Radiation Chemistry

41 Hydrated Electron and Thermoluminescent Dosimetry of Pulsed X-ray Beams 1

E. M. FIELDEN and EDWIN J. HART Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch041

Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Ill. 60439

Two dosimeters suitable for monitoring single pulses of x-rays with doses in the range 1 to 100 rads at dose rates greater than 10 rads/sec. are described. Both systems were independently referred to Fricke dosimetry as the absolute standard and cross checked under pulse conditions. In one system the transient hydrated electron absorption produced by the pulse is measured by kinetic spectropho tometry as an indication of the dose. In the other, doped L i F crystals of about 50 mg. are irradiated in sealed poly ethylene bags under conditions of electronic equilibrium. Readout of the irradiated crystals was done on a standard commercial machine. Both methods were readily capable of 5% precision and with a little care better than 3% is obtainable. 3

T ^ V o s i m e t r y of l i n e a r accelerator

r a d i a t i o n presents

special

problems

a n d i n p a r t i c u l a r t h e d o s i m e t r y of a single p u l s e c a n b e difficult. D o s e rates u s e d i n p u l s e r a d i o l y s i s are as h i g h as 1 0

1 1

rads/sec. b u t the

largest dose p e r p u l s e u s e d is t y p i c a l l y of the o r d e r of 2 X

10

4

rads.

A l t h o u g h s u c h a p u l s e is d e t e c t a b l e b y t h e m a j o r i t y of dosimeters, its h i g h dose rate takes i t o u t of t h e l i n e a r range of m a n y of t h e m .

With

pulses l / 1 0 0 t h as great b u t of s i m i l a r d u r a t i o n , dose rate ceases to b e a p r o b l e m b u t t h e n t h e a m o u n t of change p r o d u c e d b y a single p u l s e is too s m a l l f o r accurate d e t e r m i n a t i o n b y most dosimeters.

Doses i n t h e

range 1 to 100 rads are b e i n g i n c r e a s i n g l y u s e d i n p u l s e r a d i o l y s i s studies a n d t h e n e e d for a n accurate single p u l s e dosimeter i n this range p r o m p t e d the w o r k d e s c r i b e d here. Present address : Physics Department, Institute of Cancer Research, Belmont, Sutton, Surrey, England.

1

585 In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

586

RADIATION CHEMISTRY

Radiation-induced thermoluminescence

has

been

1

d e v e l o p e d as

a

g e n e r a l system of d o s i m e t r y a n d i n p a r t i c u l a r has b e e n a p p l i e d to per s o n n e l a n d t h e r a p y d o s i m e t r y . T h e t e c h n i q u e has b e e n r e v i e w e d (3,

13,

T h e p u b l i s h e d d a t a i n d i c a t e that l i t h i u m fluoride s h o u l d be a n

15, 16).

excellent m a t e r i a l f o r single p u l s e d o s i m e t r y as it is dose rate i n d e p e n d e n t u p to 10 rads/sec. (10)

a n d is p r a c t i c a l l y linear u p to t o t a l doses of 10

8

rads (12, u p to 2 Χ

S i n c e this w o r k w a s p e r f o r m e d , dose rate i n d e p e n d e n c e

14). 10

5

11

rads/sec. has b e e n r e p o r t e d ( 5 )

m a k i n g this m e t h o d of

d o s i m e t r y a p p l i c a b l e over the f u l l range of p u l s e d r a d i a t i o n dose.

Pow

d e r e d L i F is n o r m a l l y u s e d b u t a spurious b a c k g r o u n d s i g n a l f r o m the Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch041

p o w d e r becomes i n c r e a s i n g l y i m p o r t a n t at doses b e l o w 10 rads

(10).

S i n g l e crystals of L i F w e r e u s e d i n this w o r k a n d it was f o u n d that t h e i r m u c h greater f r e e d o m f r o m spurious signals a l l o w e d their use d o w n to 1 r a d without any special precautions. T h e r e p o r t e d i r r a d i a t i o n energy i n d e p e n d e n c e of the L i F dosimeter ( I ) p e r m i t s its use as a secondary s t a n d a r d w h e n c a l i b r a t e d against γ-rays i n m e a s u r i n g the parameters

r e q u i r e d for a h y d r a t e d

6 0

Co

electron

dosimeter. T h e m a j o r i t y of laboratories

u s i n g the p u l s e r a d i o l y s i s

use the m e t h o d of k i n e t i c s p e c t r o p h o t o m e t r y r a d i a t i o n - i n d u c e d changes.

