, 168, 169, 214 definition, 143 obtainable with several dosemeters of same

type, 147-9 obtainable with one single dosemeter, 146-7 influence of cleaning on, 139 influence of dose on, 149 parameters affecting, 124

Pre-dose, 154 effect, 348 luminescence, 154 dating technique, 351, 371-2

Pre-exposure, 176, 211 Pre-heating, 209, 210 Pre-irradiation anneal, see Annealing Pre-read(out) anneal, see Annealing Pre-read treatment, see Annealing Pressure reduction valve, 69 Printer, 52 Printing calculators, 52 Primary standards, 383 Programmable heating, see Heating Properties of commercial TLD systems, see

TLD systems, commercial Proton, 151

radiator, see also Radiator, 241, 243, 244, 245

internal, 254 range, 245 recoil, 159

p-sexiphenyl, 243 PTEE, see Teflon®, 40 PTB, see Physikalisch Technische Bundesanstalt PTL 710, 716, 717, 97 PTL 717, 7LiF : Na,Mg, high dose properties,

188 Publication, TL, approximate number, 8 Pulse height analyser, 52 P^'C, see Polyvinylchlorine

Quality factor, 231 Quartz, determination of fibrogenic properties,

292-3 glass discs, 44 inclusion dating, assessment of dose rate, 377 inclusion technique, see Dating window, 110

Quenching of spurious luminescence, 212 thermal, 19

R, see Rontgen Rad, 391 Radiation Detection Company, 65 Radiation absorption, 172

background, 197, 223 damage, 188, 192 directly ionising, 151 incidence, 127, 167, 170, 193 penetrating, 167, 177 non-penetrating, 167, 177, 182 quality, 167, 177,193

Radiationless transition, see Transition Radiative transition, see Transition Radiator, see also Proton radiator, 159

liquids, 243 Radi-Guard, 61 Radioactivity, 391 Radiocarbon dating, see Dating Radiological Service TNO, 65, 185 Radiological units, 391 Radiolysis, 14-5 Radiophotoluminescence (RPL), 6, 152, 153-5

dosemeters, 6 properties, 162

glass for environmental dosimetry, 200 of TL and optical grade LiF, 305-6 reader, block diagram, 155

principles, 154 Radiotherapy (absorbed dose) measurements,

271,279-82 Radon measurements, 159 Ramp heating, see Heating

410 Index

Randall-Wilkins shift, 298 theory, 210

Random uncertainties, 143-4 Range changes, 260

changing method, 259 Ratemeter, 51 , 152 R-centre, 13 Reactor engineering, TLD in, 315-6 Reactor gamma heating measurements, 321 Reader, 39

properties, 219 quality, 202 systems, 53-64 temperature, 170

Readers, 53-64 automatised, 53, 178 main properties, 171-2, 178

Reading, method, 39 residual, 175-6, 188, 191 second, 191

Read-out (technique), 171, 175, 178 automatic, 202 instrument, operation errors, 124-6, 169

reader instability, 124-6 procedure, optimal, 40, 44 process, 132-3

errors, 132-4 second, 176 systems, 174, 233

automatic, 175, 177 performance, 180 properties, 178

time, 51 Re-assessment of dose, see Dose Recoil proton, see Proton range, 241, 243,

244 Recombination, 16, 17, 18, 91

centres, 19, 36, 152,153 coefficient, 19, 21 rate, 20

Recommendations, technical, 168 Recorder, 52

magnetic, 260 strip-chart, 259

Recordings interpretation, 260 strip-chart, 260

Reference dose irradiator, see Irradiator Reference energies for photons, 169 Reference light source, see Light source Reference medium, 86 Reflectivity, 44

changes in, 49 of planchets, 125

Reflector, 58 Regeneration, 168

Relative standard deviation, see Standard deviation

Relaxation, thermal, 16 Rem, 391

meter, 190 Repeated annealing, see Annealing

measurements, see Measurements Reporting, 198 Reproducibility, 44, 45, 48, 94, 173-5, 178,

181, 198, 199, 200, 201, 204, 204, 212,218-20,225

curves, 173, 174 evaluation, 94 frequency distribution, 181 improvement, 72 influence of cleaning, 139 of TLD systems, 197

Residual dose, see Dose Residual dose reading, see Reading Response, energy, 86-8

handling, 52 to beta-rays, 88-9 to neutrons, 89-90 to photons, 86-8 variations, 175

