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Proton angular distributions from the Be9(t, p)Be11 reaction measured at 14 MeV triton energy
Volume 9, number 2
PHYSICS LETTERS
the d a t a of r e f . 2 and the p r e s e n t d a t a cannot b e , t h e r e f o r e , e x p l a i n e d a s a r e s u l t of a s l i g h t l y d i f f e r e n t n e u t r o n e n e r g y . The c r o s s s e c t i o n i n t e g r a t e d within the f o r w a r d c r o s s of 60 ° y i e l d s 9.5 rob. T h e ang~d~r d i s t r i b u t i o n i s c o m p a r e d with the B u t l e r l = 1 c u r v e f o r t o = 5.5 fro. The t h e o r e t i cal curve is smeared. The extracted reduced width ~2 i s 0.076. T a k i n g f o r the s p e c t r o s c o p i c f a c t o r S = 4 9), one g e t s 02 = 0.019. The O16(d,t)O 15 w a s i n v e s t i g a t e d 10) a t E d = 14.9 MeV Yi2elding A~ 2 = 2 . 4 ( A ~ 190) and t h e r e f o r e ~o ~ 0.003. The ~z e x t r a c t e d f r o m the d a t a of r e f . 8,10 and f r o m the p r e s e n t m e a s u r e m e n t a r e s m a l l e r than 02 f o r l p s h e l l . It w a s s u g g e s t e d that such a smallV 02 could be r e l a t e d to the s m a l l e n e r g i e s of the outgoing p a r t i c l e s , and to the l a r g e binding e n e r g i e s of the t r a n s f e r r e d nucleons. T h e (n,p) r e a g t i o n a t 14.4 MeV l e a d s to 4 lowlying l e v e l s of N I ° 2- (gs), 0- (0.119 MeV), 3(0.295 MeV) and 1- (0.393 MeV). T h e ang~li~r d i s t r i b u t i o n of t h e s e f o u r u n r e s o l v e d g r o u p s of p r o tons i s shown in fig. 2. The c r o s s s e c t i o n i n t e g r a t e d f r o m 0 o to 90 ° y i e l d s ~ 35 mb. T h i s r e s u l t t o g e t h e r with the t o t a l c r o s s s e c t i o n f o r O16(n,p)N 16 a r g u e s f o r the low c r o s s s e c t i o n at l a r g e a n g l e s . The a n g u l a r d i s t r i b u t i o n i s c o m p a r e d with the B u t l e r c u r v e s 7) f o r l = 1 and I = 3, u s i n g r o = 5 f m and A A o s A t s = 3 and 8 f o r I = 1 and I = 3, r e s p e c t i v e l y . Although the B u t l e r c u r v e s a r e not v e r y s a t i s f a c t o r y , it i s obvious
1 April 1964
t h a t t h e O16(n,p)N 16 l e a d i n g to low l y i n g s t a t e s i s mainly a direct reaction. The a u t h o r s e x p r e s s t h e i r t h a n k s to D r . M. C e r i n e o , M r . V. Valkovi~ and J. T u d o r t 6 - G h e m o f o r t h e i r help d u r i n g t h i s w o r k and f o r s t i m u l a t i n g discussions.
RcfP.~'ences 1) T. Lauritsen and F. Ajzenberg-Selove, Energy levels of light nuclei (NRC, 1962); M.D. Goldberg, V.M.May and J.R.Stehn, Angular distributions in neutron induced reactions, BNL400, Vol. 1 (1962). 2) E. Gadioli and S. Micheletti, Physics Letters 6 (1963) 229. 3) G. Marcazzan, A.M. Sona and M. Pignanelli, Nuovo Cimento 10 (1958) 155; F. L.Ribe and J.D~Seagrave, Phys. Rev. 94 (1954)
934; R. N. Glover, K. M. p u r s e r and E. Weigold, Nuclear Instr. and Methods 10 (1961) 343. 4) L.G.Kuo, M.Petravi6 and B.Turko, Nuclear Instr. and Methods 10 (1961) 53. 5) A. B. Lfllie, Phys. Rev. 87 (1952) 716. 6) G.D.Joanou and H. Fenech, Reactor Sci. Techn. 17 (1963) 425; D. G. Gardner, Nuclear Phys. 29 (1962) 373. 7) S.T.Butler, Phys. Rev. 106 (1957) 272. 8) J.L.Wefl and K.W.Jones, Phys. Rev. 112 (1960) 1975. 9) M. H. Macfarlane and J. B. French, Revs. Modern Phys. 32 (1960) 567. 10) E . L . K e l l e r , Phys. Rev. 121 (1961) 820.
