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UBC Theses and Dissertations

THE 0¹⁶(p,૪)F¹⁷ REACTION Robertson, Lyle Purmal 1957

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THE 0  l 6  (p^)F  1 7  REACTION  by LYLE PURMAL-ROBERTSON  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS  i n t h e Department of PHYSICS  We a c c e p t t h i s t h e s i s as conforming t o t h e standard r e q u i r e d from candidates f o r the degree of MASTER OF ARTS  Members o f t h e department  of P h y s i c s  THE UNIVERSITY OF BRITISH COLUMBIA November, 1957  ABSTRACT  The  d i f f e r e n t i a l cross s e c t i o n f o r d i r e c t r a d i a t i v e 16  c a p t u r e o f p r o t o n s by 0  has been measured u s i n g i c e t a r g e t s o f  known t h i c k n e s s and 800 kev.  protons.  The d i f f e r e n t i a l  cross  s e c t i o n f o r t h e gamma r a y t r a n s i t i o n t o t h e f i r s t e x c i t e d i n F ? was found t o be (lO.W- - 1.3) x 10"32 1  c m  2  a t 90° t o t h e i n c i d e n t p r o t o n beam d i r e c t i o n .  #  p  e  r  s  state  teradian  A t t h e same energy,  the r a t i o o f t h e d i f f e r e n t i a l c r o s s s e c t i o n a t 90° f o r t r a n s i t i o n s t o t h e ground s t a t e t o t h a t f o r t r a n s i t i o n s t o t h e f i r s t e x c i t e d s t a t e i n F ? was found t o be Q»lh - 0 . 0 3 . 1  The  energy o f t h e f i r s t e x c i t e d s t a t e i n  was d e t e r -  mined b y measuring t h e energy o f t h e gamma r a y f r o m t h i s t o the ground s t a t e .  level  T h i s method i s d i f f i c u l t because o f t h e  presence o f p o s i t r o n a n n i h i l a t i o n r a d i a t i o n of t h e same energy, w i t h i n e x p e r i m e n t a l e r r o r s , f r o m the decay o f F ? . 1  The f i r s t  e x c i t e d s t a t e energy was a l s o measured by n o t i n g t h e d i f f e r e n c e between the c a p t u r e gamma r a y s t o t h i s s t a t e and t o t h e ground state.  The energy o f t h i s l e v e l was found t o be 0 . 5 0 ± 0 . 0 1 Mev.  i n agreement w i t h the r e s u l t s o f M a r i o n and Bonner (55) and w i t h e a r l i e r r e s u l t s obtained  i n this laboratory  (Warren e t . a l . , (5k),  An attempt t o c o n f i r m t h a t t h e s o u r c e o f t h e 873 k e v . r a d i a t i o n f r o m p r o t o n bombardment o f n a t u r a l o x i d e t a r g e t s above 1.8 Mev. bombarding energy was t h e 0 ^ ( p , made u s i n g s e p a r a t e d O ^ and 1  0 ? targets. 1  p ' j ^ J O ^ r e a c t i o n , was 1  The r e s u l t s were  incon-  c l u s i v e due t o t h e s m a l l p e r c e n t a g e o f oxygen t h a t s t u c k t o t h e t a r g e t s and t o t h e presence of s e v e r a l c o n t a m i n a n t s .  In the  presenting  requirements  of  British  it  freely  agree for  that  available  thesis  I agree  by h i s  copying  or  gain  s h a l l not  of  may  November  the  be  this  by t h e It  is  thesis  Columbia,  I of  of  University  Library shall  make  further this  Head o f  thesis my  understood for  my w r i t t e n  PW^s>cs  1957  the  copying  granted  allowed without  at  and s t u d y .  extensive  publication of be  degree  representative.  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8, C a n a d a . Date  that  for reference  purposes  or  in partial fulfilment  an advanced  permission for  that  Department  for  Columbia,  scholarly  Department  this  financial  permission.  ACKNOWLEDGEMENTS  The  a u t h o r w i s h e s t o thank Dr. G.M.  h i s d i r e c t i o n of t h i s work and Dr. B.L.  most h e l p f u l .  thanked f o r h i s s u g g e s t i o n s  on 0  1 7  of the t h e s i s p r e p a r a t i o n  and  White whose a s s i s t a n c e and guidance w i t h the e x p e r -  i m e n t a l , work was  topic.  Griffiths for  The  D r . J.B. Warren i s a l s o  and d i s c u s s i o n s of the t h e s i s  d i r e c t i o n of D r . C.A.  Barnes d u r i n g the e a r l y Work  i s g r a t e f u l l y acknowledged. Thanks are due  t o Mr.  P. R i l e y and Mr. E. L a r s o n f o r  h e l p i n o p e r a t i n g the Van de G r a a f f g e n e r a t o r  and t o Mr.  G.  Jones f o r many h e l p f u l d i s c u s s i o n s of e l e c t r o n i c problems. The  a u t h o r a l s o w i s h e s t o thank Mrs. G. Conway f o r  a s s i s t a n c e i n the p r e p a r a t i o n of t h i s t h e s i s . The  f i n a n c i a l a s s i s t a n c e g i v e n by the N a t i o n a l  s e a r c h C o u n c i l i n the form of a B u r s a r y and g r a t e f u l l y acknowledged.  a Studentship  Reis  TABLE OF CONTENTS  CHAPTER I II  PAGE INTRODUCTION  1  O (p,^)F  6  l 6  1.  CROSS SECTION MEASUREMENTS  6  Apparatus (a) (b) (c)  6 7 10  T a r g e t Arrangement Gamma Ray D e t e c t o r Electronics  2.  T a r g e t T h i c k n e s s Measurements  11  3.  Experimental  16  (a) (b)  Background Procedure  16 17  (c)  Measurement o f t h e E f f e d t i v e C e n t r e .......  18  k.  III  1 7  Cross S e c t i o n C a l c u l a t i o n s  20  (a) (b) (c) (d)  20 21 22 2k  Carbon C o n t a m i n a t i o n C o r r e c t i o n S o l i d Angle C a l c u l a t i o n o f Cross S e c t i o n Errors  5. R e s u l t s  25  ENERGY DETERMINATION .OF ^ 3  27  1. 2.  Introduction Apparatus  27 28 28 30  3.  (a) T a r g e t (b) D e t e c t o r and E l e c t r o n i c s (b . Experimental (a) " ^ i and  k.  ^?  (b) Energy D e t e r m i n a t i o n (c) G a i n S h i f t s Results  ofYo  30 30 36  hi ^3  CHAPTER IV  PAGE A LOOK FOR 0  1 7  (p,^)F  l 8  1.  Introduction  2.  E x p e r i m e n t a l Procedure (a) (b) (c) (d) (e)  3.  Mf  GAMMA RAYS  M+ ..  3  Gamma-Ray D e t e c t i o n System h$ Targets *fo The 8 7 2 Kev. R a d i a t i o n . . . . . . . ^6 Contamination Spectra Measurement o f A n n i h i l a t i o n R a d i a t i o n ..... *+9 50  Conclusions  53  APPENDIX - M e r c u r y R e l a y P u l s e Generator BIBLIOGRAPHY  h*  ...  60  LIST OF ILLUSTRATIONS  FIGURE NUMBER  FACING PAGE  SUBJECT.  1.  F  2.  T a r g e t Chamber Arrangement ..................  3.  D 0 I c e Target Thickness C a l i b r a t i o n  13  D 0 Dispenser C a l i b r a t i o n  1^  5.  P h o t o m u l t i p l i e r Head A m p l i f i e r  10  6.  S p e c t r a From I c e T a r g e t s  7.  E f f e c t i v e Centre D e t e r m i n a t i o n  19  8.  0 ^(p,^i)Fl  31  1 7  L e v e l Scheme  ...  *f  2  2  9.  1  7  Gamma Ray E n e r g i e s  E n e r g y D e t e r m i n a t i o n .....................  10.  Mercury P u l s e G e n e r a t o r C i r c u i t s  11.  M u l t i v i b r a t o r D r i v e C i r c u i t and D.C. Circuit  12.  •••••  P u l s e Shapeing  6  18  37 53  Supply 56 •••  57  CHAPTER I INTRODUCTION  S h e l l model predictions indicate that the F^? nucleus can be considered as a single proton moving i n the p o t e n t i a l of the doubly closed s h e l l O ^ core. 1  This system i s therefore a  simple one t o consider t h e o r e t i c a l l y and should-lend i t s e l f to a quantitative test of s h e l l model predictions.  Comparison with  i t s mirror nucleus 017 should indicate a one to one correspondence between l e v e l energies (corrected f o r coulomb and neutronproton mass difference e f f e c t s ) and spin and p a r i t y values. The s h e l l model predicts that the ground state of both 0^7 and p i ? should be a D5/2 l e v e l consisting of the odd nucleon +  with i t s spin aligned p a r a l l e l to the o r b i t a l angular momentum. Experimental r e s u l t s are consistent .with D^/ + f o r both ground 2  states.  The  ground state spin has been measured d i r e c t l y hy  Alder and Yu (51)  and the assumption that the p i ? ground state  spin i s 5/2 i s consistent with the allowedfi and with O ^ ^ n )  +  t r a n s i t i o n to 17  stripping data (Ajzenburg, 51).  0  The f i r s t ex-  cited state of the mass 17 system appears not t o be the D3/2 member of the ground state doublet but to be an S1/2+ l e v e l of the odd nucleon. predictions.  This i s consistent with present s h e l l model  The experimental evidence comes from stripping an-  gular d i s t r i b u t i o n s of O ^ d . p ) © ? (Burrows et a l , 50) and 1  0 6(d,n)F 7 (Ajzenburg, 5D• 1  1  A d d i t i o n a l evidence f o r O ? spins 1  i s a v a i l a b l e from a n g u l a r c o r r e l a t i o n between protons s t a t e ( T h i r i o n , 53)  from t h e f i r s t e x c i t e d  and^-rays  and from i n t e r n a l c o n E-2  v e r s i o n measurements on t h e same"^ - r a y s w h i c h i n d i c a t e an 52).  t r a n s i t i o n (Thomas and L a u r i t s e n ,  T h i s c o n s i s t e n c y between the s h e l l model and e x p e r i m e n t a l evidence f o r l e v e l parameters and the f i r s t e x c i t e d s t a t e o f F ^ for  the 0 ^(p," b)F- 1  >  1  7  f i g u r a t i o n f o r F^ ,  7  predictions  o f ground  suggests t h a t a c a l c u l a t i o n  r e a c t i o n parameters  assuming t h i s s i m p l e  should g i v e reasonable r e s u l t s .  7  state  con-  Theoretical  c a l c u l a t i o n s f o r t h e c r o s s s e c t i o n and a n g u l a r d i s t r i b u t i o n o f t h i s r e a c t i o n a r e b e i n g made by a C a l i f o r n i a I n s t i t u t e of Techn o l o g y group ( p r i v a t e communication from N. T a n n e r ) . r a d i a t i v e c a p t u r e p r o c e s s i s b e i n g assumed, c o n s i s t e n t r e s u l t s of Warren e t a l (5*+) was non-resonant  A direct w i t h the  t h a t the " ^ - r a d i a t i o n from'O^Cp,"^)  i n the r e g i o n E  p  0.9  t o 2.1  Mev.  Since a  t a i l e d comparison between the t h e o r e t i c a l c a l c u l a t i o n s and  deaccurate  e x p e r i m e n t a l r e s u l t s s h o u l d p r o v i d e a s e n s i t i v e check on t h e assumed models i t was f e l t t h a t more a c c u r a t e d a t a on the c r o s s s e c t i o n and a n g u l a r d i s t r i b u t i o n of the gamma r a d i a t i o n would be of v a l u e a t t h i s t i m e . The 0 l 6 ( ^ ) 1 7 c r o s s s e c t i o n i s a l s o o f i n t e r e s t i n p  astrophysics.  F  I n hydrogen s t a r s w i t h c e n t r a l temperatures  t h a n about f i f t e e n m i l l i o n degrees K e l v i n , t h e main energy  greater source  i s t h e c o n v e r s i o n of p r o t o n s i n t o a l p h a p a r t i c l e s by means o f t h e  carbon-nitrogen c y c l e .  I n t h i s process v a r i o u s i s o t o p e s o f c a r -  bon and n i t r o g e n a c t as c a t a l y s t s i n t h e c o n v e r s i o n , as f o l l o w s :  C 3(p,-t)N .> N '(p,iS)0 5—^N ? 1  11+  v  ^ I  1 t r  1 4  1  1  W—s-He* + 2(r + Q where Q ^ 27 Mev. 1  J  Nl5(p,60cl2  Some o f t h e c a t a l y s t i s l o s t f r o m t h e c y c l e b y t h e r e a c t i o n However t h e r e a c t i o n0 l 6 ( p , i ) F l 7 , followed by  Fl7(^3 )ol7, 0 7(p,<A)N r  1  The 0 l 6 ( p , ^ ) F l 7  1,f  puts oxygen n u c l e i back i n t o t h e c y c l e .  cross s e c t i o n determine^,after a long period,  the c a r b o n oxygen r a t i o i n t h e s t a r a s a f u n c t i o n o f t h e s t e l l a r temperature  (Cameron, 57)•  S i n c e c r o s s s e c t i o n measurements a t  s t e l l a r e n e r g i e s a r e n o t a c c e s s i b l e t o l a b o r a t o r y measurement i n t h i s case, t h e o r e t i c a l estimates extrapolated from experimental c r o s s s e c t i o n s a t h i g h e r e n e r g i e s must be u s e d .  I n order t o make  t h i s e x t r a p o l a t i o n w i t h any r e a s o n a b l e a c c u r a c y i t i s n e c e s s a r y t h a t t h e energy dependence a s w e l l as t h e a b s o l u t e t h e o r e t i c a l c r o s s s e c t i o n be checked e x p e r i m e n t a l l y i n t h e h i g h energy r e g i o n . P r e v i o u s l y t h e o r e t i c a l c a l c u l a t i o n s b y S a l p e t e r (55) and i n t e r p r e t a t i o n o f e x p e r i m e n t a l r e s u l t s b y L a u b e n s t e i n and L a u b e n s t e i n (5D assumed t h a t t h e r e a c t i o n proceded b y compound n u c l e u s  for-  m a t i o n through t h e t a i l s o f r e s o n a n c e s above h Mev. and a t 0.5 Mev.  i n Fl7.  However, t h e r e s u l t s of Warren e t a l (5%) f o r t h e  0l°(pp£) r e a c t i o n i n d i c a t e t h a t t h e r e a c t i o n proceeds by d i r e c t r a d i a t i v e c a p t u r e o f t h e p r o t o n s , s i m i l a r t o t h e D(p,tf)He3 reaction. :  (3A)  4.73  ft///////////////////////////*  4.35  Y//////W//////w,wm llllllllllllllllh  3.56  (>/*-)  3.10  . 5 99 •+ 16 & p 17 '  .599  1/2 +  .499  0 +p , 6  J = 5/2+  £'  7  I n t h i s case t h e energy dependence o f t h e r e a c t i o n w i l l  differ  f r o m t h a t p r e d i c t e d by t h e compound n u c l e u s a s s u m p t i o n . The 0l (p,^)pl7 r e a c t i o n was f i r s t observed b y measur6  i n g t h e a n n i h i l a t i o n quanta f r o m t h e decay o f F a l , 38)•  1 7  (DuBridge e t  L a u b e n s t e i n e t a l (5D measured t h e r e l a t i v e c r o s s  s e c t i o n as a f u n c t i o n o f energy f r o m 1.*+ t o * f . l Mev. b y measuring the y i e l d of a n n i h i l a t i o n r a d i a t i o n .  The r e l a t i v e e l a s t i c  s c a t t e r i n g c r o s s s e c t i o n o f p r o t o n s was a l s o measured f r o m 0.6 t o h.5 Mev.  Warren e t a l (5 *-), i n t h i s l a b o r a t o r y , f i r s t ob1  served t h e gamma r a d i a t i o n d i r e c t l y and a t t r i b u t e d t h e n o n - r e s o nant c h a r a c t e r  o f t h e c r o s s s e c t i o n f o r gamma e m i s s i o n i n t h e  r e g i o n 0.8 t o 2.1 Mev. t o a d i r e c t r a d i a t i v e c a p t u r e p r o c e s s and not t o t h e e f f e c t o f a broad l o w l e v e l nor t h e r e s o n a n t l e v e l s o f The 0l6( ,^) 17 c r o s s s e c t i o n was measured a t  h i g h e r energy.  p  F  1.90 Mev. and found t o be 6 ± 3 x 10"30 2. The l o , , energy l e v e l s Gm  and s p i n s of F- - o b t a i n e d f r o m p r e v i o u s work a r e shown i n F i g u r e 1  1.  7  I n t h e work o f Warren e t a l (5k), t h e f o l l o w i n g gamma r a y s  were observed i n a d d i t i o n t o t h e a n n i h i l a t i o n r a d i a t i o n f r o m t h e F  1 7  p o s i t r o n decay: A ground s t a t e t r a n s i t i o n , ' ' ^ , o f energy  16/17 Ep + Q; a t r a n s i t i o n t o a n e x c i t e d s t a t e a t about 0.5 Mev., ~^2; and t h e t r a n s i t i o n f r o m t h i s e x c i t e d s t a t e t o t h e ground s t a t e o f F ,H^2. 