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THE 0¹⁶(p,૪)F¹⁷ REACTION 1957

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THE 0 l 6 ( p ^ ) F 1 7 REACTION by LYLE PURMAL-ROBERTSON A THESIS SUBMITTED IN THE REQUIREMENTS MASTER PARTIAL FULFILMENT OF FOR THE DEGREE OF OF ARTS i n the Department of PHYSICS We accept t h i s t h e s i s as conforming t o the standard r e q u i r e d from candidates f o r the degree of MASTER OF ARTS Members of the department of Physics 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 capture of protons by 0 has been measured using i c e t a r g e t s of known th 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 the gamma ray t r a n s i t i o n t o the f i r s t e x c i t e d s t a t e i n F 1 ? was found to be (lO.W- - 1.3) x 10"32 c m 2 # p e r s t e r a d i a n at 90° t o the i n c i d e n t proton beam d i r e c t i o n . At the same energy, the r a t i o of 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 at 90° f o r t r a n s - i t i o n s to the 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 the f i r s t e x c i t e d s t a t e i n F 1 ? was found t o be Q»lh - 0 . 0 3 . The energy of the 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 by measuring the energy of the gamma ray from t h i s l e v e l t o the ground s t a t e . This method i s d i f f i c u l t because of the presence of 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 the same energy, w i t h i n experimental e r r o r s , from the decay of 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 no t i n g the d i f f e r e n c e between the capture gamma rays t o t h i s s t a t e and t o the ground s t a t e . The energy of t h i s l e v e l was found t o be 0 .50 ± 0 .01 Mev. i n agreement w i t h the r e s u l t s of Marion 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 t h i s l a b o r a t o r y (Warren e t . a l . , (5k), An attempt t o confirm that the source of the 873 kev. r a d i a t i o n from proton bombardment of n a t u r a l oxide t a r g e t s above 1.8 Mev. bombarding energy was the 0 ^ ( p , p ' j ^ J O 1 ^ r e a c t i o n , was made using separated O 1^ and 0 1 ? t a r g e t s . The r e s u l t s were incon - c l u s i v e due to the small percentage of oxygen that stuck t o the ta r g e t s and to the presence of s e v e r a l contaminants. I n p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f t h e r e q u i r e m e n t s f o r a n a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d b y t h e H e a d o f my D e p a r t m e n t o r b y h i s r e p r e s e n t a t i v e . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f PŴ s>cs T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , V a n c o u v e r 8, C a n a d a . D a t e November 1957 ACKNOWLEDGEMENTS The author wishes to thank Dr. G.M. G r i f f i t h s f o r h i s d i r e c t i o n of t h i s work and of the t h e s i s p r e p a r a t i o n and Dr. B.L. White whose a s s i s t a n c e and guidance w i t h the exper- i m e n t a l , work was most h e l p f u l . Dr. J.B. Warren i s a l s o thanked f o r h i s suggestions and d i s c u s s i o n s of the t h e s i s t o p i c . The d i r e c t i o n of Dr. C.A. Barnes during the e a r l y Work on 0 1 7 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. Larson f o r help i n operating the Van de Graaff generator 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 author a l s o wishes to 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 given by the N a t i o n a l Re- search C o u n c i l i n the form of a Bursary and a Studentship i s g r a t e f u l l y acknowledged. TABLE OF CONTENTS CHAPTER PAGE I INTRODUCTION 1 I I O l 6 ( p , ^ ) F 1 7 CROSS SECTION MEASUREMENTS 6 1 . Apparatus 6 (a) Target Arrangement 6 (b) Gamma Ray Detector 7 (c) E l e c t r o n i c s 1 0 2 . Target Thickness Measurements 11 3 . Experimental 16 (a) Background 16 (b) Procedure 17 (c) Measurement of the E f f e d t i v e Centre ....... 18 k. Cross S e c t i o n C a l c u l a t i o n s 20 (a) Carbon Contamination C o r r e c t i o n 2 0 (b) S o l i d Angle 21 (c) C a l c u l a t i o n of Cross S e c t i o n 22 (d) E r r o r s 2k 5. R e s u l t s 25 I I I ENERGY DETERMINATION .OF ^ 3 27 1 . I n t r o d u c t i o n 2 7 2 . Apparatus 2 8 (a) Target 28 (b) Detector and E l e c t r o n i c s 3 0 (b . 3. Experimental 3 0 (a) " ^ i and ^ ? 3 0 (b) Energy Determination o f Y o 36 (c) Gain S h i f t s hi k. R e s u l t s ^3 CHAPTER PAGE IV A LOOK FOR 0 1 7 ( p , ^ ) F l 8 GAMMA RAYS M f 1. I n t r o d u c t i o n M+ 2 . Experimental Procedure .. h3* (a) Gamma-Ray D e t e c t i o n System h$ (b) Targets *fo (c) The 872 Kev. R a d i a t i o n . . . . . . . ^6 (d) Contamination Spectra (e) Measurement of A n n i h i l a t i o n R a d i a t i o n ..... *+9 3 . Conclusions 50 APPENDIX - Mercury Relay Pulse Generator 53 BIBLIOGRAPHY ... 6 0 LIST OF ILLUSTRATIONS FIGURE FACING NUMBER SUBJECT. PAGE 1. F 1 7 L e v e l Scheme ... *f 2 . Target Chamber Arrangement .................. 6 3 . D 2 0 Ice Target Thickness C a l i b r a t i o n 13 D 2 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 . Spectra From Ice Targets ••••• 18 7. E f f e c t i v e Centre Determination 19 8 . 0 1 ^ ( p , ^ i ) F l 7 Gamma Ray Energies 31 9 . Energy Determination ..................... 37 10. Mercury Pulse Generator C i r c u i t s 53 11. M u l t i v i b r a t o r D r i ve C i r c u i t and D.C. Supply C i r c u i t 56 12. Pulse Shapeing ••• 57 CHAPTER I INTRODUCTION Shell model predictions indicate that the F^? nucleus can be considered as a single proton moving i n the potential of the doubly closed shell O1^ core. This system i s therefore a simple one to consider theoretically and should-lend i t s e l f to a quantitative test of shell model predictions. Comparison with i t s mirror nucleus 017 should indicate a one to one correspon- dence between level energies (corrected for coulomb and neutron- proton mass difference effects) and spin and parity values. The shell model predicts that the ground state of both 0̂ 7 and pi? should be a D5/2+ level consisting of the odd nucleon with i t s spin aligned parallel to the orbital angular momentum. Experimental results are consistent .with D^/2+ for both ground states. The ground state spin has been measured directly hy Alder and Yu (51) and the assumption that the pi? ground state spin i s 5/2 i s consistent with the allowedfi+ transition to 017 and with O ^ ^ n ) stripping data (Ajzenburg, 51). The f i r s t ex- cited state of the mass 17 system appears not to be the D3/2 member of the ground state doublet but to be an S1/2+ level of the odd nucleon. This i s consistent with present shell model predictions. The experimental evidence comes from stripping an- gular distributions of O^d.p)© 1? (Burrows et a l , 50) and 0 16(d,n)F 17 (Ajzenburg, 5D• Additional evidence for O1? spins i s a v a i l a b l e from angular c o r r e l a t i o n between protons and^-rays from the f i r s t e x c i t e d s t a t e ( T h i r i o n , 53) and from i n t e r n a l con- v e r s i o n measurements on the same"^ -rays which i n d i c a t e an E - 2 t r a n s i t i o n (Thomas and L a u r i t s e n , 5 2 ) . This consistency between the s h e l l model p r e d i c t i o n s and experimental evidence f o r l e v e l parameters of ground s t a t e and the f i r s t e x c i t e d s t a t e of F^ 7 suggests t h a t a c a l c u l a t i o n f o r the 01^(p,">b)F-1-7 r e a c t i o n parameters assuming t h i s simple con- f i g u r a t i o n f o r F^ 7, should give reasonable r e s u l t s . T h e o r e t i c a l c a l c u l a t i o n s f o r the cross s e c t i o n and angular d i s t r i b u t i o n of t h i s r e a c t i o n are being made by a C a l i f o r n i a I n s t i t u t e of Tech- nology group ( p r i v a t e communication from N. Tanner). A d i r e c t r a d i a t i v e capture process i s being assumed, c o n s i s t e n t w i t h the r e s u l t s of Warren et a l (5*+) t h a t the " ^ - r a d i a t i o n from'O^Cp,"^) was non-resonant i n the r e g i o n E p 0 . 9 t o 2 .1 Mev. Since a de- 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 accurate experimental r e s u l t s should provide a s e n s i t i v e check on the assumed models i t was f e l t t h a t more accurate data on the cross s e c t i o n and angular 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 value a t t h i s time. The 0 l 6 ( p ^ ) F 1 7 cross s e c t i o n i s a l s o of i n t e r e s t i n a s t r o p h y s i c s . I n hydrogen s t a r s w i t h c e n t r a l temperatures greater than about f i f t e e n m i l l i o n degrees K e l v i n , the main energy source i s the conversion of protons i n t o alpha p a r t i c l e s by means of the carbon-nitrogen c y c l e . I n t h i s process v a r i o u s isotopes of c a r - bon and n i t r o g e n act as c a t a l y s t s i n the conversion, as f o l l o w s : C 13(p,-t)N 1 1 + .> v 1 t r ^ I W—s-He*1- + 2(r + Q N 1 4 ' ( p , i S ) 0 1 5 — ^ N 1 ? where Q ^ 27 Mev. Nl5(p,60cl2 J Some of the c a t a l y s t i s l o s t from the c y c l e by the r e a c t i o n However the r e a c t i o n 0 l 6 ( p , i ) F l 7 , followed by Fl7(^3r)ol7, 017(p,<A)N1,f puts oxygen n u c l e i back i n t o the c y c l e . The 0l6(p,^)Fl7 cross s e c t i o n determine^,after a long p e r i o d , the carbon oxygen r a t i o i n the s t a r as a f u n c t i o n of the s t e l l a r temperature (Cameron, 57)• Since cross s e c t i o n measurements a t s t e l l a r energies are not 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 e x t r a p o l a t e d from experimental cross s e c t i o n s a t higher energies must be used. 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 reasonable accuracy i t i s necessary t h a t the energy dependence as w e l l as the absolute t h e o r e t i c a l cross 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 the high 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 by Sal p e t e r (55) and i n t e r - p r e t a t i o n of experimental r e s u l t s by Laubenstein and Laubenstein (5D assumed t h a t the r e a c t i o n proceded by compound nucleus f o r - mation through the t a i l s of resonances above h Mev. and a t 0.5 Mev. i n Fl7. However, the r e s u l t s of Warren et a l (5%) f o r the 0l°(pp£) r e a c t i o n i n d i c a t e t h a t the 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 capture of the protons, s i m i l a r t o the D(p,tf)He3 :reaction. (3A) (>/*-) ft///////////////////////////* Y//////W//////w,wm 3 . 5 6 1/2 + J = 5/2+ 4.73 4.35 llllllllllllllllh 3.10 . 499 .5 99 •+ 16 & p 17 ' .599 0 , 6 + p £ ' 7 I n t h i s case the energy dependence of the r e a c t i o n w i l l d i f f e r from that p r e d i c t e d by the compound nucleus assumption. The 0l 6(p,^)pl7 r e a c t i o n was f i r s t observed by measur- in g the a n n i h i l a t i o n quanta from the decay of F 1 7 (DuBridge et a l , 38)• Laubenstein et a l (5D measured the r e l a t i v e cross s e c t i o n as a f u n c t i o n of energy from 1.