OFFICIAL ORGAN OF THE RADIATION RESEARCH SOCIETY

RADIATION RESEARCH

EDITOR-IN-CHIEF: ODDVAR F. NYGAARD

Volume 64, 1975

Academic Press • New York and London

filBUOTH^ ITIerarztlicheiy^akultSt

Kon igjrvstr ä$e 10 ÖOOO Münzen 22

— — Universitäts-

Bibliothek München

Copyright © 1975 by Academic Press, Inc .

All rights reserved

No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the copyright owner.

Made in the United States of America

RADIATION R E S E A R C H

OFFICIAL ORGAN OF THE RADIATION RESEARCH SOCIETY

Editor-in-Chief: O D D V A R F . N Y G A A R D , Department of Radiology, Case Western Reserve University, Cleveland, Ohio 44106

ASSOCIATE EDITORS

S. I . A U E R B A C H , Oak Ridge National L a b - G . J . K O L L M A N N , Albert Einstein Medical oratory Center

M . A B E N D E R , Brookhaven National L a b - J . B . L I T T L E , Harvard University oratory R . L O E V I N G E R , National Bureau of

J . A . B E L L I , Harvard University Standards

W. A. B E R N H A R D , University of Rochester L - S ^ g sR S ' U n i v e r s i t y o f California, Los

J - ^ U d ^ ^ m , ' M ^ n ^ b a n e r g y * ^ * N E T ^ C a r n e g i e Mellon University ^ v n A T m n f m r . T T • -x r A i R . B . P A I N T E R , University of California, G . V . D A L R Y M P L E , University of Arkansas g a n Francisco

P . W . D U R B I N , University of California, G # S I L I N I , Centro di Studi Nucleari della Berkeley Cassaccia, Roma, I taly

R . A . H O L R O Y D , Brookhaven National L . D . S K A R S G A R D , British Columbia Laboratory Cancer Institute, Vancouver, Canada

A . M . K E L L E R E R , Universität Wurzburg, H . R . W I T H E R S , M . D . Anderson Hospital Germany T M Y U H A S , Oak Ridge National L a b -

D . J . K I M E L D O R F , Oregon State University oratory

OFFICERS OF THE SOCIETY

President: R O B E R T H . SCHÜLER, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

Vice President (and President Elect): R O B E R T F . K A L L M A N , Department of Radiology, Stanford University, Palo Alto, California 94305

Secretary-Treasurer: W I L L I A M C . D E W E Y , Dept. R a d . and R a d . Biol. Colorado State University, Fort Collins, Colorado 80521

Editor-in-Chief: O D D V A R F . N Y G A A R D , Case Western Reserve University, Cleveland, Ohio 44106

Executive Director: R I C H A R D J . B U R K , J R . , 4720 Montgomery Lane, Suite 506, Bethesda, Maryland 20014

ANNUAL MEETINGS

1976: June 27-July 2, San Francisco, California 1977: San Juan, Puerto Rico

VOLUME 64, 1975

Councilors Radiation Research Society 1975-1976

P H Y S I C S

E . R . E p p , H a r v a r d U n i v e r s i t y A . M . Kel lerer , Universität W u r z b u r g , G e r m a n y

B I O L O G Y

E . J . A i n s w o r t h , A r g o n n e N a t i o n a l L a b o r a t o r y R . J . M . F r y , Argonne N a t i o n a l L a b o r a t o r y

M E D I C I N E

J . I . F a b r i k a n t , U n i v e r s i t y of Connect i cut S. J . Ade ls te in , H a r v a r d U n i v e r s i t y

C H E M I S T R Y

J . G . B u r r , U n i v e r s i t y of O k l a h o m a M . S. M a t h e s o n , Argonne N a t i o n a l L a b o r a t o r y

A T - L A R G E

H . I . Ad le r , Oak R idge N a t i o n a l L a b o r a t o r y A . P . Casarett , Corne l l U n i v e r s i t y

R A D I A T I O N R E S E A R C H 6 4 , V - V U ( 1 9 7 5 )

CONTENTS OF VOLUME 64

N U M B E R 1 , O C T O B E R 1 9 7 5

S Y M P O S I U M O N T H E D I S T R I B U T I O N O F S E C O N D A R Y E L E C T R O N S F R O M I O N I Z I N G C O L L I S I O N S

J . W M . M C G O W A N . Introduction and Summary. 1 M I T I O I N O K U T I . Ionization in Gases under Electron Irradiation. 6 F . H . R E A D . Displaced Electron Energies and the "Shake-Down" Effect. 2 3 C . E . B R I O N . llPholoelectronn Spectroscopy by Electron Impact. Coincidence Studies of Scattered

and Ejected Electrons. 3 7 T H O M A S A . C A R L S O N . The Nature of Secondary Electrons Created as the Result of Electron

Shake-Off and Vacancy Cascades. 5 3 E A R L C . B E A T Y . Measurements of the Energy and Angular Distribution of Secondary Electrons. 7 0 N O B U O O D A . Energy and Angular Distributions of Electrons from Atoms and Molecules by

Electron Impact. 8 0 Y O N G - K I K I M . Energy Distribution of Secondary Electrons. 9 6 S. P . K H A R E . Ionizing Collisions of Electrons with Atoms and Molecules. 1 0 6 A . E . S. G R E E N . The Role of Secondary Electrons in Charged Particle Degradation. 1 1 9 D . A . D O U T H A T . Energy Deposition by Electrons and Degradation Spectra. 1 4 1 M . E . R U D D . Mechanisms of Electron Production in Ion-Atom Collisions. 1 5 3 R . H . R I T C H I E A N D C . J . T U N G , V . E . A N D E R S O N , A N D J . C . A S H L E Y . Electron Slowing-Down

Spectra in Solids. 1 8 1

N U M B E R 2 , N O V E M B E R 1 9 7 5

Y O N G - K I K I M . Energy Distribution of Secondary Electrons. I I . Normalization and Extrapola­tion of Experimental Data. 2 0 5

U . F A N O . Platzman's Analysis of the Delivery of Radiation Energy to Molecules. 2 1 7 P . N E T A A N D R O B E R T H . S C H Ü L E R . The Mode of Reaction of 0~ Radical with Aromatic Systems. 2 3 3 B . K . L Y D E R S E N A N D E R N E S T C . P O L L A R D . The Induction of \-Prophage by Ionizing Radiation. 2 3 7 E R I C J . H A L L , J U D Y K . N O V A K , A L B R E C H T M . K E L L E R E R , H A R A L D H . R O S S I , S T E P H E N

