- Email: [email protected]
Excited states in 6Be
Volume 20, number 2
PHYSICS LETTERS
The c a l c u l a t e d l o w - l y i n g s p e c t r a a r e shown in fig. 1 which g i v e s a l s o the e x p e r i m e n t a l r e sults. The g e n e r a l a g r e e m e n t v a r i e s f r o m poor to r a t h e r good (in the c a s e of the B. G. T. f o r c e ) . It will be noted that the lowest c a l c u l a t e d T = 2 e n e r g i e s a r e 2 to 4 MeV l o w e r than the o b s e r v e d ones (0 + at 22.9 MeV, 2 + at 24.7 MeV) and in fact a d i s c r e p a n c y of the s a m e s o r t is found also for the o d d - p a r i t y l e v e l s . This d i s c r e p a n c y h o w e v e r l a r g e l y d i s a p p e a r s if we c o n s i d e r the r e l a t i v e p o s i t i o n s of t h e s e s t a t e s with r e s p e c t to o th er l e v e l s r a t h e r than to the ground s t a t e above. This would imply then that the c a l c u l a t e d g r o u n d - s t a t e e n e r g y is too high or in o th e r w o r d s for e x a m p l e that the a t t e m p t to p r o d u c e an e n e r g y gap not much l a r g e r than 6 MeV (the v a l u e is 7.5 MeV f o r B. G. T.) i m p r o p e r l y d i s t o r t s o th e r a s p e c t s of the s t r u c t u r e . This would be c o n s i s t e n t with the s u g g e s t i o n s made by o th e r a u th o r s on the b a s i s of m o r e r e s t r i c t e d c a l c u l a t i o n s . It would imply that the one- and t w o - p a r t i c l e - h o l e a d m i x t u r e s in the ground s t a t e a r e s o m e w h a t s m a l l e r than c a l cu l at ed above and would be c o n s i s t e n t with the c u r r e n t view [7] that m u l t i - p a r t i c l e - h o l e a d m i x t u r e s a r e i m p o r t a n t in the l o w - l y i n g s t a t e s . In line with that, a m a j o r f a i l u r e in the c a l cu l at i o n s as they stand at the p r e s e n t t i m e is that they do not give the s t r o n g e n h a n c e m e n t of the E2 t r a n s i t i o n s b et w e e n s t a t e s of the "K = 0 r o t a t i o n a l band" which b e g i n s with the 6 MeV l e v e l . We p r o p o s e to extend our c a l c u l a t i o n s to c o n s i d e r the m o r e c o m p l e x e x c i t a t i o n s . B e c a u s e of the l a r g e d i m e n s i o n a l i t i e s , it is i m p o s s i b l e to include the 4~w c o n f i g u r a t i o n s and s t i l l demand the s a m e de-
EXCITED
1 F eb r u ar y 1966
g r e e of the c o m p l e t e n e s s as we have done f o r the (0 + 2)~w s t a t e s . We a r e p r e s e n t l y engaged in applying s u m - r u l e m e t h o d s f o r c a l c u l a t i n g the c e n t r o i d s (and widths) of m u l t i - p a r t i c l e - m u l t i hole e x c i t a t i o n s and hope t h e r e b y to obtain s o m e u n d e r s t a n d i n g s of the r e l a t i v e i m p o r t a n c e of t h e s e configurations. B e s i d e s that we p r o p o s e also to extend the s h e l l - m o d e l c a l c u l a t i o n s by including in the s h e l l - m o d e l v e c t o r s p a c e s c e r t a i n of the apparently important m u l t i - h o l e - p a r t i c l e states. The author w i s h e s to acknowledge the guidance and e n c o u r a g e m e n t of P r o f e s s o r J. B. F r e n c h and the many v e r y helpful d i s c u s s i o n s with D r s . E. C. Halbert, L.S. Hsu, D. Koltun and J. B. McG r o r y throughout this work.
References 1. J . B . F r e n c h , E.C.Halbert, J.B.McG r o r y and S.S.M.Wong, to be published. 2. J . P . Elliott and B. H. Flowers, Proc. Roy. Soc. (London) A 242 (1957) 57. 3. A.M. Green, A. Kallio and K. Kolltveit, Physics Letters 14 (1965) 142. 4. B.M. Spicer and J. M. Eisenberg, Nucl. Phys. 63 (1965) 520. 5. C.R. Browne and J. Michael, Phys. Rev. 134 (1964) B133. 6. D.S.Devons, G . Goldring and G. R. Lindsay, Proc. Roy. Soc. (London) A67 (1954) 134. 7. W.H. Bassichis and G.Ripka, Physics Letters 15 (1965) 323; I.Kelson, Physics Letters 16 (1965) 143; G. E. Brown and A. M. Green, preprint Princeton University.