17  The observed Q v a l u e was c o n s i s t e n t w i t h  Mev. deduced f r o m t h e 0 ^ ( d , n ) F 7 t h r e s h o l d 1  measurements o f  B u t l e r (5D a t a d e u t e r o n energy o f I.63I Mev. corrected  t o I.626 Mev.  0.59^  ( T h i s has been  (Bonner, 55) thus p u t t i n g t h e Q a t  - 5 -  0.599 - .006 Mev.). Because o f t h e need f o r a more a c c u r a t e d e t e r m i n a t i o n of t h e a b s o l u t e c r o s s s e c t i o n t o check t h e t h e o r e t i c a l c a l c u l a t i o n s b e i n g performed  b y t h e C a l i f o r n i a I n s t i t u t e o f Technology  group, t h e d i f f e r e n t i a l c r o s s s e c t i o n has been measured a t 800 kev. i n t h e p r e s e n t work.  The gamma r a y e n e r g i e s have a l s o been  remeasured i n o r d e r t o r e s o l v e some o f t h e i n c o n s i s t e n c y i n t h e literature. Warren e t a l (5*0 observed t h a t gamma r a d i a t i o n o f 873 kev. appeared f o r e n e r g i e s g r e a t e r t h a n 1.8 Mev. d u r i n g p r o t o n bombardment o f oxygen t a r g e t s .  The r a d i a t i o n was a t t r i b u t e d t o  the i n e l a s t i c p r o t o n s c a t t e r i n g from t h e 873 l e v e l i n 0  1  7  .  D u r i n g t h e p r e s e n t work s e p a r a t e d t a r g e t s o f Q16 and 0 ^ were bom7  barded w i t h p r o t o n s t o attempt t o c o n f i r m t h i s  assignment.  CONNECTED  TARC-ET CHAMBER  "  / f G-LAS5W0OL  H0 DISPEM5G-R 2  MANOMETER  F I G U R E  2  T A - R G E T  C U A M B l t R  M ^ P M v l & E - M t N T  - 6  -  CHAPTER I I 0 (p.~o)F l 6  The  1 7  CROSS SECTION MEASUREMENTS  aim of the p r e s e n t work has been t o measure the  c r o s s s e c t i o n of t h e O ^ C p ^ F r a t i o f o r the two  1 7  r e a c t i o n and  the  gamma r a y s f r o m the c a p t u r i n g  a c c u r a c y of about ±10  per c e n t a t 800 kev.  800 kev. was  state to  an  p r o t o n bombarding  e n e r g y , u s i n g an i c e t a r g e t whose t h i c k n e s s reasonably accurately.  branching  c o u l d be measured  chosen as the bombarding  energy i n o r d e r t o m i n i m i z e the e f f e c t of c a r b o n c o n t a m i n a t i o n and  s t i l l be a b l e t o r e s o l v e d ) 2 f r o m t h e 0.51  Mev.  annihilation  radiation. 1.  Apparatus. (a)  T a r g e t Arrangement. The  t a r g e t was  copper b a c k i n g 1/16  d i s t i l l e d w a t e r f r o z e n onto a g o l d  i n . t h i c k w h i c h was  of a l i q u i d n i t r o g e n c o l d t r a p . arrangement a r e shown i n F i g u r e  The 2.  attached  plated  t o the bottom  t a r g e t chamber and  stop  Since f l u o r i n e targets  had  been used i n t h e chamber f o r t h e t a r g e t t h i c k n e s s c a l i b r a t i o n s , the w a l l s of t h e chamber i n t h e r e g i o n of t h e t a r g e t were g o l d plated with a thickness  s u f f i c i e n t t o s t o p 10 Mev.  protons.  s t o p s u s e d , t o ensure a c c u r a t e p o s i t i o n i n g of the beam on  The  the  t a r g e t , were g o l d p l a t e d copper t o reduce coulomb e x c i t e d gamma r a y s produced by o t h e r low atomic number m a t e r i a l s y  McGowan, 55)•  The  t a r g e t and  (Stelson  s u p p o r t were i n s u l a t e d w i t h a  and lucite  - 7 -  r i n g t o enable beam c u r r e n t measurements t o be made; a p o s i t i v e p o t e n t i a l o f 135 v o l t s was a p p l i e d t o t h e t a r g e t system t o r e duce secondary e l e c t r o n e r r o r i n t h e beam measurement.  The beam  c u r r e n t was i n t e g r a t e d w i t h a c u r r e n t i n t e g r a t o r (Edwards,  50).  T h i s i n t e g r a t o r i s e s s e n t i a l l y a m i l l e r I n t e g r a t o r c i r c u i t whose v o l t a g e o u t p u t i s a f u n c t i o n o f t h e charge f e d onto t h e i n t e g r a t i n g condensers.  The o u t p u t o f t h e m i l l e r i n t e g r a t o r o p e r a t e s  a sehmidt t r i g g e r c i r c u i t ; t h e l e v e l a t w h i c h t h i s t r i g g e r s c o r responds t o a c e r t a i n amount o f charge on t h e i n t e g r a t i n g c o n densers.  Each time t h i s charge i s reached i t o p e r a t e s a r e l a y  which discharges  t h e condensers and d r i v e s a m e c h a n i c a l r e g i s t e r .  Two s e t s o f i n t e g r a t i n g condensers i n p a r a l l e l p e r m i t d i s c h a r g i n g of one s e t w h i l e m a i n t a i n i n g tive.  the i n t e g r a t i o n c i r c u i t  Thus no c o r r e c t i o n i s r e q u i r e d f o r t h e r e c o v e r y  the c i r c u i t .  opera-  time o f  The i n t e g r a t o r was c a l i b r a t e d by f e e d i n g i t w i t h a  known c u r r e n t s u p p l i e d from a 100 v o l t b a t t e r y .  The c u r r e n t was  d e t e r m i n e d by measuring t h e v o l t a g e a c r o s s a s t a n d a r d w i t h a Rubicon potentiometer.  resistance  The 100 v o l t source was r e q u i r e d  t o ensure t h a t t h e v a r i a t i o n i n c u r r e n t , due t o t h e 0.5 v o l t swing on t h e i n p u t o f t h e i n t e g r a t o r over each i n t e g r a t i n g c y c l e , was s m a l l .  T h i s measurement i n d i c a t e d t h a t over a one year  the i n t e g r a t o r c a l i b r a t i o n was c o n s t a n t (b)  period  t o w i t h i n one per c e n t .  Gamma Ray D e t e c t o r . A. c y l i n d r i c a l sodium i o d i d e , t h a l l i u m a c t i v a t e d (Harshaw)  c r y s t a l , 2.5  i n . d i a m e t e r by 3*5 i n . l o n g , mounted on a Dumont  - 8 -  6363 p h o t o m u l t i p l i e r was used a s a gamma r a y d e t e c t o r .  Dow C o r n i n g  200,000 c e n t i s t o k e s s i l i c o n e o i l was used a s a n o p t i c a l c o u p l i n g between t h e c r y s t a l and t h e p h o t o m u l t i p l i e r ; a v u l c a n i z e d r u b b e r s l e e v e h e l d t h e p h o t o m u l t i p l i e r and c r y s t a l t o g e t h e r and p r e vented t h e s i l i c o n e o i l f r o m l e a k i n g .  No n o t i c e a b l e r e d u c t i o n  i n r e s o l u t i o n was n o t i c e d over a p e r i o d o f one and a h a l f y e a r s . P i t t e d w i t h a mu-metal s h i e l d , t h e p h o t o m u l t i p l i e r w i t h t h e c r y s t a l was mounted i n a 3*5 i n . b r a s s tube w h i c h a l s o housed t h e preamplifier• The e f f i c i e n c y o f t h e l a r g e c r y s t a l f o r gamma r a y s has been measured a t 6.1k Mev. and a t 1.17 and 1.33 Mev. The 6 Mev. measurement was c a r r i e d o u t u s i n g t h e Fl9( o(^>)ol6reaction a t P)  energy 3+0 k e v . ( L a r s o n , 57). The method was f i r s t suggested by 1  Van A l l e n and Smith (^1). The apparatus used i n t h e p r e s e n t measurements was developed  by G.M. G r i f f i t h s a t t h e C a v e n d i s h  L a b o r a t o r y , Cambridge and t h e measurements were made by E . L a r s o n and t h e author and a r e more f u l l y r e p o r t e d b y L a r s o n (57). A thin  t a r g e t was bombarded w i t h 3^0 k e v . p r o t o n s and t h e <X  p a r t i c l e s were counted  i n a t h i n window p r o p o r t i o n a l c o u n t e r w i t h  an a c c u r a t e l y known s o l i d a n g l e subtended a t t h e t a r g e t . b o t h t h e a l p h a p a r t i c l e s and t h e ^ - r a y s a r e e m i t t e d and t h e r e i s one 6,1k Mev. gamma r a y f o r each a l p h a  Since  isotropically particle  c o u n t e d , a measurement o f t h e number o f a l p h a p a r t i c l e s g i v e s a measure o f t h e number o f 6.1^ Mev gamma r a y s s i m u l t a n e o u s l y e m i t t e d b y t h e t a r g e t . A c o r r e c t i o n must be made f o r t h e 2.3 per c e n t y i e l d o f 6.9 and 7.1 Mev. gamma r a y s f o r w h i c h t h e c o r -  - 9 -  responding  a l p h a p a r t i c l e s a r e n o t s u f f i c i e n t l y e n e r g e t i c t o count  i n the a l p h a c o u n t e r  (Dosso, 57)• P. S i n g h and H. Dosso (57)  i n t h i s l a b o r a t o r y have measured t h e e f f i c i e n c y f o r Co^° gamma r a y s u s i n g a source c a l i b r a t e d of Canada.  b y the N a t i o n a l R e s e a r c h  Council  P. S i n g h has e s t i m a t e d t h e o r e t i c a l l y the e f f i c i e n c y  a t t h i s energy and agreement i s w i t h i n e x p e r i m e n t a l e r r o r s p l a c e d on t h e Co^O s o u r c e .  The agreement between t h e o r e t i c a l  calcula-  t i o n s and measured e f f i c i e n c y f o r 6 Mev. gamma r a y s i s w i t h i n 5 per c e n t .  E x t e n d i n g t h e e s t i m a t e s t o lower e n e r g i e s i s c o n s i d e r e d  r e a s o n a b l e because o f t h e good agreement i n t h e s e upper r a n g e s . A more e x a c t e f f i c i e n c y v a l u e i n t h i s l o w r e g i o n i s expected f r o m pending r e s u l t s i n C o ^ c o i n c i d e n c e work. For 800 kev. p r o t o n bombardment of oxygen, t h e O ^ ( p ^ ) F  1 7  gamma r a y s , " ^ ^ and""^* have e n e r g i e s o f 1.32 and 0 . 8 1 Mev.  respectively.  The e f f i c i e n c y f o r gamma r a y s ,  £(E, E ) , d e f i n e d b  as the r a t i o o f t h e number o f counts o f a gamma r a y of e n e r g y , E, above a g i v e n b i a s e n e r g y , E^, t o t h e number o f gamma r a y s i n c i d e n t on t h e c r y s t a l , has been o b t a i n e d f o r these gamma r a y s by c o n s i d e r i n g t h e t h e o r e t i c a l a b s o r p t i o n c o e f f i c i e n t s and t h e d e t a i l e d shape o f t h e e x p e r i m e n t a l gamma r a y s p e c t r a .  Since the  shapes o f t h e ^ i and "^2 s p e c t r a were n o t a c c u r a t e l y ob-  detailed  served i n the experiment  due t o t h e presence  o f s e v e r a l gamma  r a y s , a d d i t i o n a l comparisons were made w i t h the s p e c t r a f r o m N a and C s ^ . 1  7  The f o l l o w i n g  r a d i a t i o n s were o b t a i n e d ;  r e s u l t s relevent t o the 0  1  ^(p7^)F  17  f o r ~ 6 \ the e f f i c i e n c y i s €.,(1.32, 1 . 0 )  2 2  TEST IN  + MI.  300 V  -I M i 470 /3KV  WW  I00K  IM 470 / 3 K Y  6K  1000 I0O  • 330K  I  I0O / 3 K V  •HH-e  6J6  '470K  COUECTOR  TRIODES PARALLELED  ^ 330K 470 / 3 K V <  •iHhj-4  T  > 330K 470 /3KV ?  DUMONT  •'Hr4-€ >  i  5K  y  IOOO  IN 34  6 5 6 3  330 K  33 n  I.5K  DYNODfrS  9"' i) o  -ll 650^  OUT PUT  1^  L  270J\  330K  P0CU55ING 11  <  CRID (ALL  ~T" PUOTOCATWODE  ON 2.5 x 3.5 CRYSTAL  CAPACITIES IN jjuf  UNLESS SPECIFIED)  4- 250 V TEST IK) 9  _0  OUT PUT  PULSE TRANSFORMER. PT-3300 150 K  R&fc5  I  D£LkY LINE-  Ro = IK  RCA 6342  (  COLLECTOR  6A.K5  IOO / 3 K V  IOOK OYNODE  ON SMALL  SUPPLY  CRYSTAL  DETECTOR  A5 ft-BOVE  tIGUIX  ,25 uf  5  PUOTOMULTIPLICR  Ut/VD UMPLICIkR  10 = (38 * 5) per cent and f o r ~ ^  t  h  e f f i c i e n c y i s £ 2 (0.8l, 0.65)  e  2  = (31 * *+) per c e n t . A 1.75  i n . by 2 i n . sodium i o d i d e t h a l l i u m  activated  Harshaw c r y s t a l was a l s o a v a i l a b l e , b u t t h e l a r g e r c r y s t a l was used because i t s e f f i c i e n c y was more a c c u r a t e l y  known.  Also,  being l a r g e r i n a r e a , f o r a g i v e n s o l i d angle the l a r g e r count e r c o u l d be p l a c e d f u r t h e r f r o m t h e t a r g e t to the target t o c r y s t a l distance  so t h a t e r r o r s due  measurements and i n t h e po-  s i t i o n i n g o f t h e beam on t h e t a r g e t were r e l a t i v e l y s m a l l e r . Room r a d i a t i o n p r e s e n t e d about t h e same s h i e l d i n g problem t o b o t h crystals. (c)  Electronics. The  was s u p p l i e d supply.  1000 v o l t s f o r t h e p h o t o m u l t i p l i e r by an I s o t o p e s Development L i m i t e d  dynode c h a i n s t a b i l i z e d power  A 6J6 cathode f o l l o w e r was used as a p r e a m p l i f i e r  with  a d i o d e c l i p p e r t o l i m i t t h e s i z e o f l a r g e cosmic r a y p u l s e s . (Figure  5).  The n e g a t i v e p u l s e s f r o m t h e p r e a m p l i f i e r  were f e d  t o a N o r t h e r n E l e c t r i c wide band a m p l i f i e r Type l W f t h e n t o a biased a m p l i f i e r .  The output f r o m t h e b i a s e d a m p l i f i e r was f e d  t o a M a r c o n i 30 c h a n n e l k i c k s o r t e r .  The k i c k s o r t e r c h a n n e l edges  were s e t up by f e e d i n g p u l s e s f r o m t h e mercury p u l s e g e n e r a t o r (Appendix I I ) onto t h e g r i d o f t h e cathode f o l l o w e r .  Then any  n o n - l i n e a r i t y i n t h e components o t h e r t h a n t h e p h o t o - m u l t i p l i e r were compensated f o r i n s e t t i n g up t h e k i c k s o r t e r w i t h t h e a c c u r a t e l y l i n e a r pulse generator.  The p u l s e s f e d i n t o t h e head  a m p l i f i e r a l s o checked t h e s t a b i l i t y of t h e e l e c t r o n i c s .  The  11  -  e l e c t r o n i c s w i t h t h e e x c e p t i o n of the p h o t o m u l t i p l i e r proved t o be a d e q u a t e l y s t a b l e . 2.  Target Thickness  Measurements.  I n o r d e r t o measure t h e a b s o l u t e c r o s s s e c t i o n t o a n a c c u r a c y of about 10 per c e n t , the number o f t a r g e t n u c l e i per square c e n t i m e t e r must be known t o an a c c u r a c y b e t t e r t h a n t h i s . To produce such a t a r g e t w i t h a known number of oxygen atoms per square c e n t i m e t e r a system f o r p r o d u c i n g i c e l a y e r s of r e p r o duceable t h i c k n e s s was c o n s t r u c t e d .  A f i x e d volume of water  vapour whose p r e s s u r e was measured by an o i l manometer was densed onto a c o o l e d copper s u p p o r t a t t a c h e d t o a l i q u i d gen r e s e r v o i r .  con-  nitro-  The t h i c k n e s s of the t a r g e t as a f u n c t i o n o f  p r e s s u r e of water vapour i n t h e d i s p e n s e r was determined i n terms o f t h e energy l o s s o f 3 ^ 0 k e v . p r o t o n s i n p a s s i n g t h r o u g h t h e ice layer.  T h i s was found by measuring the c e n t r o i d s h i f t o f  the 3 ^ 0 k e v . Fl9(p.of^S) resonance when t h e i c e l a y e r was densed on a t h i n f l u o r i n e t a r g e t .  