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Unbound energy levels in 4H
Volume 25B. number 2
PHYSICS
LETTERS
7August1967
Table 1 Source
Run 1 6 x 107
Run 2 6 x 10 7
Run 3 6 x 107
Run 4 6 x 10 7
Lead filter thickness
2 mm
1 mm
2 mm
2 mm
-3.3 ± 0.7
-2.9 i 0.65
-2.4 ± 0.65 +0.8 ± 0.4
181Hf 46Sc 82Br
+ 0.5 =e 0.3
Runs 1-4 weighted average 6 × 10 7
Circular polarization Ry
-3.8 ± 1.2
-2.95 ± 0.36
-(6 =e 1) × 10 - 6 .
-0.9 ± 0.4
+0.2
± 0.3
* E r r o r with allowance made for control e x p e r i m e n t s . 1. V.M.Lobashov, V.A.Nazarenko, L.F.Saenko and L.M. Smotritsky, P i s ' m a J . E . T . P . 3 (1966) 76. 2. F. Boehm and E. Kankeleit, Phys.Rev. L e t t e r s 14 (1965) 312. 3. V.M.Lobashov, V.A.Nazarenko, L.F.Saenko, L.M. S m o t ritsky and G.I.Kharkevitch, Pis'ma J . E . T . P . 3 (1966) 274. 4. Y.G.Abov, P.A.KrupchitskyandY.A.Oratovsky, Y a d e r naya Physics 1 (1965) 479. 5. V.M.Lobashov, Yadernaya Physica 2 (1965) 957. 6. F.C.Michel, Phys.Rev. B 133 (1964) 329. 7. S.Wahlborn, Phys.Rev. B 138 (1964) 530.
Table 2 Isotope
P~ = RF
181Ta E~=482keV -(6~1) x10-6 175Lu
ET= 396 keV
+(4±1)×10 -5
RLheor.
F
15±150
-(0.4±4) x10 -7
50±200
(2
±8) Xl0 -7
* * * * *
UNBOUND
ENERGY
LEVELS
IN
4H *
D. F . M E A S D A Y **
Cyclotron Laboratory, Harvard University, Cambridge, Massachusetts and J . N. P A L M I E R I
Cyclotron Laboratory, Harvard University, Cambridge, Massachusetts and Department of Physics, Oberlin College, Oberlin, Ohio Received 12 June 1967
The reaction 4He(n,p)4H has been studied with 152 MeV monokinetic neutrons. Structure in the proton s p e c t r a is i n t e r p r e t e d as excitation of 1- unbound levels in 4H. No bound levels of 4H are observed.
T h e e x p e r i m e n t a l s t a t u s of t h e f o u r - n u c l e o n s y s t e m h a s r e c e n t l y b e e n r e v i e w e d by A r g a n e t al. [1] a n d by M e y e r h o f a n d M c E l e a r n e y [2]. T h e e x p e r i m e n t a l d a t a h a v e b e e n a n a l y z e d by d e S h a l i t a n d W a l e c k a [3] a n d by B a r r e t t e t a l . [4] i n t e r m s of t h e s h e l l m o d e l . It s e e m s f a i r l y c e r t a i n t h a t 4H i s p a r t i c l e u n s t a b l e . T h e r e h a s b e e n p o s i t i v e i d e n t i f i c a t i o n [5] of a r e s o n a n c e c o r r e s 106
p o n d i n g to 4H f r o m t h e a b s o r p t i o n of n e g a t i v e p i o n s on 6 L i a n d 7Li. T h i s r e s o n a n c e h a s b e e n known for a long t i m e f r o m the total neutron * This work was supported by the Office of Naval R e s e a r c h and the National Science Foundation. ** P r e s e n t a d d r e s s : CERN, 1211 Geneva 23, S w i t z e r land.