M A R I N O , A N D L E O N J . G O O D M A N . RBE as a Function of Neutron Energy. I . Experimental Observations. 2 4 5

C A M E R O N J . K O C H A N D R O B E R T B . P A I N T E R . The Effect of Extreme Hypoxia on the Repair of DNA Single-Strand Breaks in Mammalian Cells. 2 5 6

J . W . H A R R I S , J . A . P O W E R , A N D C . J . K O C H . Radiosensilizalion of Hypoxic Mammalian Cells by Diamide. I . Effects of Experimental Conditions on Survival. 2 7 0

B J Ö R N R Y D B E R G A N D K A R L J . J O H A N S O N . Radiation-Induced DNA Strand Breaks and Their Rejoining in Crypt and Villous Cells of the Small Intestine of the Mouse. 2 8 1

J . G . S H A R P A N D D . B R Y N M O R T H O M A S . Thymic Regeneration in Lethally X-Irradiated Mice. 2 9 3 R U T H R O O T S A N D S H I G E F U M I O K A D A . Estimation of Life Times and Diffusion Distances of

Radicals Involved in X-Ray-Induced DNA Strand Breaks or Killing of Mammalian Cells. 3 0 6 R . E . J . M I T C H E L . Involvement of Hydroxyl Radicals in the Release by Ionizing Radiation of

a Cell Surface Nuclease from Micrococcus radiodurans. 3 2 1 L A N C E S . E V A N S A N D J A C K V A N ' T H O F . Dose Rate, Mitotic Cycle Duration, and Sensitivity of

Cell Transitions from Gl —> S and G2 —> M to Protracted Gamma Radiation in Root Meristems. 3 3 1

P . Z A R Ä N D . On the Class "A" Fission-Neutron Irradiation of Small Laboratory Animals. 3 4 4 C . J . G I L L E S P I E , J . D . C H A P M A N , A . P . R E U VERS, A N D D . L . D U G L E . The Inactivation of

Chinese Hamster Cells by X Rays: Synchronized and Exponential Cell Populations. 3 5 3

v

v i C O N T E N T S O F V O L U M E 64

J . D . C H A P M A N , C . J . G I L L E S P I E , A . P . R E U V E R S , A N D D . L . D U G L E . The Inactivation of

Chinese Hamster Cells by X Rays: The Effects of Chemical Modifiers on Single- and Double-Events. 305

C O R R E S P O N D E N C E

M . S . S . M U R T H Y , N . M . S . R E D D Y , B . S. R A O , P . S U B R A H M A N Y A M , A N D U . M A D H V A -N A T H . On the Repairable Sublethal Damage Induced by mPo Alpha Rays and mCo Gamma Rays in Dipolid Yeast. 376

R . E . J . M I T C H E L . Origin of Cell Surface Proteins Released from Micrococcus radiodurans by Ionizing Radiation. 380

N E A L K . C L A P P A N D L O U C . S A T T E R F I E L D . Modification of Radiation Lethality by Previous Treatment with Butylated Hydroxy toluene. 388

L I O N E L C O H E N . A Comment on the Note by Green and Burki on Survival Curves. 393 S H I R L E Y L E H N E R T . Intracellular Cyclic AMP Levels and Radiosensitivity in Synchronized

V-79 Cells. 394: F R A N K R . M R A Z A N D G E R H A R D R . E I S E L E . Increased U4Ce Uptake in Fetal Rats after

the Addition of Carrier. 399) J . F . D I C E L L O , R . D . C O L V E T T , W . GROSS, A N D U . K R A L J E V I C . Beta Emission from

Encapsulated Sources of Californium-252. 4011

A N N O U N C E M E N T S 405)

N U M B E R 3, D E C E M B E R 1975

H I D E H I K O A R A I A N D H I R O S H I H O T T A . The Dependence of Self-Focusing of a High-Intensity

Pulsed Electron Beam on Gaseous Media as Studied by Depth-Dose Distributions. I I I . Hydrocarbons and Halogenomethanes. 4077

D . W . W H I L L A N S A N D P . N E T A . Radiation Chemical Studies of the Sensitizer Diamide. 4166 O R A Z I O S A P O R A , E . M A R T I N F I E L D E N , A N D P A M E L A S. L O V E R O C K . The Application of Rapid

Lysis Techniques in Radiobiology. I . The Effect of Oxygen and Radiosensitizers on DNA Strand Break Production and Repair in E. coli B/r. 4311

H E L E N B . S T O N E A N D H . R O D N E Y W I T H E R S . Enhancement of Radiation Response of a Murine Mammary Carcinoma by Two Nitrofuran Derivatives. 4433

M . F A R A G G I , J . L . R E D P A T H , A N D Y . T A L . Pulse Radiolysis Studies of Electron Transfer Reaction in Molecules of Biological Interest. I . The Reduction of a Disulfide Bridge by Peptide Radicals. 4.5S2

V . S V O B O D A A N D V . K O F R Ä N E K . Response of Femoral and Vertebral Colony-Forming Units to 2URa Internal Contamination in Mice. 4t6?>7

D . G . S T E T K A A N D P . L . W E B S T E R . Triliated-Thymidine Induced Changes in Cell Population Kinetics in Root Meristems of Pisum sativum. 4 7£/5

C H A N G W . S O N G A N D S E Y M O U R H . L E V I T T . Immunotherapy with Neuraminidase-Trealeä

Tumor Cells after Radiotherapy. 48555 J A C K W . P A T R I C K , L L O Y D D . S T E P H E N S , A N D R A L P H P I . T H O M A S A N D L O L A S. K E L L Y . The

Design of an Experiment to Study Leukemogenesis in Mice Irradiated by Energetic Heavy/ Ions. 4'9S92

N A N C Y O L E I N I C K A N D R O N A L D C . R U S T A D . The Sensitivity of Ribosomal Functions in Tetra-hymena pyriformis to Varying Doses and Dose Rates of Gamma-Radiation. 5*0939

J A M E S C . N E W T O N , M A R Y C . B A R S A - N E W T O N , J E R R Y H . JACOBSON, A N D D A V I D L . K R O H N . Biochemical Effects of X-Radiation on the Retina of the Albino Rabbit. 5>1H8

T H O M A S M . K O V A L , W . C . M Y S E R , A N D W . F . H I N K . Effects of X-Irradiation on Cell Division?, Oxygen Consumption, and Growth Medium pH of an Insect Cell Line Cultured in Vitro. 5>2424

G E O R G E M . H A H N . Radiation and Chemically Induced Potentially Lethal Lesions in Non-cycling Mammalian Cells: Recovery Analysis in Terms of X-Ray and Ultraviolet-Like Systems.

