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The gamma rays of radium (B and C) Pearce, Michael 1949

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THE GAMMA RAYS  OF RADIUM (B + C) M i c h a e l Pearce T h e s i s S u b m i t t e d i n P a r t i a l F u l f i l l m e n t o f the Requirements f o r t h e Degree o f M a s t e r o f A r t s i n t h e Department o f P h y s i c s - 3 -A B S T R A C T : S e v e r a l n e w t e c h n i q u e s f o r i m p r o v i n g t h e s t a t i s -t i c s o f c o u n t i n g s m a l l p e a k s o n l a r g e b a c k g r o u n d s i n a b e t a -s p e c t r o m e t e r a r e p r e s e n t e d . T h e s e a r o s e f r o m a n a t t e m p t t o r e -c o n c i l e t h e c o n f l i c t i n g r e p o r t s i n t h e l i t e r a t u r e o f g a m m a -t r a n s i t i o n s i n R a (B + 0) a b o v e 1.02 M e v . S o m e s u c c e s s h a s b e e n m e t w i t h . - 1 -ACKNOWLEDGMENT: T h i s work was c a r r i e d out under a G r a n t - i n - A i d o f R e s e a r c h t o Dr. K.C. Mann from t h e N a t i o n a l R e s e a r c h C o u n c i l . The a u t h o r would l i k e t o acknowledge the N a t i o n a l R e s e a r c h C o u n c i l S c h o l a r s h i p w h i c h made t h e i n v e s t i g a t i o n p o s s i b l e . The B r i t i s h Columbia I n d u s t r i a l R e s e a r c h C o u n c i l k i n d l y p l a c e d a 5 0 0 mc, radium source a t our d i s p o s a l . L a s t l y , t h e a u t h o r would l i k e t o acknowledge t h e c o n t i n u e d a s s i s t a n c e o f Dr. K. C. Mann, w i t h o u t whose i n t e r e s t t h e p r o j e c t c o u l d n o t have been u n d e r t a k e n . - 2 -TABLE OF CONTENTS: Acknowledgment Page 1 C o n t e n t s 2 A b s t r a c t 3 I n t r o d u c t i o n 4 E x p e r i m e n t a l Techniques <-(1) The S p e c t r o m e t e r 5 , ( i i ) The G e l g e r Counter 5 ( i i i ) The Source 9 ( i v ) The C u r r e n t R e g u l a t o r 9 (v) The C o u n t i n g Equipment 10 R e s u l t s 10 C o n c l u s i o n s 13 B i b l i o g r a p h y 14 - 4- -INTRODUCTION; Al e c h a n o v and L a t y s h e v 1 u s i n g i n t e r n a l p a i r p r o d u c t i o n r e p o r t , e l e v e n gamma-lines of RaB and RaCVof energy more than 1.02 Mev. E l l i s ? and Mann and O z e r o f f ^ e ach r e p o r t s i x l i n e s i n t h i s r e g i o n . E l l i s used i n t e r n a l c o n v e r s i o n t e c h n i q u e s . The l a t t e r group used a t h i n l e n s b e t a - s p e c t r o -meter and 10 mc. o f Ra (B -f C) w i t h a l e a d r a d i a t o r ( i . e . a t h i n l e a d l a m i n a as a source o f p h o t o e l e c t r o n s . ) * Because a 500 mc. s o u r c e o f Ra (B + C) has s i n c e been made a v a i l a b l e by the B r i t i s h Columbia R e s e a r c h C o u n c i l , t h e spectrum has been r e p e a t e d i n an attempt t o c o n f i r m t h e work of A l e c h a n o v and L a t y s h e v . T h i s has been done i n a t h i n l e n s s p e c t r o m e t e r o f the t y pe d e s c r i b e d by D e u t s c h , E l l i o t t , and Evans^. The spec-t r o m e t e r i s shown i n F i g . / and i s shown s c h e m a t i c a l l y i n F i g . 