STATES
IN 6 B e
t
S. F. E C C L E S , C. WONG and J. D. ANDERSON Lawrence Radiation Laboratory, University of California, L i v e r m o r e , California Received 11 January 1966
In a r e c e n t l e t t e r to this j o u r n a l [1] r e s u l t s f r o m a 6Li(p, n)6Be e x p e r i m e n t w e r e g iv e n in d i cating nine e x c i t e d s t a t e s in 6Be up to an e x c i t a tion e n e r g y of 16.3 MeV, the f i r s t t h r e e l e v e l s a p p e a r i n g b el o w 4 MeV e x c it a t io n . P r e v i o u s int Work performed under the auspices of the U. S. Atomic Energy Commission. 190
v e s t i g a t i o n s [2, 3] s e e m e d to indicate that up to 3.3 MeV e x c i t a t i o n in 6Be, a s i n g l e e x c i t e d state at 1.5 MeV was p r e s e n t with a width l e s s than 100 keV. O t h er e x p e r i m e n t s [4] i n d i cat ed no e x c i t e d s t a t e s in 6Be with width l e s s than 1.0 MeV up to an e x c i t a t i o n e n e r g y of about 2.8 MeV, and no s t a t e s with width l e s s than 1.5 MeV up to an e x c i t a t i o n e n e r g y of 4 MeV.
V o l u m e 20, n u m b e r 2
PHYSICS
4He (SHe, n)6Be
LETTERS
1 F e b r u a r y 1966
TIME-OF-FLIGHT SPECTRUM E(3He) = 26 MeV
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GAMMA RAYS FROM GAS CELL AND COLLIMATORS
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250 HEIGHT
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5 EXITATION ENERGY IN 6 B e -
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Fig. 1. T i m e - o f - f l i g h t s p e c t r u m at a l a b o r a t o r y angle of 15.0 °, f r o m the 4He(3He,n) r e a c t i o n . The incident 3He e n e r g y was 26 MeV. At e a c h edge o f the f i g u r e a r e peaks due to g a m m a r a y s f r o m the g a s s cell and c o l l i m a t o r s . The b a c k g r o u n d r u n was taken with the g a s r e m o v e d f r o m the cell. The t i m e c a l i b r a t i o n of the s y s t e m was 1.44 n s / c h a n n e l .
6Li (p,n)6Be Ep= 11.6 MeV, 8 L = 3,=
Ep=I2.6 MeV, # L =60 °
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6Re (GROUN[) STATE)
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Fig. 2. 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 at l a b o r a t o r y a n g l e s of 3.0 ° and 60.0 ° d e r i v e d f r o m t i m e - o f - f l i g h t s p e c t r a f r o m t h e r e a c t i o n 6Li(p,n). T h e n e u t r o n e n e r g y was 11.6 and 12.6 MeV, r e s p e c t i v e l y . The r e s u l t of s u b t r a c t i n g a f o u r - b o d y breakup s p e c t r u m is a l s o shown. 191
Volume 20, number 2
PHYSICS LETTERS
Table 1 T = 1 states in the triad 6 H e - 6 L i - 6 B e (a) Isotope
Eexc (MeV)
Fcm (keV)
jTr
6He 6Li 6Be 6He 6Li 6Be
0 3.562 * 0.004 0 1.795 • 0.025 5.36 -~ 0.015 1.6 ± 0.1
0o) <5 92 ± 6 113 + 20 320 =~50 1100 =e200
0+ (b)+ (2) (b) (b)
0+
(a) All data except for the second T = I state in 6Be taken from ref. 8. (b) Not measured. In o r d e r to c l a r i f y this s i tu a t io n with r e g a r d to l o w - l y i n g s t a t e s in 6Be, we wish to r e p o r t the r e s u l t s of two e x p e r i m e n t s [5] (3He + 4He 6Be + n and 6Li + p ~ UBe + n) done with the L i v e r m o r e v a r i a b l e e n e r g y c y c l o t r o n and the t i m e - o f - f l i g h t f a c i l i t y . The e x p e r i m e n t a l method was g e n e r a l l y the s a m e as d e s c r i b e d in o t h e r work [6]. In the f i r s t e x p e r i m e n t , 3He p a r t i c l e s of 26 MeV incident e n e r g y b o m b a r d e d a t a r g e t of 4He and the r e s u l t i n g n e u tr o n s p e c t r u m was a n a l y s e d by t i m e - o f - f l i g h t t e c h n i q u e s . In addition, a 4He b e a m at 28 MeV was u s e d to b o m b a r d a t a r g e t of 3He. The c o m b i n a t i o n of t h e s e two b o m b a r d m e n t s gave r e a c t i o n n e u t r o n s throughout the complete angular range. These laboratory beam e n e r g i e s a r e not e q u i v a l e n t in the c e n t e r of m a s s , t h e r e being roughly a 20% d i f f e r e n c e . T h i s d i f f e r ence is not i m p o r t a n t f o r the r e s u l t s r e p o r t e d here. A t i m e - o f - f l i g h t s p e c t r u m at a l a b o r a t o r y angle of 15.0 ° is shown in fig. 1 f o r incident 3He p a r t i c l e s of 26 MeV. The ground state n e u tr o n group (no) is e a s i l y identified f r o m it s absolute e n e r g y and f r o m the k i n e m a t i c e n e r g y shift with angle. A l s o ev i d e n t is a b r o a d group (nl) which p eak s at an e x c i t a t i o n e n e r g y of 1.6 MeV in 6Be. The only o t h e r peak in this or any o t h e r s p e c t r u m a n a l y z e d was due to an i m p u r i t y of 160 in the 4He g a s