M I C H A E L S . K I L B E R G A N D O T T O W . Neuhaus. Accumulation of a-Aminoisobutyric Acid by Rat Tissues after Whole-Body ^-Irradiation.

C O N T E N T S O F V O L U M E 64 v i i

E I L E E N W . B R A D L E Y , P . C . C H A N , A N D S. J . A D E L S T E I N . The Radiotoxicity of Iodine-125

in Mammalian Cells. I . Effects on the Survival Curve of Radioiodine Incorporated into DNA. 555

A . O . F R E G E N E . A Study of the Cavity Displacement Effect in "Thimble Chambers" Using L i F Dosimeters. 564

G . A R M A N D , P . J . B A U G H , E . A . B A L A Z S , A N D G . O . P H I L L I P S . Radiation Protection of Hy­

aluronic Acid in the Solid State. 573 S A L V A T O R E C A N N I S T R A R O , Y V E S L I O N , A N D A L B E R T V A N D E V O R S T . Formyl Radicals in X-Ir-

radialed Frozen Aqueous Solutions of DNA and Nucleic Acids Constituents. 581 G . C A N C E L L I E R E , P . G I A C C H I , P . M I S T I I - D O R E L L O , A N D M . Q U I N T I L I A N I . The Influence

of Agents That Enhance Lethal Effects of Radiation on the Damage to Bacterial M embranes by X Rays and Ultraviolet Light. 593

A H A R O N Y E R U S H A L M I . Cure of a Solid Tumor by Simultaneous Administration of Microwaves and X-Ray Irradiation. 602

L E O E . G E R W E C K , E D W A R D L . G I L L E T T E , A N D W I L L I A M C . D E W E Y . Effect of Heat and Radia­

tion on Synchronous Chinese Hamster Cells: Killing and Repair. 611 K . A . S A V A G A O N A N D A . S R E E N I V A S A N . Radiation-Induced Increase Melanosis in Crustacean

Shellfish. 624 J . M A R T I N B R O W N . Selective Radiosensitization of the Hypoxic Cells of Mouse Tumors with

the Nilroimidazoles Metronidazole and Ro 7-0582. 633 R . B . P A I N T E R A N D B . R . Y O U N G . X-ray-Induced Inhibition of DNA Synthesis in Chinese

Hamster Ovary, Human HeLa, and Mouse L Cells. 648

C O R R E S P O N D E N C E

G . N . A . N A Y A R A N D S . S R I N I V A S A N . The Effects of Gamma Radiation on Solutions of A cetylcholinesterase. 657

J I U N N - T S A I R W u A N D R O B E R T R . K U N T Z . The Reactions of Hydrogen Atoms in Aqueous Solutions: Effect of pH on Reaction with Cysteine and Penicillamine. 662

A U T H O R I N D E X FOR V O L U M E 64. 667

C U M U L A T I V E S U B J E C T I N D E X FOR V O L U M E S 61-64. 669

K A . D I A T 1 0 N R E S E A R C H 6 4 , 245-255 (1075)

RBE as a Function of Neutron Energy 1

I. Experimental Observations

E R I C J . H A L L , J U D Y K . N O V A K , A L B R E C H T M . K E L L E R E R , H A R A L D H . R O S S I , S T E P H E N M A R I N O , A N D L E O N J . G O O D M A N

Radiological Research Laboratory, College of Physicians and Surgeons of Columbia University, New York, New York 10032

H A L L , E . J . , N O V A K , J . K , K E L L K R E R , A . M . , Rossi , H . H . , M A R I N O , S., A N D G O O D ­M A N , J . R B E as a Funct ion of Neutron Energ} ' . I . Experimental Observations. Radial. Res. 6 4 , 245-255 (1975).

The survival of Chinese hamster cells in culture was used as a test system to determine the R B E of neutrons over a wide energy range. The Radiological Research Accelerator Fac i l i ty ( R A R A F ) at Brookhaven Nat iona l Laboratory was used for experiments i n ­vo lv ing nine neutron energies between 110 keV and 15 M e V ; for a l l b u t the lowest energy the beams were essentially monoenergetic. Add i t i ona l experiments were performed w i t h high energy cyclotron-produced neutrons at the N a v a l Research Laboratory , Washing­ton, D . C , and at the Texas A & M Variable Energy Cyclotron ( T A M V E C ) . I n b o t h cases broad neutron energy spectra were involved. I n each experiment, survival curves were obtained for one neutron energy and compared w i t h 250 k V p X rays, using cells f rom the same suspension and common controls. I n this way a detailed study was made of the relation between R B E and neutron absorbed dose for each neutron energy. A t any given cell surv ival level, R B E varies w i t h neutron energy. Neutrons at 350 keV are biologically the most effective, the R B E fal l ing off for bo th higher and lower neutron energies.

I N T R O D U C T I O N

T h e radiobio logical properties of neutrons are of interest f r o m several v i e w ­po ints . F i r s t , there is concern a b o u t t h e effectiveness of smal l absorbed doses of neutrons t o w h i c h large sections of the pub l i c m a y be exposed f r o m fission reactions. Second, neutrons are a lready i n use a t a n u m b e r of centers for the t r e a t m e n t of cancer pat ients . T h i r d , experiments w i t h neutrons m a y help t o shed l i g h t on the basic mechanisms of the ac t ion of r a d i a t i o n , because t h e y are r e l a t i v e l y densely i on iz ing a n d 3̂ et able to penetrate t o considerable depths i n absorbing m a t e r i a l .

The re lat ive bio logical effectiveness ( R B E ) of neutrons , a n d the manner i n w h i c h i t varies w i t h n e u t r o n energy, has been s tud ied b y a n u m b e r of workers . The widest interest concerns effects on m a m m a l i a n cells, a n d the now-classical studies were repor ted b y Broerse, Barendsen, a n d v a n Kersen (1) and b y

1 This investigation was supported b y Contract A T - ( l l - l ) - 3 2 4 3 f r om the U . S. Atomic Energy Commission and by Public H e a l t h Service Research Grant No . CA-12536.

