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Study of the 13C(3He, γo)16O reaction in the region of the giant dipole resonance
Volume 20, number 3
IN
P H Y S I C S L E TT E RS
STUDY OF THE REGION
15 February 1966
THE 13C(3He,7o)160 REACTION OF THE GIANT DIPOLE RESONANCE
*
N. G. PUTTASWAMY
Department of Physics, Stanford University, Stanford, California and D. KOHLER **
Department of Physics, Stanford University, Stanford, California and Research Laboratories, Lockheed Missiles and Space Company, Palo Alto, California Received 25 October 1965
The giant r e s o n a n c e e n e r g y r e g i o n in 1 6 0 has b e e n e x t e n s i v e l y studied in the p a s t with a n u m b e r of d i f f e r e n t r e a c t i o n s [1-6, 9-11]. In t h e s e st udi e s s e v e r a l r e s o n a n c e s have b e e n s e e n . The two m o s t p r o m i n e n t ones j u s t above an e x c i t a t i o n e n e r g y of 20 MeV in 1 6 0 and c o n s i d e r e d to be p a r t of the giant dipole r e s o n a n c e [7] a r e o b s e r v e d at 22.3 and 24.4 MeV [1]. Since the r a d i a t i v e capt u r e of 3He by 13C has a l a r g e Q - v a l u e , t h e s e s a m e high e x c i t a t i o n e n e r g i e s in 1 6 0 can be r e a c h e d with a l o w - e n e r g y a c c e l e r a t o r and a study of this r e a c t i o n could give additional i n f o r mation about the c o m p o u n d - n u c l e u s s y s t e m . In this e x p e r i m e n t , the y i e l d of the g r o u n d - s t a t e y - r a y in the 13C(3He, 7)160 r e a c t i o n has been studied fo r incident 3He e n e r g i e s b e t w e e n 1.0 and 3.5 MeV c o r r e s p o n d i n g to e x c i t a t i o n e n e r g i e s in 160 b e t w e e n 23.60 and 25.64 MeV. The 3He b e a m was a c c e l e r a t e d in a 3 MV Van de Graaff g e n e r a tor. The b e a m e n e r g y was c a l i b r a t e d u s in g the 1.747 MeV r e s o n a n c e in the 13C(p, 7)14N r e a c t i o n . The t a r g e t u s e d was 13C d e p o s i t e d on 0.01" copper backing [8] and of t h i c k n e s s about 50 keV f o r 3 MeV 3He ions. The r - r a y d e t e c ti o n s y s t e m c o n s i s t e d of a 5" dia. x 6" long NaI(T1) c r y s t a l s u r r o u n d e d by a p l a s t i c a n t i c o i n c i d e n c e shield. P u l s e - p i l e u p r e j e c t i o n was a c c o m p l i s h e d by the u s e of a f a s t tunnel-diode discriminator. A typical spectrum r e c o r d e d in a V i c t o r e e n 4 0 0 - c h a n n e l a n a l y z e r is shown in fig. 1. The y - r a y t r a n s i t i o n to the ground sta te in 1 6 0 is w e l l s e p a r a t e d f r o m t h o s e going to
the e x c i t e d s t a t e s . The a r e a u n d er the peak, shown shaded in the f i g u r e , was u s e d to obtain an e x c i t a t i o n c u r v e . B a c k g r o u n d s u b t r a c t i o n was made u s i n g the counts above the shaded r e g i o n . The background is m o s t l y due to r e s i d u a l pileup a r i s i n g f r o m the high y i e l d of l o w er e n e r g y r - r a y s which pileup onto the 15.11 MeV 7 - r a y f r o m the 13C(3He, a r ) 1 2 C r e a c t i o n . The c o s m i c r a y r e j e c tion is b e t t e r than 100 to 1. 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 d~/d~2 at 90 ° f o r E3He(lab) between 1.0 and 3.5 MeV is shown in fig. 2. Two peaks a p p e a r at e n e r g i e s of about 1.8 and 2.92 MeV. The e r r o r s shown a r e due to s t a t i s t i c s in counting r a t e , background s u b t r a c t i o n and n o r m a l i z a t i o n with r e s p e c t to one t a r g e t spot.
* Supported in part by the National Science Foundation and the U. S. Army Research Office (Durham). ** Currently at Lockheed Missiles and Space Company. The work reported here was initiated while this author was still at Stanford University.
