I L NUOVO C I M E N T 0
VOL. L X V A , N. 2
21 Gennaio 1970
Experimental Study of (3He, g) Reactions on 2s-ld and 1]._ Nuclei. II. - The s~P(~He, ~)3op Reaction (*). ~F . 13ELLEGRI~I~ I. FILOSOFO a n d F . GENTILI~N Istituto di Fisica dell' Universit~t - Padova Istituto Nazionale di Fisica Nueleare - Sezione di Padova .Laboratori Nazionali di Legnaro - Legnaro (Padova)
(ricevuto il 6 0 t t o b r e
1969)
Summary. - - Angular distributions for the 31P(ZHe, a)z~ reaction induced by 10 MeV 3He particles have been measured. We have found experimental evidence t h a t the 3 + ( T : 0) states at 2.54 and 1.98 MeV excitation energy and the 2 + ( T = 0) 1.46 MeV state, arise from the excitation of the 2ssi core. The other state up to an excitation energy of 3 MeV are well described by the 2s89 t configuration.
1.
-
Introduction.
In this paper we present the experimental results obtained in the 31p(3He ' ~)30p r e a c t i o n . I n t h e l i t e r a t u r e t h e m o s t r e c e n t n e u t r o n p i c k - u p e x p e r i m e n t on 31p is t h e (p, d) w o r k p e r f o r m e d b y B]~C~ET (1) a t a n i n c i d e n t p r o t o n e n e r g y of 18.6 MeV. T h i s i n v e s t i g a t i o n w a s l i m i t e d t o t h e a n a l y s i s of d e u t e r o n g r o u p s t o t h e g r o u n d s t a t e a n d t h e 700 k e V d o u b l e t . T h e r e a r e s h e l l - m o d e l c a l c u l a t i o n s (2) w h i c h t r e a t t h e 3~p n u c l e u s as a ~sSi core l d j c l o s e d shell p l u s t h e t h r e e r e m a i n i n g n u c l e o n s l i m i t e d t o t h e 2s89 orbits. As
(') Work performed as a p a r t of the NU2 program of the Istituto NazionMe di Fisica Nucleare. (1) E. ]?. BE•NET: Phys. Rev., 122, 595 (1961). (2) p. W. )~. GLAUDEMANS, G. WIECIIERS and P. J. ]~RUSSAARD: ~t/~C~. Phys., 56, 529, 548 (1964). 311
312
F. P E L L E G R I N I , I. FILOSOFO &rid F. GENTILIN
mentioned in the I n t r o d u c t i o n of ref. (3)there is experimental information thab nuclei in the 2s-ld shell contain in their ground states appreciable components of core excitation. In order to obtain information on d~ core-excited components the present (SHe, a) experiment was u n d e r t a k e n ~t a bombarding energy of 10 MeV.
2. - Experimental procedure and results.
The doubly ionized 10 MeV age beam, used to s t u d y the reaction reported in this paper was provided b y the C17 Van de Graaff of Legnaro (Padova). The experimental equipment and technique were identical to those described in ref. (3). The phosphorus target was produced in this l a b o r a t o r y b y v a c u u m e v a p o r a t i o n of n a t u r a l red phosphorus onto a thin carbon backing. The evaluution of absolute cross-section was done b y a comparison with the elastic scuttering of nile from phosphorus target ut 3 and 4 MeV incident energy. The t o t a l u n c e r t a i n t y of the absolute cross-section presented in this work has already been e s t i m a t e d to be 15% (see ref. (a)). Figure 1 shows a typicul cr spectrum obtained at an angle of 60 degrees
I00
(A. S. 3~
)
80
~+ (J~ T)
2+.
(A. S.3~ 60f-
3+0
Lu
r
40
3+0 '
1
rl
/L ~
20
ooooo
o
~ 3
o
b
2
o l
L ~o~
o
o 0
E(MeV) Fig. 1. - T h e *r spectrum for the alP(SHe, ~)aop reaction obtained ~t 01~b= 60~
(3) F. PELLEGRINI, I. FILOSOFO, F. G]~NTILIN, I. SCO~ONI and I. GABRiELLI: _N'UOVO Cimento, 65 A, 297 (1970).
EXPERIM]ENTAL
STUDY
OF
(3He, o~)
in the l a b o r a t o r y system. At doublet h~s been obtained. for an excitation energy up (T - 0 ) and b y the 0 + (T
[_
gnound state
REACTIONS
ON
1/~
AND
NUCLEI
-
ZI
313
this angle clear eu of the 0.68 and 0.71 MeV The 2s89 n e u t r o n pick-up t r a n s i t i o n is e x h a u s t e d to 3 MeV, b y the 1~- ground and 0.71 MeV states 1) isobaric analogue of the 3~ ground state.