*+ t o * f . l Mev. by 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 cross s e c t i o n of protons was a l s o measured from 0.6 t o h.5 Mev. Warren et a l (51*-), i n t h i s l a b o r a t o r y , f i r s t ob- served the 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 the non-reso- nant character of the cross s e c t i o n f o r gamma emission i n the 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 capture process and not t o the e f f e c t of a broad low l e v e l nor the resonant l e v e l s of higher energy. The 0l6(p,^)F17 cross s e c t i o n was measured a t 1.90 Mev. and found t o be 6 ± 3 x 10"30Gm2. The lo,, energy l e v e l s and spins of F-1-7 obtained from previous work are shown i n Figure 1. I n the work of Warren et a l (5k), the f o l l o w i n g gamma rays were observed i n a d d i t i o n t o the a n n i h i l a t i o n r a d i a t i o n from the 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 , ' ' ^ , of energy 16/17 Ep + Q; a t r a n s i t i o n t o an e x c i t e d s t a t e a t about 0.5 Mev., ~̂ 2; and the t r a n s i t i o n from t h i s e x c i t e d s t a t e t o the ground st a t e of F 1 7,H^2. The observed Q value was c o n s i s t e n t w i t h 0.59^ Mev. deduced from the 0^(d,n)F 17 t h r e s h o l d measurements of B u t l e r (5D at a deuteron energy of I.63I Mev. (This has been corrected t o I.626 Mev. (Bonner, 55) thus p u t t i n g the Q at - 5 - 0.599 - .006 Mev.). Because of the need f o r a more accurate determination of the absolute cross s e c t i o n t o check the t h e o r e t i c a l c a l c u l a - t i o n s being performed by the C a l i f o r n i a I n s t i t u t e of Technology group, the d i f f e r e n t i a l cross s e c t i o n has been measured at 800 kev. i n the present work. The gamma ray energies have a l s o been remeasured i n order t o r e s o l v e some of the i n c o n s i s t e n c y i n the l i t e r a t u r e . Warren et a l (5*0 observed t h a t gamma r a d i a t i o n of 873 kev. appeared f o r energies greater than 1.8 Mev. during proton bombardment of 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 proton s c a t t e r i n g from the 873 l e v e l i n 0 1 7 . During the present work separated t a r g e t s of Q16 and 0 ^ 7 were bom- barded w i t h protons t o attempt t o con f i r m t h i s assignment. C O N N E C T E D TARC-ET " / CHAMBER f G-LAS5W0OL H20 DISPEM5G-R 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 l 6 ( p . ~ o ) F 1 7 CROSS SECTION MEASUREMENTS The aim of the present work has been t o measure the cross s e c t i o n of the O ^ C p ^ F 1 7 r e a c t i o n and the branching r a t i o f o r the two gamma rays from the c a p t u r i n g s t a t e t o an accuracy of about ±10 per cent a t 800 kev. proton bombarding energy, using an i c e t a r g e t whose th i c k n e s s could be measured reasonably a c c u r a t e l y . 800 kev. was chosen as the bombarding energy i n order t o minimize the e f f e c t of carbon contamination and s t i l l be able t o resolved) 2 from the 0 .51 Mev. a n n i h i l a t i o n r a d i a t i o n . 1. Apparatus. (a) Target Arrangement. The t a r g e t was d i s t i l l e d water f r o z e n onto a gold p l a t e d copper backing 1/16 i n . t h i c k which was attached t o the bottom of a l i q u i d n i t r o g e n c o l d t r a p . The t a r g e t chamber and stop arrangement are shown i n F i g u r e 2 . Since f l u o r i n e t a r g e t s had been used i n the chamber f o r the 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 the chamber i n the r e g i o n of the t a r g e t were gold plated w i t h a t h i c k n e s s s u f f i c i e n t t o stop 10 Mev. protons. The stops used, t o ensure accurate p o s i t i o n i n g of the beam on the t a r g e t , were gold p l a t e d copper t o reduce coulomb e x c i t e d gamma rays produced by other low atomic number m a t e r i a l s ( S t e l s o n and y McGowan, 55)• The t a r g e t and support were i n s u l a t e d w i t h a l u c i t e - 7 - r i n g t o enable beam current measurements t o be made; a p o s i t i v e p o t e n t i a l of 135 v o l t s was a p p l i e d t o the 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 the beam measurement. The beam current was i n t e g r a t e d w i t h a cu r r e n t i n t e g r a t o r (Edwards, 50). This 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 volta g e output i s a f u n c t i o n of the charge fed onto the i n t e - g r a t i n g condensers. The output of the m i l l e r i n t e g r a t o r operates a sehmidt t r i g g e r c i r c u i t ; the l e v e l at which 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 of charge on the i n t e g r a t i n g con- densers. Each time t h i s charge i s reached i t operates a r e l a y which discharges the condensers and d r i v e s a mechanical r e g i s t e r . Two sets of i n t e g r a t i n g condensers i n p a r a l l e l permit d i s c h a r g - ing of one set wh i l e maintaining the i n t e g r a t i o n c i r c u i t opera- t i v e . Thus no c o r r e c t i o n i s re q u i r e d f o r the recovery time of the c i r c u i t . The i n t e g r a t o r was c a l i b r a t e d by feeding i t w i t h a known current s u p p l i e d from a 100 v o l t b a t t e r y . The cu r r e n t was determined by measuring the voltage across a standard r e s i s t a n c e w i t h a Rubicon potentiometer. The 100 v o l t source was r e q u i r e d t o ensure th a t the v a r i a t i o n i n c u r r e n t , due t o the 0.5 v o l t swing on the in p u t of the 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 . This measurement i n d i c a t e d t h a t over a one year period the i n t e g r a t o r c a l i b r a t i o n was constant t o w i t h i n one per cent. (b) Gamma Ray Detector. 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 . diameter 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 as a gamma ray d e t e c t o r . Dow Corning 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 as an o p t i c a l c o u p l i n g between the c r y s t a l and the 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 rubber sleeve held the p h o t o m u l t i p l i e r and c r y s t a l together and pre- vented the s i l i c o n e o i l from 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 noticed over a period of one and a h a l f years. P i t t e d w i t h a mu-metal s h i e l d , the p h o t o m u l t i p l i e r w i t h the c r y s - t a l was mounted i n a 3*5 i n . brass tube which a l s o housed the p r e a m p l i f i e r • The e f f i c i e n c y of the l a r g e c r y s t a l f o r gamma rays has been measured at 6.1k Mev. and at 1.17 and 1.33 Mev. The 6 Mev. measurement was c a r r i e d out using the Fl9( P )o(^>)ol6reaction a t energy 31+0 kev. (Larson, 57). The method was f i r s t suggested by Van A l l e n and Smith (^1). The apparatus used i n the present measurements was developed by G.M. G r i f f i t h s a t the Cavendish Laboratory, Cambridge and the measurements were made by E. Larson and the author and are more f u l l y reported by Larson (57). A t h i n t a r g e t was bombarded w i t h 3̂ 0 kev. protons and the <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 counter w i t h an a c c u r a t e l y known s o l i d angle subtended at the t a r g e t . Since both the alpha p a r t i c l e s and t h e ^ - r a y s are emitted i s o t r o p i c a l l y and there i s one 6,1k Mev. gamma ray f o r each alpha p a r t i c l e counted, a measurement of the number of alpha p a r t i c l e s g ives a measure of the number of 6.1^ Mev gamma rays simultaneously emitted by the t a r g e t . A c o r r e c t i o n must be made f o r the 2.3 per cent y i e l d of 6.9 and 7.1 Mev. gamma rays f o r which the c o r - - 9 - responding alpha p a r t i c l e s are not s u f f i c i e n t l y energetic t o count i n the alpha counter (Dosso, 57)• P. Singh and H. Dosso (57) i n t h i s l a b o r a t o r y have measured the e f f i c i e n c y f o r Co^° gamma rays using a source c a l i b r a t e d by the N a t i o n a l Research C o u n c i l of Canada. P. Singh has estimated 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 experimental e r r o r s placed on the Co^O source. The agreement between t h e o r e t i c a l c a l c u l a - t i o n s and measured e f f i c i e n c y f o r 6 Mev. gamma rays i s w i t h i n 5 per cent. Extending the estimates to lower energies i s considered reasonable because of the good agreement i n these upper ranges. A more exact e f f i c i e n c y value i n t h i s low r e g i o n i s expected from pending r e s u l t s i n C o ^ coincidence work. For 800 kev. proton bombardment of oxygen, the O ^ ( p ^ ) F 1 7 gamma r a y s , " ^ ^ and""^* have energies of 1.32 and 0 . 8 1 Mev. r e s p e c t i v e l y . The e f f i c i e n c y f o r gamma r a y s , £(E, E b ) , d e f i n e d as the r a t i o of the number of counts of a gamma ra y of energy, E, above a given b i a s energy, E^, t o the number of gamma rays i n - c i d e n t on the c r y s t a l , has been obtained f o r these gamma rays by considering the 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 the de- t a i l e d shape of the experimental gamma ray s p e c t r a . Since the d e t a i l e d shapes of t h e ^ i and "^2 spectra were not a c c u r a t e l y ob- served i n the experiment due t o the presence of s e v e r a l gamma ra y s , a d d i t i o n a l comparisons were made w i t h the spectra from N a 2 2 and C s 1 ^ 7 . The f o l l o w i n g r e s u l t s r e l e v e n t t o the 0 1^(p7^)F 1 7 r a d i a t i o n s were obtained; for~ 6 \ the e f f i c i e n c y i s €. ,(1.32, 1.0) + MI. TEST IN 300 V -I M i - 4 7 0 /3KV IM 1 0 0 0 • 330K 4 7 0 / 3 K Y •HH-e I 0 0 K I 0 O 6 K WW I0O / 3 K V COUECTOR ^ 3 3 0 K 4 7 0 / 3 K V < •iHhj-4 > 3 3 0 K 4 7 0 / 3 K V ? • 'Hr4 -€ > 3 3 0 K ' 4 7 0 K I i 6 J 6 TRIODES PARALLELED T D U M O N T 6 5 6 3 5 K I.5K IOOO y IN 34 33 n -ll 9"' 6 5 0 ^ 1 ^ OUT PUT i)Lo D Y N O D f r S 270J\ 3 3 0 K P0CU55ING C R I D 11 < ~T" PUOTOCATWODE ON 2.5 x 3.5 CRYSTAL ( A L L CAPACITIES IN jjuf UNLESS SPECIFIED) 4- 250 V TEST IK) 9 R C A 6 3 4 2 ( COLLECTOR 150 K IOO /3KV IOOK OYNODE SUPPLY A5 ft-BOVE _0 OUT PUT PULSE TRANSFORMER. PT-3300 I R&fc5 D£LkY LINE- Ro = IK 6A.K5 ,25 uf ON SMALL CRYSTAL DETECTOR t I G U I X 5 PUOTOMULTIPLICR Ut/VD UMPLICIkR 10 - = (38 * 5) per cent and f o r ~ ^ 2 t h e e f f i c i e n c y i s £ 2(0.8l, 0.65) = (31 * *+) per cent. A 1.75 i n . by 2 i n . 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 was a l s o a v a i l a b l e , but the 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. A l s o , being l a r g e r i n area, f o r a g i v e n s o l i d angle the l a r g e r coun- t e r could be placed f u r t h e r from the t a r g e t so t h a t e r r o r s due to the t a r g e t t o c r y s t a l d i s t a n c e measurements and i n the po- s i t i o n i n g of the beam on the 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 presented about the same s h i e l d i n g problem t o both c r y s t a l s . (c) E l e c t r o n i c s . The 1000 v o l t s f o r the p h o t o m u l t i p l i e r dynode c h a i n was supplie d by an Isotopes Development L i m i t e d s t a b i l i z e d power supply. 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 w i t h a diode c l i p p e r t o l i m i t the s i z e of la r g e cosmic r a y pulses. (Figure 5). The negative pulses from the p r e a m p l i f i e r were fed to a Northern E l e c t r i c wide band a m p l i f i e r Type l W f then t o a biased a m p l i f i e r . The output from the biased a m p l i f i e r was fed to a Marconi 30 channel k i c k s o r t e r . The k i c k s o r t e r channel edges were set up by feeding pulses from the mercury pulse generator (Appendix I I ) onto the g r i d of the 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 the components other than the 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 the k i c k s o r t e r w i t h the a c - c u r a t e l y l i n e a r pulse generator. The pulses fed i n t o the head a m p l i f i e r a l s o checked the s t a b i l i t y of the e l e c t r o n i c s . The 1 1 - e l e c t r o n i c s w i t h the exception of the p h o t o m u l t i p l i e r proved t o be adequately s t a b l e . 