IL NUOVO CIMENTO VOL. 85 A, N. 4 21 Febbraio 1985

Break-Up Mechanisms in the 12C(6Li, d~) Reaction

at 34 MeV Incident Energy.

~. CUI~SOLO, A. FOTI, G. I ~ , G. PAPPALARDO, G. ]~ACITI and F . ]~IZZO

Dipartimento di Fisica dell' Universitd - Corse Italia 57, 95129 Catania Istituto 2(azionale di Fisica Nucleare- Sezione di Catania Laboratorio Nazionale del Sud - Catania Centre Siciliano di Fisica Nucleare e Struttura della Materia - Catania, Italia

•. SAUNIER

Ddpartment de Physique Nueldaire, C E N Saelay, 91191 Gif-sur-Yvette, France

(ricevuto il 3 0 t t o b r e 1984)

S u m m a r y . - Deuteron-alpha angular correlations for the 12C(6Li, de) reaction have been measured at E(eLi)~--34 McV and o ~ b = - 10 ~ The analysis of the d-~ coincidences has shown the existence of elastic and inelastic break-up mechanisms, depending on whether in the e l i break- up process the residual nucleus is left in the ground or in the excited states. With this definition, the well-known sequential mechanism involv- ing the 3 + state of the e l i nucleus at 2.18 MeV and a direct (nonsequential) mechanism with a maximum at a relative A0d.~--~ 30 ~ angle contribute to the elastic break-up process. Two contributions are also found to the inelastic e l i break-up process: one due to the sequential eli(3+) mecha- nism and the other to a nonsequential mechanism. A rough estimation of the ratio of the inelastic to the elastic eLi break-up yields gives about 0.5.

PACS. 25.70. - Heavy-particle-induced reactions and scattering.

1 . - I n t r o d u c t i o n .

The wel l -pronounced and weak ly bounded ~-d c lus ter s t ruc ture of the SLi

nucleus has or ig ina ted , in the las t decade, much in te res t on reac t ion induced

b y th i s l ight , b u t s t i l l heavy- ion projec t i le . I n the (eLi, d) react ions , these

features expla in the different reac t ion mechanisms t h a t genera te , in the energy

343

344 4. CUNSOLO, A. FOTI, G, IMM~, G. PAPPALAI~DO, G. R~CITI, F. RlZZO ~TC.

spectra of forward-emitted deuterons, discrete peaks and bumps centred at the beam velocity even at rather low incident energies.

The discrete peaks have been associated to ~-cluster states of the final nucleus (~), while the bumps have been attributed to the 6Li break-up (2).

In the frame of a systematic study on the 6Li~-light nuclei collisions we have recently investigated both the discrete (3) and the continuous (4) com- ponents of the deuteron inclusive spectra emitted in the ~C~6Li collision at 34 !KeV incident energy. In particular i n ref. (4) the experimental forward- angle deuteron inclusive spectra were compared with the predictions of the zero-range distorted-wave Born approximation break-up theory of Baar et

al. (5) and, despite ~he rather crude approximations made, the comparison seemed encouraging. However, two points remained to be clarified, first of all, experimentally: the first concerning the contribution from the sequential break-up mechanism of ~ Chrough its 2.18 ~ e u (J- ---- 3 +) state (2,~) and the second one concerning the contribution from the (( inelastic ~ break-up process already experimentally pointed up (7) and theoretically expected (8). Therefore, in order to gain more detailed information on the reaction mechanisms that contribute to the inclusive deuteron continuous energy spectra, deuteron- alpha angular-correlation experiments have been performed for the eLi~2C system again at 34 ~eV incident energy.

Section 2 of the present work refers to the experimental procedures and sect. 3 and 4 deal with the analysis of the data. The conclusions are drawn in sect. 5.

2. - E x p e r i m e n t a l procedure.

The 34 ~eV eli3+ beam of about 100 n~_ current intensity was produced by the Super F.:N. Tandem Van de Graaff accelerator of the C.E.SL-Saclay.