0.70 MeV
[
1.46 MeV
~-},
_
/
2s-ld
/
F
__
i
I
i
:
1 b
\L ]0 -I
\
1.98 MeV
/
i
.
2.54 MeV
2.73 MeV
P /i
_
\ /
L i
1
;02~ 0
i
40
80
120
0
40 gO 8 (degrees)
~20
o
io
8'0
120
cm
Fig. 2. - Angular distributions from the 31P(3He, ~)30p reaction. The continuous lines are drawn through the experimental points and they have no further significance. Representative statistical uncertainties are given by error bars.
Tile ld~ and l d l n e u t r o n pick-up transitions, corresponding to the t r a n s f e r of an l - - 2 n e u t r o n way% are m o s t responsible for the excitation of the remaining states of sop, observed in the present e x p e r i m e n t . Figure 2 shows t h e angular distributions of a-particles leading to the ground, 0.68 and 0.71 (unresolved doublet) 1.46, 1.98, 2.54 and 2.73 MeV states of sop.
314
3. -
F. P E L L E G R I N I ,
I. FILO3OFO a n d
F. G E N T I L I N
Discussion.
GLAUDEMAI~S et al. (2) t r e a t e d the 3op as ~ closed she]/=sSi core with the two extr~ nucleons limited to the 2s89 t orbits. The results of this calculation are summarized in Fig. 3. The observed 2 + and 3 + states, at 1.46 and 1.98 MeV respectively, ~re not reproduced b y this model. As suggested b y these authors, such states m a y involve 28Si core-excited components. The other states are
3,02 2.23
2§
- -
2.94
(])
2+ 7=1 - --
2.84 --2.72
2 . 9 1 ~
(2) 2.54
1.9B
1.46
0+
3.00
0.71 0.68
2.73
exp.
305 i
3Op
3+
3+
3+
2+
1+
0.71
1+
0+ 7 = 1 - - -
0.68
0 + T=-I
1+
exp..
2* T = 1 2+
1+ talc.
3~
Fig. 3. - Comparison of the experimental level energies and spins with the theoretical calculations of ref. (2).
well reproduced b y the theoreticul calculations and for a n excitation energy up to 3 1KeV, are well described b y the 2s89 t configuration. Figure 4 shows the ~ spectrum obtained at 60 degrees, in the laboratory system, together with t h e spectroscopic factors for the l , = 2 and l ~ = 0 transitions, e v a l u a t e d from the wave functions t a b u l a t e d b y GLA-ODE~A~S et al. (2). These spectroscopic f~ctors have been normalized to t h e 2 + ( T - - 1), 2.94 MeV isobaric analogue of the ~~ first excited state. The model reproduces well the relative excitation of the 2 + s t a t e ut 2.73 MeV ~nd, t o w less extent, the doublet, a,t 0.70 lV[eV. T h e experimental results strongly
EXFEBZMENTAL
O F ( 3 H e , ~) R E A C T I O N S
~TUDY
ON
2~-Id A N D
l/~ N U C L E I - II
315
disagree for the excitation of the 3 + state at 2.54 ]V[eV and for the 1 + ground state. Although for a better understanding of the experimental results, we need evaluation of absolute value of spectroscopic factors extracted from the 100
(A. 5.3~ 80
1"0
2+1/ (A.
(J~, T)
7
3§ o
/~
3~
~c
o
3+o
1
2+I
2 /%=
2 0+1
1
1+0~ /. = 0
(2) i
0
3
L:O
Lo=2 2
E'(MeV)
1
n
1+
0
0
Fig, 4. - Comparison of the a spectrum obtained at 01~b = 60 ~ with the spectroscopic factors calculated, with the wave functions tabulated in refi (2).
