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The ‘quasielastic scattering’ of 6Li ions determined by the charge-exchange spin-flip forces
Volume 3I_B. number 1,0
THE DETERMINED
PHYSICS LETTERS
l l May 1970
'QUASIELASTIC SCATTERING' BY T H E C H A R G E - E X C H A N G E
OF 6Li IONS SPIN-FLIP FORCES
V. I. CHUEV, V.V. DAVYDOV. V.I. MANGO. B. G. NOVATSgY. S. B. SAKUTA and D. N. STEPANOV I, V.ti~rchatoc lnstil~dc of the AIov~ic E n e r g y . Moscom. USSR
Received 20 March 1970
The 'quasielastic scattering' of 6Li-ions. determined by the 171 72)(alO'2)-forces in the nucleon-nucleon interaction, is considered. The experimental results on the 13C(6Li. 6tte)t3N and 9Be(6Li. 6He)9B r e actions, which appear to prove that these reactions are such 'quasielastic scattering' processes, are presented.
The direct charge-exchange (p. n) and (3He, t) nuclear reactions were widely investigated in recent years [1.21 . These reactions were explained by Lane as determined by the chargeexchange (71 ~'2)-forces in the interaction of nucleons ~3I. Lane noted also that the chargeexchange spin-flip (~I ~2)(71 72)-forces in this interaction could contribute to such processes, but their contribution should be considerably smaller. The experimental results, which seem to point out the importance of these forces for (p.n)-reactions. were obtained in the 170. 180. and Ti-isotopes cases [4.51 . The ((~i cr2)(7172)-forces should play an important role in some nuclear reactions, induced by the 6Li-ions. The investigations of these r e actions can give valuable information about the nuclear forces and about the nucleus structure. The (6Li, 6He)-reaction and (6Li, 6Li* (0+,T = I, 3.56 MeV))-reaction. can be considered as 'quasielastic scattering' direct processes, due to the effect of the charge-exchange spin-flip forces. The advantages of the reactions above a r e : 1) the other exchange f o r c e s ((ct1 cr2) or (7172)) cannot contribute to t h e m ; 2) the other m e c h a n i s m s (the c o m p o u n d - n u c l e u s f o r m a t i o n or s o m e other i n d i r e c t p r o c e s s e s ) appear to be i m p r o b a b l e for t h em ; 3) the l e v e l s of r e l e v a n t s t r u c t u r e of r e s i d u a l nuclei a r e ' s e l e c t e d ' in t h e s e r e a c t i o n s . The detailed t r e a t m e n t of th e s e p r o c e s s e s will be published e l s e w h e r e . H e r e we b r i e f l y c o n s i d e r the ' q u a s i e l a s t i e s c a t t e r i n g ' of the 6 L i - i o n s . The e x p e r i m e n t a l r e s u l t s on the 13C(6Li. 6He)13N and 9Be(6Li. 6He)913 r e a c t i o n s . which show that t h e s e r e a c t i o n s a r e indeed such 'quasielastic scattering' processes, are presented. 624
Let us c o n s i d e r f i r s t l y the r e a c t i o n 6Li+zAN - 6He + z A N _ I . w h e r e z A N i S a n u cleus with Z p r o t o n s and N neutrons. The ground s t a t e s of the 6Li and 6He nuclei a r e v e r y s i m i lar. This can be concluded f r o m the fact that the ;~-decay of the 6 H e - n u c l e u s to the ground state of the 6 L i - n u c l e u s is the s u p e r - a l l o w e d t r a n s i t i o n ( l o g f t = 2 . 