The Dosimetry of Very High Intensity Pulsed Electron Sources Used

37 The Dosimetry of Very High Intensity Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch037

Pulsed Electron Sources Used for Radiation Chemistry: II. Dosimetry for Gaseous Samples C. WILLIS, O. A. MILLER, A. E. ROTHWELL, and A. W. BOYD Research Chemistry Branch, Chalk River Nuclear Laboratories, Atomic Energy of Canada Ltd., Chalk River, Ontario, Canada

Adiabatic calorimetry has been used to measure the mean dose in a cell used for the irradiation of gaseous samples with very high intensity electron pulses at a dose rate of about 10 e.v./gram sec. The calorimeter was a thin alumi num disc, and the temperature rise caused by the pulse was measured by an attached thermocouple. By using discs of different thicknesses and extrapolating to zero thickness, the dose in an infinitely thin absorber was obtained. This was corrected for the difference in electron stopping power be tween aluminum and the gases. Using this dose, the nitrogen yield from nitrous oxide was found to be G(N ) = 12.4 ± 0.2. The temperature of the nitrous oxide was initially 25°C. but increased to ~50°C. on irradiation. The hydro gen yield from ethylene was found to be pressure dependent. At one atm., G(H ) = 1.9 and decreases to 1.0 at 10 atm. 27

2

2

>Tphe o b s e r v a t i o n b y o p t i c a l spectroscopy of transient species f o r m e d i n short bursts of r a d i a t i o n requires a reasonable n u m b e r of these transients to b e f o r m e d . F o r t h e p u l s e r a d i o l y s i s of l i q u i d s t h e dose p e r p u l s e r e q u i r e d is 1 - 1 0 K r a d . d e l i v e r e d i n a b o u t 0.1 /xsec. A t a pressure of a f e w atmospheres, h o w e v e r , t h e s t o p p i n g p o w e r of gases p e r u n i t v o l u m e is s m a l l a n d to p r o d u c e a reasonable n u m b e r of transient species, a m u c h h i g h e r dose p e r p u l s e is r e q u i r e d . R e c e n t l y several types of m a c h i n e s c a p a b l e of p r o d u c i n g intense electron pulses h a v e b e c o m e a v a i l a b l e . T h e s e are b e i n g u s e d f o r p u l s e 539 Hart; Radiation Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

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RADIATION CHEMISTRY

studies a n d dose rate studies i n the r a d i a t i o n c h e m i s t r y of gases.

1

The

m a c h i n e s are t y p i c a l l y c a p a c i t o r - d i s c h a r g e c i r c u i t s u s i n g the field emission effect to o b t a i n a h i g h c u r r e n t of electrons f r o m fine t u n g s t e n needles i n a v a c u u m tube. T h e m e a s u r e m e n t of the r a d i a t i o n energy a b s o r b e d i n a sample, a necessity for accurate r a d i a t i o n c h e m i s t r y studies, c a n n o t be d o n e b y Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch037

c o n v e n t i o n a l m e t h o d s at these v e r y h i g h intensities. I o n c h a m b e r s , the c o m m o n s t a n d a r d f o r d o s i m e t r y of gases at l o w e r intensities, c a n n o t b e u s e d . A 1 M r a d p u l s e i n air p r o d u c e s a b o u t 2 Χ

10

6

esu. p e r cc. i n 100 n s e c , w h i c h is orders of m a g n i t u d e h i g h e r t h a n c a n be used i n ion chambers

(5).

T h e e x t r a p o l a t i o n of a n y c h e m i c a l dosimeter c a l i b r a t e d at m u c h l o w e r dose rates cannot be u s e d d i r e c t l y because the r e l a t i v e c o n c e n t r a t i o n of r a d i c a l s p r o d u c e d w i t h s u c h pulses is so h i g h that r a d i c a l - r a d i c a l reactions m a y p r e d o m i n a t e so that the y i e l d s of p r o d u c t s m a y b e different. A d i a b a t i c c a l o r i m e t r y is the o b v i o u s absolute m e t h o d f o r use these dose rates a n d i n a p r e v i o u s p a p e r (16) d o s i m e t r y of l i q u i d samples.

at

w e r e p o r t e d o n its use f o r

It is the object of the present p a p e r to

r e p o r t u p o n its use f o r gas phase d o s i m e t r y . W i t h l i q u i d s it w a s not possible to stop the t o t a l b e a m i n the sample, a n d therefore the s t o p p i n g thickness of the c a l o r i m e t e r w a s m a t c h e d to the s a m p l e . T h i s w a s necessary as the electrons i n a p u l s e are not m o n o energetic.