PROTON ANGULAR DISTRIBUTIONS FROM THE REACTION MEASURED A T 14 M e V T R I T O N
Be9(t,p)Be ENERGY
11
S. HINDS and R. MIDDLETON Atomic Weapons Research Establishment, Aldermaston, Berkshire, England
Received 17 February 1964 T a l m i and Unna 1) have p r e d i c t e d that the s p i n and p a r i t y of the ground s t a t e of Be l l should be ~+. T h e s i m p l e s h e l l m o d e l a s s i g n m e n t f o r t h i s s t a t e i s ½-, which T a l m i and Unna a s s i g n to the first excited state. The study of the E - d e c a y of Be l l b y W i l k i n s o n and A l b u r g e r 2):, t o g e t h e r with the m o s t p r o b a b l e s p i n s and p a r i t i e s f o r the s t a t e s 3) of B e l l s u g g e s t that the ground s t a t e of B e l l p r o b a b l y h a s p o s i t i v e
p a r i t y . To t r y to c o n f i r m t h i s t e n t a t i v e a s s i g n m e n t P u l l e n e t al. 4) studied the B e 9 ( t , p ) B e r l r e a c t i o n a t i n c i d e n t t r i t o n e n e r g i e s of 5.96 and 10.08 MeV. At t h e s e e n e r g i e s the p r o t o n a n g u l a r d i s t r i b u t i o n s f o r the g r o u p s l e a d i n g to the ground and f i r s t e x c i t e d s t a t e s of Be l l did not show good s t r i p p i n g p a t t e r n s and in p a r t i c u l a r d i s p l a y e d l a r g e b a c k w a r d p e a k s . Since the e x c i t a t i o n of the compound s t a t e f o r t h i s r e a c t i o n i s only about 13 149
Volume 9, number 2
PHYSICS LETTERS
MeV p l u s the i n c i d e n t e n e r g y in the c e n t r e - o f - m a s s , it i s p o s s i b l e that the a n g u l a r d i s t r i b u t i o n s of PUllen et ai. show the effects of i n t e r f e r e n c e between the c o n t r i b u t i o n s f r o m s t r i p p i n g and f r o m the c o m pound n u c l e u s p r o c e s s . It was t h e r e f o r e thought worthwhile to r e p e a t the m e a s u r e m e n t s on the Be9(t,p) r e a c t i o n at a higher b o m b a r d i n g e n e r g y w h e r e compound n u c l e u s effects should be l e s s pronounced. Also following the s u r v e y of Middleton and P u l l e n 5) t h e r e i s now g r e a t e r confidence in the i n t e r p r e t a t i o n of (t,p) r e a c t i o n s when studied at i n c i d e n t e n e r g i e s g r e a t e r than 10 MeV. T r i t o n s w e r e a c c e l e r a t e d to an e n e r g y of 14.0 MeV in the A l d e r m a s t o n t a n d e m e l e c t r o s t a t i c g e n e r a t o r and the r e a c t i o n p r o d u c t s w e r e a n a lysed in the m u l t i - g a p m a g n e t i c s p e c t r o g r a p h 5). The t a r g e t was a thin s e l f - s u p p o r t i n g f o i l of b e r y l l i u m which was about 80 ~ g / c m z thick. The proton g r o u p s c o r r e s p o n d i n g to the ground and f i r s t excited s t a t e s of Be l l w e r e identified at a l l "angles of o b s e r v a t i o n by t h e i r c h a r a c t e r i s t i c change of e n e r g y with angle. The a n g u l a r d i s t r i b u t i o n s of t h e s e two g r o u p s i s shown in fig. 1 w h e r e both p l o t s have the s a m e a r b i t r a r y unit of c r o s s - s e c t i o n . The c u r v e s in the f i g u r e a r e p l a n e wave two n u c l e o n s t r i p p i p g c u r v e s c a l c u l a t e d f r o m eq. (14) of Newns 7). The r a d i i of i n t e r a c tion r o used for a l l c u r v e s was 6.0 fro. T h i s value was previously_ found to give b e s t a g r e e m e n t for the Beg(He3,p)B 11 r e a c t i o n 8). It m a y be s e e n that for the f i r s t excited state d i s t r i b u t i o n t h e r e i s no b a c k w a r d peak and good
9Bt(t,p) HB¢ , Et ' 1 4 . 0 ~/-\ h
~'°t
~ L 1 ,
References
- 2 L'3
i ,\ i
1) I. Talmi and I. Unna, Phys. Rev. Letters 4 (1960) 469. 2) D.H.Wilkinson and D.E.Alburger, Phys. Rev. 113 (1959) 563.
