LETTERE AL NUOVO CIM~NTO VOL 34, ~. 9 26 Giugno 1982

Inclusive Deuteron Spectra from the 6Li~-12C Collision at 34 MeV.

A. CUNSOLO, A. FOTI, G. IMM~3, G. ])APPALARDO, G. RACITI and F. RIzzo

l s t i t u to Naz iona le di F i s i ea Nueleare - Sezione d i Catania Centro S ic i l iano d i i~isica Nucleate e di S t ru t tura della Matev ia - Catania I s t i tu to di F i s i ca dell 'Universit~t - Corso I t a l i a 57, 95129 Catania, I t a l i a

N. SAUM~R

D~partement de Phys ique ~'uel~aire, C E N Saelay - B . P . 2, 91190 Gi] sur Yvette, ~ranee

G. BAUR and R. SHYAM (*)

I n s t i t u t /iir K e r n p h y s i k der Kern]orschungsanlage Ji~lich - D-5170 Ji~lieh, Wes t Germany

F. R6SEL and D. TRAUTMANN

I n s t i t u t ]iir Theoretische P h y s i k der Univers i td t Basel - CH-4056 Basel, Swi t zer land

(ricevuto il 21 Aprile 1982)

S u m m a r y . - The inclusive deuteron spectra from the 6Li-induced reaction on 12C at 34 MeV have been measured from 4 ~ to 100 ~ laboratory angles. The forward-angle data, that show a dominant bump centred at the beam velocity, have been analysed with a ZRDWBA break-up formalism, based on the deuteron spectator model. The overall agreement with data is good except for the spectra at 0~ab < 10 ~ for which a significant fraction of cross-section is not accounted.

Continuous energy spectra of particles emitted in light-ion as well as heavy-ion- induced reactions have been the subject of much interest recently (1). In addition to the particle evaporation from the fully equilibrated compound nucleus and the pre- equilibrium particle emission, the break-up of the projectile in the field of target nucleus has a dominant contribution to the spectra in this region (e). The broad bumps centred around the energies corresponding to the beam velocity are the characteristic features of the continuous energy spectra at forward emission angles. This characteristic of the

(*) N o w a t Sc ience R e s e a r c h Counci l , D a r e s b u r y L a b o r a t o r y , U K . (1) A. BUDZANAWSKI: Dynamics of Heavy Ion Collisions, H A V A R , 1981, e d i t e d b y N. CINDRO, R . A. RICCI ~,n(]. "yr. GREINER ( A m s t e r d a m , 1981). (a) C. K . GELBKE: Continuum Spectra in Heavy Ion Reactions, San Antonio, 1979, e d i t e d b y W. TAMURA, J . B. NATOWITZ, D. H . YOUNGBLOOD (New Y o r k , N . Y . , 1980).

229

230 A. CUNSOLO, A. FOTI , G. IMM]~, G. PAPPALAI:~DO :ETC.

spectra can be understood in terms of a simple spectator mechanism of the break-up of the projectile in the field of the target nucleus (3).

During the past years an extensive amount of experiments has been performed to measure the inclusive spectra of the particles emitted in the nuclear reactions induced by deuteron, 3He, u, eLi, 9Be etc. projectiles (1,4). Numerous experiments with other heavier systems have also been performed (2). However, the eLi-induced reactions are rather specific. On the one hand, e l i is a simple enough projectile and the DWBA theory of break-up which has been employed successfully to understand the break-up of deuteron, 3He and cr can still be used to perform calculations for this case without adding any addit ional numerical problem. On the other band, there are certain fea- tures in this reaction which are typical for heavy ions. The well-developed ud cluster structure of 6Li and its rather low-binding energy (E = 1.47 MeV) indicates the large probabil i ty of break-up in u and deuteron channels.

Several at tempts have been made to measure the deuteron and the u-particles in the eLi-induced reactions for incident energies in the range of (12--156)MeV (5), However, very few measurements have been performed for light target nuclei. For light target nuclei the nuclear field of the target nucleus is expected to play a dominant par t in the fragmentation process even at moderate incident energies.

We have performed a systematic investigation of the inclusive charged-particle spectra observed in the reaction of e l i projectile with light target nuclei (A < 30) at 34 lVieV incident energy. Experiments were performed at the Supcr-FN tandem accelerator of the CEN Saclay.