t a r g e t . E v e n though the t a r g e t g a s was p u r e , s o m e oxygen can c o m e f r o m the t a n t a l u m gas c e l l f o i l s b e c u a s e of the a c t i o n of the incident b eam . T h e r e was no a m b i g u i t y in the d e t e r m i n a tion of s t a t e s in 6Be due to t h i s i m p u r i t y . S p e c t r a w e r e t a k e n at l a b o r a t o r y a n g l e s of 3.0 °, 15.0 °, 30.0 ° and 45.0 ° with incident 3He, and at a n g l e s of 15.0 ° and 30.0 ° with incident p a r t i c l e s . At e a c h angle a ground state group and a b r o a d e x c i t e d state group w e r e o b s e r v e d , and each s p e c t r u m was c o n s i s t e n t with a n a r r o w ground state and an e x c i t e d state in 6Be at 1.6 + 0.1 MeV with a width of 1.1 + 0.2 MeV. The e r r o r s a r e a r e s u l t of the u n c e r t a i n t i e s in the s u b t r a c t i o n of the continuum background n e u t r o n s . 192
1 F eb r u ar y 1966
The b r o a d peak is a s y m m e t r i c a l in the t i m e s p e c t r u m , but t h i s a s y m m e t r y can be accounted f o r by the background continuum. N o e v i d e n c e f o r o t h e r e x c i t e d s t a t e s was o b s e r v e d up to an e x c i t a t i o n e n e r g y of 5.5 MeV (for e x a m p l e see fig. 1). It should be noted that broad, w eak l y e x c i t e d s t a t e s would be o b s c u r e d by the continuu m of breakup n e u t r o n s . The width of the ground st at e group eq u al ed the e x p e r i m e n t a l width of the t i m e - o f - f l i g h t s y s t e m (~ 540 keV at t h e s e e n e r gies). A second e x p e r i m e n t i n v o l v e d the 6Li(p, n) r e a c t i o n and u s e d incident p r o t o n s of 11.6 and 12.6 MeV. S p e c t r a w e r e taken at lab a n g l e s of 3.0 o, 15.0 o, 30.0 o, 45.0 ° and 60.0 °. In fig. 2 a r e shown two t y p i c a l c e n t e r of m a s s c r o s s s e c t i o n s d e r i v e d f r o m t i m e - o f - f l i g h t data. T h e s e c u r v e s a l s o show the ground state and a b r o a d e x c i t e d st at e at about 1.6 MeV e x c i t a t i o n in 6Be with a width of about 1 MeV. A f o u r - b o d y breakup s p e c t r u m f r o m phase space c o n s i d e r a t i o n s was subt r a c t e d f r o m t h i s s p e c t r u m as shown. This shape h as been shown to be r e a s o n a b l e f o r the continuum n e u t r o n s f r o m the r e a c t i o n 9Be(p, n) which a l s o can lead to f o u r - b o d y breakup [7]. With the a s s u m p t i o n of a f o u r - b o d y breakup shape we do not i m p l y d i r e c t f o u r - b o d y breakup si n ce s e q u e n t i a l d ecay through b r o a d l e v e l s would yield a s i m i l a r shape when only one r e a c t i o n p r o d u c t is o b s e r v e d . Independent of the ex act shape of the background, it is c l e a r t h e r e e x i s t s a broad n eu t r o n group f r o m an e x c i t e d state at 1.6 MeV in 6Be. T h e r e is u n c e r t a i n t y in the width e s t i m a t e b e c a u s e of l a c k of knowledge of the ex act shape of the b r eak u p continuum. T h e r e was roughly a 5% c o n t a m i n a t i o n of 7Li in the 6Li t a r g e t , but this led to no am b i g u i t y in the i d en ti f i c a t i o n of s t a t e s in 6Be. The n eu t r o n group identified as f r o m an ex c i t e d state in 6Be could a l s o be due to a final state i n t e r a c t i o n between 5Li and a proton. Howe v e r , the i n t e r p r e t a t i o n a s a l e v e l f i t s nicely with T = 1 l e v e l s in 6He at 1.80 MeV ex ci t at i o n , and in 6Li at 5.36 MeV e x c i t a t i o n [8]. In table 1 we s u m m a r i z e d the known T = 1 s t a t e s in the t r i a d 6 H e - 6 L i - 6 B e . Again, f r o m our data we find no e v i d e n c e f o r o t h er s t a t e s in 6Be up to an e x c i t a t i o n e n e r g y of about 5.5 MeV. T h i s r e s u l t is c o n s i s t e n t with the known T = 1 l e v e l s in 6Li
[8]. The l a r g e width of the e x c i t e d state in 6Be c o m p a r e d to the ground state is not s u r p r i s i n g si n ce the state is unstable to p a r t i c l e breakup into both 5Li +p and 4He + 2p, w h e r e a s the ground state is unstable only to breakup into 4He + 2p. T h i s o t h e r open p a r t i c l e channel would c e r t a i n l y
Volume 20, number 2
PHYSICS LETTERS