9 2 [61). In the shell model the 6 L i a n d 6He nuclei ground s t a t e s a r e the 1S 3 (T=0) and 1So ( T= 1) states r e s p e c t i v e l y . In the r e a c t i o n above the c h a r g e - e x c h a n g e between one proton of the 6 L i - n u c l e u s and one of the t a r g e t nucleus n e u t r o n s takes place. This c h a r g e - e x c h a n g e should be followed by the sp i n - f l i p b e c a u s e of the Pauli exclusion principle (it is impossible for two extra neutrons of the 6He-nucleus to be in the state with the parallel spins as are the proton and neutron in the 6Li-nucleus). Consequently this reaction has to be determined exactly by the (~I ~2)(71 T2)-f°rces in the interaction of the 6Li-nucleus proton with the target nucleus neutrons, namely by the nondiagonal (~i ~2)(7(+I) 7~2)) term. The features of the 'quasielastic' (6Li, 6He) -reaction can be understood qualita tively by using a rather simple model for its description. Assuming that the orbital angular momentum is not transferred to the target nucleus neutron, one can readily obtain that for the neutron in the state, labeled by the quantum numbers l, j, m, the only allowed transitions are that to the proton states with the quantum n u m b e r s [ , j , m; [ , j , } n + l ; and l, j I m ; [ , j I ~n=L1. H e r e ,
jl
= I
j+l
at
j=l-½
j-1
at
j=l+½
Volume 3lB. n u m b e r 10
PHYSICS
It f o l l o w s f r o m t h i s t h a t in t h i s r e a c t i o n t h e residual nucleus levels, which can be obtained from the target nucleus ground state as a result of s u c h s i n g l e - p a r t i c l e t r a n s i t i o n s , h a v e to b e e x c i t e d in p r e f e r e n c e . F o r e x a m p l e , in t h e c a s e of t h e e v e n - o d d t a r g e t n u c l e u s , t h e t r a n s i t i o n s of t h e odd u n p a i r e d n e u t r o n c a n in g e n e r a l l e a d to two s t a t e s , T h e f i r s t i s t h e p r o t o n l , j - s t a t e , w h i c h i s t h e a n a l o g of t h e t a r g e t n u c l e u s g r o u n d s t a t e . T h i s a n a l o g u e s t a t e s h o u l d b e e x c i t e d in a n y c a s e . T h e s e c o n d i s t h e p r o t o n l, j l - s t a t e . It c a n b e e x c i t e d o n l y in t h e c a s e w h e n in t h e t a r g e t n u c l e u s t h e p r o t o n j l - s u b s h e l l i s not c o m p l e t e l y filled. T h e r e a c t i o n (6Li. 6Li* ( 0 - , T = 1, E * = 3.56 M e V ) ) w i t h t h e e x c i t a t i o n of t h e f i r s t T = 1 s t a t e in t h e 6 L i - n u c l e u s s h o u l d a l s o b e t h e d i r e c t r e action determined by the (~71a2)(r 1 r2)-forces, t h e d i a g o n a l (or1 cr2)(~(1)~(2))-term. C o n s e q u e n t l y . it s h o u l d h a v e t h e s a m e f e a t u r e s a s t h e (6Li, 6 H e ) - r e a c t i o n . T h e o n l y d i f f e r e n c e i s t h a t t h e s i n g l e - p a r t i c l e t r a n s i t i o n s of t h e t a r g e t n u cleus nucleons occur without charge changing. It i s w o r t h - w h i l e to m e n t i o n h e r e t h a t t h e c h a r g e - e x c h a n g e s p i n - f l i p f o r c e s s h o u l d l e a d to t h e s a m e d i r e c t n u c l e a r r e a c t i o n s of t h e ' q u a s i e l a s t i c s c a t t e r i n g ' of d e u t e r o n s w i t h t h e f o r m a t i o n of t h e b i n e u t r o n o r t h e d e u t e r o n in t h e T= 1,0 + state. The 13C(6Li, 6He)13N and 9Be(6Li. 6He)9B r e a c t i o n