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AM~LE (C of .)9e,sees Fig. 1 ~ Angular distribution of protons measured from the Be~(t,p) reacttoq~leading to the ground and first excited states of Be ~ . The incident triton e~ergy was 14.0 MeV and the curves were oaleulated from the t w o nucleon stripping theory of Newns 7) using r o = 6.0 fm.
150
2
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a g r e e m e n t i s obtained with an L = 2 s t r i p p i n g curve. T h i s a g r e e s with the t e n t a t i v e fit of P u l len et a l . , although t h e i r d i s t r i b u t i o n had a l a r g e backward peak. T h i s a s s i g n m e n t of L = 2 is in a g r e e m e n t with the p r e d i c t i o n of T a l m i and Unna for He f i r s t excited state (½-) t r a n s i t i o n . T h i s should p r o c e e d by t r a n s f e r r i n g a l p $ l p ! n e u t r o n p a i r which • 2 would allow L = 0 o r ~ , except that L = 0 i s f o r bidden for a _3_ i 2 to a ~ t r a n s i t i o n . The 319 keV f i r s t excited state m a y t h e r e f o r e f a i r l y confidently be a s s i g n e d a s n e g a t i v e p a r i t y ? with p o s s i b l e spins of ½, ~ , ~ or ~, w •t h ~3 u n f a v o u r e d since no L = 0 c o n t r i b u t i o n i s o b s e r v e d . The ground state at 14 MeV i n c i d e n t e n e r g y does not appear to be f o r m e d by stripping. The v e r y slight a g r e e m e n t with an L = 3 c u r v e to the f o r w a r d a n g l e s i s p r o b a b l y s p u r i o u s . It has a m a x i m u m i n t e n s i t y of about 20% that of the f i r s t excited state, i . e . , it is only a little s t r o n g e r than the l a r g e angle background of the l a t t e r . It m a y be r e m a r k e d that T a l m i and Unna expected the ground state of Be l l to have a 2s_~ c o n f i g u r a tion, so that it should be f o r m e d fror~ Be 9 by c a p t u r i n g a 1 p~ 1 s~_ n e u t r o n p a i r which c o r r e sponds to a L 2 1 s~ripping p a t t e r n . Since this L - v a l u e i s s m a l l e r than that for the f i r s t excited s t a t e , the ground state would " a p r i o r i " be expected to be s t r o n g e r than the f i r s t excited state. It i s difficult to u n d e r s t a n d why the ground state of Be l l is not f o r m e d by (t,p) s t r i p p i n g if it has the p r e d i c t e d 2 s ! configuration.
0.319 McV
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z t'2
1 April 1964
1
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3) P.T.Donovan, J.V.Kane, R.E.Plzley and D.H.Wilkinson, Phys. Rev. 123 (1961) 589. 4) D. J. Pullen, A.E. Litherland, S. Hinds and R. Middle ton, Nuclear Phys. 36 (1962) 1. 5) R.Middleten and D.J.PuUen, Nuclear Phys. 51 (1964) 50. 6) R. Middleton and S. Hinds, Nuclear Phys. 34 (1962) 404. 7) H . C . N e w n s , Proc. Phys. Soc. 76 (1960) 489: 8) S. Hinds and R. Middleten, Proc. Phys. Soc. 75 (1960) 754.