con-  The d i s p e n s i n g system was  cal-  i b r a t e d f o r heavy w a t e r , however the c a l i b r a t i o n a p p l i e s e q u a l l y w e l l t o o r d i n a r y water s i n c e t h e number of m o l e c u l e s of gas c o n t a i n e d i n t h e d i s p e n s e r f o r a g i v e n p r e s s u r e i s t h e same f o r b o t h gasses.  N e g l i g i b l e d i f f e r e n c e s c o u l d have r e s u l t e d f r o m t h e  d i f f e r e n c e i n t h e heat of f u s i o n and the Van d e r W a a l s c o n s t a n t 1  of t h e two g a s e s .  U s i n g the v a l u e s of the m o l e c u l a r s t o p p i n g  c r o s s s e c t i o n f o r heavy water measured e x p e r i m e n t a l l y by Wenzel and Whaling ( 5 2 ) the number of i c e m o l e c u l e s per square c e n t i m e t e r  - 12  -  as a f u n c t i o n of manometer p r e s s u r e can be d e t e r m i n e d  from meas-  urements of t h e energy l o s s o f p r o t o n s i n p a s s i n g t h r o u g h t h e i c e l a y e r f o r t a r g e t s c o r r e s p o n d i n g t o s e v e r a l d i f f e r e n t manometer r e a d i n g s . The d i s p e n s e r , F i g u r e 2 , was made o f g l a s s w i t h g l a s s t a p s ground t o g i v e a vacuum t i g h t f i t . G c t o i l Vacuum Pump G i l , (vapour p r e s s u r e 10"? mm. a t 20°C, d e n s i t y 0 . 9 gm. per c c . ) heated under vacuum t o remove h i g h vapour p r e s s u r e was used i n t h e manometer.  The f l u o r i n e t a r g e t s were prepared  by e v a p o r a t i n g c a l c i u m f l u o r i d e onto c l e a n e d copper ( l a r s o n , 57);  impurities,  sheets  t h e s e were i n d i u m s o l d e r e d onto a 1/16  i n . thick  copper s u p p o r t a t t a c h e d t o t h e l i q u i d n i t r o g e n r e s e r v o i r . I n o r d e r t o make a r e p r o d u c e a b l e t a r g e t t h e f o l l o w i n g procedure was adopted.  The manometer was evacuated  arm p r e s s u r e w i t h t a p s T i and T and T  2  2  open.  t o the side  See F i g u r e 2 .  With T i  e l o s e d , T3 was opened u n t i l t h e d e s i r e d p r e s s u r e o f water  vapour was r e a c h e d and T3 was c l o s e d .  When e q u i l i b r i u m was e s -  t a b l i s h e d i n t h e manometer, t h e temperature manometer r e a d i n g were r e c o r d e d . t a t e d so t h a t t h e c o o l e d copper dispenser.  o f t h e a i r and t h e  The t a r g e t assembly was r o plate faced the o u t l e t of the  Tap T i was opened c a r e f u l l y t o l e a k t h e water s l o w l y  i n t o t h e t a r g e t chamber and t h e manometer r e a d a g a i n when t h e water had been e x p e l l e d from t h e d i s p e n s e r .  T^ was t h e n c l o s e d .  The s i d e arm p r e s s u r e measured a t t h e magnet box was a p p r o x i m a t e l y 10"^ mm.  of mercury.  I f the pressure rose higher than  Protons on Fluorine  Protons on Fluorine  Proton  Energy  KEV. F i g . 3 - L\0 Ice T a r g e t  Thickness  Calibration  - 13 -  10**5 mm. due t o l e a k s , the r e p r o d u c e a b i l i t y i n t a r g e t t h i c k n e s s was  c o n s i d e r a b l y reduced and the t h i c k n e s s measured f o r manometer  r e a d i n g s o f 10 - 20 cm. o f o i l was l o w e r . of water a t 20°C corresponded  The vapour p r e s s u r e  t o a manometer r e a d i n g o f 2h cm.  By u s i n g p r e s s u r e s o f water l e s s t h a n the vapour p r e s s u r e , e r r o r s due t o c o n d e n s a t i o n on the w a l l s o f t h e d i s p e n s e r were The  reduced.  l i n e a r i t y and a b s o l u t e c a l i b r a t i o n o f the Van de  G r a a f f energy s c a l e measured b y the g e n e r a t i n g v o l t m e t e r were determined  b y n o t i n g the p o s i t i o n s o f the 0.22^, 0.3J+0, 0.*+80 and  the 0.669 Mev. resonances  o f F 9(p,^,~ ^)o ^. 1  1  1  F i g u r e 3 shows the c e n t r o i d s h i f t o f the 3*+0 kev. r e s o nance. cm.  An i c e l a y e r c o r r e s p o n d i n g t o a water p r e s s u r e o f 13.6  o f o i l was l a y e d down on t h e f l u o r i n e t a r g e t .  The i n c r e a s e d  w i d t h o f the resonance f o r t h e t a r g e t w i t h t h e i c e l a y e r i s due t o the n o n - u n i f o r m i t y o f the t a r g e t w h i c h amounts t o about 10 kev.  P r o t o n beams o f 0.5 microamperes were used d u r i n g these  c a l i b r a t i o n runs.  C u r r e n t s h i g h e r t h a n 1 microampere caused  d e t e r i o r a t i o n o f the t a r g e t s w h i c h appeared as a broadening o f the resonance c u r v e .  Since the p o s i t i o n of the c e n t r o i d d i d not  s h i f t w i t h bombardment and the a r e a under t h e resonance c u r v e r e mained c o n s t a n t t h e broadening was a t t r i b u t e d t o a decrease i n the u n i f o r m i t y o f the t a r g e t and not t o e i t h e r a r e d u c t i o n i n t h e mean heavy i c e t h i c k n e s s or a d e t e r i o r a t i o n o f the f l u o r i d e get.  tar-  (20  -I  M/VNOME-TtR  P  O  R  E  4  0 , 0  at\DIN&  D I 5 P E N S & R  P IN  CM  C A - L I P R A T I O W  -11+  -  The r e s u l t s of t h e measured energy l o s s o f p r o t o n s i n the i c e t a r g e t s as a f u n c t i o n o f the manometer r e a d i n g , P,  are  shown i n F i g u r e *+.  To determine  f r o m t h i s d a t a the number o f  heavy i c e m o l e c u l e s  per square c e n t i m e t e r on the t a r g e t a know-  ledge o f the m o l e c u l a r s t o p p i n g c r o s s s e c t i o n a t these i s required.  Wenzel and W h a l i n g (52)  energies  have measured the molec-  u l a r s t o p p i n g power,^0^0, o f heavy water as a f u n c t i o n of e n e r g y . They i n d i c a t e t h a t t h e i r r e s u l t s agree t o w i t h i n e x p e r i m e n t a l e r r o r w i t h the t h e o r e t i c a l v a l u e s c a l c u l a t e d by H i r s c h f e l d e r and Magee (*f8)  u s i n g the s e m i - e m p i r i c a l t h e o r y o f Bethe (37)  s t o p p i n g power.  W h a l i n g (57)  f o r the  has c o l l e c t e d d a t a f o r hydrogen  and oxygen and f o r water w h i c h g i v e s l i g h l y h i g h e r v a l u e s  than  the e x p e r i m e n t a l measurements on heavy water f o r the p r o t o n ergy range 300 t o 500 kev.  en-  I n t h i s work t h e measured s t o p p i n g  c r o s s s e c t i o n s o f Wenzel and W h a l i n g were used f o r the c a l c u l a t i o n of the number of m o l e c u l e s  per square c e n t i m e t e r .  The m o l e c u l a r s t o p p i n g c r o s s s e c t i o n as a f u n c t i o n o f p r o t o n e n e r g y , E, i n the r e g i o n f r o m 3*fO k e v . t o *f60 kev. be approximated  b y C ( E ) = A + B(E) where E ^ E  E' i s the bombarding energy.  1  T h i s f u n c t i o n was  - 3*K) k e v .  can and  f i t t e d t o the  e x p e r i m e n t a l c u r v e g i v e n i n Wenzel and W h a l i n g (51)  a t 36O and  ^30 kev., w i t h t h e r e s u l t t h a t CT Now  ( l l f . 9 - 0.0238 E) l O " ^ ev.-cm./ m o l e c u l e 1  of  D 0. 2  the m o l e c u l a r s t o p p i n g c r o s s s e c t i o n i s d e f i n e d as - 1 N  dE dx  - 15 -  where N i s t h e number o f m o l e c u l e s I dx —  ~\>j <JE  per c u b i c c e n t i m e t e r .  Then  where E i s t h e t o t a l energy l o s s f o r  p r o t o n s p a s s i n g t h r o u g h a d i s t a n c e x o f heavy w a t e r and b i s a constant.  Assuming t h a t t h e t h i c k n e s s o f i c e i s p r o p o r t i o n a l  t o t h e d i s p e n s e r manometer r e a d i n g , t h e n where c i s a p r o p o r t i o n a l i t y c o n s t a n t .  P —  <=: J  ~  Substituting for C  p = j - f aE E  CB  A '  T h i s f u n c t i o n i s p l o t t e d i n F i g u r e h f o r t h e case o f CB = - 0 . 0 1 0 7 , chosen t o f i t t h e e x p e r i m e n t a l p o i n t s .  From t h i s  curve  one c a n determine t h e water t a r g e t t h i c k n e s s i n k e v . f o r p r o t o n s i n t h e energy r e g i o n j u s t above 3M) k e v . W i t h t h i s and t h e m o l e c u l a r s t o p p i n g c r o s s s e c t i o n f o r heavy water (Wenzel and W h a l i n g , 52) t h e number o f m o l e c u l e s was  per c u b i c c e n t i m e t e r ,  obtained.  HD 0 9  .  =  T x 103 XT  where T i s t h e t a r g e t t h i c k n e s s i n k e v , O " i s t h e m o l e c u l a r s t o p p i n g power f o r Ep = 3*+0 + T/2 i n e v . - c m / m o l e c u l e . 2  nD 0, 2  - 16 -  Ice  t a r g e t s f o r t h e oxygen c r o s s s e c t i o n measurements  were made u s i n g d i s t i l l e d w a t e r f r o m t h e U.B.C. C h e m i s t r y Department . 3* E x p e r i m e n t a l . (a)  Background. A c c u r a t e measurement o f gamma y i e l d s become i n c r e a s -  i n g l y d i f f i c u l t t h e lower t h e gamma energy because t h e background r i s e s q u i c k l y w i t h d e c r e a s i n g energy below about 2 Mev. The background i n t h i s r e g i o n i s due t o secondary cosmic r a d i a t i o n , r a d i a t i o n from n a t u r a l r a d i o a c t i v e s a l t s i n t h e c o n c r e t e ( K ° l,h Mev., RdTh 2 . 6 2 Mev.), r e a c t i o n s due t o contaminants i n 4  the  t a r g e t , and machine X - r a y background.  Since these a r e r e l -  a t i v e l y low energy r a d i a t i o n s , a p p r o x i m a t e l y h i n . o f l e a d i s s u f f i c i e n t t o reduce t h i s background t o a r e a s o n a b l e l e v e l . When r u n n i n g a t 800 k e v . machine e n e r g y , however, t h e c o u n t e r must be s h i e l d e d on a l l s i d e s t o reduce r a d i a t i o n f r o m t h e room. T h e r e f o r e , h i n . o f l e a d was p l a c e d around t h e c o u n t e r and t h e t a r g e t chamber s o t h a t none of t h e room was v i s i b l e t o t h e counter.  W i t h t h e arrangement used t h e time dependent c o u n t i n g r a t e  was 60 eounts p e r minute i n t h e energy range 0.5 t o 3 Mev. Lead and p a r a f f i n b l o c k s were p l a c e d between t h e magnet  box and t h e c o u n t e r t o reduce background f r o m t h e (d,n)  r e a c t i o n s i n t h e magnet box. P r o t o n bombardment o f c a r b o n c o n t a m i n a t i o n on t h e t a r get  produced 2.37 Mev. gamma r a y s from c l ( p ^ ) N l 3 and a n n i h i l a 2  -  17  -  t i o n r a d i a t i o n f r o m the subsequent p o s i t r o n decay of the N  .  The assignment of the gamma r a y t o C-- was c o n s i s t e n t w i t h the 1 2  energy measurement of 2 . 3 6 - .0*f Mev.  and t h e p o s i t i o n of a r e s o -  nance f o r t h i s gamma r a y a t h-60 k e v .  Some of the c a r b o n contam-  i n a t i o n was p r e s e n t i n the g o l d p l a t e d b a c k i n g but the l a r g e s t source was f r o m t h e h y d r o c a r b o n pump o i l s and vacuum g r e a s e s used i n the vacuum system w h i c h condensed  on the c o l d t a r g e t p l a t e .  P r i o r t o l a y i n g down a t a r g e t t h e t a r g e t chamber was baked a t 150°C f o r 2k hours t o reduce the amount of o i l s w h i c h on t h e c o l d t a r g e t d u r i n g the r u n s . ip the  Because  condensed  of the presence o f  C ^ c o n t a m i n a t i o n , i t was d e c i d e d not t o measure an a b s o l u t e  c r o s s s e c t i o n a t h i g h e r e n e r g i e s w i t h the i c e t a r g e t but t o make a r e l a t i v e c r o s s s e c t i o n measurement as a f u n c t i o n of p r o t o n energy u s i n g a s t a b l e o x i d e t a r g e t w h i c h c o u l d be heated t o p r e vent carbon d e p o s i t i o n .  Then a comparison of the y i e l d s a t 8 0 0  kev. p r o t o n energy from the i c e t a r g e t of known t h i c k n e s s and f r o m the o x i d e t a r g e t , would determine the o x i d e t a r g e t t h i c k n e s s . T h i s work has n o t y e t been c o m p l e t e d . (b)  Procedure. P r o t o n s of 8 3 0 k e v . produced by the U.B.C. Van de G r a a f f  g e n e r a t o r were used t o bombard two d i f f e r e n t i c e t a r g e t s .  The  beam energy measured by the g e n e r a t i n g v o l t m e t e r and a n a l y z e d by the  9 0 ° d e f l e c t i o n magnet was known t o w i t h i n about - 3 kev.  With  one t a r g e t , measurements were made a t t h r e e d i f f e r e n t d i s t a n c e s t o determine any c o r r e c t i o n t o the s o l i d t h a t the a n g u l a r d i s t r i b u t i o n o f ^ form a ( b + S i n 6 ) . 2  w a s  n o  a n g l e due t o the f a c t ^  i s o t r o p i c but of the  The t a r g e t was p o s i t i o n e d a t  *+5°  t o t h e beam  looo  MOO  ~I  20  PIQURE 6  1  i  ;  1  1—  25  SPECTRA- EROM.ICE T A T O 5  - 18 -  a f t e r l a y i n g down t h e i c e so t h a t the. r a d i a t i o n was n o t a t t e n uated b y t h e copper s u p p o r t .  The a n g u l a r p o s i t i o n of t h e t a r g e t  was o b t a i n e d f r o m a degree c i r c l e a t t a c h e d t o t h e t a r g e t chamber. The  e r r o r i n a n g u l a r p o s i t i o n i n g was - 0 . 5  g i v i n g a probable  e r r o r o f - 1 p e r cent i n t h e number o f atoms p e r square c e n t i m e t e r seen b y t h e beam due t o u n c e r t a i n t y i n t a r g e t o r i e n t a t i o n . Beams o f h t o 5 microamperes o f p r o t o n s were u s e d . The beam was d e f o c u s s e d  s o t h a t t h e s t o p system d e f i n e d t h e s i z e  and p o s i t i o n o f t h e beam s p o t .  No d e t e r i o r a t i o n o f t h e t a r g e t  even a f t e r b e i n g s u b j e c t e d t o t o t a l beams of 10^  was observed  microcoulombs o t h e r t h a n f o r t h e b u i l d up o f some c a r b o n on t h e surface. S p e c t r a were r e c o r d e d w h i c h i n c l u d e d " ^ ]_ and ^2 also'w'i,  a  n  d  t  n  e  2  » 3 7 Mev. r a d i a t i o n f r o m C ( p , " ^ ) . 12  an(  *  This  per-  m i t t e d t h e background due t o c a r b o n c o n t a m i n a t i o n , w h i c h i n c r e a s e d i n t h i c k n e s s w i t h t h e amount of bombardment, t o be e s t i mated.  P e r i o d i c checks o f t h e c h a n n e l p o s i t i o n s o f t h e k i c k -  s o r t e r f o r each s p e c t r a l range showed t h e e l e c t r o n i c s were s t a b l e t o b e t t e r t h a n 0 . 