Fig. 1. Typicaly-ray spectrum from 13C(3He,~160 reaction at 90° and E3He = 2.75 MeV; 7o indicates the transition to the ground state, r l 2 the transitions to the first and second excited state~ and so on. The shaded region was used for obtaining the excitation function.
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Volume 20, number 3 40
P HYSI CS L E T T E RS
15 February 1966
THIS EXPT.
13C(3He.Yo)IS00
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Fig. 2. Differential c r o s s section at 90 ° for the 13C (3He,~o) 160 reaction as a function of 3He lab. energy. The dafa points for different target spot positions are shown by different symbols and were normalized to each other near 2.9 MeV 3He energy. See text for f u r ther details. T h e u n c e r t a i n t i e s in b a c k g r o u n d s u b t r a c t i o n and c h a n g e s in the a n t i c o i n c i d e n c e e f f i c i e n c y a r e e s t i m a t e d to be 15%. The a b s o l u t e c r o s s s e c t i o n d(r/d~2 at 90 ° i s d e t e r m i n e d u s i n g the c r o s s s e c t i o n of the 15.11 MeV ~ - r a y f r o m the 13C(3He, ol~,) 12C r e a c t i o n known to 50% [11] and i s found to b e 0.7 ~ b / s r at the p e a k e n e r g y of 2.92 MeV. The f i n a l e r r o r in t h i s c r o s s s e c t i o n i s e s t i m a t e d to b e about +80%, -50%. S i n c e the p e n e t r a b i l i t y d e c r e a s e s r a p i d l y b e low a b o u t 2.5 MeV 3He e n e r g y , a c o m p a r i s o n w i t h o t h e r d a t a c a n be m a d e by p l o t t i n g the q u a n t i t y (E/P)(da/d~2) v e r s u s e n e r g y w h e r e E i s the c e n t e r - o f - m a s s e n e r g y of i n c i d e n t 3He i o n s and P the p e n e t r a b i l i t y . On the a s s u m p t i o n that the p e a k s s e e n in t h i s e x p e r i m e n t a r e r e s o n a n c e s t h a t can be d e s c r i b e d by the B r e i t - W i g n e r o n e - l e v e l f o r mula [12] the structure exhibited by (E/P)(d(~/d~) can then be compared to that obtained in other r e actions leading to the same compound nucleus. In particular, ff there is a resonance in the compound nucleus, the peak energies should be the same for all the different reactions. In this argument it is
23.5
24.0 24.5 25,0 25.5 EXCITATION ENERGY IN ~SO (MeV)
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Fig. 3. Comparison of the 13C(3He,~o) 160 data with the results of other experiments. (a) Results of this experiment at 90 ° shown as a plot of (E/Po)(d(~/d~) versus excitation energy in 160. The data taken for different target spots were averaged. See text for details. (b) 15N(P,7o)160 reaction at 90 ° [1]. {Only the differential cross section is plotted as a function of energy because neither the c e n t e r - o f - m a s s energy of the incident proton nor the penetrability vary significantly over the range of excitation energy considered here.) (c) 160(e,e'p}lSN at 0= 76 ° and E)= 48 ° [2]. (d) 160(r,n)150 at 70° [3]. a s s u m e d t h a t the t o t a l c r o s s s e c t i o n ~ i s p r o p o r t i o n a l to (d~/d~2) at 90 ° and that the r e d u c e d w i d t h i s i n d e p e n d e n t of e n e r g y . Such a p l o t o b t a i n e d by u s i n g an S - w a v e p e n e t r a b i l i t y P o ( c h a n n e l r a d i u s R = 5.3 fm) is shown in fig. 3. A s i m i l a r plot i s o b t a i n e d w i t h a P - w a v e p e n e t r a b i l i t y . Two r e s o n a n c e s a r e s e e n at 1 6 0 e x c i t a t i o n e n e r g i e s of 24.05 ± 0.10 and 25.12 ± 0.05 MeV; the t o t a l w i d t h s a r e about 0.45 and 0.60 MeV (c.m.) r e s p e c t i v e l y . T h e s e n u m b e r s v a r y s l i g h t l y if a d i f f e r e n t r a d i u s R i s a s s u m e d . A l s o d i s p l a y e d in fig. 3 a r e d a t a f r o m o t h e r r e a c t i o n s . The r e s o n a n c e at 25.12 MeV r e e s w e l l w i t h the 1 5 N ( P , ~ o ) 1 6 0 d a t a [1], the 60(7, n ) 1 5 0 d a t a [3, 4] and the 7 - a b s o r p t i o n d a t a [5]; but it i s not s e e n p r o m i n e n t l y in the 1 6 0 ( e , e ' p ) 289