245

C o p y r i g h t © 1975 b y A c a d e m i c P r e s s , I n c . A l l r i g h r s of r e p r o d u c t i o n i n a n y form r e s e r v e d .

246 H A L L E T AL.

B e r r y (2, 8). These experiments were constrained b y the l i m i t e d range of n e u ­t r o n energies avai lable , a n d t h e i r i n t e r p r e t a t i o n was g r e a t l y compl i cated b y the fac t t h a t most of the beams used i n v o l v e d a b r o a d s p e c t r u m of n e u t r o n energies. Other invest igat ions , r epor ted b y Dennis a n d B o o t (4) a n d b y U n -d e r b r i n k a n d Sparrow (5), u t i l i z e d largety monoenergetic neutrons b u t con­cerned somatic m u t a t i o n s i n Tradescant ia , w h i c h are scored as color changes i n the stamen hairs . I t is n o t cer ta in how these effects re late t o m a m m a l i a n cell k i l l i n g . I t was against th i s background t h a t the present s t u d y was m o u n t e d , t o invest igate I I B E as a f u n c t i o n of n e u t r o n energy us ing m a m m a l i a n cells i n cu l ture .

The s t u d y was made possible b y the a v a i l a b i l i t y of the Radio log ica l Research Accelerator F a c i l i t y ( R A R A F ) , a unique source of monoenergetic neutrons covering a wide range of energies. R A R A F is a j o i n t enterprise between t h e Radio log ica l Research L a b o r a t o r y of C o l u m b i a U n i v e r s i t y a n d the M e d i c a l Research Center of the B r o o k h a v e n N a t i o n a l L a b o r a t o r y . I t consists of a large v a n de Graaff accelerator, capable of accelerating protons or deuterons up t o a n energy of 4.2 M e V . B3' e m p l o y i n g suitable targets , a n d using neutrons e m i t t e d a t a n appropr ia te angle f r o m the inc ident charged par t i c l e beam, es­sent ia l ly monoenergetic n e u t r o n beams can be generated, r a n g i n g i n energy f r o m 15.4 M e V d o w n t o 220 k e V . I n a d d i t i o n , a lower energy n e u t r o n beam is avai lable w h i c h consists of a w ide spectrum of energies described as t h e "110 k e V s p e c t r u m . "

T h i s f a c i l i t y has been used b y a number of invest igators , a n d radiob io log ica l findings have been reported i n the l i t e r a t u r e (6-9). T h e present paper describes experiments i n w h i c h the re la t ive biological effectiveness ( R B E ) was s tud ied over the entire range of n e u t r o n energies avai lable at R A R A F , us ing m a m ­m a l i a n cells i n cu l ture . I n a d d i t i o n , using the same bio logical test system a n d compat ib le dos imetry , v i s i t s were made t o the Texas A & M Var iab le E n e r g y Cyc lo t ron ( T A M V E C ) a n d t o the N a v a l Research L a b o r a t o r y ( N R L ) C y c l o ­t r o n . These machines generate n e u t r o n beams b y b o m b a r d i n g b e r y l l i u m targets w i t h deuterons ; the neutrons have a wide range of energies w i t h m a x i m a of 50 and 35 M e V , respectively , for the t w o fac i l i t ies .

M A T E R I A L S A N D M E T H O D S

Culture of the Cells

V79 Chinese hamster cells were cu l tured b y s tandard techniques a n d g r o w n i n n u t r i e n t m e d i u m F10 supplemented w i t h 1 0 % f e t a l calf serum (10), L - g l u -t a m i n e , and ant ib io t i cs . Because of the l i m i t e d dose rates a t the R A R A F accelerator i t was essential to expose cell samples close to the n e u t r o n - p r o d u c i n g target . T o achieve th i s end w i t h o u t sacrificing dose u n i f o r m i t y , i t was neces­sary for the cells t o occupy a smal l vo lume d u r i n g i r r a d i a t i o n . T h i s was achieved i n the f o l l ow ing way .

For each exper iment , a n ac t ive ly growing , p a r t l y confluent stock b o t t l e was chosen, the cells were removed b y t r y p s i n i z a t i o n (2 m i n a t 0 . 2 5 % t r y p s i n ) , and prepared i n t o a suspension i n complete g r o w t h m e d i u m consisting of 1.5

N E U T R O N R B E ' s 247

T A B L E I

C H A R A C T E R I S T I C S OF N E U T R O N B E A M S S T U D I E D

Facility Energy Maximum energy spread

N eutron production

process

Average dose rale

T A M V E C N R L R A R A F

50 M e V max 35 M e V max 15 M e V 5.8 M e V 2.0 M e V 1.0 M e V

680 keV 430 keV 340 keV 220 keV

110 keV spectrum

0-50 M e V 0-35 M e V ± 4 % ± 8 % ± 7 % + 15 - 1 2

± 1 3 ± 1 5 ± 1 5 + 2 8 - 2 5 0-110 keV

• Be • Be • T • D • T > T

• T • T • T • T

p - > T

70 r a d / m i n 65 r a d / m i n

1500 r a d / h r 475 r a d / h r 450 r a d / h r 625 r a d / h r

550 r a d / h r 300 r a d / h r 250 r a d / h r 130 r a d / h r

85 r a d / h r

X 10 4 c e l l s / m l . O n e - t h i r d of a m i l l i l i t e r of th i s suspension was p i p e t t e d i n t o each of 90 smal l plast ic v ia ls . The v ia ls were fabr i cated f r o m Falcon 1-ml dis­posable pipets i n the f o l l ow ing way . T h e pipets were cut i n t o 7-cm lengths a n d heat-sealed at one end. The i n t e r n a l d iameter of each v i a l is about 2 m m . A f t e r being filled w i t h the cell suspension, the open end of each v i a l was heat-sealed, a n d t h e n the v ia ls were g e n t l y centr i fuged so t h a t the cells sett led o u t i n t o a smal l vo lume at one end of each v i a l . I n t h i s w a y the cell samples oc­cupied a smal l vo lume and could be located accurately close t o the n e u t r o n -p r o d u c i n g targe t . P l a t i n g efficiencies were i n excess of 8 0 % , a n d d i d n o t de­crease s igni f i cant ly for the experiments i n v o l v i n g the longest exposure t imes .