2 . Target Thickness Measurements. I n order t o measure the absolute cross s e c t i o n t o an accuracy of about 10 per cent, the number of t a r g e t n u c l e i per square centimeter must be known t o an accuracy b e t t e r than 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 centimeter a system f o r producing 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 pressure was measured by an o i l manometer was con- densed onto a cooled copper support attached t o a l i q u i d n i t r o - gen r e s e r v o i r . The thickness of the t a r g e t as a f u n c t i o n of pressure of water vapour i n the dispenser was determined i n terms of the energy l o s s of 3 ^ 0 kev. protons i n passing through the i c e l a y e r . This was found by measuring the c e n t r o i d s h i f t of the 3^0 kev. Fl9(p.of^S) resonance when the i c e l a y e r was con- densed on a t h i n f l u o r i n e t a r g e t . The d i s p e n s i n g system was c a l - i b r a t e d f o r heavy water, 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 ord i n a r y water s i n c e the number of molecules of gas con- t a i n e d i n the dispenser f o r a given pressure i s the same f o r both gasses. N e g l i g i b l e d i f f e r e n c e s could have r e s u l t e d from the d i f f e r e n c e i n the heat of f u s i o n and the Van der Waal 1s constant of the two gases. Using the values of the molecular stopping cross 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 molecules per square centimeter - 12 - as a f u n c t i o n of manometer pressure can be determined from meas- urements of the energy l o s s of protons i n passing through the i c e l a y e r f o r t a r g e t s corresponding t o s e v e r a l d i f f e r e n t mano- meter readings. The dispenser, Figure 2 , was made of g l a s s w i t h g l a s s taps ground t o give a vacuum t i g h t f i t . G c t o i l Vacuum Pump G i l , (vapour pressure 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 high vapour pressure i m p u r i t i e s , was used i n the manometer. The f l u o r i n e t a r g e t s were prepared by evaporating calcium f l u o r i d e onto cleaned copper sheets ( l a r s o n , 57) ; these were indium soldered onto a 1/16 i n . t h i c k copper support attached t o the l i q u i d n i t r o g e n r e s e r v o i r . In order t o make a reproduceable t a r g e t the f o l l o w i n g procedure was adopted. The manometer was evacuated t o the s i d e arm pressure w i t h taps T i and T 2 open. See Fig u r e 2 . With T i and T 2 e l o s e d , T3 was opened u n t i l the d e s i r e d pressure of water vapour was reached and T3 was c l o s e d . When e q u i l i b r i u m was es- t a b l i s h e d i n the manometer, the temperature of the a i r and the manometer reading were recorded. The t a r g e t assembly was r o - ta t e d so t h a t the cooled copper p l a t e faced the o u t l e t of the dispenser. Tap T i was opened c a r e f u l l y t o l e a k the water s l o w l y i n t o the t a r g e t chamber and the manometer read a g a i n when the water had been e x p e l l e d from the dispenser. T^ was then c l o s e d . The side arm pressure measured at the magnet box was a p p r o x i - mately 10"^ mm. of mercury. I f the pressure rose higher than Protons on Fluorine Protons on Fluorine Proton E n e r g y KEV. Fig. 3 - L\0 Ice T a r g e t Th ickness C a l i b r a t i o n - 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 readings of 10 - 20 cm. of o i l was lower. The vapour pressure of water at 20°C corresponded t o a manometer reading of 2h cm. By using pressures of water l e s s than the vapour pressure, e r r o r s due to condensation on the w a l l s of the dispenser were reduced. The l i n e a r i t y and absolute c a l i b r a t i o n of the Van de Graaff energy s c a l e measured by the generating voltmeter were determined by no t i n g the p o s i t i o n s of the 0.22^, 0.3J+0, 0.*+80 and the 0.669 Mev. resonances of F 19(p,^,~ 1^)o 1^. F i g u r e 3 shows the c e n t r o i d s h i f t of the 3*+0 kev. r e s o - nance. An i c e l a y e r corresponding t o a water pressure of 13.6 cm. of o i l was layed down on the f l u o r i n e t a r g e t . The increased width of the resonance f o r the t a r g e t w i t h the i c e l a y e r i s due t o the non-uniformity of the t a r g e t which amounts t o about 10 kev. Proton beams of 0.5 microamperes were used during these c a l i b r a t i o n runs. Currents higher than 1 microampere caused d e t e r i o r a t i o n of the t a r g e t s which appeared as a broadening of the resonance curve. 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 area under the resonance curve r e - mained constant the 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 of 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 the mean heavy i c e thickness or a d e t e r i o r a t i o n of the f l u o r i d e t a r - get. (20 -I M/VNOME-TtR at\DIN& P IN CM P O R E 4 0 , 0 D I 5 P E N S & R C A - L I P R A T I O W -11+ - The r e s u l t s of the measured energy l o s s of protons i n the i c e t a r g e t s as a f u n c t i o n of the manometer r e a d i n g , P, are shown i n Figure *+. To determine from t h i s data the number of heavy i c e molecules per square centimeter on the t a r g e t a know- ledge of the molecular stopping cross s e c t i o n a t these energies i s r e q u i r e d . Wenzel and Whaling (52) have measured the molec- u l a r stopping power,^0^0, of heavy water as a f u n c t i o n of energy. 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 experimental e r r o r w i t h the t h e o r e t i c a l values 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) using the s e m i - e m p i r i c a l theory of Bethe (37) f o r the stopping power. Whaling (57) has c o l l e c t e d data f o r hydrogen and oxygen and f o r water which give s l i g h l y higher values than the experimental measurements on heavy water f o r the proton en- ergy range 300 t o 500 kev. I n t h i s work the measured stopping cross s e c t i o n s of Wenzel and Whaling were used f o r the c a l c u l a - t i o n of the number of molecules per square centimeter. The molecular stopping cross s e c t i o n as a f u n c t i o n of proton energy, E, i n the r e g i o n from 3*fO kev. t o *f60 kev. can be approximated b y C ( E ) = A + B(E) where E ^ E 1 - 3*K) kev. and E' i s the bombarding energy. This f u n c t i o n was f i t t e d t o the experimental curve given i n Wenzel and Whaling (51) a t 36O and ^30 kev., w i t h the r e s u l t t h a t CT ( l l f . 9 - 0.0238 E) l O " 1 ^ ev.-cm./ molecule of D 2 0 . Now the molecular stopping cross s e c t i o n i s d e f i n e d as - 1 dE N dx - 15 - where N i s the number of molecules per cubic centimeter. Then I d x — ~\>j <JE where E i s the t o t a l energy l o s s f o r protons passing through a d i s t a n c e x of heavy water and b i s a constant. Assuming th a t the thickness of i c e i s p r o p o r t i o n a l to the dispenser manometer r e a d i n g , then P — <=: J ~ where c i s a p r o p o r t i o n a l i t y constant. S u b s t i t u t i n g f o r C p = j - f E a E CB A ' This f u n c t i o n i s p l o t t e d i n Figure h f o r the case of CB = - 0 . 0 1 0 7 , chosen t o f i t the experimental p o i n t s . From t h i s curve one can determine the water t a r g e t thickness i n kev. f o r protons i n the energy r e g i o n j u s t above 3M) kev. With t h i s and the molecular stopping cross s e c t i o n f o r heavy water (Wenzel and Whaling, 52) the number of molecules per cubic centimeter, nD 2 0 , was obtained. HD 9 0 = T x 103 . XT where T i s the t a r g e t thickness i n kev, O " i s the molecular stopping power f o r Ep = 3*+0 + T/2 i n ev.-cm 2/molecule. - 16 - Ice t a r g e t s f o r the oxygen cross s e c t i o n measurements were made using d i s t i l l e d water from the U.B.C. Chemistry De- partment . 3* Experimental. (a) Background. Accurate measurement of 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 the lower the gamma energy because the background r i s e s q u i c k l y w i t h decreasing 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 the concrete (K 4° l,h Mev., RdTh 2.62 Mev.), r e a c t i o n s due t o contaminants i n the t a r g e t , and machine X-ray background. Since these are r e l - a t i v e l y low energy r a d i a t i o n s , approximately h i n . of lead i s s u f f i c i e n t to reduce t h i s background t o a reasonable l e v e l . When running at 800 kev. machine energy, however, the counter must be shielded on a l l s i d e s t o reduce r a d i a t i o n from the room. Therefore, h i n . of lead was placed around the counter and the ta r g e t chamber so tha t none of the room was v i s i b l e to the coun- t e r . With the arrangement used the time dependent counting r a t e was 60 eounts per minute i n the energy range 0.5 t o 3 Mev. Lead and p a r a f f i n blocks were placed between the mag- net box and the counter to reduce background from the (d,n) r e a c t i o n s i n the magnet box. Proton bombardment of carbon contamination on the t a r - get produced 2.37 Mev. gamma rays from c l 2 ( p ^ ) N l 3 and a n n i h i l a - - 17 - t i o n r a d i a t i o n from the subsequent p o s i t r o n decay of the N . The assignment of the gamma ray to C-1-2 was c o n s i s t e n t w i t h the energy measurement of 2 . 3 6 - .0*f Mev. and the p o s i t i o n of a r e s o - nance f o r t h i s gamma ray at h-60 kev. Some of the carbon contam- i n a t i o n was present i n the gold p l a t e d backing but the l a r g e s t source was from the hydrocarbon pump o i l s and vacuum greases used i n the vacuum system which 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 the 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 which condensed on the cold t a r g e t during the runs. Because of the presence of i p the C ^ contamination, i t was decided not t o measure an absolute cross s e c t i o n a t higher energies 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 cross s e c t i o n measurement as a f u n c t i o n of proton energy using a s t a b l e oxide t a r g e t which could be heated t o pre- vent carbon d e p o s i t i o n . Then a comparison of the y i e l d s at 8 0 0 kev. proton energy from the i c e t a r g e t of known thic k n e s s and from the oxide t a r g e t , would determine the oxide t a r g e t t h i c k n e s s . This work has not yet been completed. (b) Procedure. Protons of 8 3 0 kev. produced by the U.B.C. Van de Graaff generator 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 generating voltmeter and analyzed by the 9 0 ° d e f l e c t i o n magnet was known to w i t h i n about - 3 kev. With one t a r g e t , measurements were made at three 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 angle due t o the f a c t t h a t the angular d i s t r i b u t i o n o f ^ w a s n o ^ i s o t r o p i c but of the form a(b + S i n 2 6 ) . The t a r g e t was p o s i t i o n e d at *+5° t o the beam looo MOO ~ I 1 i ; 1 1— 20 25 PIQURE 6 SPECTRA- EROM.ICE T A T O 5 - 18 - a f t e r l a y i n g down the i c e so t h a t the. r a d i a t i o n was not a t t e n - uated by the copper support. The angular p o s i t i o n of the t a r g e t was obtained from a degree c i r c l e attached t o the t a r g e t chamber. The e r r o r i n angular 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 of - 1 per cent i n the number of atoms per square centimeter seen by the 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 of h t o 5 microamperes of protons were used. The beam was defocussed so that the stop system defined the s i z e and p o s i t i o n of the beam spot. No d e t e r i o r a t i o n of the t a r g e t was observed even a f t e r being subjected t o t o t a l beams of 10^ microcoulombs other than f o r the b u i l d up of some carbon on the surf a c e . Spectra were recorded which i n c l u d e d " ^ ]_ and ^2 an(* also'w'i, a n d t n e 2 » 3 7 Mev. r a d i a t i o n from C 1 2(p,"^). This per- mitted the background due t o carbon contamination, which i n - creased i n th i c k n e s s w i t h the amount of bombardment, t o be e s t i - mated. P e r i o d i c checks of the channel p o s i t i o n s of the k i c k - s o r t e r f o r each s p e c t r a l range showed the e l e c t r o n i c s were s t a b l e to b e t t e r than 0 . 