Volume 25B, number 2
PHYSICS LETTERS
c r o s s section of 3H. A r e c e n t p h a s e - s h i f t a n a l y s i s [6] of n e u t r o n - t r i t o n s c a t t e r i n g has e s t a b l i s h e d two l e v e l s in 4H at 24.0 MeV (2-) and at 25.7 MeV (1-), m e a s u r e d with r e s p e c t to the ground state of 4He (not c o r r e c t e d for Coulomb or m a s s differences). T h e s e l e v e l s c o r r e s p o n d [6] to unbound r e s o n a n c e s in the n-3H s y s t e m at 3.4 and 5.1 MeV in the c e n t r e - o f - m a s s s y s t e m . C o r r e s p o n d i n g l e v e l s a r e known [7] in 4Li. The m o s t d i r e c t way to produce 4H is by the (n, p) r e a c t i o n on 4He. Since the expected Q v a l ue for the r e a c t i o n to the unbound "ground state" (2-level) is -23.2 MeV, it is n e c e s s a r y to use a m e d i u m - to h i g h - e n e r g y n e u t r o n b e a m . We have i n v e s t i g a t e d the 4He(n, p)4H r e a c t i o n u s i n g the 152 MeV monokinetic n e u t r o n beam [8] of the H a r v a r d s y n c h r o - c y c l o t r o n . We detected the p r o t o n s with a t r i p l e s c i n t i l l a t o r t e l e s c o p e in coincidence with a NaI detector. Since the 4He(n, d)3H r e a c t i o n c o n s t i t u t e s a m a j o r b a c k ground, it was n e c e s s a r y to d i s t i n g u i s h between outgoing d e u t e r o n s and p r o t o n s . The p a r t i c l e identification was a c c o m p l i s h e d by m e a s u r i n g , for each c h a r g e d p a r t i c l e t r a v e r s i n g the telescope its r a t e of energy l o s s (dE/dx) in a 1.27 cm thick p l a s t i c s c i n t i l l a t o r and the e n e r g y (E) is deposited i n t h e N a I d e t e c t o r . The p u l s e s for E and Edx w e r e digitized and s t o r e d on m a g n e t i c tape by a c o m p u t e r . An e x a m i n a t i o n of the d i s t r i b u t i o n of p a i r s of v a l u e s (E, dE/dx) for p r o t o n s and d e u t e r o n s knocked out of v a r i o u s t a r g e t s placed in the n e u t r o n b e a m , r e v e a l e d two d i s t i n c t bands of e v e n t s , one for p r o t o n s , the other for d e u t e r o n s . The t a r g e t for the 4He(n, p)4H r e a c t i o n was liquid h e l i u m ; it was contained in a 5 cm d i a m e t e r v e r t i c a l c y l i n d e r with walls of 50 gm kapton polyimide f i l m . The t a r g e t d i a m e t e r was such that a 120 MeV proton would lose about 4.8 MeV in t r a v e r s i n g the h e l i u m . When the s p r e a d in e n e r g y (6 MeV, f.w.h.h.) of the i n c i d e n t n e u t r o n beam [8] was c o m b i n e d with other s o u r c e s of e n e r g y s p r e a d (angular r e s o l u t i o n and l o s s of e n e r g y by outgoing p r o t o n s in the target), the e n e r g y r e s o l u t i o n was found to be 10 MeV (f.w.h.h.). T h i s r e s o l u t i o n was c o n f i r m e d e x p e r i m e n t a l l y by o b s e r v i n g f r e e n - p s c a t t e r i n g f r o m liquid hydrogen in the s a m e t a r g e t . The e n e r g y c a l i b r a t i o n of the NaI d e t e c t o r was also d e t e r mined from free n-p scattering. Fig. 1 shows proton s p e c t r a at a n g l e s between 5o and 40o (lab). The c r o s s s e c t i o n s have been n o r m a l i z e d to known n - p c r o s s section [9] at this e n e r g y . T h e r e i s an u n c e r t a i n t y of 30% in the a b solute value of the 4He(n, p)4H c r o s s s e c t i o n s . We o b s e r v e that each of the s p e c t r a f r o m 50 to 20o exhibits a single b r o a d peak at an e n e r g y
PDP-I
7August 1967
oa %*.,s'*
•
I
i
I
u,.I ~ 1 -
o
i*
i
i
I ~J_
.~
0.8 ~E
z
***
20 °
"7-5° o~
¢.,,*'7
, -
,.,.'~,~:.~
I
• .~o%
/
i o.4. O ~"I~ '
.,
i "i
~44.