i n c r e a s e the width. The l a r g e width of the excited state in 6Be c o m p a r e d to the c o r r e s p o n d i n g state in 6He is also not s u r p r i s i n g , s i n c e the state in 6He is stable a g a i n s t decay to 5He + n and only has the open channel 4He + 2n. Qualitatively, one expects the width in 6Be to be l a r g e r than the c o r r e s p o n d i n g state in 6Li; however, the d i f f e r ence between our m e a s u r e d width of the 6Be state and the r e p o r t e d width of the 6Li state is l a r g e r than one would expect f r o m s i m p l e phase space considerations. Our r e s u l t s a r e c o n s i s t e n t with the work of Whaling [4], but i n c o n s i s t e n t with the other data on s t a t e s in 6Be ([1], [2] and [3]). Some v e r y r e c e n t work at the L a w r e n c e Radiation L a b o r a t o r y in B e r k e l e y [9] on the 6Li(3He,t) r e a c t i o n is in a g r e e m e n t with the above e x p e r i m e n t a l r e s u l t s . The a n g u l a r d i s t r i b u t i o n of n e u t r o n s leading to the ground state of 6Be a p p e a r s to be m o r e or l e s s i s o t r o p i c f r o m the 4He(3He, n) r e a c t i o n within the r a t h e r l a r g e e r r o r s in the d e t e r m i n a t i o n of the c r o s s s e c t i o n s (~ 30%). A typical value for the c r o s s s e c t i o n is 0.3 m b / s r . The i n t e n s i t y of
ELECTRODISINTEGRATION LOW MOMENTUM TRANSFER:
1February 1966
the excited state in 6Be is m u c h l a r g e r , in both r e a c t i o n s , than the ground state (see figs. 1 and 2). A m o r e c o m p l e t e account of this work, along with a study of s t a t e s in 6Li, will be published elsewhere. 1. C.J. Batty, E. Friedman, P.C. Rowe and J. B. Hunt, Physics Letters 19 (1965) 35. 2. F. Ajzenberg-Selove, C.F. Osgood and C. P. Baker, Phys.Rev. 116 (1959) 1521. 3. H.C. Bryant, J.G. Berry, E.R. Flyrm and W. T. Leland, Nuclear Phys.53 (1964} 97. 4. J.L. Honsaker and W. Whaling, Bull. Am. Phys. Soc. 9 (1964) 627. 5. A part of these results have been given previously in S.F.Eccles, J.D.Anderson, H.F.Lutz and C. Wong, Bull. Am. Phys. Soc. 11 (1965) 693. 6. J.D. Anderson, C. Wong, J. W. McClure and B.D. Walker, Phys. Rev. 136 (1964) B 118. 7. R. W. Bauer, J.D. Anderson and C. Wong, Nuclear Phys. 56 (1964) 117. 8. T. Lauritsen and F. Ajzenberg-Selove, Energy Levels of Light Nuclei VII, submitted to Nuclear Phys., Sept. 1965. 9. N. Mangelson, F. Ajzenberg-Selove and M. Reed, private communication.
OF THE DEUTERON AT MAGNETIC TRANSITIONS
*
J. GOLDEMBERG ** and C. SCHAERF ***
High-Energy Physics Laboratory, Stanford University Stanford, California Received 12 January 1966
The c r o s s s e c t i o n for e l a s t i c e l e c t r o n - d e u t e r o n s c a t t e r i n g has b e e n r e c e n t l y i n v e s t i g a t e d by v a r ious people [1, 2]. Since the d e u t e r o n has isotopic spin 0, only the i s o s c a l a r p a r t of the e l e c t r o m a g n e t i c field c o n t r i b u t e s to the e l a s t i c s c a t t e r i n g c r o s s section. F o r this r e a s o n s i n g l e - m e s o n exchange c o n t r i b u t i o n s a r e i d e n t i c a l l y zero. A d l e r and D r e l l [3] have e s t i m a t e d the i m p o r t a n c e of t h r e e pion exchanges and found them to be s m a l l and in r e a s o n a b l e a g r e e m e n t with experiments. U n f o r t u n a t e l y t h e r e i s no such s e l e c t i o n r u l e * Work supported in part by the U. S. Office of Naval Research, Contract [Nonr 225(67)]. ** Now at Universidade de S. Paulo, Caixa Postal 8105, S. Paulo, Brazil. *** Now at Laboratori Nazionali di Fraseati, Frascati, Italia.
for the c o n t r i b u t i o n to the i n e l a s t i c s c a t t e r i n g c r o s s s e c t i o n and the p r o b l e m i s c o m p l i c a t e d by the final state i n t e r a c t i o n of the two nucleons. We have m e a s u r e d the s p e c t r u m of e l e c t r o n s inelastically scattered from a deuterium target at 180 ° . At this angle the t r a n s i t i o n i s m a i n l y m a g n e t i c (spin-flip) and t h e r e f o r e the dominant state of the two n u c l e o n s after the t r a n s i t i o n should be a s i n g l e t with z e r o o r b i t a l a n g u l a r m o m e n t u m (1S^). We used theUsame e x p e r i m e n t a l s e t - u p as in ref. 1 including the liquid d e u t e r i u m target. The absolute v a l u e s of the c r o s s s e c t i o n were obt a i n e d by c o m p a r i s o n with e l a s t i c e l e c t r o n p r o t o n s c a t t e r i n g . The s t a t i s t i c a l e r r o r in the n o r m a l i z a t i o n should be as s m a l l as 3%. The m e a s u r e m e n t s were p e r f o r m e d at two values of the i n c i d e n t e l e c t r o n e n e r g y : 54 and 70 MeV. 193
PHYSICS LETTERS