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PHYSICS LETTERS
15N data [2]. The width of 0.6 MeV c o m p a r e s well with 0.5 MeV obtained by Tanner et al. [1]. Howe v e r , the r e s o n a n c e at 24.05 MeV a p p e a r s to be at a l ow er e n e r g y as c o m p a r e d to the one o b s e r v e d at 24.4 MeV in the o t h e r r e a c t i o n s [1-3, 5]; s e e h o w e v e r [4]. Also the width of 0.45 MeV is m u c h s m a l l e r than 1 MeV obtained by T a n n e r et al. [1]. Our r e s u l t s can a l s o be c o m p a r e d with the work of Suffert [6] who does not s e e any p r o m i n e n t s t r u c t u r e o v e r this e x c i t a t i o n e n e r g y r e g i o n in the 14N(d, 7o)160 r e a c t i o n . In addition one might note that the s t r u c t u r e s e e n h e r e in the (3He,~o) r e a c t i o n is m u c h d i f f e r e n t f r o m what is s e e n in the (3He, no) and the (3He, Po) r e a c t i o n s in 13C. In the (3He, no) r e a c t i o n at 0o [9] only one r e s o nance is s e e n at 3He lab. e n e r g y of 1.95 MeV and in the (3He, Po) r e a c t i o n [10] t h e r e is no e v i d e n c e of any s t r u c t u r e at 0 ° and 90 °. The a p p a r e n t d i f f e r e n c e s in the s t r u c t u r e around 24 MeV a l r e a d y pointed out could be taken to indicate that the (3He, Yo) peak at about 24.05 MeV should not be i d e n t i f i e d with the component Of the giant e l e c t r i c dipole r e s o n a n c e at 24.4 MeV. If this is g r a n t e d it is a p p a r e n t that the 3He r e duced width of the 24.4 MeV s t a t e m u s t be m u c h s m a l l e r than that c o r r e s p o n d i n g to the 25.1 MeV 3He c a p t u r e r e s o n a n c e . This in turn might indicate that the p r o m i n e n t 3He c a p t u r e r e s o n a n c e at 25.1 MeV should not be c o n s i d e r e d as p a r t of the giant dipole s t r u c t u r e o b s e r v e d in the s i n g l e p a r ticle reactions since such very different particle r e d u c e d widths a m o n g this c o m p o n e n t s would s e e m r a t h e r i m p r o b a b l e . Finally, if as s e e m s quite r e a s o n a b l e , the 25.1 MeV 3He c a p t u r e r e s o n a n c e is i d e n t i f i e d with the 25.1 MeV r e s o n a n c e s e e n in the ot he r r e a c t i o n s [1, 3-5], then this l a t t e r f e a t u r e would p r o b a b l y not be p a r t of the giant e l e c t r i c dipole r e s o n a n c e . On the o t h e r hand, one might be t e m p t e d to identify the two r e s o n a n c e s s e e n in this 3He capt u r e work with those s e e n in the o t h e r r e a c t i o n s [1-5]. Using the 15N(P,7o)160 c r o s s s e c t i o n ~(P, 7o) [1] and a s s u m i n g i s o t r o p i c a n g u l a r d i s t r i -
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butions, one then finds [~(p,~,o)/a(3He,vo)] ~ 180 and ~ 3 . 6 a t E 7 ~ 24 and ~ 2 5 MeV, r e s p e c t i v e l y . A s s u m i n g a channel r a d i u s of 4.85 fm for the 15N(p, ~o)160 r e a c t i o n and using S - w a v e penet r a b i l i t i e s one obtains for the r a t i o s of r e d u c e d widths [72(p)/72(3He)] ~ 3.6 and ~ 1.4 at E y ~ 24 and ~ 25 MeV, r e s p e c t i v e l y . H o w e v e r , these l a r g e 3He r e d u c e d widths again lend su p p o r t to the a r g u m e n t above that the r e s o n a n c e s s e e n in the (3He, Vo) r e a c t i o n a r e p r o b a b l y not components of the giant dipole r e s o n a n c e . It is a p l e a s u r e to acknowledge the help and guidance of P r o f e s s o r W. E. M e y e r h o f throughout this work. Our thanks a r e extended to P r o f e s s o r s S.S. Hanna and P. Paul for t h e i r i n t e r e s t in this work and to Mr. W. F e l d m a n f o r his a s s i s t a n c e during the r u n s.