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Q9 -
.
-~-zo
15 °
40 °
o
o- ~ue ~ev)
Fig. 1. Proton spectra for the reaction 4He(n,p)4H at 152 MeV. Note that at the smaller angles the abscissa represents the Q value, while at the larger angles the proton energy is indicated. The curves are the expected shapes for excitation of the giant dipole levels. c o r r e s p o n d i n g to a Q value of -29 :E 3 MeV. The peaks a r e about 20 MeV wide, twice the energy r e s o l u t i o n of the e x p e r i m e n t a l s y s t e m . On the b a s i s of the p o s s i b l e 4H l e v e l s d i s c u s s e d above, a r e a c t i o n to the "ground state" should have a Q value of -23.2 MeV, and to a bound state must have a Q value of -19.8 MeV or higher. F r o m the s p e c t r a we conclude that there a r e no bound s t a t e s of 4H and that the "ground state" is not s t r o n g l y excited in the (n, p) r e a c t i o n . F o r the l a r g e r a n g l e s the shape of the s p e c t r a is different due to a change in r e a c t i o n m e c h a n i s m to q u a s i - e l a s t i c s c a t t e r i n g , i.e. the s c a t t e r i n g of the i n c i d e n t n e u t r o n off a single proton in the h e l i u m n u c l e u s . F o r angles l e s s than 25o, the Q value of the peak r e m a i n s constant. This s t r o n g l y s u g g e s t s that the peaking is due to a f i n a l - s t a t e i n t e r a c t i o n in 4H. A s i m i l a r r e s u l t has been found by T y r 6 n et al. [10] for the i n e l a s t i c s c a t t e r i n g of 181 MeV p r o t o n s off h e l i u m . Now L a n g s f o r d et al. [11] have studied the (p,n) r e a c t i o n on 12C and 5:iV at 138 MeV, and they find a s t r o n g peak at a Q value of -25 MeV. The Q value of the peak s u g g e s t s the giant dipole r e sonance. B a r r e t t et al. [4] have r e c e n t l y p r o p o s e d 107
Volume 25B, number 2
PHYSICS
10
+
O
~8
+
6
~4 O
~2
ANC4.E(degrees C.M )
Fig. 2. Differential c r o s s section for the reaction 4He(n,p)4H as a function of the angle of the emitted p r o ton. The c r o s s section is calculated from the number of protons found between a Q value of -40 and -20 MeV. There is a 30% uncertainty on the absolute valu of the c r o s s - s e c t i o n scale. two 1-, T=I l e v e l s in 4He. One w o u l d e x p e c t the (p, p') r e a c t i o n to e x c i t e t h e s e l e v e l s w h i l s t the (n,p) r e a c t i o n s h o u l d e x c i t e the e q u i v a l e n t l e v e l s in ~H. On the f o u r s m a l l - a n g l e s p e c t r a in fig. 1, we h a v e d r a w n the s h a p e that one w o u l d o b t a i n if the (n, p) r e a c t i o n e x c i t e d j u s t the g i a n t d i p o l e r e s o n a n c e l e v e l s . T o o b t a i n t h i s s h a p e we d i v i d e d the p h o t o n u c l e a r c r o s s s e c t i o n by E 7 3 ; t h i s g i v e s the r e d u c e d r a d i a t i v e t r a n s i t i o n r a t e [12]. T h e c u r v e w a s t h e n s m o o t h e d with a G a u s s i a n of 10 MeV f . w . h . h , and the e n e r g y a x i s a d j u s t e d by 2 M e V to fit the h i g h - e n e r g y e d g e p e r f e c t l y (this is just within the experimental error). We see that the f i t i s v e r y good up to Q = -35 M e V . A s i m i l a r f i t i s o b t a i n e d f o r the i n e l a