5 per c e n t . (c)  Measurement o f t h e e f f e c t i v e c e n t r e a t 0 . 5 1 and 1.28 Mev. A knowledge of t h e e f f e c t i v e c e n t r e p o s i t i o n f o r t h e  gamma r a y e n e r g i e s measured was r e q u i r e d . a r e absorbed  throughout  S i n c e t h e gamma r a y s  t h e 3 . 5 i n . l o n g d e t e c t o r c r y s t a l , measure-  ments o f t h e c o u n t i n g r a t e w i l l n o t show a n i n v e r s e square dependence on t h e d i s t a n c e measured f r o m a source t o t h e end of t h e  -  crystal.  19  -  However e x p e r i m e n t a l l y i t has been shown t h a t i f d i s -  tance measurements a r e made f r o m t h e source t o a p o i n t i n s i d e the c r y s t a l w h i c h we c a l l t h e e f f e c t i v e c e n t r e t h e i n v e r s e square r e l a t i o n between c o u n t i n g r a t e and d i s t a n c e w i l l h o l d , e x c e p t f o r d i s t a n c e s o f t h e same order as t h e c r y s t a l d i m e n s i o n s . T h i s p o i n t c a n be thought o f a s t h e mean d i s t a n c e i n s i d e t h e c r y s t a l a t which absorption takes place.  T h i s p e r m i t s one t o  d e f i n e a n e f f i c i e n c y w h i c h i s independent o f d i s t a n c e .  Also the  t h e o r e t i c a l e f f i c i e n c y c a l c u l a t e d f o r plane i n c i d e n t gamma r a y f l u x e s w i l l correspond  t o t h i s d e f i n i t i o n of e f f i c i e n c y .  The  d i s t a n c e t o t h e e f f e c t i v e c e n t r e depends on t h e gamma r a y energy s i n c e t h e a b s o r p t i o n c o e f f i c i e n t i s energy dependent. The e f f e c t i v e c e n t r e was determined e x p e r i m e n t a l l y f o r gamma r a y e n e r g i e s c l o s e t o those  obtained  u s i n g a N a ^ source and measuring t h e c o u n t i n g r a t e as a f u n c t i o n of d i s t a n c e f r o m t h e end o f t h e c r y s t a l f o r b o t h t h e 0.51 Mev. and t h e 1.28 Mev. gamma r a y s .  A Na  2 2  source c o n t a i n e d  i na 0.25  i n . d i a m e t e r aluminum r o d was p l a c e d i n t h e t a r g e t chamber a t the p o s i t i o n where t h e beam s t r u c k t h e i c e t a r g e t and t h e c o u n t ing  r a t e o f t h e 0.51 and 1 . 2 8 Mev. gamma r a y s f o r d i f f e r e n t  counter  t o . s o u r c e d i s t a n c e s were measured.  shown i n F i g u r e 7 .  The r e s u l t s a r e  From t h e s t r a i g h t l i n e f i t t e d by l e a s t  squares, the d i s t a n c e s from the c r y s t a l face t o the e f f e c t i v e c e n t r e f o r 0.51 and  1.28  Mev. r a d i a t i o n s were  I.V32  -  .08  i n . and  l,k? - . 1 0 i n . r e s p e c t i v e l y . These a r e t o be compared t o t h e 1 . 6 0 - . 1 0 i n . found f o r t h e e f f e c t i v e c e n t r e d i s t a n c e a t 6.1M-  - 20 -  Mev.  To w i t h i n the p r o b a b l e e r r o r t h e e f f e c t i v e c e n t r e d i s t a n c e  f o r t h e t h r e e e n e r g i e s a r e t h e same. An e s t i m a t e o f t h e e f f e c t i v e c e n t r e p o s i t i o n may be made assuming t h a t t h i s i s t h e d i s t a n c e , x , f r o m t h e f r o n t f a c e of t h e c r y s t a l such t h a t h a l f t h e t o t a l a b s o r p t i o n t a k e s p l a c e i n f r o n t o f x (assuming a p l a n e i n c i d e n t f l u x and n e g l e c t i n g m u l t i p l e p r o c e s s e s and l o s s e s t h r o u g h t h e c r y s t a l s i d e s ) .  Then  x = - I n ( 0 . 5 ( 1 + exp -JU-1) ). where M i s t h e t o t a l a b s o r p t i o n  c o e f f i c i e n t , and ^.l^f*  x  1.28  s  x o ^ l  i s t h e l e n g t h of t h e c r y s t a l . a  r  e  The r a t i o s  Is 0 . 6 : 0.^5 which d i f f e r from those  measured e x p e r i m e n t a l l y .  T h i s d i f f e r e n c e has n o t been accounted  f o r and f u r t h e r measurements a r e needed t o d e t e r m i n e t h e cause o f the d i s c r e p a n c y . k.  Cross S e c t i o n C a l c u l a t i o n s . (a)  Carbon C o n t a m i n a t i o n C o r r e c t i o n . S i n c e t h e number of 2.37 Mev. gamma r a y s f r o m  C^ (p 2  ,^) 13 N  depends on the d u r a t i o n o f t h e bombardment, and because much o f t h i s c o n t a m i n a t i o n e v a p o r a t e d a t t h e same t i m e t h a t t h e t a r g e t was a l l o w e d t o e v a p o r a t e , beam dependent  backgrounds t a k e n w i t h  no i c e on t h e t a r g e t d i d n o t g i v e a t r u e measure o f t h e C ground e f f e c t .  back-  1 2  T h e r e f o r e , i n making background c o r r e c t i o n s t h e  f o l l o w i n g procedure Was a d o p t e d .  F i r s t , t h e beam dependent  background was s u b t r a c t e d under each of t h e peaks ( f r o m 0 . 6 5 2 t o 0 . 9 9 5 Mev. i n t h e case case oftfj).  ofY2,  and f r o m 0 . 9 9 5 t o l . V ? Mev. i n t h e  I n t h e case where t h e s p e c t r a i n c l u d e d t h e C I  2  K^-rays  -21 -  the background was a l s o s u b t r a c t e d f r o m the 2.37 Mev. peak. r e s i d u e i n the spectrum between 1.^5  The  and 2.5 Mev. w i t h beam and  time dependent background s u b t r a c t e d was assumed t o be due t o b u i l d up d u r i n g t h e r u n .  On the b a s i s of t h e 2.37 Mev.  gamma r a y spectrum shape, a n amount p r o p o r t i o n a l t o t h i s r e s i d u e was  s u b t r a c t e d f r o m t h e H ^ i and 1 ^ r e g i o n s o f t h e spectrum.  p r o p o r t i o n s s u b t r a c t e d were e s t i m a t e d f r o m a C-*- spectrum 2  p r e v i o u s l y during contamination s i g n i f i c a n t i n the case of ifi had a much h i g h e r (b)  checks.  T h i s G^  2  The taken  c o r r e c t i o n was  but n o t i n the case o f 1^2 w h i c h  yield.  S o l i d Angle From r u n s t a k e n a t t h r e e counter d i s t a n c e s on one o f  the i c e t a r g e t s t h e counts per i n t e g r a t o r fovo^  followed an i n -  v e r s e square p l o t as a f u n c t i o n o f the d i s t a n c e t o the  effective  c e n t r e measured f o r t h e .51 Mev. gamma r a y s t o b e t t e r t h a n 3 per cent.  This i n d i c a t e s t h a t t h e s o l i d angle  subtended b y the coun-  t e r d u r i n g t h e c r o s s s e c t i o n measurements i s s u f f i c i e n t l y s m a l l t h a t t h e s i n ^ c o m p o n e n t o f o ^ does n o t s i g n i f i c a n t l y a f f e c t t h e c a l c u l a t i o n o f t h e c r o s s s e c t i o n i f c o n s t a n t f l u x per u n i t s o l i d angle i s assumed.  I n t e g r a t i n g a s i n 6 y i e l d over t h e c o u n t e r 2  a r e a a t t h e e f f e c t i v e c e n t r e f o r t h e d i s t a n c e used above g i v e s a r e s u l t d i f f e r i n g b y the order o f 3 per cent f r o m t h a t o f a n isotopic  distribution.  - 22 -  (c)  C a l c u l a t i o n of cross s e c t i o n The  s e r i e s o f r u n s t a k e n on each t a r g e t were summed  and t h e c r o s s s e c t i o n e s t i m a t e d f o r t h e two cases s e p a r a t e l y and t h e n compared.  The c a l c u l a t i o n s f o r one o f t h e t a r g e t s a r e  g i v e n below. The d i f f e r e n t i a l c r o s s s e c t i o n i s d e f i n e d by: do"  =  dSV,  _1_ <*i  ^*1 (diA)  .  r£  .  1_ n  1_ n  .  p  Q  where £jA i s t h e e f f i c i e n c y times a r e a o f t h e f a c e o f t h e counter  r  i s the target t o e f f e c t i v e centre distance  np  i s t h e number of p r o t o n s i n c i d e n t on t h e t a r g e t  c<\  i s t h e t r a n s m i s s i o n c o e f f i c i e n t t h r o u g h the 1/16 i n c h b r a s s w a l l s w h i c h i s .916 f o r .8 Mev.,  .935  f o r 1.3 Mev. gamma r a y s no  i s t h e number o f G  x  atoms per square  centimeter  W i t h a manometer r e a d i n g o f 1 8 . 7 5 c e n t i m e t e r s o f o i l f o r the t a r g e t used, the corresponding  t h i c k n e s s o f D2O was 107  kev t o 390 k e v p r o t o n s as r e a d f r o m t h e c a l i b r a t i o n curve k»  The s t o p p i n g c r o s s s e c t i o n per D 0 molecule 2  f o r protons of  t h i s energy as g i v e n by Wenzel and W h a l i n g (52) i s x 10""^ ev-cm . 2  Figure  ^020*13.5  T h e r e f o r e , t h e number o f oxygen atoms o f mass  16 per c e n t i m e t e r seen by t h e beam, i n c l u d i n g a >f2 f a c t o r s i n c e the t a r g e t was a t ^5° t o .the beam, i s g i v e n b y  - 23 -  n  = f  0  .  Jl  107xl0 x 13.5 x 10-15 3  = 1.118 x 10^9 atoms per cm  2  .9976  where f i s r e l a t i v e n a t u r a l abundance o f G^= n  = 95 i n t e g r a t o r c o u n t s x 107.2 microcoulombs  p  1.602  xlO"  1 3  = 3.5*+ i n c h e s  r  N j 2 = 7982 counts above a b i a s o f 1.0 Mev., a f t e r background = ^.909 i n .  A (do-)  . c£  2  corrections  2  = 7982 x ( 3 . 5 * f )  (<*A^90°  .916  x 1.602 x lo"  2  95  *f.909  13  x  107.2  x  = 3*15 x 1 0 ~ 3 c e n t i m e t e r s 2  1  1.118 x 1 0  1 9  per s t e r a d i a n  2  P u t t i n g i n t h e e f f i c i e n c y o f 3i per c e n t t h i s g i v e s t h e d i f f e r e n t i a l cross  section  (d£2)  ='0.2 x 1 0 " 3  2 c m  (dr\V90°  2  p Q r  s  teradian.  The r a t i o o f t h e r e l a t i v e y i e l d s a t 90 degrees of ^ (d_£T) ^ * l (  d  and T ^ i s c a l c u l a t e d as f o l l o w s : 4  \  =  J_  .  2  .  ±  •  .916 .935  2  (d£!) =  .1751  .  JL2 £1  P u t t i n g i n t h e e f f i c i e n c i e s .38 f o r f ^ and . 3 1 f o r £  2  we g e t t h e  r a t i o of t h e number of ^ 2 . t o V 2 observed a t 90 degrees as O.lh.  -  (d)  -  2k  Errors The s o u r c e s o f e r r o r i n the d i f f e r e n t i a l c r o s s s e c t i o n  measurement o f Tf  2  a  r  e  t a b u l a t e d below.  Source  Probable Error  c a l i b r a t i o n of dispenser  n,  a  Probable E r r o r i n Cross S e c t i o n Measurement  1.5!  ~T> 0  k'  /v  2  ±k.\  Angular p o s i t i o n of target 1% e f f e c t i v e c e n t r e and - . 0 2 ^ 1 beam p o s i t i o n *05%) ±  ±12% ±12%  12^  The p r o b a b l e e r r o r i n t h e number o f counts f r o m 0 p e r 2  i n t e r g r a t o r count e s t i m a t e d f r o m the c o n s i s t e n c y between the r u n s on a g i v e n t a r g e t g i v e ± 2% f o r the 107 k e v . t a r g e t and 3.7  per c e n t f o r the 93 k e v . t a r g e t i n r e a s o n a b l e agreement w i t h  the s t a t i s t i c s on the gamma-ray counts i n each r u n . The p r o b a b l e e r r o r i n t h e r a t i o o f the c r o s s s e c t i o n s £or~^i and u^2 c a l c u l a t e d from t h e c o n s i s t a n c y o f the r a t i o o f >  N^]_ t o N y f o r a l l r u n s i s ± 1 . 3 per c e n t . 2  W i t h the e r r o r o f 17  per cent i n the r a t i o £ i t o £ , t h i s g i v e s a r e s u l t i n g 2  probable  e r r o r o f 17 per c e n t i n the r a t i o o f the d i f f e r e n t i a l c r o s s sections.  - 25 5.  Results. The d i f f e r e n t i a l c r o s s s e c t i o n was measured u s i n g  pro-  t o n s o f i n c i d e n t energy 830 -5 k e v . S i n c e t h e p r o t o n s l o s e energy i n t r a v e r s i n g t h e i c e t a r g e t t h e mean p r o t o n energy a t w h i c h t h e r e a c t i o n o c c u r s i s a better indication  a t what energy t h e r e a c t i o n was measured.  Assuming t h e O ^ C p ^ F ^ c r o s s s e c t i o n i s r e l a t i v e l y c o n s t a n t f o r p r o t o n s o f energy over t h e range a p p e a r i n g i n t h e t a r g e t , t h e n Ep = Ep + A where Ep i s t h e i n c i d e n t p r o t o n energy i n k e v . and  2  A i s the target thickness  i n kev.  F o r t h e t a r g e t s used i n t h e  c r o s s s e c t i o n measurement t h e t h i c k n e s s was a p p r o x i m a t e l y 60 kev, f o r 800 k e v . p r o t o n s ; t h e r e f o r e Ep = 800 ±10 k e v . The r e s u l t s  o f t h e measurements on t h e two d i f f e r e n t  i c e t a r g e t s gave t h e d i f f e r e n t i a l c r o s s s e c t i o n a t 90 degrees o f "i>2  a  ^  s  , I f  ~  x  10~  32  cm  per s t e r a d i a n .  2  Using the angular d i s t r i b u t i o n  o f "0^ found b y Warren  et a l (55) a s b e i n g p r o p o r t i o n a l t o 1 + 5 s i n © 2  for the angular d i s t r i b u t i o n  a t 1.9 Mev.  a t 800 k e v . , t h e t o t a l y i e l d f o r  2 i n t e g r a t e d over a l l a n g l e s i s g i v e n b y :  ^2 .  • •  -  cc  x  *2  =  ^77 (1+2/3 x 5) x (d£s) 6 ( d R ) 90° 9 . S x 10-31 2 f o r m a t 800 kev. c m  The r e l a t i v e  of i  x  t o "^2  i  s  °'  l l f  io  d i f f e r e n t i a l c r o s s s e c t i o n a t 90 degrees  »93  - 26 The t o t a l r e l a t i v e y i e l d s , a g a i n u s i n g t h e a n g u l a r distribution for i f 2 <Hl  <5-fc2  a-fc  1#  90  Mev i s g i v e n by  6  x 0.1^ + 0.19  1+2/3.5  From t h i s and t h e v a l u e f o r ^ t 2 , <Til i s l . S x The i n t e n s i t y r a t i o of "B^ t o " ^  i s  10""-3  . cm.  significantly  h i g h e r t h a n .1 found p r e v i o u s l y i n t h i s l a b a t 1.90  Mev.  How-  ever b e f o r e t h e s e r e s u l t s can be a c c u r a t e l y compared w i t h t h e e a r l i e r work, a n g u l a r d i s t r i b u t i o n s ured.  a t 800 k e v . need t o be meas-  - 27 -  CHAPTER I I I ENERGY DETERMINATION OF  1.  Introduction. The measurement o f t h e gamma r a y energy from t h e e x -  c i t e d l e v e l a t a p p r o x i m a t e l y 0.5 Mev. i n O ^ i s o f i n t e r e s t b e 1  cause o f d i s c r e p a n c i e s between v a r i o u s p r e v i o u s measurements o f t h i s energy.  A knowledge of t h e p o s i t i o n of t h i s l e v e l i s n e c e s -  s a r y f o r t h e c a l c u l a t i o n s of t h e c r o s s s e c t i o n a t low e n e r g i e s . A l s o r e s u l t s d i f f e r i n g g r e a t l y by d i f f e r e n t methods of measurement i n d i c a t e t h a t e i t h e r t h e a c c u r a c y o f some methods a r e n o t as good as supposed o r t h e l e v e l s t r u c t u r e i s more complex t h a n believed. The l e v e l energy was f i r s t measured by F.A. A j z e n b e r g (5D  f o r the r e a c t i o n 0-^(d,n).  The n e u t r o n a n g u l a r  was measured by p h o t o g r a p h i c p l a t e t e c h n i q u e .  