References 1. N.W. Tanner, G.C. Thomas and E. D. Earle, Nuclear Phys. 52 (1964) 45. Other references are found in this paper. 2. W. R. Dodge and W. C. Barber, Phys. Rev. 127 (1962) 1746. 3. F. W. K. Firk and K. H. Lokan, Phys. Rev. Letters 8 (1962) 321. 4. R.L. Bramblett, J. T. Caldwell, R.R. Harvey and S. C. Fultz, Phys. Rev. 133 (1964) B 869. 5. N.A. Burgov, G.V. Danilyan, B.S. Dolbilkin, L.E. Lazareva and F. A. Nikolaev, Soviet Physics J. E. T. P. 16 (1963) 50; J.M.Wyekoff, B. Ziegler, H. W. Koch and R. Uhlig, Phys. Rev. 137 (1965) B576. 6. M.Suffert, Nuclear Phys. 75 (1965) 226. 7. See for example V. Gillet and N. Vinh Mau, Nuclear Phys. 54 (1964) 321 and F. H. Lewis Jr., Phys. Rev. 134 (1964) B 331. 8. Supplied by A. E. R. E., Harwell, England. 9. J. L. Well and G. U. Din, Bull. Am. Phys. Soc. 7 (1962) 111. 10. J . P . Sehiffer, T. W. Bonner, R.H. Davis and F. W. Prosser Jr., Phys. Rev. 104 (1956) 1964. 11. H.M.Kuan, P.R. Almond, G.U.Dinand T,W. Bonner, Nuclear Phys. 60 (1964) 509. 12. See for example A. M. Lane and R. G. Thomas, Rev. Mod. Phys. 30 (1958) 257 p. 322.
IN
P H Y S I C S L E TT E RS
STUDY OF THE REGION
15 February 1966
THE 13C(3He,7o)160 REACTION OF THE GIANT DIPOLE RESONANCE
*
N. G. PUTTASWAMY
Department of Physics, Stanford University, Stanford, California and D. KOHLER **
Department of Physics, Stanford University, Stanford, California and Research Laboratories, Lockheed Missiles and Space Company, Palo Alto, California Received 25 October 1965
The giant r e s o n a n c e e n e r g y r e g i o n in 1 6 0 has b e e n e x t e n s i v e l y studied in the p a s t with a n u m b e r of d i f f e r e n t r e a c t i o n s [1-6, 9-11]. In t h e s e st udi e s s e v e r a l r e s o n a n c e s have b e e n s e e n . The two m o s t p r o m i n e n t ones j u s t above an e x c i t a t i o n e n e r g y of 20 MeV in 1 6 0 and c o n s i d e r e d to be p a r t of the giant dipole r e s o n a n c e [7] a r e o b s e r v e d at 22.3 and 24.4 MeV [1]. Since the r a d i a t i v e capt u r e of 3He by 13C has a l a r g e Q - v a l u e , t h e s e s a m e high e x c i t a t i o n e n e r g i e s in 1 6 0 can be r e a c h e d with a l o w - e n e r g y a c c e l e r a t o r and a study of this r e a c t i o n could give additional i n f o r mation about the c o m p o u n d - n u c l e u s s y s t e m . In this e x p e r i m e n t , the y i e l d of the g r o u n d - s t a t e y - r a y in the 13C(3He, 7)160 r e a c t i o n has been studied fo r incident 3He e n e r g i e s b e t w e e n 1.0 and 3.5 MeV c o r r e s p o n d i n g to e x c i t a t i o n e n e r g i e s in 160 b e t w e e n 23.60 and 25.64 MeV. The 3He b e a m was a c c e l e r a t e d in a 3 MV Van de Graaff g e n e r a tor. The b e a m e n e r g y was c a l i b r a t e d u s in g the 1.747 MeV r e s o n a n c e in the 13C(p, 7)14N r e a c t i o n . The t a r g e t u s e d was 13C d e p o s i t e d on 0.01" copper backing [8] and of t h i c k n e s s about 50 keV f o r 3 MeV 3He ions. The r - r a y d e t e c ti o n s y s t e m c o n s i s t e d of a 5" dia. x 6" long NaI(T1) c r y s t a l s u r r o u n d e d by a p l a s t i c a n t i c o i n c i d e n c e shield. P u l s e - p i l e u p r e j e c t i o n was a c c o m p l i s h e d by the u s e of a f a s t tunnel-diode discriminator. A typical spectrum r e c o r d e d in a V i c t o r e e n 4 0 0 - c h a n n e l a n a l y z e r is shown in fig. 1. The y - r a y t r a n s i t i o n to the ground sta te in 1 6 0 is w e l l s e p a r a t e d f r o m t h o s e going to
