B ARC-1546

>aCJI

DEVELOPMENT OF SCINTILLATION AND LUMINESCENT DETECTORSAT BARC

Edited by

Dr. A. S. PradhanDivision of Radiological Protection

1991

B.A.R.C. -

GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION

<CD

DEVELOPMENT OF SCINTILLATION AND LUMINESCENT

DETECTORS AT BARC

Edited byA.S. Pradhan

Division of Radiological Protection

BHABHA ATOMIC RESEARCH CENTREBOMBAY, INDIA

1991

B.A.R.C.-1546

Ol

O2

O3

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B.A.R.C.-1546

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10 Title and subtitle : Development of scintillation andluminescent detectors at BARC

11 Collation :

13 Project No. :

20 Personal author(s) :

110 p., figs.

A.S. Pradhan (ed.)

21 Affiliation of author(s) 1 Division of Radiological Protection,Bhabha Atomic Research Centre,Bombay — 400 085

22 Corporate author(s)

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Bhabha Atomic Research Centre,Bombay-400 085

Division of Radiological Protection,B.A.R.C., Bombay - 400 085

30 Date of submission :

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Department of Atomic Energy

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February 1991

March 1991

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60 Abstract : Research and development work carried out at theBhabha Atomic Research Centre, Bombay, in the field qf radiationdetectors for various applications, particularly in the area ofscintillation and luminescent detectors.is reviewed. The reviewis presented in the form of 7 articles.

70 Keywords/Descriptors : BARC; LIQUID SCINTILLATORS; PLASTICSCINTILLATORS; INORGANIC PHOSPHORS; DOPED MATERIALS; RAREEARTHS; CRYSTAL GROWTH; THERMOLUMINESCENT DOSIMETRY;THERMOLUMINESCENT DOSEMETERS; LIQUID SCINTILLATION. DETECTORS;PLASTIC SCINTILLATION DETECTORS; PRODUCTION; CHEMICALPREPARATION; TRITIUM; RADIATION MONITORING; THIN FILMS

71 Class No. s INIS Subject Category : B24.10; E41.10; E41.40

99 Supplementary elements :

POUEVAKI)

With the ever increasing use of ionis ing rad ia t ions in

several f i e ld s such as indust ry , medicine, ag r i cu l tu r e and

pure and app l ied r e s e a r c h , a v a i l a b i l i t y of a p p r o p r i a t e

d e t e c t o r s fo r t h e measurement of m a n i f o l d a s p e c t s of

d i f f e r e n t i o n i s i n g r a d i a t i o n s has g a i n e d s i g n i f i c a n t

re levance and importance. In l i n e wi th i t s t r a d i t i o n , the

Bhabha Atomic Research Centre (BARC) has made p i o n e e r i n g

e f f o r t s in t h e deve lopmen t of d e t e c t o r s fo r v a r i o u s

a p p l i c a t i o n s . Since t h i s work has been in d i f f e ren t groups of

BARC, there has been a long- fe l t need to c o l l a t e the vast

information and e x p e r t i s e g e n e r a t e d a t t he C e n t r e . The

p r e s e n t r e p o r t i s i n t e n d e d t o meet t h i s r e q u i r e m e n t ,

p a r t i c u l a r l y in the area of S c i n t i l l t i t i o n and Luminiscent

d e t e c t o r s . The mater ia l covered in the repor t i s based on the

l ec tu re s given by the s t a f f members of BARC in a Workshop

organised by the Bombay Chapter of the Indian Society of

Had int. ion Physics and Western Regional I n s t r u m e n t a t i o n

C e n t r e . I t i s s i n c e r e l y hoped Lhut t h e r e p o r t would

con t r ibu te to the wide dissemination of valuable information

generated a t the Centre in th i s important area of Radiation

Physics .

4s/" G4(D.V. Gopin.it h)

DirectorHealth, Safety & Environment. Group

C O N T E N T S

I J p e r Title and AuthorsNo.1 CRYSTAL GROWTH AND EVALUATION OF MATERIALS FOR SCINTI-

LLATION DETECTION AND THERMOLUMINESCENCE DOSIMETRY

M.P. Chougaonkar, V.H. GokhaJe, S.M.D. Rao and R.V. Srikantaiah

2 DEVELOPMENT OF THERMOLUMINESCENT MATERIALS FOR RADIATIONDOSIMETRIC APPLICATIONS AT DRP, BARC.

Bhuwan Chandra Bhatt

3 PRODUCTION OF CaSO. : Dy PHOSPHOR FOR DOSIMETRIC APPLI-CATIONS

R.K. Iyer

k PREPARATION AND CHARACTERISATION OF RARE EARTH BASEDPHOSPHORSG. Alexander

5 DEVELOPMENT OF LIQUID SCINTILLATION CHEMICALS IN BARC

K.A. Noras and CM. Paul

6 DEVELOPMENT OF PLASTIC SCINTILLATORS IN BARC

A.N. Rangarajan and CM. Paul

7 PLASTIC SCINTILLATOR SPONGE AND THIN FILM DETECTORS DEVE-LOPED FOR CONTINUOUS ON-LINE MONITORING OF TRITIUM IN WATERAND AIR

A.N. Singh, C.K.G. Nair and M. Rathnakaran

PAPER - 1

CRYSTAL GROWTH AND EVALUATION OF MATERIALS FOR SCINTILLATION

DETECTION AND THERMOLUMINESCENCE DOSIMETRY

M.P. Chougaonkar, V.H. Gokhale, S.M.D. Raoand R.V. Srikantaiah

CRYSTAL GROWTH AND EVALUATION OF MATERIALS FOR SCINTILLATION

DETECTION AND THERMOLUMINESCENCE DOSIMETRY

M.P. CHOUGAONKAR, V.H. GOKHALE*, S.M.D. RAO*and R.V. SRIKANTAIAH*

Environmntal Assessment Division^Technical Physics and Prototype Engg. Division

Bhabha Atomic Research CentreBombay 400 085

SYNOPSIS

Lithium Fluoride (TLD-100) and Calcium Fluoride (TLD-2OO)

are two well known TL phosphors marketed by Harshaw. TLD grade

LiF doped with 100 ppm of each Mg and Dy has been developed. The

commercially available LiF was used for the work after- repeated

distillation. The crystals grown were then crushed to 100-200

tayler mesh size. It was found that the phosphor thus developed

compares well with the TLD-100 phosphor. The performance as

regards the gamma ray sensitivity, linearity, etc. are

discussed.

Calcium fluoride doped with about 0.04% Dy was developed.

The starting material was CaF2 ultrapure obtained from the

Chemistry division. The Dy doped crystals were grown by the well

known Bridgmann technique under high vacuum. It was found that

the phosphor in the form of either cleaved plates or powder were

better than TLD 200. The performance is discussed.

Scintillator materials like CaF2 and BaFs also have been

grown. CaF2 (Eu) crystals were grown with about 0.6% of Eu3 as

dopant. The crystals thus grown were then cut and polished to get

the optical transparency. The performance studies like the alpha

and gamma ray spectra, linearity of response, have been carried

out. The details are discussed.

Barium Fluoride crystals also have been grown with the same

technique. The starting material '.as laboratory made BaF2.

Crystals upto 40 ram dia have been grown. Their properties have

been under study. Some of the experiments and their results are

discussed.

1. INTRODUCTION

Growth of good quality crystals is an important tool for

materials to be used in nuclear radiation detection. This is

mainly due to the optical transparency required for scintillation

detectors and also the uniform distribution of dopants. The

fluorides of alkali and alkaline earth metals pose special

problems due to the fact that they decompose when heated in air.

The paper discusses the growth technique of some of these

materials and also their performance as regards their use in

nuclear detection.

2. INSTRUMENTS

Two separate equipments will be described here. The first

is a distillation set up for lithium fluoride and the other is

vacuum furnace for growth of the crystals.

2.1 Lithium fluoride distillation set up:- It has been found

that the commercially available LiF is not at all good for ob-

taining transparent crystals. It is essential that the LiF thus

obtained is purified. A simple set up was designed by SMD Rao< i >

and was used for the work. Figure 1 shows a schematic of the set

up. It essentially consists of a vacuum chamber in which a

nickel cylinder partly filled with LiF is placed. The open end

of nickel tube faces a water cooled cold finger. The chamber un-

der a high vacuum of the order of 10-5 Torr is heated externally

by resistance heater furnace to a temperature slightly above the

melting point of LiF. After a prolonged heating for about 7-8

hours the chamber is slowly cooled and the cold finger removed.

The distillate collected on it and also on the upper wall of Ni

cylinder is collected. Lithium fluoride thus distilled is then

subjected to next cycle of distillation. Three or four such dis-

tillations are needed to obtain a sufficiently good starting

material. Table-1 shows a typical impurity analysis of LiF by

ICP method after repeated distillations.

Table -1 : Impurity Analysis of LiF

(Impurity content given in r>v>m)

LiF Fe tin Pb Mg Al Ti Ni Ca Na K

Starting

material 100 10 50 50 200 20 10 200 50 50

After 2nd

distillation <10 <10 <10 10 <10 <10 <10 10 10 <10

2.2 Vacuum furnace for the crystal growth*2> :- Figure-2 shows

a schematic of the set up. This is a high temperature high vacuum

furnace where these crystals are grown by Bridgmann technique.

It consists of the following parts : The design details are dis-

cussed earlier*2) .

The high temperature high vacuum furnace consists of a

double wall jacketed vacuum chamber(l). A side port (2) is

provided to it for inserting R.F. heater. Another side part (3)

connects the chamber to pumping system through L bend (4). The

dome shaped top (5) is for covering as well as making the furnace

suitable for Czochralski technique crystal growth through the

port(6). The viewing port (7) is used for viewing the sample as

well as monitoring the temperature by optical pyrometer.

Two water cooled electrodes (9), crucible support (10),

thermocouple seals etc. all pass through base plate (8).

A squirrel cage type graphite heater (11) is fixed on the

graphite plates (12) which are fastened to the electrodes. This

heater is surrounded by graphite felt cylinders which act as

radiation shields. Typical temperature profile of a heater is as

shown in a figure-3 and the heater in figure-4. The graphite

crucible (13) sits inside the heater, on the crucible support

which can be lowered through a vacuum tight wilson seal by lead

screw arrangement.

3. CRYSTAL GROWTH :-

The sample with its dopants is accurately weighed and is

throughly mixed. It is then transferred to a graphite crucible

of required dimensions. The crucible is then placed inside the

heater and the furnace is evacuated. After the desired vacuum

has been achieved, the heating is started. Initially the sample

is heated to about 100 -150»C at a very slow rate so that all the

moisture present is evaporated and the chances of hydrolysis are

reduced. In addition, except in the case of LiF about 0.5% of

PbF2 is also added to charge for the same purpose. The charge is

then further heated to about 15-20°C above the melting point and

is kept at that temperature for a time ranging from 1/2 hour to 1

1/2 hours depending on amount of charge and the size of crystal.

The temperature is then slightly reduced to the level of about 3-

5<>C above the melting point and the growth is then started at the

predetermined rate. After the desired length has been grown, the

crystal growth is stopped and the furnace is then cooled down to

room temperature in 24-96 hours depending on the size of

crystals. Figure-5 shows some of the crystals grown in the

furnace.

3.1 Growth of scintillator crystals:- Crystals like CaF2(Eu)

and BaF2 are grown typically in 25 mm, 40 mm or 50 mm dia

crucibles. About 0.6% Eu+3 is added to CaF2 along with about .5%

PbF2 as a. scavenger. As for Barium fluoride, since it is an

intrinsic scintillator, only adequate amount of PbFa is added as

a scavenger. From our experience, commercially available BaFs

has been found to be unsatisfactory. Hence it is being prepared

in the laboratory using following reaction :

Ba(N03)2 + NH4HF2 = BaF2 + NH4NO3 + HNO3 O>

Table-2 shows a typical impurity analysis of BaF2

prepared.carried out by ICP method.

Table - 2 : Impurity Analysis of BaF2

Raw Material

Sample

Ba(NOs )2

Pb(N03)2

Prepared Material

BaF2-TB1

BaF2-TB2

Ca

<5

- BaF2

«.°

Cu

<5

<5

<5

<5

Fe

<25

<25

6

6

Ni

<50

<50

a

Pb

100

<5

7

3.2 Growth of TLD crystals:- These crystals are grown typically

in 6 mm or 10 mm dia 7 hole crucibles. Each of the seven holes

acts as a separate crucible and seven crystals are grown simul-

taneously under identical conditions. For LiF (Mg, Dy), 100 ppm

of each of Mg+2 and Dy+3 are used as dopents while for CaF2 (Dy),

about .04% of Dy+3 is used as a dopant.

4. EVALDTION :-

Fig.6 shows a typical glow curve of LiF (Mg, Dy) grown by above

technique. It is seen here that the performance is comparable to

that with commercially available TLD 100 from M/s Harshaw. The

other studies like fading with time, response vs annealing cycles

etc. are discussed earlier < *) .

Fig. 7 shows a typical glow curve of CaF2 (Dy) thus grown.

The fading studies were carried out with these samples. The

crystals were powdered and sieved between 100 - 200 taylor mesh

and 30 mg packets were used for the work. The samples were all

irradiated simultaneously and the TL output was recorded after

various intervals. Fig. 8 shows the fading characteristics of

CaF2 (Dy) prepared.

Fig. 9 shows a x ray spectra of CaFz(Eu) for various

energies. Fig.10 shows the Am24* alpha spectrum*5> .While the idea

was only to optimise the Eu+8 concentration, after the Eu+3 con-

centration has been standardised, the work for making these scin-

tillator crystals for tritium detection etc. will be taken up.

Various studies have been carried out for BaF2 crystals

grown. Fig. 11 shows an optical transmission spectra of a 40 mm

dia x 50 mm crystal. It is seen that in the UV region the trans-

mission is around 30%. While the good crystal should show about

30% of transmission, the best resolution at 660 KeV gamma (Cs137)

has been found to use about 25% whereas the best reported value

is about 13%. Efforts are being made to improve the quality of

crystals.

Fig. 12 shows a typical gamma spectra of Cs*37 . It is

felt that the resolution is affected by Pb present in the

crystal. However, it is very difficult to grow a good quality

large dia BaF2 crystal without PbF2 as a scavenger. Optimisation

of PbF2 content is under investigation.BaF2 scintillates in UV

range. Its two components being at 220 nm (decay time .6ns) and

310 nm (decay time 600 ns). This requires special quartz window

PMTs, suitable optical couplings, UV reflectors etc. We are plan-

ning to work in the direction of shifting the light to near

visible range where common PMTs can safely be used. This could be

acheived by doping BaF2 with Ce in which case the wavelength is

shifted to 365nm with a decay time of 50 ns.(6)It could also be

achieved by vacuum coating of p-terphenyl on the BaF2 crystal.*7)

After the satisfactory performance of the small crystals,

growth of large crystals will be initiated.

REFERENCES:-

1. S.M.D. Rao, Ph.D. Thesis.

2. A versatile high temperature high vacuumfurnace for crystal

growing, S.M.D.Rao and M.P. Chougaonksr, 1st National Sem.

on Crystal Growth, Madras, 1982.

3. S.M.D. Rao, M.P. Chougaonkar et al, Proc. of National Symp.

on Nuclear Electronics and Instrumentation, Feb.15-17, 1989

PP 72-77.

4. Development of Lithium fluoride (Mg, Dy) phosphor for Ther-

moluminescent dosimetry, P. Ayappan etal. INd. Jl. of Pure

and Appl. Physics, Vol.19 (1981), P P 323-325.

5. Preparation of CaF2(Eu) single crystals and study of their

scintillation properties, S.M.D. Rao, L.H. Peshori, Proc.

of National Symp. on Rad. Physics, Vol. XXVI, JUne 1976,

PP 389-393.

6.- R.C.Tailor et.al. IEEE Trans.Nucl Sci.NS 33.

7. E.Dafni Nucl ins.and methods.A254 (1987) 54-60.

( 3 ) TO VACUUM *SYSTEM

NICKEL FOIL

(L)NICKEL CONTAINER

WATER COOLER COILS@

LID®

STAINLESS STEEL VESSEL(T

COLO FINGER ( ? )

STAiNLESS STEEL TUBE

FIG. 1 LiF DISTILLATION SET UP

FIG. 2 SCHEMATIC DIAGRAM OF VACUUM FURNACE

oen

O(Xu.IDU2

28 r-

20

16

8

0600

| l__~700 X800 900 1000

TEMPERATURE °C

1100

FIG. 3 TEMPERATURE PROFILE OFA GRAPHITE HEATER

FIG. 4 GRAPHITE HEATER FOR CRYSTAL GROWTH

BaF2

AS GROWN

g 1 ' 7 1 ' ? ' ' 9 ! ' 110 ' 111 ' 112

FIG. 5 PHOTOGRAPHS OF BaF2 CRYSTALS

zo_ Jr—1

o1

oo

IAR

SH

/

*~*

PCD

—1

]0o1—

oO

oURV

ES

o

m"0m>

mo°

o

oo

>JOn

LIGHT EMITTED (ARB. UNITS)

5> o ro

<0

CD

oo

300*

QUJ

UJ

100 200

TEMPERATURE °C300

FIG. 7 GLOW CURVE OF CaF2 : Dy GROWN

BY BRIDGMANN TECHNIQUE

O

o

CM

(O

CMO»

2 </>

sga:a.