technique

to detect a n d

measure

W i t h s u c h apparatus it is thus possible to

use transient as w e l l as p e r m a n e n t o p t i c a l changes as a measure of the effects of a r a d i a t i o n pulse. h y d r a t e d electron (6, 11)

A c o n v e n i e n t transient

a b s o r p t i o n is

the

w h i c h has the v e r y h i g h e x t i n c t i o n coefficient

of 1.85 Χ 1 0 c m . " at 7000 A . U s e of the h y d r a t e d e l e c t r o n as a d o s i m e t e r 4

1

has a l r e a d y b e e n discussed (4,9),

a n d it has f o u n d w i d e s p r e a d use a m o n g

w o r k e r s i n p u l s e r a d i o l y s i s as a n a p p r o x i m a t e dosimeter.

T h i s w o r k set

out to measure the parameters u n d e r w h i c h the h y d r a t e d electron c o u l d be u s e d as a n accurate dosimeter for l o w dose pulses. A s the a m o u n t of h y d r a t e d electron present at the e n d of the i r r a d i a t i o n p u l s e is u s e d as a measure of dose, it is i m p o r t a n t that the l i f e t i m e of the e~

m

is l o n g c o m

p a r e d w i t h the p u l s e l e n g t h . T h e r e a c t i o n of e~

with H

m

+

c a n b e suppressed b y u s i n g a n a l k a l i n e

s o l u t i o n a n d t h e n the l i m i t i n g d e c a y process i s : 2H 0 -> H 2

e~

m

+ e-

m

2

+ 20H

(1)

S a t u r a t i n g the s o l u t i o n w i t h h y d r o g e n converts the O H r a d i c a l s i n t o H atoms: OH + H -» H + H 0 2

2

(2)

a n d i n a l k a l i n e s o l u t i o n the H atoms are c o n v e r t e d i n t o e aqH + O H " - » e-

aq

+ H 0 2

In Radiation Chemistry; Hart, E.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

(3)

4L

FIELDEN AND HART

587

Pulsed X-TCiy BeCLTYlS

T h e a d d i t i o n of H to a n a l k a l i n e s o l u t i o n thus doubles t h e y i e l d of e~ 2

m

w h i c h is o b s e r v e d . H o w e v e r , t h e h a l f - l i f e of R e a c t i o n 2 w i t h H

2

dissolved

at 1 a t m . is some 25 /xsec. a n d i f t h e loss of O H r a d i c a l s b y t h e c o m p e t i n g reaction, OH

w i t h 2k

4

+ OH-H>H 0 2

(4)

2

= 9 Χ 1 0 M' sec." , is to b e k e p t s m a l l , t h e n t h e dose p e r p u l s e 9

1

1

m u s t n o t exceed about 100 rads. T h u s , w h e n t h e d o u b l e d y i e l d p r o v i d e d b y Reactions 2 a n d 3 is d e s i r e d , o n l y s m a l l doses c a n b e m e a s u r e d . A t p H 12, R e a c t i o n 3 is r a p i d a n d i t has b e e n s h o w n (2) that at this p H a l l Η a n d O H r a d i c a l s are c o n v e r t e d i n t o e~

i n a h y d r o g e n saturated s o l u

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch041

m

t i o n . S u c h a s o l u t i o n also reaches a r e p r o d u c i b l e e'

m

decay w h e n irradi

a t e d (2, 7) b y c o n t i n u o u s o r r e p e t i t i v e p u l s e i r r a d i a t i o n a n d h y d r o g e n p e r o x i d e is n o t b u i l t u p b e y o n d a v e r y l o w c o n c e n t r a t i o n (8).

Similarly,

i n i t i a l traces of o x y g e n a n d h y d r o g e n p e r o x i d e are r e d u c e d b y p r e i r r a d i a t i o n to t h e i r v e r y l o w e q u i l i b r i u m concentrations. net c h e m i c a l c h a n g e i n the a l k a l i n e H

2

A s there is n o

saturated dosimeter s o l u t i o n , i t

c a n b e p e r m a n e n t l y sealed i n a n o p t i c a l c e l l . Experimental D o s i m e t r y . T h e G v a l u e f o r t h e p r o d u c t i o n of f e r r i c i o n i n a s t a n d a r d F r i c k e s o l u t i o n w a s t a k e n as 15.6 f o r C o γ-rays a n d x-rays p r o d u c e d b y p r i m a r y electron b e a m s w i t h energies of 3 a n d 14 M e v . F e r r i c i o n w a s estimated s p e c t r o p h o t o m e t r i c a l l y i n a R e c k m a n D U spectrophotometer u s i n g a n e x t i n c t i o n coefficient of 2197 M " c m . " at 304 m/x at 25 ° C . 6 0