the e x c i t e d s t a t e s . The a r e a u n d er the peak, shown shaded in the f i g u r e , was u s e d to obtain an e x c i t a t i o n c u r v e . B a c k g r o u n d s u b t r a c t i o n was made u s i n g the counts above the shaded r e g i o n . The background is m o s t l y due to r e s i d u a l pileup a r i s i n g f r o m the high y i e l d of l o w er e n e r g y r - r a y s which pileup onto the 15.11 MeV 7 - r a y f r o m the 13C(3He, a r ) 1 2 C r e a c t i o n . The c o s m i c r a y r e j e c tion is b e t t e r than 100 to 1. 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 d~/d~2 at 90 ° f o r E3He(lab) between 1.0 and 3.5 MeV is shown in fig. 2. Two peaks a p p e a r at e n e r g i e s of about 1.8 and 2.92 MeV. The e r r o r s shown a r e due to s t a t i s t i c s in counting r a t e , background s u b t r a c t i o n and n o r m a l i z a t i o n with r e s p e c t to one t a r g e t spot.
* Supported in part by the National Science Foundation and the U. S. Army Research Office (Durham). ** Currently at Lockheed Missiles and Space Company. The work reported here was initiated while this author was still at Stanford University.
Fig. 1. Typicaly-ray spectrum from 13C(3He,~160 reaction at 90° and E3He = 2.75 MeV; 7o indicates the transition to the ground state, r l 2 the transitions to the first and second excited state~ and so on. The shaded region was used for obtaining the excitation function.
288
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NUMBER
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360
400 v
Volume 20, number 3 40
P HYSI CS L E T T E RS
15 February 1966
THIS EXPT.
13C(3He.Yo)IS00
41-
15c
(3He,YO)160
(o)
e - 90 °
30
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2 3 5He ENERGY (LAB) IN MeV
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Fig. 2. Differential c r o s s section at 90 ° for the 13C (3He,~o) 160 reaction as a function of 3He lab. energy. The dafa points for different target spot positions are shown by different symbols and were normalized to each other near 2.9 MeV 3He energy. See text for f u r ther details. T h e u n c e r t a i n t i e s in b a c k g r o u n d s u b t r a c t i o n and c h a n g e s in the a n t i c o i n c i d e n c e e f f i c i e n c y a r e e s t i m a t e d to be 15%. The a b s o l u t e c r o s s s e c t i o n d(r/d~2 at 90 ° i s d e t e r m i n e d u s i n g the c r o s s s e c t i o n of the 15.11 MeV ~ - r a y f r o m the 13C(3He, ol~,) 12C r e a c t i o n known to 50% [11] and i s found to b e 0.7 ~ b / s r at the p e a k e n e r g y of 2.92 MeV. The f i n a l e r r o r in t h i s c r o s s s e c t i o n i s e s t i m a t e d to b e about +80%, -50%. S i n c e the p e n e t r a b i l i t y d e c r e a s e s r a p i d l y b e low a b o u t 2.5 MeV 3He e n e r g y , a c o m p a r i s o n w i t h o t h e r d a t a c a n be m a d e by p l o t t i n g the q u a n t i t y (E/P)(da/d~2) v e r s u s e n e r g y w h e r e E i s the c e n t e r - o f - m a s s e n e r g y of i n c i d e n t 3He i o n s and P the p e n e t r a b i l i t y . On the a s s u m p t i o n that the p e a k s s e e n in t h i s e x p e r i m e n t a r e r e s o n a n c e s t h a t can be d e s c r i b e d by the B r e i t - W i g n e r o n e - l e v e l f o r mula [12] the structure exhibited by (E/P)(d(~/d~) can then be compared to that obtained in other r e actions leading to the same compound nucleus. In particular, ff there is a resonance in the compound nucleus, the peak energies should be the same for all the different reactions. In this argument it is