7 . A r e d u c t i o n o f background count t o improve t h e c o u n t i n g s t a t i s t i c s has been atte m p t e d i n the f o l l o w i n g ways: ( i ) C o i n c i d e n c e c o u n t e r s were c o n s t r u c t e d so as t o count o n l y t h o s e p a r t i c l e s w h i c h e n t e r e d the c o u n t e r window i n the p r o p e r d i r e c t i o n as de-f i n e d by t h e s p e c t r o m e t e r b a f f l e s , (see F i g . 2) ( i i ) E x t r a l e a d was added t o b a f f l e A (see F i g . 7) a t t h e source end t o reduce t h e background from Compton e l e c t r o n s . ( i i i ) Compton e l e c t r o n s produced i n t h e s o u r c e con-- 5 -t a i n e r made a l a r g e c o n t r i b u t i o n t o t h e background c o u n t . An attempt was made t o d e f l e c t t h e s e e l e c t r o n s away from t h e l i n e o f f o c u s e d e l e c t r o n s by means o f a magnet p l a c e d between t h e source c o n t a i n e r and r a d i a t o r . EXPERIMENTAL TECHNIQUES: (1) The S p e c t r o m e t e r , (see F i g s . 1 and 7) The s p e c t r o m e t e r used was o f t h e t h i n l e n s type d e s c r i b e d by Deutsch, E l l i o t and Evans. The s p e c t r o m e t e r tube was 40 i n c h e s i n l e n g t h and 8 i n c h e s i n d i a m e t e r . I t was a l i g n e d p a r a l l e l t o t h e magnetic m e r i d i a n i n o r d e r t h a t t h e h o r i z o n t a l component of t h e e a r t h ' s f i e l d s h o u l d not d e f o c u s the b e t a - p a r t i c l e s . The v e r t i c a l component o f t h e e a r t h ' s f i e l d was e l i m i n a t e d by p a s s i n g a c u r r e n t t h r o u g h h o r i z o n t a l H e l m h o l t z c o i l s above and below t h e s p e c t r o m e t e r , (see F i g . 1) The magnet c o n s i s t s o f f o u r water c o o l e d c o i l s , (see F i g . 7) I n t h i s spectrum o n l y t h e t h r e e o u t e r c o i l s were used. T h i s has t h e advantage o f l o w e r i n g t h e s p h e r i c a l a b e r -r a t i o n and t h u s i n c r e a s i n g t h e r e s o l u t i o n . ( i i ) The G e i g e r Counter. I n t h e o p e r a t i o n o f any c o u n t e r where i t i s de-s i r e d t o measure a c o u n t i n g r a t e N which i s superimposed upon a background c o u n t i n g r a t e N ^ a c^, i t i s apparent t h a t two sep-a r a t e measurements be made - t h e t o t a l count and N^Q^Q^. N i s t h e n o b t a i n e d by s u b t r a c t i o n . Thus N = N t o t a l + Nback (1) I t f o l l o w s t h a t t h e s t a n d a r d d e v i a t i o n o f N i s CT 2 = ( T 2 t o t a l + <T 2back , and assuming a P o i s s o n d i s t r i b u t i o n <T = V N t o t a l t Nback , and from (1) = V 2 N b a c k +• N (2) To reduce OTand t h e r e b y improve t h e s t a t i s t i c a l a c c u r a c y i t was n e c e s s a r y t o reduce N D a c k . T h i s was accomp-l i s h e d by two c o u n t e r s o p e r a t i n g i n c o i n c i d e n c e so as t o r e -c o r d o n l y f o c u s s e d b e t a - r a y s . The arrangement i s shown i n F i g . 