5 per cent. (c) Measurement of the e f f e c t i v e centre a t 0 .51 and 1.28 Mev. A knowledge of the e f f e c t i v e centre p o s i t i o n f o r the gamma r a y energies measured was r e q u i r e d . Since the gamma rays are absorbed throughout the 3 . 5 i n . long d e t e c t o r c r y s t a l , measure- ments of the counting r a t e w i l l not show an in v e r s e square depend- ence on the d i s t a n c e measured from a source t o the end of the  - 19 - c r y s t a l . However exp e r i m e n t a l l y i t has been shown th a t i f d i s - tance measurements are made from the source t o a point i n s i d e the c r y s t a l which we c a l l the e f f e c t i v e centre the inve r s e square r e l a t i o n between counting r a t e and d i s t a n c e w i l l h o l d , except f o r di s t a n c e s of the same order as the c r y s t a l dimensions. This point can be thought of as the mean di s t a n c e i n s i d e the c r y s t a l at which absorption takes p l a c e . This permits one t o defi n e an e f f i c i e n c y which i s independent of d i s t a n c e . A l s o 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 dist a n c e t o the e f f e c t i v e centre depends on the gamma ray energy since the 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 centre was determined e x p e r i m e n t a l l y f o r gamma r a y using a N a ^ source and measuring the counting r a t e as a f u n c t i o n of d i s t a n c e from the end of the c r y s t a l f o r both the 0.51 Mev. and the 1.28 Mev. gamma r a y s . A N a 2 2 source contained i n a 0 . 2 5 i n . diameter aluminum rod was placed i n the t a r g e t chamber at the p o s i t i o n where the beam s t r u c k the i c e t a r g e t and the count- ing r a t e of the 0.51 and 1 . 2 8 Mev. gamma rays f o r d i f f e r e n t counter to.source d i s t a n c e s were measured. The r e s u l t s are shown i n Figure 7 . From the 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 dis 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 centre f o r 0.51 and 1 . 2 8 Mev. r a d i a t i o n s were I . V 3 2 - . 0 8 i n . and l,k? - . 1 0 i n . r e s p e c t i v e l y . These are t o be compared t o the 1 . 6 0 - . 1 0 i n . found f o r the e f f e c t i v e centre d i s t a n c e at 6.1M- energies c l o s e t o those obtained - 20 - Mev. To w i t h i n the probable e r r o r the e f f e c t i v e centre d i s t a n c e f o r the three energies are the same. An estimate of the e f f e c t i v e centre p o s i t i o n may be made assuming t h a t t h i s i s the d i s t a n c e , x, from the f r o n t face of the c r y s t a l such t h a t h a l f the t o t a l a b s o r p t i o n takes place i n f r o n t of x (assuming a plane 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 processes and lo s s e s through the 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 the 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 i s the le n g t h of the c r y s t a l . The r a t i o s ^ . l ^ f * x 1 . 2 8 s x o ^ l a r e 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 . This d i f f e r e n c e has not been accounted f o r and f u r t h e r measurements are needed t o determine the cause of the discrepancy. k. Cross S e c t i o n C a l c u l a t i o n s . (a) Carbon Contamination C o r r e c t i o n . Since the number of 2.37 Mev. gamma rays from C^ 2(p ,^) N 13 depends on the d u r a t i o n of the bombardment, and because much of t h i s contamination evaporated at the same time t h a t the t a r g e t was allowed t o evaporate, beam dependent backgrounds taken w i t h no i c e on the t a r g e t d i d not give a true measure of the C 1 2 back- ground e f f e c t . Therefore, i n making background c o r r e c t i o n s the f o l l o w i n g procedure Was adopted. F i r s t , the beam dependent background was subtracted under each of the peaks (from 0 .652 t o 0.995 Mev. i n the case ofY2, and from 0 .995 t o l.V? Mev. i n the case oftfj). I n the case where the spectra included the C I 2 K^-rays - 2 1 - the background was a l s o subtracted from the 2.37 Mev. peak. The residue i n the spectrum between 1.̂ 5 and 2.5 Mev. w i t h beam and time dependent background subtracted was assumed t o be due t o b u i l d up during the run. On the b a s i s of the 2.37 Mev. gamma ray spectrum shape, an 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 subtracted from theH^i and 1 ^ regions of the spectrum. The proportions subtracted were estimated from a C-*-2 spectrum taken p r e v i o u s l y during contamination checks. This G^2 c o r r e c t i o n was s i g n i f i c a n t i n the case of ifi but not i n the case of 1̂ 2 which had a much higher y i e l d . (b) S o l i d Angle the i c e t a r g e t s the counts per i n t e g r a t o r fovo^ followed an i n - verse square p l o t as a f u n c t i o n of the d i s t a n c e t o the e f f e c t i v e centre measured f o r the .51 Mev. gamma rays t o b e t t e r than 3 per cent. This i n d i c a t e s t h a t the s o l i d angle subtended by the coun- t e r during the cross 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 the sin^component o f o ^ does not s i g n i f i c a n t l y a f f e c t the c a l c u l a t i o n of the cross s e c t i o n i f constant 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 2 6 y i e l d over the counter area at the e f f e c t i v e centre f o r the di s t a n c e used above gives a r e s u l t d i f f e r i n g by the order of 3 per cent from that of an i s o t o p i c d i s t r i b u t i o n . From runs taken a t three counter d i s t a n c e s on one of - 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 of runs taken on each t a r g e t were summed and the cross s e c t i o n estimated f o r the two cases s e p a r a t e l y and then compared. The c a l c u l a t i o n s f o r one of the t a r g e t s are given below. The d i f f e r e n t i a l c ross s e c t i o n i s defined by: do" = _1_ ^ * 1 . r£ . 1_ . 1_ dSV, <*i ( d i A ) n p n Q where £jA i s the e f f i c i e n c y times area of the face of the coun- t e r r i s the t a r g e t t o e f f e c t i v e centre d i s t a n c e np i s the number of protons i n c i d e n t on the t a r g e t c<\ i s the t r a n s m i s s i o n c o e f f i c i e n t through the 1/16 i n c h brass w a l l s which i s .916 f o r .8 Mev., .935 f o r 1.3 Mev. gamma rays no i s the number of G x atoms per square centimeter With a manometer reading of 18 .75 centimeters of 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 of D2O was 107 kev t o 390 kev protons as read from the c a l i b r a t i o n curve F i g u r e k» The stopping cross s e c t i o n per D 2 0 molecule f o r protons of t h i s energy as gi v e n by Wenzel and Whaling (52) i s ^ 0 2 0 * 1 3 . 5 x 10""^ ev-cm 2. Therefore, the number of oxygen atoms of mass 16 per centimeter seen by the 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 at ^5° t o .the beam, i s given by - 23 - n 0 = f . 1 0 7 x l 0 3 x Jl 13.5 x 10-15 = 1.118 x 10^9 atoms per cm 2 where f i s r e l a t i v e n a t u r a l abundance of G^= .9976 n p = 95 i n t e g r a t o r counts x 107.2 microcoulombs 1.602 x l O " 1 3 r = 3.5*+ inches Nj2 = 7982 counts above a b i a s of 1.0 Mev., a f t e r background c o r r e c t i o n s A = ^.909 i n . 2 (do-) . c £ 2 = 7982 x ( 3 . 5 * f ) 2 x 1.602 x lo" 1 3 x 1 (<*A^90° .916 *f.909 95 x 107.2 1.118 x 1 0 1 9 = 3*15 x 10~ 3 2 c e n t i m e t e r s 2 per s t e r a d i a n P u t t i n g i n the e f f i c i e n c y of 3i per cent t h i s gives the d i f f e r - e n t i a l cross s e c t i o n (d£2) ='0.2 x 10 " 3 2 c m 2 p Q r s t e r a d i a n . (dr\V90° The r a t i o of the r e l a t i v e y i e l d s at 90 degrees of ̂  and T ^ i s c a l c u l a t e d as f o l l o w s : (d_£T) 4 ( d ^ * l \ = . J_2 . ±2 (d£!) = .1751 . JL2 • .916 £ 1 .935 P u t t i n g i n the e f f i c i e n c i e s .38 f o r f ^ and .31 f o r £ 2 we get the r a t i o of the number of ^ 2 . t o V 2 observed at 90 degrees as O.lh. - 2k - (d) E r r o r s The sources of e r r o r i n the d i f f e r e n t i a l cross s e c t i o n measurement of Tf2 a r e t a b u l a t e d below. Source Probable E r r o r Probable E r r o r i n Cross S e c t i o n Measurement n, c a l i b r a t i o n of dispenser 1.5! a~T>20 k'/v Angular p o s i t i o n of ta r g e t 1% e f f e c t i v e centre and - . 0 2^1 beam p o s i t i o n ±*05%) ±12% ±12% ±k.\ 12^ The probable e r r o r i n the number of counts from 0 2 per i n t e r g r a t o r count estimated from the c o n s i s t e n c y between the runs on a given t a r g e t give ± 2% f o r the 107 kev. t a r g e t and - 3.7 per cent f o r the 93 kev. t a r g e t i n reasonable agreement w i t h the s t a t i s t i c s on the gamma-ray counts i n each run. The probable e r r o r i n the r a t i o of the cross s e c t i o n s £or~^i and>u^2 c a l c u l a t e d from the consistancy of the r a t i o of N^]_ t o Ny 2 f o r a l l runs i s ± 1 . 3 per cent. With the e r r o r of 17 per cent i n the r a t i o £ito£ 2 , t h i s gives a r e s u l t i n g probable e r r o r of 17 per cent i n the r a t i o of 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 s . - 25 - 5. R e s u l t s . The d i f f e r e n t i a l cross s e c t i o n was measured using pro- tons of i n c i d e n t energy 830 -5 kev. Since the protons l o s e energy i n t r a v e r s i n g the i c e ta r g e t the mean proton energy at which the r e a c t i o n occurs i s a b e t t e r i n d i c a t i o n a t what energy the r e a c t i o n was measured. Assuming the O^Cp^F^ cross s e c t i o n i s r e l a t i v e l y constant f o r protons of energy over the range appearing i n the t a r g e t , then Ep = Ep + A where Ep i s the i n c i d e n t proton energy i n kev. and 2 A i s the t a r g e t t h i c k n e s s i n kev. For the t a r g e t s used i n the cross s e c t i o n measurement the thic k n e s s was approximately 60 kev, f o r 800 kev. protons; th e r e f o r e Ep = 800 ±10 kev. The r e s u l t s of the measurements on the two d i f f e r e n t i c e t a r g e t s gave the d i f f e r e n t i a l cross s e c t i o n at 90 degrees of "i>2 a s ^ , I f ~ x 10~ 3 2 cm 2 per s t e r a d i a n . Using the angular d i s t r i b u t i o n of "0̂  found by Warren et a l (55) as being p r o p o r t i o n a l t o 1 + 5 sin 2© a t 1.9 Mev. f o r the angular d i s t r i b u t i o n a t 800 kev., the 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 angles i s given by: ^ 2 - 7̂7 (1+2/3 x 5) x (d£s) . ccx 6 ( d R ) 90° • • *2 = 9 . S x 10-31 c m 2 format 800 kev. The r e l a t i v e d i f f e r e n t i a l cross s e c t i o n a t 90 degrees of ix t o "^2 i s ° ' l l f i o » 9 3 - 26 - The t o t a l r e l a t i v e y i e l d s , again u s i n g the angular d i s t r i b u t i o n f o r i f 2 a - f c 1#90 Mev i s given by < H l 6 x 0.1̂  + 0.19 <5-fc2 1+2/3.5 From t h i s and the value f o r ^ t 2 , <Til i s l . S x 10""-3 . cm. The i n t e n s i t y r a t i o of "B^ t o " ^ i s s i g n i f i c a n t l y higher than .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 before these r e s u l t s can be a c c u r a t e l y compared w i t h the e a r l i e r work, angular d i s t r i b u t i o n s at 800 kev. need t o be meas- ured. - 27 - CHAPTER I I I ENERGY DETERMINATION OF 1. I n t r o d u c t i o n . The measurement of the gamma ray energy from the ex- c i t e d l e v e l at approximately 0.5 Mev. i n O1^ i s of i n t e r e s t be- cause of d i s c r e p a n c i e s between v a r i o u s previous measurements of t h i s energy. A knowledge of the p o s i t i o n of t h i s l e v e l i s neces- sary f o r the c a l c u l a t i o n s of the cross s e c t i o n at low energies. 