CM

IDO•

O

o

auu.o

UJ

a

z

i00

S2

(D N O «r- «- ©

(O <4 (M• • •o o o

aav) AIISNSINI 11

90

80

70

60

CD

a. 50<

30

20

10 / \ f\

. i

50 100 150

CHANNEL NUMBER

30

20o

zUJ

200

FIG. 9 GAMMA RAY SPECTRA OF CaF2 (Eu)

50 100

CHANNEL NUMBER

150

FIG. 10 Am241 ALPHA SPECTRUM

80

190

BaF2 CRYSTAL F8

4Omm0 x A8mm HEIGHT

GROWN ON 23-10-1990

474 616

WAVELENGTH A (nm)

758 900

FIG. 11 OPTICAL TRANSMISSION OF BaF2

V)»-zz>oou.octUJ00

s

137SOURCE : Cs (660keV)P. M. TUBE : EMI 9556 QBBIAS : U00 VAMPLIFIER : ORTEC 572TIME CONSTANT 2/ULS

BaF2 CRYSTAL F8

40 mm 0 x 48 mm HEIGHT

GROWN ON 23-10-1990

CHANNEL NUMBER

FIG. 12 GAMMA RAY SPECTRA OF BaF2 CRYSTAL

PAPER - 2

DEVELOPMENT OF THERMOLUMINESCENT MATERIALS FOR RADIATION

DOSIMETRIC APPLICATIONS AT DRP, BARC.

Bhuwan Chandra Bhatt

DEVELOPMENT OF THERMOLUMINESCENT MATERIALS FOR RADTATTOIfDOSIMETRIC APPLICATIONS AT DRP, BARC.

Bhuwan Chandra BhattDivision of Radiological Protection

Bhabha Atomic Research Centt.frTrombay, Bombay 400 085

1. Introduction

The programme of development of TL materials for various

radiation dosimetrie applications was initiated in DPP in the

year 1969.The materials initially tried were Lii.-B4f7.-Mn and

CaS0->:Dy. L12B.1O- :Mn was being developed d,j«.' to i •• • tissue

equivalence (zy f i =7 . 4 j and possible applications in tht,- field of

Medical Physics. CaSCU:Dy (Ze 1 1 = 15.3) was being tried du.-i t<

its reported^'•2' high TL sensitivity (CaSO4:Dy is about 100

times more sensitive131 than LizB-iOTiMn and 33 ti:;u-s niori;

sensitive than LiF(TLD-lOO). Other materials that were tried

[4,5] subsequently were Li;>B.iO-:Cu and MgB-i O- : Dy (7,-ti = 8.4)

having their respective TL sensitivities 1.5 and 2 times than

that of LiF TLD-100. However, out of all these materials tried,

CaSO.i : Dy was found by -is to be most suitable material f<u various

radiation dosimetric applications. This is bt-i.aus*- of two main

reasons : (i) Its high TL sensitivity and (ii) the simple method

of preparation of this phosphor as compared to other TL materials

which involve firing of the mixture of the host and the

appropriate dopant at high temperatures in a muffle furnace.

Thus CaSO4 iDy/CaSOi:Tm TLD phosphors in various physical fot^s

such as loose powder (in the grain size range 74-210 urn) ,

phosphor encapsulated glass capillaries, phosphor embedded teflon

discs etc., find various applications in thw fi«ld of radiation

dosi^etry.

The aim of this review is to highlight the properties and

applications of these CaSO-i :Dy/Tm based TL dosimeters and iheir

various physical forms in the field of radiation dosimetry. The

review concentrates mainly on the developments and R « D efforts

that took place in the Division of Radiological Protection for

the use of this system in various dosimetric applications during

last twenty one years.

2. T L D Materials Investigated

2.1 LizB4O7 :Mn (0.1% by wt) was prepared by following the method

of Kirk et al* 6 ] and Christensen et alt7' by mixing Li* CO3 and

H3BO3 in the ratio of 1:2 respectively alongwith appropriate

amount of the activator and drying the wet mixture at 80°C for 16

hrs. The dried and finely powdered product is fused in a

platinum crucible at 950°C for 30 min and then quenching the

molten mass to room temperature. The phosphor is used for

dosimetric applications in the grain size range 74-210 urn. Its

usable dose range is 5 mGy to 103 Gy. The main factor in favour

of lithium borate has been its tissue equivalence. Due to its

tissue equivalence it finds applications in patient dosimetry in

the field of medical physics.

2.2 L12B4O7 :Cu

Subsequently LiaB-tOrrCu TL phosphor was prepared! i ] by

sintering at 900°C for lhr the raw L12 B4 O7 along with an

appropriate amount {0.05% by wt) of Cu2* activator. The TL

sensitivity of this phosphor was 0.64 of the reported value by

Takenaga et al f 8 ]. Its TL sensitivity was twice the sensitivity

of LiF TLD-700 (or TLD-100) and five times that of LizB-jOvrMn.

The advantages of this phosphor are: (i) its TL emission is at

360 nra as compared to red emission (600 rim) given by Li2B«07:Mn

and (ii) a linear dose response from 0.3 mGy to 3 x 102 Gy

beyond which its TL response saturates.

This phosphor has advantage over Li2B.iO7:Mn for medical

physics applications because of its higher TL sensitivity and

linear dose vs TL response. If stored for long duration (nor<-

than one year) the phosphor has a tendency to absorb moisture and

form solid lumps. In such a situation the phosphor needs to be

crushed lightly and annealed at 300°C in order to remove

moisture.

2 . 3 MgB*O7:Dy TLD Phosphor

The phosphor MgBiO-:Dy (0.5 mol%) was prepared^9' by making

a mixture of MgCOa and Ha BO3 in the weight ratio of 1 : 2.5 -=ilontj

with an appropriate amount of DyzOa (dissolved in dilute nitric

acid) was dried in a Teflon beaker at 100°C for 16 hr. The dry

residue was finely ground and then kept in a platinum crucible

and sintered in a muffle furnace at 950°C for 2 h and then it was

quickly cooled to room temperature {argon gas was flushed over

the sample during cooling). The TL sensitivity of this phosphor

was 1.5 times that of TLD-100. Its TL response was linear in the

dose range 1 mGy to 40 Gy and supralinear above 40 Gy. Its

usable range was from 1 mGy to 10" Gy. Because of its

hygroscopic nature and high fading (40% in 17 days) it could not

become a practical dosimeter.

2.4 CaSOi :Dy/Tm TLD Phosphors

CaSCU :Dy and CaSOq :Tm phosphors were initially prepared in

small batches of 10-20 g in the laboratory by fallowing th&

method described originally by Yamashita et al.' 1 2' TT i->-i<=

method CaSCU :2HzO powder (analytical reagent grade) is dissolved

in cone. H2 SO4 (analytical reagent grade) along with appropriate

amount of . the required activator. The resulting solution is

evaporated at a temperature of about 270 - 300°C to dryness in

order to obtain polycrystalline phosphor. The complt.'t>- process

of evaporation takes about 4-5 hours in order to obtain clear

crystallites of the luminescent material. The product so

Table 1. Dosimetric characteristics of common TL phosphors.

TLD Phosphor Relativegamma raysensitivity

TL emissionspectrum

(nm)

Dosimetricpeak temp.

Detectionthreshold

Effectiveatomicnumber

Photonenergyresponse'a)

TL fading ofdosimetric peakat 25°C

LiF:Mg,TiLiF:Mg,Cu,P chips .

Li2B<07:CuLi2B<07:NnMgB<07:DyCaSO<:TmCaSCU:0yCaF2:MnCaF2 (natural)CaF2:DyMg2Si04:TbAl2O3:Si ,Ti<x-Al2O3:C s i n g l e

crystals

129

2-30.47

32385

231653

5

50

400360,410

368600

480,570450

480,570500380

480,570380,552

420

420

190210

215210210220220260260

200,240195250

190

5060

100.210

55

1010

55

10

1

M<5ynGy

tiGymGypGyyGypGypGyMGyMGyMGypGy

pGy

88

778

15151616161110

10

.2

.2

.4

.4

.4

.3

.3

.0

.3

.0

.0

.2

.2

1.351.00

0.90.91.5

10-1210-1213-1513-1513-154.53.5

3.5

5%/monthno fading

in one month9%/month10% /month<10%/month

1-2%/month1-2%/month10%/month3%/month10VYear<>>>3%/month5%/tvo weeks

3%/month ( r t )

(a) TL sensitivity at 30 keV relative to that at 1.25 MeV

(b) It was subjected to post-irradiation annealing at 100°C/20 min prior to readout

(in the absence of this treatment the TL fading in this phosphor is 25% per month)

(c) Detection threshold is highly dependent on photomultiplier tube used.

(d) Akselrod, M.S. et al. Radiat. Prot. Dosim. 31, *5 (1990).

obtained is repeatedly rinsed in distilled water in order to

remove the traces of acid adhering to the crystals of CaSO4:Dy /

Tin. The product is dried to remove truces of water and then

annealed at 600 - 700°C for 2h in order to stabilize its TL

characteristics. The resultant phosphor is ground and grains in

the desired grain size are obtained by using the sieves of

appropriate mesh size. The laboratory scale preparation helped

u s f3,35] to make samples with various concentration of the

activators so as to exploit these phosphors for various

dosimetric applications. For personnel monitoring use we arrived

at a Dy3+ concentration of 0.05 mol % in CaSO<i so that the

phosphor has optimum sensitivity to gamma rays and negligible

thermal neutron response (equivalent to 3.8 mGy of Co-60 gamma

rays per 1010 ncnr2). Table 1 compares the TL sensitivities of

some common TLD phosphors which are also available commercially

from many international manufacturers.

After standardizing the phosphor, we requested chemistry

Division, BARC to devise a process for the large scale

manufacture of the phosphor for its use in personnel monitoring.

Chemistry Division developed ci°J a large scale process for the

manufacturer of this phosphor; they make 900 g CaSO4:Dy (0.05

mol%) per batch. Presently, DRP is getting about 6 kg phosphor

per year from them. The phosphor being made by them is quite

satisfactory. For personnel and environmental monitoring use,

the selection criteria for a particular phosphor batch is the

following:

If the TL sensitivity of the supplied batches is within ± 5%

(lo) of the standard batch then they are selected; the standard

batch was chosen from one of the earlier batches (typically five

batches were taken for selecting the standard) when they were

compared with a small batch (10-20 g batch) prepared by us in the

laboratory. The TL sensitivity variation within a batch (intra-

batch sensitivity) should be less than ±5%(lo). Fig.l shows the

typical TL glow curve for a large size batch (made by Chemistry

Division) for a test dose of 0.1 Gy and Table 2 shows the

comparison of TL sensitivity of 6 batches made by l?rge scale

method.

Table 2 : Relative TL sensitivity of CaSCU :Dy powder (0-74 pm.size) from different batches (900 g each)

Batch number Relative sensitivity

Standard 1.00

1 1.02

2 1.05

3 1.01

4 1.00

5 1.01

2.5 Development of CaSOi:Dy Filled Glass Capillaries

After devising a process to make CaSO^:Dy TLD powder in

large quantities, it was essential to explore its use as a

dosimeter for large scale use in personnel and environmental

monitoring. The handling of loose powder in routine use is

somewhat cumbersome. Therefore, initially phosphor fillwcj glass

capillaries (yach of about 2 rnai iflia find 10 mm length and each

containing 20 ng CdSO* :Dy) were di-.-velopedr ' ' > . They were used

for various applications in medical physics particularly in,

patient desimetry, mapping of dos«-distribution fro™ teletherapy

rnachines.

For personnel monitoring use, a pocket dosimeter containing

two CaSO.i :Dy powder {grain size 74-210 pro) filled glass

capillaries was developed. In the pocket dosimeter'12', a 0.5 ,-ii.v

thick lead filter {cylindrical in shape) with 10% perforatLons

yielded a uniform response (within ± 10%) in the energy range 33

to 1250 kaV. The response of the pocket dosimeter was reported to

be linear in the exposure range (2.5 x 10"6- 2.58 x 10-MCkg-1

i.e. 10 mR to 1000R.

2.6 Development of CaSO< :Dy Embedded Teflon TLD Discs

Since the phosphor filled glass capillaries are somewhat,

fragile and hence need extra care during their use and handling

during readout. Subsequently, phosphor embedded teflon TLD discs

{each of diameter 13.3 mm and thickness 0.8 mm) were

developed1'33 . For this CaSCu :Dy powder ( in the grain size 0-74

j.m) and Teflon powder grade 7 A {average particle size 35 \iv\) nrt-

mixed in the ratio of 1:3 respectively. Discs each of weight 280

ing are made by cold compaction of CaSO4 :Dy and Teflon mixture in

a hydraulic press. These cold pressed discs are given a moulding

treatment at 400°C for 1 hr in order to give them strength and

flexibility. The mixing of CaSO*:Dy and Teflon powder is done at

liquid nitrogen temperature as the teflon powder is available

from manufacturers in the form of lumps and at ambient

temperature it is difficult to mix them uniformly by adopting

usual blending techniques used for mixing inorganic powders.

3. Applications in Radiation Dosimetry

3.1 Personnel Monitoring TLD Badge using CaSO< :Dy Embedded

Teflon TLD Discs

A TLD badge based on CaSO4:Dy Teflon TLD discs was

developedr'4'. TLD badge (Fig. 2) consists of a plastic cassette

containing three Teflon TLD Discs (13.3 mm dia and 0.8 mm thick)

that are mechanically positioned on circular holes {12 mm dia)

in an aluminium card. A thin paper strip with printed

information about the badge and the user is inserted in the badge

as a wrapper of the card along with a plastic pouch (t£tal

thickness the paper wrapper and plastic pouch is 13.5 mg/cm2).

There are three well defined areas in the badge corresponding to

the three TLD discs. These are {1) a circular filter combination

of 1 mm Al and 1 min Cu (the copper filter is nearer to the TLD

disc), (2) a circular open-window region and (3) a region with a

3.5 mm thick plastic filter over the TLD disc in the middle

portion of the badge. A clip attachment affixes the badge to the

user's clothing. Fig.3 shows the photon energy dependence of the

TLD badge. The usable gamma dose range of the TLD badge for

personnel monitoring applications is 0.2 mSv to 10 Sv. The badge

can be reused for more than 15 cycles. The badge is meant for

monitoring, generally, beta, gamma or X-, gamma-radiations in our

nuclear installations including nuclear power stations,

industrial, medical and research institutions. Presently, out of

the total of 38,000 radiation workers in the country, about

20,000 are covered by TLD monitoring and the remaining are

monitored with photographic film badge.

The readout procedure currently being used for the personnel

monitoring TLD cards leaves certain amount of TL as the residual

TL in the TLD cards. As a result of which the second readout'1S'

of the same disc (in the TLD card) is typically about- 10-11% of

the first readout (Fig.4). This residual TL is used to advantage

for verifying the first measured dose (by taking the second

readout) in certain situations when the personnel monitoring TLD

card receives gamma doses greater than 10 mSv.

The personnel monitoring TLD badge has been usedt*6•*7' in

two International Intercomparisons of Personnel Dosimeters. The

performance of our dosimetry system was quite satisfactory in

these intercomparisons.

With the help of locally available teflon moulding companies

we have developed t » «• • « *» 1 techniques for the manufacture of thin

discs (0.1 to 0.8 mm thick) by making either CaSO« :Dy teflon tape

or by moulding CaSOi :Dy teflon rods of desired diameter and

cutting out discs of required thickness. These thin dosimeters

were prepared 120,21] either by cementing 0.1 mm thin CaSO.) :Dy

teflon discs to the 0.7mm teflon (pure) base or by moulding

CaSO4:Dy teflon discs containing certain percentage of graphite

powder. These thin dosimeters helped'z*• s8J in improving thy

beta energy response (Fig.5) as compared to the 0.8 mm thick

CaSOn :Dy Teflon discs which are being used in the present TLD

badge.

For improving the performance of the present TLD badg*.- in

terms of faster readout for a large scale personnel monitoring,

efforts are being made either to develop TLD badge with thinner

detector or by using heating methods which would enable faster

readout of the dosimeters. Following efforts are being made :

(i) TLD cardt23' based on 0.4 mm thin tape incorporating

antibuckling device (0.4 mm thin sheet of mica) in order to

avoid buckling of the teflon tape during readout. This

design enables the TLD card readout in the presently

available TLD readers (without any major modification) with

lesser time and better reusability.

(ii) The hot air/hot nitrogen method of heating is being

developed by using modified TLD card design and using < 0.4

mm thin CaSO4:Dy teflon discs. Initial experiments in the

laboratory tend to prove that the readout time could be

reduced to 5-10 seconds from the present readout time of

60 seconds.

(iii) Laser (CO2 laser emitting at 10.6 pm) heating method and

using thin teflon discs (0.1 to 0.4) as well as thin

substrate dosimeters of CaSO4:Dy either on polyimide film or

on thin alumina ceramic base. Initial experiments using COz

laser beam and thin teflon discs (0.2 mm), it is possible to

perform readout of these dosimeters in about 15 seconds.

The development of thin substrates of CaSO* :Dy detector is

underway for their trial with CO2 laser beam.