s t i c p r o t o n s p e c t r a . T h e a n g u l a r d i s t r i b u t i o n in the p r e s e n t e x p e r i m e n t (fig. 2) i s c o n s i s t e n t with an o r b i t a l a n g u l a r m o m e n t u m c h a n g e of one (we h a v e s u m m e d the c o u n t s b e t w e e n a Q v a l u e of -40 and -20 MeV). W e t h e r e f o r e s u g g e s t t h a t t h e p e a k o b s e r v e d in t h i s e x p e r i m e n t i s due to t h e e x c i t a tion of the g i a n t d i p o l e l e v e l s , a l t h o u g h we c a n not e x c l u d e t h e p o s s i b i l i t y of c o n t r i b u t i o n s f r o m o t h e r l e v e l s , e s p e c i a l l y 0 - and 2- l e v e l s . In a r e c e n t e x p e r i m e n t on the r e a c t i o n 6Li(~ +, 2p)4He, C h a r p a k et al [13] h a v e found an
108
LETTERS
7August 1967
a p p a r e n t r e s o n a n c e in 4He at 30 M e V , w h i c h t h e y [14] i n t e r p r e t a s a r i s i n g f r o m a p e r i p h e r a l m e c h a n i s m r a t h e r than an e x c i t e d s t a t e in 4He. On the o t h e r hand, T a n g [15] i n t e r p r e t s the r e s u l t a s b e i n g f u r to T=O e x c i t e d s t a t e s of h e l i u m with the s t r u c t u r e of two d e u t e r o n s in r e l a t i v e o s c i l l a t i o n . A l t h o u g h we h a v e c o n s i d e r e d v e r y d i f f e r ent r e a c t i o n s we s u g g e s t that a c o m p l e t e i n t e r p r e t a t i o n of the 6Li(~ +, 2p)4He r e a c t i o n m u s t i n c l u d e the T=I l e v e l s of h e l i u m d i s c u s s e d h e r e . W e thank P r o f e s s o r B. G o t t s c h a l k of N o r t h e a s t e r n U n i v e r s i t y f o r p r o v i d i n g the e l e c t r o n i c s y s t e m u s e d f o r d i g i t i z i n g and r e c o r d i n g the p u l s e s f r o m the c o u n t e r t e l e s c o p e . W e a l s o e x p r e s s o u r g r a t i t u d e to M i s s J. C h e v a l l e y * of Oberlin College for her assistance during the e x p e r i m e n t and with the a n a l y s i s of the data. • Supported by the National Science Foundation Undergraduate Research Participation Program.
References 1. P. E. Argan, G. C. Mantovani, P. Marazzini, A. Piazzoli and D. Scannicchio, Suppl. Nuovo Cimento 3 (1965) 245. 2. W. E. Meyerhof and J. N. McElearney, Nuel. Phys. 74 (1965} 533. 3. A. De Shalit and J . D . W a l e c k a , Phys. Rev. 147 (1966) 763. 4. B.R. Barrett, W. E. Meyerhof and J.D.Walecka, Phys. Letters 22 (1966) 450. 5. R. C. Cohen, A.D. Kanaris, S. Margulies and J. L. Rosen, Phys. Letters 14 (1965) 242. 6. T. A. Tombrello, Phys. Rev. 143 (1966) 772. 7. T.A. Tombrello, Phys. Rev. 138 (1965) B40. 8. D. F. Measday, Nucl. Instr. and Meth. 40 (1966) 213. 9. D. F. Measday, Phys. Rev. 142 (1966) 584. 10. H. Tyr~n, G. Tibell and Th. A. J. Marls, Nucl. Phys. 4 (1957) 277. 11. A. Langsford, P. H. Bowen, G. C. Cox and R. A. J. Riddle, Proc. of Padua Conf. (1962) p. 1150. 12. A.B. Clegg, High energy nuclear reactions (Clarendon P r e s s , 1965) Ch. 5. 13. G. Charpak, G. Gr6goire, L. Massonnet, J. Daudinos, J. Favier, M. Gusakow and M. Jean, Phys. Lett e r s 16 (1965) 54. 14. G. Charpak, J. Favier, L. Massonnet and ~. Zupan~i~, Proc. Intern. Conf. on Nuclear structure, Gatlinburg, Tennessee, September 1966, to be published. 15. Y. C. Tang, Phys. Letters 20 (1966) 299.