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 measure- ment i n d i c a t e t h a t e i t h e r the accuracy of some methods are not as good as supposed or the l e v e l s t r u c t u r e i s more complex than b e l i e v e d . The l e v e l energy was f i r s t measured by F.A. Ajzenberg (5D f o r the r e a c t i o n 0-^(d,n). The neutron angular d i s t r i b u t i o n was measured by photographic p l a t e technique. From the energy se p a r a t i o n of the two neutron groups the l e v e l was placed at 536 - 10 kev. Previous work done i n t h i s l a b o r a t o r y (Warren et a l ) i n d i c a t e d that the l e v e l was l e s s than 0.51 Mev. above the ground s t a t e by d i r e c t measurement of the energy of T n e energy was estimated t o be k&7 ± 15 kev. P r e l i m i n a r y unpublished work by Bonner agreed more c l o s e l y w i t h Ajzenberg (Ajzenberg and L a u r i t s e n , 55). However, as a r e s u l t of more accurate measure- 7 7 ment of the t h r e s h o l d f o r L i (p,n)Be , Bonner and Marion (55) measured the energy as *+99 - 3 kev. again using ©^(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 - hold neutrons were detected by making f a s t - s l o w neutron r a t i o measurements as a f u n c t i o n of deuteron energy, w i t h an estimated probable e r r o r of - 0.006 kev. Doyle et a l (56) using the N l l f ( c^,n)F*T r e a c t i o n give 0.53 * 0.0*+ Mev. as the l e v e l energy; the high probable e r r o r would support e i t h e r of the previous de- te r m i n a t i o n s , not only Ajzenberg's r e s u l t s as i n d i c a t e d by Doyle. The determination of the energy o f " o ^ was repeated, because of t h i s l a t e s t r e p o r t , and to check t h a t the e a r l i e r r e s u l t s obtained were reproduceable. 2. Apparatus. (a) Target. Because of the d i f f i c u l t y i n s h i e l d i n g ttie counter and t a r g e t system w i t h lead when the i c e dispenser was used and i n reducing carbon contamination on the c o l d t a r g e t support, i t was decided t a use s o l i d o x i d i z e d metal t a r g e t s . Tungsten was used because of the low energy of the coulomb e x c i t e d s t a t e s at 112 kev. ( S t e l s o n and McGowan, 55)• K X-rays (66 kev. f o r tungsten) and bremstrahlung would be present 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 tungsten p l a t e 0 .020 i n . by 1 i n . by 0 .75 i n . was cleaned i n potassium hydroxide (20 per cent s o l u t i o n ) etched, and r i n s e d thoroughly w i t h d i s t i l l e d water. The tungsten was o x i d i z e d by suspending i t i n a heater c o i l of - 29 - n i c k e l wire i n s i d e a b e l l j a r , heating the s t r i p under vacuum and admitting c y l i n d e r oxygen (commercial grade). The tung- sten surface d i s c o l o u r e d green, b r i g h t blue and then blue-gray as the oxide l a y e r thickened. A long period of heating i n an atmosphere of oxygen produced a yellow-green surface due t o the formation of tungsten t r i o x i d e . This changed back t o the b l u e - gray colour where the beam h i t i t during bombardment presumable due t o the tungsten t r i o x i d e changing back t o tungsten d i o x i d e . Therefore f o r a t a r g e t w i t h a constant oxygen content, the b l u e - gray tungsten d i o x i d e l a y e r was p r e f e r a b l e . Since bombardment of sodium gives a 0.^5 Mev. gamma ray from Na 2 3(p,p'), as w e l l as a 1.60 Mev. gamma ray f o r Na 2 3(p,<5*^)Ne 2 0, the t a r g e t was checked f o r Na 2 3 contamination by running an e x c i t a t i o n curve over the resonances at 1287.5 and 1257.5 kev. The resonance at 1257.5 kev. decays mostly by proton emission g i v i n g the 0.^5 Mev. gamma r a y . Traces of Na 2 3 were apparent. From the e x c i t a t i o n f u n c t i o n f o r Na23(p,<^^) and Na 2 3(p,p'^) given by S t e l s e n and Preston (51*) f o r proton energies greater than 1 Mev. and by R.L. B u r l i n g (^hljfor protons from 0.3 t o 1.9 Mev., the Na 2 3 y i e l d was estimated t o be l e s s than 10 counts per 100 microcoulombs of beam above a 300 kev. bias f o r 830 kev. proton energy f o r the counter-target geometry used during the measurement of This r e s i d u a l N a 2 3 e f f e c t was s u f f i c i e n t l y s m a ll that i t would produce n e g l i g i b l e d i s t o r - t i o n of the""l^ peak. - 30 - The t a r g e t chamber used was s i m i l a r t o that used by Alexander (5S)» Steam heat was a p p l i e d t o the t a r g e t assembly t o reduce the amount of o i l vapours condensing on the t a r g e t . During some bombardments no steam was used and no t y p i c a l cracked o i l deposit appeared on the t a r g e t . I t t h e r e f o r e appears t h a t i f the vacuum system i s cl e a n the heating of the t a r g e t by the beam along w i t h good c o l d trapping c l o s e t o the t a r g e t assembly are s u f f i c i e n t t o prevent o i l vapour contamination of the t a r g e t . A lead s h i e l d was moulded t o f i t the geometry of the t a r - get chamber and f i t i n t o the lead c a s t l e housing the gamma r a y counter. (b) Detector and E l e c t r o n i c s . The 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 , p o s i t i o n e d t o touch the chamber face thus subtending approximately 0.015 of a sphere at the t a r g e t . The k i c k s o r t e r was set up as described i n Chapter I I . The s t a b i l i t y of the e l e c t r o n i c s during the runs was bet t e r than 0.1 per cent as measured by the mercury pulse generator. The f u l l energy peaks of the gamma rays were measured w i t h a high d i s p e r - s i o n of the 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"^. Protons w i t h an energy of 800 kev, as read on the gener- a t i n g voltmeter were used t o bombard the oxide t a r g e t p o s i t i o n e d A flCURL 8 0'\?,vr GAMMA. RAV ENERGIES - 31 - so t h a t the gamma rays were not attenuated by the t a r g e t support i n the d i r e c t i o n of the d e t e c t o r . The 1 .12 Mev. gamma r a y from Z n ^ the 0 . 6 6 2 Mev. gamma r a y from Cs 1^ and the 1 . 2 8 Mev. gamma ray from N a 2 2 supplied c a l i b r a t i o n p o i n t s f o r the pulse generator amplitude s c a l e . The l i n e a r i t y of the c r y s t a l pulse height versus gamma ray energy was be t t e r than 1 per cent i n t h i s r e g i o n . The curves used t o determine the energies of 0^ and ° 2 are shown i n Figure 8 . The r e s u l t s of these 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 generating voltmeter was c a l i b r a t e d at the 8 7 3 . 5 kev. r e s o - nance of F^Cp,^) on a t h i n f l u o r i n e t a r g e t . and Ep. The Q value f o r the r e a c t i o n may be determined from "Ŷ Q = %v - 16 E p ° 1 17 Because the protons l o s e energy as they t r a v e r s e the t a r g e t a mean proton energy, E p , must be used and since the r e a c t i o n i s non- resonant, Up" = Ep - A/2, where A i s the t a r g e t t h i c k n e s s (expressed i n kev.) t o protons of energy E . An estimate of the number of O1^ - 32 - atoms per square centimeter can be made from the r e l a t i v e y i e l d of ^ 2 o n "the tungsten d i o x i d e t a r g e t as compared w i t h that on the 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 the tung- sten d i o x i d e t a r g e t w i l l then be given by, where i s the molecular stopping power f o r tung- wo2 sten d i o x i d e , I Q Q i s the number of 0±o atoms per square centimeter and 1 accounts f o r the f a c t that there are two oxygen • 2 atoms per molecule. cr may be c a l c u l a t e d from atomic stopping power data given by WO 2 Whaling (57) as f o l l o w s : cr - cr + 2 <3~ wo2 w o . Since Whaling does not give CT^, e x t r a p o l a t i o n from < ^ u as- suming C & Z can be used t o get cT^ Then CT =2± ^ A U - 2 CTQ = 35.0 x l O " 1 ^ e.v.-cm 2/ molecule at E p = 0.800 Mev. The number of counts i n the f u l l energy peak o f 2 f° T the oxide t a r g e t w i t h the detector 2.3 i n . from the t a r g e t was 62 counts per i n t e g r a t o r . With the same detector set at 3.6 i n . from an i c e t a r g e t of thickness ^IHVJO = 1 , 1 - L 8 x ^ P 1 ^ molecules of water per square centimeter, there were 92 counts per i n t e - grator i n the "0̂ 2 f u l l energy peak. The number of oxygen atoms per - 33 - square centimeter on the oxide t a r g e t was c a l c u l a t e d as f o l l o w s „ - N V oxide H i c e MO ' — * V\ H o w If i c e «tt oxide where i s the number of gamma rays per i n t e g r a t o r , and - H _ are the s o l i d angles f o r the two cases n = 62 x ( 2 . 3 ) 2 x 1.118 x 1 0 1 9 atoms/cm 2 •0 92 (3.6) Then the energy l o s s f o r protons passing through the tungsten d i - oxide i s ( A ) = 35.0 x l O " 1 ^ x 1 x 62 x ( 2 . 3 ) 2 x 1.118 x 1 0 1 9 E W G 2 2 92 \TZ) This r e s u l t assumed th a t the oxygen was i n the form of WO2. The t a r g e t surface was blue-brown i n colour c h a r a c t e r i s t i c of the d i o x i d e r a t h e r than the yellow colour of the t r i o x i d e . I f some WO3 was present t h i s would reduce the t a r g e t t h i c k n e s s . A 53 kev. t h i c k t a r g e t should have r e s u l t e d i n an i n - crease i n the observed widths o f ^ i and'o^ by approximately 50 kev, The observed increase i n the widths i s l e s s than 10 kev. The i n - consistency of 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 not yet explained. Assuming that tungsten t r i o x i d e i s formed, C^ E )^o^ = 38 kev., s t i l l r a t h e r wider than can be accounted f o r by the width of the observed gamma ray l i n e s . Assuming (^E) = 53 kev. - 3* - t o be c o r r e c t then f o r an i n c i d e n t proton energy Ep = 831 kev. the mean proton energy on the t a r g e t Ep = 80^ kev. For E ^ = 1.3k6 Mev. from Equation 1 we o b t a i n Q = 590 - 23 kev. This agrees w i t h i n experimental e r r o r w i t h the value 0.599 - .006 Mev. c a l c u l a t e d from the 0 ^ ( d , n ) F ^ t h r e s h o l d (Bonner and Marion, 55). I t i s i n t e r e s t i n g t o compare the above r e s u l t s on gamma ray energies w i t h s i m i l a r r e s u l t s obtained from the i c e t a r g e t s below: = 1.315 - .016 Mev. E ^ = .812 - .016 2 E v - = .503 - .032 Mev. °1 °2 To determine the Q value fromV1? the t a r g e t t h i c k n e s s at an i n c i d e n t proton energy of 830 kev. must be c a l c u l a t e d i n order t o estimate the mean proton 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- tons; t h i s corresponds t o a th i c k n e s s of 60 kev. at 800 kev., c a l c u l a t e d using 0.6 f o r the r a t i o of the molecular stopping cross s e c t i o n s f o r water at 390 and at 830 kev. (Whaling 57). Then Ep = 800 kev. - 10 kev. and Q = 562 ±26 kev. The Q here i s lower than would be expected from the known mass va l u e s . The spreading of the proton energy due to the target thickness will produce a spread in the energy of ifj and~̂  2. The widths of the full energy peaks of Y2 andlT̂  are about *f0 - 15 - 35 - kev. greater than the widths of the f u l l energy peaks of the .661 Mev.Y-ray of Cs 1^ and the 1.28 Mev. if-ray of N a 2 2 respec- t i v e l y . Since the 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 kev., the measured width of the 01^(p^)F17gamma rays should be 60 kev. wider than t h a t due t o the r e s o l u t i o n of the d e t e c t i n g system. This i s greater than the measured v a l u e , however j u s t outside the e r r o r placed on the measured width i n c r e a s e . I f the cross 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 decreasing energy i n t h i s r e g i o n of bombarding energy, the width increase would be correspondingly reduced since there 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 of 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 energy protons. This would help e x p l a i n the l a c k of observed increase i n the tungsten oxide t a r g e t s . 