~ETTERE AL NUOVO CIMENTO VOL. 37, N. 6 11Giugno 1983

States in 1SO at High Excitation Energy.

A. CUNSOLO, A. FOTI, G. IMM~, G. PAPPALARD0 and G. RACITI

I s t i tu to Naz ionale di ~ i s i c a Nuclea te - Sezione di Catania _Laboratorio Naz iona le del S u d - Catania

Centre Sic i l iano d~ E is ica Nuc lea te e S t ru t tura della Mater ia

I s t i tu to Dipar t imenta le di F i s i ca dell' Universi t~ - Corse I ta l ia 57, 95129, Catania, I t a l ia

~ . S A U N I E R

Depar tmen t de Phys ique Nuel$aire, C . E . N . - Saclay F91191, Gi] sur Yvette, Cedex, !~rance

(ricevuto il 12 Aprile 1983)

PACS. 25.70. - Heavy-part icle-induced reactions and scattering.

S u m m a r y . - Deuteron-alpha angular correlations for the 14C(eLi, d)lS0--~ ~-14Cg.,. reaction have been measured at E(eLi) = 34 MeV and O~ ab= 10 ~ The data analysis per- formed under the hypothesis of a sequential process allows spin and pari ty assignments for many unknown is0 states at h igh excitation energy. In addition the 14C(~Li, d)l~O reaction is confirmed to proceed essentially via direct ~-particle transfer. A 180 negative- par i ty band, corresponding to the well-known band in 2~ is suggessed.

In a recent work (i) we have investigated is0 states populated through the 14C(8Li, d)lS0 reaction at E(SLi) = 34 MeV, by studying the deuteron angular distri- butions. Unfortunately, the experimental angular distributions for unbound 180 states resulted structureless, which prevents unambiguous spin and pari ty assignments.

This information can be obtained by measuring the a-decay from these states, i.e. by studying the deuteron-alpha angular correlation (2.s). Moreover, information on the populating reaction mechanism can be gained, if the first emit ted particle is de-

(~) A. CUNSOLO, A. FOTI, G. I~'r G. PAPPALAI~DO, G. RXCITI and N. SAUNIER: Phys. Rev. C, 24, 476 (1981). (~) A. ]~. LITHERLAND and A. J. FEROUSSON: Can. J. Phys., 39, 788 (1961). (8) E . F . DA SILVEIR~-: Prec. o] the X I V Winter Meeting on Nuclear Physics, Bormio, Italy, 1976 (unpublished), p. 18. (~) A. STRAZZERI: NUOVO Cimento, A, 34, 229 (1976). (a) A . CUNSOLO, A. FOTI, G. PAPPALARDO, G. I:~ACITI, ~ . SAUNIER a n d E . F . DA SILVEIRA.* Nuovo Cimcnto A, 40, 293 (1977). (e) ~k. CUNSOLO, A. FOTI, G. IMM]~, G. PAPPALARDO, G. I~ACITI a n d 1~. SAUNIER: Phys. Rev. C, 21, 2345 (1980).

193

194 A. CU-NSOLO, A. FOTI, G. IMME, G. PAPPALARDO~ G. lZACITI a n d X. SAUNI:ER

t e c t e d a t ~b 0 o(3,6) Od # . F r o m b o t h these fac t s some i n f o r m a t i o n on t he a - s t r t l c tu r e of t he ~s0 s ta tes , ly ing a t h i g h e x c i t a t i o n energy, can be deduced . T h e r e f o r e i t s eemed w o r t h w i l e m e a s u r i n g t he d-~ a n g u l a r cor re la t ions for t he l*C(6Li, d ) l S 0 - + z + laC~.~. r e a c t i o n a t E(~Li) = 34 MeV a n d O~'~h = 10 o. P r e l i m i n a r y re su l t s conce rn ing t h e i den t i f i ca t ion of t h e 8 + m e m b e r of t h e 4p-2h lsO p o s i t i v e - p a r i t y r o t a t i o n a l b a n d h a v e been r e c e n t l y p u b l i s h e d (7).