PHYSICS LETTERS
the d a t a of r e f . 2 and the p r e s e n t d a t a cannot b e , t h e r e f o r e , e x p l a i n e d a s a r e s u l t of a s l i g h t l y d i f f e r e n t n e u t r o n e n e r g y . The c r o s s s e c t i o n i n t e g r a t e d within the f o r w a r d c r o s s of 60 ° y i e l d s 9.5 rob. T h e ang~d~r d i s t r i b u t i o n i s c o m p a r e d with the B u t l e r l = 1 c u r v e f o r t o = 5.5 fro. The t h e o r e t i cal curve is smeared. The extracted reduced width ~2 i s 0.076. T a k i n g f o r the s p e c t r o s c o p i c f a c t o r S = 4 9), one g e t s 02 = 0.019. The O16(d,t)O 15 w a s i n v e s t i g a t e d 10) a t E d = 14.9 MeV Yi2elding A~ 2 = 2 . 4 ( A ~ 190) and t h e r e f o r e ~o ~ 0.003. The ~z e x t r a c t e d f r o m the d a t a of r e f . 8,10 and f r o m the p r e s e n t m e a s u r e m e n t a r e s m a l l e r than 02 f o r l p s h e l l . It w a s s u g g e s t e d that such a smallV 02 could be r e l a t e d to the s m a l l e n e r g i e s of the outgoing p a r t i c l e s , and to the l a r g e binding e n e r g i e s of the t r a n s f e r r e d nucleons. T h e (n,p) r e a g t i o n a t 14.4 MeV l e a d s to 4 lowlying l e v e l s of N I ° 2- (gs), 0- (0.119 MeV), 3(0.295 MeV) and 1- (0.393 MeV). T h e ang~li~r d i s t r i b u t i o n of t h e s e f o u r u n r e s o l v e d g r o u p s of p r o tons i s shown in fig. 2. The c r o s s s e c t i o n i n t e g r a t e d f r o m 0 o to 90 ° y i e l d s ~ 35 mb. T h i s r e s u l t t o g e t h e r with the t o t a l c r o s s s e c t i o n f o r O16(n,p)N 16 a r g u e s f o r the low c r o s s s e c t i o n at l a r g e a n g l e s . The a n g u l a r d i s t r i b u t i o n i s c o m p a r e d with the B u t l e r c u r v e s 7) f o r l = 1 and I = 3, u s i n g r o = 5 f m and A A o s A t s = 3 and 8 f o r I = 1 and I = 3, r e s p e c t i v e l y . Although the B u t l e r c u r v e s a r e not v e r y s a t i s f a c t o r y , it i s obvious
1 April 1964
t h a t t h e O16(n,p)N 16 l e a d i n g to low l y i n g s t a t e s i s mainly a direct reaction. The a u t h o r s e x p r e s s t h e i r t h a n k s to D r . M. C e r i n e o , M r . V. Valkovi~ and J. T u d o r t 6 - G h e m o f o r t h e i r help d u r i n g t h i s w o r k and f o r s t i m u l a t i n g discussions.
RcfP.~'ences 1) T. Lauritsen and F. Ajzenberg-Selove, Energy levels of light nuclei (NRC, 1962); M.D. Goldberg, V.M.May and J.R.Stehn, Angular distributions in neutron induced reactions, BNL400, Vol. 1 (1962). 2) E. Gadioli and S. Micheletti, Physics Letters 6 (1963) 229. 3) G. Marcazzan, A.M. Sona and M. Pignanelli, Nuovo Cimento 10 (1958) 155; F. L.Ribe and J.D~Seagrave, Phys. Rev. 94 (1954)
934; R. N. Glover, K. M. p u r s e r and E. Weigold, Nuclear Instr. and Methods 10 (1961) 343. 4) L.G.Kuo, M.Petravi6 and B.Turko, Nuclear Instr. and Methods 10 (1961) 53. 5) A. B. Lfllie, Phys. Rev. 87 (1952) 716. 6) G.D.Joanou and H. Fenech, Reactor Sci. Techn. 17 (1963) 425; D. G. Gardner, Nuclear Phys. 29 (1962) 373. 7) S.T.Butler, Phys. Rev. 106 (1957) 272. 8) J.L.Wefl and K.W.Jones, Phys. Rev. 112 (1960) 1975. 9) M. H. Macfarlane and J. B. French, Revs. Modern Phys. 32 (1960) 567. 10) E . L . K e l l e r , Phys. Rev. 121 (1961) 820.