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 cross s e c t i o n would change so r a p i d l y over t h i s s m a l l energy range at 800 kev. A l s o the e f f e c t should be observable e q u a l l y on the tungsten oxide t a r g e t as on the i c e t a r g e t since the energy l o s s f o r protons i n the two ta r g e t s was approximately the same. Another reason why r e s u l t s from the tungsten oxide t a r g e t do not show as large a broadening of the f u l l energy peaks as expected assuming a l l the oxygen i s i n the form of tungsten oxide i s t h a t some oxygen may be occluded as a surface l a y e r on the tungsten so that the r e l a t i v e energy l o s s of the protons due t o the tungsten atoms i s reduced; f o r a pure oxygen t a r g e t of such thickness t o give an equ i v a l e n t y i e l d as measured, the energy l o s s f o r 800 kev. protons would be approximately 10 kev. - 36 - A l s o i t should be mentioned t h a t f o r t h i s c r y s t a l mounted on the Dumont 6363 p h o t o m u l t i p l i e r the width of a f u l l energy peak of the 1.28 Mev gamma r a y of N a 2 2 v a r i e d as much as 20 per cent amongst sets of runs taken under approximately the same c o n d i t i o n s . This v a r i a t i o n i n the r e s o l u t i o n although not accompanied by gain s h i f t s of the c e n t r o i d , reduces g r e a t l y the i n f o r m a t i o n a v a i l a b l e from the width of a peak i n a gamma ray spectrum. (b) Energy determination o f " ^ Since the energy of 7 ^ i s very c l o s e t o the 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 the p o s i t r o n decay of F 1 ? which i s a l s o produced i n the r e a c t i o n , the r e s u l t i n g spectrum w i l l be due t o the sum of the appropriate number of gamma rays of the two energies. The shape of the r e s u l t i n g spectrum and the p o s i t i o n of the maximum of the curve w i l l depend on the r e s o l u t i o n of the de t e c t o r , the d i f f e r e n c e i n energy between the two gamma r a y s , and the r e l a t i v e i n t e n s i t y of the gamma r a y s . Assuming the f u l l energy peak due t o a s i n g l e gamma ray i s gaussian i n shape, the spectrum shape f o r a gamma ray whose energy width i s very s m a l l compared t o the r e s o l u t i o n of the detector may be described by the u s u a l gaussian equation dN(x) = N e - 1/2 ( x - i . 2 2 dx 2TTVT where C i s the h a l f width a t .606 peak he i g h t . -tooo 500 J 1 ' 1 1 1 1 1 1 1 1 1 1 1 1 l ! I l 1 I L I I I I I I I I S IO 13 ZO ZS J O C H A N N E L N U M B E R - 37 - (the width at h a l f peak height i s ^ , x v l / 2 A 1 — 2 c f (inif) ) 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 the t o t a l number of counts i n the peak Two gamma rays whose energies are c l o s e together, i . e . w i t h i n the r e s o l u t i o n , of the d e t e c t o r , w i l l produce a shape which can be approximated by the sum of two gaussian f u n c t i o n s . Therefore to determine 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 se 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 the gamma r a y s , we consider the sum of two gaussians: y = c fexp - 1 / 2 ( x - / ^ ) 2 + r . exp -l/2(x-/Q 2 A This represents two gaussians of the same w i d t h , one w i t h cen- t r o i d a t and area c, the second w i t h c e n t r o i d a t / A x and area r c . (Figure 9 ). S o l v i n g f o r the maximum, /la , of t h i s curve Let <S Considering the f i r s t order i n i . e . & t ju^a^^x ^ \ Then - 38 - This r e s u l t can be used t o determine the p o s i t i o n of the maximum, Eo, of a complex gamma ray spectrum, of two gamma rays of f u l l energy E-|_ and E 2 , where E 2 = E-j_ + S , and the i n - t e n s i t i e s are i n the r a t i o 1 t o r w i t h a dete c t o r r e s o l u t i o n such that S i s l e s s than the width of a monenergetic spectrum at .606 peak h e i g h t . Then the p o s i t i o n of the maximum of the composite spectrum Eo i s given by E 0 = + r_|_ 1+r 5 f o r ^ < 1 C The d i f f e r e n c e i n the width a t h a l f maximum between E^ and E 2 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 at .606 maximum of a curve of the form of equation 2 i s most e a s i l y obtained g r a p h i c a l l y . Figure 9. shows 22 the r e s u l t of adding two f u l l energy peaks from a Na spectrum i n the c r y s t a l , one placed a t 0.51 Mev., the other moved t o 0.^93 Mev. w i t h an 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 of the gamma r a y s h i f t ; the s h i f t estimated from equa- t i o n 5 i s 0.33> i n reasonable agreement. With these c o n s i d e r - a t i o n s i n mind we may analyse the spectrum from 0 1^(p,"^)F 1 7 i n the r e g i o n of 0.5 Mev. gamma r a y energy. - 3 9 - The F* 7 nucleus produced by the O 1 ^ p,"̂ ) r e a c t i o n decays by p o s i t r o n emission w i t h a h a l f l i f e of 6 6 seconds (Ajzenberg and L a u r i t s e n ( 5 5 ) . The number of i f 3 gamma rays pro- duced can be c a l c u l a t e d from the branching r a t i o ; "a" f o r ̂  t o ^ 2 : where Np i s the number of F 1 7 n u c l e i produced by the r e a c t i o n . The number of a n n i h i l a t i o n gamma rays i s approximately equal t o twice the number of F 1 7 n u c l e i which decay during the counting p e r i o d . 17 To estimate the number of F ' atoms which decay i n a given period consider the equation which gives the t o t a l r a t e of change of atoms. (d_Np) - \ N + c i ( d t . ) t o t a l F where c i i s the r a t e of production of F^ 7 n u c l e i which i s propor- t i o n a l t o the beam current i and which we w i l l assume t o be con- s t a n t ; at t = G, N p = 0 . The number of F 1 7 decaying i s equal t o - \ Np whereas the number ^ 3 being produced i s p r o p o r t i o n a l t o the proton c u r r e n t . The r a t i o , r , of the number of counts f r o m ^ 3 t o the number from a n n i h i l a t i o n r a d i a t i o n can be determined f o r the time - 1+0 - i n t e r v a l s below. a) o t o t Jo r = 1 . t 2(l+a) ( t - T ( l - e x p - t / T ) where T = 1 = 95 second, the mean l i f e t i m e . 7s b) f o r the 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 the bombardment r = 1 9 2(l+a) 7 I n f i g u r e 9 curve ( i ) was taken during the f i r s t 63 seconds of beam bombardment of the t a r g e t . Curve ( i i ) was ob- tained w i t h a constant beam of approximately (10±.5) microamperes bombarding the t a r g e t f o r approximately 8 minutes (that i s , ap- proximately 5 l i f e times) a f t e r which the spectrum was recorded f o r 10.2 minutes keeping the beam constant d u r i n g t h i s time. The e q u i l i b r i u m spectrum taken f o r the f i r s t 66 seconds a f t e r the beam was shut o f f i s shown i n curve ( i i i ) . E s t i m a t i o n of the energy d i f f e r e n c e between the 0.51 Mev. gamma ray 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 (Ajzenberg and L a u r i t s e n (55))> then t i s approximately equal t o the h a l f l i f e . -1+1 - The r a t i o ~& 1 as measured above was 0.2. S u b s t i t u t i n g these T 2 values i n t o equation 6 gives r = V3. The measured s h i f t i s then (8.5 ± 5.2) kev. Therefore from equation 5 0~ = 1 + x 8.5 V 3 ^ = -15 kev. ± 12 kev. From curve ( i i ) With a = V3» equation 7 gives r = .1+2. The measured s h i f t i s (5.2 ± 5.2) kev. from the .51 Mev. l i n e . This gives S = -18 kev. ±18 kev. waio^Vftcd w a i n -Co*- % i s i ^ c o -v(o±v^,\<ev (c) Gain S h i f t s The d i f f e r e n c e i n the p o s i t i o n of the a n n i h i l a t i o n r a d i a t i o n recorded immediately a f t e r the beam was o f f the t a r - get and tha t from N a 2 2 (Figure 9) recorded a few minutes l a t e r i n d i c a t e s a gain s h i f t i n the d e t e c t i o n system. Since the counting r a t e during the runs i n the r e g i o n about *f00 kev. was low ( l e s s than 22 counts per second) the g a i n s h i f t was thought to occur due t o the high counting r a t e from coulomb e x c i t e d gamma rays from tungsten of 112 kev. and 66 kev. tungsten X-rays as w e l l as the sm a l l c o n t r i b u t i o n due t o brem- s t r a h l u n g . A t e s t was run t o reproduce t h i s s h i f t . The l a c k of d.c. s h i f t i n the e l e c t r o n i c s (not i n c l u d i n g the 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 the mercury popper pulses i n t o the head a m p l i f i e r and re c o r d i n g these pulses on the K.S. When a C s ^ 7 source was placed up t o the c r y s t a l so tha t the counting - >f2 - r a t e was greater than 1500 counts per second above 200 kev., the change i n the popper pulse height on the k i c k s o r t e r was l e s s than .1 per cent. Eu?-55 gives among other gamma rays strong l i n e s a t 330 and 87 kev.; therefore t h i s source w i t h N a 2 2 were used t o approximate the counting c o n d i t i o n s occuring d u r i n g the runs on the tungsten oxide t a r g e t s . The counting r a t e above a 50 kev. b i a s was increased from 100 counts per second to 1500 counts per second by moving the E u 1 ^ source c l o s e r to the c r y s t a l . During these runs no s h i f t was n o t i c e d i n the c e n t r o i d of the 0.51 Mev. f u l l energy peak from N a 2 2 . However, on removing the E u 1 ^ source the 0.51 Mev. r a d i a t i o n showed a g a i n s h i f t of +8 kev. approximately equal t o that observed between the a n n i h i l a t i o n r a d i a t i o n from the t a r g e t and N a " during 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 to assume that there e x i s t e d two g a i n s h i f t a f f e c t s In the p h o t o m u l t i p l i e r w i t h high counting r a t e s i n the low energy r e g i o n of the spec- trum ( l e s s than 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 which had a long decay time of the order of a few hours, the other a negative s h i f t which had a short de- cay time of the order of a minute. 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 counting r a t e , but the g a i n increase would be observed when the low energy counting r a t e was reduced. - h3 - h. R e s u l t s . Summarizing the d i f f e r e n t measurements of Re s u l t .503*.032 Mev. .^9^.012 Mev. .^95-.012 Mev. The mean of these r e s u l t s i s 0.50^.01 Mev. i n agree- ment w i t h .