T h e 34 MeV 5Li b e a m was p r o d u c e d b y t h e _FN9 T a n d e m Van de Graaff of t h e Cen t r e & E t u d e Nuc l6a i re of Saclay. Se l f - suppor ted ~4C+~2C t a r g e t s o b t a i n e d w i t h a e r ak ing p r o c e d u r e (s), 180 ~g/cm 2 t h i c k a n d en r i ched to 7 0 % in 14C, were used.

D e u t e r o n s were de t ec t ed b y m e a n s of a D E - - E si l icon te lescope (DE = 200 ~m, E - : 5000 ~m), f ixed a t Od~b= 10 ~ (q~d = 0 ~ azimuthal angle) . T h e co inc iden t a lpha- pa r t i c les were de t ec t ed b y m e a n s of two D E - - E si l icon te lescopes (DE~ = 15 ~ m

lab E ~ = 6 0 0 B m ; DE~= 13 .5Bm, E 2 = 6 0 0 B m ) in t h e a n g u l a r r a n g e 20 ~ < 107 ~ (~va = 180~ A si l icon c o u n t e r 200 Bm t h i c k f ixed a t 30 ~ was used as moni to r . P a r t i c l e i den t i f i c a t i on was p e r f o r m e d ana log ica l ly in a s t a n d a r d way (9). Coincidences were a n a l y s e d w i t h t h e TAC t e c h n i q u e a l lowing a 2 ns ful l w i d t h a t ha l f m a x i m u m ( F W H M ) reso lu t ion .

TABLE I. - 1so levels presently investigated and the respective experimental and theor- etical angular shi/ts.

E* (MeV) I ~ ($exp (a) (degrees) 6mVBA(degrees)

11.4 4 + 16.0 15.6

11.69 6 + 13.5 14.4

12.9 5- 16.0 15.7

13.3 5- 15.5 15.6

14.2 4 + 16.0 14.7

14.6 5- 18.0 15.2

15.9 5- 18.0 15.2

17.0 7- 9.0 10.0

17.6 8 + 6.5 8.0

18.2 7- 9.0 10.0

18.8 7- 10.0 10.0

19.3 8 + 5.0 8.0

19.7 4 + 13.5 12.0

21.5 6 + 9.0 11.6

22.1 6 + 8.0 10.5

23.1 7- 10.0 10.0

(a) A n i n d e t e r m i n a t i o n of ~ 0 . 5 ~ for 6ex p is e x p e c t e d f r o m the a p p l i e d ch i - sq ] l a r ed p r o c e d u r e .

(7) A. CUNSOLO, A. FOTI, G. IMMI~, G. ]~APPALAI%DO, G. I:~ACITI a n d N. S&WNIER: Phys. Lett. B, 112, 121 (1982). (s) L. BIANCItI a n d P . ]3OUYER: R e p o r t No. CEA-.W-1522, (1970-71) , p. 93. (9) R . CHA3IINADE, J . C. F.~*IVRE a n d J . ~[a.~I*NT: Nuel . Ins lrum. Methods, 49, 217 (1967).

S T A T E S I N i s 0 AT H I G H E X C I T A T I O N E N E R G Y 1 ~

12 001

6 00~

2 75

o

75

90

~)

r m

v,..: ~ ..2-+

I IT II o,

7"

r

e,i r ~ I I '~.~< ,,/ c6 -=

160 230 300 370 channe l

F i g . 1. - S p e c t r a f r o m t h e r e a c t i o n 1~0~4C(6Li, d)I~,180 - * ~2,14Cg.s" a t E ( S L i ) = 3 4 M e V . a ) S i n g l e - d e u - t e r o n s p e c t r u m d e t e c t e d a t O d = 10 ~ E n e r g y c a l i b r a t i o n w a s o b t a i n e d f r o m t r a n s i t i o n s l e a d i n g t o leO a n d 180 k n o w n s t a t e s , b) D e u t e r o n s p e c t r u m i n c o i n c i d e n c e w i t h a - p a r t i c l e s d e c a y i n g t o t h e ~Cg.s . , d e t e c t e d a t O ~ = 55 ~ C o n t r i b u t i o n s f r o m t h e ~ C t a r g e t i m p u r i t y a r e a l s o p r e s e n t , c) S a m e a s b) b u t f o r I~C(6Li, d ) ~ O - * ~+ l~Cg . s , . d) S p e c t r u m b) s u b t r a c t e d of t h e ~ C c o n t r i b u t i o n ( s p e c - t r u m c) ) .