(1) H.W. FULBRIGHT: Anna. Rev. ~q~cl. Part. Sci., 29, 161 (1979), and references therein. (2) K. 0. PFEIF]~R, E. SPETI{ and K. BETnG]~ :Nucl. Phys. A, 206, 545 (1973) ; D. SCHOLZ }I. G]~MM~X~, L. LASS]~lV, R. OST and K. B]~THGE: )[UCl. Phys. A, 288, 351 (1977); B. NEUMANN, H. REBEL, H. J. GILS, R. PLANETA, J. BUSCHMANN, II. KL~WE-NEBENIUS, S. ZAG~O~SKI, R. SHYA~ ~nd H. MACHNER: 1VUCl. Phys. A, 382, 296 (1982). (3) A. CUNSOLO, fi. FOTI, G. PAPPALARDO, G. RACITI and N. SAVNI~R: Phys. Rev. C, 18, 856 (1978). (4) A. CuNSOLO, A. FOTI, G. IMM~, G. PAPPALARDO, G. RACITI, F. RIZZO, N. SAITNIER, G. BArn, R. SHYAM, 1 ~, ROSEL and D. TRAUTMANN: ~et$. Nuo'vo Cimento, 34, 229

(1980). (5) G. BAV~, R. SHYAM, F. ROS]~T. ~nd D. TICAUTMAN~: Helv. Phys. Acta, 53, 503 (1980). (6) C.M. (!~' STAN:EDA, H.A. SMITH, P. 19. SINGH ~%l]d H. KARWOSKI: Phys. Rev. C, 21, 179 (1980). (7) A. CUNSOLO, fix. FOTI, G. IMM~, G. PAPFALARDO, G. RACITI, F. RIzzo and N. SAU- CIER: Proceedings o] the I I International Con]erenee on s I~eaction Mechanism,

BREAK-UP MECIt&:NISMS :ETC, ~

Self-supporting foils of natura l carbon with thickness ranging from 100 r

300 Fg/cm ~ were used as targets. Deuterons were detected by means of a

AE-E telescope of silicon detectors (AE = 137 Fm, E----5000 ~m) fixed at

0~b-- - - - 1 0 ~ In-plane coincident alpha-particles were detected in the lab-

oratory angular ranges - - 3 0 ~ - 19 ~ and 7 ~ ~ by means of two telescopes AE-E of silicon detectors (AE~ = 23.6 ~m; E~ = 460 ~m; AE~-=

-~ 17.9 ~m, E2 ---- 455 ~m). Final ly a fixed silicon detector (E = 200 ~m) was

used as monitor.

Iqote tha t in the present work the negative-angle nota t ion refers to azi-

mutha l angle ~ = 0 ~ the posit ive one to ~ = 180 ~

I I

4000

3000

O2ooo

1000

0 I 5 10 15 20 25

Ea(M eV)

Fig. 1. - Inclusive deuteron energy spectrum from the reaction I~C(6Li, d) at 0~b= = - -10 ~ and E(6Li)= 34 ~leV. (Negative-angle notation refers to azimuthal angle

---- 0 ~ the positive one to ~---- 180~

For each coincidence event, the five parameters --d~'t~ Id, Et= ~ I s and At,

being E~ "~ the to ta l energy, I~ the analogical derived identification and At the

coincidence t ime, were analysed on-line and stored on magnet ic tape for off-line

analysis. The energy calibration was obtained from known transit ions to leO.

The deuteron overall energy resolution (FWttlV[) was about 100 keV and the

Varenna, 17-21 June 1979, edited by CLUED-Milano. (s) T. UDAGAWA and T. TA~URA: Lecture Notes; RCNP-KIKUCHI Summer School on Nuclear Physics, edited by T. YAMAZAKI, K. ANDO and K. ITO~AGA (Osaka University, 1980), p. 171.

3 4 ~ A. CUNSOLO, A. t~OTI, G. IMM~, G. PAPPALARDO, G. RACITI , Xr RIZZO ETC.

coincidence t ime resolution about 8 ns. The inclusive deuteron spectrum is shown in fig. 1.

A typ ica l to ta l kinetic energy (TKE) spectrum of d-a coincidences is shown

in fig. 2.

r162

-.t

I t I I I I I f

0 2 z,. 6 8 10 12 lZ,. E*(t2C) (MeV)

Fig. 2. - The total kinetic energy spectrum of d-~ coincides from the 12C(6Li, ~) reac- tion, being the deuterons detected at -- 10 ~ and a-particles at 20 ~ laboratory angles.