Retrapping coefficient, 19, 21 rate, 20

Re-use of phosphors, 85, 94, 134, 138, 173-5, 168,188

RF heating, see Heating Ribbons, extruded, 40, 167

cleaning of, 138 Ring dosemeter, see Dosemeter Rocks, dating of heated, see Dating Rods, glass-encapsulated, 58 Rod handling, 74 Rontgen, WC,4 Rontgen, unit, 392 Room temperature, influence on readings, 124 Routine dosimetry, see Dosimetry

measurements, 41 RPL, see Radiophotoluminescence

systems, reproducibility, 181 Ruby, 118

Sample changer, 53 preparation for dating, 364 thickness, 88

Sandwiches, of TL-phosphors and hydrogenous plastic, 244-5

SAPHYMO-SRAT, 65 Sapphire, 118 Saturation, 83, 85-6 Scattering of ionising radiation, 151 Schottky effect, 13

Index 411

Scintillation dosemeter, see Dosemeter Second (dose) evaluation, 176, 191 Second-order kinetics, 23 Secondary electronic equilibrium, 198 Secondary radiation, 229

Standards, 383 Sedimentation studies, 294 Selection rule, 17 Self absorption, see Absorption Self-irradiation, 198, 199, 210

shielding (detector), 233 trapped holes, 12, 33 trapped hole model, 33

Semiconductors, 152 Sensitisation and dating, 369-70 Sensitivity,

detector, 49, 201 check, 51-2 factor, see also Calibration factor, 147, 149 loss, 78 reduction by dirt, 138 variations, reasons for, 147, 213 variation within batch, 172 to beta-radiation, 182

Sensors, infrared, 44 Service

centralised, 177 experience, 180

Shells, dating of, see Dating Sherd

collection, 366 treatment, 366

Shrink foil, 185 Shielding, 198 Sievert, unit, 391 Sieves, 75 Silicon diodes, 161 Signal amplifier, see Amplifier

background, 44 conditioning system, 50 non-radiation induced, 42 spurious, 42, 44, 212

creation of, 93 Signal-to-noise ratio, 49, 51 Silicon diodes, 161

fading, 161 Silver phosphate glass, see Phosphate glass

bromide, AgBr, 156 Single-crystal method, 98 Single-sphere albedo dosemeter, see Albedo

dosemeter SI units, 391

unit multiples, 391 symbols, 391

Skin dose, see Dose Skyshine, 224, 225

Slags, dating of, see Dating Sodium chloride, 12 Sodium-stabilised LiF PTL, 100

annealing, 100 preparation, 98

Sodium-stabilising effect, 100 Solidification method, 97-8 Solid state dosemeter, 8

properties of, 162 Solid state dosimetry, effects used in, 152 Solubility of TL detectors, 139 Source fluctuations, 222 Spark counter, 160 Spatial resolution, 271, 278 Specification for TLD systems, 198 Specific energy imparted, 391 Spectral transmission factor, 125 Spectrometer, 244, 247 Spurious TLD signals, see also Signal, 212,

349 Stability, 170

long term, 201, 204-5 of TL dosimetry, 39, 94

Stabiliser, 55 Standard annealing technique, see Annealing Standard deviation, 144, 170, 173, 174, 175,

181, 200, 202, 203, 218, 219, 220 against exposure curve, 219, 220 for the dark current, 219

Standard error, 144 Standardisation, 383-91 Standard,

dissemination, 383 test programme, see Test programme

Sterilising of dosemeters, 278 Stimulated exoelectron emission, see also

thermally stimulated exoelectron emission, 15 7-9

Stimulation, UV light, 175 visible light, 175

Storage of phosphors, 128, 216 Stray radiation fields, 190, 205 Strip-chart recordings, 260 Studsvik automatic TLD reader, 62, 63 Studsvik Energiteknik AB, 65, 79, 178, 180 Studsvik TL dosemeter, 179 Supralinearity, 83-5, 187, 191, 233, 236, 298

and dating, 369-70 function of LET, 84

Subtraction dating technique, see Dating Surface absorbed dose, see Dose Sv, unit, 391 Swiss badge, 179 Swiss Public Health Department TL dosemeter,

179

412 Index

Tandem dosemeter, see Dosemeter Teflon, 40 ,44, 113, 167, 171, 178, 179, 183,

206, 213, 244 disc dosemeters, annealing of, 78 dosemeters

annealing, 175 cleaning, 75

Teledyne Isotopes Inc., 52, 65, 71-2, 77, 79, 99, 115,178, 185

Teledyne Isotopes Automatic TLD reader, 61, 68

Teledyne TLD reader, 54-5, 67 Temperature build-up, 69

effects, 170 influence on PM overall sensitivity, 49 on TL, 275-6

influences on TL read-out instruments, 124, 201

maximum, 41 profile, 55 rise, linear, 41 shift, 27, 29 time profile, 40, 43

Test exposure, 199 methods, 197 procedures, 168, 199 programme, 169,178, 180, 193, 197, 201,