usual D.W.B.A. method; we can say that the ground state of ~ P must contain an admixture of [(2s89 o configuration, much larger than the 23% quoted b y the authors of ref. (~). This is also supported b y the experimental results obtained in the alp(p, d)3op reaction (1). In that work, the angular distributions leading to the ground and 700 KeV doublet of 30p, analysed in terms of P.W.B.A. theory, indicate a sizeable 2s 89 strength for the ground-state transition. For the 3 + state a~ 2.54 MeV, we mast invoke core excitation,
316
F. PELLEGRI:NI~
I. FILO3OFO
and
iv. GENTILIN
3"1. E v i d e n c e o] 2~Si core-excited components. - As already nlentioned, the 2 + and 3 + states at 1.46 and 1.98 MeV are not predicted b y the 2s89 configuration model. Therefore, the observation of such states suggests the presence of 2sSi core-excited components in the 3~p ground state. I n a pure shell-model picture the r e m o t i o n of a ]d~ n e u t r o n from the ~xP target gives rise to the excitation of 2h-states, coming from the [(2s89 configuration, with J ' ~ - - 2 + and 3 +, respectively. The spectroscopic factor for t h e transition [(2s 89189189247247 is therefore given, in this simple picture, b y
:89log), where U 2 is a normulized Raeah coefficient, for the recoupling of the t h r e e angular momentu, corresponding to the ad~, 2st neutrons and to the 89 state of the target nucleus. Hence the spectroscopic factors for the excitation of the J ' : 3+ ~nd 2 + states are in the ratio S~_~(3+)/S~(2+) ~ 1.40. The ratio between the corresponding integrated cross-sections is 1.76 which is 25 % higher t h a n the predicted one. The 3+ state at 2.54 MeV, although predicted b y the model, has a spectroscopic f~ctor, evaluated with the wave fanctions of ref. (2), equal to zero. The
exl~erimenta]
e~eit~ti~l~ of such u state is theref~r~ ~ lurther indication
of the presence of ~'Si core-excited components. 3"2. Conclusion. - The 31P(aHe, ~)3op reaction has been studied at a bombarding energy of 10 ~SeV for levels of the residual ,op nucleus up to an excit a t i o n energy of 3 MeV. The experimental results have shown t h a t the 3+ (T----0) states at 2.54 and 1.98 MeV and the 2+ ( T = 0) state at 1.46 MeV arise from the excitation of the ~sSi core. The other states are well described b y the 2 s v l d ! configuration. I n addition there is experimental evidence t h a t the ground state of s0/), contains ~ [(2s~)~]j,_L.~_o a d m i x t u r e larger t h a n ~h~t ]~red/cte4 ~n ref. (~). *:,k*
The authors are deeply indebted to Prof. R. A. ]{ICCI for his interest in t h e present work. Finally t h e y would like to t h a n k Mr. P. G. BEzzo~ for his kind technical assistance during the experiment, as well as the accelerator operation staff.
EXPEI~IMENTAL
STUDY
OF
(~He, ~) I~.~r
ON
2s-ld AND
I/~ NUCLEI
-
II
317
RIASSUNTO
Si g s t u d i a t a la r e a z i o n e alp(atIe, ~)a0p a d u n a e n e r g i a del fascio i n c i d e n t e di 10 MeV e p e r livelli del ntteleo residuo al di s o t t o di 3 MeV di eeeitazione. I r i s u l t a t i s p e r i m e n t a l i i n d i e a n o ehe gli s t a t i 3 + ( T = 0) a 2.54 e 1.98 MeV e lo s t a t o 2 + ( I ' = 0) a 1.46 MeV, p r o v e n g o n o d a l l ' e e e i t a z i o n e del core di -~ssi. Gli a l t r i s t a l l v e n g o n o d e s e r i t t i b e n e d a l l a e o n f i g u r a z i o n e 2s89 .
3KcnepnMenraqbnoe n3yqentle peaI~um3 (3He, ~) n a 2s-ld H 1/~ n~pax. I I : PeaKuua a~P(~He, ~)~0p. Pe3ioMe (*). - - Bl,t n g ~3MepeH~I yrnoBbie pacrrpe~eneHH~ ~ n n pearur~rI 3~P(aHe, ~)~0p, Br~BBanHO~ 3He uacTnUaMn c 3Hepr~efi 10 MaB. Mbt o 6 H a p y ~ J I a 3KcIIepHMeHTaJIbl-IOe rIo)ITBep~aeHne, nTO 3 + ( T ~ 0) c o c r o n n H ~ C 9neprnefi a o 3 6 y ~ e n n ~ 2.54 n 1.98 M a B n COCTOnHI~n 2 + (T ~ 0) c 9 n e p r n e ~ 1.46 M a B BO3nnKa~OT rI3 Bo36y>K~ennn OCTOBa 2sSi. Taroxe Om4CbIBarOTC~ ~pyrHe cocToaHnn BYIYIOTb ~ 0 3HeprJ~H BO36yg~leH~i~ 3 MgB C rroMombro Konqb~rypaw~i~ 2s89
(') Hepeee3eno peOamtue(t.