A l s o , b y u s i n g t h i n l i q u i d samples so that the d e p t h of the

c e l l is s m a l l c o m p a r e d w i t h the d i v e r g e n c e of the b e a m it w a s possible to m a t c h the g e o m e t r y of the c a l o r i m e t e r a n d the sample f a i r l y w e l l . S u c h a p r o c e d u r e is not possible for gases. A f a i r l y large v o l u m e

50

m l . ) of gas m u s t b e i r r a d i a t e d to g i v e a m e a s u r a b l e y i e l d of p r o d u c t s ; hence, c o n s i d e r a b l e differences i n g e o m e t r y exist b e t w e e n the c a l o r i m e t e r a n d sample. T h e m a t c h i n g of s t o p p i n g thicknesses is also not possible. F o r the c e l l u s e d i n o u r experiments, e t h y l e n e at one a t m . pressure corre sponds to a n a l u m i n u m c a l o r i m e t e r 0.0013 c m . t h i c k . S u c h a c a l o r i m e t e r w o u l d not h a v e sufficient s t r u c t u r a l strength. W e h a v e m e a s u r e d the dose i n calorimeters of v a r i o u s thicknesses a n d e x t r a p o l a t e d to zero thickness. T h e calorimeters w e r e u s e d i n v a r i o u s positions w i t h i n the c e l l to o b t a i n a n average dose over the c e l l v o l u m e . I n attempts to find a suitable c h e m i c a l dosimeter, b o t h the n i t r o g e n y i e l d f r o m the i r r a d i a t i o n of nitrous o x i d e (7,8) f r o m t h e i r r a d i a t i o n of e t h y l e n e (3, 12)

a n d the h y d r o g e n y i e l d

h a v e b e e n u s e d to measure the

doses a b s o r b e d i n gases. W e h a v e i r r a d i a t e d b o t h of these gases a n d , u s i n g the dose f r o m the c a l o r i m e t r y , w e h a v e m e a s u r e d the y i e l d s at v e r y h i g h dose rates.

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

37.

Dosimetry

WILLIS E T A L .

for Gaseous

541

Samples

Apparatus

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 28, 2018 | https://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0081.ch037

Pulsed Electron Accelerator. T h e v e r y h i g h intensity p u l s e d elec t r o n accelerator u s e d w a s a 705 F e b e t r o n ( F i e l d E m i s s i o n C o r p o r a t i o n , M c M i n n v i l l e , O r e g o n , U . S . A . ) . T h i s is n o m i n a l l y a 2 M e v . accelerator. Its m o d e of o p e r a t i o n has b e e n d e s c r i b e d p r e v i o u s l y (16). T h e c u r r e n t - t i m e profile a n d t h e energy s p e c t r u m of t h e p u l s e are shown i n Figures l a a n d l b . This current-time profile was obtained

TIME Figure la.

IN

NANOSECONDS

Current waveform

of electron

pulse

50

%

OF

4

0

3

0

TOTAL ENERGY

20

10

0

0.4

0.6

0.8

1.0

1.2

ENERGY OF ELECTRONS

1.4

1.6

(Mev.)

Figure lb. Energy spectrum of electron pulse after passing through 0.005 inch stainless steel window