distribution  From t h e energy  s e p a r a t i o n of t h e two n e u t r o n groups t h e l e v e l was p l a c e d a t 536 - 10 k e v . P r e v i o u s work done i n t h i s l a b o r a t o r y (Warren e t a l ) i n d i c a t e d t h a t the l e v e l was l e s s t h a n 0.51  Mev. above t h e  ground s t a t e by d i r e c t measurement of t h e energy of  T n e  energy was e s t i m a t e d t o be k&7 ± 15 k e v . P r e l i m i n a r y u n p u b l i s h e d work by Bonner agreed more c l o s e l y w i t h A j z e n b e r g L a u r i t s e n , 55).  ( A j z e n b e r g and  However, as a r e s u l t o f more a c c u r a t e measure7 7  ment of t h e t h r e s h o l d f o r L i (p,n)Be  , Bonner and M a r i o n  (55)  measured t h e energy as *+99 - 3 k e v . a g a i n u s i n g ©^(d,n) t h r e s h o l d s ,  - 28 -  confirming the estimates obtained i n t h i s l a b o r a t o r y .  The t h r e s -  h o l d n e u t r o n s were d e t e c t e d by making f a s t - s l o w n e u t r o n r a t i o measurements as a f u n c t i o n o f d e u t e r o n energy, w i t h an e s t i m a t e d p r o b a b l e e r r o r o f - 0.006 k e v . Doyle e t a l (56) u s i n g t h e N ( c^,n)F*T  r e a c t i o n g i v e 0 . 5 3 * 0.0*+ Mev. as t h e l e v e l  l l f  energy;  the h i g h p r o b a b l e e r r o r would s u p p o r t e i t h e r o f t h e p r e v i o u s d e t e r m i n a t i o n s , n o t o n l y A j z e n b e r g ' s r e s u l t s as i n d i c a t e d b y Doyle. The d e t e r m i n a t i o n o f t h e energy o f " o ^ was r e p e a t e d , because of t h i s l a t e s t r e p o r t , and t o check t h a t t h e e a r l i e r r e s u l t s o b t a i n e d were r e p r o d u c e a b l e . 2.  Apparatus. (a)  Target. Because of t h e d i f f i c u l t y i n s h i e l d i n g ttie c o u n t e r and  t a r g e t system w i t h l e a d when t h e i c e d i s p e n s e r was used and i n r e d u c i n g c a r b o n c o n t a m i n a t i o n on t h e c o l d t a r g e t s u p p o r t , i t was d e c i d e d t a use s o l i d o x i d i z e d m e t a l t a r g e t s . Tungsten was used because o f t h e l o w energy o f t h e coulomb e x c i t e d s t a t e s a t 112 k e v . K X-rays  ( S t e l s o n and McGowan, 5 5 ) •  (66 k e v . f o r t u n g s t e n ) and b r e m s t r a h l u n g would be p r e s e n t  w i t h a l l m a t e r i a l s used; t i n has n e g l i g i b l e coulomb e x c i t a t i o n but i s d i f f i c u l t t o o x i d i z e .  A t u n g s t e n p l a t e 0 . 0 2 0 i n . by 1  i n . by 0 . 7 5 i n . was c l e a n e d i n p o t a s s i u m h y d r o x i d e (20 per cent s o l u t i o n ) e t c h e d , and r i n s e d t h o r o u g h l y w i t h d i s t i l l e d  water.  The t u n g s t e n was o x i d i z e d by suspending i t i n a h e a t e r c o i l o f  - 29 n i c k e l w i r e i n s i d e a b e l l j a r , h e a t i n g t h e s t r i p under vacuum and a d m i t t i n g c y l i n d e r oxygen (commercial g r a d e ) .  The t u n g -  s t e n s u r f a c e d i s c o l o u r e d g r e e n , b r i g h t b l u e and t h e n b l u e - g r a y as t h e o x i d e l a y e r t h i c k e n e d .  A long period o f heating i n an  atmosphere o f oxygen produced a y e l l o w - g r e e n s u r f a c e due t o t h e formation of tungsten t r i o x i d e .  T h i s changed back t o t h e b l u e -  g r a y c o l o u r where t h e beam h i t i t d u r i n g bombardment presumable due t o t h e t u n g s t e n t r i o x i d e changing back t o t u n g s t e n d i o x i d e . T h e r e f o r e f o r a t a r g e t w i t h a c o n s t a n t oxygen c o n t e n t , t h e b l u e g r a y t u n g s t e n d i o x i d e l a y e r was p r e f e r a b l e . S i n c e bombardment o f sodium g i v e s a 0.^5 Mev. gamma r a y from N a 2 3 ( p , p ' ) , as w e l l a s a 1.60 Mev. gamma r a y f o r Na 2 3(p,<5*^)Ne , t h e t a r g e t was checked 20  f o r Na 2 3  contamination  by r u n n i n g a n e x c i t a t i o n curve over t h e r e s o n a n c e s a t 1287.5 and 1257.5 kev.  The resonance a t 1257.5 kev. decays m o s t l y by  p r o t o n e m i s s i o n g i v i n g t h e 0.^5 Mev. gamma r a y . were a p p a r e n t .  Traces of Na23  From t h e e x c i t a t i o n f u n c t i o n f o r Na 2 3(p,<^^)  and N a 2 3 ( p , p ' ^ ) g i v e n by S t e l s e n and P r e s t o n (51*) f o r p r o t o n e n e r g i e s g r e a t e r t h a n 1 Mev. and b y R.L. B u r l i n g (^hljfor p r o t o n s from 0.3 t o 1.9 Mev., t h e N a 2 3 y i e l d was e s t i m a t e d t o be l e s s t h a n 10 counts p e r 100 microcoulombs o f beam above a 300 k e v . b i a s f o r 830 k e v . p r o t o n energy f o r t h e c o u n t e r - t a r g e t geometry used d u r i n g t h e measurement o f  This r e s i d u a l N a  2 3  effect  was s u f f i c i e n t l y s m a l l t h a t i t would produce n e g l i g i b l e d i s t o r t i o n o f the""l^ peak.  - 30 The  t a r g e t chamber used was s i m i l a r t o t h a t used by  A l e x a n d e r (5S)»  Steam heat was a p p l i e d t o t h e t a r g e t assembly  t o reduce t h e amount o f o i l vapours c o n d e n s i n g on t h e t a r g e t . D u r i n g some bombardments no steam was used and no t y p i c a l o i l d e p o s i t appeared on t h e t a r g e t .  cracked  I t t h e r e f o r e appears t h a t i f  the vacuum system i s c l e a n t h e h e a t i n g  o f t h e t a r g e t by t h e beam  a l o n g w i t h good c o l d t r a p p i n g c l o s e t o t h e t a r g e t assembly a r e s u f f i c i e n t t o p r e v e n t o i l vapour c o n t a m i n a t i o n o f t h e t a r g e t . A l e a d s h i e l d was moulded t o f i t t h e geometry o f t h e t a r get chamber and f i t i n t o t h e l e a d c a s t l e h o u s i n g t h e gamma r a y counter. (b)  Detector The  and E l e c t r o n i c s .  l a r g e 2.5 i n . by 3.5 i n c y l i n d r i c a l Harshaw sodium  i o d i d e t h a l l i u m a c t i v a t e d c r y s t a l was used as a d e t e c t o r ,  positioned  t o t o u c h t h e chamber f a c e t h u s s u b t e n d i n g a p p r o x i m a t e l y 0.015 o f a sphere a t t h e t a r g e t . The The  k i c k s o r t e r was s e t up as d e s c r i b e d  i n Chapter I I .  s t a b i l i t y o f t h e e l e c t r o n i c s d u r i n g the r u n s was b e t t e r t h a n  0.1 per cent as measured by t h e mercury p u l s e g e n e r a t o r .  The f u l l  energy peaks o f t h e gamma r a y s were measured w i t h a h i g h d i s p e r s i o n o f t h e k i c k s o r t e r t o a f f o r d maximum energy r e s o l u t i o n . 3«  Experimental, ( a ) ! ^ and"^. P r o t o n s w i t h an energy o f 800 k e v , as r e a d on t h e gener-  a t i n g voltmeter  were used t o bombard t h e oxide t a r g e t  positioned  A  flCURL 8  0'\ ,vr ?  GAMMA. RAV ENERGIES  - 31 -  so t h a t t h e gamma r a y s were n o t a t t e n u a t e d by t h e t a r g e t s u p p o r t i n the d i r e c t i o n of the d e t e c t o r . Z n ^  The 1 . 1 2 Mev. gamma r a y from  t h e 0 . 6 6 2 Mev. gamma r a y from Cs ^ 1  gamma r a y from N a  2 2  and t h e 1 . 2 8 Mev.  supplied c a l i b r a t i o n points f o r the pulse  generator amplitude s c a l e .  The l i n e a r i t y o f t h e c r y s t a l p u l s e  h e i g h t v e r s u s gamma r a y energy was b e t t e r t h a n 1 per c e n t i n this region. The c u r v e s used t o determine  t h e e n e r g i e s o f 0^ and °  2  a r e shown i n F i g u r e 8 . The r e s u l t s o f t h e s e measurements i n d i c a t e = 1.3k6 i 0.018 Mev. E ^ = 0 . 8 5 2 - 0 . 0 0 8 Mev. °2  E^ - E ^  = 0.h9k - 0.012 Mev.  The g e n e r a t i n g v o l t m e t e r was c a l i b r a t e d a t t h e 8 7 3 . 5 k e v . r e s o nance o f F ^ C p , ^ ) on a t h i n f l u o r i n e t a r g e t . The Q v a l u e f o r t h e r e a c t i o n may be determined  from "Y^  and Ep. Q = %v  °1  - 16 E  p  17  Because t h e p r o t o n s l o s e energy a s t h e y t r a v e r s e t h e t a r g e t a mean p r o t o n energy, E , must be used and s i n c e t h e r e a c t i o n i s n o n p  r e s o n a n t , Up" = Ep - A / 2 , where A i s t h e t a r g e t t h i c k n e s s ( e x p r e s s e d i n kev.) t o p r o t o n s o f energy E . An e s t i m a t e o f t h e number o f O ^ 1  - 32 -  atoms per square c e n t i m e t e r can be made from t h e r e l a t i v e of ^ 2 the  o n  yield  "the t u n g s t e n d i o x i d e t a r g e t as compared w i t h t h a t on  i c e t a r g e t of known t h i c k n e s s .  The energy l o s s i n t h e t u n g -  s t e n d i o x i d e t a r g e t w i l l t h e n be g i v e n b y ,  where  i s t h e m o l e c u l a r s t o p p i n g power f o r t u n g -  wo  2  sten dioxide, I Q Q i s t h e number of 0  atoms per square c e n t i m e t e r  ±o  and 1 a c c o u n t s f o r t h e f a c t t h a t t h e r e a r e two oxygen • 2 atoms per m o l e c u l e . cr  may be c a l c u l a t e d from atomic s t o p p i n g power d a t a g i v e n by WO 2  W h a l i n g (57)  as f o l l o w s : cr  wo  -  + 2  cr  w o<3~ .  2  S i n c e Whaling does n o t g i v e suming C Then  CT  &  CT^, e x t r a p o l a t i o n from  ^  A  U  -  2  CT  u  as-  cT^  Z can be used t o g e t =2±  <^  Q  = 35.0 x l O " ^ e.v.-cm / m o l e c u l e a t E 1  2  p  = 0.800 Mev.  The number o f counts i n t h e f u l l energy peak o f 2 the  o x i d e t a r g e t w i t h t h e d e t e c t o r 2.3  62 counts per i n t e g r a t o r .  T  i n . f r o m t h e t a r g e t was  W i t h t h e same d e t e c t o r s e t a t 3.6 i n .  f r o m a n i c e t a r g e t o f t h i c k n e s s ^IHVJO of  f°  =  1 , 1  -  L 8  x  ^P ^ molecules 1  water per square c e n t i m e t e r , t h e r e were 92 counts per i n t e -  g r a t o r i n the "0^2 f u l l energy peak.  The number of oxygen atoms per  - 33 -  square c e n t i m e t e r on t h e o x i d e t a r g e t was c a l c u l a t e d a s f o l l o w s  „ MO  H ice  - V oxide N  ' w  If i c e  where  * V\  —  H  o  «tt o x i d e  i s t h e number o f gamma r a y s per i n t e g r a t o r ,  and - H _ a r e t h e s o l i d a n g l e s f o r t h e two cases n = •0  62 92  x (2.3) (3.6)  2  x 1.118 x 1 0  1 9  atoms/cm  2  Then t h e energy l o s s f o r p r o t o n s p a s s i n g t h r o u g h t h e t u n g s t e n d i oxide i s (A ) E  W  = 35.0 x l O " ^ x 1 x 62 x ( 2 . 3 ) 2 92 \TZ) 1  G  2  2  x 1.118 x 1 0  1 9  T h i s r e s u l t assumed t h a t t h e oxygen was i n t h e form o f WO2.  The t a r g e t s u r f a c e was blue-brown i n c o l o u r c h a r a c t e r i s t i c  of t h e d i o x i d e r a t h e r than t h e y e l l o w c o l o u r o f t h e t r i o x i d e .  If  some WO3 was p r e s e n t t h i s would reduce t h e t a r g e t t h i c k n e s s . A 53 k e v . t h i c k t a r g e t s h o u l d have r e s u l t e d i n an i n c r e a s e i n the observed w i d t h s o f ^ i and'o^ by a p p r o x i m a t e l y 50 kev, The  observed i n c r e a s e i n the w i d t h s i s l e s s t h a n 10 k e v . The i n -  c o n s i s t e n c y o f the two r e s u l t s i s r a t h e r p u z z l i n g and i s n o t y e t explained.  Assuming t h a t t u n g s t e n t r i o x i d e i s formed, C^ )^o^ E  = 38 kev., s t i l l r a t h e r w i d e r t h a n c a n be accounted w i d t h o f the observed gamma r a y l i n e s .  f o r by t h e  Assuming ( ^ E ) = 53 k e v .  - 3* -  t o be c o r r e c t t h e n f o r an i n c i d e n t p r o t o n energy E p = 831 k e v . the mean p r o t o n energy on t h e t a r g e t Ep = 80^ k e v .  For E ^ =  1.3k6 Mev. from E q u a t i o n 1 we o b t a i n Q = 590 - 23 k e v .  This  a g r e e s w i t h i n e x p e r i m e n t a l e r r o r w i t h t h e v a l u e 0.599 - .006 Mev. c a l c u l a t e d from t h e 0 ^ ( d , n ) F ^ t h r e s h o l d (Bonner and M a r i o n ,  55).  I t i s i n t e r e s t i n g t o compare t h e above r e s u l t s on gamma r a y e n e r g i e s w i t h s i m i l a r r e s u l t s o b t a i n e d f r o m t h e i c e t a r g e t s below:  = 1.315 - .016 Mev. = .812 - .016  E^ 2  Ev  °1  °2  = .503 - .032 Mev.  To determine t h e Q v a l u e  fromV  1?  the t a r g e t thickness  a t an i n c i d e n t p r o t o n energy of 830 kev. must be c a l c u l a t e d i n order t o e s t i m a t e t h e mean p r o t o n energy. The i c e t a r g e t s were 100 kev. t h i c k f o r 390 kev.  pro-  t o n s ; t h i s c o r r e s p o n d s t o a t h i c k n e s s o f 60 kev. a t 800 k e v . , c a l c u l a t e d u s i n g 0.6 f o r t h e r a t i o of t h e m o l e c u l a r s t o p p i n g c r o s s s e c t i o n s f o r water a t 390 and a t 830 k e v . (Whaling Then Ep = 800 kev. - 10 kev. and Q = 562 ±26 kev.  57).  The Q here i s  lower t h a n would be expected f r o m t h e known mass v a l u e s .  The spreading of the proton energy due to the ta thickness will produce a spread in the energy of ifj and~ widths of the full energy peaks of Y andlT^ are about 2  - 35 -  kev. g r e a t e r t h a n the w i d t h s o f the f u l l energy peaks o f the .661 Mev.Y-ray o f Cs ^ 1  tively. kev.,  and  the 1.28 Mev.  if-ray of N a  2 2  respec-  S i n c e t h e t a r g e t t h i c k n e s s was c a l c u l a t e d t o be 60  the measured w i d t h o f the 0 ^(p^)F 7gamma r a y s s h o u l d be 1  1  60 kev. w i d e r t h a n t h a t due t o the r e s o l u t i o n o f the system.  detecting  T h i s i s g r e a t e r t h a n t h e measured v a l u e , however j u s t  o u t s i d e the e r r o r p l a c e d  on the measured w i d t h i n c r e a s e .  I f the c r o s s s e c t i o n i s f a l l i n g r a p i d l y w i t h energy i n t h i s r e g i o n o f bombarding energy, the w i d t h would be c o r r e s p o n d i n g l y  decreasing increase  reduced s i n c e t h e r e i s a r e l a t i v e l y  l a r g e r y i e l d i n the f r o n t p o r t i o n s o f the t a r g e t compared w i t h the back p o r t i o n where the r e a c t i o n s are produced by lower e n e r g y protons.  T h i s would h e l p e x p l a i n the l a c k o f observed  i n the t u n g s t e n oxide t a r g e t s .  increase  However i t seems r a t h e r u n l i k e l y  t h a t the r a d i a t i v e c a p t i v e c r o s s s e c t i o n would change s o r a p i d l y over t h i s s m a l l energy range a t 800 kev.  A l s o the e f f e c t s h o u l d  be o b s e r v a b l e e q u a l l y on the t u n g s t e n o x i d e t a r g e t as on the i c e t a r g e t s i n c e the energy l o s s f o r p r o t o n s i n the two t a r g e t s was a p p r o x i m a t e l y the same.  