2. The two c o u n t e r t u b e s a r e c u t i n b r a s s s t o c k a t r i g h t a n g l e s t o one a n o t h e r and t o t h e a x i s o f t h e s p e c t r o m e t e r t u b e . The h o l e between c o u n t e r s and t h e mica window l a y on t h e a x i s o f t h e t u b e . Maximum t r a n s m i s s i o n was o b t a i n e d by f o c u s s i n g on t h e w i r e o f c o u n t e r No. 2. K o v a r - g l a s s s e a l s were used t o p l u g the c o u n t e r t u b e s . No. 40 copper w i r e was t h r e a d e d f o r the anode. A 2.5 mg./cm? m i c a window was s e a l e d w i t h P l i c e n e cement. A conven-t i o n a l f i l l i n g o f 9 cm. o f argon and 1 cm. o f a l c o h o l was used. Each c o u n t e r e x h i b i t e d a 100 v o l t p l a t e a u w i t h a r i s e o f l e s s than 0.2^ per v o l t . However, t h e e f f i c i e n c y o f t h e c o u n t e r s went down s e v e r a l p e r c e n t i n a few hours o f o p e r a t i o n at a c o u n t i n g r a t e o f 10,000 per mi n u t e . T h i s was thought t o r e s u l t - 7 -from the r e l a t i v e l y s m a l l volume o f a l c o h o l a v a i l a b l e i n t h e t u b e s . However, when a s i m i l a r c o u n t e r was o p e r a t e d w i t h a c o u n t i n g gas r e s e r v o i r , t h e same b e h a v i o u r was o b s e r v e r e d . T h i s s u g g e s t s t h a t the i n s t a b i l i t y I s a cathode e f f e c t , and some e v i d e n c e t o support t h i s v i e w was o b t a i n e d by t h e w r i t e r a t t he Chalk R i v e r l a b o r a t o r i e s o f the N a t i o n a l R e s e a r c h C o u n c i l . A t e s t was conducted t o i n s u r e t h a t t h e o p e r a -t i o n o f one tube was independent o f t h e o p e r a t i o n o f t h e o t h e r . The p o s s i b i l i t y e x i s t e d t h a t when one tube r e c o r d e d an i o n i z -i n g e v e n t , t h a t a photon from t h e e l e c t r o n a v a l a n c h e i n t h i s tube s h o u l d pass t h r o u g h t o t h e second t u b e , c a u s i n g a s p u r i -ous c o u n t . T h i s was I n v e s t i g a t e d by o b s e r v i n g t h e c o u n t i n g r a t e o f tube No. 1 w i t h t h e v o l t a g e on and w i t h t h e v o l t a g e o f f tube No. 2 ( i . e . w i t h and w i t h o u t No. 2 c o u n t i n g . ) . Typ-i c a l f i g u r e s f o r tube No. 1 were 1824 ( s . d . = 6) c o u n t s per minute w i t h tube No. 2 o f f and 1882 ( s . d . = 8) c o u n t s p e r min-ut e w i t h tube No. 2 r e c o r d i n g 3 0 0 0 c o u n t s per m i n u t e . Assum-i n g t h a t t h e change i n r g t e o f 58 comes from tube No. 2, we may say t h a t 2% o f the a v a l a n c h e s s p r e a d from tube No. 2 t o tube No. 1. The use o f t h e c r o s s e d c o u n t e r s i n c o i n c i d e n c e as compared t o the use o f tube No. 2 by i t s e l f was f o u n d t o r e -duce t h e background count by a f a c t o r o f n i n e . At t h e same ti m e however, t h e n e t count was re d u c e d by a f a c t o r o f t h r e e . - 8 -/ The change I n t h e s t a t i s t i c a l a c c u r a c y , w h i c h these, two new c o u n t i n g r a t e s b r i n g about, i s i n v e s t i g a t e d below. C o n s i d e r ^/fras a f i g u r e o f m e r i t , where 0~"ls the s t a n d a r d d e v i a t i o n o f t h e n e t c o i n c i d e n c e count N. S i n c e < T - V N t o t a l + Nback and N - N t o t a l + N D a c k » t h e r e f o r e ^/N = V N + 2 N b a c k /N. L e t t h e net count f o r t h e s i n g l e c o u n t e r be Nt?, and l e t t h e background f o r t h e s i n g l e c o u n t e r be Noack. Then p u t t i n g N 1 - 3N and Nfeack s 9 N D A C ^ we o b t a i n f o r t h e s i n g l e c o u n t e r <VN« = V N « 4 2 N b a c k /N« 8 8 V ^ N + l 8 N b a c k /3N , . (3) where