s w e r e s t u d i e d w i t h t h e 6 L i - i o n s b e a m of t h e c y c l o t r o n of t h e I. V. K u r c h a t o v I n s t i t u t e of Atomic Energy. The beam energy was 30.8 MeV. T h e e x p e r i m e n t a l a r r a n g e m e n t w a s a s in t h e r e c e n t e x p e r i m e n t s w i t h 6 L i - i o n s a n d w a s d e s c r i b e d e a r l i e r [7]. In t h e e x p e r i m e n t t h e d E - E - s y s t e m of m a s s - s e p a r a t i o n was used. The e n e r g y s p e c t r a of t h e 6 H e - i o n s f r o m t h e two r e a c t i o n s a r e s h o w n in fig. 1, It i s s e e n t h a t o n l y f e w l e v e l s of t h e r e s i d u a l n u c l e i a r e s t r o n g l y e x c i t e d in t h e r e a c t i o n s . In t h e c a s e of t h e 13C target nucleus these are the ground state, which i s t h e p r o t o n P12 s t a t e a n d c a n b e o b t a i n e d a s a r e s u l t of t h e odd u n p a i r e d P 1 / 2 - n e u t r o n t r a n s i t i o n s , a n d o n e of t h e two s t a t e s of t h e ~-+ ~2 d o u b l e t a t 3.5 MeV. It a p p e a r s to b e t h e ~- l e v e l of t h i s d o u b l e t , w h i c h c a n b e e x c i t e d in t h e r e a c t i o n a s a r e s u l t of t h e p a i r e d P : ~ / 2 - n e u t r o n s t r a n s i t i o n s . In t h e c a s e of t h e 9 B e t a r g e t n u c l e u s t h e p r o t o n P:~,2-state ( g r o u n d s t a t e ) i s s t r o n g l y e x c i t e d in t h e r e a c t i o n . It c a n b e o b t a i n e d a s a r e s u l t of t h e odd u n p a i r e d P ~ / ~ - n e u t r o n t r a n s i t i o n s . T h e o t h e r p e a k s , s e e n in t h e s p e c t r u m , a p p e a r to c o r r e s p o n d to ~ - l e v e l , w h i c h c a n b e e x c i t e d a s a r e s u l t of t h e p a i r e d P : ~ / ~ - n e u t r o n s t r a n s i tions.
LETTEtlS
11 May 1970
N 5O
25
•
50
":" "~
I00
.P i
450 Bho~neB'nu~nBe~
'3C(6LiNe) lN 25
.
0
.....
50
•
.'
I00
OcM--8 50
I1..-
?"
150
I 400
450 ghanne~ndmgez
l Fig. 1. T h e s p e c t r a s h o w n in fig. 1 a r e q u a l i t a t i v e l y in f a i r a g r e e m e n t w i t h t h e a s s u m p t i o n a b o u t t h e ' q u a s i e l a s t i c s c a t t e r i n g ' m e c h a n i s m of t h e r e a c t i o n . T h e p r e s e n c e of t h e w e a k l i n e . c o r r e s p o n d i n g to t h e ½ + - l e v e l e x c i t a t i o n in t h e 13C(6Li. 6He)13N reaction, suggests that the transitions with the orbital angular momentum t r a n s f e r a l s o c o n t r i b u t e to t h e r e a c t i o n .
625
Volume 3lB. number l0
PHYSICS
References 1. J. D. A n d e r s o n and C . W o n g . P h y s . Rev. L e t t e r s 7 (1961) 250. 2. P . G . R o o s . C . A . L u d e m a n n and I, I. W e s e l o w s k i . P h y s . L e t t e r s 24B (1967) 656. 3. A . M . L a n e . Nucl. P h y s . 35 (1962) 676. 4. J. D. A n d e r s o n , S. D. B l o o m . C. Wong. W . E . H o r n y a k and V. A. M a d s e n . P h y s . Rev, 177 (1969) 1416.
626
LETTERS
11 May 1970
5. C . D . G o o d m a n . J . D . A n d e r s o n a n d C. W o n g . P h y s . Rev. 156 (1967) 1249. 6. T. L a u r i t s e n a n d F. A j z e n b e r g - S e l o v e , Nucl. P h y s . 78 (1966) i. 7. V, Z. Goldberg. V. V. Davydov. A. A. Oglohlin. S.B. Sakuta and V. I. Chuev, Izv, AN SSSR, ser. fiz. 33 (1969) 566.