The c a l c u l a t e d l o w - l y i n g s p e c t r a a r e shown in fig. 1 which g i v e s a l s o the e x p e r i m e n t a l r e sults. The g e n e r a l a g r e e m e n t v a r i e s f r o m poor to r a t h e r good (in the c a s e of the B. G. T. f o r c e ) . It will be noted that the lowest c a l c u l a t e d T = 2 e n e r g i e s a r e 2 to 4 MeV l o w e r than the o b s e r v e d ones (0 + at 22.9 MeV, 2 + at 24.7 MeV) and in fact a d i s c r e p a n c y of the s a m e s o r t is found also for the o d d - p a r i t y l e v e l s . This d i s c r e p a n c y h o w e v e r l a r g e l y d i s a p p e a r s if we c o n s i d e r the r e l a t i v e p o s i t i o n s of t h e s e s t a t e s with r e s p e c t to o th er l e v e l s r a t h e r than to the ground s t a t e above. This would imply then that the c a l c u l a t e d g r o u n d - s t a t e e n e r g y is too high or in o th e r w o r d s for e x a m p l e that the a t t e m p t to p r o d u c e an e n e r g y gap not much l a r g e r than 6 MeV (the v a l u e is 7.5 MeV f o r B. G. T.) i m p r o p e r l y d i s t o r t s o th e r a s p e c t s of the s t r u c t u r e . This would be c o n s i s t e n t with the s u g g e s t i o n s made by o th e r a u th o r s on the b a s i s of m o r e r e s t r i c t e d c a l c u l a t i o n s . It would imply that the one- and t w o - p a r t i c l e - h o l e a d m i x t u r e s in the ground s t a t e a r e s o m e w h a t s m a l l e r than c a l cu l at ed above and would be c o n s i s t e n t with the c u r r e n t view [7] that m u l t i - p a r t i c l e - h o l e a d m i x t u r e s a r e i m p o r t a n t in the l o w - l y i n g s t a t e s . In line with that, a m a j o r f a i l u r e in the c a l cu l at i o n s as they stand at the p r e s e n t t i m e is that they do not give the s t r o n g e n h a n c e m e n t of the E2 t r a n s i t i o n s b et w e e n s t a t e s of the "K = 0 r o t a t i o n a l band" which b e g i n s with the 6 MeV l e v e l . We p r o p o s e to extend our c a l c u l a t i o n s to c o n s i d e r the m o r e c o m p l e x e x c i t a t i o n s . B e c a u s e of the l a r g e d i m e n s i o n a l i t i e s , it is i m p o s s i b l e to include the 4~w c o n f i g u r a t i o n s and s t i l l demand the s a m e de-
EXCITED
1 F eb r u ar y 1966
g r e e of the c o m p l e t e n e s s as we have done f o r the (0 + 2)~w s t a t e s . We a r e p r e s e n t l y engaged in applying s u m - r u l e m e t h o d s f o r c a l c u l a t i n g the c e n t r o i d s (and widths) of m u l t i - p a r t i c l e - m u l t i hole e x c i t a t i o n s and hope t h e r e b y to obtain s o m e u n d e r s t a n d i n g s of the r e l a t i v e i m p o r t a n c e of t h e s e configurations. B e s i d e s that we p r o p o s e also to extend the s h e l l - m o d e l c a l c u l a t i o n s by including in the s h e l l - m o d e l v e c t o r s p a c e s c e r t a i n of the apparently important m u l t i - h o l e - p a r t i c l e states. The author w i s h e s to acknowledge the guidance and e n c o u r a g e m e n t of P r o f e s s o r J. B. F r e n c h and the many v e r y helpful d i s c u s s i o n s with D r s . E. C. Halbert, L.S. Hsu, D. Koltun and J. B. McG r o r y throughout this work.
References 1. J . B . F r e n c h , E.C.Halbert, J.B.McG r o r y and S.S.M.Wong, to be published. 2. J . P . Elliott and B. H. Flowers, Proc. Roy. Soc. (London) A 242 (1957) 57. 3. A.M. Green, A. Kallio and K. Kolltveit, Physics Letters 14 (1965) 142. 4. B.M. Spicer and J. M. Eisenberg, Nucl. Phys. 63 (1965) 520. 5. C.R. Browne and J. Michael, Phys. Rev. 134 (1964) B133. 6. D.S.Devons, G . Goldring and G. R. Lindsay, Proc. Roy. Soc. (London) A67 (1954) 134. 7. W.H. Bassichis and G.Ripka, Physics Letters 15 (1965) 323; I.Kelson, Physics Letters 16 (1965) 143; G. E. Brown and A. M. Green, preprint Princeton University.
STATES
IN 6 B e
t
S. F. E C C L E S , C. WONG and J. D. ANDERSON Lawrence Radiation Laboratory, University of California, L i v e r m o r e , California Received 11 January 1966
In a r e c e n t l e t t e r to this j o u r n a l [1] r e s u l t s f r o m a 6Li(p, n)6Be e x p e r i m e n t w e r e g iv e n in d i cating nine e x c i t e d s t a t e s in 6Be up to an e x c i t a tion e n e r g y of 16.3 MeV, the f i r s t t h r e e l e v e l s a p p e a r i n g b el o w 4 MeV e x c it a t io n . P r e v i o u s int Work performed under the auspices of the U. S. Atomic Energy Commission. 190
v e s t i g a t i o n s [2, 3] s e e m e d to indicate that up to 3.3 MeV e x c i t a t i o n in 6Be, a s i n g l e e x c i t e d state at 1.5 MeV was p r e s e n t with a width l e s s than 100 keV. O t h er e x p e r i m e n t s [4] i n d i cat ed no e x c i t e d s t a t e s in 6Be with width l e s s than 1.0 MeV up to an e x c i t a t i o n e n e r g y of about 2.8 MeV, and no s t a t e s with width l e s s than 1.5 MeV up to an e x c i t a t i o n e n e r g y of 4 MeV.