3.2 Environmental Gamma Dosimetry

Due to photon energy dependence of CaSO«i :Dy teflon TL

dosimeters, their response should be compensated appropriately so

as to get nearly a flat response for low energy photons (<200

keV). An environmental dosimeter pack based on CaSO<:Dy phosphor

embedded teflon TLD discs was developed!2*' for background dose

measurements in our laboratory. It consists of eight CaS04:Dy

Teflon TLD discs which are sandwiched between a pair of 1 mm

thick copper discs (dia 50 mm) and contained in a plastic box of

2 mm wall thickness. The minimum measurable gamma ray exposure

with these discs is 3± 0.2 mR(lo). We participated in three

International Intercomparisons of Environmental Dosimeters held

in USA. Table 3 shows that the results of our participation in

these International Intercomparisons were quite satisfactory.

Subsequently efforts, have been made1s6> s 7 ] in our laboratory

to arrive at suitable filter thickness/combination alongwith

CaSOi :Dy detectors so that a compact dosimeters could fulfill the

ANSI requirementsi251 for environmental dosimeters. A TLD badge

containing pair of detectors were used ts6.s7]f. o n e under metal

filter combination (0.8 mm Al + 0.55 mm Sn + 0-35 rcm Cu) and

other under a composite filter (1.6 mm plastic + 0.8 mm Al). It

Table : 3. Results of the International Intercoiparison Experiieots onEmroniental Dosiieters""

Intercos- Field Exposure Laboratory Exposureparison — - -site DRF Hein of Organizers value DRP Mean- of a l l Organi-

value a l l value dosi ie ters zers(iR) dosi ie- Exposure Exposure UR) (iR) value

ter (iR| rate liR)

(iK! (pR/h)

S.Y.,1976 13.8+2.12 16.4+3.85 17.1+0.98 7.9 Id.4+2.94 18.8+3.82 21.3+1.1

Oak Ridge,1977 30.8+1.9 31.9 34.9 45.4 83.5+5.04 86.2 91.7Houston,Texas, 13.7+2.16 16.0+4.5 14.1+0.7 5.6 (I) 10.5+2.57 12.0+3.8 12.2+1.21979 (11)41.8+5.63 43.916.6 45.814.6

was found t h a t from the two s e t of d o s i m e t e r s ( i n the TLD b a d g e ) ,

the energy independent response (R) can be e v a l u a t e d by the

following equation:

R = 0.9A + O.I C (1)

where A is the readout of the disc under combined filter O.8 mm

Al + 0.55 nun Sn + 0.35 mm SS, and C is the readout of the disc

under 1.6 mm plastic + 0.8 mm Al filter. These readouts provide

response *R' with an uncertainty of ±20% in the energy range 27-

1250 keV. Thus there exists a possibility to develop an

environmental dosimeter based on CaSCM :Dy Teflon TLD discs.

Recently Vinten Instruments (UK) have developed'26' an

environmental TL dosimeter based on four 0.4 nun thick Cascu :Dy

teflon TL discs and two sets of filters(i) 3 mm Cu and 0.9 mm Al

and (ii) 4 mm PTFE and combined response is obtained by using an

algorithm

D3 o = 0.1 Dp + 0.9 D (2)

where D30 is true dose from 30 keV to 3 MeV; DPis dose

indicated under plastic filter and Da is dose indicated under

metal filter. By using the above eq. (2) the dosimeter response

could be made energy independent within ±30% from 30 keV to 3

MeV.

3.3 Ultraviolet (UV) Dosimetry

In view of the NIOSH1871 report recommending standards for

UV exposures and the likely adoption of similar standards, it was

expected that new techniques and instruments will be needed to

measure low level U.V. radiation. Thermoluminescence radiation

dosimeters and readers have been developed . over the past 3

decades and are now widely used for the detection of ionizing

radiation in personnel and environmental monitoring as well as in

medical physics applications. Many TL phosphors have been

investigated as thermoluminescent UV radiation dosimeters. Among

them are CaSO* :Dy, CaSCU :Tm, CaSO« :Dy, Cut 29 J , Mg2SiO<i:Tb,

L^BiChiCu, and CaF2 :Dy (thermally treated at 900°C) . Table 4

gives'281 the comparison of intrinsic TL response of common TLD

phosphors to UV radiation of wavelength 254 nm.

Another technique that is generally adopted for UV dosimetry

is by using photo-transferred TL {PTTL). This involves [31-34]

exposing the TL phosphor to a high gamma dose and post-annsaling

at a higher temperature so that the main dosimetry peak is

completely erased but leaving a higher temperature peak as

residual TL (RTL) peak in the sample. On giving a UV exposure

(generally at 253.7 nm) to the treated sample, electrons/holes

trapped at the high temperature peak are released and become

mobile to get trapped at lower temperature TL peaks {which were

empty). The intensity of PTTL is proportional to UV exposure

within a certain range of UV exposure.

used for a number of TL phosphors.

CaSCU :Ttn, LiF:Mg,Ti and MgzSiC^iTb, etc.

PTTL technique has been

For example, CaSO< ;Dy,

Table 4. Intrinsic Response of TL Phosphors to 254 nm UV Radiation12a

Phosphor

CaSCM :Dy

CaSCu:Tm

CaSCM :Dy, Cu

Liz Bi 0- : Cu

AI2 O3 :Si,Ti

ActivatorConcentration

0 .1 mol .%

0.05 mol %

0.1 mol %

Origin

DRP, BARC

-do-

-do-

MatsushitaElectricCo., Japan

H.P. DivisionB.A.R.C.

Relative TL response

1

80

40f 2 9 3

50[3 °'

30

CaF2 :Dy (TLD-2C0)

2 SiO-t :Tb

Harshaw chemicalCo., U.S.A.

Dai Nippon ToryoCo. Ltd., Japan

308

3.4 Microwave Dosimetry

Microwave radiation which forms a significant part of the

non-ionizing radiations and has been in use in telecommunications

and diathermy, has now entered into modern kitchens through the

extensive use of microwave ovens. Recently, in India also such

ovens have been marketed by some manufacturers for domestic use.

Microwaves have in,sufficient quantum energy to cause ionization

in biological systems but the absorbed energy is transformed into

kinetic energy in the absorbed molecules thereby producing tissue

heating. This, in turn, can result in biological damage when an

excessive exposure to microwave radiation occurs. With a view to

develop a microwave dosimetry system, the effect of microwave

radiation on certain thermoluminescent phosphors has been

investigated^34 1 .

Gamma irradiated TL phosphors (in powder form) when exposedto microwave radiation (using CW diathermy unit : 2425 + 25 MHz,maximum irradiance level 2000 Wm- 2), a reduction in gamma inducedTL output was observed. The percentage reduction in integratedTL outputs (integrated between 25-300°C), for 1 h microwaveexposure at an irradiance level of 500 Wm-2 is given in Table 5.

Table 5. Reduction in integrated TL output of variousphosphors for microwave irradiation. Gamma exposure: 2.58 x 10"3 Ckg-l; microwave exposure : Ihat 500 Wm-2 < 3«>

Phosphor Percentage reduction

BaSO-i :Tb 78

CaSO4 :Mn 96

MgF2 :Mn 9

MgF2:Tb 25

MgFz :Tm 60

The reduction varies from one phosphor to another. CaSO-i :Mn

exhibits maximum reduction (96%) whereas BaSO« :Tb shows a

reduction of 78% for a similar microwave exposure. Reduction in

integrated TL output of BaSOa:Tb was proportional to the

microwave radiant exposure (product of the irradiance level and

the exposure time) in the range 250 - 2000 Wm-2.

3.5 Neutron Dosimetry

3.5.1 Thermal Neutron Response

TL phosphors such as LizB^O-rMn, Li2B4O-:Cu LiF:Mg,Ti (TLD

100) and 6LiF:Mg, Ti(TLD 600) are highly satisfactory for the

dosimetry of thermal neutrons. Table 6 shows the thermal neutron

sensitivity of some common TL phosphors'3!. Since many other TL

phosphors have negligible sensitivity to thermal neutrons, pairs

of dosimeters can be used for mixed field (gamma ray and thermal

neutron) dosimetry. Very low thermal neutron doses can also be

measured by mixingf3sJ TL phosphors (e.g. CaSOi:Dy or Mg2SiO<i:Tb

etc. having high sensitivity to alpha or beta or gamma rays with

pure chemical powders containing elements of high thermal neutron

cross-section (6 Li2 SO4 etc).

3.5.2 Fast Neutron Dosimetry

In view of the current concern for low dose effects of high

LET radiations and the consequent proposals for increasing the

quality factor for neutron irradiations by a factor of 2, a lot

of importance is being attached to neutron monitoring.

One of the methods of fast neutron dosimetry is based on the

activatison of some of the constituent elements of the material

using threshold nuclear reactions!36 I . The TL detectors are

annealed after neutron irradiation to erase the TL induced during

irradiation. Then they are stored to permit self-irradiation

from the internal radioactivity (induced due to neutron

irradiation) for time comparable to the half-life of the

activated element. The post-irradiation induced TL provides a

measure of neutron dose. Among the various available TL

phosphors, CaSd :Dy (using the 3 2S (n, p ) 3 2 P reaction) is most

suitable (o(n,p) at 14 MeV = 250 mb); the 3aS(n,p)3zP reaction

offers the broadest energy range (2.5 - 14 MeV). The 14.3 days

half-life" of 3 2P is advantageous for long exposure time and for

handling after neutron irradiation. The minimum measurable fast

neutron dose by this method*37i was of the order to 0.04 Gy. The

sensitivity of this method has been increased by mixing small

amounts of CaSO<:Dy phosphor (grain size less than 75 um) with

pure sulphur powder'381. The fast neutron efficiency for pellets

with 0.1% CaSO«:Dy has been found to be about 100 times that of

bare CaSO*:Dy powder {Table 7). This method is quite sensitive

and can be used for low level fast neutron dosimetry. However,

this cannot become a practical device for large scale personnel,

monitoring due to tedious procedure of post-irradiation annealing

and subsequent long duration of storage for accumulation of TL in

the sample.

Table 6. Thermal neutron sensitivity of some TL Phosphors' 3 • s3>

Phosphor Main reaction Thermal neutronresponse in Co-60equivalent dost?in tissue (Oy)per 1010 n cm-2

CaS04:Dy (0 .05 mol%)

Liz B« O7 :Mn

LiF(TLD-600)

LiF(TLD-lOO)

LiF(TLD-700)

CaFz :Dy

1 6 <Dy(n ,

6 L i ( n , a )

6 L i ( n , a )

6 L i (n ,a)

6 L i ( n , a )

l 6 « D y ( n ,

T ) t » D y

3HLi

3H

3H

3H

T ) 1 6 S D y

3 . 8

3 .

15

3

1 . 1

5 . 9

X

9

. 2

. 3

X

X

1 0 - 3

io-z

10-3

3.5.3 Albedo Neutroja Dosimetry

In the recent years, TLD albedo neutron dosimetry has become

the most acceptable tool. This technique has two main

advantages: (i) it has no energy threshold and (ii) sufficiently

small doses of neutrons can be measured by using suitable TLDs.

Fig. 6 compares the energy response of albedo technique with that

of the NTA and the CR-39 detectors.

In the albedo method we practically measure the albedo

thermal neutrons and therefore employ a pair*39' of TLDs, one

sensitive to thermal neutrons and the other insensitive to them

as much as possible. In order to develop an albedo technique for

neutron dosimetry in addition to normal CaSO<t :Dy Teflon TLD

discs, thermal neutrons sensitive CaSO*:Dy Teflon discs were made

by adding 6Li-compound (non-TLD grade) to the normal discs*4°5

Table 7. TL accumulation efficiency of the CaSO*:Dy - mixed sulphurpellets, CaSO4 :Dy loose powder and CaSO*:Dy embeddedteflon discs <5 s >

Type of TL per Gy for Accumulated TL per Gy Effici-dosimeter 90Sr-90y for fast neutrons ency %

Ep E u R.i /Efs

10 pel le ts feach lg) 1 0.026 2.6with 0.1% CaSO<t10 pellets (each of lg) 11.5 0.094 0.82with 1% CaSO4 :DyCaSO* :Dy loose powder - - 0.025CaS04:Dy embedded - - 0.013teflon discs

during mixing. Initial experiments (Table 8) with CaS0i:Dy

teflon discs mixed with 25% 6LiF by weight show that the thermal

neutron response of these discs is 2.2 equivalent Gy of Co-60

gamma rays per 1010 ncnr2 which is about l/7th of the

corresponding thermal neutron response of LiF TLD-600. It may be

noted that the thermal neutron response of CaSO4:Dy Teflon discs

is l/4th the response of LiF TLD-700 under similar conditions

(table 6). Therefore, the net differentiation between thermal <

neutron sensitive and insensitive CaSO*:Dy detectors is

compareable to that between TLD-600 and TLD-700. A pair of TLD-

700 and TLD-600 detectors are used in the albedo neutron TLD

badge developed by M/s. Harshaw/Filtrol, USA. Since the TL

sensitivity of CaSO4:Dy is 38 times higher than that of TLD-600,

CaSO«:Dy Teflon discs loaded with 25-30% 6LiF would give

satisfactory response as an albedo dosimeter. The detailed

results of a preliminary experiment*«°> with CaSO* :Dy Teflon disc

detectors are reported elsewhere.

Table 8. Thermal neutron responses of different typesof Teflon TLD discs'«°>

Teflon Discs Equivalent Gy of 60Co gamma

rays per 1010 n cm-2

CaS04 :Dy 2 . 5 x 1 0 - "

6LiF + CaSO< :Dy 2.2LiF + CaSO4:Dy 0.46

LizB-jO? + CaSO< :Dy 0.26

* Lakshmanan A.R. and Bhatt R.C. Int. J. Appl. Radiat.Isot. 28, 665 (1977)

3.6 High Level Gamma Dosimetry

Although the TL output of the dosimetry peak ( " 230°C)

in CaSOi:Dy and CaSOiTm saturates at a gamma dose level of about

2 x 103Gy, the high temperature peak (~400°C) saturates^ - ^ J at

a dose level of about 106Gy (Fig. 7); the TL vs gamma dose

response curve for the high temperature peak rises sublinearly in

the dose range 102 to 106 Gy. Alternatively, photo-transferred

TIJ (PTTL) respose for the dosimetry peak could also be used as it

avoids infrared problems which are prevalent during the readout

of such high temperature TL peaks. For CaSO4:Dy, the PTTL

response for the dosimetry peak saturates at about 5 x 10s Gy.

The materials that were tried for high level gamma dosimetry were

caSOi:Dy, CaSO<:Tm and Li2B«O7:Mn. The saturation dose for high

temperature TL peaks in CaSO^:Tm (400°C) and Li2B4O7:Mnf* * 3

(370°C) were 10$and 10s Gy respectively. The corresponding PTTL

response at the dosimetry peak saturates at doses of 3 x 105 and

10" Gy.

CaSO* :Dy samples containing high Dy-concentration (2 mol %)

show better TL vs dose characteristics and lesser tendency f«ai

for saturation against gamma radiation dose as compared to the

samples containing normal Dy-concentration (0.05 mol % ) . Thus

high temperature TL peak (" 400°C) in CaSOt:Dy (2 mol %) does not

show saturation in its TL vs dose response even upto the studied

dose of 3 x 106 Gy.

4. Recent R & D Efforts leading to New Techniques in

TL Radiation Dosimetry

Investigations have been done in our laboratory in order to

study the effects of additional dopants (co-dopants) on the TL

characteristics of CaSO4 :Dy and CaSO-j :Tm phosphors. Effect of

co-doping in CaSO<:Dy phosphor for augmenting its TL

characteristics as well as for increasing TL sensitivity were

studied. Activator concentration dependent effects were also

studied.

4.1 Effect of Monovalent Cations on the TL Characteristics of

CaSO< :Dy and CaSOi :Tm Phosphors.

It has been observed*43] by us earlier that the addition of

Na* in CaSOi:Dy reduces the intensity of dosimetry peak (230°C)

but enhances the intensity of low temperature {"130°O pe*k (Fig.

8). This study was basically initiated to see the effect of

monovalent charge compensators on the TL characteristics of

CaSOi:Dy phosphor as well as to arrive at the Na4 concentration

below which it does not affect the useful characteristics of the

phosphor. Na* in concentrations < 0.02 mol % in CaSCU :Dy (0.1

mol%) does not affect the TL characteristics of dosimetry peak.

:20:

In addition, at a suitable concentration of Na* (0.5 mol %)

in CaSOi:Dy (0.1 mol % ) , the intensities of low and high

temperature peaks become comparable as well as they are also well

separated. Therefore, due to relatively high sensitivity and two

peak structure, the ratio**61 of low^and high temperature TL

peaks can be used to estimate the elapsed time after acute gamma

exposure {Fig. 9) as well as the magnitude of radiation exposure

from the high temperature TL peak intensity.

The effect of addition of monovalent cations (Na4) has been

investigated in other CaSO*:RE3 + systems. The addition of Na* in

CaSO<:Tm and CaSO«:Sm reduces the dosimetry peak but enhances the

low temperature peak similar to that observed earlier in CaSO4 :Dy

doped with Na* , although the quantitative factors were somewhat

different.