THE DETERMINED
PHYSICS LETTERS
l l May 1970
'QUASIELASTIC SCATTERING' BY T H E C H A R G E - E X C H A N G E
OF 6Li IONS SPIN-FLIP FORCES
V. I. CHUEV, V.V. DAVYDOV. V.I. MANGO. B. G. NOVATSgY. S. B. SAKUTA and D. N. STEPANOV I, V.ti~rchatoc lnstil~dc of the AIov~ic E n e r g y . Moscom. USSR
Received 20 March 1970
The 'quasielastic scattering' of 6Li-ions. determined by the 171 72)(alO'2)-forces in the nucleon-nucleon interaction, is considered. The experimental results on the 13C(6Li. 6tte)t3N and 9Be(6Li. 6He)9B r e actions, which appear to prove that these reactions are such 'quasielastic scattering' processes, are presented.
The direct charge-exchange (p. n) and (3He, t) nuclear reactions were widely investigated in recent years [1.21 . These reactions were explained by Lane as determined by the chargeexchange (71 ~'2)-forces in the interaction of nucleons ~3I. Lane noted also that the chargeexchange spin-flip (~I ~2)(71 72)-forces in this interaction could contribute to such processes, but their contribution should be considerably smaller. The experimental results, which seem to point out the importance of these forces for (p.n)-reactions. were obtained in the 170. 180. and Ti-isotopes cases [4.51 . The ((~i cr2)(7172)-forces should play an important role in some nuclear reactions, induced by the 6Li-ions. The investigations of these r e actions can give valuable information about the nuclear forces and about the nucleus structure. The (6Li, 6He)-reaction and (6Li, 6Li* (0+,T = I, 3.56 MeV))-reaction. can be considered as 'quasielastic scattering' direct processes, due to the effect of the charge-exchange spin-flip forces. The advantages of the reactions above a r e : 1) the other exchange f o r c e s ((ct1 cr2) or (7172)) cannot contribute to t h e m ; 2) the other m e c h a n i s m s (the c o m p o u n d - n u c l e u s f o r m a t i o n or s o m e other i n d i r e c t p r o c e s s e s ) appear to be i m p r o b a b l e for t h em ; 3) the l e v e l s of r e l e v a n t s t r u c t u r e of r e s i d u a l nuclei a r e ' s e l e c t e d ' in t h e s e r e a c t i o n s . The detailed t r e a t m e n t of th e s e p r o c e s s e s will be published e l s e w h e r e . H e r e we b r i e f l y c o n s i d e r the ' q u a s i e l a s t i e s c a t t e r i n g ' of the 6 L i - i o n s . The e x p e r i m e n t a l r e s u l t s on the 13C(6Li. 6He)13N and 9Be(6Li. 6He)913 r e a c t i o n s . which show that t h e s e r e a c t i o n s a r e indeed such 'quasielastic scattering' processes, are presented. 624
Let us c o n s i d e r f i r s t l y the r e a c t i o n 6Li+zAN - 6He + z A N _ I . w h e r e z A N i S a n u cleus with Z p r o t o n s and N neutrons. The ground s t a t e s of the 6Li and 6He nuclei a r e v e r y s i m i lar. This can be concluded f r o m the fact that the ;~-decay of the 6 H e - n u c l e u s to the ground state of the 6 L i - n u c l e u s is the s u p e r - a l l o w e d t r a n s i t i o n ( l o g f t = 2 . 