PHYSICS
LETTERS
7August1967
Table 1 Source
Run 1 6 x 107
Run 2 6 x 10 7
Run 3 6 x 107
Run 4 6 x 10 7
Lead filter thickness
2 mm
1 mm
2 mm
2 mm
-3.3 ± 0.7
-2.9 i 0.65
-2.4 ± 0.65 +0.8 ± 0.4
181Hf 46Sc 82Br
+ 0.5 =e 0.3
Runs 1-4 weighted average 6 × 10 7
Circular polarization Ry
-3.8 ± 1.2
-2.95 ± 0.36
-(6 =e 1) × 10 - 6 .
-0.9 ± 0.4
+0.2
± 0.3
* E r r o r with allowance made for control e x p e r i m e n t s . 1. V.M.Lobashov, V.A.Nazarenko, L.F.Saenko and L.M. Smotritsky, P i s ' m a J . E . T . P . 3 (1966) 76. 2. F. Boehm and E. Kankeleit, Phys.Rev. L e t t e r s 14 (1965) 312. 3. V.M.Lobashov, V.A.Nazarenko, L.F.Saenko, L.M. S m o t ritsky and G.I.Kharkevitch, Pis'ma J . E . T . P . 3 (1966) 274. 4. Y.G.Abov, P.A.KrupchitskyandY.A.Oratovsky, Y a d e r naya Physics 1 (1965) 479. 5. V.M.Lobashov, Yadernaya Physica 2 (1965) 957. 6. F.C.Michel, Phys.Rev. B 133 (1964) 329. 7. S.Wahlborn, Phys.Rev. B 138 (1964) 530.
Table 2 Isotope
P~ = RF
181Ta E~=482keV -(6~1) x10-6 175Lu
ET= 396 keV
+(4±1)×10 -5
RLheor.
F
15±150
-(0.4±4) x10 -7
50±200
(2
±8) Xl0 -7
* * * * *
UNBOUND
ENERGY
LEVELS
IN
4H *
D. F . M E A S D A Y **
Cyclotron Laboratory, Harvard University, Cambridge, Massachusetts and J . N. P A L M I E R I
Cyclotron Laboratory, Harvard University, Cambridge, Massachusetts and Department of Physics, Oberlin College, Oberlin, Ohio Received 12 June 1967
The reaction 4He(n,p)4H has been studied with 152 MeV monokinetic neutrons. Structure in the proton s p e c t r a is i n t e r p r e t e d as excitation of 1- unbound levels in 4H. No bound levels of 4H are observed.
T h e e x p e r i m e n t a l s t a t u s of t h e f o u r - n u c l e o n s y s t e m h a s r e c e n t l y b e e n r e v i e w e d by A r g a n e t al. [1] a n d by M e y e r h o f a n d M c E l e a r n e y [2]. T h e e x p e r i m e n t a l d a t a h a v e b e e n a n a l y z e d by d e S h a l i t a n d W a l e c k a [3] a n d by B a r r e t t e t a l . [4] i n t e r m s of t h e s h e l l m o d e l . It s e e m s f a i r l y c e r t a i n t h a t 4H i s p a r t i c l e u n s t a b l e . T h e r e h a s b e e n p o s i t i v e i d e n t i f i c a t i o n [5] of a r e s o n a n c e c o r r e s 106
p o n d i n g to 4H f r o m t h e a b s o r p t i o n of n e g a t i v e p i o n s on 6 L i a n d 7Li. T h i s r e s o n a n c e h a s b e e n known for a long t i m e f r o m the total neutron * This work was supported by the Office of Naval R e s e a r c h and the National Science Foundation. ** P r e s e n t a d d r e s s : CERN, 1211 Geneva 23, S w i t z e r land.