204,210,220 results, 198

Thermal emission, 49 The Harshaw Chemical Company, see Harshaw Theory of TL, 11-38 Therados, see Instrument AB Therados Therados TLD system, 58-9 Therapy dosimetry, accuracy, 140

dosimetry, see Dosimetry machine output, 271

Thermal activation energy, see Activation energy

capacity, 42, 43, 69 conductivity, see Conductivity contact, 40, 44, 45, 46, 48, 77-8, 134 energy, 16, 19 excitation, see Excitation neutron detectors, 232-40 neutron response, 189

Thermally stimulated conductivity (TSC), 11, 17 ,27 ,32

Thermally stimulated exoelectron emission (TSEE), 11, 17, 157

curve for BeO, 159 gas flow proportional counter, 158 sensitivity, 162

Thermally stimulated luminescence glow curve, 12

Thermalox 995, 84, 309

energy response, 87 Thermal quenching, see Quenching Thermal separation, 49 Thermocouple, 43, 44 Thermodynamic equilibrium, 16

model, 16 Thermoluminescence, TL, 3, 32, 39

dosimetry, see Dosimetry glow curve, see Glow curve

Thermoplates, see also Furnaces, 76 Three and four element cards, 179 Three element dosemeter, see Dosemeter Threshold dose, see Dose Thresholds, 252 Till, dating of, see Dating Time from exposure, determination of, 268 Tissue, 182

effective atomic number, 103 Tissue equivalence, 167, 171, 198

equivalent detectors, 171,177, 182, 192,193 directional dependence, 172

of BeO, see Beryllium oxide of LiF, see Lithium fluoride of LijB407, see Lithium borate

TLD, see Thermoluminescence dosimetry and/ or Dosimetry

TLD 100, see Lithium fluoride, LiF': Mg,Ti chips, energy response, 186 sandwiches, 244

TLD 2 00, see Calcium fluoride, CaF,: Dy TLD 300, see Calcium fluoride, CaF,: Tm TLD 400, see Calcium fluoride, CaF2: Mn TLD 600,6LiF : Mg,Ti, 97, 178, 181, 189,

235,237,240 glow curve, 238

TLD 700, 7LiF : Mg,Ti, 97, 178, 181, 184, 186, 188, 189, 199, 235, 237, 252, 253

ribbons, response to 9 MeV neutrons, 187 TLD 800, see Lithium borate, LL,B407: Mn TLD, applications, 182

for animal habit studies, 295 for dating, 347-81 for sedimentation studies, 294 in biochemistry, 293 in biology, 289-95 in ecology, 293-4 for environmental studies, 197-227 in medicine, 271 to personnel dosimetry, 167-95 in reactor engineering, 315-46

TLD badge reading, 59 card, 60 equipment, manufacturers, 64-5 phosphors, see Phosphors

Index 413

TLD badge (continued) method, applications, 39 systems, relative standard deviation against

exposure, 203 systems, commercial,

properties of, 180, 198-213 maximum permissible errors, 169-70 reproducibility, 178

TL dating, see Dating dosemeters, see Dosemeters (TL) efficiency, 102

intrinsic, 102 increase of, 84

intensity, 20, 29 materials, characteristics, 84-96, 199

properties of commercial TLD systems, 198-213

specific requirements, 170 thermal neutron response, 234

maximum shift, 32 measurements, accuracy, see also Accuracy,

149-50 precision, see also Precision, 146-9

natural, 7 output, effect of light on, 128-9 phosphors for high level dosimetry, 311

photon energy response, 274 properties, 170 sensitivity to fast neutrons, 91 sensitivity to slow neutrons, 90

products, preparation and properties, 97-122

powder, see also Powder, 67, 71 publications, 8 reader systems, see Reader systems reader temperature, 170 yield, definition, 83, 102

non-radiation induced, 94 TNO, see Radiological Service TNO TNO automatic TLD system, 62, 63 TNO badge, energy response, 186 TNO hot gas automatic TLD reader, 56, 57 TOLEDO, 55, 56, 217 Toxicity, biological of TL materials, 139

determination, 292-3 Track counting, 160

detection, 152, 159-60 sensitivity, 162 technique, 160

latent, 160 Transfer dating, see Dating Transfer standard, 384 Transformer, 42 Transit exposure, 198, 216 Transition coefficient, 17 Transition probability, 19, 91