542

radiation chemistry 1

f r o m a F a r a d a y c u p p l a c e d d i r e c t l y over t h e electron w i n d o w o f t h e Febetron. T h e energy spectrum s h o w n was obtained b y correcting the i n i t i a l energies o f t h e electrons f o r a b s o r p t i o n b y t h e stainless steel w i n d o w o f t h e i r r a d i a t i o n c e l l . T h e i n i t i a l energy s p e c t r u m w a s d e t e r m i n e d both f r o m measurement of the voltage a n d current waveforms a n d t h e r a n g e o f t h e electrons i n a l u m i n u m b y t h e F i e l d E m i s s i o n C o r p o r a t i o n . T h e e l e c t r o n b e a m has a d i a m e t e r o f b e t w e e n 3 a n d 4 c m . at t h e w i n d o w a n d is d i v e r g i n g . I n a n y h u n d r e d c o n s e c u t i v e pulses, f o r a g i v e n t u b e , there is a r e p r o d u c i b i l i t y o f ± 3 % ( s t a n d a r d d e v i a t i o n ) o f t h e dose p e r pulse, b u t over several h u n d r e d pulses there appears t o b e a g r a d u a l increase i n dose p e r p u l s e . Irradiation Cells and Calorimeters. T h e c e l l u s e d f o r gas phase i r r a d i a t i o n s w i t h t h e F e b e t r o n w a s m a d e o f stainless steel. T h e gas v o l u m e w a s a c y l i n d e r 4.0 c m . i n d i a m e t e r a n d 3.0 c m . d e e p b e h i n d a 0.0127 c m . stainless steel electron w i n d o w . T h e e l e c t r o n w i n d o w w a s w e l d e d a r o u n d t h e edge o f t h e c e l l . W e l d e d t o t h e rear of t h e c e l l w a s a s m a l l filling t u b e w i t h a stainless steel H o k e v a c u u m v a l v e . T h e w h o l e a s s e m b l y c o u l d b e p u m p e d a n d b a k e d . Gases w e r e f r o z e n i n t o t h e filling t u b e volume w i t h l i q u i d nitrogen a n d allowed to expand into the cell o n warming. 3.0 cm-

D Figure

2.

Ε Irradiation cell used for calorimetry

(A) 0.0127 cm. thick stainless steel electron win dow, (B) calorimeter, (C) thermocouple, (D) stainless steel spacer ring, and (E) nylon calo rimeter mounting ring T h e c e l l u s e d f o r c a l o r i m e t r i c measurements s h o w n i n F i g u r e 2 w a s essentially i d e n t i c a l t o that u s e d f o r t h e gas i r r a d i a t i o n s . T h e f r o n t stain less steel w i n d o w w a s w e l d e d t o t h e c e l l b o d y a n d t h e b a c k p l a t e w a s r e m o v a b l e . T h e c e l l i n t e r n a l d i a m e t e r w a s 4.13 c m . so as t o a l l o w 4.0 c m .


37.

WILLIS E T A L .

Dosimetry

for

Gaseous

543

Samples


d i a m e t e r calorimeters to b e u s e d w i t h a 0.065 c m . gap for a n y l o n calo r i m e t e r m o u n t that also a c t e d as t h e r m a l i n s u l a t i o n . T h e calorimeters fitted s n u g l y i n t o the m o u n t a n d w e r e s e c u r e d b y a s m a l l a m o u n t of G l y p t a l cement at v a r i o u s points a r o u n d the c i r c u m f e r e n c e . T h e c a l o rimeters w e r e m a d e of s u p e r - p u r e a l u m i n u m a n d h a d thicknesses of 0.0203, 0.0280, 0.076, a n d 0.127 c m . T h e assembly fitted t i g h t l y into the c e l l a n d w a s p u s h e d against a stainless steel spacer (0.04 c m . t h i c k ) that d e f i n e d the p o s i t i o n of the c a l o r i m e t e r r e l a t i v e to the f r o n t w i n d o w . Spacers of lengths 0.2, 1.0, 1.5, 2.0, a n d 2.7 c m . w e r e u s e d . A 0.1 c m . h o l e w a s d r i l l e d t h r o u g h e a c h c a l o r i m e t e r at a b o u t h a l f r a d i u s to a l l o w a i r i n t e r c h a n g e b e t w e e n the rear a n d front of the c e l l . T h i s w a s f o u n d to b e necessary as the t h e r m a l e x p a n s i o n because of the h e a t i n g b y the electron pulse of the air t r a p p e d i n the f r o n t section w a s sufficient, i n some cases, to d i s p l a c e the c a l o r i m e ter assembly. T h e t h e r m o c o u p l e s w e r e m a d e f r o m c o p p e r a n d constantan w i r e 0.0127 c m . i n d i a m e t e r . T h e j u n c t i o n w a s s o l d e r e d w i t h silver a n d the t h e r m o c o u p l e was either p e e n e d i n t o or spot w e l d e d to the c a l o r i m e t e r r o u g h l y at its center. T h e leads of the t h e r m o c o u p l e w e r e l e d o u t t h r o u g h a s m a l l h o l e i n the e n d p l a t e of the c e l l . I d e n t i c a l t e m p e r a t u r e rises w e r e o b t a i n e d w i t h a n d w i t h o u t a n i c e - b a t h c o l d - j u n c t i o n a n d i n general the w i r e s w e r e j o i n e d d i r e c t l y to the r e c o r d e r i n p u t s .