196 A. CUNSOLO, A. FOTI , G. ISIM]~, G. PAPPALARDO, G. RACITI a n ~ N. SAUNIER

Further details on the experimental apparatus and procedure are reported in ref. (6). As the 1~C al~d 1~C related events cannot be always kiuematically resolved, each run

was repeated by using a natural carbon target in order to subtract ~C contributions. Figure 1 shows typical inclusive and ~-coincidcnt deuteron spectra.

10 0

lOZ ~ !

10 0

13.3 ~4eV(5 §

12.9(5-)

10 o 11.69 (6 §

10 -z

10 0

Q~

10 -2

,11.4(4+)

10 -~ 20 60

�9 17.0 MeV(7 +)

14.6(5-)

- ~

14.2 (4 + )

100 140 20 60 100 140 e 2 (degrees)

:Fig. 2. - Angu la r - co r r e l a t i on func t ions for the t r ans i t ions ana lysed in the p r e s e n t work . Th e e r ro r b a r s t ake into accoun t the s u b t r a c t i o n of 12C i m p u r i t y . The cu rves r ep r e sen t theo re t i ca l ca lcu la t ions done b y a s suming a d i rec t R-transfer for the p r i m a r y reac t ion . 02 is the ~-par t ic le anglo defined in the recoi l nuc leus cen t re -o f -mass f r a m e , b u t w i th r e spec t to the b e a m direct ion. E(6Li) = 34 MeV, @ b = 10 o, ~ b = 0 o, ~ab = i s 0 o.

STATES ]N 180 AT H I G H EXCITATION E N E R G Y 197

The 1~0 levels analysed in t he present work arc ind ica ted in fig. 1, and repor t ed in tab le I.

The F W H M energy resolut ion was about 100 KeV. The angular corre la t ion d a t a are r epor t ed in fig. 2 and 3. The theore t i ca l curves ,

also shown in fig. '2 and 3, are eva lua t ed in the fo rmal i sm of ref. (3) assuming a sequent ia l process l~C(6Li, d)lsO(1 ~) -+ ~ -~ 14C'g.~..

W e describe the (~ d) p r ima ry reac t ion as a d i rec t a - t ransfer lead ing to a well- 4ef ined ~sO s ta te which co-decays to t he 14C g.s.

10 0

10 -~

j_10

10 0

10 -2

1.o

10

lO

19.3MeV (8 §

__ , I i l i I ~- I . e ! ,

�9 18.8(7-)

20 60 100

"*' ~ 23.1 MeV(7-)

r.... j

OeQ . + / - N - ~+*~, ' , , ,o

. _ . . . . L - I , ~ . J___ ~ I , I i I .A . . . _

19.7(4 +)

_ l . I i I . _ L _ I J L ' I 14-0 20 60 I00 14-0

02 (d.egrees)

Fig. 3. - Same as in fig. 2.

198 ~. CUNSOLO, A. FOTI, G. 1MMI~, G. PAPPALARDO, G. RACITI a n d N. 8AUNIBR

Under these hypotheses the W(D1, ~2~) angular-correlation function is proportional to the square of the Legendre polynomial of first order, shifted over an angle 6. In general, different reaction mechanisms produce different values of the angular 6 shift (a,6). In particular the (6Li, d) reaction is analysed in the exact finite-range distorted-wave Born approximation (EFR-DWBA) franlework. The used optical-model parameters are the same as in ref, 0). The calculations are carried out by using the code COR- RELA (10).

As can be seen in fig. 2 and 3 the strong oscillating behaviour of data is well ac- counted for by the theoretical curves. The only relevant discrepancy is seen at forward angles for the higher excitation energy states and it is probably due to the contribu- t ion from the ~Li break-up (n).

Table I summarizes the theoretical shifts and thc experimental ones deduced from a Z2-procedure to the data. The overall good agreement indicates that all these transi- tions proceed mainly through a direct a-transfer and allows unambiguous spin and pari ty assignments, as reported in table I. Remark that the I ~ ~ 5- for the 14.6 MeV and I .~ ~ 7- for the 17.0 g e V states spin and parity values correct the previous sug- gestion ( I ~ = 4+ for both these states) deduced through the angular-distribution analysis (~).