Volume 25B, number 2
PHYSICS LETTERS
c r o s s section of 3H. A r e c e n t p h a s e - s h i f t a n a l y s i s [6] of n e u t r o n - t r i t o n s c a t t e r i n g has e s t a b l i s h e d two l e v e l s in 4H at 24.0 MeV (2-) and at 25.7 MeV (1-), m e a s u r e d with r e s p e c t to the ground state of 4He (not c o r r e c t e d for Coulomb or m a s s differences). T h e s e l e v e l s c o r r e s p o n d [6] to unbound r e s o n a n c e s in the n-3H s y s t e m at 3.4 and 5.1 MeV in the c e n t r e - o f - m a s s s y s t e m . C o r r e s p o n d i n g l e v e l s a r e known [7] in 4Li. The m o s t d i r e c t way to produce 4H is by the (n, p) r e a c t i o n on 4He. Since the expected Q v a l ue for the r e a c t i o n to the unbound "ground state" (2-level) is -23.2 MeV, it is n e c e s s a r y to use a m e d i u m - to h i g h - e n e r g y n e u t r o n b e a m . We have i n v e s t i g a t e d the 4He(n, p)4H r e a c t i o n u s i n g the 152 MeV monokinetic n e u t r o n beam [8] of the H a r v a r d s y n c h r o - c y c l o t r o n . We detected the p r o t o n s with a t r i p l e s c i n t i l l a t o r t e l e s c o p e in coincidence with a NaI detector. Since the 4He(n, d)3H r e a c t i o n c o n s t i t u t e s a m a j o r b a c k ground, it was n e c e s s a r y to d i s t i n g u i s h between outgoing d e u t e r o n s and p r o t o n s . The p a r t i c l e identification was a c c o m p l i s h e d by m e a s u r i n g , for each c h a r g e d p a r t i c l e t r a v e r s i n g the telescope its r a t e of energy l o s s (dE/dx) in a 1.27 cm thick p l a s t i c s c i n t i l l a t o r and the e n e r g y (E) is deposited i n t h e N a I d e t e c t o r . The p u l s e s for E and Edx w e r e digitized and s t o r e d on m a g n e t i c tape by a c o m p u t e r . An e x a m i n a t i o n of the d i s t r i b u t i o n of p a i r s of v a l u e s (E, dE/dx) for p r o t o n s and d e u t e r o n s knocked out of v a r i o u s t a r g e t s placed in the n e u t r o n b e a m , r e v e a l e d two d i s t i n c t bands of e v e n t s , one for p r o t o n s , the other for d e u t e r o n s . The t a r g e t for the 4He(n, p)4H r e a c t i o n was liquid h e l i u m ; it was contained in a 5 cm d i a m e t e r v e r t i c a l c y l i n d e r with walls of 50 gm kapton polyimide f i l m . The t a r g e t d i a m e t e r was such that a 120 MeV proton would lose about 4.8 MeV in t r a v e r s i n g the h e l i u m . When the s p r e a d in e n e r g y (6 MeV, f.w.h.h.) of the i n c i d e n t n e u t r o n beam [8] was c o m b i n e d with other s o u r c e s of e n e r g y s p r e a d (angular r e s o l u t i o n and l o s s of e n e r g y by outgoing p r o t o n s in the target), the e n e r g y r e s o l u t i o n was found to be 10 MeV (f.w.h.h.). T h i s r e s o l u t i o n was c o n f i r m e d e x p e r i m e n t a l l y by o b s e r v i n g f r e e n - p s c a t t e r i n g f r o m liquid hydrogen in the s a m e t a r g e t . The e n e r g y c a l i b r a t i o n of the NaI d e t e c t o r was also d e t e r mined from free n-p scattering. Fig. 1 shows proton s p e c t r a at a n g l e s between 5o and 40o (lab). The c r o s s s e c t i o n s have been n o r m a l i z e d to known n - p c r o s s section [9] at this e n e r g y . T h e r e i s an u n c e r t a i n t y of 30% in the a b solute value of the 4He(n, p)4H c r o s s s e c t i o n s . We o b s e r v e that each of the s p e c t r a f r o m 50 to 20o exhibits a single b r o a d peak at an e n e r g y