~2,1a,~4C, le0, 2~A1 and eesi targets were bombarded by a 34McV eLi-beam. The resulting charged particles (A <eLi) were detected by using two A E - - E silicon tele- scopes. The angular range for the measurements was from 10 ~ to 100 ~ in steps of 5 ~ for forward angles and in steps of 10 ~ for backward angles.

In this letter the results of our analysis of the deuteron spectra from a ~2C target is presented. For this case additional measurements in the more forward direction (0~ b ~> 4 ~ in steps of 1 ~ were done by using a A E ~ - A E ~ - E silicon telescope. The deuteron spectra at 4 ~ 35 ~ and 70 ~ of laboratory angle are shown in fig. 1. At forward angles, the bump in the spectra at energies corresponding roughly to the beam velocity is quite evident. This indicates the presence of the projectile break-up mechanism. I t is also clear from this figure that certain discrete peaks are also strongly excited in this reaction. These peaks have been interpreted as the u-cluster levels of ~e0 and these have been described as the direct transfer of an cr to the discrete levels of leO (e).

The experimental double differential cross-section for the inclusive deuteron spectra has been analysed by using a DWBA break-up theory of (( elastic ~> (7) and (, inelastic ~) break-up (s) of the projectile in the field of target nucleus. We shall mean by elastic break-up those processes in which the target remains in the ground state during the collision; whereas for inelastic break-up we mean all those process in which one of the fragments interacts inelastically with the target. Clearly for the inclusive spectra, both of these modes will contribute. Previously this theory has been successfully applied to

(a) G. BAUd: S t u d y week-end on * Topics in Heavy I o n React ions ,~ ( D a r e s b u r y , 4-5 O c t o b e r , 1980) . (4) G. BAUR, F. ~6SEL, D. TRAUTMANN a n d R . S~YA~: Deep-Inelas t ic and F u s i o n React ions wi th H e a v y l o n s , Ber l in , 1979 (Ber l in , 1980). (6) K . O . PFEIFFER, E. SPETH, K . BETHGE: N u c l . P h y s . . , 4 , 2 0 6 , 545 (1973); B. NEUMANN, H . REBEL, ft. B U S C ~ A N N , H . J . GILS, H. KLRWF,-NEBENIUS a n d S. ZAGBOMSKI: Z . P h y s . , 296 , 113 (1980). (~) A. CUNSOLO, A. FOTI, G. I~M~, G. PAPPALARDO, G. RACITI a n d N. SAUNIER: P h y s . Rev. C, 21, 2345 (1980). (7) G. BAUR, D. TRAUTMANN: P h y s . Rep . G, 25, 293 (1976). (0 G. BAVa, R . SHYA~, F. R()SEL a n d D. TRAUTMANN: Helv. P h y s . Ac ta , $3, 503 (1980).

I N C L U S I V E D E U T E R O N S P E C T R A F R O M T H E ~Li~J2C C O L L I S I O N AT 34MeV

80

2 3 1

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F i g . 1. - I n c l u s i v e d e u t e r o n s p e c t r a f r o m t i l e 12C(+Li, d x ) r e a c t i o n . EeLi = 34 M e V a) 01a b = 4 ~

b) 01a b = 35 ~ , c) 01~ b = 70 ~ .

the analysis of the continuous spectra of the particles emitted in d-, 3He- and m-induced nuclear reactions. Recently, charged-particle spectra in eLi-induced reaction at 156 MeV incident energy have been analysed by using this theory (9). Some calculations for light- (lo) as well as heavy-ion (u)-induced reactions using another mechanism of break-up,

(9) B. NEUMANN, H. REBEL, H. J. GILS, R. PLANETA, J. BUSCHMANN, H. K L E W E - N E B E N I U S , S. ZAC~RO~SKI, R . SHYA~f a n d I t . )]IACHNER: Nucl . Phys . ( in p r e s s ) . (lo) .}- KLEINFELLER, J . BISPLINGttOFF, J . ERNST, W. ]~AYER-KUCKUK, (~. BAUR, B. ~[OFFMANN, R . SHYAM, F . R S S E L a n d D . TRAUTMANN: Nucl . Phys . A , 370 , 205 (1981) . (,1) T . UDAGAWA: Continuum Spectra in Heavy lou Reactions ( s e e r e f . (2)); T . UDAQAWA a n d T . TA- MURA: Phys. Rev. Left. 15, 1311 (1980) .