4.2 Effect of Co-dopants in CaSO< ;Dy TL Phosphors

4.2(a) Recent characterization studies in our laboratoryt«7 J

have shown that the linear dose-response relationship and simple

glow curve structure of materials like LiF:Mg,Cu and Li2B.»O7:Cu

(unlike the bulk of TL materials) could be attributed to copper

doping/codoping. In view of this, the materials development

studies have been undertaken to analyse the likely influence of

such impurities (Cu2< and others including rare earths) in

favourably augmenting the TL characteristics of some common TL

materials. Therefore, samples of CaSO4:Dy3 • ,Cu2+ were prepared

and their TL properties have been investigated^« 8 i . Addition of

copper in concentrations less than 0.1 mol% in CaSO^ :Dy (0.1

mol*) effectively eliminates the conspicuous and broad high

temperature peak structure above 275°C normally present in CaSO4

samples doped with Dy alonet33'. CaSO4:Dy,Cu samples show

extended linear response up to 102 Gy (in CaSCU :Dy, the onset of

supralinearity starts from the gamma of 1-2 Gy). For uv

dosimetry (Table 4), with proper manipulation of Cu2 •

concentration (> 0.2 mol %) CaSO«:Dy, Cu could be made as one of

the very sensitive TL materials (> 40 times of CaSO4:Dy).

Table 9. Radiation induced sensitization (S/So)values for CaSO«:Dy samples along withhigh temperature residual TL (RTL) peakintensities. Pre-dose was 103 Gy, Post-annealing treatment 300°C, 1 h and testgamma dose 1 Gy.

Sample

CaSCU :Dy

1 0.05 mol %

2

3

4

5

6

0.1 mol %

0.2 mol %

0.5 mol %

1 mol %

2 mol %

S/So value

2.33

1.86

1.67

1.44

1.82

1.62

RTL intensity(arbitiary units)

22.53

22.71

21.87

12.00

11.37

8.08

4.2(b) It is well known that the optimum concentration of an

individual dopant (e.g. Dy3 • in CaSOi) is determined by the onset

of concentration quenching; beyond this concentration the TL

sensitivity declines. One possible wayt*9J of overcoming this

limitation in sensitivity is to co-dope CaSO4:Dy with another

activator which after gamma irradiation has a TL sensitivity ^nd

glow curve shape comparable to that of Dy (when doped

individually in CaSO« matrix). Initial experiments in this

direction using Tm3• as a co-dopant show that CaSO*:Dy (0.05mol%)

doped additionally with 0.011 mol % of Tm3• show 14 % increase in

:22:

TL sensitivity when compared with either CaSO4;Dy (0.05 mol %) or

CaSO<:Tin (0.011 mol % ) . Efforts are underway in order to

increase further the TL sensitivity of CaSO<:Dy by trying various

combinations of dopants.

4.3 Activator Concentration Dependent Radiation Induced

Sensitizatison and LET Effects in CaSO* :Jy and CaSOi :Tnt

Phosphors.

The radiation induced sensitization factors (S/So) for the

dosimetric peak in CaSOi:Dy and CaSO^:Tm decrease slowly with

concentration (Table 9) from the starting concentration of 0.05

mol %. The decrease of S/So values with increasing activator

concentration yielded extended linearitytso' and lesser

supralinearity in both the phosphors.

In CaSOi:Dy, concentration dependent LET effects have been

observed!s!3 for irradiations (in the grain size range 74-210 pm)

to Am-241 alpha particles and Co-60 gamma rays (Table 10). The

peak height ratio (alpha to gamma response) of the dosimetric

peakc9l 1 for 1.0 mol % doped Dy samples was 1.7 times that of 0.1

mol % Dy doped samples. Similar to CaSO<:Dy, concentration

dependent LET effects have been observed!"2' in CaSO4:Tm. The

corresponding ratio for 1.0 mol % doped Tm samples was 1.5 times

that of 0.1 mol % Tm doped samples. Thus these phosphors could

be exploited for high LET dosimetry.

Additionally, in CaSO4:Dy dependence of TL peak width on

stopping power has been investigated*s3' in the LET range 0.55 to

237 keV pnr* . Analysis of peak width parameters (i.e. TL peak

widths at a quarter (w, ) , half (w,) and three quarters {W3/4 )

of the peak maximum as a function of radiation dose) shows that

the amount of increase in peak width decreases with increasing

stopping power. That is, peak width is maximum for radiation

:23:

Table 10. TL sensitivity of CaSO-i :Dy (height of peak II)for alpha and gamma radiations as a function ofDy concentration. The sensitivity values areonly relative and not absolute because thesample thicknesses used are slightly largerthan the a-particle rangefs* ' .

Dy-Concentration

(mol%)

0.01

0.05

0.1

0.2

0.5

1.0

2.0

5.0

Alpha

2.26

6.82

7.78

9.59

14.06

13.97

12.78

8.05

Height of peak

Gamma

27.6

73.0

71.0

70.0

80.0

75.0

78.0

49.0

II (arbitrary units)

Alpha

Gamma

0.032

0.093

0.110

0.137

0.176

0.186

0.164

0.164

Alpha

Gamma(Normalisedat 0.1 mol%)

0.75

0.85

1.00

1.25

1.60

1.69

1.49

1.49

with low stopping power (0.55 keV unrl ) whereas, no increase in

peak width is observed for radiation with very high stopping

power ("240 keV pnr* ) . This dependence of peak width on stopping

power can be used as a measure of LET of radiation. However, the

additional dependence of peak width on the radiation dose at low

LET has to be considered^s"•'«I.

:24:

5. Conclusions

In addition to the various applications of the TL phosphors

in the field of radiation dosimetry stated above, numerous

studies have been done, in DRP, on CaSO4:Dy, CaSO-i:Tm,

Li2B<0?:Mn, Li2B4O7tCu, LiF:Mg,Ti, CaF2:Tm and many other common

TL phosphors in order to understand the basic TL phenomena,

nature of trapping entities involved in the TL process, role of

non-radiative processes on the TL behaviour of materials,

mechanism of radiation induced sensitization and supralinearity,

activator concentration dependent TL behaviour, photo-transfer TL

(PTTL), study of emission spectra of various TL peaks in

materials, determination of TL parameters for various TL

materials etc. In addition, we have gained from the numerous

detailed investigations on CaSO«:RE3', CaFz:RE3< LiF:Mg,Ti and

AlzO3:Ti TL phosphors carried out in Health Physics Division,

BARC for understanding the basic TL mechanism in these materials.

All these investigations have given us confidence to use

CaSO<:Dy , CaSCU :Tm and other TL materials for various dosimetrio

applications as well as in the development of new and sensitive

TL materials. In future we wish to develop a-Al2Oa:C, CaSO-j :Dy

and CaSO4:Tm phosphors co-doped with suitable metal ions ss

sensitizers of luminescence and rare earth doped double sulphate

systems.

Acknowledgements

I am grateful to Dr. U. Madhvanath and Dr.S.J. Supe for

encouragement and helpful discussions. I wish to thank Or. A.S.

Pradhan for going through the manuscript and giving suggestions

for modification.

:25:

References

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2. Yamashita, T., Nada, N., Onishi, H. and Kitamura, S. HealthPhys. 21, 295 (1971).

3. Ayyangar, K., Lakshmanan, A.R., Bhuwan Chandra and Ramadas,K., Phys. Med. Biol 19., 665 (1974).

4. Pradhan A.S., Bhatt, R.C. and Vohra, K.G. Radiochere.Radioanal. Letters 53, 103 (1982).

5. Lakshmanan, A.R., Bhuwan Chandra, Pradhan, A.S. and Bhatt,R.C. Radiochem. Radioanal. Letters 3_7_, 377 (1979).

6. Kirk, R.D., Schulman, J.H., West, E.J. and Nask, A.E. inProc. Symp. on Solid State and Chemical Radiation Dosimetryin Medicine and Biology, Vienna, (Vienna, IAEA), SM-78/23,p. 91 (1966).

7. Christensen, P. in Proc. 2nd. Int. Conf. on LuminescenceDosimetrys, USAEC-CONF 680920, p. 90 (1968).

8. Takenaga, M., Yamamoto, O. and Yamashita, T. Nucl. Instrum.Meth. 175, 77 (1980).

9. Kazanskaya, V.A., Kuzmin, V.V., Minaeva, E.E. and Sokolov,A.D. Proc. 4th. Int. Conf. on Luminescence Dosimetry,Krakow, Poland, p. 581 (1974).

10. Rao, G.S., Iyer, R.K., Gokhale, Y.W., Gupta, S.K.,Deshpande, S.G. and Gupta, S.S. Report No. BARC/I-591,Bhabha Atomic Research Centre, Bombay 400 085 (1980).

11. Pradhan, A.S., Ayyangar, K. and Madhvanath, U. in Proc.Natl. Symp. on Thermoluminescence and its applications,IGCAR (RRC), Kalpakkam, Madras, p. 404 (1975).

12. Vohra, K.G., Nagpal, J.S., Pendurkar, H.K. and Gangadharan,P. Association of Medical Physicists of India (AMPI),Medical Physics Bulletin, Vol 4 (No. 2), p. 108 (1979).

13. Pradhan, A.S., Bhuwan Chandra and Ayyangar, K. in Proc.Natl. Symp. on Thermoluminescence and its applications,IGCAR (RRC), Kalpakkam, Madras, p. 409 (1975).

:26:

14. Vohra, K.G., Bhatt, R.C., Bhuwan Chandra, Pradhan, A.S.,Lakshmanan, A.R. and Shastry, S.S. Health Phys. 38., 193(1980).

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17. Bhatt, B.C. Paper presented at IAEA/RCA workshop onIntercomparison of Personnel Dosimeters held at JAERI,Tokai-mura, Japan, 22-26 October, 1990.

18. Bhuwan Chandra, Pradhan, A.S., Lakshmanan, A.R., Popli, K.L.and Bhatt, R.C. IARP Bulletin (India), 2, 57 (1979).

19. Bhuwan Chandra, Pradhan, A.S., Lakshmanan, A.R. Diwakaran,T., Bhatt, R.C. and Supe, S.J. Presented at the 3rd AnnualAMPI Conference on Medical Physics, Bombay, March 19-21,1980. Also in Medical Physics Bulletin (AMPI), Vol. 5 {No.20, p. 73 (1980).

20. Pradhan, A.S. and Bhatt, R.C. Phys. Med. Biol.22., 873(1977).

21. Lakshmanan, A.R., Bhuwan Chandra, Pradhan, A.S., Kher, R.K.and Bhatt, R.C. Int. J. Appl. Radiat. Isotopes 3JL, 10"7

(1980) .

22. Lakshmanan, A.R., Bhuwan Chandra, Pradhan, A.S., and Supe,S.J. Radiat. Protect. Dosim. .17. 49 (1986).

23. Lakshmanan, A.R., Kher, R.K. and Madhvanath, U. Radiat.Protect. Dosim. 3C>, 179 (1990).

24. Vohra, K.G., Bhatt, R.C., Bhuwan Chandra, Pradhan, A.S. andLakshmanan, A.R. Health Phys. 42, 217 (1982).

25. American National Standard Institute ANSI Report N545-1975,New York (1975).

26. Vinten Instruments Ltd., U.K. Technical Note No. 107.

27. National Institute for Occupational Safety and Health(NIOSH), Criteria for a Recommended Standard OccupationalExposure to U.V. Radiation, HSM. 73-11009 (1972).

28. Nagpal, J.S. Radiat. Effects Letters 43, 173 (1979).

:27:

29. Srivastava, J.K. Bhatt, B.C. and Supe, S.J. Paper presentedin National Seminar on Thermolumineseence and itsApplications, Feb. 7-9, 1991. M.S. University of Baroda,Baroda - 390 001, India.

30. Lakshmanan, A.R., Bhuwan Chandra and Bhatt, R.C. Radiat.Protect. Dosim. 1_, 191 (1981).

31. Mason, E.W. Phys. Med. Biol. 16., 303 (1971).

32. Sunta, CM. and Watanabe, S.J. Phys. D : Appl. Phys. 9, 1271(1976) .

33. Nambi, K.S.V. and Higashimura, T. Proc. 3rd. Int. Conf. onLuminescence Dosimetry, Riso Report 249 (ARC, RJso,Denmark?, p. 1107 {1971).

34. Nagpal, J.S., Varadharajan, Geetha and Gangadharan, P. Phys.Med. Biol. 27_, 145 (1982).

35. Ayyangar, K., Bhuwan Chandra and Lakshmanan, A.R. Phys. Med.Biol. 19, 656 (1974).

36. Pearson, D.W. and Moran, P.R. USERDA Technical Report COO1105-227 (1975).

37. Bhatt, R.C., Lakshmanan, A.R., Bhuwan Chandra and Pradhan,A.S. Nucl. Instrum. Meth. 152, 527 (1978).

38. Fradhan, A.S., Bhatt, R.C., Lakshmanan, A.R., Bhuwan Chandraand Shinde, S.S. Phys. Med. Biol. 22, 723 (1978).

39. Storm, E., Buslee, P.L., Blackstock, A.W., Littlejohn, G.J.,Cortez, J.R., Fultyn, R.V. and Lawrence, J.N.F. Radiat.Prot. Dosim. 1, 209 (1981).

40. Pradhan, A.S., Bhatt, B.C. and Bhatt, R.C. Bulletin ofRadiat. Prot. (India), 13L, 47 (1988).

41. Alsmiller, R.G. and Barish, J. Health Phys. 26, 13 (1974).

42. Bhuwan Chandra, Bhatt, R.C. and Supe, S.J. Int. J. Appl.Radiat. Isot. 32, 553 (1981).

43. Lakshmanan, A.R. and Bhatt, R.C. Phys. Med. Biol- 2_4, 1258(1979).

44. Pradhan, A.S., Bhatt, R.C. and Supe, S.J. Int. J. Appl.Radiat. Isot:. 3_1, 671 (1980).

45. Bhuwan Chandra, Shinde, S.S., Lakshmanan A.R. .-m<:3 Blvitt,R.C. Phys. Stat. Sol. (a) 1JD3, 599 (1987).

:28:

46. Lakshmanan, A.R., Bhatt, B.C. and Bhatt, R.C. Radiat. Prot.Dosim. 27, 15 (1989).

47. Srivastava, J.K. and Supe, S.J. J. Phys. D : Appl. Phys. 22,1537 (1989).

48. Srivastava, J.K. Bhatt, B.C., Bhatt, R.C. and Supe, S.J.Presented at 3rd CECRI Research conference on Luminescence;,12-14 January, 1990, CECRI, Karaikudi, India.

49. Bhatt, B.C., Srivastava, J.K. and Shinde S.S. Paperpresented in National Seminar on Thermoluminescence and itsApplications, Feb. 7-9, 1991, M.S. University of Baroda,Baroda 390 001, India.

50. Bhuwan Chandra and Bhatt, R.C. Nucl. Instrusm. Meth. 164,571 (1979).

51. Lakshmanan, A.R. and Bhatt, B.C. Radiat. Prot. Dosim. 2J5, 31(1988).

52. Bhatt, B.C., Shinde, S.S. and Bhatt, R.C. 9th Int. Conf. onSolid State Dosimetry, Vienna, Nov. 6-10, 1989 (Abstractonly).

53. Srivastava J.K. and Supe, S.J. J. Phys. D: Appl. Phys. 13_,2337 (1980) .

54. Shinde, S.S. and Shastry, S.S. Int. J. Appl, Radiat.Isotopes 30, 75 (1979).

55. Pradhan, A.S. Radiat. Protec. Dosim. 1, 153 (1981).

56. Pradhan, A.S. and Bhatt, R.C. Nucl. Instrum. Meth.166., 497 (1979) .

57. Vohra, K.G., Pradhan, A.S. and Bhatt, R.C. Health Phys. 4J3,391(1982).

58. Charles, M.W. In Proc. 5th Int. Conf. on LuminescenceDosimetry, Sao Paulo, Brazil, p313(1977).

25 100 200

TEMPERATURE (*C)

300 400

Fig.1. Typical TL glow curve of CaSO^ .'Dy powderfor gamma dose of 0.1 Gy.

FIG. 2 . T L D BADGE SYSTEM

oCARD HOLDER CARD TLD BADGE

(Front view)

1. COPPER FILTER2. TEFLON TLD DISC3. OPEN WINDOW4. TRANSPARENT PLASTIC

WINDOW5. STAINLESS STEEL CUP

ASSEMBLY OF TLD BADGE(Back view partially open)

10

LL)

I

UJ

UJDC

0.10.01 0.1

PHOTON ENERGY (MeV )

Fig.3. Photon energy dependence of the TLD badge( a ) response under the Al*Cu filter,(b) response under the open window.

S 5

II III Iv v

Readout times

vl vll

Fig. 4. T L output of a card exposed to10 R of ^Co gamma rays and read outrepeatedly without subjecting to annealingor exposure.

HI

zoQ.CO

UJ>

UJCf.

1.0

0.8

0.6

0.4

0.2

n

y

A

/B

/

I

r

//

2 M TL11 I

u VQ

/

/

32 90P Sr/Y1 1 i

0.2

BETA ENERGY E M A X IMeV)

Fig. 5.. THE ENERGY DEPENDENCE OF TL DOSIMETERS

TO BETA RADIATION.(A) 0.025mm LiF TEFLON

DISC BONDED TO 0.2 mm TEFLON BASE I Charles

1977).(B) 0.1mm CaSO .'Dy DISC ON 0.7mm PURE

TEFLON BASE AND (C) 0.8mm CaSO^ :Dy TEFLON

DISC.

io3

i 10*

io5

16 V

10 I I

TLD-ALBEDO

1.0

0>

13

0.1 ItilDC

0.01

103 10* Id1 101 10'

NEUTRON ENERGY (MeV)

Fig.6. Energy dependence of T L D albedo, CR-39and NTA Films.