*f99-.003 Mev. given by Bonner and Marion (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 et a l , 5*0•• Target Measurement i c e £\ _£*2 W02 _Etf2 W02 C e n t r o i d s h i f t s - hh - CHAPTER IV A LOOK FOR 0 1 7 ( p ^ ) F 1 8 GAMMA RAYS 1. I n t r o d u c t i o n . According to the s h e l l model the mass 18 n u c l e i con- s i s t of two nucleons outside the closed O1^ core. Because of t h i s r e l a t i v e l y simple assumptions are p o s s i b l e f o r the d e t e r - mination of wave f u n c t i o n s f o r the ground s t a t e and some of the ex c i t e d s t a t e s . E l l i o t and Flowers (5*0 have c a l c u l a t e d the p o s i t i o n s of the low l y i n g l e v e l s of the mass 18 and 19 systems of an intermediate coupling s h e l l model of the nucleus. I n o r - der t o check the v a l i d i t y of the theory, experimental values of •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 of the e x c i t e d s t a t e s of F are of i n t e r e s t a t the present time. A study of the o l ? ( p , ^ ) F l 8 r e a c t i o n w i l l supply some of the parameters of the F l 8 s t a t e s which can 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 . Since a sep- arated O1^ t a r g e t was a v a i l a b l e , i t was decided t o look f o r gamma r a d i a t i o n from t h i s r e a c t i o n i n the r e g i o n 1.0 Mev. t o 2.2 Mev. proton 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 l 8 between 6.5 and 7.7 l e v . Warren et a l (51*-) found the existence of an 873 kev gamma r a y during the bombardment of n a t u r a l oxygen t a r g e t s w i t h protons of energy greater than 1.8 Mey. This 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 of the 872 kev. l e v e l i n 0 1 7 ( A j z e n b e r g and - i+5 - L a u r i t s e n ,55) e x c i t e d by i n e l a s t i c s c a t t e r i n g of protons. I n order t o check t h i s assignment, separated t a r g e t s of 0 and 0-*-7 were bombarded w i t h protons i n the energy r e g i o n around 2 Mev. From the r e s u l t s , no capture gamma rays from 0^7 could p o s i t i v e l y be i d e n t i f i e d . The l i m i t of d e t e c t a b i l i t y was set by contaminants. The spectra of capture gamma rays from the contam- ina n t s were studied and the r e s u l t s are reported on b r i e f l y here as i t i s f e l t t h a t they should be u s e f u l i n connection w i t h f u r - ther low cross 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 Procedure. (a) Gamma Ray 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 as described i n Chapter I I I . A g a i n s h i f t phenomenon was observed on the Du- mont 6363 which was a f u n c t i o n of the counting r a t e , the spec- trum shape and the H.T. voltage on the dynode c h a i n . The ga i n tended t o increase when the detector was exposed t o a high count- i n g r a t e and d i d not r e t u r n immediately t o i t s o r i g i n a l value on decreasing the count r a t e , but sl o w l y dropped over a period of a few hours or more. The amount of the s h i f t was much greater f o r comparable counting r a t e s above a f i x e d bias when caused by high energy gamma rays c o n t r i b u t i n g t o the f a s t counting r a t e than when due t o low energy gamma r a y s . A l s o the amount of ga i n s h i f t f o r a given counting r a t e and a given spectrum was de- creased 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 the - i f 6 - g a i n of the system was independent of pulse s i z e i n the r e g i o n from 2 t o 6 Mev. gamma r a d i a t i o n . The maximum g a i n s h i f t was reached only a f t e r approximately 1/2 hours f o r a given counting 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 approaching an e q u i l i b r i u m value more s l o w l y . The dependence of t h i s g a i n s h i f t on pulse amplitude and dynode H.T. vol t a g e would i n d i c a t e some dependence on the instantaneous pulse current drawn not only on the mean cu r r e n t . This g a i n s h i f t n e c e s s i t a t e d frequent c a l i b r a t i o n checks when observing the 6 t o 10 Mev. r e g i o n of the spectrum to ensure t h a t the s h i f t d i d not mask some of the spectrum s t r u c - t u r e . A f t e r 20 minutes w i t h a counting r a t e of 2000 counts per second above a 0.51 Mev. bias energy w i t h 6.1^ Mev. gamma rays and 1000 v o l t s across the dynode chain the g a i n increased by approximately 8 per cent. (b) Targets. The t a r g e t chamber was described i n Chapter I I I . Four t a r g e t s of m a g n e t i c a l l y separated 0 ^ layed down on .020 i n c h Tungsten backings, were made by A.E.R.E. Harwell England. The thi 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 the t a r g e t s were approximately 50 micrograms per square centimeter f o r t a r g e t s number 1 and 2 and 20 and 15 micrograms per square centimeter f o r t a r g e t s num- ber 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 . Proton bombardment of number 1 and 2 t a r g e t s at 1.90 Mev. proton energy i n d i c a t e d t h a t there was l e s s than 0.1 micro- - ±7 - grams per square centimeter of 0 assuming the 872 kev. was due to 0 1 7(p,p'X). This was c a l c u l a t e d from the r e l a t i v e y i e l d s of the 872 kev. gamma ray from the separated O1^ t a r g e t s and from n a t u r a l oxygen t a r g e t s w i t h a known number of oxygen atoms per square centimeter and t a k i n g the percentage of 0 1 7 i n n a t u r a l oxygen as 0.0V per cent. Since an e x c i t a t i o n f u n c t i o n over the 873 kev. resonance of F^ 9 i n d i c a t e d that i n these t a r g e t s there was greater than 0.1 micrograms per square centimeter of F l o u r i n e no f u r t h e r search f o r ca p t i v e gamma rays from 0^7 was made using these two t a r g e t s . The number 3 and h t a r g e t s showed no 872 kev. r a d i a t i o n f o r proton bombarding energies of 1.75 t o 2.2 Mev. An 0 X 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 separated by A.E.R.E. Harwell was a l s o bombarded w i t h protons of energy 1.8 t o 2.1 Mev. No 872 kev. gamma r a d i a t i o n was observed, however the t a r - 16 get contained only a small amount of 0 as judged from the y i e l d of^ 2« (d) Contamination Spect r a . Contamination spectra 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 the energy range .8 t o 1.9 Mev. was measured i n 20 kev. i n t e r v a l s . More accurate e x c i t a t i o n f u n c t i o n s were then made i n regions where resonances were i n d i c a t e d . The p o s i t i o n of these resonances and the energy of the gamma rays appearing i n the spectrum were used t o determine the contaminants present. The f o l l o w i n g nuclear contaminants were found: - HQ - The y i e l d of 6,lh Mev. gamma rays a t the 873 kev. 19 resonance i n d i c a t e d t h a t the amount of F ' contamin- a t i o n on the t a r g e t s was of the order of .1 mi H i micrograms per square centimeter, much l e s s than t h a t i n t a r g e t s number 1 and 2. Resonances were observed at proton energies of .898 1.2 and 1.650 Mev. w i t h gamma rays of energy ^ .^l Mev. This was a t t r i b u t e d t o N 1^(p,^,"^)C 1 2, (Hagedorn and Marion (57) and Schardt et a l (52)). The apparent l a c k of resonances i n Nllf(p,~^) of comparable height t o the N ^ - ^ p , ^ , ^ ) ^ 2 resonance i n d i c a t e d t h a t the amount of R l 5 was greater than 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 isotopes would suggest, and th e r e f o r e p o s s i b l y was layed down as NH2+ a t the mass 17 focus of the separator. The cl3(p,~^)N I'+reaction has a resonance at I .76 Mev. proton energy (Ajzenberg and L a u r i t s e n (55)) which was observed as was the 9.18 Mev. gamma r a y from the N l ^ decay. The carbon contamination was not i n t r o - duced onto the t a r g e t by the beam since c l e a n tungsten backings which had been bombarded f o r comparable lengths of time showed l e s s c!3 contamination. - k9 - The separated Oie> t a r g e t had showed the I . 6 3 Mev. gamma r a d i a t i o n from N a 2 3 ( p , 0 * ^ ) ^ 2 0 between 1 .9 and 2 . 0 Mev. proton bombardment energy. There i s considerable sodium on t h i s t a r g e t . (e) Measurement of a n n i h i l a t i o n r a d i a t i o n Since the F-*-8 decays by p o s i t r o n emission w i t h a h a l f l i f e of 112 minutes (Blaser, ( ^ 9 ) ) , the existence of a n n i h i l a t i o n 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 of 112 1ft minutes would i n d i c a t e that F - had been formed. Therefore an attempt was made to detect the decay through a measurement of the counting r a t e as a f u n c t i o n of time a f t e r the beam was turned o f f . The F 1 7 produced from 0l6(p,l£) and N 1 3 from C 1 2 ( p ^ ) would c o n t r i b u t e t o the a n n i h i l a t i o n r a d i a t i o n since both a l s o decay by p o s i t r o n emission. However t h e i r a s s o c i a t e d h a l f l i v e s are 66 seconds (Wong ^5^.) and 10 minutes ( C h u r c h i l l et a l ̂  53! ) r e s p e c t i v e l y and w i l l not i n t e r f e r e w i t h measurements made of 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 using coincidence methods, the sm a l l c r y s t a l d e t e c t o r being used as the 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 Figure 5 . The output from the small detector was fed t o an E.K. Cole l O ^ A a m p l i f i e r which i n t u r n fed an Atomic Instruments s i n g l e channel analyser. The window of the analyser was set to see r a d i a t i o n between the 300 and 600 kev. energy r e g i o n of the spectrum by using the mercury pulse generator which 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 kicksorter. The output of the analyser triggered the gated biased amplifier through which the pulse from the large crystal passed. The pul- ses from the large crystal were delayed by approximately 1.5 microseconds going into the Moody amplifier in order to compen- sate for 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 crystals. The annihila- tion radiation counting rate observed indicated a fast 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 or- der of a few hours, possibly due to F decay. The counting rate from this long l i f e portion was very small indicating that the F1* yield for the target was very low. 3. Conclusions. No 872 kev. gamma radiation 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 did appear at the higher bombarding energies. On the other two (targets numbered 3 and k-) the radiation was not observed; however this i s consis- 1 o tent with the very small amount of F annihilation radiation. But the amount of 0^ and 0^ on the two types of separated tar- gets appears to be much smaller than Harwell estimated. This was - 51 - c o n f i r m e d f o r t h e 0 ± 0 t a r g e t b y c o m p a r i s o n o f t h e y i e l d o f 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 s e p a r a t e d t a r g e t 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 s e p a r a t e d t a r g e t compared t o a n a t u r a l o x i d e t a r g e t . T h e r e f o r e , t h e c o n - 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 p r e s e n c e or a b - sence 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 c a s e because of t h e s m a l l number o f t a r g e t atoms i n t h e t u n g s t e n b a c k i n g s . The s m a l l amounts o f o x y g e n i s o t o p e s on the t u n g s t e n t a r g e t s s u g g e s t s e i t h e r t h a t t h e oxygen i s n o t r e t a i n e d as a n o x i d e b u t o c c l u d e d i n t h e s u r f a c e and t h e n l o s t w i t h s u b s e q u e n t bombardment , o r t h a t t h e p e r c e n t a g e o f t h e mass 17 beam i n t h e e l e c t r o m a g n e t i c s e p a r a t o r w h i c h i s 0*7 i s n o t a s l a r g e a s a s s u m e d . I f t h e f i r s t r e a s o n i s t h e c a u s e o f t h e d i f f i c u l t y i n t h e s e p - a r a t o r , 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 f o r m t h e o x i d e m i g h t be a n improvement because 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 t u n g s t e n . W i t h r e f e r e n c e t o t h e s e c o n d s u g g e s t i o n , A h n l u n d e t a l (5*0 m e n t i o n t h a t f o r the e l e c t r o m a g n e t i c s e p a r a t i o n of 0̂ 7 9 t h e h y d r o g e n a l w a y s p r e s e n t i n t h e i o n s o u r c e combines e a s i l y 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 amounts o f o 1^H+ a t t h e mass 17 f o c u s . A h n l u n d e t a l c o l l e c t e d t h e 0 ^ a t mass number 31 i n t h e f o r m of N ^ O - ^ , by u s i n g gas e n r i c h e d i n o!7(a few p e r c e n t ) i n t h e i o n s o u r c e m i x e d w i t h n a t u r a l n i - t r o g e n f o r w h i c h t h e N-14" t o r a t i o i s v e r y h i g h . T h i s - 5 2 - r e s u l t e d i n enriched t a r g e t s i n which the composition was approx- imat e l y 90 per cent N l l f0 1 7, 7 per cent N 1 ^ 1 ^ and l e s s than 3 per cent C^O 1 8. P o s s i b l y t h i s i s the only method t o get reasonable en- richment of O^7. C e r t a i n l y before any u s e f u l i n f o r m a t i o n about l e v e l parameters of F-*-8 can be extracted much b e t t e r t a r g e t s w i l l have t o be obtained. M K I DC S O U R C E P o L A R i r y SWITCH Volt-melve -?0- T 16 V 0 - J — Q -o—ox> ,tOOK ' HaUpot < 3 — s A\A/--—> WE 50K ^[flflJX 267E > K-d-, , AA/W S O O S L 100 K OUTPUT ~5> 400 M K I TO R E L A Y DRIVE 5 0 0 J \ (w) A W v A - -S) Hi OUTPUT iOOO?i> DECAY TIME RISE TIME f lOJRt 10 MCRCURY PUL5L GEICRATOR - 53 - APPENDIX Mercury Relay Pulse Generator. I n order t o set up the Marconi 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 ray d e t e c t i n g e l e c t r o n i c s , a pulse generator whose accuracy and s t a b i l i t y was be t t e r than 0 . 