Another r e a s o n why r e s u l t s f r o m the  t u n g s t e n oxide t a r g e t d o not show a s l a r g e a b r o a d e n i n g o f the f u l l energy peaks as expected assuming a l l the oxygen i s i n the f o r m o f t u n g s t e n oxide i s t h a t some oxygen may be o c c l u d e d a s a s u r f a c e l a y e r on the t u n g s t e n so t h a t the r e l a t i v e energy l o s s of the p r o t o n s due t o the t u n g s t e n atoms i s r e d u c e d ; f o r a pure oxygen t a r g e t o f such t h i c k n e s s t o g i v e an e q u i v a l e n t y i e l d a s measured, the energy l o s s f o r 800 kev. p r o t o n s would be a p p r o x i m a t e l y  10  kev.  - 36 -  A l s o i t s h o u l d be mentioned t h a t f o r t h i s  crystal  mounted on t h e Dumont 6363 p h o t o m u l t i p l i e r t h e w i d t h o f a f u l l energy peak o f t h e 1.28 Mev gamma r a y o f N a  2 2  v a r i e d as much as  20 p e r cent amongst s e t s o f runs t a k e n under a p p r o x i m a t e l y t h e same c o n d i t i o n s . T h i s v a r i a t i o n i n t h e r e s o l u t i o n a l t h o u g h n o t accompanied by g a i n s h i f t s o f t h e c e n t r o i d , r e d u c e s  greatly  the i n f o r m a t i o n a v a i l a b l e from t h e w i d t h of a peak i n a gamma ray  spectrum. (b)  Energy d e t e r m i n a t i o n o f " ^ S i n c e t h e energy of 7 ^ i s v e r y c l o s e t o t h e 0 . 5 1 Mev.  a n n i h i l a t i o n r a d i a t i o n from t h e p o s i t r o n decay o f F ? w h i c h i s 1  a l s o produced  i n t h e r e a c t i o n , t h e r e s u l t i n g spectrum w i l l be  due t o t h e sum o f t h e a p p r o p r i a t e number o f gamma r a y s o f t h e two energies.  The shape of t h e r e s u l t i n g spectrum and t h e p o s i t i o n  of t h e maximum o f t h e curve w i l l depend on t h e r e s o l u t i o n o f t h e d e t e c t o r , t h e d i f f e r e n c e i n energy between t h e two gamma r a y s , and t h e r e l a t i v e i n t e n s i t y o f t h e gamma r a y s . Assuming t h e f u l l energy peak due t o a s i n g l e gamma ray  i s g a u s s i a n i n shape, t h e spectrum shape f o r a gamma r a y  whose energy w i d t h i s v e r y s m a l l compared t o t h e r e s o l u t i o n o f the d e t e c t o r may be d e s c r i b e d by t h e u s u a l g a u s s i a n e q u a t i o n dN(x) = N e - 1/2 ( x - i . dx 2TTVT where C  2  i s t h e h a l f w i d t h a t .606 peak h e i g h t .  2  -tooo  500  J  1  '  1  1 S  1  1  1  1  1  1  1  1  IO  1  1  l  13  CHANNEL  NUMBER  !  I  l  1  ZO  I  L  I  I  I  ZS  I  I  I  I  I  JO  - 37 -  (the  w i d t h a t h a l f peak h e i g h t i s  A 1 —  2 c^ f (, i n ixf )v l / 2 )  2 x  i s the pulse height i n v o l t s  u i s the c e n t r o i d of the d i s t r i b u t i o n N  i s t h e t o t a l number o f counts i n the peak  Two  gamma r a y s whose e n e r g i e s a r e c l o s e t o g e t h e r , i . e .  w i t h i n the r e s o l u t i o n , o f t h e d e t e c t o r , w i l l produce a shape w h i c h c a n be approximated by the sum o f two g a u s s i a n f u n c t i o n s . T h e r e f o r e t o d e t e r m i n e how the maximum f o r such a measured  spec-  trum v a r i e s as a f u n c t i o n of energy s e p a r a t i o n and i n t e n s i t y r a t i o s of t h e gamma r a y s , we c o n s i d e r t h e sum o f two g a u s s i a n s : y = c  fexp  -1/2(x-/^)  T h i s r e p r e s e n t s two g a u s s i a n s troid at rc.  2  + r.  exp  -l/2(x-/Q A 2  o f t h e same w i d t h , one w i t h  and a r e a c , t h e second w i t h c e n t r o i d a t / A  x  and  cenarea  ( F i g u r e 9 ).  S o l v i n g f o r t h e maximum, /la , o f t h i s  curve  L e t <S C o n s i d e r i n g the f i r s t Then  order i n  i . e . & ju^a^^x ^ \ t  - 38 T h i s r e s u l t c a n be used t o determine t h e p o s i t i o n o f the maximum, E o , o f a complex gamma r a y spectrum, o f two gamma r a y s o f f u l l energy E-|_ and E , where E 2  2  = E-j_ + S  , and t h e i n -  t e n s i t i e s are i n the r a t i o 1 t o r with a detector  resolution  such t h a t S i s l e s s t h a n t h e w i d t h o f a monenergetic  spectrum  a t .606 peak h e i g h t . Then t h e p o s i t i o n o f t h e maximum o f t h e composite spectrum Eo i s g i v e n by E  0  for  =  + r_|_ 1+r ^ < 1 C  The d i f f e r e n c e E^ and E  2  5  i n t h e w i d t h a t h a l f maximum between  i s n e g l i g i b l e f o r these s e p a r a t i o n s . The w i d t h a t .606 maximum of a c u r v e o f t h e form o f  e q u a t i o n 2 i s most e a s i l y o b t a i n e d g r a p h i c a l l y .  F i g u r e 9. shows  22 the r e s u l t o f adding two f u l l energy peaks f r o m a Na  spectrum  i n t h e c r y s t a l , one p l a c e d a t 0.51 Mev., t h e o t h e r moved t o 0.^93 Mev. w i t h a n i n t e n s i t y r a t i o o f . 5 . The maximum i s s h i f t e d 0.^ - .1 o f t h e gamma r a y s h i f t ; t h e s h i f t e s t i m a t e d from equat i o n 5 i s 0.33> i n r e a s o n a b l e agreement.  With these c o n s i d e r -  a t i o n s i n mind we may a n a l y s e t h e spectrum f r o m 0 ^ ( p , " ^ ) F i n 1  the r e g i o n of 0.5 Mev. gamma r a y energy.  17  -  39  The F * n u c l e u s produced 7  -  by the O ^ 1  p,"^) r e a c t i o n  decays by p o s i t r o n e m i s s i o n w i t h a h a l f l i f e o f 6 6 seconds ( A j z e n b e r g and L a u r i t s e n ( 5 5 ) . The number of i f 3 gamma r a y s p r o duced c a n be c a l c u l a t e d from t h e b r a n c h i n g r a t i o ; " a " f o r ^ t o ^ 2  where Np i s t h e number o f F  1 7  n u c l e i produced  by t h e r e a c t i o n .  The number of a n n i h i l a t i o n gamma r a y s i s a p p r o x i m a t e l y e q u a l t o t w i c e t h e number of F  1 7  n u c l e i w h i c h decay d u r i n g t h e  counting period. 17 To e s t i m a t e t h e number of F ' atoms w h i c h decay i n a g i v e n p e r i o d c o n s i d e r t h e e q u a t i o n w h i c h g i v e s t h e t o t a l r a t e of change of  atoms. (d_Np) -\ N ( .) total d t  + c i  F  where c i i s t h e r a t e o f p r o d u c t i o n of F ^ n u c l e i w h i c h i s p r o p o r 7  t i o n a l t o t h e beam c u r r e n t i and w h i c h we w i l l assume t o be cons t a n t ; a t t = G, N  p  = 0 . The number o f F  1 7  - \ Np whereas t h e number ^ 3 b e i n g produced  decaying i s equal t o i s proportional to  the p r o t o n c u r r e n t . The r a t i o , r , of t h e number o f c o u n t s f r o m ^ 3 number from a n n i h i l a t i o n r a d i a t i o n c a n be determined  t o the  f o r t h e time  :  - 1+0  -  i n t e r v a l s below. a)  o to t  Jo  1  r =  .  2(l+a) where T = 1  = 95 second, t h e mean l i f e t i m e .  7s  b)  t (t-T(l-exp-t/T)  f o r t h e time i n t e r v a l t measured a t l e a s t 5 h a l f  l i v e s a f t e r s t a r t i n g t h e bombardment r =  1  9  2(l+a)  7  I n f i g u r e 9 curve ( i ) was t a k e n d u r i n g t h e f i r s t seconds of beam bombardment of t h e t a r g e t .  63  Curve ( i i ) was ob-  t a i n e d w i t h a c o n s t a n t beam o f a p p r o x i m a t e l y (10±.5) microamperes bombarding t h e t a r g e t f o r a p p r o x i m a t e l y 8 minutes  (that i s , ap-  p r o x i m a t e l y 5 l i f e t i m e s ) a f t e r w h i c h t h e spectrum was r e c o r d e d for  10.2 minutes k e e p i n g t h e beam c o n s t a n t d u r i n g t h i s t i m e .  The  e q u i l i b r i u m spectrum t a k e n f o r t h e f i r s t 66 seconds a f t e r t h e beam was shut o f f i s shown i n curve  (iii).  E s t i m a t i o n of t h e energy d i f f e r e n c e between t h e 0.51 Mev. gamma r a y a n d ^ ^  w  a  s  m a  d e as f o l l o w s :  From curve ( i ) Since the h a l f l i f e of F  1 7  i s 66 seconds ( A j z e n b e r g and  L a u r i t s e n (55))> t h e n t i s a p p r o x i m a t e l y e q u a l t o t h e h a l f l i f e .  -1+1  The r a t i o ~& 1 T  -  as measured above was 0.2.  Substituting  these  2  values i n t o equation 6 gives r = V3. (8.5 ± 5.2)  kev.  The measured s h i f t i s t h e n  T h e r e f o r e from e q u a t i o n 5 0~ = 1 + x 8.5 V3  ^ = -15 kev. ± 12 k e v . From curve ( i i ) W i t h a = V3» e q u a t i o n 7 g i v e s r = . +2. 1  s h i f t i s (5.2 ± 5.2)  kev. f r o m t h e .51 Mev. l i n e .  S = -18 kev. ±18 kev. (c)  The measured This gives  waio^Vftcd w a i n -Co* % i s i ^ c o -  -v(o±v^,\<ev  Gain S h i f t s The d i f f e r e n c e i n t h e p o s i t i o n o f t h e a n n i h i l a t i o n  r a d i a t i o n r e c o r d e d i m m e d i a t e l y a f t e r t h e beam was o f f t h e t a r get and t h a t f r o m N a  2 2  ( F i g u r e 9) r e c o r d e d a few minutes l a t e r  i n d i c a t e s a g a i n s h i f t i n t h e d e t e c t i o n system. Since the counting r a t e d u r i n g the runs i n the r e g i o n about *f00 kev. was low ( l e s s t h a n 22 counts per second) t h e g a i n s h i f t was thought  t o occur due t o t h e h i g h c o u n t i n g r a t e from  coulomb e x c i t e d gamma r a y s f r o m t u n g s t e n of 112 k e v . and 66 k e v . t u n g s t e n X - r a y s as w e l l as t h e s m a l l c o n t r i b u t i o n due t o bremstrahlung.  A t e s t was r u n t o reproduce  this shift.  The l a c k  of d.c. s h i f t i n t h e e l e c t r o n i c s (not i n c l u d i n g t h e p h o t o m u l t i p l i e r ) was  e s t a b l i s h e d by p u t t i n g t h e mercury popper p u l s e s i n t o t h e  head a m p l i f i e r and r e c o r d i n g these p u l s e s on t h e K.S. When a Cs^  7  source was p l a c e d up t o t h e c r y s t a l so t h a t t h e c o u n t i n g  - >f2 -  r a t e was  g r e a t e r t h a n 1500  c o u n t s per second above 200  kev.,  the change i n the popper p u l s e h e i g h t on the k i c k s o r t e r was t h a n .1  per  less  cent.  Eu?-55 g i v e s among o t h e r gamma r a y s s t r o n g l i n e s a t 330  and 87 kev.;  t h e r e f o r e t h i s source w i t h N a  2 2  were used t o  approximate t h e c o u n t i n g c o n d i t i o n s o c c u r i n g d u r i n g the r u n s the t u n g s t e n oxide t a r g e t s . b i a s was  i n c r e a s e d f r o m 100  second by moving the E u ^ 1  these runs no s h i f t was  2 2  approximately  Mev.  c o u n t i n g r a t e above a 50  counts per second t o 1500 source  kev.  c o u n t s per  c l o s e r to the c r y s t a l .  During  n o t i c e d i n the c e n t r o i d of the 0.51  f u l l energy peak f r o m N a . source the 0.51  The  However, on removing the  r a d i a t i o n f r o m the t a r g e t and N a "  Mev.  Eu ^ 1  r a d i a t i o n showed a g a i n s h i f t of +8  e q u a l t o t h a t observed between the  on  kev.  annihilation  d u r i n g the bombardment of the  oxide. One  e x p l a n a t i o n of t h i s behaviour  would be t o assume  t h a t t h e r e e x i s t e d two g a i n s h i f t a f f e c t s I n the p h o t o m u l t i p l i e r w i t h h i g h c o u n t i n g r a t e s i n the low energy r e g i o n of the trum ( l e s s t h a n 100  kev.) w i t h d i f f e r e n t decay time  constants;  one a p o s i t i v e s h i f t w h i c h had a l o n g decay time of the of a few h o u r s , the other a n e g a t i v e cay time of the order of a m i n u t e .  spec-  order  s h i f t w h i c h had a s h o r t These a f f e c t s c a n c e l l e d  each other w i t h a l a r g e low energy c o u n t i n g r a t e , but the  gain  i n c r e a s e would be observed when the low energy c o u n t i n g r a t e reduced.  de-  was  - h3 -  h.  Results. Summarizing Target ice  W0 W0  t h e d i f f e r e n t measurements of Measurement  £\  _Etf  .503*.032 Mev. .^9^.012 Mev.  Centroid shifts  .^95-.012 Mev.  2  2  Result  _£*2 2  The mean o f t h e s e r e s u l t s i s 0.50^.01 Mev. i n a g r e e ment w i t h .*f99-.003 Mev. g i v e n by Bonner and M a r i o n (55) > and w i t h i n the probable e r r o r of the previous r e s u l t s from t h i s l a b o r a t o r y (Warren e t a l , 5*0••  - hh -  CHAPTER I V A LOOK FOR 0 7 ( p ^ ) F 1  1.  1 8  GAMMA RAYS  Introduction. A c c o r d i n g t o t h e s h e l l model t h e mass 18 n u c l e i c o n -  s i s t o f two n u c l e o n s o u t s i d e t h e c l o s e d O ^ c o r e . 1  Because o f  t h i s r e l a t i v e l y s i m p l e assumptions a r e p o s s i b l e f o r t h e d e t e r mination  o f wave f u n c t i o n s f o r t h e ground s t a t e and some o f t h e  excited states.  E l l i o t and F l o w e r s (5*0 have c a l c u l a t e d t h e  p o s i t i o n s o f t h e l o w l y i n g l e v e l s o f t h e mass 18 and 19 systems of a n i n t e r m e d i a t e  c o u p l i n g s h e l l model o f t h e n u c l e u s .  In or-  der t o check t h e v a l i d i t y o f t h e t h e o r y , e x p e r i m e n t a l v a l u e s o f •I  Q  the s p i n , p a r i t y and i s o t p p i c s p i n o f t h e e x c i t e d s t a t e s o f F are o f i n t e r e s t a t t h e p r e s e n t t i m e .  A study of the o l ? ( p , ^ ) F l  r e a c t i o n w i l l s u p p l y some o f t h e parameters o f t h e F w h i c h c a n be compared t o t h e o r e t i c a l p r e d i c t i o n s .  l 8  8  states  Since a sep-  a r a t e d O ^ t a r g e t was a v a i l a b l e , i t was d e c i d e d t o l o o k f o r gamma 1  r a d i a t i o n f r o m t h i s r e a c t i o n i n t h e r e g i o n 1.0 Mev. t o 2.2 Mev. p r o t o n energy l e a d i n g t o e x c i t e d s t a t e s i n F  between 6.5 and  l 8  7.7 l e v . Warren e t a l (5*-) found t h e e x i s t e n c e 1  o f a n 873 k e v  gamma r a y d u r i n g t h e bombardment o f n a t u r a l oxygen t a r g e t s  with  protons o f energy g r e a t e r t h a n 1.8 Mey. T h i s was a t t r i b u t e d t o gamma d e - e x c i t a t i o n o f t h e 872 k e v . l e v e l i n 0 ( A j z e n b e r g and 1 7  -  i+5 -  L a u r i t s e n ,55) e x c i t e d b y i n e l a s t i c s c a t t e r i n g o f p r o t o n s . order t o check t h i s a s s i g n m e n t , s e p a r a t e d 0-*- were bombarded w i t h protons 7  t a r g e t s of 0  In  and  i n the energy r e g i o n around 2  Mev. From the r e s u l t s , no c a p t u r e p o s i t i v e l y be i d e n t i f i e d . contaminants.  