PROTON ANGULAR DISTRIBUTIONS FROM THE REACTION MEASURED A T 14 M e V T R I T O N
Be9(t,p)Be ENERGY
11
S. HINDS and R. MIDDLETON Atomic Weapons Research Establishment, Aldermaston, Berkshire, England
Received 17 February 1964 T a l m i and Unna 1) have p r e d i c t e d that the s p i n and p a r i t y of the ground s t a t e of Be l l should be ~+. T h e s i m p l e s h e l l m o d e l a s s i g n m e n t f o r t h i s s t a t e i s ½-, which T a l m i and Unna a s s i g n to the first excited state. The study of the E - d e c a y of Be l l b y W i l k i n s o n and A l b u r g e r 2):, t o g e t h e r with the m o s t p r o b a b l e s p i n s and p a r i t i e s f o r the s t a t e s 3) of B e l l s u g g e s t that the ground s t a t e of B e l l p r o b a b l y h a s p o s i t i v e
p a r i t y . To t r y to c o n f i r m t h i s t e n t a t i v e a s s i g n m e n t P u l l e n e t al. 4) studied the B e 9 ( t , p ) B e r l r e a c t i o n a t i n c i d e n t t r i t o n e n e r g i e s of 5.96 and 10.08 MeV. At t h e s e e n e r g i e s the p r o t o n a n g u l a r d i s t r i b u t i o n s f o r the g r o u p s l e a d i n g to the ground and f i r s t e x c i t e d s t a t e s of Be l l did not show good s t r i p p i n g p a t t e r n s and in p a r t i c u l a r d i s p l a y e d l a r g e b a c k w a r d p e a k s . Since the e x c i t a t i o n of the compound s t a t e f o r t h i s r e a c t i o n i s only about 13 149
Volume 9, number 2
PHYSICS LETTERS
MeV p l u s the i n c i d e n t e n e r g y in the c e n t r e - o f - m a s s , it i s p o s s i b l e that the a n g u l a r d i s t r i b u t i o n s of PUllen et ai. show the effects of i n t e r f e r e n c e between the c o n t r i b u t i o n s f r o m s t r i p p i n g and f r o m the c o m pound n u c l e u s p r o c e s s . It was t h e r e f o r e thought worthwhile to r e p e a t the m e a s u r e m e n t s on the Be9(t,p) r e a c t i o n at a higher b o m b a r d i n g e n e r g y w h e r e compound n u c l e u s effects should be l e s s pronounced. Also following the s u r v e y of Middleton and P u l l e n 5) t h e r e i s now g r e a t e r confidence in the i n t e r p r e t a t i o n of (t,p) r e a c t i o n s when studied at i n c i d e n t e n e r g i e s g r e a t e r than 10 MeV. T r i t o n s w e r e a c c e l e r a t e d to an e n e r g y of 14.0 MeV in the A l d e r m a s t o n t a n d e m e l e c t r o s t a t i c g e n e r a t o r and the r e a c t i o n p r o d u c t s w e r e a n a lysed in the m u l t i - g a p m a g n e t i c s p e c t r o g r a p h 5). The t a r g e t was a thin s e l f - s u p p o r t i n g f o i l of b e r y l l i u m which was about 80 ~ g / c m z thick. The proton g r o u p s c o r r e s p o n d i n g to the ground and f i r s t excited s t a t e s of Be l l w e r e identified at a l l "angles of o b s e r v a t i o n by t h e i r c h a r a c t e r i s t i c change of e n e r g y with angle. The a n g u l a r d i s t r i b u t i o n s of t h e s e two g r o u p s i s shown in fig. 1 w h e r e both p l o t s have the s a m e a r b i t r a r y unit of c r o s s - s e c t i o n . The c u r v e s in the f i g u r e a r e p l a n e wave two n u c l e o n s t r i p p i p g c u r v e s c a l c u l a t e d f r o m eq. (14) of Newns 7). The r a d i i of i n t e r a c tion r o used for a l l c u r v e s was 6.0 fro. T h i s value was previously_ found to give b e s t a g r e e m e n t for the Beg(He3,p)B 11 r e a c t i o n 8). It m a y be s e e n that for the f i r s t excited state d i s t r i b u t i o n t h e r e i s no b a c k w a r d peak and good
9Bt(t,p) HB¢ , Et ' 1 4 . 0 ~/-\ h
~'°t
~ L 1 ,
References
- 2 L'3
i ,\ i
1) I. Talmi and I. Unna, Phys. Rev. Letters 4 (1960) 469. 2) D.H.Wilkinson and D.E.Alburger, Phys. Rev. 113 (1959) 563.