23.5
24.0 24.5 25,0 25.5 EXCITATION ENERGY IN ~SO (MeV)
26.0
Fig. 3. Comparison of the 13C(3He,~o) 160 data with the results of other experiments. (a) Results of this experiment at 90 ° shown as a plot of (E/Po)(d(~/d~) versus excitation energy in 160. The data taken for different target spots were averaged. See text for details. (b) 15N(P,7o)160 reaction at 90 ° [1]. {Only the differential cross section is plotted as a function of energy because neither the c e n t e r - o f - m a s s energy of the incident proton nor the penetrability vary significantly over the range of excitation energy considered here.) (c) 160(e,e'p}lSN at 0= 76 ° and E)= 48 ° [2]. (d) 160(r,n)150 at 70° [3]. a s s u m e d t h a t the t o t a l c r o s s s e c t i o n ~ i s p r o p o r t i o n a l to (d~/d~2) at 90 ° and that the r e d u c e d w i d t h i s i n d e p e n d e n t of e n e r g y . Such a p l o t o b t a i n e d by u s i n g an S - w a v e p e n e t r a b i l i t y P o ( c h a n n e l r a d i u s R = 5.3 fm) is shown in fig. 3. A s i m i l a r plot i s o b t a i n e d w i t h a P - w a v e p e n e t r a b i l i t y . Two r e s o n a n c e s a r e s e e n at 1 6 0 e x c i t a t i o n e n e r g i e s of 24.05 ± 0.10 and 25.12 ± 0.05 MeV; the t o t a l w i d t h s a r e about 0.45 and 0.60 MeV (c.m.) r e s p e c t i v e l y . T h e s e n u m b e r s v a r y s l i g h t l y if a d i f f e r e n t r a d i u s R i s a s s u m e d . A l s o d i s p l a y e d in fig. 3 a r e d a t a f r o m o t h e r r e a c t i o n s . The r e s o n a n c e at 25.12 MeV r e e s w e l l w i t h the 1 5 N ( P , ~ o ) 1 6 0 d a t a [1], the 60(7, n ) 1 5 0 d a t a [3, 4] and the 7 - a b s o r p t i o n d a t a [5]; but it i s not s e e n p r o m i n e n t l y in the 1 6 0 ( e , e ' p ) 289
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15N data [2]. The width of 0.6 MeV c o m p a r e s well with 0.5 MeV obtained by Tanner et al. [1]. Howe v e r , the r e s o n a n c e at 24.05 MeV a p p e a r s to be at a l ow er e n e r g y as c o m p a r e d to the one o b s e r v e d at 24.4 MeV in the o t h e r r e a c t i o n s [1-3, 5]; s e e h o w e v e r [4]. Also the width of 0.45 MeV is m u c h s m a l l e r than 1 MeV obtained by T a n n e r et al. [1]. Our r e s u l t s can a l s o be c o m p a r e d with the work of Suffert [6] who does not s e e any p r o m i n e n t s t r u c t u r e o v e r this e x c i t a t i o n e n e r g y r e g i o n in the 14N(d, 7o)160 r e a c t i o n . In addition one might note that the s t r u c t u r e s e e n h e r e in the (3He,~o) r e a c t i o n is m u c h d i f f e r e n t f r o m what is s e e n in the (3He, no) and the (3He, Po) r e a c t i o n s in 13C. In the (3He, no) r e a c t i o n at 0o [9] only one r e s o nance is s e e n at 3He lab. e n e r g y of 1.95 MeV and in the (3He, Po) r e a c t i o n [10] t h e r e is no e v i d e n c e of any s t r u c t u r e at 0 ° and 90 °. The a p p a r e n t d i f f e r e n c e s in the s t r u c t u r e around 24 MeV a l r e a d y pointed out could be taken to indicate that the (3He, Yo) peak at about 24.05 MeV should not be i d e n t i f i e d with the component