A first result (v) of the present investigation concerns the identification of the 8 + member of the (sdp(pi) -2 ~sO positive-parity rotational band. This band should correspond (P-} to the well-kimwn (l~) (sd)4 positive-parity 2~ one.

Analogously, a negative-parity rotational band could be expected in ~sO under the assumption of weak coupling (~4) of the two ~4C pt holes to the (p])l(sd)a "~176 negative- pari ty band based on the 5.78 MeV (1-) state (13). Calculations based on the Za- mick's formula (~5), taking a = - -0 .4 , give excitation energies of about 8.5 MeV (1-), 9.9 MeV (3-), 13.0 MeV (5-) and 18.0 MeV (7-) for the members of such a band.

On the other hand, BU-CK and I)~LT (~6), in the folded cluster model framework, sug- gested the existence of negative-parity ~SN-triton cluster states in the ~so nucleus.

Therefore, in order to study the structure of the ~so negative-parity states a com- parison of the ~SN(~Li, aite)lSO at E(6Li) = 40 MeV (1~) and the present ~4C(~Li, d)~so spectra can be useful. In fact, assunling pure configurations, final states strongly pop- ulated in the (~Li, ~He) reaction and weakly in the (6Li, d) one, should have strong 3p-lh parentage. Conversely, peaks inhibited in the (qA, SHe) reaction, but populated in the (SLi, d) one probably reflect major (p/)~(sd)~(p~) -~ components. Such a comparison selects, between the negative-parity states reported in table I, the 12.9 ~eV or the 13.3 MeV and the 18.2 }IeV as the 5- and the 7- members, respectively, of the suggested (p])l(sd)S(p~) ~ ~sO negative-parity rotational band. The lower members could be the 7.62 MeV (1-) and the 8.~8 MeV (3-) or the 10.11 McV (3-) states not populated in the (SLi, SHe) reaction, but seen in the (6Li, d) one 0). On the other hand, the 14.6 MeV (5-) strongly populated in the (SLi, SHe) reaction should be the 5- member of the 3p-lh ~sO rotational band (16).

(lo) E. F. DA ~ILVEIRA: p r i v a t e c o m m u n i c a t i o n . (11) A. CUNSOLO, A. FOTI, (~. I-",ISl]~, G. PAPPAL~kRDO, G. ~ACITI, F. IXIZZO an d N. ~AUNI~I:t: Lelt . Nuovo Cimeuto, 34, 229 (1982). (~) T. ERIKSON an4 G. E. BRO'~VN: N u c t . P h y s . A , 277, 1(I977) . (13) F. AJZE.~'BERC~-SEI, OVF: N u c l . p h y s . A , 392, 1 (1983) an d referenccs there in . (1,) A. At~I~A, H . HORIUCH~ a nd T. SEBE: P h y s . Let t . B , 24, 129 (1967). (~G) L. ZA~CK: P h y s . Lett . , 19, 580 (1965). (~) B. BUCK and A. A. PILT: N u c l . P h y s . _4, 295, 1 (1978). (~) C. J . SAUNDERS, L. ~r M~RTZ a nd P. D. PARI<ER: Proc. o / the In ternat ional Con]erence or~ Nuc lear ~truclure, Tokyo, J a p a n , 1977, p. 177.

STATES IN 180 AT H I G H EXCITATION ENERGY 199

In summary, angular-correlat ion da ta from the 140(~ d)lSO --~ ~-~140~.~. react ion have been analysed allowing spin and pa r i ty assignment to many unknown 180 states. These states are populated mainly through a direct-~-transfer reaction, as evident ia ted by both the strongly oscillating shapes of the da ta and the analysis of the a-shifts.

The present assignments and a comparison between (6Li, d) and (eLi, SHe) reactions support the suggestion of a (p])l(sd)3(pt)-~ 1sO negat ive-par i ty ro ta t ioa l band based on the 7.62 MeV (1-) state, being the 8.28 MeV (3-) or the 10.11 MeV(3-) , the 12.9 MeV (5-) or the 13.3 MeV (5-) and the 18.9 MeV (7-) the other members.