210

190

170

> 150H;

2130 —

110UJ

5 soUJ

70

50

30

10

O -GAMMA RESPONSE(250°C Peak )

X - PTa(25O°C Rak)

O -RTL(400°C P«a<)

£ ~RTL(485°C Pec*)

10 10 10 10

GAMMA DOSE ( Gray)

10

20.000

16.000

12,000

8000

4.000

5x10

F ig . 7. RELATIVE TL INTENSITIES OF THE GLOW PEAKS IN CaSO^ :Oy {0.05 moUS) AS A

FUNCTfON OF PRE-GAMMA DOSE. MULTIPLICATION FACTOR INDICATED ON THE CURVES

WAS APPLIED TO BRING THE INTENSITY WITHIN THE SC/UE.

130 #C Peak

225 °C Peak

375 °C Peak

0.5 1-0 2.0

Na CONCENTRATION (mol •/•)

Fig. 8. Effect of Na4 concentration onTL intensities of (a) A130°C,(b)«225#Cand • 375 *C TL peaks In CaSO4 'Dy(O.imot0 / .).

joO

8

2.0

1.8

1.6

u 1 A

o

o? 1.2inCM

2 1.0o1 0.8

0.6

0.4

(25* T O

t lh )«10(1 .625 -R) /0 .43

34*3 C

t(h)sK)(1.36-R)/0.43

10 10'

POST IRRADIATION INTERVAL, t (h)

Figure. 9. Dependence of glow curve area ratio (25-180 °C / 180-300'C)of CaSOA (0.1mot*/o Dyt 0.5mol% Na) on post-irradiation intervalfor dosemeters stored at RT and at 34£3°C. The plot between Rand log t is linear in both cases.

PAPER - 3

PRODUCTION OF CaSO^ : Dy PHOSPHOR FOR DOSIMETRIC APPLICATIONS

R.K. Iyer

PRODUCTION OF CaSO4;Dy PHOSPHOR FOR DOSIMETRIC APPLICATIONS

R.K.IYER

Applied Chemistry Division

Bhabha Atomic Research Centre

Bombay - 400 085

Solid state dosimeters based on thermoluminescence are

now being widely used in place of film badges for personnel

monitoring. CaSO<:Dy phosphor is one of the prominent materials

used for this purpose due to its high gamma ray sensitivity,

remarkable stability under extreme climatic conditions and

relatively low cost. To meet the requirement of this phosphor for

the countrywide personnel monitoring programme undertaken by the

Division Of Radiological Protection) B.A.R.C., work was taken up

to develop a suitable process for preparation of the phosphor in

large batches (300g or more per batch). The method of preparation

involves dissolution of the requisite quantities of Ca.SO4.2H2O

and Dy2O3 in cone.H2 SO<, removal of H2SO4 by evaporation,

isothermal annealing of the product at 700°C followed by grinding

and sieving to get powder of particle size <75 vim (average

particle size " 38um). The gamma ray sensitivity of the phosphor

prepared on a large scale by the open evaporation method differed

considerably from batch to batch due to contamination with

impurities. In addition, release of the sulphuric acid into the

environment resulted in severe corrosion and pollution problems.

Hence detailed studies were carried out to circumvent these

problems and a simple process was developed employing a simple

distillation assembly with, air as carrier gas and gradient

heating. The sulphuric acid is recovered and a major part of it

is recycled. The process gives a pure product of superior quality

with batch to batch variation in gamma ray sensitivity within ±

5%. With a 10 litre assembly, 600g of the phosphor can be

produced per batch. The phosphor is now being produced in

Chemistry Division, B.A.R.C. and supplied to the Division Of

Radiological Protection (6Kg/annum) for making CaSCU:Dy teflon

discs and other dosimetric applications.

INTRODUCTION

During the last three decades, a new type of dosimeter has

been added to the family of radiation measuring devices(1).

These are solid state dosimeters based on a phenomenon called

thermoluminescence(TL).The choice of a TL phosphor for dosimetric

use depends on several factors and hence only a handful of them

find commercial application. LiF:Mg,Ti; BeO:Na,Li; MgB4O7:Tb ;

Li2B4O7 :Mn,Si; AI2O3 ; Mg2SiO4 :Tb ; CaF2 :Mn; CaSO* :Tm and CaS04

:Dy are some of these phosphors. Among these, lithium fluoride is

the most widely used TL phosnhor.• The disadvantages of most

lithium fluoride phosphors are : (i) they can be reused only

after a relatively complicated and time consuming annealing

procedure (ii) permanent loss in sensitivity after exposure to a

high radiation dose (2). In later years, use of CaSO*:Dy phosphor

in gamma ray dosimetry attained prominence due to its

comparatively low cost and remarkable stability under extreme

climatic conditions(3,4). It is 20 to 30 times as sensitive as

LiF:Mg,Ti and radiation doses as low as 0.3 mrad can be

detected(5). It is used successfully in environmental dose

measurement in tropical(6) as well as arctic countries(7).

The Division of Radiological Protection, B.A.R.C. developed

TLD badges for personnel raonitering using CaSO4:Dy phosphor. The

estimated annual requirement of the phosphor in 1975 was 5 to 10

kg. Due . to the exhorbitantly high cost of the imported phosphor

(present cost is approx.. Rs 4 lakhs/kg), the Pure Materials

Section of Chemistry Division, B.A.R.C. was entrusted with the

task of developing a suitable method for production of the

phosphor.As a result of detailed studies,a simple process was

developed for preparation of the phosphor in large batches(8).

The salient feature of the process is the use of air as carrier

gas and gradient heating for removal of sulphuric acid by

distillation.The highlights of the above process are

incorporated in a review on thermoluminescence of calcium based

phosphors(9).

The specifications of the phosphor are i) high gamma

ray sensitivity ii) particle size < 75 um (average particle size

38ura) and iii) batch to batch variation of .gamma ray

sensitivity within ± 5%, The phosphor should be of high purity

since impurities such as iron act as poisons while impurities

like manganese are sensitive activators which cause fading of TL

output.

PREPARATION

The details of the work carried out for developing the

above process and also the standardised procedure for preparation

of the phosphor are given below.

The method of preparation of the phosphor on a few

grams scale has been reported earlier(10-12).Yamashita et al(10)

prepared the phosphor by dissolving the required quantities of

CaSO4.2H2O and Dy2O3 in the minimum quantity of sulphuric acid

and evaporating the sulphuric acid at about 300°C. Though the

method is very simple, major problems are associated with the

preparation of the material on a large laboratory scale.

Open Evaporation

400 g of CaSO4.2HaO and 0.217 g Dy2O3 were dissolved in 3

litres of Cone.H2SO4 in a shallow china or silica dish and heated

using a burner inside a fumehood.The problems encountered were :

i) corrosion of parts of fumehood and environmental pollution due

to the copious fumes of H2SO4 ii) increase in volume due to

absorption of moisture on leaving the partially evaporated

solution overnight iii) occassional breakage of dish during the

final stages of evaporation and iv) contamination of the product

with external impurities. The batch to batch variation in gamma

ray sensitivity was ±10X(Table I).

Table I

TL sensitivities and spectrographic analyses of CaSOi:Dy samples

Sample

No.

Time

for

of H2

required

removal

SCU (Hrs)

Spectrographic

Fe Pb

(in parts per

analyses

Mg Mn

million)

Relative res-

ponse to gamma

radiation

12

3

4

5

6

830

35

35

16

16

10

. 30

50

50

25

10

<5<5

<5

<5

<5

<5

1010

• it;

10

25

10

<5CS

<5

<5

10

<5

1.00.9

0.85

0.8-*

1.0

* peak structure of glow curve altered.

Remarks: Sample Nos 1 to 4: prepared by open evaporation.

Sample Nos 5 and 6: sulphuric acid removed by distillation.

Distillation

Bumping was severe during distillation of the solution.

Distillation in presence of air as carrier gas

This was reasonablly satisfactory, but occassional bumping

was observed during the later stages of distillation due to

increase in concentration of solution.

When use of air as carrier gas was coupled with gradient

heatingi distillation was smooth and no problem was encountered.

METHOD STANDARDISED

The details of the method standardised for obtaining the

phosphor of the required purity and also the recovery of

sulphuric acid by distillation under suitable conditions for a

typical batch, are given below:

Batch scale :300 g

Calcium sulphate dihydrate :400 g

(BDH Analar grade)

Dysprosium oxide(99.99%) :0.217 g

Cone. Sulphuric acid :4 litres

The dysprosium oxide was dissolved in 400 ml of hot

sulphuric acid and transferred to a 5 litre 2 necked round

bottomed flask. 2.85 litres of sulphuric acid were added to the

flask which was then heated on an isoraantle to about 200°C. A

slurry of the calcium sulphate dihydrate made in about 750 ml of

cone.sulphuric, acid was added in small quantities to the contents

of the flask with constant stirring. * The heating was continued,

with occasional stirring till a clear solution was obtained.

Distillation of the sulphuric acid without bumping was

achieved by using air as carrier gas and gradient heating. The

assembly employed is shown in Fig.l. Dry air was drawn through

the system at the rate of 1.5 to 2 litres/min. using a water

suction pump. The isomantle used contained two heating coils with

independent controls so that the temperature at the bottom and

the side could be controlled as desired. Heating tape was used to

heat the top part of the flask. The temperatures Tl, T2 &nd T3

at different parts were monitored using thermocouples. Tl and T2

were maintained at 275-300°C and 325-350°C respectively. A double

walled condenser was used to condense the vapours. During

collection of the first fraction (*200ml) T3 was 290-310°C.

The temperature subsequently rose to 316-318°C. The distillation

rate was 250-275 ml/hour.When there was no liquid in the flask,

both Tl and T2 were raised to « 3750c to decompose the calcium

bisulphate formed into CaSC-4 and H2SO4.Nearly 98X of the

sulphuric acid was recovered out of which more than 90X was 35

-36 N.This was recycled for processing subsequent batches.The

time required was 16 hours.

The resulting crystalline product of calcium sulphate

( doped with dysprosium) was powdered and heated to remove the

residual sulphuric acid. The product was then isothermally

annealed in air at 700°C for two hours, ground and sieved to get

powder of particle size < 75um (average particle size 38um).The

powder was then blended and homogenised using a horizontal

blending machine.

CHARACTERISATION OF THE PHOSPHOR

This was carried out by the Division of Radiological

Protection. The method consists of exposing a sample (100 mg) of

the phosphor to a known and fixed dose of Co60 gamma rays and

measuring the TL output, 24 hours after irradiation, using a

'Thermoluminescent Reader' assembly. The suitability of the

phosphor is determined by comparision of the TL peak structure

and the total TL output with those of a standard. The glow curves

of the sample and the standard are given in fig.2. These were

obtained at a non-linear heating rate, the average heating rate

being 14°/sec.

The conditions of preparation of the phosphor were

optimised such that the batch to batch variation in gamma ray

sensitivity was within ±5%.

EFFECT OF VARIOUS PARAMETERS ON GAMMA RAY SENSITIVITY

Gamma ray sensitivity of the phosphor is considerably

affected by the concentration of impurities during the process of

its preparation as well as the particle size of the final

product. It has been found that the presence of manganese and

iron beyond the limits of 5 and 10 p.p.m. respectively in the

final product is detrimental to the sensitivity of the phosphor

(Table I). These impurities generally creep in from the bulk of

the sulphuric acid used and get concentrated during its removal

by distillation (although they may be present at sub p.p.m. level

in the bulk used). Therefore one has to be careful to chovje the

starting materials of the desired purity, especially with respect

to iron and manganese. The effect of particle size on the

sensitivity of the phosphor has. been studied in detail

earlier(13,14). The optimum particle size suitable for TLD badges

is <75u (average particle size ~ 38um).

The phosphor is now being produced in Chemistry Division,

B.A.R.C and supplied to the Division of Radiological Protection

(6Kg/annum) for making CaSOi :Dy teflon discs and other dosimetric

applications.

Methods of preparation of the phosphor similar to the one

given above have been reported subsequently (15,16). However, the

basic feature remains the same. In these, the sulphuric acid

obtained by distillation is neutralised with NaOH. It is not.

advisable to do this since 90% of the sulphuric acid recovered

has a strength of 35-36N which is suitable for reuse.

The work reported here was carried out in Chemistry

Division, B.A.R.C in collaboration with S/S Y.W.Gokhale,

S.K.Gupta, S.G.Deshpande and S.S.Gupta and Dr G.S.Rao.

REFERENCES

1. K. Becker, Solid State Dosimetry, CRC Press, Cleveland, Ohio,

U.S.A.(1973).

2. K.P.Doppke and J.R.Cameron, Report COO-1105-119, USAEC,

Phys.Med.Biol., i£i 571 (1970).

3. A.S.Pradhan, Bhuvan chandra and K.Ayyangar, Proceedir>,;» >i

National Symposium on Thermoluminescence and its Applications,

R.R.C, Madras, India, 409 (1975),

4. Teledyne Isotopes, 50, Van Buren Avenue, Westwod, N. J. 07675,

U.S.A.

5. K. Becker, Progress in Luminescence Dosimetry, Science, 177

539 (1972).

6. K. Becker, R.H. Lu and P.S.Weng, Proc Third Int.

Conf.Luminescence Dosiraetry,Riso-Rep. 249, Vol.2, Danish

AEC,Riso,Roskilde,960 (1971).

7. B.L.Nielsen and V.Mejdahl, Riso-Rep.219,Danish

AEC,Riso,Rokilde(1970).

8. G.S.Rao, R.K.Iyer, Y.W.Gokhale, S.K.Gupta, S.G.Deshpande and

S S.Gupta, Preparation of CaSO4; Dy phosphor Report No.

BARC/I- 591, Bhabha Atomic Research Centre, Bombay -

400 085 (1980).

9. C.M.Sunta, Nucl. Tracks, 10, 47(1985).

10.T.Yamashita, N.Nada, H. Onishi and S. Kitamura, Proc.2nd Int.

Conf. on Luminescence Dosimetry, USAEC-CONF 680920,4(1968).

U.K. Becker, Nucl. Instrum. Meth. , ifiit 405 (1972).

12.K.Ayyangar, Bhuwan Chandra and A.R.Lakshmanan, Phys.Med.Biol.,

H , 656 (1974).

13.Bhuwan Chandra, K. Ayyangar and A.R. Lakshmanan,

Phys.Med.Biol., ZX, 67 (1976).

14.A.S.Pradhan and R.C.Bhatt, Nucl. Instrum. Meth., 161. 243

(1979).

15.L.L. Campos, Journal of Luminescence, £&, 481 (1983).

16.J. Azorin and A. Gutierrez, Health Physics, ££, 551 (1989).

AIR—=5

F = - * T O WATERSUCTION PUMP

ISOMANTLE

FK3.1 EXPERWENTAL ASSEMBLY FOR PREPARATION OF CaSO4 : Dy PHOSPHOR

> 2

HI

enEXPOSURE : 2-5 R OF CoY-fWSAVERAGE HEATING RATE: 14*/SEC.

210 *C

STANDARDSAMPLE

15 30 45TIME (SECONDS)

FIG. 2 GLOW CURVES OF Ca$O4*Dy PHOSPHOR

PAPER - *

PREPARATION AND CHARACTERISATION OF RARE EARTH

BASED PHOSPHORS

G. Alexander

PREPARATION AND CHARACTERISATION OF RARE EARTHBASEP- PHOSPHORS

BY

G. Alexander,Uranium Extraction Division,

Bhabha Atomic Research Centre,Bombay - 400 085.

Abstract

Rare Earth phosphors of the Europium .activated typo findextensive application in colour T.V. tubes and mercury vapourlamps. Potential uses of Terbium activated phosphors are alsofound in medical radiology involving X-rays. Several crores atRupees of precious foreign exchange are spent every year for i tsjimport. The technology for the preparation of these phosphors isa closely guarded secret. The basic raw materials of the highpurity Rare Earths are now being produced witin the country bythe Indian Rare Earths in active collaboratin with the UraniumExtraction Division of BARC. With a view to commercially exploitthese raw materials for indigenous production of the valuablephosphor, appropriate facilities have been created for thepreparation and characterisation of these phosphors. Some of thephosphors successfully synthesised are the Eu(3+) activatedYttrium vanadate, Yttrium oxide. Yttrium oxysulphide and Eu(2+)activated Aluminosilicate, Tb(3+) activated Yttrium oxysulphideetc. The emission and excitation spectra of the phosphors havebeen examined and their quantum and c^thodoluminescentefficiencies have been evaluated.