1 per cent was r e q u i r e d . I f the s t a b i l i t y of both pulse generator and counter system i s good, a gamma r a y energy can be determined i n terms of s e t t i n g s of the pulse generator voltage which can be related t o a gamma ray energy s c a l e using sources w i t h gamma rays of known energies. The shape of the pulse from the pulse generator must approximate t h a t of the pulses due t o gamma ray i n t e r a c t i o n i n the d e t e c t o r so that 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 pulse 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 of va r y i n g the pulse shape of the generator was r e q u i r e d . The pulses should be able t o be fed i n t o high or low impedance points i n o r - der t o t e s t at d i f f e r e n t p o i n t s of the counting system. 1. Pulse Generator Mark I . The Mk I model (Figure 1 0 ) , was designed to produce pulses t o be fed i n t o the g r i d of the head p r e a m p l i f i e r cathode f o l l o w e r . This was a high impedance point p e r m i t t i n g the de- s i g n as shown. The i n t e g r a t i n g time constant was f i x e d at the r i s e time of the pulse from 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 the f a l l time was ad- ded e x t e r n a l l y t o the generator u s u a l l y i n the input t o the head a m p l i f i e r . - 9+ - The Western E l e c t r i c 276 series of r e l a y s switch i n one d i r e c t i o n when the current through the c o i l becomes greater than +1.5 milliamperes and switch back when the c o i l current drops t o l e s s than -1.5 m i l l i a m p e r e s . On the a l t e r n a t e h a l f c y c l e from the pulse production, the coupling and pulse shaping c a p a c i t o r s were discharged through a 100K r e s i s t o r ; t h i s produced a pulse of opposite p o l a r i t y approximately one t h i r d the amplitude of the d e s i r e d p u l s e . Increasing the 100K r e s i s t o r t o reduce the s i z e of t h i s discharge pulse introduced p i c k up i n the moving arm of the r e l a y during the 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 output. The 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 cent. 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 the 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 small b a t t e r y of dry c e l l s . 2 . Pulse Generator Mark I I . The Mk I I design (Figure 10) was b u i l t t o provide ad- d i t i o n a l f a c i l i t i e s ; f i r s t , the output pulses can be fed i n t o both low or high 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 imes, and a l a r g e r amplitude range are provided, 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 provides voltage r i s i n g l i n e a r l y i n time which when chopped by the mercury sw i t c h provides a s l i d i n g pulse generator f o r t e s t i n g the k i c k s o r t e r . The design was s i m i l a r to that described by B a t t e l and Chapman ( 5 D « - 55 - The linearly varying direct current source was pro- duced by driving a single turn 5K Helipot Model L, (linearity 0.1$), with a synchronous motor and feeding the voltage from the sliding arm to the pulse shaper. A single turn helipot was chosen since this eliminates the need for limit and motor rever- sing switches which are required for a multiturn helipot. The single turn helipot however does suffer from the fact that an additional ripple is introduced by the slider passing over the single turns of wire on the resistor (1800 turns), however for the present pulse generator this ripple was less than 0.05$ and consequently was negligible. The motor drives the helipot through a reduction gear at a speed of 6 r.p.m. The direct current power supply delivered 10 m i l l i - amperes at 85 volts with 2 millivolts 10 cycle ripple and less than 1 m i l l i v o l t 60 cycle ripple, peak to peak, at the input to the voltage divider. The voltage divider supplied 60, 15, and 6 volt output for the helipot, variable by adjustable "trimpots", and 50, 30, and 10 volt outputs for the sliding helipot. The Western Electric 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 like pulse to appear on the t a i l of the shaped pulse and also when the relay returns on the alternate half cycles. This could be made small by increasing the direct current source impedance. But i f the source impedance was too high there would be a relatively large liOV AC <: t " V MOTOR +400V 2693 P O L A R I T Y SWITCH I.5K I W v V - 35K oc O U T P U T F1XE0 S W E E P . 3 K S 6 _^o_l 1 + I50V -5" + g^)65A2 PIXED VOLTAOE DIVIDED T 'K IK IK TuiMPor 35^ TRIM POT TKIWFOT - W » — W — V v / v — v W — v v y h- 6Y I0OK HELIPOT A W v V I—yx&A 5K WW —'vwv—o~ \{ 5 K HELIPOT T W M S O r O b VOLT M E T E R I0K S 4 OC OUTPUT SWEEP VOLTAGE DIVIDED. DC 5UPPLY RELAV COIL |0 0 o +2.40 1 n c l fc5V EXTERNAL SUPPLY T-IGURt II MULTIVIBRATOR CIRCUIT' - 56 - e r r o r i n the pulse height produced, due t o the charging time of the i n t e g r a t i n g condenser, which v a r i e d w i t h the p o s i t i o n of the h e l i p o t . This i s the reason t h a t a 10K ohm r e s i s t o r i s placed between the d i r e c t current output and the h e l i p o t , t h i s was the best compromise f o r minimum pulse due t o the 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 to v a r y i n g charging time constant. This could be improved by using 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 the d i r e c t current output; the time constant e r r o r would be n e g l i g i b l e and the current pulse would be reduced over t h a t obtained w i t h the c i r c u i t shown i n Figure 10. The current pulse now causes approximately 1 per cent decrease i n d.c. l e v e l at 60 v o l t s on the 60 v o l t range and 0.1 per cent at 6 v o l t s on the 6 v o l t s range. 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 5 6volts the s h i f t i s 0 .5 per cent and 0.25 per cent at kO v o l t s f o r the 60 v o l t range. A m u l t i v i b r a t o r (Figure 11) was b u i l t t o permit v a r i - able pulse r e p e t i o n r a t e s t o be used. This was introduced not only because of greater f l e x i b i l i t y of t e s t i n g but because i n de- t a i l e d t e s t i n g of pulse amplitude analysers such as the Marconi 30 channel k i c k s o r t e r used i n t h i s work, i f pulses are fed i n at mains frequency, then the time c o r r e l a t i o n between the input pulses and mains frequency r i p p l e voltages i n the 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 the r e s u l t s . This e f f e c t was confirmed f o r the present apparatus by n o t i n g that i f the mercury sw i t c h on the s l i d i n g pulser (whose l i n e a r l y r i s i n g d i r e c t current voltage was produced by the 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 '̂sî Kct' - 57 - motor and th e r e f o r e locked t o the mains frequency) was run at mains frequency, then the v a r i a t i o n i n k i c k s o r t e r channel widths appeared d i f f e r e n t than when the mercury switch was operated from the m u l t i v i b r a t o r at a frequency not commensurate w i t h the mains frequency. The pulse shape i s determined by the RC networks used. The response of the c i r c u i t s can be c a l c u l a t e d by u s i n g Laplace transform methods. The low impedance, "L o M , output c i r - c u i t form i s e s s e n t i a l l y t h a t of (a) Figure 12. S o l u t i o n by Laplace transforms gives e 0 ( t ) = Vo_ Rg e - * / ^ ^ c t T 2C R ; L 2 T ( 2 \ ̂ 2 1 - hT \ T i T 2 / *1 = R 1 C 1 T 2 = R 2 C 2 1 = 1 + 1 + 1_ T T X T 2 C 2 R l V 0 i s i n i t i a l v o ltage on Figure 12 shows a p l o t of exp -t/2T s i n h Ct/2T. From t h i s p l o t the r i s e time i s approximately 2T, n e a r l y independent of C and the f a l l time constant i s approximately 2T/1-C. - 58 - The components were chosen so t h a t f eeding i n t o a 75 ohm impedance the "Lo" output would give the time constants i n - d i c a t e d on Figure 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) Figure 12. I f the time constant formed by the c o u p l i n g condenser and the load i s l a r g e r than T^ or T 2 then the voltage at the output e 0 ( t ) i s given by: e 0 ( t ) = V 0 e " t / 2 T s i n h Ct/2T 2T 2C which has the same time dependence as above. For the " H i " out- put the minimum load impedance which w i l l not a f f e c t the time constants i s R^ v R 2 C 2 where C c i s the s i z e of the output c o u p l i n g s . y ~~z— condenser. u c The accuracy w i t h which the K.S. can be set up w i t h the s l i d i n g popper depends on how the pulse 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 pulses were completely random then about 10,000 counts would have to be accumulated i n each channel to set the edges to 1 per cent accuracy ( t h i s i s what must be done i f a comp- ton spectrum from an anthracene c r y s t a l i s used as a f l a t spectrum). I f the pulse 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 that 10 counts per sweep per channel were recorded, - 59 - then since the sweep h e l i p o t i s d r i v e n by a synchronous motor the channel edges could be set t o only 1 0 per cent no matter how many counts were recorded. Therefore t o get 1 per cent accuracy e i t h e r a sweep speed which permits 1 0 0 counts per channel per sweep i s necessary or a r e p e t i t i o n r a t e which i s a few c y c l e s o f f 6 0 c y c l e s . For t h i s reason the v a r i a b l e frequency d r i v e should be used, set at a frequency c l o s e t o 6 0 c y c l e s . This can 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 at 6 0 c y c l e s per second and feeding the t e s t pulses i n t o the v e r t i c a l a m p l i f i e r . I f the t e s t pulse r a t e i s j u s t o f f 6 0 c y c l e s per second then the t e s t pulses as seen on the o s c i l l i s c o p e w i l l d r i f t s l o w l y w i t h respect t o the h o r i z o n t a l t r a c e . - 60 - Bibliography Ahnlund, K., Thulin, S. and Pauli, H., ( 1 9 5 1 * - ) Arkiv For Fysik 8 , 4 8 9 . Ajzenberg, F. and Lauritsen, T., ( 1 9 5 5 ) Rev. Mod. Phys. 2 2 , 7 7 . Ajzenberg, F., ( 1 9 5 D Phys. Rev. £3, 6 9 3 . 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Edwards, M., ( 1 9 5 0 ) M.A. Thesis, University of Br i t i s h Columbia. Hagedorn, F. and Marion, J. - Kellog Radiation Lab. Preprint. Hirschfelder, J. and Magee, J., ( 1 9 W Phys. Rev. 23? 2 0 7 . Larson, E., ( 1 9 5 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 . Laubenstein, R., Laubenstein, M., Koester, 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 . Schardt, Fowler and L a u r i t s e n , (1952) Phys. Rev. 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 Preston, (195^) Phys. Rev. 21, 97^. T h i r i o n , J . , (1953) Annales de Physique 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 Smith, N., (19^1) Phys. Rev. £9_, 618 Warren, J . , L a u r i e , K., James, D. and Erdman, K., (195^) Can. J . 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