Transitions radiationless, 17, 29 radiative, 18

Transparency, 145 Trap (centres), 11, 16, 19, 40, 152, 153, 156,

157,261,362 creation, 84 depth, 17,24, 152-3,373

determination, 375 mechanism, 135 parameters, determination, 24-6

Trapping model, 16 levels, 19

Traps, thermally disconnected, 31 Trap-to-trap migration, 111 Tray, 42, 44

infrared emission, 44 Tribo(thermo)luminescence, 4, 67, 86, 95, 116,

175,212 elimination, 95 of AL03. 120 ofCaS04 :Dy/Tm, 116

Trichlorethylene, 69, 139 toxicity, 140

Trimers, 103 Tritium, neutron-induced, 192 Tritons, 235, 236 TSC, see Thermally stimulated conductivity

data analysis, 32-6 intensity, 29 shift of maxima, 28, 30, 32

TSEE, see Thermally stimulated exoelectron emission

Tunnelling process, 374-5 Turntable, 56 Tweezers, mechanical, 74, 138, 212, 213

manipulation of phosphors with, 212 Tweezers, vacuum, 74-5, 138 Two element card, 179

Ultrasonic bath, 139 cleaners, see Cleaners

Ultra-thin bonded discs, 183 Ultraviolet (UV) light, 253 Uncertainty of measurement, 221, 223

of TLD systems for environmental monitoring, 221

overall, 169,218-20,223 Uncertainties

random assessment, 143-4 systematic assessment, 144-5

United States Energy Research and Development Administration (USERDA), 197

Uranium, 245 UTB, see Ultra-thin bonded discs

414 Index

UV dosimetry, 8 light, effect of, 212 photo-transfer technique, 176 radiation, discovery, 4 re-estimation, 176 stimulation, 175, 212

Vacancies, 12, 13 Vacancy aggregates, 13 Vacuum

needle, 45 ,53 , 56 tweezers, see Tweezers

Valence band, 15, 18, 152 Variance, 144 V,-centre, 155 Vj." centre, 13 V^-centre, 27

in alkali halides, model, 12 ,27 ,33

VEB RFT-Messelektronik, 65 Vibrator, 71

time-setting, 73 Victoreen Instrument Division, 65, 206 Victoreen TLD reader, 58, 59 Visible light stimulation, see Light stimulation Volcanic events, dating of, sec Dating

V/O 'Licensintorg', 65 Voltage, 170

Wavelength, 49 Whole-body dose, see Dose Window, 182, 183, 184, 193 Wblsendorf fluorite, 7

X-rays, 151, 177,237,253 discovery and TL, 4, 237

X, t (X, Y) recorder (plotter), 52

Yield, see TL yield

Z-centre, 103 Z2-centre, 103 Zero-dose, 86, 175

reading, 69, 146, 170, 173, 174, 175, 176, 178, 192, 198, 200, 201, 202, 204, 212,216,219,220

reduction of, 213 Zinc oxide, ZnS, 252, 253 Zircon dating, see Dating Zircon inclusion technique, 350 ZnO : Tm, see Zinc oxide

Of related interest Thermoluminescence Dosimetry (Medical Physics Handbooks 5) A F McKinlay (National Radiological Protection Board, Chilton, UK) A concise introduction to the increasing use of thermoluminescence dosemeters for the accurate measurement of the exposure and dose of ionising radiations used in the diagnosis and treatment of disease. Covers basic theory, practical applications of dosemeters, calibration and operating problems, design and operation of the associated read-out equipment. 1981 x+170pp ISBN 0-85274-520-6

Fundamentals of Radiation Dosimetry (Medical Physics Handbooks 6) J R Greening (Royal Infirmary, University of Edinburgh) An introductory textbook on the principles and methods of radiation dosimetry providing the pertinent information required by a newcomer to the field. Particular emphasis is placed on the special quantities and units encountered in radiation dosimetry, the relationships between them, and the principles underlying their measurement. For workers in radiation therapy, radiodiagnosis, radiation protection, radiation physics and related fields. 1981 xii + 160pp ISBN 0-85274-519-2

Physics in Medicine and Biology A monthly journal (from 1982) published by The Institute of Physics containing original research papers and review articles on many aspects of radiation physics and other topics including TLD September 1981 issue contains a review article:

The theoretical and microdensimetric basis of thermoluminescence and applications to dosimetry, by Y S Horowitz, Ben Gurion University of the Negev, Israel

Subscription enquiries to The Institute of Physics, Techno House, Redcliffe Way, Bristol BS1 6NX, UK

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ISBN 0-85274-544-3