I. Introduction

Of late, interest in luminescent materials based on rareearths either as host lattices or as activators is increasing.The year 1964 marked the. beginning of a new era in phosphorpreparations and applications based on rare earths. The longfelt need of the colour T.V. industry to have a narrow band redemitting phosphor with the correct eye fesponse characteristics

was realised with the discovery of Eu(3+) activated YV04 byPalilla etal.(l) Thereafter, the emergence of a broadtheoretical framework based On crystal field theory as applied torare earth crystalline lattices incorporating rare .earthactivators coupled with the modern purification methods ofindividual rare earths, resulted in the multifaceled growth ofthe rare earth phosphor industry covering many areas of presentday applications. Several variants of the original YV04:Eu(3+)have since appeared. New phosphor host lattices with various.combinations of activators and co-activators have beendiscovered. A peculiar feature is that host lattices played asignificant role in deciding the colour of the emission likethe blue emission of Eu(2+) and the i«d emission of Eu(3+) indifferent crystalline matrices. Characteristic red emission ofEu(3+) and the green emission of Tb(3+) have led to the developmentof many efficient group of phosphors incorporating these ions forthese two colours. The task of a phosphor scientist is to findthe right type of crystalline matrix to incorporate the. activatorin optimum concentrations with suitable combinati< 'i-s ofco-activators which will help to enhance the omissioncharacteristics of the activator, for the sole purpose ofextracting maximum efficiency of emission of a particular colour.The modern crystal field theory gave a thrust to thee.se

developments.

Applications in which rare earth phosphors are put to useare numerous. Mention must be made at this stage that yttriumthoi"~h strictly not a rare earth is clubbed with other rareearths primarily because of its close chemical identity andbecause of its ability to form crystalline compounds of similarcrystal structure like Gadolinium, lutetium etc. Tonic siso-i ofyttrium also matches closely with that of the rare earths. Inaddition it is optically inert and in this respect helps not tointerfere with any activator emission. Activator ions likeEu(3+), Tb(3+) etc. find easy substitution in lattices formedwith yttrium and other rare earth compounds with almost identicallattice surroundings. An examination of the emissioncharacteristics of Eu(3+) in Gadolinium and yttrium vanadatebears ample testimony to this fact (2). Thus modern phosphorterminology does not made any distinction between yttrium andother rare, earths and treats both as belonging to the samefamily. Mixed crystal lattices like (Y,Gd)2O2S., (Y,Gd)2 03 etc.are easily realizable with varying ratios of these

A broad subdivision can be made of rare earth phosphors ascoherent and incoherent rare earth optical materials. In thecoherent group, we have Ndodymium activated Yttrium AluminiumGarnet as the shining example of a laser material. Singlecrystals of YVC4 and Y2O3 have also been found to be excellentlaser hosts for ions like Eu(3+) and Nd(3+). Area of our presentactivity is centred on the development of incoherent group ofmaterials such as powder phosphors, mainly from the ind<jenoussources of raw materials. Rare earth oxide materials o.f up tofour nine purity are now available from Indian Rare Earths. Themost important new application of Rare Earth phosphors hasoccured in the fields mentioned below:1. Cathodoluminescent devices2. Fluorescent lamps3. X-ray image intensifier tubes and dosimetry films4. Thermoluminescent dosimeters5. Up converting phosphors

Since the bulk of the phosphor requirement is in the fieldcathodoluminescence and fluorescent lighting, such phosphors willbe highlighted in this presentation. The mechanism ofluminiscence being the same in X-ray and cathodoluminescentphosphors and similar phosphors are also used in X-rays, thisalso will be briefly discussed.

II. Phosphors for Cathodoluminescent Scieens:-

Phoophors for cathodoljminescent applications such us incolour T.V. tubes, only the red emitting europium activated typehave so far been found successful. No replacement for the (Zn,Cd)S type phosphors for the blue and green primary colours hasbeen found acceptance yet. This is so mainly because (Zn, Cd)Svariety have unmatched efficiency figures and superior colourcharacteristics. However, one area in which the Tb activatedgreen rare earth phosphor holds promise is in its saturationcharacteristics. The emission efficiency of this phosphor islinear with respect to current density to a great, extent where asthat of (Zn, Cd)S type is not. Non linearity with current densitymay lead -to errors in colour balance - a limitation standing inthe way of improved picture quality by increasing current.density. Thus search is still on for an improved Tb(3+) activatedgreen phosphor. After the intioduction of YV0^:Eu(3+) in W64 asa red colour primary phosphor, new materials of the Eu(3+)

activated variety like Y,03 and Y2O2S with superior efficiencyvalues have been discovered. Addition of several co-doi»>inL-; suchas Tb, Bi, etc. in these lattices have been tried for efficiencyimprovement. The relative brightness figures of these phosphoi sare given in Table I. By the present reckoning,Y202S:Eu(3+) phosphor appears to have an edge over others.Cathodoluminiscent efficiency of several phsophot .-, with Eu and Tbactivation are given in Table II. All the above phosphors withEu(3+) activation give narrow line emission in the range 611 to627 nm.

Table I (Red emitting Phosphors)

Material Relative Brightness Relative Enoi <jy el f ii-i

Zn ,Cd nS:Ag

:Eu3 +

Y2O3:Eu"

Y2O2S:Eu3 +

48%

.57%

100%

100%

180%

86%

100%

120%

(Data Taken from Electro. Chem. tech. Vol. 4, 21-24 (1966))

Table II

Material Cathodoluminescent ef f icit-ncy

YV04:Eu(3+)

Y203:Eu(3+)

Y202S:Eu(3+)

LaO2S:Tb(3+)

6dO,S:Tb(3+)

5 . 4 - 7 . 1 %

6 . 5 - 1 0 . 3 % ( 1 2 )

16% ( 1 3 , 1 4 )

13ut,

18%

Ill Phosphors for fluorescent lighting

Rare Earth phosphor* are extensively used as colourcorrectors in High Pressure Mercury Vapour lamps. H.P.M.V lampsgive intense light emission in the blue, green and yellow regionbut defficient in the red. It also emit intensa ultra violentradiation which goes uruitlised. The net effect is that goodcolour retention is affected in addition to a loss in energyefficiency. Eu(3+) activated red phosphor has come in handy as acolour corrector in H.P.M.V. lamps. The inner surface of theenvelope of the lamp is coated with a powder layer of thephosphor which absorbs the ultraviolent radiation and emit redlight, thereby compensating for the deffic iency of the lamp inthe red region of the spectrum. The desirable properties of asuch a phosphor are (1) high quantum efficiency (2) a broad andstrong ultraviolet absorption band (3) high quenching temperatureof luminescence. High quantum efficiency is essential to ensure alarge visible light output for all the ultraviolent radiationthat is absorbed by. it'. Quantum efficiency is defined as thenumber of emitted photons to that absorbed by it. Quantumeffciency as high as 90% have been realised for YV04:Eu(3+)phosphor. Fig. 1 shows the excitation spectrum of this phosphorwhich is broad and covers the entire region of the mercury vapourlamp ultraviolent emission. A notable feature of the Eu(3+)activated YV04 and Y203 is that its luminous efficiency remainsunaffected at high temperature, an essential criteria forselection for HPMV lamps since its operating temperature is above300*C •

IV Rare Earth phosphors for X-ray Application

The basic similarity of the excitation mechanism of cathoderays and X-rays in powder phosphors led to its adoption in X-rayintensifying screens and image intensifier tubes„ The twoimportant properties which suggest most strongly the use of rareearth phosphors in X-ray intersifying applications (4,5) are(1) the high cathode-ray efficiency of the terbium activated raiu

earth oxysulphide and(2) the high average atomic number and the consequent increase in

absorption coefficient for the Lanthanum and Gadoliniumbased compounds.The reported cathode ray efficiencies for

La2O2S.Tb(3+) - 13% and Gd2O2S:Tb(3+) - 18% compare favourablywith the till now used (Cd, Zn)S: Ag of 15 to 20% in x-ray

intensifiar tubes.' The replacement of (Cd,Zn)&.A:; by one or theother of the terbium activated oxysulphide should increasesubstantially the number of X*ray photons absori .•<•_ i by the X-rayintensifier input screen and may lead to a significant increasein net light output from the tube for a given operating conditionand incident X-ray flux. The two important advantages resultingfrom the use of rare earth oxysulphidos are (1) increase in thefractional utilisation of incident X-ray photon, tho majorbenefit of which is a lower dosage for equivalent data content inthe output image (2)substantial net gain in tube brightness atany given dosage level due to easier development of rare earthphosphor powder of the required particle size.

Since 1896, CaWO4 is the only phosphor used in X-ray filmintensifying screens. Though it . has a relatively high x-rayabsorption coefficient, its light conversion efficiency of 3 to5% is very poor. The two rare earth • oxysulphidesLa20j,S:Tb(3+) and Gd2O2S:Tb(3+) have very high light conversionefficiencies in comparison. However, its major disadvantage isthat the present day X-ray film developed solely for CaWQ4(emission ir. the blue) is not sensitive to the green emissionfrom these phosphors. If a green sensitive film equivalent to thepresent blue sensitive one is made available, the X-ray exposurereduction by as much as a factor of 20 could become feasibl«(5).This is a phenomenal gain considering the risk factors thataffect health. Indeed green sensitive film is already on its wayto development.

V An introduction to the theory of Rare Earth Luminescence

Tripositive rare earth ions when incorporated in crystallinematrix possess unique spectroscopic properties. The 4f shell isincomplete and the electrons in it are protected by the 5s and 5psubshells from interaction due to surrounding ions. he energylevel transition within the 4f shell are spin and paarityf orbidden(7,8,9) .• The crystal field splitting of the electronicenergy levels is of the order of 100cm" whereas that in atransition metal ion is of the order of 104cm~J. Inspite of thispoor crystal field interaction, many rare earth ions like Eu(3+),Tb(3+), Dy(3+) etc, are found to be optically active. The reasonfor this observed phenomenon is the strong spin orbit coupling inconjunction with the weak crystal field interaction - leading toa partial lifting of the spin and parity prohibition rules. Forefficient luminescence to occur from such ions, the followingconditions are to satisfied.

(1) Activator ions, should occupy a lattice site devoid ofinversion symmetry.(2) Activator ion should have"*comparable ionic size to that ofthe host lattice cation which it replaces and also similarcrystal structure.(3) Activator concentration should not exceed a certain maximumpercentage of the host cation.(4) A certain classes of ions known as sensitisers in verysmall concentrations will enhance the emission efficiency of theactivator whereas certain others will quench the activatoremission. Hence, purity of the materials used for preparation ofthe phosphor.

In the light of the above v requirements fo< activatoremission, the characteristics of a typical phosphor likeYV04:Eu(3+) with reference to the energy level diagram of theEu(3+)(1) ion will be briefly discussed below. The» energy leveldiagram of Eu(3+) ion is given in Fig. 2.

There are 6 electrons in the 4f shell of the Eu(3+) ionwhich is less by one electron for a half filled stable state. Theexcitation of the Eu(3+) ion will result in the jumping over ofan electron from the surrounding oxygen ion to the 4f shell,which is termed as the charge transfer state, the state of theexcitation being 4f72p~1. The charge transfer state slowly losesits energy to the lattice in small packets as phonons and finallyarrives at the luminescent level. Luminescent levels are denotedby black half circles. There are 4 such levels - SD O to5D 3 from- where luminescent transitions can occur to the groundlevels 7FO to

7Ffi. .Symmetry considerations of the lattice, willdecide the criteria for transition between these levels. Anexample of an electric dipole transition from 5 Do to 7F2 willalways result in red emission irrespective of which lattice theion is located. Thus the charge transfer state play a part in theenergy of the excitation but tha emission, colour is alwaysindependent of this state, the site symmetry consideration forthe electric dipole transition 5no~*>

?'r2 is alwayssatisfied in the yttrium vanadate lattice which crystalises inthe tetragonal system with zircon structure. Europium alsocrystallises as orthovanadate with the same crystal structure andsimilar lattice constants, hence Eu3+ can readily substitute forY in the YV04 lattice.

The energy level transitions within the 4f shell is given inFig 2. All fluorescent transitions are found to originate fromthe 5D O level and the terminating levels except 7FQ are all

degenerate. A case of the degeneracy of the 7F 2 level beinglifted by the crystal field of YV04 is shown in this fig- andresults in a group of linens which are very closely spaced. Twomost prominent lines in this group are 615nm and 6l9nm andalmost 70% of the energy of emission occurs here. In additionthere are emission bands centred at 698nm due to the electricdipole transition 5De--

7F4 and 595nm due to the magnetic dipoletransition Do~~ (% • °ne typical feature of the electric dipoleemission is that it is characterised by a larger than usual levellife time of the order of 10 3 sec. which is indicative' of theforced nature of the transition.

VI Preparation of rare earth based phosphors

a)GdV04:Eu(3+), YV0,,:Eu(3+)

The phosphor is synthesised by mixing stoichiometricproportions of Gadolinium or yttrium oxide with a little excessof V205» and Eu203 as a small percenage of Gd or Y and firing attemperatures above 1000°C. Mixing of the constitutents is carriedout in a ball mill and on a laboratory scale by using a mortarand pestle. The compound YV04: Eu(3+) is formed by the process ofsolid state reaction. Purity of the starting material is 99.99%(obtained from Indian Rare Earths), Vanadium Pentoxide >99.9% andEuropium oxide >99.9%. After firing at temperature above 1000'Cfor 2 to 3 hours, the material is taken out, ground to a finepowder and washed in ammonia solution for the removal of anyunreacted excess vanadium pentoxide. After rinsing several timesin distilled water, a second firing for one hour is carried out.Measurements are carried out after grindiny' the material to afine powder.

b) Y203:Eu(3+)

Europium activated yttrium oxide phosphor is prepared bydissolving both Y203 and Eu2Os in appropriate concentrations inconcentrated' nitric acid and precipitating the homogeneousmixture of both yttrium and europium oxalate by the addition of aconcentrated solution of oxalic acid. The mixed oxalate is driedand fired.for 1 hour at 800*C in a quartz crucible and refired at1000 C for 3 hours. Since there is no flux used for thepreparation, no cleaning process is involved in the preparationof Y203 Eu(3+) phosphor.

c) Y202S:Eu(3+)

The Y202S:Eu(3+) phosphor*is prepared by the sulphur carbonateflux technique. The chemical constituentsY203,Eu203, Na2C03 and excess quantity of sulphur overstoichiometric compositons are mixed together and loaded in aplatinum crucible and fired under nitrogen atmosphere in a quartzchamber using an R.F. heater as the heating source. Thetemperature of firing is about 1100°C for about 2 hours duration.The material after cooling to room temperature is ground to afine poweder and washed in distilled water a number of times anddried.

d) A12O3: SiO2 : Eu(2+)

An aluminosilicate phosphor activated by the Eu(2+) ionhaving quantum efficiency close to unity with emission peaking at450 nm was described by A. Wacht(lO). From quantum efficiencyvalues and emission characteristics, the phosphur appears to beof excellent quality as a blue component lamp phosphur. Thisphosphor is prepared in the laboratory by the addition of excessalcoholic NH4OH to a solution of Al(N03)3 + Eu(N03)3 i-Si(OC2H5)4 in about 70% alcohol. The gel thus formed is waterwashed, dried at 950'C and fired in air at 1250*C for 2 hours.After grinding the product to a fine powder, it is refired inhydrogen atmosphere in a quartz chamber at 1100'C for 2 hoursusing an R.F. heater. The phosphor is cooled to room temperatureand ground to a fine powder.

VII Evaluation of Phosphor Characteristics

Phosphors have to be evaluated for its efficiency andspectral emission charactristics. In our laboratory, facilitieshav been created for ' the measurement of photo andcathodoluminescent efficiencies and cathodoluminescent spectra ofa phosphor.

a) Quantum efficiency

An apparatus- has been designed and fabricated for themeasurement of quantum efficiency of a phosphor using aphotodiode as a sensing element. A block diagram of the apparatusis given in Fig. 3. It consists of a high pressure mercury vapourlamp as a source of ultraviolet light, an interference filter toisolate a narrow band of ultraviolet radiation from the mercury

vapour lamp, a quartz lens to focus ultraviolet light to t^ephosphor plaque and the silicon phototfiode electrometercombination to detect and m&asure radiation intensity. Theintensity of the u.v. light used'for excitation of the phosphoris measured by reflecting it ^ith MgO powder. The quantumefficiency is computed(ll) using the formula

o = (f /i-r)*(iuv/ivi)*(c2/cl)

where " ("" is the reflectin coefficient of MgO for U.V.radiation "r" is the reflection coefficient of the phosphor foru.v. radation. I and Iv. are the currents produced by thephotodiode per photon p&r sec. for ultraviolet and visibleradiation respectively, Cx and C2 are the measured currents bythe electrometer detector combination for ultra violet andfluorescent radiation signals respectively.

b) Cathodoluminescent efficiency and spectra

A demountable all metal cathodoluminescent apparatus hasbeen designed and fabricated to record the cathodoluminescentspectra and cathodoluminescent efficiency of the phosphors. Theapparatus consists of a S.S vacuum system which can give a vacuumof the order of 10"6mm of Hg, an electron gun which can give abeam current of a few microampers at a continuously variableacclerating voltage' up to 20kv, a 1\4 meter Jarrel Ashmonochromabor to scan the cathodoluminescent spectra and arecorder to record the spectrum. The phosphor is deposited on analuminium plate of lxl inch and mounted on a cubic block ofcopper. Four such samples can be mounted on the block at the sametime which can be rotated from outside and made to face thedefocused electron beam at an angle of 45°. The light emitted bythe phosphor enters' the •entrance slit 'of- the tnonochromatorthrough a quartz windiow and the monochromator output is fed to arecorder for recording the spectra. The cathodolumeniscentefficiecncy of the phosphor can be computed by measuring the beamcurrent, accelerating voltage and the intensity of the lightemitted by the phosphor. A block diagram of the apparatus isgiven in Fig. 4.