F o r each neutron energy s tudied , f our v ia ls at each of seven absorbed doses were used. On each occasion t h a t a n e u t r o n exper iment was per formed, a p a r ­al le l series of v ia ls , filled f r o m the same cell suspension, was exposed t o graded doses of 250 k V p X rays. T h i s p l a n was fo l lowed because i t was assumed, a n d subsequently conf irmed (11), t h a t var ia t i ons mithin a g iven exper iment are smaller t h a n between separate experiments. Consequently each exper iment was a sel f -contained comparison of one n e u t r o n energy versus 250 k V p X rays . A l l i r rad ia t i ons were per formed at r o o m tempera ture .

F o l l o w i n g the complet ion of a set of i r rad ia t i ons , each v i a l was ag i ta ted i n a mechanical v i b r a t o r t o resuspend the cells, a f ter w h i c h var ious fract ions of i t s contents were seeded i n t o p e t r i dishes conta in ing fresh g r o w t h m e d i u m . F o l l o w i n g an 8-day i n c u b a t i o n per iod , the cells were fixed a n d stained, a n d the number of visible colonies per dish counted.

B y comparison w i t h u n i r r a d i a t e d controls , the f r a c t i o n of cells s u r v i v i n g each absorbed dose of X rays or neutrons was calculated.

248 H A L L E T AL.

Methods of Irradiation

A t the R A R A F F a c i l i t y nine n e u t r o n energies were s tudied . T h e y were 15.4, 6, 2, and 1 M e V ; 680, 440, 340, a n d 220 k e V ; i n a d d i t i o n , a s p e c t r u m w i t h

F I G . 1. Arrangement used to irradiate Chinese hamster cells w i t h neutrons of various energies at the Radiological Research Accelerator Fac i l i ty ( R A R A F ) . A vert ica l ly upward charged particle beam was used to bombard the target. Cells were contained i n small sealed plastic vials, and were gently centrifuged to one end. The cell samples were arranged around the circumference of a circle. The diameter of the circle and its position w i t h respect to the target varied w i t h neutron energy. The vials were rotated about the axis of the charged particle beam to average out small f luctuations i n absorbed dose rate. They were also rotated about their own axes to achieve a more uni form absorbed dose. The 6-mm diameter tissue equivalent ionization chamber used for dosim­etry is shown i n place of one of the vials. The monitor ionization chamber used for all irradiations can be seen above the array of vials.

N E U T R O N R B E ' s 249

0.50 A \ 011 MeV

0 .20-1 \ QIO- \ \ 005; \ \ 002r \ \ 001 h \ \

0.005 ;-. 100 ' 050

< 020- 4,

CT : \ u- 0.10- \ o 005- \ - 1 \ > 002r \ > 0.01 j- \

\ 0.43 MeV

1 0.005

I 00: 0.50

V Q20 r - \ N

010- \ 005-

0.02; 001 r

^ N , 0 22 MeV

\ \ 0.66 MeV

\ ^ _ ... ......

; \ \ 2 M e V

\ \

\

1 \

onns' ; — i — t - - i ••• .* —I '—i—K—s _ j — • — 1—i—s • -i—•—«- - j — 0 400 800 1200 0 400 800 1200 0 400 800 1200

ABSORBED DOSE (rod)

F I G . 2 . Surv iva l data for Chinese hamster cells exposed to neutrons of various energies at R A R A F . I n each experiment corresponding data were obtained for 2 5 0 k V p X rays.

a m a x i m u m energy of 110 k e V was used. T h e characteristics of the beams used are summarized i n T a b l e I . T o a t t a i n the required doses i n exposures of reasonable durat ions , the plast ic v ials w i t h the cells centr i fuged t o one end were pos i t ioned a r o u n d the circumference of a circle coaxial w i t h t h e charged par t i c l e beam and a t 2.6 t o 6.2 c m f r o m the neutron -produc ing target , de­pend ing on the n e u t r o n energy desired. T h e v ia ls were r o t a t e d a r o u n d the beam axis a n d about t h e i r o w n axes t o compensate for smal l var ia t i ons i n a b ­sorbed dose rate due t o t a r g e t asymmetr ies , f r o m inverse square l a w effects, a n d f r o m se l f -a t tenuat ion . T h e f ix ture shown i n F i g . 1 was designed to a l low the ends of the cell v ials t o be posit ioned at specific angles w i t h respect t o the beam d i rec t i on , rang ing f r o m 15 t o 130°, t o o b t a i n the range of n e u t r o n energies employed. A t each n e u t r o n energ ty, 16 v ia ls could be i r r a d i a t e d s imultaneously at a u n i f o r m absorbed dose r a t e ; some were removed at var ious t imes and others inserted, so t h a t i n a l l , seven graded absorbed doses, w i t h four replicate v ia l s per absorbed dose, were avai lable .

D o s i m e t r y at R A R A F was per formed using a tissue equiva lent , 6 - m m diame­ter , spherical m u l t i p l i c a t i o n i on i za t i on chamber placed i n one of the v i a l holes a n d centered at a pos i t i on n o r m a l l y occupied b y cells. These measurements were correlated t o the response of a m o n i t o r i on izat i on chamber used d u r i n g a l l cell i r rad ia t i ons . Separate measurements w i t h an energy compensated G e i g e r - M u e l l e r dosimeter y ie lded g a m m a - r a y absorbed doses rang ing f r o m 1.5 t o 8 % of the t o t a l absorbed dose, depending on n e u t r o n energy.

F o r comparison, the characteristics of the t w o h igh energy cyclotrons at w h i c h experiments were per formed are also l i s ted i n Table 1. These machines are c u r r e n t l y being used for n e u t r o n t h e r a p y of cancer pat ients (12} 18). T h e

250 H A L L E T AL.

10.0

DOSE ( r o d s ) 0 200 4 0 0 600 800 1000 1200 1400

.001 -

- i— i— i— i— i— i— i— i— i— i— i— i i r

HAMSTER C E L L S

X-rays

N R L x « \ TA MV EC -*2 neut rons \ \ neutrons 3 5 MeV \ ^ 0 MeV

\ \

i I • I i I • I • I •> I i

F I G . 3. Survival data for Chinese hamster cells exposed to h igh energy neutrons a t T A M V E C and N R L . I n each case corresponding data for 250 k V p X rays were obtained.

p r o d u c t i o n of neutrons b y b o m b a r d i n g a b e r y l l i u m targe t w i t h deuterons gives rise t o a wide range of energies. The N R L cyc l o t ron and T A M V E C use 35 a n d 50 M e V deuteron beams, respectively. T h e maximum n e u t r o n energies are s l i g h t l y higher, and the mean energy of the neutrons is about 0.4 of the d e u ­t e r o n energy.