V o l u m e 20, n u m b e r 2
PHYSICS
4He (SHe, n)6Be
LETTERS
1 F e b r u a r y 1966
TIME-OF-FLIGHT SPECTRUM E(3He) = 26 MeV
104 $
=.
ii
Id
I
• x. ~
-"• : ... " •: b~# '
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160 IMPURITY
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iO2 ........... / ~,,.,~,.~,~nwu~u
GAMMA RAYS FROM GAS CELL AND COLLIMATORS
i01
i
i
I00
150
i
I
5.5
i
200 - -
-
,
PULSE
250 HEIGHT
i
5 EXITATION ENERGY IN 6 B e -
I
4
I
i
i
i
3,00
350
400
I
3,
•'•~Z;~L "x• x• ~ • ~ , " ~ x • "x=x-~•.~#,.~'Y. ' ~ .~••'x.~ ~ ..
2
I
I
GAMMA RAYS
J
450
I
0
Fig. 1. T i m e - o f - f l i g h t s p e c t r u m at a l a b o r a t o r y angle of 15.0 °, f r o m the 4He(3He,n) r e a c t i o n . The incident 3He e n e r g y was 26 MeV. At e a c h edge o f the f i g u r e a r e peaks due to g a m m a r a y s f r o m the g a s s cell and c o l l i m a t o r s . The b a c k g r o u n d r u n was taken with the g a s r e m o v e d f r o m the cell. The t i m e c a l i b r a t i o n of the s y s t e m was 1.44 n s / c h a n n e l .
6Li (p,n)6Be Ep= 11.6 MeV, 8 L = 3,=
Ep=I2.6 MeV, # L =60 °
I000
SBe*
6Re (GROUN[) STATE)
1
,
SBi*
GROUND STATE
SBe
_>100
"~-o
BIAS SETTING
,~ •
CUTOFF TO PREVENT FOLDOVER OF NEUTRONS - - INTO ADJACENT CYCLE
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4
i
=
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5
4
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i
6
i
Fig. 2. 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 at l a b o r a t o r y a n g l e s of 3.0 ° and 60.0 ° d e r i v e d f r o m t i m e - o f - f l i g h t s p e c t r a f r o m t h e r e a c t i o n 6Li(p,n). T h e n e u t r o n e n e r g y was 11.6 and 12.6 MeV, r e s p e c t i v e l y . The r e s u l t of s u b t r a c t i n g a f o u r - b o d y breakup s p e c t r u m is a l s o shown. 191
Volume 20, number 2
PHYSICS LETTERS
Table 1 T = 1 states in the triad 6 H e - 6 L i - 6 B e (a) Isotope
Eexc (MeV)
Fcm (keV)
jTr
6He 6Li 6Be 6He 6Li 6Be
0 3.562 * 0.004 0 1.795 • 0.025 5.36 -~ 0.015 1.6 ± 0.1
0o) <5 92 ± 6 113 + 20 320 =~50 1100 =e200
0+ (b)+ (2) (b) (b)
0+
(a) All data except for the second T = I state in 6Be taken from ref. 8. (b) Not measured. In o r d e r to c l a r i f y this s i tu a t io n with r e g a r d to l o w - l y i n g s t a t e s in 6Be, we wish to r e p o r t the r e s u l t s of two e x p e r i m e n t s [5] (3He + 4He 6Be + n and 6Li + p ~ UBe + n) done with the L i v e r m o r e v a r i a b l e e n e r g y c y c l o t r o n and the t i m e - o f - f l i g h t f a c i l i t y . The e x p e r i m e n t a l method was g e n e r a l l y the s a m e as d e s c r i b e d in o t h e r work [6]. In the f i r s t e x p e r i m e n t , 3He p a r t i c l e s of 26 MeV incident e n e r g y b o m b a r d e d a t a r g e t of 4He and the r e s u l t i n g n e u tr o n s p e c t r u m was a n a l y s e d by t i m e - o f - f l i g h t t e c h n i q u e s . In addition, a 4He b e a m at 28 MeV was u s e d to b o m b a r d a t a r g e t of 3He. The c o m b i n a t i o n of t h e s e two b o m b a r d m e n t s gave r e a c t i o n n e u t r o n s throughout the complete angular range. These laboratory beam e n e r g i e s a r e not e q u i v a l e n t in the c e n t e r of m a s s , t h e r e being roughly a 20% d i f f e r e n c e . T h i s d i f f e r ence is not i m p o r t a n t f o r the r e s u l t s r e p o r t e d here. A t i m e - o f - f l i g h t s p e c t r u m at a l a b o r a t o r y angle of 15.0 ° is shown in fig. 1 f o r incident 3He p a r t i c l e s of 26 MeV. The ground state n e u tr o n group (no) is e a s i l y identified f r o m it s absolute e n e r g y and f r o m the k i n e m a t i c e n e r g y shift with angle. A l s o ev i d e n t is a b r o a d group (nl) which p eak s at an e x c i t a t i o n e n e r g y of 1.6 MeV in 6Be. The only o t h e r peak in this or any o t h e r s p e c t r u m a n a l y z e d was due to an i m p u r i t y of 160 in the 4He g a s t a r g e t . E v e n though the t a r g e t g a s was p u r e , s o m e oxygen can c o m e f r o m the t a n t a l u m gas c e l l f o i l s b e c u a s e of the a c t i o n of the incident b eam . T h e r e was no a m b i g u i t y in the d e t e r m i n a tion of s t a t e s in 6Be due to t h i s i m p u r i t y . S p e c t r a w e r e t a k e n at l a b o r a t o r y a n g l e s of 3.0 °, 15.0 °, 30.0 ° and 45.0 ° with incident 3He, and at a n g l e s of 15.0 ° and 30.0 ° with incident p a r t i c l e s . At e a c h angle a ground state group and a b r o a d e x c i t e d state group w e r e o b s e r v e d , and each s p e c t r u m was c o n s i s t e n t with a n a r r o w ground state and an e x c i t e d state in 6Be at 1.6 + 0.1 MeV with a width of 1.1 + 0.2 MeV. The e r r o r s a r e a r e s u l t of the u n c e r t a i n t i e s in the s u b t r a c t i o n of the continuum background n e u t r o n s . 192