VIII Results

Quantum efficiency values of the phosphors prepared in our

10

activated variety like Y203 anc' Yi°2S w* t n superior efficiency

values have been discovered. Addition of several co-dopants suchas Tb, Bi, etc. in these lattices have been tried for efficiencyimprovement. The relative brightness figures of these phosphorsare given in Table I. , By the present reckoning,Y202S:Eu(3+) phosphor appears to have an edge over others.Cathodoluminiscent efficiency of several phsophors with Eu and Tbactivation are given in Table II. All .the above phosphors withEu(3+) activation give narrow lino emission in the range 611 to627 nm.

Table I (Red emitting Phosphors)

Material Relative Brightness Relative Energy efficiency

Zn 2Cd eS:Ag 48% 180%

YVO4:Eu3f 57% 86%

Y203:Eu3* 100% 100%

Y202S:Eu3+ 100% 120%

(Data Taken from Electro. Chem. tech. Vol. A, 21-24 (1966))

Table II

Material Cathodoluminescent efficiency

YVO4:Eu(3+) 5.4 - 7.1%

YiO3:Eu(3+) 6.5 -10.3% (12)

Y202S:Eu(3+) 16% (13,14)

LaO2S:Tb(3+) 13%

GdO2S:Tb(3+)

11

laboratory are given in Table 3. Detailed preparation techniqueshave been studied so' far for YVO4:Eu(3+) phosphor only. Theoptimum quantum efficiency obtained for the phosphor is 85 - 90%which compares favourably with the value reported in theliterature. Few preliminary experiments carried out forY203:Eu(3+) has yielded a quantum efficiency of 60%. Thequantum efficiency value obtained forA12O3:SiO2:Eu(2+) is of the order of 50%. A lot more work is yetto be done on the preparation of Y202S:Eu(3+)• The evaluation ofcathodoluminescent efficiency is also at the preliminary stages.The recorded cathodoluminescent spectra of YV04:Eu(3+) taken inour apparatus is given in Fig.5.

Table III

Material Quantum Efficiency

YVO4 : Eu(3+) 70 - 90 %

Y203 : Eu(3+) 50 - 60 %

A12O3 : SiO2 : Eu(3+) 50 %

Y2O2S : Eu(3+) very low

Acknowledgements

The author wish to thank Shri K.S. Koppiker, Head, UraniumExtraction Division and Shri U.R. Marwevh, Head, ProductDevelopment Section for all the support and technical guidanceextended for this programme. Author also wish to thank Shri S.D.Samant, Health Physics Division, BARC for many encouragingdiscussions and technical help extended, for this work.

References

1) Levine A.K. and Palilla F.C.Appl. Phys. Letters 5,118 (1964)

2) Palilla F.C.. Levine. A.K. and Rinkevin M.J.J. of Elec. Chem. Soc. 112, 776 (1965)

12

3) Bril A and Wanmaker W.L.Philips Tech. Rev. 27, 22 (1966)

• * •

4) Wickershiem K.A., Alves R.V. and Buchanan R.A.IEEE Trans. NS-17, 57 (1970)

5) Wang S.P. etalI-EEE Trans. NS-17, 49 (1970)

6) Buchanan R.A., Finkelstein S.I. and Wickershin K.A.Radiology 105, 185 (1972)

7) Blasse G. and Bril APhilips Tech. Review 31, 304 (1970)

8) Judd B.R.Phys. Rev. 127, 750 (1962)

9) 0 Felt C.S.J. Chem. Phys. 37, 511 (1962)

10) Watchel A,J. Elec. Chem. Soc.. 116, 61 (1969)

11) A paper on the design of the apparatus and evaluation of•quantum effi"c"6ncyi'tJf'~iTrof"g«f«««*i««»t*«k*#«»fj»+>6>ai>hors is due forpublication by the author.

12) Buohanan R.A. et alJ. Appl. Phys. 39, 4343 (1960)

13) Shrader R.E. and Yoecom P.N.J. of Luminescence 1,2, 14 (1970)

14) Lehmen WJ. Electro. Chem. Soc. 1164 (1970)

13

35

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FIG. 5.

PAPER - 5

DEVELOPMENT OF LIQUID SCINTILLATION CHEMICALS IN BARC

K.A. Noras and CM. Paul

DEVELOPMENT OF LIQUID,SCINTILLATION CHEMICALS IN BARC

K. A. NORAS and C. M. PAULChemical Engineering DivisionBhabha Atomic Research Centre

Bombay-400 08b

INTRODUCTION

One of the oldest techniques for the detection of radiation

is by the observation of scintillation light produced by

the action of radiation on certain materials. The scintillation

phenomenon is employed as a useful method for the detection •and

measurement of a wide assortment of radiations. Various types of

scintillators are available but this paper discusses the

development in BARC. of only liquid organic solutions as

seintillators.

In liquid scintillation counting, the radioactive labelled

sample is usually dissolved or kept in suspension in an

appropriate solvent. A small percentage of a primary solute

added fluoresces when excited by energy tr«tns±er processes taking

place within the mixture. Sometimes a secondary solute is added

as a wavelength shifter to tailor the emission spectrum for a

particular purpose. The sample, solvent and solute ifluori

mixture is taken in u glass or plastic vial and photomultiplier

tubes detect the light emitted from the vial. Electrical pulses

are produced and the pulse height is proportional to the energy

of the initiating particle. The liquid scintillation counting

technique is widely used for counting low level beta activity

such as from tritium and carbon 14. However, a number of

isotopes which emit radiations other than betas are also counted

by this method.

To assay tritium in heavy water moderated reactors,

development work on the preparation of scintillation chemicals

for use in liquid scintillation counting was undertaken. Their

preparation encompasses the field of synthetic organic chemistry

and as these chemicals have commercial importance most of the

methods of preparation reported in literature are patented and

hence other methods of synthesis were developed for some

1

chemicals. Over the years, the work carried out could be

classifiod as follows:

(1/ preparation of scintillation solvents

(2) preparation of scintillation solutes

(3) purification

(4) analytical and counting data of the scintillation chemicals

prepared.

1. SOLVENTS

Whereas a number of solvents have been used in liquid

scintillation counting, toluene and dioxane are those most

commonly used for non-aqueous and aqueous radioactive samples

respectively. The preparation of scintillation grade naphthalene

used as a secondary solvent in liquid scintillation counting is

also included in this paper. Naphthalene is used along with

dioxane.

The preparation of these scintillation grade solvents was

essentially the purification of the respective commercial grade

solvents.

(a) Purification of toluene : Commercial toluene contains

methyl thiophene (thiotoluenea) b.p. 112" - 113' 0 which camiol

be separated by distillation. The separation is achieved by

making use of the fact that sulfcmation of methyl thiophene at

.30° C with cone, sulphuric acid takes place more readily as

compared to toluene. Thus, commercial toluene is treated with

about 10% of its volume of cone, sulphuric acid at 30° C. The

lower acid layer is separated and the procedure is repeated until

the acid layer remains colourless. The toluene layer after

separation is distilled over small amounts of sodium hydroxide

pellets to remove last traces of acid. Finally, toluene is

refluxed over metallic sodium and fractionally distilled at

110.6" C.

(b) Purification of dioxan-s Commercial dioxane contains small

amounts of acetaldehyde and glyool acetal together with water.

On keeping, the acetal tends to undergo hydrolysis and the

liberated acetaldehyde leads; to some peroxide formation.

Purification involves the removal of peroxide by refluxing

dioxane with 5% ferrous sulphate in dilute hydrochloric acid

solution followed by drying with sodium hydroxide pellets and

removal of aqueous layer. Finally, it is rei'luxed over sodium

metal and then fractionally distilled at 101.5" C.

(c) Purification of naphthalene : Commercially available

naphthalene contains thionaphthene as impurity. It was purified

by fractional crystallization in ethyl alcohol (95%). The

procedure was repeated 3 to 4 times to get the requisite

purity.

These chemicals were being supplied to- various units of DAE

but later discontinued with the availability of AR grade which is

cheaper and comparable to the scintillation grade.

(2) SOLUTES

Although a number of compounds serve as seintillators, only

a few find practical use. The principal solutes used in liquid

scintillation counting for monitoring tritium in heavy water

moderated reactors is a combination of primary solute PPO i 2. £•

diphenyl oxasole) and secondary solute POPOP (.1,4 di-2(5 phetiyl

oxasolyl) benzene. Other solutes have specific applications.

The preparation of a few scintillation solutes prepared by the

group are as follows:

(a) p-Terphenyl

This primary scintillation solute was obtained from

commercially available Santowax F (a mixture of o, m and p-

terphenyl). A unit was setup to produce 5 kg of p-terphenyl per

month based on the differential solubility of the isomers in

benzene.

(b) 1,1,4,4- Tetraphenyl 1,3 butadiene

The preparation of this secondary solute involved the

following steps:

(i) preparation of diethyl succinate from succinic acid and

ethyl alcohol

(ii) the interaction of diethyl succinate with phenyl magnesium

to give 1,1,4,4 tetraphenyl butane diol-1,4

(iii )dehydration of the diol with acetic aci:f containing 6%

sulphuric acid to give 1,1,4,4 tetraphenyl 1,3 butadiene.

(c) 2,5 Diphenyloxaaole (PPO)

A ne.v and convenient method(1,2) with overall high yield was

developed in the Chemical Engineering Division of BARG for the

synthesis of PPO. The method involves the conversion of hippurie

acid by phosphorus pentachloride to hippuryl chloride and the

acylation of benzene with hippuryl chloride (using anhydrous

aluminium chloride as catalyst) to give the intermediate N-phen-

acylbensamide, which is cyclodehydrated to PPO by cone, sulphuric

acid.

This new method of synthesising PPO has an advantage over

the methods reported earlier in literature because it eliminates

the use of lachrymatory w-bromoacetophenone and avoids the

difficult experimental conditions of making PPO via asalaetone.

(d) 1,4 Di-2(5 phenyloxaaolyl) benzene (POPOP)

It is also synthesised by a new method{2) developed in the

Chemical Engineering Division of BARC. It involves the acid

hydrolysis of N-phenacylbensamide to give w-amino acetophenone

hydrochloride which is then coupled to terephthalolyl chloride in

presence of aqueous sodium carbonate to yield N, H' diphenaeyl

terephthalamide which is subsequently cyclodehydrated by cone,

sulphuric acid to POPOP.

The main advantages of this new method of synthesising POPOP

are fewer steps in synthesis and higher yield.

(e) Other substituted oxaaoles and bisoxasoies

The method developed for the. synthesis of PPO and POPOP was

extended to the preparation of other substituted oxaaoles and

bisoxasoles, some of them for the first time. NPO, BFO, TFO.

BBO, TOPOT and 2, 2' p phenylene bis 5-p ethyl phenyl oxasole

(3,4) are examples of scintillation solutes prepared in the

laboratory of the Chemical Engineering Division.

(£) Oxadiasoles

The oxadiaaoles commonly' used in liquid scintillation

counting via. 2 phenyl 5 (4 biphenylyl) 1,3,4 oxadiasole (PBT>.?

and 2,5 phenyl 1,3,4 oxadiasole (PPD) were also successfully

synthesised. The method consists essentially of the following

steps:

(i) preparation of benahydraside from ethyl bensoate and 8b%

aqueous hydrasine hydrate.

(ii'i coupling of ben,3oyi hydrasine with either p-phesiyl bensoyl

chloride or bensoyl chloride or benaoyl chloride in presence of

pyridine to give 1 bensoyl-2-t4-p-phenyl benaoyl) hydrasine or

1,2 bensoyl hydrasine respectively.

(iii)dehydration with excess phosphorus oxyeliloride to give FbD

or PPD respectively.

(g) Organo tin compounds

These are compounds which do not scintillate but are loaded

in liquid or plastic scintillators for gamma detection like

iodine-125. Tetra-n butyl tin, triphenyl 4 ethyl phenyl tin and

tetraphenyl tin were synthesised.

(h) 9 Vinyl anthracene

A new method of synthesis was developed for preparing this

high potential sciritllator. The method was based on the

methylenation of 9 anthraldehyde by adding a benaene solution of

9 anthraldehyde along with an ethereal solution <.>£ ruethylene

iodide to magnesium amalgan>( 5 ) .

(3) PURIFICATION

Scintillation chemicals need a very high degree of purity

for optimum performance in liquid scintillation counting. Most

of the purification procedures involved crystallisation through

different solvents. Soxhlet extraction and final purification

through a modified Sangaster's apparatus which enveloped

extraction, adsorption and crystallisation all in one with a

minimum quantity of solvent.

(4) ANALYTICAL AND COUNTING DATA ~

(i) All compounds synthesised were subjected to a quantitative

determination of some of the constituent elements and the

observed values were comparable to the calculated values.

(ii) Melting points were determined in open glass capillaries on

Buchis' melting point apparatus and found correct.

{iii)Infra-red spectra of the compounds were recorded either in

the form of a mull in Hujol or KBr pellet on a Parkin Elmer Infra

Cord 137 model.

(iv) Proton magnetic resonance spectra were recorded on a Varian

A BOA model NMR spectrometer.

(v) Absorption and fluorescence spectra were recorded on an

Aminco Bonman Spectrophotofluorometer and found matching-

(vi) The relative performance of the scintillation solutes were

determined on LS spectrometer of Packard Instrument Co (Model

3255) and found acceptable.

CONCLUSION

Though hundreds of organic compounds have been classified as

scintillation solutes PPO is till today the most popular primary

organic scintillator. However, a recently reported compound PHP

(1 phenyl 3 mesityl 2 pyra^oline) offers promising potential as a

solute in liquid and plastic large volume scintillators.

ACKNOWLEDGEMENT

The authors wish, to thank Shri S. Sen, Director, Chemical

Engineering Group, E^RC and Shri V. S. Keni.. Head, Chemical

Engineering Division, BARC for their keen interest and

encouragement in the development of these scintillation

chemicals.

REFERENCES

1. S. D. Paul, D. L. Dhane, K. A. Noras and A. U. Mushrif,

J. Ind. Chern. Society 49, 579 (1972).

2. D. L. Dhane, K. A. Noras, A. U. Mushrif, G. M. Gaiki and

S. P. Hirve, BARC Report No. 792 (1975).

3. D. L. Dhane and G. M. Gaiki, BARC Report No. 1030 (1979).

4. D. L. Dhane, K. A. Noras and A. U. Mushrif,

BARC Report No. 1217 (1984).

5. D. I. Dhane, K. A. Noras and G. M. Gaiki,

BARC Report No. 835 (1975).

PAPER - 6

DEVELOPMENT OF PLASTIC SCINTILLATORS IN BARC

A.N. Rangarajan and CM. PauJ

DEVELOPMENT OF PLASTIC SCINTILLATORS IN BARC

A.N. Rangarajan and C M . Paul

Chemical Engineering Division

Bhabha Atomic Research Centre

Bombay - 400 085

INTRODUCTION

The plastic scintillator as a nuclear particle detector is

unique amongst different types of detectors. It is ehemically

inert, mechanically stable and can be prepared or moulded in

different sizes and shapes. The simplicity of its manufacture

and comparatively low cost are added advantages. The process for

the large scale production of plastic scintillators with

polystyrene as base has been successfully developed in Chemical

Engineering Division, BARC. The basic materials are locally

available. Machined and polished pieces of various sizes of

plastic scintillators were supplied to various research

laboratories of BARC, universities and other research

institutions. Besides larger blocks of 50 x 50 x 5 cm were

supplied to TIFR and NRL, for studies in cosmic rays.

PLASTIC SCINTILLATOR SOLVENTS

Monomeric styrene is a viscous liquid which polymerises on

heating to form a solid thermoplastic polymer polystyrene. The

vinyl group <-CH=CH2) forms the basis of all aromatic polymers.

Although vinyl toluene is a more efficient scintillation solvent

(10-15% higher) than polystyrene, the latter is preferred because

of its low cost and availability in the country. In acrylic base

scintillators employed for high transparency applications methyl

methacrylate monomer is used as a solvent.

SOLUTES

P-terphenyl (PTP) is an efficient primary solute with a

1

relative scintillation efficiency of 30% at an optimum

concentration of 1-3%. Its emission spectrum ( max) is 360 nm.

Since this emission is illmatched to S 11 photomultiplier

response, a secondary solute or a quartz window photomultiplier

is used to improve the scintillation efficiency. 2-phenyl-5- (4

biphenylyl) 1,3,4 oxadiazole (PBD) appears to be slightly more

efficient primary solute than PTP, although it is costlier.

TPB (1 I1 4 4' tetraphenyl butadiene) was the first

efficient substance discovered for use as a secondary solute in

plastic scintillators. Its emission spectrum ( max 445 nm)

matches well with the response of Sll photomultipliers. Though

Anthracene as a single crystal is the best known organic

scintillator, it is a mediocre solute for plastic scintillator.