A t these accelerators the absorbed dose rate was n o t a l i m i t a t i o n ; a t a d i s ­tance of 125 c m f r o m the targe t t h e absorbed dose rates were 50 r a d / m i n a t t h e N R L cyc l o t ron a n d 70 r a d / m i n at T A M V E C . T h e cells, conta ined i n t h e sealed plast ic v ia ls , were i r r a d i a t e d i n a fixture fabr i ca ted of tissue e q u i v a l e n t plastic , w i t h a thickness of plast ic ahead of the cells a p p r o x i m a t e l y equal t o the m a x i m u m range of the protons produced b y the neutrons (1 c m a t N R L , and 2.5 c m at T A M V E C ) .

D o s i m e t r y a t N R L a n d T A M V E C was per formed using a tissue e q u i v a l e n t , 2.5-cm diameter , disc-shaped i o n i z a t i o n chamber. These measurements were correlated t o t h e response of t h e m o n i t o r i o n i z a t i o n chambers a t these fac i l i t i es w h i c h were used d u r i n g the cell i r rad ia t i ons . A separate measurement of t h e g a m m a - r a y absorbed dose was per formed a t N R L using a G e i g e r - M u e l l e r dosimeter.

R E S U L T S

The s u r v i v a l curves obta ined for Chinese hamster cells exposed t o neutrons of var ious energies a t R A R A F are shown i n F i g . 2. T h e comparable d a t a f or the h i g h energy neutron beams a t T A M V E C a n d N R L are shown i n F i g . 3.

N E U T R O N R B E ' s 251

W i t h each n e u t r o n exper iment , a n X - r a y s u r v i v a l curve was established using cells f r o m t h e same i n i t i a l suspension a n d c o m m o n controls . These curves are i n c l u d e d i n Figs. 2 a n d 3. T h e repeated d e t e r m i n a t i o n of the X - r a y s u r v i v a l curve was necessary because of the poss ib i l i ty of changes i n cel lular sens i t iv i ty o c curr ing over t h e 12-mo per i od d u r i n g w h i c h t h e n e u t r o n data were accu­m u l a t e d . I n a d d i t i o n , i t was considered useful t o o b t a i n exper imenta l da ta w h i c h w o u l d p e r m i t a s ta t i s t i ca l eva luat i on of such fluctuations i n cel lular s e n s i t i v i t y . T h e s ta t i s t i ca l analysis, presented i n P a r t I I of t h i s s t u d y (11), does indeed lead t o t h e conclusion t h a t a systematic v a r i a t i o n of cel lular sen­s i t i v i t y occurred d u r i n g t h e per iod over w h i c h experiments were per formed. F o r t h i s reason i t is appropr ia te t o derive t h e R B E of neutrons a n d i ts de­pendence on n e u t r o n absorbed dose f r o m comparisons of each i n d i v i d u a l n e u t r o n dose-effect curve w i t h the corresponding X - r a y curve established on the same day .

T h e v e r t i c a l bars shown i n F i g . 2 represent t h e s ta nda rd deviat ions , der ived f r o m a comparison of the s u r v i v i n g clones resu l t ing f r o m replicate v ials of cells exposed t o t h e same absorbed dose. Theore t i ca l s tandard deviat ions , based mere ly on the Poissonian fluctuations due t o the f in i te number of s u r v i v i n g clones a t the var ious dose levels, have somewhat smaller values. However , i t has been f o u n d (11) t h a t the difference is smal l . T h e s u r v i v a l data for n e u ­trons of in termed ia te energies are closely a p p r o x i m a t e d b y exponential funct ions of dose. A t higher energies, i n v o l v i n g protons w h i c h are r e la t i ve ly sparsely i on iz ing , or a t v e r y l o w energies where m a n y of the secondary protons produced b y the n e u t r o n beams have i n i t i a l energies below 100 keV , the s u r v i v a l curves have s igmoidal shape.

T h e R B E values, p l o t t e d i n F i g . 4, were obta ined f r o m the s u r v i v a l curves of Figs. 2 a n d 3 b y d e t e r m i n i n g dose rat ios a t 0.8, 0.37, 0 .1 , a n d 0.01 s u r v i v a l b y i n t e r p o l a t i n g between the t w o nearest da ta po ints . A detai led s tat i s t i ca l analysis of the R B E as a f u n c t i o n of dose a n d n e u t r o n energy w i l l be g iven separately (11).

D I S C U S S I O N

T h e n e u t r o n beams a t R A R A F h a v i n g energies between 220 k e V and 15 M e V are essentially monoenergetic (at least o n l y a n a r r o w range of energies exists). F r o m 220 k e V to 15 M e V , R B E varies as a f u n c t i o n of n e u t r o n energy w i t h a b r o a d m a x i m u m near 340 k e V (see F i g . 4 ) . T h e l o w energy spec t rum a t R A R A F a n d the cy c l o t ron produced beams a t N R L a n d T A M V E C conta in a broad spectrum of neut ron energies, a n d i t is a n a r b i t r a r y choice whether to p l o t R B E values against the m a x i m u m energy, the mean energy, or some more compl icated f u n c t i o n such as a dose-mean. I n F i g . 4 maximum energy is used and so i t is no t surpr is ing t h a t these da ta po ints do n o t f a l l on the same smooth curve together w i t h the monoenergetic n e u t r o n data .

T h e R B E m a x i m u m a t a round 340 k e V is i n good agreement w i t h the d a t a reported previous ly for Vicia seedlings i r r a d i a t e d a t R A R A F (9). Indeed , the overal l shape of the d o s e - R B E re lat ionship is m u c h the same for these t w o

252 H A L L E T AL.

100

50 -

20 Lü CD 10 et

5 0 100

NEUTRON ENERGY MeV

F I G . 4 . R B E as a funct ion of neutron energy. R B E values were calculated at four levels of cell surv ival , namely 8 0 , 3 7 , 10 , and 1 % . The curves result f rom interpolat ion w i t h o u t statist ical analysis.

biological systems, a l t h o u g h the absolute values of the H B E are m u c h higher i n the case of Vicia (F ig . 5 ) .