1

1

L i F t h e r m o l u m i n e s c e n c e w a s c a l i b r a t e d against F r i c k e d o s i m e t r y w i t h C o γ-rays b y i r r a d i a t i n g a single L i F c r y s t a l sealed i n a s m a l l p o l y e t h y l e n e b a g h e l d i n fixed geometry i n a 30 m l . c a p a c i t y t h i n - w a l l e d b o r o s i l i c a t e glass b u l b c o n t a i n i n g F r i c k e s o l u t i o n . T h e b u l b w a s i m m e r s e d i n a beaker of w a t e r a n d i r r a d i a t e d — 14 c m . f r o m t h e source. T o a v o i d i m p u r i t y effects f r o m t h e p o l y e t h y l e n e , F r i c k e d o s i m e t e r r e a d ings w e r e t a k e n w i t h t h e b u l b i n t h e same i r r a d i a t i o n g e o m e t r y b u t w i t h o u t t h e L i F dosimeter. 6 0

A p p a r a t u s . T h e L i F a n d F r i c k e dosimeters w e r e separately c o m p a r e d w i t h t h e h y d r a t e d electron a b s o r p t i o n u s i n g t h e i r r a d i a t i o n assem b l y s h o w n i n F i g u r e 1. T h i s is s i m i l a r to that p r e v i o u s l y d e s c r i b e d (2) a n d makes use o f a m u l t i p l e reflexion c e l l ( 6 4 c m . p a t h l e n g t h u s e d ) f o r h y d r a t e d electron a b s o r p t i o n spectroscopy. T h e a b s o r p t i o n d e t e c t i o n e q u i p m e n t f o r these transient species w a s a c o n v e n t i o n a l oscilloscope p h o t o m u l t i p l i e r c o m b i n a t i o n as d e s c r i b e d p r e v i o u s l y ( 8 ) . X - r a y s w e r e p r o d u c e d b y e x t e r n a l targets, 7 m m . of t u n g s t e n f o r t h e 14 M e v . l i n a c electron b e a m a n d 3 m m . of l e a d f o r the 3 M e v . V a n d e G r a a f f b e a m . I m m e d i a t e l y i n f r o n t of t h e a b s o r p t i o n c e l l was a l 3 u i l d u p b l o c k ' t o b r i n g the X - r a d i a t i o n i n t o electronic e q u i l i b r i u m t h r o u g h o u t t h e v o l u m e of t h e d o s i m e t r y p o s i t i o n . T h e w a t e r jacket of t h e m u l t i p l e reflexion c e l l a c t e d i n the same w a y f o r t h e i r r a d i a t i o n v o l u m e . T h e b l o c k consisted of a r e c t a n g u l a r container of w a t e r 11.3 c m . i n d e p t h w i t h respect to t h e


588

RADIATION CHEMISTRY

1


d i r e c t i o n of r a d i a t i o n . E l e c t r o n e q u i l i b r i u m w a s c o m p l e t e after pene t r a t i n g 5 c m . of w a t e r . F r i c k e s o l u t i o n w a s c o n t a i n e d i n a t h i n - w a l l e d s p h e r i c a l b o r o s i l i c a t e glass b u l b of 30 m l . c a p a c i t y i m m e r s e d i n the water c a n o n the b e a m axis. L i F crystals sealed i n p o l y e t h y l e n e bags w e r e i m m e r s e d i n F r i c k e s o l u t i o n i n the same b u l b f o r the c o m p a r i s o n e x p e r i ments. F i g u r e 1 shows the L i F c r y s t a l h o l d e r m o u n t e d i n the dosimeter bulb.