232 A. CUNSOLO~ A. FOTI , G. IMM:E, G. PAPPALARDO :ETC.

known as (( sequential break-up )), has also been reported in the literature. In this mechanism it is assumed that the projectile is first inelastically excited to some state which subsequently decays into various fragments. We have not performed the cal- culation assuming such a mechanism in this paper.

The inputs into our theoretical calculations are the optical potentials in the entrance and the exit channels and a zero-range normalization constant for the vertex a --~ b + x, where a is the projectile and b and x are its fragments. The optical potentials used in the present calculation are shown in table I. The zero-range normalization constant was determined by normalizing the peak of the experimental cross-section at 20 ~ by

60

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30

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Fig. 2. - Compar i son b e t w e e n the e x p e r i m e n t a l inclus ive d e u t e r o n sp ec t r a (I~C(6Li, 4x) ; E,LI = 34 l~leV) a n d theore t i ca l p red ic t ions of t he D W B A B U n o r m a l i z e d a t the 20 ~ e x p e r i m e n t a l cross-sect ion (see t ex t ) . The con t inuous a n d the dashed c u r v e s r e p r e s e n t the t o t a l an d the elast ic t heo re t i ca l b r e a k - u p , respect ive ly �9

INCLUSIVE DEUTERON SPECTRA FROM THE e L i ~ 2 C COLLISION AT 34MeV 233

TABLE I. -- Optical potent ial (Saxon-Woods I]orm) used i n "the analys is o/ the break-up cross-section.

Channel V rv av W r W a w BD rD a , rr Ref.

~Li-I~C 170 1.21 0.802 8.9 2.17 0.945 1.30 (19)

~-19C 163 1.10 0.744 61.5 1.31 0.145 1.30 (la)

d-~N (~) 1.15 0.810 (b) 1.34 0.68 1.15 (~4)

(a) 86 .8--0 .22E. (b) 14.4 -b 0.24E.

the theoretical calculations. This normalization constant was then used for all the other angles. In fig. 2 a comparison of our calculated cross-sections and the experimental data is presented for various deuteron angles. I t should be noted that in our calcula- t ions we have not included the contributions of particle evaporation and pre-equi- l ibrium processes. I t is clear from this figure that apart from the very forward angles the absolute magnitude as well as the shape of the experimental spectra is nicely reproduced by the DWBA break-up calculations with a single normalization constant. The difficulty of the DWBA calculations of not being able to reproduce the experi- mental spectra at alla the angles may be due to certain approximations made in our calculations. 1Kost crucial is the so-cMled zero-range approximation which is clearly not as well fulfilled for 6Li as it is for l ighter projectiles. A finite-range calculation seems to be necessary for a better understanding of the data. Similar observations were also made in the analysis of the 156 MeV eLi-induced reaction on a 96spb target (9).

The contribution of the inelastic-break-up mode to the spectra is roughly equal to that of inelastic mode at 4 ~ . With increasing angle this contribution becomes smaller and smaller. This can be understood from the spectator mechanism of the break-up. One expects tha t if the observed fragment is emit ted in a more forward direction, the other fragment and the target nucleus recoil at more backward angles thus having more nonelastic collisions. However, a better understanding of these processes is likely to emerge from coincidence measurements.

In conclusion, the measurements were made for the inclusive deuteron spectra in the ~Li-induced reaction on a 19C target at 34 ~r incident energy. The double differen- t ia l cross-section for deuterons at various emission angles were analysed by means of a DWBA break-up theory. The theory in general provides a good description of the energy spectra. However, i t fails to reproduce the angular distribution of the experi- mental cross-section. The inclusion of finite-range and recoil effects appears to be necessary.

(1.~) p. SCHU~iACHER, N. UETA, H. H. ][)UH~I, K. I. KUBO, Vv r. J. KLAGES: Nucl. Phys. `4, 212, 573 (1973). (t~) j . B. A. ENGLAND, E. CASAL, A. GARCIA, T. PIGAZO, J. ANGUILAR and H. M. SEN GUPTA: NUC[~ Phys . .4 , 284, 29 (1977). ('*) C. M. PEREr and F. G. PEREY: Atomic Data aud Nuclear Data Tables, 17, 1 (1976).