The Tradescantia da ta of Dennis and B o o t (4) and also of U n d e r b r i n k a n d Sparrow (5), b o t h of w h i c h cover a wide range of n e u t r o n energies a n d i n v o l v e large ly monoenergetic beams, are l ikewise s imi lar i n general shape t o data i n the present paper. I t is more d i f f i cu l t t o make a mean ing fu l comparison of the present hamster cell da ta w i t h m a m m a l i a n cell data of Broerse et al. (1) a n d of B e r r y (3), since i n these studies a m u c h more l i m i t e d energy range was covered a n d most of the beams i n v o l v e d a wide spec t rum of energies. There is no m a r k e d confl ict over R B E i n the l i m i t e d energy range for w h i c h com­parisons are possible. A l l the studies prev ious ly repor ted i n the l i t e r a t u r e are presented i n F i g . 5 for purposes of comparison.

The characterist ic R B E - e n e r g y dependence was pred i c ted b y Ke l l e rer a n d Rossi (14) on the basis of microdos imetr ic data . A l t h o u g h the detai ls of t h i s analysis have been presented*earlier, i t m a y be useful t o repeat the essential arguments i n s impl i f ied f o r m . F r o m var ious experiments on higher organisms i t was suggested t h a t the cellular effect of i on iz ing rad iat ions m i g h t depend on the square of the energy absorbed i n sensitive sites w h i c h are of the size of the nucleus or somewhat smaller. A t l ow doses of neutrons the r a d i a t i o n effect m u s t be m a i n l y due t o single charged part ic les . Accord ing t o the q u a ­dra t i c dependence on energy concentrat ion , the R B E should t h e n be p r o p o r t i o n a l t o the p r o d u c t of the mean event frequency per u n i t absorbed dose a n d the square of the energy t rans ferred per event. I n a s impl i f i ed analysis w h i c h is based on the L E T - c o n c e p t t h i s p r o d u c t can be shown 2 t o be p r o p o r t i o n a l t o

2 The event frequency, 4>(o), per u n i t absorbed dose is inversely proport ional to the track average L E T , which is given by

where l(L) is the d is tr ibut ion of L E T i n t rack length ( I C R U Report 1G). The expectation value, e2, of the square of the energy deposition, on the other hand, is proport ional to the second moment

N E U T R O N R B E ' s 253

t h e so-called dose average l inear energy transfer Lx>. T h e R B E of neutrons a t s m a l l doses should therefore be p r o p o r t i o n a l t o th i s dose average L E T .

I n the microdos imetr i c analysis, L E T is s u b s t i t u t e d b y the q u a n t i t y y, w h i c h is def ined as the energy absorbed d i v i d e d b y the mean t raversa l l e n g t h i n a microscopic region. T h i s q u a n t i t y of l inea l energy, y, corresponds closely t o L E T ; the on ly difference is t h a t y accounts for the ac tua l s ta t i s t i ca l fluctuations of energy deposit ion a long the t racks a n d for such compl i ca t ing factors as the r a d i a l extension of the de l ta nxys and the finite l e n g t h of part i c l e t racks . The d i s t r i b u t i o n of the values of y, a l though i t is i n m a n y cases closely re lated t o t h e d i s t r i b u t i o n of L E T , is therefore a more accurate measure of energy de­pos i t i on i n c r i t i c a l sites i n the cell . T h e dose averages of y w h i c h correspond t o t h e dose averages, L D ) of L E T have been de termined for var ious n e u t r o n energies a n d for spherical vo lumes of the order of several micrometers i n d iame­t e r . These values are p l o t t e d i n F i g . 5 ( top panel ) .

There is one a d d i t i o n a l difference between the microdos imetr i c values w h i c h are p l o t t e d i n F i g . 5 a n d the dose averages of L E T . T h i s difference lies i n t h e fac t t h a t the microdos imetr i c q u a n t i t y has been ad justed for the sa tura ­t i o n effect w h i c h occurs w h e n a charged part i c le has such h igh s topping power t h a t i t deposits more energy i n the cell t h e n is necessary to inac t i va te the cell . A q u a n t i t a t i v e assessment of th i s correct ion w h i c h is based on data obta ined b y Barendsen (15) a n d b y T o d d 3 has been g iven earlier (14); i t should be p o i n t e d ou t t h a t the correct ion factor is smal l up t o n e u t r o n energies of several mega-electron vo l t s , wh i l e i t is qu i te s ignif icant a t the higher n e u t r o n energies, such as 14 M e V . T h i s is due t o the increased role of heavier recoi l ing part ic les a t these h i g h n e u t r o n energies. F i n a l l y , i t should be no ted t h a t the m i c r o ­dos imetr ic q u a n t i t y y* does no t s t rong ly depend on the d iameter of the region of reference. T h e values p l o t t e d i n F ig . 5 refer t o a site d iameter of 2 jum; h o w ­ever, the values are n o t great ly changed i f t h e y are re lated t o regions closer t o t h e d iameter of a cell nucleus.

of the d is tr ibut ion of L E T i n track lengths:

T h e effect per un i t dose, a n d ; therefore; the R B E at low doses, is equal to the product of event frequency and effect probabi l i ty per event. I f i t is assumed (14) t h a t the effect probabi l i ty per event is proportional to e2, one has

Accordingly, the R B E at low doses is proportional to Lo> 3 P. W. Todd, "Reversible and Irreversible Effects of Ionizing Radiations on. the Reproductive

I n t e g r i t y of M a m m a l i a n Cells cultured in Vilm," Thesis, Univers i ty of California, Lawrence Radiat ion Laboratory U C R L 11014.

T h e so-called dose average of L E T is defined as

2 5 4 H A L L ET AL.

h- 100 c —1 —

u j e 6 0 |

> ^ 4 0 1

y~ > O <u 2 0 -u . u . UJ

I 0 r

6 : UJ 4 -en cc

2 -

1 -I 0 F

NEUTRON ENERGY (MeV)

0.1 I 10 100

1 P 3 8 8 LEUKEMIA CELLS : (Berry 1974) ^ F i s s i o n d — Be : : (Berry 1974) ^ F i s s i o n

cyclotron

d—-T" * * b

T , CELLS, 8 0 % SURVIVAL (Broerse, Barendsen 8 van Kersen, 1967)

100 6 0 4 0

2 0

: TRAOESCANTIA M I M ) 1 1 1 1 1 H l | 1 1 1 I | 1 1 .