XT i s t h e s t a n d a r d d e v i a t i o n o f N'. E x a m i n a t i o n shows t h e r e i s no N 1 sueh- t h a t <*7N < <T/N'. That i s , no b e t t e r s t a t i s t i c s a r e o b t a i n a b l e from the use o f the two counters'. However i f i t were t h a t N r = 2 N , ' then o b v i o u s l y " -<?7N <°7N* f o r a l l N ^ 5 N b a c k . So the e f f i c i e n c y o f the c o i n c i d e n c e c o u n t e r s f o r f o c u s s e d e l e c t r o n s must be s l i g h t l y i n c r e a s e d i f t h e s t a t -i s t i c s a r e t o be improved. T h i s might be a c c o m p l i s h e d by more c a r e f u l f o c u s s i n g and b e t t e r c o l l i m a t i o n . I f t h e c o u n t i n g - 9 -l o s s e s were due t o s c a t t e r i n g i n t h e window, t h i n n e r m i c a might improve t h e e f f i c i e n c y . ( i i i ) The Source. The 500 mc. radium s o u r c e was i n a s i l v e r con-t a i n e r whose w a l l s were o f s u f f i c i e n t t h i c k n e s s t o s t o p t h e p r i m a r y b e t a - r a y s . An attempt t o s e p a r a t e t h e Compton e l e c -t r o n s o f t h e c o n t a i n e r from t h e gamma-rays by means o f a s t r o n g permanent magnet was made. The gap o f t h e magnet, w h i c h had a f l u x o f 1 ,500 gauss, was p l a c e d between t h e l e a d r a d i a t o r and t h e s o u r c e . I t was n e c e s s a r y t o s h i e l d the s p e c t -rometer tube from the permanent magnet by an i r o n sheet p l a c e d p e r p e n d i c u l a r t o the a x i s o f the s p e c t r o m e t e r . T h i s sheet had a 5/16 i n c h h o l e o p p o s i t e t h e l e a d r a d i a t o r , w h i c h was 3/16 i n c h e s i n d i a m e t e r . However, t h e magnet was u n a b l e t o remove b e t a - p a r t i c l e s s c a t t e r e d from the p o l e f a c e s and from t h e i r o n s h e e t , w i t h t h e r e s u l t t h a t t h e background was not s u c c e s s f u l l y e l i m i n a t e d . The background was v e r y e f f e c t i v e l y r educed by e n l a r g i n g the b a f f l e a t the c o u n t e r end o f t h e s p e c t r o m e t e r "tube and by b e t t e r c o l l i m a t i o n a t t h e sou r c e end, ( i v ) The C u r r e n t R e g u l a t o r . The magnet c u r r e n t was r e g u l a t e d i n a c o n v e n t i o n -a l manner by p a s s i n g i t t h r o u g h 38 t w i n t r i o d e s (6AS7's) i n p a r a l l e l , (see F i g . 3) A s i g n a l i s t a k e n from a c o n s t a n t r e -- 1 0 -s i s t a n c e I n the magnet c i r c u i t , a m p l i f i e d by an A. C. and a D. C. a m p l i f i e r , (see P i g . 4 ) t h e n a p p l i e d 1 8 0 ° out o f phase onto th e g r i d s o f t h e 6 A S 7's. The i n p u t o f t h e D.C. i s a Brown c o n v e r t e r w h i c h d e t e c t s t h e d i f f e r e n c e between t h e s i g -n a l and a p o t e n t i o m e t e r v o l t a g e . The p o t e n t i o m e t e r s e t t i n g ;. may be cheeked a g a i n s t a s t a n d a r d c e l l . (v) C o u n t i n g Equipment. C o u n t i n g was done w i t h t h e c o i n c i d e n c e c o u n t e r d e s c r i b e d i n s e c t i o n ( i i ) and a r e g u l a t e d h i g h t e n s i o n s e t . A c o n v e n t i o n a l cathode f o l l o w e r was used i n c o n j u n c t i o n w i t h an "Atomic" s c a l e r and a p r e a m p l i f i e r , (see F i g . 