gamma r a y s f r o m 0^  7  could  The l i m i t o f d e t e c t a b i l i t y was s e t b y  The s p e c t r a o f c a p t u r e gamma r a y s f r o m the contam-  i n a n t s were s t u d i e d and t h e r e s u l t s a r e r e p o r t e d on b r i e f l y here as i t i s f e l t t h a t t h e y s h o u l d be u s e f u l i n c o n n e c t i o n w i t h f u r t h e r l o w c r o s s s e c t i o n s t u d i e s i n t h i s energy r e g i o n . 2.  Experimental (a)  Procedure.  Gamma R a y D e t e c t i o n Systems The  l a r g e 2.5 by 3.5 i n c h t h a l l i u m a c t i v a t e d sodium  i o d i d e c r y s t a l was used w i t h the same e l e c t r o n i c s a s d e s c r i b e d i n Chapter I I I . A g a i n s h i f t phenomenon was observed on the Dumont 6363 w h i c h was a f u n c t i o n o f the c o u n t i n g r a t e , t h e s p e c trum shape and the H.T. v o l t a g e on the dynode c h a i n .  The g a i n  tended t o i n c r e a s e when the d e t e c t o r was exposed t o a h i g h c o u n t i n g r a t e and d i d n o t r e t u r n i m m e d i a t e l y  t o i t s o r i g i n a l value  on d e c r e a s i n g the count r a t e , b u t s l o w l y dropped over a p e r i o d of a few hours or more.  The amount o f the s h i f t was much g r e a t e r  f o r comparable c o u n t i n g r a t e s above a f i x e d b i a s when caused by h i g h energy gamma r a y s c o n t r i b u t i n g t o the f a s t c o u n t i n g t h a n when due t o low energy gamma r a y s .  rate  A l s o the amount o f g a i n  s h i f t f o r a g i v e n c o u n t i n g r a t e and a g i v e n spectrum was d e c r e a s e d f o r lower  p h o t o m u l t i p l i e r H.T. v o l t a g e .  I n a l l cases t h e  -  if6  -  g a i n o f t h e system was independent o f p u l s e s i z e i n t h e r e g i o n f r o m 2 t o 6 Mev. gamma r a d i a t i o n . reached  The maximum g a i n s h i f t was  o n l y a f t e r a p p r o x i m a t e l y 1/2 hours f o r a g i v e n c o u n t i n g  r a t e , i n c r e a s i n g q u i c k l y i n i t i a l l y , and a p p r o a c h i n g v a l u e more s l o w l y .  an e q u i l i b r i u m  The dependence of t h i s g a i n s h i f t on p u l s e  a m p l i t u d e and dynode H.T. v o l t a g e would i n d i c a t e some dependence on t h e i n s t a n t a n e o u s p u l s e c u r r e n t drawn n o t o n l y on t h e mean current.  This gain s h i f t necessitated frequent  calibration  checks when o b s e r v i n g t h e 6 t o 10 Mev. r e g i o n of t h e spectrum t o ensure t h a t t h e s h i f t d i d n o t mask some o f t h e spectrum s t r u c ture.  A f t e r 20 minutes w i t h a c o u n t i n g r a t e o f 2000 counts per  second above a 0.51  Mev. b i a s energy w i t h 6.1^ Mev. gamma r a y s  and 1000 v o l t s a c r o s s the dynode c h a i n t h e g a i n i n c r e a s e d by a p p r o x i m a t e l y 8 per c e n t . (b)  Targets. The t a r g e t chamber was d e s c r i b e d i n Chapter  t a r g e t s of m a g n e t i c a l l y separated 0 ^  I I I . Four  l a y e d down on .020 i n c h  Tungsten b a c k i n g s , were made by A.E.R.E. H a r w e l l E n g l a n d . t h i c k n e s s f i g u r e s s u p p l i e d w i t h t h e t a r g e t s were  The  approximately  50 micrograms per square c e n t i m e t e r f o r t a r g e t s number 1 and 2 and 20 and 15 micrograms per square c e n t i m e t e r f o r t a r g e t s number 3 and h r e s p e c t i v e l y . (c)  The 872 Kev. R a d i a t i o n . P r o t o n bombardment o f number 1 and 2 t a r g e t s a t 1.90  Mev. p r o t o n energy i n d i c a t e d t h a t t h e r e was l e s s t h a n 0.1 m i c r o -  - ±7 -  grams per square c e n t i m e t e r o f 0 to  0 (p,p'X). 17  assuming t h e 872 k e v . was due  T h i s was c a l c u l a t e d f r o m t h e r e l a t i v e  yields  of t h e 872 k e v . gamma r a y f r o m t h e s e p a r a t e d O ^ t a r g e t s and 1  from n a t u r a l oxygen t a r g e t s w i t h a known number o f oxygen atoms per square c e n t i m e t e r and t a k i n g t h e percentage of 0 oxygen as 0.0V per c e n t .  1 7  i n natural  S i n c e a n e x c i t a t i o n f u n c t i o n over t h e  873 k e v . resonance o f F ^ i n d i c a t e d t h a t i n t h e s e t a r g e t s t h e r e 9  was g r e a t e r t h a n 0.1 micrograms  per square c e n t i m e t e r o f F l o u r i n e  no f u r t h e r s e a r c h f o r c a p t i v e gamma r a y s from 0^ was made u s i n g 7  these two t a r g e t s .  The number 3 and h t a r g e t s showed no 872 k e v .  r a d i a t i o n f o r p r o t o n bombarding An 0  X  e n e r g i e s of 1.75 t o 2.2 Mev.  t a r g e t e l e c t r o m a g n e t i c a l l y s e p a r a t e d b y A.E.R.E.  H a r w e l l was a l s o bombarded w i t h p r o t o n s o f energy 1.8 t o 2.1 Mev.  No 872 k e v . gamma r a d i a t i o n was o b s e r v e d , however t h e t a r -  16 get c o n t a i n e d o n l y a s m a l l amount o f 0 yield  as judged f r o m t h e  of^ « 2  (d)  Contamination Spectra. C o n t a m i n a t i o n s p e c t r a on t a r g e t number 3 was s t u d i e d .  An e x c i t a t i o n f u n c t i o n over t h e energy range .8 t o 1.9 Mev. was measured i n 20 k e v . i n t e r v a l s .  More a c c u r a t e e x c i t a t i o n f u n c t i o n s  were t h e n made i n r e g i o n s where resonances were i n d i c a t e d .  The  p o s i t i o n of t h e s e resonances and t h e energy o f t h e gamma r a y s a p p e a r i n g i n t h e spectrum were used t o d e t e r m i n e t h e c o n t a m i n a n t s present.  The f o l l o w i n g n u c l e a r contaminants were f o u n d :  - HQ -  The y i e l d o f 6,lh Mev. gamma r a y s a t t h e 873 k e v . 19  resonance  i n d i c a t e d t h a t t h e amount o f F ' contamin-  a t i o n on the t a r g e t s was of t h e order of .1 mi H i micrograms p e r square c e n t i m e t e r , much l e s s t h a n t h a t i n t a r g e t s number 1 and 2. Resonances were observed a t p r o t o n e n e r g i e s of .898 1.2 and 1.650 Mev. w i t h gamma r a y s o f energy ^ . ^ l Mev. T h i s was a t t r i b u t e d t o N ^ ( p , ^ , " ^ ) C , (Hagedorn and 1  12  M a r i o n (57) and S c h a r d t e t a l (52)). l a c k o f resonances the N ^ - ^ p , ^ , ^ ) ^  2  The apparent  i n N (p,~^) o f comparable h e i g h t t o llf  resonance  i n d i c a t e d t h a t t h e amount  of R l 5 was g r e a t e r t h a n the n a t u r a l abundance r a t i o s f o r n i t r o g e n i s o t o p e s would s u g g e s t , and t h e r e f o r e p o s s i b l y was l a y e d down a s NH2+ a t t h e mass 17 f o c u s of t h e s e p a r a t o r . The cl3(p,~^)N I'+reaction has a resonance  a t I . 7 6 Mev.  p r o t o n energy ( A j z e n b e r g and L a u r i t s e n (55)) was  observed  N l ^ decay.  which  as was t h e 9.18 Mev. gamma r a y from t h e The c a r b o n c o n t a m i n a t i o n was n o t i n t r o -  duced onto the t a r g e t by the beam s i n c e c l e a n t u n g s t e n b a c k i n g s w h i c h had been bombarded f o r comparable l e n g t h s of time showed l e s s c!3 c o n t a m i n a t i o n .  - k9  The s e p a r a t e d O  ie>  -  t a r g e t had showed t h e I . 6 3 Mev.  gamma r a d i a t i o n f r o m N a 3 ( p , 0 * ^ ) ^ 2 0 between 1 . 9 and 2 . 0 Mev. 2  p r o t o n bombardment energy.  There i s c o n s i d e r a b l e sodium on t h i s  target. (e)  Measurement of a n n i h i l a t i o n r a d i a t i o n S i n c e t h e F-*- decays by p o s i t r o n e m i s s i o n w i t h a h a l f 8  life  o f 112 minutes ( B l a s e r , ( ^ 9 ) ) , t h e e x i s t e n c e o f a n n i h i l a t i o n o f 112  r a d i a t i o n f o l l o w i n g p o s i t r o n decay w i t h a h a l f l i f e 1ft  minutes would i n d i c a t e t h a t F  - had been formed.  Therefore an  attempt was made t o d e t e c t t h e decay t h r o u g h a measurement o f the off.  c o u n t i n g r a t e as a f u n c t i o n of time a f t e r t h e beam was t u r n e d The F  1 7  produced from 0 (p,l£) and N l6  1 3  from C  1 2  (p ^)  would c o n t r i b u t e t o t h e a n n i h i l a t i o n r a d i a t i o n s i n c e b o t h a l s o decay by p o s i t r o n e m i s s i o n . However t h e i r a s s o c i a t e d h a l f are  lives  66 seconds (Wong ^5^.) and 10 minutes ( C h u r c h i l l e t a l ^ 53! )  r e s p e c t i v e l y and w i l l n o t i n t e r f e r e w i t h measurements made o f the  2 hour h a l f l i f e  of F « l 8  The a n n i h i l a t i o n spectrum was observed u s i n g c o i n c i d e n c e methods, t h e s m a l l c r y s t a l d e t e c t o r b e i n g used a s t h e second counter.  The head a m p l i f i e r c i r c u i t i s shown i n F i g u r e 5 .  The  output from t h e s m a l l d e t e c t o r was f e d t o an E.K. C o l e l O ^ A a m p l i f i e r w h i c h i n t u r n f e d an Atomic I n s t r u m e n t s s i n g l e c h a n n e l analyser.  The window of t h e a n a l y s e r was s e t t o see r a d i a t i o n  between t h e 300 and 600 k e v . energy r e g i o n of t h e spectrum by u s i n g t h e mercury p u l s e g e n e r a t o r w h i c h had been c a l i b r a t e d i n  - 50 terms of energy with a gamma ray displayed on the k i c k s o r t e r . The output of the analyser triggered the gated biased amplifier through which the pulse from the large c r y s t a l passed.  The pul-  ses from the large c r y s t a l were delayed by approximately 1.5 microseconds  going into the Moody amplifier i n order to compen-  sate f o r the delay i n the trigger pulse from the single channel kicksorter. After a two hour bombardment with approximately 8 microamperes of 1.8 Mev. protons, the target was removed from the chamber and placed between the two c r y s t a l s .  The a n n i h i l a -  t i o n r a d i a t i o n counting rate observed indicated a f a s t decay component probably due to F^? produced from O^Cp,^) reaction and also a slowly decaying component with a half l i f e of the order of a few hours, possibly due to F  decay.  The counting  rate from t h i s long l i f e portion was very small indicating that the F * y i e l d f o r the target was very low. 1  3.  Conclusions. No 872 kev. gamma r a d i a t i o n was observed from proton 16  bombardment of the separated 0  target. Also on two of the four  separated 0^ targets the 872 kev. gamma ray d i d appear at the higher bombarding energies.  On the other two (targets numbered  3 and k-) the r a d i a t i o n was not observed; however this i s consis1o  tent with the very small amount of F  annihilation radiation.  But the amount of 0^ and 0 ^ on the two types of separated t a r gets appears to be much smaller than Harwell estimated.  This was  - 51 -  confirmed f o r the 0  ±  0  target  by comparison of the  f r o m t u n g s e t n o x i d e and f r o m t h e  separated  yield  target  and f o r  b y t h e r e l a t i v e l y s m a l l amount o f 872 k e v . f o r t h e target  compared t o a n a t u r a l o x i d e t a r g e t .  separated  Therefore,  c l u s i o n w h i c h c o u l d n o r m a l l y be d r a w n f r o m t h e  of  the  presence  con-  or  ab-  s e n c e o f t h e 872 k e v . gamma r a y i s d o u b t f u l i n t h i s  case  of the  backings.  s m a l l number o f t a r g e t  atoms i n t h e  The s m a l l amounts o f t a r g e t s suggests  either  that  oxide but occluded i n the bombardment,  or t h a t  electromagnetic If  the  the  on the  tungsten  oxygen i s not r e t a i n e d as  surface  and t h e n l o s t w i t h  which i s  the f i r s t r e a s o n i s the  arator,  oxygen i s o t o p e s  the percentage  separator  tungsten  because  an  subsequent  o f t h e mass 17 beam i n 0*7 i s n o t a s  l a r g e as  the assumed.  cause of the d i f f i c u l t y i n the  t h e n u s i n g b a r i u m m e t a l as a b a c k i n g o n w h i c h t o  sepform  o x i d e m i g h t be a n i m p r o v e m e n t b e c a u s e b a r i u m f o r m s a n o x i d e  more r e a d i l y t h a n  tungsten.  W i t h r e f e r e n c e t o the mention that  (5*0  for  the  second s u g g e s t i o n ,  electromagnetic  the hydrogen always present  i n the  Ahnlund et  s e p a r a t i o n o f 0^7  i o n source  combines  1  a t t h e mass 17 f o c u s .  Ahnlund et a l c o l l e c t e d  a t mass number 31 i n t h e f o r m o f N ^ O - ^ , i n o!7(a  few per c e n t ) i n the  t r o g e n f o r w h i c h t h e N- " t o 14  by u s i n g gas  9  easily  w i t h o x y g e n t o f o r m 0H+ OH2+ OH3+; t h i s w o u l d g i v e l a r g e o f o ^H+  al  amounts the  0 ^  enriched  i o n source mixed w i t h n a t u r a l n i ratio is very high.  This  - 52 r e s u l t e d i n e n r i c h e d t a r g e t s i n w h i c h the c o m p o s i t i o n was a p p r o x i m a t e l y 90 per cent N per c e n t C ^ O  1 8  llf  0 , 7 p e r c e n t N ^ ^ and l e s s t h a n 3 17  1  1  .  P o s s i b l y t h i s i s t h e o n l y method t o get r e a s o n a b l e e n r i c h m e n t o f O^ . 7  l e v e l parameters  C e r t a i n l y b e f o r e any u s e f u l i n f o r m a t i o n about o f F-*- can be e x t r a c t e d much b e t t e r t a r g e t s 8  w i l l have t o be o b t a i n e d .  MKI DC S O U R C E  P o L A R i r y SWITCH  Q  Volt-melve  ,tOOK  ' HaUpot  <3  -?0-  A\A/--—> 50K  WE  OUTPUT  ^[flflJX 267E  >  —s  K-d-,  T 16 V 0- — J  ,  AA/W  ~5>  SOOSL  100 K  400  -o—ox>  M K TO R E L A Y  I  DRIVE 5 0 0 J \ (w)  -S) Hi OUTPUT  AWvA-  iOOO  DECAY  TIME  f l O J R t 10 MCRCURY  RISE  TIME  PUL5L GEICRATOR  ?i>  -  53  -  APPENDIX  Mercury Relay Pulse  Generator.  I n o r d e r t o s e t up the M a r c o n i t h i r t y  channel k i c k -  s o r t e r a c c u r a t e l y and check the a s s o c i a t e d gamma r a y d e t e c t i n g e l e c t r o n i c s , a p u l s e g e n e r a t o r whose a c c u r a c y and s t a b i l i t y was b e t t e r t h a n 0 . 1 per c e n t was r e q u i r e d .  I f the s t a b i l i t y of  b o t h p u l s e g e n e r a t o r and c o u n t e r system i s good, a gamma r a y energy c a n be determined  i n terms o f s e t t i n g s o f t h e p u l s e  g e n e r a t o r v o l t a g e w h i c h c a n be related t o a gamma r a y energy s c a l e u s i n g sources w i t h gamma r a y s o f known e n e r g i e s .  