Set t ing appropria te windows on the T K E spectra and project ing on the •d axis one gets the a-coincident deuteron spectra leading e i ther to the ground or to the 4.43 iVfeu {2+), or the 9.64 ~VfeV (3-) exci ted states of the 1~C nu-

cleus (being the state at 7.65 ~ e V ve ry weakly populated). Figures 3-5 show

selected examples of these ~d.~~ Ed.~ and Ed.~, spectra. F r o m inspection of fig. 1-5, one can realize tha t the 12C(eLi, d~) react ion

feeds the d-~o, d-~l and d-a3 channels with comparable strengths even at forward 0~ angles, and t h a t some deuteron continuous energy components can be found in all the three channels. F r o m these findings, as already pointed out (7,9),

(9) A. CVNSOLO, A. FOTI, G. IMM~, G. PAPPALAI~DO, G. RACITI and N. SAUlqIE:R: Phys. t~ev. C, 21, 2345 (1980), and references therein.

B R E A K - U P MECHANISMS ETC. 3 4 7

1000

2000

5

0 Lw

50,

50

J

- - - - t -

--21 ~

7 ~

ol

50

Iv 5 10 15 20 25

Ed(MeV)

Fig. 3. - Ed.~0 coincidence spectra from the 12C(eLi, dce0)12Cg.s" reaction at some selected 0~ ~b angles and 0~b-- - - - 1 0 ~ The arrows indicate the k inemat ica l ly predic ted posi- tions of peaks due to sequential 6Li(3+) break-up. The continuous curves represent the contr ibut ion due to a nonsequential break-up mechanism (see text) .

3 ~ A. CUNSOLO, A. FOTI, G. IMM~;, G. PAPI'ALARDO, G. RACITI, F. RIZZO ~:'I'C.

200

1 1

ii t i

.-21 o

I

7 ~

z, O0

D

. ~ 200 b

200

0

160

0

I !

]1~ 16.5 ~

,l t I

- ,fti '~'~~

10 15 20 25 Ea(NeV)

F ig . 4. - As in fig. 3, for t h e E,~.a~ c o i n c i d e n c e e v e n t s in l :C(SLi, d~1)~:(!4".4 3.

BREAK-liP MECHANISMS ETC. 349

3ooi 1 -21 ~

7 ~

300 [ -

,

~ i AAr~. {

150 -- 16"5~

%

~ 120

i lO 15 Ed(MeV)

19.5 ~

I l

27 ~

I I 20 25

0

150

Fig. 5. - As in fig. 3, for the Ea.a~ coincidence events in 12C(6Li, d%)r-'Cg.6~.

350 A. CUNSOLO, A. FOTI, G. IMME, G. ]?APP~LLARDO, G. RACITI, F. RIZZO :ETC.

one can infer t ha t there is appreciable contr ibut ion to the projecti le break-up process tha t comes f rom (( inelastic ~ mechanisms (5,8).

3. - Analysis of the d-% data and elastic e l i break-up.

In the angular range present ly explored there are two regions in which the d-go spectra show dist inct features. In fact (see fig. 3) for relat ive angles

A0d. ~ --~ l0 s -- 0~[<20 ~ the spectra are dominated by a two-peak structure, whose energy spacing decreases when AOd. ~ increases, while for A0d. ~ > 20 ~ the spectra show m a n y peaks, more or less isolated, bu t with angle-independent energies.

For these peaks a good energy correspondence can be generally established with those seen in the inclusive deuteron spectrum.

l~or bo th these kinds of peaks the leading react ion mechanisms are well known (~,6,9) and correspond to sequential processes t h a t will be described in

the nex t two sections.

3"1. Sequential break-up o/ the e l i nucleus. - In this process, the incident e l i nucleus is inolastically exci ted by the Coulomb and nuclear fields of the ta rge t to the unbound 3+ level at 2.18 l~IeV, f rom which subsequent ly decays into the ~ d channel.

In this process, once fixed in the labora tory f rame the direction at which deuterons are detected, the kinematics allows us to define a cone around this direction (having in our case an angular aper ture of about 20 ~ ) in which the associated alpha-particles can be emit ted. For relat ive A0a. ~ angles smaller t ha n the l imit ing value, there are two dist inct couples of E~-E a correlated energies t ha t contr ibute to the exper imenta l spectra ( that originate f rom two slightly different angles of scat ter ing of the 6Li*(3+; 2.18 i~eV) unbound nu- cleus). These two couples of energies converge to a unique one when the rel- at ive A0d. ~ angle approaches the l imit ing value. As can be inferred f rom fig. 3, these kinemat ical features of the sequential break-up of the e l i nucleus through the 2.18 l~eV (3 +) state are confirmed. ~lote tha t , in the present data, no

evidence has been found for contr ibut ions f rom sequential break-up involving

any other known T---- 0 e l i exci ted states.