PDP-I
7August 1967
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Fig. 1. Proton spectra for the reaction 4He(n,p)4H at 152 MeV. Note that at the smaller angles the abscissa represents the Q value, while at the larger angles the proton energy is indicated. The curves are the expected shapes for excitation of the giant dipole levels. c o r r e s p o n d i n g to a Q value of -29 :E 3 MeV. The peaks a r e about 20 MeV wide, twice the energy r e s o l u t i o n of the e x p e r i m e n t a l s y s t e m . On the b a s i s of the p o s s i b l e 4H l e v e l s d i s c u s s e d above, a r e a c t i o n to the "ground state" should have a Q value of -23.2 MeV, and to a bound state must have a Q value of -19.8 MeV or higher. F r o m the s p e c t r a we conclude that there a r e no bound s t a t e s of 4H and that the "ground state" is not s t r o n g l y excited in the (n, p) r e a c t i o n . F o r the l a r g e r a n g l e s the shape of the s p e c t r a is different due to a change in r e a c t i o n m e c h a n i s m to q u a s i - e l a s t i c s c a t t e r i n g , i.e. the s c a t t e r i n g of the i n c i d e n t n e u t r o n off a single proton in the h e l i u m n u c l e u s . F o r angles l e s s than 25o, the Q value of the peak r e m a i n s constant. This s t r o n g l y s u g g e s t s that the peaking is due to a f i n a l - s t a t e i n t e r a c t i o n in 4H. A s i m i l a r r e s u l t has been found by T y r 6 n et al. [10] for the i n e l a s t i c s c a t t e r i n g of 181 MeV p r o t o n s off h e l i u m . Now L a n g s f o r d et al. [11] have studied the (p,n) r e a c t i o n on 12C and 5:iV at 138 MeV, and they find a s t r o n g peak at a Q value of -25 MeV. The Q value of the peak s u g g e s t s the giant dipole r e sonance. B a r r e t t et al. [4] have r e c e n t l y p r o p o s e d 107
Volume 25B, number 2
PHYSICS
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Fig. 2. Differential c r o s s section for the reaction 4He(n,p)4H as a function of the angle of the emitted p r o ton. The c r o s s section is calculated from the number of protons found between a Q value of -40 and -20 MeV. There is a 30% uncertainty on the absolute valu of the c r o s s - s e c t i o n scale. two 1-, T=I l e v e l s in 4He. One w o u l d e x p e c t the (p, p') r e a c t i o n to e x c i t e t h e s e l e v e l s w h i l s t the (n,p) r e a c t i o n s h o u l d e x c i t e the e q u i v a l e n t l e v e l s in ~H. On the f o u r s m a l l - a n g l e s p e c t r a in fig. 1, we h a v e d r a w n the s h a p e that one w o u l d o b t a i n if the (n, p) r e a c t i o n e x c i t e d j u s t the g i a n t d i p o l e r e s o n a n c e l e v e l s . T o o b t a i n t h i s s h a p e we d i v i d e d the p h o t o n u c l e a r c r o s s s e c t i o n by E 7 3 ; t h i s g i v e s the r e d u c e d r a d i a t i v e t r a n s i t i o n r a t e [12]. T h e c u r v e w a s t h e n s m o o t h e d with a G a u s s i a n of 10 MeV f . w . h . h , and the e n e r g y a x i s a d j u s t e d by 2 M e V to fit the h i g h - e n e r g y e d g e p e r f e c t l y (this is just within the experimental error). We see that the f i t i s v e r y good up to Q = -35 M e V . A s i m i l a r f i t i s o b t a i n e d f o r the i n e l a s t i c p r o t o n s p e c t r a . T h e a n g u l a r d i s t r i b u t i o n in the p r e s e n t e x p e r i m e n t (fig. 2) i s c o n s i s t e n t with an o r b i t a l a n g u l a r m o m e n t u m c h a n g e of one (we h a v e s u m m e d the c o u n t s b e t w e e n a Q v a l u e of -40 and -20 MeV). W e t h e r e f o r e s u g g e s t t h a t t h e p e a k o b s e r v e d in t h i s e x p e r i m e n t i s due to t h e e x c i t a tion of the g i a n t d i p o l e l e v e l s , a l t h o u g h we c a n not e x c l u d e t h e p o s s i b i l i t y of c o n t r i b u t i o n s f r o m o t h e r l e v e l s , e s p e c i a l l y 0 - and 2- l e v e l s . In a r e c e n t e x p e r i m e n t on the r e a c t i o n 6Li(~ +, 2p)4He, C h a r p a k et al [13] h a v e found an
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a p p a r e n t r e s o n a n c e in 4He at 30 M e V , w h i c h t h e y [14] i n t e r p r e t a s a r i s i n g f r o m a p e r i p h e r a l m e c h a n i s m r a t h e r than an e x c i t e d s t a t e in 4He. On the o t h e r hand, T a n g [15] i n t e r p r e t s the r e s u l t a s b e i n g f u r to T=O e x c i t e d s t a t e s of h e l i u m with the s t r u c t u r e of two d e u t e r o n s in r e l a t i v e o s c i l l a t i o n . A l t h o u g h we h a v e c o n s i d e r e d v e r y d i f f e r ent r e a c t i o n s we s u g g e s t that a c o m p l e t e i n t e r p r e t a t i o n of the 6Li(~ +, 2p)4He r e a c t i o n m u s t i n c l u d e the T=I l e v e l s of h e l i u m d i s c u s s e d h e r e . W e thank P r o f e s s o r B. G o t t s c h a l k of N o r t h e a s t e r n U n i v e r s i t y f o r p r o v i d i n g the e l e c t r o n i c s y s t e m u s e d f o r d i g i t i z i n g and r e c o r d i n g the p u l s e s f r o m the c o u n t e r t e l e s c o p e . W e a l s o e x p r e s s o u r g r a t i t u d e to M i s s J. C h e v a l l e y * of Oberlin College for her assistance during the e x p e r i m e n t and with the a n a l y s i s of the data. • Supported by the National Science Foundation Undergraduate Research Participation Program.
References 1. P. E. Argan, G. C. Mantovani, P. Marazzini, A. Piazzoli and D. Scannicchio, Suppl. Nuovo Cimento 3 (1965) 245. 2. W. E. Meyerhof and J. N. McElearney, Nuel. Phys. 74 (1965} 533. 3. A. De Shalit and J . D . W a l e c k a , Phys. Rev. 147 (1966) 763. 4. B.R. Barrett, W. E. Meyerhof and J.D.Walecka, Phys. Letters 22 (1966) 450. 5. R. C. Cohen, A.D. Kanaris, S. Margulies and J. L. Rosen, Phys. Letters 14 (1965) 242. 6. T. A. Tombrello, Phys. Rev. 143 (1966) 772. 7. T.A. Tombrello, Phys. Rev. 138 (1965) B40. 8. D. F. Measday, Nucl. Instr. and Meth. 40 (1966) 213. 9. D. F. Measday, Phys. Rev. 142 (1966) 584. 10. H. Tyr~n, G. Tibell and Th. A. J. Marls, Nucl. Phys. 4 (1957) 277. 11. A. Langsford, P. H. Bowen, G. C. Cox and R. A. J. Riddle, Proc. of Padua Conf. (1962) p. 1150. 12. A.B. Clegg, High energy nuclear reactions (Clarendon P r e s s , 1965) Ch. 5. 13. G. Charpak, G. Gr6goire, L. Massonnet, J. Daudinos, J. Favier, M. Gusakow and M. Jean, Phys. Lett e r s 16 (1965) 54. 14. G. Charpak, J. Favier, L. Massonnet and ~. Zupan~i~, Proc. Intern. Conf. on Nuclear structure, Gatlinburg, Tennessee, September 1966, to be published. 15. Y. C. Tang, Phys. Letters 20 (1966) 299.