The  shape o f the p u l s e f r o m the p u l s e g e n e r a t o r must approximate t h a t of t h e p u l s e s due t o gamma r a y i n t e r a c t i o n i n t h e d e t e c t o r so t h a t any n o n - l i n e a r i t y i n the e l e c t r o n i c s due t o d i f f e r e n t shapes w i l l be e l i m i n a t e d .  Therefore the p o s s i b i l i t y  the p u l s e shape of t h e g e n e r a t o r was r e q u i r e d .  pulse  of varying  The p u l s e s  s h o u l d be a b l e t o be f e d i n t o h i g h or l o w impedance p o i n t s i n o r der t o t e s t a t d i f f e r e n t p o i n t s of t h e c o u n t i n g system. 1.  P u l s e Generator  Mark I .  The Mk I model ( F i g u r e 1 0 ) , was d e s i g n e d  t o produce  p u l s e s t o be f e d i n t o the g r i d o f t h e head p r e a m p l i f i e r cathode follower.  T h i s was a h i g h impedance p o i n t p e r m i t t i n g t h e d e -  s i g n as shown.  The i n t e g r a t i n g time c o n s t a n t was f i x e d a t t h e  r i s e time of t h e p u l s e f r o m a sodium i o d i d e ( t h a l l i u m a c t i v a t e d ) s c i n t i l l a t i o n counter, 0 . 2 5 microseconds,  and t h e f a l l time was a d -  ded e x t e r n a l l y t o t h e g e n e r a t o r u s u a l l y i n the i n p u t t o t h e head amplifier.  -  9+  -  The W e s t e r n E l e c t r i c 2 7 6 s e r i e s o f r e l a y s s w i t c h i n one d i r e c t i o n when t h e c u r r e n t t h r o u g h t h e c o i l becomes g r e a t e r t h a n +1.5 m i l l i a m p e r e s and s w i t c h back when t h e c o i l c u r r e n t drops t o l e s s t h a n -1.5 m i l l i a m p e r e s .  On t h e a l t e r n a t e h a l f c y c l e f r o m  the p u l s e p r o d u c t i o n , t h e c o u p l i n g and p u l s e shaping were d i s c h a r g e d  t h r o u g h a 100K r e s i s t o r ; t h i s produced a p u l s e  of o p p o s i t e p o l a r i t y a p p r o x i m a t e l y the d e s i r e d p u l s e .  one t h i r d t h e amplitude o f  I n c r e a s i n g t h e 100K r e s i s t o r t o reduce t h e  s i z e of t h i s discharge arm  capacitors  p u l s e i n t r o d u c e d p i c k up i n t h e moving  o f t h e r e l a y d u r i n g t h e a l t e r n a t e h a l f c y c l e , adding r i p p l e  to the pulse The  output. f i f t e e n t u r n 100K H e l i p o t Model B had a l i n e a r i t y  r a t i n g of - 0 . 0 5 per c e n t .  Only 6 0 c y c l e r e p e t i t i o n r a t e was  a v a i l a b l e f o r t h e mercury s w i t c h .  The d i r e c t c u r r e n t was s u p p l i e d  by a s m a l l b a t t e r y of d r y c e l l s . 2.  P u l s e Generator Mark I I . The Mk I I d e s i g n ( F i g u r e 10) was b u i l t t o p r o v i d e a d -  d i t i o n a l f a c i l i t i e s ; f i r s t , t h e output p u l s e s can be f e d i n t o b o t h low or h i g h impedance p o i n t s , second, v a r i a b l e r i s e and f a l l t i m e s , and a l a r g e r amplitude  range a r e p r o v i d e d , and t h i r d , a  motor d r i v e n s i n g l e t u r n h e l i p o t p r o v i d e s v o l t a g e r i s i n g  linearly  i n time w h i c h when chopped by t h e mercury s w i t c h p r o v i d e s a s l i d i n g p u l s e g e n e r a t o r f o r t e s t i n g t h e k i c k s o r t e r . The d e s i g n was s i m i l a r t o t h a t d e s c r i b e d by B a t t e l and Chapman ( 5 D «  - 55 The l i n e a r l y varying d i r e c t current source was produced by d r i v i n g a single turn 5K Helipot Model L, ( l i n e a r i t y  0.1$), with a synchronous motor and feeding the voltage from the s l i d i n g arm to the pulse shaper.  A single turn helipot was  chosen since t h i s eliminates the need for l i m i t and motor reversing switches which are required for a multiturn h e l i p o t . The single turn helipot however does suffer from the fact that an additional r i p p l e i s introduced by the s l i d e r passing over the single turns of wire on the r e s i s t o r (1800 turns), however f o r the present pulse generator  t h i s r i p p l e was less than 0.05$  and consequently was n e g l i g i b l e . The motor drives the helipot through a reduction gear at a speed of 6 r.p.m. The d i r e c t current power supply delivered 10 m i l l i amperes at 85 v o l t s with 2 m i l l i v o l t s 10 cycle r i p p l e and l e s s than 1 m i l l i v o l t 60 cycle r i p p l e , peak to peak, at the input to the voltage d i v i d e r .  The voltage divider supplied 60, 15, and  6 v o l t output for the helipot, variable by adjustable  "trimpots",  and 50, 30, and 10 v o l t outputs f o r the s l i d i n g h e l i p o t . The Western E l e c t r i c 276 series relays used, have the property that for 1 millisecond after the switching has occurred a l l contacts are shorted.  This causes a pedestal l i k e pulse to  appear on the t a i l of the shaped pulse and also when the r e l a y returns on the alternate half c y c l e s .  This could be made small  by increasing the d i r e c t current source impedance.  But i f the  source impedance was too high there would be a r e l a t i v e l y large  +400V  + I50V  -5" + g^)65A2  liOV AC <:  t " V MOTOR  2 6 9 3 PIXED VOLTAOE  POLARITY SWITCH  oc  OUTPUT  'K TuiMPor  DIVIDED  IK IK 35^ TRIM POT TKIWFOT  T  h-  - W » — W — V v / v — v W — v v y  F1XE0  6Y  SWEEP  I0OK  .3K  HELIPOT  A W v V  —'vwv—o~ I.5K I  WvV-  35K  1  _^o_l S6  SWEEP  VOLTAGE  I—yx&A 5K WW  I0K  S  \{ 5 K HELIPOT T  W  M  S  Or Ob VOLT M E T E R  DIVIDED.  DC 5UPPLY  RELAV COIL  |0 +2.40  1  0  ncl  EXTERNAL  o fc5V  SUPPLY  T-IGURt II MULTIVIBRATOR CIRCUIT'  OC OUTPUT  4  - 56 e r r o r i n t h e p u l s e h e i g h t produced, due t o t h e c h a r g i n g time o f the i n t e g r a t i n g condenser, w h i c h v a r i e d w i t h t h e p o s i t i o n of t h e helipot.  T h i s i s t h e r e a s o n t h a t a 10K ohm r e s i s t o r i s p l a c e d  between t h e d i r e c t c u r r e n t output and t h e h e l i p o t , t h i s was t h e b e s t compromise f o r minimum p u l s e due t o t h e r e l a y s h o r t i n g e f f e c t and minimum e r r o r due t o v a r y i n g c h a r g i n g time  constant.  T h i s c o u l d be improved by u s i n g a 10K ohm h e l i p o t and a 100K ohm r e s i s t a n c e between the h e l i p o t and t h e d i r e c t c u r r e n t  output;  the time c o n s t a n t e r r o r would be n e g l i g i b l e and t h e c u r r e n t p u l s e would be reduced  over t h a t o b t a i n e d w i t h t h e c i r c u i t shown  i n F i g u r e 10.  The c u r r e n t p u l s e now causes a p p r o x i m a t e l y 1 per  cent decrease  i n d.c. l e v e l a t 60 v o l t s on the 60 v o l t range and  0.1 per c e n t a t 6 v o l t s on t h e 6 v o l t s r a n g e .  This e r r o r i s not  a l i n e a r f u n c t i o n of h e l i p o t s e t t i n g f o r a t 56volts t h e s h i f t i s 0.5  per cent and 0.25 per c e n t a t kO v o l t s f o r the 60 v o l t r a n g e . A m u l t i v i b r a t o r ( F i g u r e 11) was b u i l t t o permit  a b l e p u l s e r e p e t i o n r a t e s t o be used.  vari-  T h i s was i n t r o d u c e d n o t  o n l y because o f g r e a t e r f l e x i b i l i t y of t e s t i n g but because i n det a i l e d t e s t i n g of p u l s e a m p l i t u d e  a n a l y s e r s such as t h e M a r c o n i  30 c h a n n e l k i c k s o r t e r used i n t h i s work, i f p u l s e s a r e f e d i n a t mains f r e q u e n c y , t h e n t h e time c o r r e l a t i o n between t h e i n p u t p u l s e s and mains f r e q u e n c y r i p p l e v o l t a g e s i n t h e d i s c r i m i n a t o r s may i n v a l i d a t e c e r t a i n of t h e r e s u l t s . for  t h e p r e s e n t apparatus  T h i s e f f e c t was  confirmed  by n o t i n g t h a t i f t h e mercury s w i t c h on  the s l i d i n g p u l s e r (whose l i n e a r l y r i s i n g d i r e c t c u r r e n t v o l t a g e was produced by t h e s i n g l e t u r n h e l i p o t , d r i v e n by a synchronous  tlCUDt 12  PUL5E 5UAPE PL0T5 Ot e^'si^Kct'  - 57 -  motor and t h e r e f o r e l o c k e d t o t h e mains f r e q u e n c y ) was r u n a t mains f r e q u e n c y , t h e n t h e v a r i a t i o n i n k i c k s o r t e r c h a n n e l w i d t h s appeared d i f f e r e n t t h a n when the mercury s w i t c h was operated f r o m the m u l t i v i b r a t o r a t a f r e q u e n c y n o t commensurate w i t h t h e mains f r e q u e n c y . The p u l s e shape i s determined b y t h e RC networks used.  The response o f t h e c i r c u i t s c a n be c a l c u l a t e d by u s i n g  L a p l a c e t r a n s f o r m methods.  The l o w impedance, " L o , output M  cir-  c u i t form i s e s s e n t i a l l y t h a t of ( a ) F i g u r e 1 2 . S o l u t i o n b y Laplace transforms gives e (t)  = Vo_  0  T C 2  ( T 1 T V  0  =  R  -  1 1 C  2  =  2 \ ^2 hT \ TiT / 2  R C  = 2  ct 2T  R ; L  1  *1  e - * / ^ ^  Rg  1 T  2  + X  1 T  + 2  1_ C  2 R l  i s i n i t i a l v o l t a g e on  F i g u r e 12 shows a p l o t o f exp -t/2T s i n h Ct/2T. t h i s p l o t t h e r i s e t i m e i s a p p r o x i m a t e l y 2T, n e a r l y of  From  independent  C and t h e f a l l time c o n s t a n t i s a p p r o x i m a t e l y 2T/1-C.  - 58 -  The components were chosen so t h a t f e e d i n g i n t o a  75  ohm impedance the "Lo" output would g i v e the time c o n s t a n t s i n d i c a t e d on F i g u r e  10.  The " H i " output c i r c u i t i s e s s e n t i a l l y t h a t of (b) F i g u r e 12.  I f the time c o n s t a n t formed by the c o u p l i n g condenser  and t h e l o a d i s l a r g e r t h a n T^ or T  2  t h e n the v o l t a g e a t the  output e ( t ) i s g i v e n by: 0  e (t)  =  0  V  e"  0  t / 2 T  s i n h Ct/2T  2T C 2  w h i c h has the same time dependence as above.  For the " H i " o u t -  put the minimum l o a d impedance w h i c h w i l l not a f f e c t the time constants i s R^  v  y  R C ~~z— c 2  u  2  where C i s the s i z e of the output c o u p l i n g s . condenser. c  The a c c u r a c y w i t h w h i c h the K.S. s l i d i n g popper depends on how  can be s e t up w i t h t h e  the p u l s e r e p e t i t i o n r a t e and  speed of h e l i p o t sweep compare. I f the p u l s e s were c o m p l e t e l y random t h e n about 10,000 counts would have t o be accumulated  i n each c h a n n e l t o s e t the  edges t o 1 per c e n t a c c u r a c y ( t h i s i s what must be done i f a compton  spectrum from an anthracene c r y s t a l i s used as a f l a t  spectrum).  I f t h e p u l s e r e p e t i t i o n r a t e were 60 c y c l e s and the sweep such t h a t 10 counts per sweep per c h a n n e l were r e c o r d e d ,  - 59 then s i n c e t h e sweep h e l i p o t i s d r i v e n by a synchronous  motor t h e  c h a n n e l edges c o u l d be s e t t o o n l y 1 0 per c e n t no matter how many counts were r e c o r d e d .  T h e r e f o r e t o get 1 per cent a c c u r a c y  e i t h e r a sweep speed w h i c h p e r m i t s 1 0 0 counts per c h a n n e l per sweep i s n e c e s s a r y or a r e p e t i t i o n r a t e w h i c h i s a few c y c l e s off  6 0 cycles. For t h i s r e a s o n the v a r i a b l e f r e q u e n c y d r i v e s h o u l d be  used, s e t a t a f r e q u e n c y c l o s e t o 6 0 c y c l e s .  T h i s c a n be checked  on an o s c i l l o s c o p e by t r i g g e r i n g the sweep a t 6 0 c y c l e s per second and f e e d i n g t h e t e s t p u l s e s i n t o t h e v e r t i c a l a m p l i f i e r . I f t h e t e s t p u l s e r a t e i s j u s t o f f 6 0 c y c l e s per second t h e n t h e t e s t p u l s e s as seen on t h e o s c i l l i s c o p e w i l l d r i f t r e s p e c t t o the h o r i z o n t a l t r a c e .  slowly with  - 60 -  Bibliography  Ahnlund, K., Thulin, S. and P a u l i , H., ( 1 9 5 * - ) 1  8, 4 8 9 .  Ajzenberg, F. and Lauritsen, T., ( 1 9 5 5 ) Ajzenberg, F., ( 1 9 5 D  Rev. Mod. Phys. 2 2 , 7 7 .  Phys. Rev. £3, 6 9 3 .  Alder, F. and Yu, F., ( 1 9 5 D Alexander, T., ( 1 9 5 5 )  Arkiv For Fysik  Phys. Rev. 8 2 , 1 0 5 .  M.A. Thesis, University of B r i t i s h Columbia.  B a t t e l , W.J. and Chapman, E,E., ( 1 9 5 1 ) Atomic Energy of Canada Report No C R E L - 1 + 6 7 . ' . . Bethe, H. and Livingstone, M . , ( 1 9 3 7 ) Blaser, Boehm and Marmier ( 1 9 ^ 9  Rev. Mod. Phys. 9 _ , 2 ^ 5 .  Phys. Rev. 25 > 1 9 5 3 .  Bonner, T. and Butler, J . , ( 1 9 5 D  Phys. Rev. 8 3 , 1 0 9 1 .  Bonner, T. and Marion, J . , ( 1 9 5 5 )  Phys. Rev. 1 0 0 , k6.  Burling, R., ( 1 9 " + 1 )  Phys. Rev. 6 0 , 3 ^ 0 .  Burrows, Gibson and Rotblat, ( 1 9 5 0 )  Phys. Rev. 8 0 , 1 0 9 5 .  Cameron, A. ( 1 9 5 7 ) Atomic Energy of Canada, A.E.C.L. No k$k Report No C R L A l . C h u r c h i l l , Jones, and Hunt, ( 1 9 5 3 ) Dosso, H., ( 1 9 5 7 )  9  Nature 1 2 2 , ^ 6 0 .  M.A. Thesis, University of B r i t i s h Columbia.  Doyle, W. and Robbins, A., ( 1 9 5 6 )  Phys. Rev. 1 0 1 ,  DuBridge, S., Barnes, S., Buck, J . and S t r a i n , C , Rev. £ 3 , hh-7. Edwards, M., ( 1 9 5 0 )  1056. (1938)  Phys.  M.A. Thesis, University of B r i t i s h Columbia.  Hagedorn, F. and Marion, J . - Kellog Radiation Lab. Preprint. Hirschfelder, J . and Magee, J., ( 1 9 W Larson, E., ( 1 9 5 7 )  Phys. Rev. 23? 2 0 7 .  M.A. Thesis, University of B r i t i s h Columbia  Laubenstein, R. and Laubenstein, M., ( 1 9 5 1 )  Phys. Rev. 8k, 1 8 .  L a u b e n s t e i n , R., L a u b e n s t e i n , M., K o e s t e r , L. and Mobley, R., (195D Phys. Rev. 8]±, 12. S a l p e t e r , E., (1955) A s t r o p h y s i c a l J . 121, l 6 l . S c h a r d t , Fowler and L a u r i t s e n , (1952) Phys. R e v . 86, 527. S t e l s o n , P. and McGowan, F., (1955) Phys. Rev. 9_2, 112. S t e l s o n , P. and P r e s t o n , (195^) Phys. Rev. 21, 97^. T h i r i o n , J . , (1953) A n n a l e s de P h y s i q u e 8, *f89. Thomas, R. and L a u r i t s e n , T., (1952) Phys. Rev. 88, ^69. Van A l l e n , J . and S m i t h , N., (19^1) Phys. R e v . £9_, 6 1 8 Warren, J . , L a u r i e , K., James, D. and Erdman, K., (195^) Can. J . Phys. 32, 563. Wenzel, W. and W h a l i n g , W., (1952) Phys. Rev. 8Z, 1+99. W h a l i n g , W., (1957) K e l l o g R a d i a t i o n L a b o r a t o r y P r e p r i n t . Wong, C , ( 1 9 5 ^ ) Phys. Rev. 21, 761.  

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