5) RESULTS: The c u r v e s o b t a i n e d a r e shown i n F i g . 6 . The upper c u r v e was t a k e n w i t h t h e l e a d r a d i a t o r and i s the sum o f p h o t o e l e c t r o n s from the l e a d r a d i a t o r , Compton e l e c t r o n s from t h e source h o l d e r , and e l e c t r o n s s c a t t e r e d i n t h e s p e c t -r ometer. The l o w e r c u r v e was t a k e n w i t h o u t t h e l e a d r a d i a t o r . The 0 , 6 0 7 Mev gamma-line was used f o r . c a l i b r a -t i o n . The r e s u l t s appear i n T a b l e I t o g e t h e r w i t h t h e r e s u l t s o f E l l i s , Mann and O z e r o f f , and o f A l e c h a n o v and L a t y -shev. I t s h o u l d be n o t e d t h a t t h e gamma l i n e o f energy 1.52 r e p o r t e d by A l i c h a n o v and L a t y s h e v has been c o n f i r m e d . The 11 v a l u e o b t a i n e d was 1.54 Mev. A f a i n t l i n e d e t e c t e d a t 1.94 Mev may be t h e 1.82 Mev l i n e r e p o r t e d by Alechanov and L a t y -shev. The gamma l i n e r e p o r t e d a t 0.448 Mev by Mann • and O z e r o f f has not been f o u n d . Note t h a t i n t h e r e g i o n below 1.02 Mev, l i n e s r e p o r t e d a t 0.498 Mev and 0.933 Mev by E l l i s have been found a t 0.511 Mev and 0.950 Mev r e s p e c t i v e l y . - 12 -TABLE I GAMMA LIMES OF RADIUM IN MEV. E l l i s A l e c h a n o v  and L a t y s h e v Mann and O z e r o f f Mann and Pearce 0.0529 0.2406 0.8571 0.2937 0.3499 0.4260 0*4980 0.6067 0.766 0.933 1*120 1*238 1.379 1*414 1.761 2.193 • • • • . . . • • • • • • • • • . . . • • • * • • • . . . • • • • • • • • • . 0.291 • < • . 0.283 • • • • . 0.352 • • . 0.351 • • . 0.426 (9). . 0.425 • • * • . 0.450 (4). . 0.483 • • • • . 0.496 (14). . 0.511 0.606 • • . 0.607 (74). . 0.697 0.766 • • . 0.781 (13). . 0.780 0.933 • • . . . • • . 0.950 1.11 • • . . . • • • • • 1.120 • • . 1.12 ( 9 ) . . 1.12 1.21 (23). . 1.22 (24). . 1.22 1.234 • • • • • • • • • • 1.370 • • • • • • • • • • 1.39 (49). . 1.40 (23). .1.38 1.414 • • • • • • • . . . 1.52 (29). • • • • • . 1.54 1.62 (22). * • • • • • • • 1.69 (17). • • • • • • t • 1.75 (100). . 1.77 (100). . 1.79 1.761 • • • • • • • . . . 1.82 (17). • • • • • .1.94 2.09 (15). • • • • • • • 2.20 (41). . 2.21 (25). . 2.23 2.40 (21). . 2.40 • • F i g u r e s i n p a r e n t h e s e s a r e r e l a t i v e i n t e n s i t i e s . - 13 -CONCLUSIONS: The p r e s e n t i n v e s t i g a t i o n has I n d e n t i f i e d gamma r a y s w h i c h a r e i n agreement w i t h t h e o v e r a l l p i c t u r e o f E l l i s . One, and p r o b a b l y two, l i n e s o f Alechanov and L a t y s h e v h i t h e r -t o u n c o n f i r m e d have been f o u n d . - 14 -BIBLIOGRAPHY: 1. A l e c h a n o v , A., and L a t y s h e v , G. D., Compt. r e n d . acad. s c i . U . S. S. R. 2 0 : 2 4 9 . 1948. 2 . E l l i s , C D . , P r o c . Roy. Soc. (London), A , 143:350. 1934. 3 . Mann, K. C , and O z e r o f f , M. J . , Can. J o u r , o f Res., A , V o l . 27, p. 164. 1949.• 4 . D e u t s c h , M., E l l i o t t , L. G., and Evans, R ., Rev. S c i . I n s t r u m e n t s , 15 :7 . 1944. - 15 -F i g . 1 The Thin Lens Beta Spectrometer - 16 -- 17 -v -<> n 4 W W - 1 8 -F i g . 4 - 1 9 -^ 1 9 •^ T-JJ - OS -M A G N E T I C W A T E R 

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