•Underbr inkS Sparrow, 1974

* Dennis 8 Boot, 1967 ". m -~. ^

d

2 0 . V ICIA

10 -6 ; 4 : (Hall et ol., 1973)

: HAMSTER CELLS

(Holl et al., 1975) i • i i •

\ f

> ' •

O.Ol 0.1 I 10 100

NEUTRON ENERGY (MeV)

F I G . 5. The six panels, reading f rom top to bo t tom are: (a) Dose-mean energy density corrected for the saturat ion effect, (y*) as a funct ion of neutron energy. Redrawn from Kellerer and Rossi (14). (b) R B E as a funct ion of neutron energy for P 3 8 8 lymphocyt ic leukemia cells assayed in vivo. The d —» T neutrons are monoenergetic at 14 M e V . The remaining energies are spectra, fission neutrons, or cyclotron produced d —> Be neutrons. D a t a f rom (3). (c) R B E as a funct ion of neutron energy for T i human kidney cells. The point at 15 M e V refers to monoenergetic d —» T neutrons, the remainder to spectra of neutrons. D a t a f rom (1). (d) R B E as a funct ion of neutron energy for the production of somatic mutations i n Tradescantia, expressed as color changes i n stamen hairs. D a t a f rom (4) and (5). (e) R B E as a funct ion of neutron energy determined w i t h Vicia seedlings, and calculated as the rat io of absorbed doses to produce 5 0 % inh ib i t i on of root growth . D a t a f rom (9). (f) R B E as a funct ion of neutron energy, determined w i t h Chinese hamster cells and calculated as the rat io of absorbed doses to result i n a surv iv ing fract ion of 3 7 % . D a t a f rom present paper.

T h e results of t h e present experiments are consistent w i t h earlier observa­t ions , for example, on bean roots (9), Tradescant ia (7, 8), a n d the m u r i n e lens (6), insofar as t h e y agree w i t h the overal l shape of the theore t i ca l l y pre ­d i c ted dependence of R B E on n e u t r o n energy. C e r t a i n deviat ions of the observed curves f r o m the theoret ica l dependence m a y be due t o the fac t t h a t the theo ­ret i ca l curve applies s t r i c t l y on ly t o the l i m i t of v e r y s m a l l absorbed doses,

N E U T R O N R B E ' s 2 5 5

w h i l e t h e exper imenta l d a t a have been obta ined f r o m a n intermediate range of t h e s u r v i v a l curves where a t least a t v e r y l o w a n d a t v e r y h i g h n e u t r o n energies the i n t e r a c t i o n of several charged particles cannot be completely neglected.

A discussion of the dependence of R B E on absorbed dose w i l l be g iven i n the context of the more detai led s tat i s t i ca l analysis of these d a t a i n P a r t I I of t h i s s t u d y (11).

R E C E I V E D : J u l y 2 9 , 1 9 7 4 ; R E V I S E D : M a r c h 1 0 , 1 9 7 5

R E F E R E N C E S

1. J . J . B R O E R S E , G. W . B A R E N D S E N , and G . R. V A N K E R S E N , Surv iva l of cultured human cells after i r rad iat ion w i t h fast neutrons of different energies i n hypoxic and oxygenated con­dit ions. Int. J . Radiat. Biol. 13 , 559-572 (1967).

2. R . J . B E R R Y , Hypoxic protection against fast neutrons of different energies: A Review. Eur. J . Cancer 7, 145-152 (1971).

3. R. J . B E R R Y , Modi f i cat ion of neutron effects upon cells by repair, and by physical and chemical means. I n Biological Effects of Neutron Irradiation: pp . 257-271. Internat ional Atomic Energy Agency, Vienna, 1974. S T I / P U B / 3 5 2 .

Jh J . A . D E N N I S and S. J . B O O T , Dependence of the oxygen enhancement rat io on neutron energy. Nature {London) 215, 310-311 (1967).

5. A . G . U N D E R B R I N K and A . H . SPARROW, The influence of experimental endpoint, dose, dose-rate, neutron energy, nitrogen ions, hypoxia, chromosome volume and plo idy level on R B E i n Tradescantia stamen hairs and pollen. I n Biological Effects of Neutron Irradiation. I n t e r ­nat ional Atomic Energy Agency, Vienna, 1974.

6. J . L . B A T E M A N and M . R . S N E A D , Current research i n neutron R B E i n mouse lens opacity. Symposium on Neutrons i n Radiobiology, Oak Ridge, Tenn . N o v . 1969.

7. A . G . U N D E R B R I N K and A . H . SPARROW, Power relations as an expression of relative biological effectiveness ( R B E ) in Tradescantia stamen hairs. Radial. Res. 4 6 , 580-587 (1971).

8. A . G . U N D E R B R I N K , R . C. SPARROW, A . H . SPARROW, and H . H . Rossi , R B E values of X rays and 0.43 M e V monoenergetic neutrons on somatic mutat ions and loss of reproductive in tegr i ty i n Tradescantia stamen hairs. Radiat. Res. 44) 187-203 (1970).

0. E . J . H A L L , H . H . R O S S I , A . M . K E L L E R E R , L . G O O D M A N , and S. M A R I N O , Radiobiological studies w i t h monoenergetic neutrons. Radial. Res. 5 4 , 431-443 (1973).

10. R. G . H A M and T . T . P U C K , Quant i tat ive colonial growth of isolated mammalian cells. I n Methods in Enzymology (S. P. Colowick and N . O. Kap lan , Eds.) , Vo l . V , pp . 99-119. Academic Press, New Y o r k , 1962.

11. A . M . K E L L E R E R , H . H . R O S S I , E . J . H A L L , and P. T E E D L A . R B E as a funct ion of neutron energy. P t . I I . Statistical analysis of the results. Radial. Res., i n press.

12. P. R. A L M O N D , J . B . S M A T H E R , G . D . O L I V E R , J R . , E . B . H R A N I T Z K Y , and K . R O U T T , Dos i ­metric properties of neutron beams produced by 16-60 M e V deuterons on bery l l ium. Radiat. Res. 5 4 , 24-34 (1973).

18. F. H . A T T I X , R. B . T H E U S , P. S H A P I R O , R. E . S U R R A T , A . E . N A S H , and S. G . C O R B I C S , N e u t r o n beam dosimetry at the N R L cyclotron. Phys. Med. Biol. 16 , 497-507 (1973).

14. A . M . K E L L E R E R and H . H . ROSST, The theory of dual radiat ion action. Curr. Top. Radiat. Res. Q 8, 85-158 (1972).

15. G . W . B A R E N D S E N , Mechanism of action of different ionizing radiations on the prol i ferative capacity of mammalian cells, Theor. Exp. Biophys. 1 , 167-231 (1967).