1 F eb r u ar y 1966
The b r o a d peak is a s y m m e t r i c a l in the t i m e s p e c t r u m , but t h i s a s y m m e t r y can be accounted f o r by the background continuum. N o e v i d e n c e f o r o t h e r e x c i t e d s t a t e s was o b s e r v e d up to an e x c i t a t i o n e n e r g y of 5.5 MeV (for e x a m p l e see fig. 1). It should be noted that broad, w eak l y e x c i t e d s t a t e s would be o b s c u r e d by the continuu m of breakup n e u t r o n s . The width of the ground st at e group eq u al ed the e x p e r i m e n t a l width of the t i m e - o f - f l i g h t s y s t e m (~ 540 keV at t h e s e e n e r gies). A second e x p e r i m e n t i n v o l v e d the 6Li(p, n) r e a c t i o n and u s e d incident p r o t o n s of 11.6 and 12.6 MeV. S p e c t r a w e r e taken at lab a n g l e s of 3.0 o, 15.0 o, 30.0 o, 45.0 ° and 60.0 °. In fig. 2 a r e shown two t y p i c a l c e n t e r of m a s s c r o s s s e c t i o n s d e r i v e d f r o m t i m e - o f - f l i g h t data. T h e s e c u r v e s a l s o show the ground state and a b r o a d e x c i t e d st at e at about 1.6 MeV e x c i t a t i o n in 6Be with a width of about 1 MeV. A f o u r - b o d y breakup s p e c t r u m f r o m phase space c o n s i d e r a t i o n s was subt r a c t e d f r o m t h i s s p e c t r u m as shown. This shape h as been shown to be r e a s o n a b l e f o r the continuum n e u t r o n s f r o m the r e a c t i o n 9Be(p, n) which a l s o can lead to f o u r - b o d y breakup [7]. With the a s s u m p t i o n of a f o u r - b o d y breakup shape we do not i m p l y d i r e c t f o u r - b o d y breakup si n ce s e q u e n t i a l d ecay through b r o a d l e v e l s would yield a s i m i l a r shape when only one r e a c t i o n p r o d u c t is o b s e r v e d . Independent of the ex act shape of the background, it is c l e a r t h e r e e x i s t s a broad n eu t r o n group f r o m an e x c i t e d state at 1.6 MeV in 6Be. T h e r e is u n c e r t a i n t y in the width e s t i m a t e b e c a u s e of l a c k of knowledge of the ex act shape of the b r eak u p continuum. T h e r e was roughly a 5% c o n t a m i n a t i o n of 7Li in the 6Li t a r g e t , but this led to no am b i g u i t y in the i d en ti f i c a t i o n of s t a t e s in 6Be. The n eu t r o n group identified as f r o m an ex c i t e d state in 6Be could a l s o be due to a final state i n t e r a c t i o n between 5Li and a proton. Howe v e r , the i n t e r p r e t a t i o n a s a l e v e l f i t s nicely with T = 1 l e v e l s in 6He at 1.80 MeV ex ci t at i o n , and in 6Li at 5.36 MeV e x c i t a t i o n [8]. In table 1 we s u m m a r i z e d the known T = 1 s t a t e s in the t r i a d 6 H e - 6 L i - 6 B e . Again, f r o m our data we find no e v i d e n c e f o r o t h er s t a t e s in 6Be up to an e x c i t a t i o n e n e r g y of about 5.5 MeV. T h i s r e s u l t is c o n s i s t e n t with the known T = 1 l e v e l s in 6Li
[8]. The l a r g e width of the e x c i t e d state in 6Be c o m p a r e d to the ground state is not s u r p r i s i n g si n ce the state is unstable to p a r t i c l e breakup into both 5Li +p and 4He + 2p, w h e r e a s the ground state is unstable only to breakup into 4He + 2p. T h i s o t h e r open p a r t i c l e channel would c e r t a i n l y
Volume 20, number 2
PHYSICS LETTERS