Figure 1. X-irradiation set-up for simultaneous Fricke or LiF and hydrated electron dosimetry. Left to right: Linac, tungsten target, an 11.3 cm. water container with dosimeter bulb on the beam axis and a multiple reflexion cell for transient absorption spectrophotometry. From the multiple reflexion cell the light beam passes into a monochromator-photomultiplier assembly T h e dose ratio b e t w e e n t h e b u l b a n d the a b s o r p t i o n c e l l w a s o b t a i n e d b y i r r a d i a t i n g w i t h F r i c k e s o l u t i o n i n b o t h the b u l b a n d the cell. T o c a l i b r a t e t h e h y d r a t e d e l e c t r o n a b s o r p t i o n against t h e F r i c k e d o s i m e t e r the system w a s i r r a d i a t e d to a total dose of about 1.5 k r a d w i t h 300 pulses at 10 p e r second. T h e transient a b s o r p t i o n f o l l o w i n g e a c h p u l s e w a s r e c o r d e d o n a c o m p o s i t e p h o t o g r a p h a n d one t h r e e - h u n d r e d t h o f t h e dose r e c o r d e d b y t h e F r i c k e s o l u t i o n w a s t a k e n to b e t h e dose r e q u i r e d to p r o d u c e the m e a n a b s o r p t i o n r e c o r d e d o n the p h o t o g r a p h w h e n cor r e c t e d f o r t h e dose factor b e t w e e n the c e l l a n d b u l b . T h e t h e r m o l u m i n e s cent dosimeter w a s s i m i l a r l y c o m p a r e d w i t h t h e h y d r a t e d e l e c t r o n a b s o r p t i o n b y p h o t o g r a p h i n g f r o m one to f o u r pulses a n d r e l a t i n g the m e a n a b s o r p t i o n p e r p u l s e to the m e a n t h e r m o l u m i n e s c e n t r e a d o u t p e r pulse. F i g u r e 2 shows f o u r oscilloscope traces that w e r e r e c o r d e d f o r c o m p a r i s o n w i t h s i m u l t a n e o u s l y i r r a d i a t e d t h e r m o l u m i n e s c e n t crystals. T h e r m o l u m i n e s c e n t r e a d o u t w a s p e r f o r m e d i m m e d i a t e l y after exposure w i t h a standard unit made b y M a d i s o n Research a n d Development Laboratories Inc., M o d e l S 1 2 L .



41.

FiELDEN AND HART

Pulsed X-ray

Beams

589

Figure 2. Four consecutive oscilloscope traces taken under identical conditions at pH 12 with doses of 5 rads/pulse. The short horizontal trace represents the 100% light level preceding the radiation pulse (break in curve) followed by the decay of the hydrated electron absorption at 7000 A. The readout of the LiF crystal exposed to those four pulses was 1728 counts D e - a e r a t e d , h y d r o g e n - s a t u r a t e d solutions w e r e p r e p a r e d a n d h a n d l e d b y t h e syringe t e c h n i q u e w h i c h has b e e n d e s c r i b e d p r e v i o u s l y ( 2 ) . Materials. W a t e r was prepared b y the standard triple distillation t e c h n i q u e . A l k a l i n e solutions w e r e p r e p a r e d b y d i s s o l v i n g 5N grade p u r e s o d i u m i n degassed w a t e r (2). A l l other materials w e r e of A n a l a R grade. S i n g l e crystals of l i t h i u m f l u o r i d e T L D 100 grade f r o m H a r s h a w C h e m i c a l C o . , a p p r o x i m a t e l y 4 m m . square a n d w e i g h i n g f r o m 17 t o 65 m g . w e r e u s e d . T h e crystals w e r e c l e a v e d f r o m larger units a n d crystals f r o m different parents h a d differences i n sensitivity of u p to a factor of t w o . F r a g m e n t s f r o m t h e same p a r e n t c r y s t a l g e n e r a l l y s h o w e d n e g l i g i b l e variations i n sensitivity. T h e b a c k g r o u n d w a s 10 ± 1 counts c o m p a r e d w i t h a t y p i c a l readout of 72 counts p e r r a d . A t the l o w doses u s e d t h e r e a d o u t process restored t h e b a c k g r o u n d to its f o r m e r v a l u e a n d t h e c r y s t a l c o u l d b e re-used w i t h o u t f u r t h e r a n n e a l i n g . Effects o f a c c i d e n t a l over-exposure w e r e r e m o v e d b y a n n e a l i n g at 400 ° C . f o r o n e hour.


590

RADIATION CHEMISTRY

1

Results F i g u r e 3 presents a t y p i c a l γ-ray c a l i b r a t i o n of L i F t h e r m o l u m i n e s cence u s i n g a dose rate of 0.90 rads/sec.

T h e γ-ray source was c a l i b r a t e d

against the F r i c k e dosimeter. 1400 1200


1000 CA

Radiation Chemistry

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