Of the giant e l e c t r i c dipole r e s o n a n c e at 24.4 MeV. If this is g r a n t e d it is a p p a r e n t that the 3He r e duced width of the 24.4 MeV s t a t e m u s t be m u c h s m a l l e r than that c o r r e s p o n d i n g to the 25.1 MeV 3He c a p t u r e r e s o n a n c e . This in turn might indicate that the p r o m i n e n t 3He c a p t u r e r e s o n a n c e at 25.1 MeV should not be c o n s i d e r e d as p a r t of the giant dipole s t r u c t u r e o b s e r v e d in the s i n g l e p a r ticle reactions since such very different particle r e d u c e d widths a m o n g this c o m p o n e n t s would s e e m r a t h e r i m p r o b a b l e . Finally, if as s e e m s quite r e a s o n a b l e , the 25.1 MeV 3He c a p t u r e r e s o n a n c e is i d e n t i f i e d with the 25.1 MeV r e s o n a n c e s e e n in the ot he r r e a c t i o n s [1, 3-5], then this l a t t e r f e a t u r e would p r o b a b l y not be p a r t of the giant e l e c t r i c dipole r e s o n a n c e . On the o t h e r hand, one might be t e m p t e d to identify the two r e s o n a n c e s s e e n in this 3He capt u r e work with those s e e n in the o t h e r r e a c t i o n s [1-5]. Using the 15N(P,7o)160 c r o s s s e c t i o n ~(P, 7o) [1] and a s s u m i n g i s o t r o p i c a n g u l a r d i s t r i -
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butions, one then finds [~(p,~,o)/a(3He,vo)] ~ 180 and ~ 3 . 6 a t E 7 ~ 24 and ~ 2 5 MeV, r e s p e c t i v e l y . A s s u m i n g a channel r a d i u s of 4.85 fm for the 15N(p, ~o)160 r e a c t i o n and using S - w a v e penet r a b i l i t i e s one obtains for the r a t i o s of r e d u c e d widths [72(p)/72(3He)] ~ 3.6 and ~ 1.4 at E y ~ 24 and ~ 25 MeV, r e s p e c t i v e l y . H o w e v e r , these l a r g e 3He r e d u c e d widths again lend su p p o r t to the a r g u m e n t above that the r e s o n a n c e s s e e n in the (3He, Vo) r e a c t i o n a r e p r o b a b l y not components of the giant dipole r e s o n a n c e . It is a p l e a s u r e to acknowledge the help and guidance of P r o f e s s o r W. E. M e y e r h o f throughout this work. Our thanks a r e extended to P r o f e s s o r s S.S. Hanna and P. Paul for t h e i r i n t e r e s t in this work and to Mr. W. F e l d m a n f o r his a s s i s t a n c e during the r u n s.
References 1. N.W. Tanner, G.C. Thomas and E. D. Earle, Nuclear Phys. 52 (1964) 45. Other references are found in this paper. 2. W. R. Dodge and W. C. Barber, Phys. Rev. 127 (1962) 1746. 3. F. W. K. Firk and K. H. Lokan, Phys. Rev. Letters 8 (1962) 321. 4. R.L. Bramblett, J. T. Caldwell, R.R. Harvey and S. C. Fultz, Phys. Rev. 133 (1964) B 869. 5. N.A. Burgov, G.V. Danilyan, B.S. Dolbilkin, L.E. Lazareva and F. A. Nikolaev, Soviet Physics J. E. T. P. 16 (1963) 50; J.M.Wyekoff, B. Ziegler, H. W. Koch and R. Uhlig, Phys. Rev. 137 (1965) B576. 6. M.Suffert, Nuclear Phys. 75 (1965) 226. 7. See for example V. Gillet and N. Vinh Mau, Nuclear Phys. 54 (1964) 321 and F. H. Lewis Jr., Phys. Rev. 134 (1964) B 331. 8. Supplied by A. E. R. E., Harwell, England. 9. J. L. Well and G. U. Din, Bull. Am. Phys. Soc. 7 (1962) 111. 10. J . P . Sehiffer, T. W. Bonner, R.H. Davis and F. W. Prosser Jr., Phys. Rev. 104 (1956) 1964. 11. H.M.Kuan, P.R. Almond, G.U.Dinand T,W. Bonner, Nuclear Phys. 60 (1964) 509. 12. See for example A. M. Lane and R. G. Thomas, Rev. Mod. Phys. 30 (1958) 257 p. 322.