i n c r e a s e the width. The l a r g e width of the excited state in 6Be c o m p a r e d to the c o r r e s p o n d i n g state in 6He is also not s u r p r i s i n g , s i n c e the state in 6He is stable a g a i n s t decay to 5He + n and only has the open channel 4He + 2n. Qualitatively, one expects the width in 6Be to be l a r g e r than the c o r r e s p o n d i n g state in 6Li; however, the d i f f e r ence between our m e a s u r e d width of the 6Be state and the r e p o r t e d width of the 6Li state is l a r g e r than one would expect f r o m s i m p l e phase space considerations. Our r e s u l t s a r e c o n s i s t e n t with the work of Whaling [4], but i n c o n s i s t e n t with the other data on s t a t e s in 6Be ([1], [2] and [3]). Some v e r y r e c e n t work at the L a w r e n c e Radiation L a b o r a t o r y in B e r k e l e y [9] on the 6Li(3He,t) r e a c t i o n is in a g r e e m e n t with the above e x p e r i m e n t a l r e s u l t s . The a n g u l a r d i s t r i b u t i o n of n e u t r o n s leading to the ground state of 6Be a p p e a r s to be m o r e or l e s s i s o t r o p i c f r o m the 4He(3He, n) r e a c t i o n within the r a t h e r l a r g e e r r o r s in the d e t e r m i n a t i o n of the c r o s s s e c t i o n s (~ 30%). A typical value for the c r o s s s e c t i o n is 0.3 m b / s r . The i n t e n s i t y of
ELECTRODISINTEGRATION LOW MOMENTUM TRANSFER:
1February 1966
the excited state in 6Be is m u c h l a r g e r , in both r e a c t i o n s , than the ground state (see figs. 1 and 2). A m o r e c o m p l e t e account of this work, along with a study of s t a t e s in 6Li, will be published elsewhere. 1. C.J. Batty, E. Friedman, P.C. Rowe and J. B. Hunt, Physics Letters 19 (1965) 35. 2. F. Ajzenberg-Selove, C.F. Osgood and C. P. Baker, Phys.Rev. 116 (1959) 1521. 3. H.C. Bryant, J.G. Berry, E.R. Flyrm and W. T. Leland, Nuclear Phys.53 (1964} 97. 4. J.L. Honsaker and W. Whaling, Bull. Am. Phys. Soc. 9 (1964) 627. 5. A part of these results have been given previously in S.F.Eccles, J.D.Anderson, H.F.Lutz and C. Wong, Bull. Am. Phys. Soc. 11 (1965) 693. 6. J.D. Anderson, C. Wong, J. W. McClure and B.D. Walker, Phys. Rev. 136 (1964) B 118. 7. R. W. Bauer, J.D. Anderson and C. Wong, Nuclear Phys. 56 (1964) 117. 8. T. Lauritsen and F. Ajzenberg-Selove, Energy Levels of Light Nuclei VII, submitted to Nuclear Phys., Sept. 1965. 9. N. Mangelson, F. Ajzenberg-Selove and M. Reed, private communication.
OF THE DEUTERON AT MAGNETIC TRANSITIONS
*
J. GOLDEMBERG ** and C. SCHAERF ***
High-Energy Physics Laboratory, Stanford University Stanford, California Received 12 January 1966
The c r o s s s e c t i o n for e l a s t i c e l e c t r o n - d e u t e r o n s c a t t e r i n g has b e e n r e c e n t l y i n v e s t i g a t e d by v a r ious people [1, 2]. Since the d e u t e r o n has isotopic spin 0, only the i s o s c a l a r p a r t of the e l e c t r o m a g n e t i c field c o n t r i b u t e s to the e l a s t i c s c a t t e r i n g c r o s s section. F o r this r e a s o n s i n g l e - m e s o n exchange c o n t r i b u t i o n s a r e i d e n t i c a l l y zero. A d l e r and D r e l l [3] have e s t i m a t e d the i m p o r t a n c e of t h r e e pion exchanges and found them to be s m a l l and in r e a s o n a b l e a g r e e m e n t with experiments. U n f o r t u n a t e l y t h e r e i s no such s e l e c t i o n r u l e * Work supported in part by the U. S. Office of Naval Research, Contract [Nonr 225(67)]. ** Now at Universidade de S. Paulo, Caixa Postal 8105, S. Paulo, Brazil. *** Now at Laboratori Nazionali di Fraseati, Frascati, Italia.
for the c o n t r i b u t i o n to the i n e l a s t i c s c a t t e r i n g c r o s s s e c t i o n and the p r o b l e m i s c o m p l i c a t e d by the final state i n t e r a c t i o n of the two nucleons. We have m e a s u r e d the s p e c t r u m of e l e c t r o n s inelastically scattered from a deuterium target at 180 ° . At this angle the t r a n s i t i o n i s m a i n l y m a g n e t i c (spin-flip) and t h e r e f o r e the dominant state of the two n u c l e o n s after the t r a n s i t i o n should be a s i n g l e t with z e r o o r b i t a l a n g u l a r m o m e n t u m (1S^). We used theUsame e x p e r i m e n t a l s e t - u p as in ref. 1 including the liquid d e u t e r i u m target. The absolute v a l u e s of the c r o s s s e c t i o n were obt a i n e d by c o m p a r i s o n with e l a s t i c e l e c t r o n p r o t o n s c a t t e r i n g . The s t a t i s t i c a l e r r o r in the n o r m a l i z a t i o n should be as s m a l l as 3%. The m e a s u r e m e n t s were p e r f o r m e d at two values of the i n c i d e n t e l e c t r o n e n e r g y : 54 and 70 MeV. 193