»

I ι 0

TIME ι 5

10

(SECONDS) Figure

3.

Typical

trace of thermocouple calorimeter

for

aluminum

A sensitive M o s e l e y strip-chart r e c o r d e r w a s u s e d to m e a s u r e the v o l t a g e o u t p u t of the t h e r m o c o u p l e . T h i s r e c o r d e r has a f u l l scale re sponse t i m e of 0.5 sec. A t y p i c a l trace is s h o w n i n F i g u r e 3. P e a k 1 is t h e instantaneous t e m p e r a t u r e registered b y the t h e r m o c o u p l e . T h i s is n o t the e q u i l i b r i u m t e m p e r a t u r e of t h e calorimeter. T h e t h e r m o c o u p l e



544

RADIATION CHEMISTRY

1

materials h a v e specific heats l o w e r t h a n a l u m i n u m a n d also, since the b e a m i n t e n s i t y is n o t u n i f o r m across its d i a m e t e r , t h e center of t h e c a l o r i m e t e r is i n i t i a l l y hotter t h a n t h e c i r c u m f e r e n c e . P e a k 2 is i n d u c e d b y t h e s w i t c h i n g off the F e b e t r o n f o c u s s i n g m a g n e t i c field — 1 sec. after the p u l s e . C u r v e 3 is t h e n o r m a l c o o l i n g c u r v e of t h e c a l o r i m e t e r assem b l y . T h e i n d u c e d v o l t a g e AV e x t r a p o l a t e d to t i m e zero u s i n g p a r t 3 of the c o o l i n g c u r v e corresponds to t h e i r r a d i a t i o n dose a b s o r b e d b y t h e c a l o r i m e t e r . T h a t this is a n accurate measure of t h e average dose over the area of t h e c a l o r i m e t e r has b e e n s h o w n (16) b y agreement b e t w e e n calorimeters of different materials. A f t e r corrections f o r electron s t o p p i n g p o w e r a n d backscatter t h e doses f r o m a l u m i n u m g r a p h i t e a n d n i c k e l calorimeters a g r e e d w i t h i n 2 % . Since these elements h a v e c o n s i d e r a b l y different t h e r m a l c o n d u c t i v i t i e s a n d specific heats it is u n l i k e l y that t h e heat losses f r o m t h e c a l o r i m e t e r w e r e significant d u r i n g t h e t i m e r e q u i r e d to r e a c h t h e r m a l e q u i l i b r i u m w i t h i n t h e calorimeter. Gases. M e d i c a l - g r a d e n i t r o u s o x i d e ( L i q u i d A i r C o m p a n y L t d . ) a n d research-grade ethylene ( P h i l l i p s ) w e r e u s e d d i r e c t l y f r o m the bottle after several f r e e z e - p u m p - t h a w cycles to r e m o v e a n y n o n - c o n d e n s i b l e impurities. T h e irradiation products were measured b y freezing out w i t h a l i q u i d n i t r o g e n c o l d t r a p a n d p u m p i n g t h e n o n - c o n d e n s i b l e gases i n t o

0.025

0.075 THICKNESS - CM.

0.125

Figure 4. Doses measured in aluminum calorimeters of different thicknesses at various distances from front window of cell


37.

WILLIS E T A L .

Dosimetry

for

Gaseous

545

Samples

a s t a n d a r d v o l u m e . W i t h ethylene a s u p p l e m e n t a r y s o l i d n i t r o g e n t r a p was u s e d . T h e m e a s u r e d v o l u m e of gas was a n a l y z e d b y mass spectrometry. I r r a d i a t i o n s w e r e c a r r i e d out at 24 ±

1°C.


T h e gas c e l l u s e d f o r the ethylene i r r a d i a t i o n s w a s b a k e d at 5 5 0 ° C . i n a i r after e a c h i r r a d i a t i o n to r e m o v e the p o l y m e r f o r m e d .

Q

The Dosimetry of Very High Intensity Pulsed Electron Sources Used

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