~
,
• i $o
IO
Io
• i° Ill
i$l
AM~LE (C of .)9e,sees Fig. 1 ~ Angular distribution of protons measured from the Be~(t,p) reacttoq~leading to the ground and first excited states of Be ~ . The incident triton e~ergy was 14.0 MeV and the curves were oaleulated from the t w o nucleon stripping theory of Newns 7) using r o = 6.0 fm.
150
2
HeY
~p',, i
a g r e e m e n t i s obtained with an L = 2 s t r i p p i n g curve. T h i s a g r e e s with the t e n t a t i v e fit of P u l len et a l . , although t h e i r d i s t r i b u t i o n had a l a r g e backward peak. T h i s a s s i g n m e n t of L = 2 is in a g r e e m e n t with the p r e d i c t i o n of T a l m i and Unna for He f i r s t excited state (½-) t r a n s i t i o n . T h i s should p r o c e e d by t r a n s f e r r i n g a l p $ l p ! n e u t r o n p a i r which • 2 would allow L = 0 o r ~ , except that L = 0 i s f o r bidden for a _3_ i 2 to a ~ t r a n s i t i o n . The 319 keV f i r s t excited state m a y t h e r e f o r e f a i r l y confidently be a s s i g n e d a s n e g a t i v e p a r i t y ? with p o s s i b l e spins of ½, ~ , ~ or ~, w •t h ~3 u n f a v o u r e d since no L = 0 c o n t r i b u t i o n i s o b s e r v e d . The ground state at 14 MeV i n c i d e n t e n e r g y does not appear to be f o r m e d by stripping. The v e r y slight a g r e e m e n t with an L = 3 c u r v e to the f o r w a r d a n g l e s i s p r o b a b l y s p u r i o u s . It has a m a x i m u m i n t e n s i t y of about 20% that of the f i r s t excited state, i . e . , it is only a little s t r o n g e r than the l a r g e angle background of the l a t t e r . It m a y be r e m a r k e d that T a l m i and Unna expected the ground state of Be l l to have a 2s_~ c o n f i g u r a tion, so that it should be f o r m e d fror~ Be 9 by c a p t u r i n g a 1 p~ 1 s~_ n e u t r o n p a i r which c o r r e sponds to a L 2 1 s~ripping p a t t e r n . Since this L - v a l u e i s s m a l l e r than that for the f i r s t excited s t a t e , the ground state would " a p r i o r i " be expected to be s t r o n g e r than the f i r s t excited state. It i s difficult to u n d e r s t a n d why the ground state of Be l l is not f o r m e d by (t,p) s t r i p p i n g if it has the p r e d i c t e d 2 s ! configuration.
0.319 McV
L ',
z t'2
1 April 1964
1
j[ iio
3) P.T.Donovan, J.V.Kane, R.E.Plzley and D.H.Wilkinson, Phys. Rev. 123 (1961) 589. 4) D. J. Pullen, A.E. Litherland, S. Hinds and R. Middle ton, Nuclear Phys. 36 (1962) 1. 5) R.Middleten and D.J.PuUen, Nuclear Phys. 51 (1964) 50. 6) R. Middleton and S. Hinds, Nuclear Phys. 34 (1962) 404. 7) H . C . N e w n s , Proc. Phys. Soc. 76 (1960) 489: 8) S. Hinds and R. Middleten, Proc. Phys. Soc. 75 (1960) 754.