9 2 [61). In the shell model the 6 L i a n d 6He nuclei ground s t a t e s a r e the 1S 3 (T=0) and 1So ( T= 1) states r e s p e c t i v e l y . In the r e a c t i o n above the c h a r g e - e x c h a n g e between one proton of the 6 L i - n u c l e u s and one of the t a r g e t nucleus n e u t r o n s takes place. This c h a r g e - e x c h a n g e should be followed by the sp i n - f l i p b e c a u s e of the Pauli exclusion principle (it is impossible for two extra neutrons of the 6He-nucleus to be in the state with the parallel spins as are the proton and neutron in the 6Li-nucleus). Consequently this reaction has to be determined exactly by the (~I ~2)(71 T2)-f°rces in the interaction of the 6Li-nucleus proton with the target nucleus neutrons, namely by the nondiagonal (~i ~2)(7(+I) 7~2)) term. The features of the 'quasielastic' (6Li, 6He) -reaction can be understood qualita tively by using a rather simple model for its description. Assuming that the orbital angular momentum is not transferred to the target nucleus neutron, one can readily obtain that for the neutron in the state, labeled by the quantum numbers l, j, m, the only allowed transitions are that to the proton states with the quantum n u m b e r s [ , j , m; [ , j , } n + l ; and l, j I m ; [ , j I ~n=L1. H e r e ,
jl
= I
j+l
at
j=l-½
j-1
at
j=l+½
Volume 3lB. n u m b e r 10
PHYSICS
It f o l l o w s f r o m t h i s t h a t in t h i s r e a c t i o n t h e residual nucleus levels, which can be obtained from the target nucleus ground state as a result of s u c h s i n g l e - p a r t i c l e t r a n s i t i o n s , h a v e to b e e x c i t e d in p r e f e r e n c e . F o r e x a m p l e , in t h e c a s e of t h e e v e n - o d d t a r g e t n u c l e u s , t h e t r a n s i t i o n s of t h e odd u n p a i r e d n e u t r o n c a n in g e n e r a l l e a d to two s t a t e s , T h e f i r s t i s t h e p r o t o n l , j - s t a t e , w h i c h i s t h e a n a l o g of t h e t a r g e t n u c l e u s g r o u n d s t a t e . T h i s a n a l o g u e s t a t e s h o u l d b e e x c i t e d in a n y c a s e . T h e s e c o n d i s t h e p r o t o n l, j l - s t a t e . It c a n b e e x c i t e d o n l y in t h e c a s e w h e n in t h e t a r g e t n u c l e u s t h e p r o t o n j l - s u b s h e l l i s not c o m p l e t e l y filled. T h e r e a c t i o n (6Li. 6Li* ( 0 - , T = 1, E * = 3.56 M e V ) ) w i t h t h e e x c i t a t i o n of t h e f i r s t T = 1 s t a t e in t h e 6 L i - n u c l e u s s h o u l d a l s o b e t h e d i r e c t r e action determined by the (~71a2)(r 1 r2)-forces, t h e d i a g o n a l (or1 cr2)(~(1)~(2))-term. C o n s e q u e n t l y . it s h o u l d h a v e t h e s a m e f e a t u r e s a s t h e (6Li, 6 H e ) - r e a c t i o n . T h e o n l y d i f f e r e n c e i s t h a t t h e s i n g l e - p a r t i c l e t r a n s i t i o n s of t h e t a r g e t n u cleus nucleons occur without charge changing. It i s w o r t h - w h i l e to m e n t i o n h e r e t h a t t h e c h a r g e - e x c h a n g e s p i n - f l i p f o r c e s s h o u l d l e a d to t h e s a m e d i r e c t n u c l e a r r e a c t i o n s of t h e ' q u a s i e l a s t i c s c a t t e r i n g ' of d e u t e r o n s w i t h t h e f o r m a t i o n of t h e b i n e u t r o n o r t h e d e u t e r o n in t h e T= 1,0 + state. The 13C(6Li, 6He)13N and 9Be(6Li. 6He)9B r e a c t i o n s w e r e s t u d i e d w i t h t h e 6 L i - i o n s b