However its derivatives with alkyl, aryl and methoxy groups in

meso positions are found to be efficient solutes. P-

guarterphenyl is another suitable secondary solute for use along

with PTP. The advent of oxazoles and 1,3,4 oxadiazole

derivatives and similar compounds led to their exploitation as

primary and secondary solutes for plastic scintillators. 4 4'

Diphenyl stilbene (DPS) and 2,5-Di-(4-Diphenylyl)-oxazole (BBO)

are the best secondary solutes. 1,4 Di-2-(5 phenyl oxazolyl)-

benzene) (POPOP) is also commonly used as a secondary solute with

PTP.

PRODUCTION OF PLASTIC SCINTILI.ATORS

Polymer grade styrene (purity 99.9%) is available with M/s.

Polychem Ltd., Bombay. The high purity scintillation solutes

like PTP, POPOP, TPB, PPO are being produced in the Chemical

Enginnering Division of BARC.

Styrene monomer is vacuum distilled to remove inhibitor and

oth3r oxygen bearing impurities. To fabricate a 50 x 50 x 5 cm

scintillator, 16 litres of styrene monomer along with 150 gms of

PTP (1%) and 7.5 gms of TPB (primary and secondary solutes

respectively) with 2 gms of Zinc stearate (mould releasing agent)

is polymerised at 140°C in a stainless ste??.! i factor vessel. The

reactor vessel is placed in an oil bath heated to 140°C. The

reactio'n starts within an hour with violent boiling. Being an

highly exothermic reaction, plenty of heat is liberated. But the

thermostatically regulated oil bath and the. condenser help to

control the react icr., Two more boilings are observed within an

hour, after which a jelly like ir.ass is formed. The heating will

be continued for another 22 hours to maximise the polymerisation-

It also helps to reduce the residual monomer content to as low as

0.1 to 0.2%. The presence of residual monomer reduces the light

output. The reactor vessel is transferred from the oil bath to a

water bath to shock .cool and release the block from the mould.

The blcok is annealed in a current of Nitrogen at 105°C for 12

hours to remove the internal stresses and strains and to improve

the mechanical properties.

CHARACTERISTICS OF A PLASTIC SCINTILLATORS

1. Composition Styrene monomerP-terphenyl

TPB

97.46%1%

0.05%

2. Resolution 20%

3. Decay time 3 n sec

4. Light output 60% of anthracene

5. Wavelength of max emission 425 nm

6. Molecular weight 80000-1,20,000 units

HEAVY ELEMENT LOADED PLASTIC SCINTILLATOR

In recent years, there has been an increasing demand for

scintillating plastics that show improved efficiency towards low

energy r-ray detection. Because of their low density and low

atomic number of its constituent elem^ntu (C & H) organic

scintillators have much lower r-ray absorption (especially at low

energies) than inorganic phosphors like Nal <Tl) or CSI (Eu).

Bur organic scintillators possess short decay time (3-4 n sec).

Since r-ray absorption in the photoelectric effect region is

proportional to the fifth power of the atomic number that is Z ,

it is reasonable to expect that by incorporating even small

amounts of heavy atoms into the matrix, a significant improvement

in the photosensitivity and resolution will result.

Since Tin (Sn) and Lead (Pb) have large cross section for

absorption of r-rays (Pb 1.68 x 10~ cm2/atom Sn 3.36 x 10~22

2cm /atom) incorporation of Tin or Lead will be desirable. Tin

compounds yield plastics with better clarity and even upto 10% of

elemental Tin could be loaded in PVT scintillator. Organo Tin

compounds were observed to quench the light output much less than

organo lead compounds. In addition, lead compounds have poor

solubility.

The organo Tin compounds like Tetraphenyl Tin and Triphenyl

4-ethyl phenyl Tin, which show good photoelectric peak resolution

in PVT or polystyrene plastics have been successfully synthesised

in the Chemical Engineering Division. A few plastic

scintillators incorporating these compounds have also been

prepared on an experimental basis.

POLYMETHYL METHACRYLATE SCINTILLATORS (PMMA)

Methyl methacrylate which is structurally similar to

nonaromatic liquid solvents like dioxane is found to be inferioi

in respect of energy transfer. But PMMA's high transparency and

good mechanical properties make them attractive as a base

material for scintillator.1 By using Naphthalene as a secondary

solvent (3%) energy transfer can be enhanced. Buty PBD and bis

MSB are being used as primary and secondary solutes respectively

(PTP is not suitable as a solute with naphthalene due to the

similarity of their spectra, thereby inhibiting energy transfer).

The primary and secondary solutes have been i-ynthesised in the

Chemical Engineering Division. A few experimental pieces of PMMA

scintillators with high transparency have made in glass vials.

PMMA (e>lso known as lucite, plexiglass or perspex)

scintillators because of their high transparency and clarity are

useful as light pipes. They are placed between the scintillator

and photo cathode and serve in helping to prevent trapping of the

light in the scintillator.

Plastic scintillating fibres command much attention these

days for applications in Hodoscopes, calorimeters, imaging trays

etc.

A STANDARD LIGHT SOURCE OF LOW INTENSITY

In thermoluminescence studies and in many photomultiplier

applications, a need exists for a standard reference light source

of constant output Cor frequent calibration of the photometric

system. A solid light source of scintillating plastic doped with14a C compound appears to have a distinct advantage over liquid

light source. It has a luminous intensity of 2 x 10

candles/cm and possesses long term stability and a very small

temperature coefficient.

To a pre-polymerised phosphor in a glass vial is added 0.1814mg of C labelled benzoic acid (25 uCi). The mixture after

stirring well is sealed in,a current of Nitrogen. Polymerisation

will be continued for 24 hours at 50-60'C. After completion of

polymerisation, the vial is broken to remove the plastic. It is

cut to the desired size and polished. Experiments with inactive

benzoic acid have been successfully carried out.

ADVANTAGES OF PLASTIC SCIBTTILLATORS

As no single scintillator has all the desired properties,

one chooses a scintillator which is most suitable for specific

applications. Some of the advantages of using plastic

scintillators are:

(1) For large size scintillators where it is impossible or too

expensive to use crystalline phosphors like anthracene or

stilbene.

(2) Simple to prepare, easy to machine and have good mechanical

and optical properties.

(3) The absence of hygroscopy and being a stable solid,

facilitates ease of handling, packaging and transportation.

(4) Compared to liquid scintillators, they have better thermal

stability and can be used over wide range of temperature (-

190°C to 70°C).

(5) They can be used in a wide variety of shapes like filaments,

thin films to count fission fragments, annular cylinders for

coincidence measurements.

Production of plastic scintillators (polystyrene base) of

50 cm x 50 cm cross section in the thickness range of 5 cm, 10 cm

and 15 cm was successfully demonstrated in the Chemical

Engineering Division of BARC. These scintillators are being

regularly supplied to various organisations to meet their

specific demands.

ACKNOWLEDGEMENT

The authors acknowledge their sincere thanks to Shri S. Sen,

Director, Chemical Engineering Group and Shri V.S. Keni, Head,

Chemical Engineering Division for their keen interest and support

in this work.*********************************

PAPER - 7

PLASTIC SCINTILLATOR SPONGE AND THIN FILM DETECTORS

DEVELOPED FOR CONTINUOUS ON-LINE MONITORING OF

TRITIUM IN WATER AND AIR

A.N. Singh, C.K.G. Nair and M. Rathnakaran

PLASTIC SCINTILLATOR SPONGE AND THIN FILM DETECTORS

DEVELOPED FOR CONTINUOUS ON-LINE MONITORING OF

TRITIUM IN WATER AND AIR

A. N. Singh, C.K.8. Nair a.nd M. Rathnaliaran

EIMVIRONNENTAL ASSESSMENT SECT I ON

BHABHA ATOMIC RESEARCH CENTRE

TROMBAY, BOMBAY 40O 085,

ABSTRACT

The paper describes the development of plastic sc::int.i 13 at or

based detectors -for continuous on line monitoring of tritium in

air and water. The detectors are prepared -from 5 uni thick plas-

tic scinfci 1 lator -films -for which a technique has been developed

that gives consistent results- The water detector shows a sen-

sitivity of 0.5 nCi/ml of tritium in water and the air detector

shown- a sensitivity o-f 1 uCi/rn3 o-f tritium in air. Both the

detractor;; have a -fast response, The detectors are in the -form

of- a flowcell made of perspex , coupled to a matched pair o-f

photonu.il tipi iers and the measurement is made-:' in the coincidence

mode o-f counting- With the introduction of mi ceo-computer based

background compensation, effect of- interfering gaseous

radionucl ides like A-41 does not a-f •feet the tritium measurement

in the air considerably. In presence of 2000 uCi./m13 o-f A-41

concentration in the air, the interference in the tritium chan-

nel is brought, down to 0.5 DAC level o-f tritium. The monitors

are designed for continuous on line operation.

1. INTRODUCTION

Continuous monitoring of- tritium presents difficulties

because? of two principal reasons. The first is due to the ex-

tremely low beta energy of tritium <1S keV ma;:..) and the second

is clue to the presence'.of usually large amounts o-f interfering

radi onucl :i des of much higher beta energy,. The low beta eneryy

could be detected only i f the sample is brought in intimate? con-

tact with the detecting surface. It has been evaluated that on

a detecting sur-face of 1 cm 2 sur-face area immersed in tritiated

water o-f 1 uCi/cm51 concentration only 62 tritium betas will

strike the surface every minuts* '•> . Detection of tritium with

reasonable sensitivity thus requires as large a detector surface;

area* as possible, in as small detector mass as -feasible, to keep

the gamma background low at the same time. For tritiated water,

a plastic scintillator sponge detector has been dev&loped that

combines these properties. <3J> The Sponge is prepared from 5 urn

thick plastic scintillator films and combines a surface arc??, o-f

3Odo cm15 in just i gm. For tritium in air detectors,about 4500

cma -film surface contained in a 65 ml flowcell has been

employed. The difference in wat&r and air detectors is mainly

in the former having wet operation and the latter having dry

operation. For reducing the effect of interfering

radionuelides, electronic cance11 ation of larger ©nergy pu1ses

and compensation technique for the residual background using

microcomputer have been employed.

2. Preparation of 5 urn Plastic Scintillator -films

The films sirs made on the surface of distilled water over

which a solution of the plastic scintillator is poured. The

material is dissolved in a solvent mixture consisting of 90%

ethyl acetate and 107. arnyl acetate. In 100 ml solvent, 3 g/ns

of the solute in the form of thin chips is dissolved. The solu-

tion gets ready within an hour. For each film sheet, 5 ml of-

the solution is measured in a measuring cylinder and poured in

quick succession over the surface of distilled water placed in a

tray of 30 cm ;•: 30 cm x S cm (depth). The solution rapidly

spreads over the water surface., After about 2 minutes, a large

number- of holes of 'varying sizes, start forming over the film

surface. The film gets- ready for removal in about 5 minutes. It

is removed by spreading •&. clean sheet of white paper an the film

surface and lifting it up. 20 such films each weighting 150 mg

and having surface area (each side) of about 250 cm2 can be

prepared from 100 ml solution. The f:i). ms are- then allowed to

dry on the paper, after covering it by another paper to prevent

the -film -from getting dirty.

3. Preparation o-f Detectors

For water detectors the cell volume is 15 ml, each ir.ell has

a diameter of 5 cm and is encased in aluminium. It consists of

two parts viz. the container and the lid with a neoprene 0-ring

in between. The two parts can bo screwed together airtight. A

nozzle on each of the two parts is provided -for water inlet and

outlet. To make the sponge; packing, the dry film is taken out

of the paper and is folded a number of times. It is then wetted

in distilled water and placed in the container part with a clean

forceps. It is positioned properly using a glass rod. For

satisfactory results, 7 or 8 such film sheets are stuffed one by

one and the lid is .tightened. After 3-4 hrs, the film packing

gets converted into a spongy disc of 48 mm dia. The holes on

the wet film surface play a role to convert the packing in to a

form of sponge that is highly porous. It permits water flow of

about 300 ml/miri in siphon mode, at i meter head of water. Fig i

shows the photograph of,the flow ceil filled with the detector.

The cell is then mounted on a pair of matched photomultipliers

type EMI 9635 A, face to face.

The cell for air detectors has a larger volume of 65 ml and

it. is stuffed only with dry film sheets. In the absence of

water wetting as was the case for water detectors, the sponge

formation does not take place. 6 to 10 films are used for each

det&ctar, depending upon the desired sensitivity. The outer

diameter of the aluminium clad of the flow cell detector is 95mm

and it is suitably t&pBrezd to conform to the requirements of 51

mm dia photomultipliers..

4. Performance of the Detectors

1. Tritiated Water Detectors:-

The important parameters to be studied are (a) flow of

sample water (b) tritium response as a function of number of

film sheets packed and (c) linearity of response for various

concentraicms o-f tritium activity in the water,

a) Flow of sample water

The detectors offer negligible resistance to flow of water

and d'o not require a peristaltic type of pump to push the water

through the detector. The detectors function quite well in

syphon mode of operation. Table-I gives the water flow rates at

various pressure heads.

Table I : Table showing the flow of water through the

detector as a function of the pressure head.

Water Flow Rate

(nil per min)

10

27

56

SI

105

120

<b) Tritium Sensitivity as a function of number of film sheets

Fig. 2 shows a plot of the count ratts obtained with various

numbers of film sheets used to prepare the sponge. There is

regular increase up to 7 film sheet after which rise becomes

slow saturating at 10 sheets.

(c) Linearity of Response

Tritiated water of various concentrations was flown through

the detector and the count rate for each concentration obtained.

The results are shown in fig.3. The response is linear.

2). Detectors for Tritium in Air

Using this detector along with the new microcomputer based

electronic system described later, it is possible to con-

tinuously monitor tritium even in presence of large levels of

gaseous interfering radi anticlines like A-41. The detector has a

• No.

1

2

3

4

5

6

Pressure

• (cms of

10

25

50

75

100

110

Headwater)

very fast response, <a few seconds only) and the sensitivity is

quite high.

The observed response is as follows:

Coincidence counting efficiency : 257.

Detector background

Observed count rate

Interference rejection

Sensitivity in presence of

2000 uCi/m* of.A-4i'

Percent of A-41 rejected

Equivalent to 1 uCi/m3

of tritium in air

50 counts/min for

luCi/nr3 tritium

80'/. rejection by pulse

height discrimination

Residual rejection by

background compensation

10 uCi/m* (0.5 DAC)

of tritium in air

99.5"/.

Gamma response

Other features

5. The Electronic System

Extremely low, a 6cm

thick lead shield could

eliminate it.

Simultaneous measurement

of A~4i activity.

Negligible attention in

continuous operation.

The electronic system incorporates the low and high voltage

power supplies, coincidence analyser and microcomputer based

counting and background compensation circuits. The entire

electronics has been accommodated in a standard siae small

cabinet. A tritium window has been provided for preferentially

separating tritium pulses from interfering pulses. The compen-

sation circuit involves digital subtraction of a predetermined

fraction of the counts above the tritium window (the upper

channel) from the counts in the tritium channel to offset the

spillover component of the interfering activity in the tritium

channel. The counting rates of the two channels (the tritium

channel and the interference channel), the compensated count

rate and the? conversion into DAC levels of tritium is printed on

a built-in minipri nt.er» The various parameters such as counting

times, number o-f readings to be averaged, the compensation

•factor, the conversion factor etc. are inserted through a built-

in keyboard. The parameters once entered are locked by the

software and cannot be disturbed unless the reset switch is

pressed, in which case the parameters will have to be entered

again.

The detector photomuitipl ier assembly .is incorporated in a

separate box. Provision is made for air inlet and outlet as

also the? power supplies for the phatomultipl ier and the -follower

circuit. A micropump of 1 1/min sampling rate, working on 6

volts DC is used for sampling. If necessary, the capacity of

the pump may be increased, though it is not essential. Fig.4

shows the photograph of phomultip!ier detector assembly with

the micropump and electronics. A flow cell without detector is

shown separately. For sampling in a remote mode, a pump of

higher capacity will have to be employed.

6. Conclusions

Plastic scinti11ator film detectors are found to be

suitable for 'tritium in water and air. The electronic: system is

common to both. In the present form, the? tritium monitors baaed

on these detectors ar& bound to be extensively used for the

monitoring of tritium in our heavy water type nuclear power

reactors.

7. References

1. Feinendecjen, L.E. (1967) "Tritium Labeled Molecules in Bio-

logy and Medicine (Academic Press, New York.)

2. Singh-, A.M., Rathnafcaran, M and Vnhra, K.G. , "An on-Line

Tritium-in-water Moni tor ".Nucl . i'nstr. and Meth A 236 < 1985)

159.

FIG. 1 . PHOTOGRAPH OF FLOWCELL ANDPLASTIC SCINTILLATOR SPONGE.

0 5 10 15NUMBER OF PLASTIC SIMULATOR FILMS

FIG. 2. COUNT RATE AS A FUNCTION OF NUMBER OF FILMSWITH 100nCi/ml OF TRITIATED WATER.

oo

i-zIDOo

50 100 150 200 250 300

TRITIUM ACTIVITY CONCENTRATION - TiCi / ml

FIG. 3 . ACTIVITY CONCENTRATION VERSUS COUNTRATE WITH 8 FILMS.

FIG.4. PHOTOGRAPH OF TRITIUM-IN-AIRMONITOR .

Published by : M. R. Baiakrishnan Head, Library & Information Services DivisionBhabha Atomic Research Centre Bombay 400 085