e a m of t h e c y c l o t r o n of t h e I. V. K u r c h a t o v I n s t i t u t e of Atomic Energy. The beam energy was 30.8 MeV. T h e e x p e r i m e n t a l a r r a n g e m e n t w a s a s in t h e r e c e n t e x p e r i m e n t s w i t h 6 L i - i o n s a n d w a s d e s c r i b e d e a r l i e r [7]. In t h e e x p e r i m e n t t h e d E - E - s y s t e m of m a s s - s e p a r a t i o n was used. The e n e r g y s p e c t r a of t h e 6 H e - i o n s f r o m t h e two r e a c t i o n s a r e s h o w n in fig. 1, It i s s e e n t h a t o n l y f e w l e v e l s of t h e r e s i d u a l n u c l e i a r e s t r o n g l y e x c i t e d in t h e r e a c t i o n s . In t h e c a s e of t h e 13C target nucleus these are the ground state, which i s t h e p r o t o n P12 s t a t e a n d c a n b e o b t a i n e d a s a r e s u l t of t h e odd u n p a i r e d P 1 / 2 - n e u t r o n t r a n s i t i o n s , a n d o n e of t h e two s t a t e s of t h e ~-+ ~2 d o u b l e t a t 3.5 MeV. It a p p e a r s to b e t h e ~- l e v e l of t h i s d o u b l e t , w h i c h c a n b e e x c i t e d in t h e r e a c t i o n a s a r e s u l t of t h e p a i r e d P : ~ / 2 - n e u t r o n s t r a n s i t i o n s . In t h e c a s e of t h e 9 B e t a r g e t n u c l e u s t h e p r o t o n P:~,2-state ( g r o u n d s t a t e ) i s s t r o n g l y e x c i t e d in t h e r e a c t i o n . It c a n b e o b t a i n e d a s a r e s u l t of t h e odd u n p a i r e d P ~ / ~ - n e u t r o n t r a n s i t i o n s . T h e o t h e r p e a k s , s e e n in t h e s p e c t r u m , a p p e a r to c o r r e s p o n d to ~ - l e v e l , w h i c h c a n b e e x c i t e d a s a r e s u l t of t h e p a i r e d P : ~ / ~ - n e u t r o n s t r a n s i tions.
LETTEtlS
11 May 1970
N 5O
25
•
50
":" "~
I00
.P i
450 Bho~neB'nu~nBe~
'3C(6LiNe) lN 25
.
0
.....
50
•
.'
I00
OcM--8 50
I1..-
?"
150
I 400
450 ghanne~ndmgez
l Fig. 1. T h e s p e c t r a s h o w n in fig. 1 a r e q u a l i t a t i v e l y in f a i r a g r e e m e n t w i t h t h e a s s u m p t i o n a b o u t t h e ' q u a s i e l a s t i c s c a t t e r i n g ' m e c h a n i s m of t h e r e a c t i o n . T h e p r e s e n c e of t h e w e a k l i n e . c o r r e s p o n d i n g to t h e ½ + - l e v e l e x c i t a t i o n in t h e 13C(6Li. 6He)13N reaction, suggests that the transitions with the orbital angular momentum t r a n s f e r a l s o c o n t r i b u t e to t h e r e a c t i o n .
625
Volume 3lB. number l0
PHYSICS
References 1. J. D. A n d e r s o n and C . W o n g . P h y s . Rev. L e t t e r s 7 (1961) 250. 2. P . G . R o o s . C . A . L u d e m a n n and I, I. W e s e l o w s k i . P h y s . L e t t e r s 24B (1967) 656. 3. A . M . L a n e . Nucl. P h y s . 35 (1962) 676. 4. J. D. A n d e r s o n , S. D. B l o o m . C. Wong. W . E . H o r n y a k and V. A. M a d s e n . P h y s . Rev, 177 (1969) 1416.
626
LETTERS
11 May 1970
5. C . D . G o o d m a n . J . D . A n d e r s o n a n d C. W o n g . P h y s . Rev. 156 (1967) 1249. 6. T. L a u r i t s e n a n d F. A j z e n b e r g - S e l o v e , Nucl. P h y s . 78 (1966) i. 7. V, Z. Goldberg. V. V. Davydov. A. A. Oglohlin. S.B. Sakuta and V. I. Chuev, Izv, AN SSSR, ser. fiz. 33 (1969) 566.