The angular dis t r ibut ion of the inelastic scat ter ing

,-'e(eLi, eli*(3 +; 2.18 ~eV))'~c

has been ex t rac ted f rom the data assuming t h a t the el i* nucleus decays isotropically in its own frame. The results are shown in fig. 6 and are well accounted by an exact finite-range DWBA calculation, done by using the

BREAX-VP MECRAN[SMS ETC. 351

101

10 o

lO' 5

b

lO ~

+

101

a)

%+++

o) q~

\ 0 10 20 30 40

Oc.m (degrees) 50

Fig. 6. - Angular distributions for single and mutual excitation for the 12C(~Li, 6Li*(3+)) The experimental errors indicate geometrical as well as statistical uncertainties. The curve represents the EFR-DWBA calculation (see text): a) 1~C*(3-; 9.64MeV), b) t2C*(2+; 4.43 MeV), c) 12C(0+; g.s.).

P t o l e m y code of M a e f a r l a n e a n d P i e p e r (lO), o p t i c a l - m o d e l p a r a m e t e r s t a k e n

f r o m ref . (a) a n d fl = 0.61 for t h e 6Li nuc l ea r d e f o r m a t i o n p a r a m e t e r (11).

3"2. Analysis o] the isolated 160 levels. - I n a p r ev ious i n v e s t i g a t i o n on t h e

12C(~Li, da0) r eac t i on (9), i t has been shown t h a t t h e i so la ted p e a k s o b s e r v e d

(lo) M . H . MAcFaRLANE and S.C. PIEPER: Report ANL-76-11 (1978). (11) H. GEMMEKE, B. DELUIGI, L. LASSEN and D. SCHOLZ: Z. Phys. A, 286, 73 (1978).

352 A . C I T I W S O L O , A . F O T I , O . I M M E , G . P A P P A L A t { D O , G . R A C I T I , 1~. R I Z Z O E T C .

+~ 0

10

-I 1o

_ ', ,, t lO ', , , ',

a) ~, e)

I I I I I I I I

10 , '.* w,- -.,,,.

r

/ ,+ ~,.. ,, , i + #+ ~

, , r ' J ' , I ~ . i i I I I b) I I , ' ' I f) " ~ ',

I I I I ' I I I I i I I I ISl~

4'0 10 o § . .

+ \ ~ '*

+ ",-" -+§ i ~ % ++ - -1 + l '+, f - ~ . / ' " ,o , , i + / + , t , . ,+, , .+. , + + , , . ,o I I I ~+, I I

I I ; o i i l I I I i 10 - 2 ' I i + --

c) ~ J , I g) i t ~l ' I I I ~, ,

I I I I I I I I

1o o .,..+.._+.~ '~ _..,_. " ' r1 ' ' "X ~ + l

' +tL+ ~o-' ' { " " ' """ h..L. J."l',, '~ , / , ,, *~ ,; #, ,~+

',t tl ~ , , , ~ r + , . , , , , , I I ~I I " ] : , ~, t, : ,

. / l ~I ~;I i l ~ ~ ; I ' , i ' , I I I I I

d) h)

I I I ~ I , I o ~o 80 o 40 80

e 2 (degrees)

~ig. 7. - Angular -cor re la t ion funct ions for some t rans i t ions analysed in the p resen t work. See t e x t for ~he mean ing of t he dashed curves. E ~ = 34 MeV, 0~+~: - - 1 0 ~ 0~ is t he ~-part icle angle defined in t he recoil nucleus centre-of-mass f rame, bu t w i th respec t to t he beam direct ion, a) 17 .4MeV, 6 = 16 ~ , 3 5 = 6 ; b) 16 .3MeY, 6 ~ 13 ~ , 3 5 = 6; c) 14 .5MeV, 6 = 15 ~ 35---- 5; d) 10.35MeV, 6---- 16 ~ 35---- 4; e) 22 .5MoV,

= 7 ~ , 3 5 = 6 ; ] ) 21 .8MeV, 6 = 5 ~ , 3 5 = 6; g) 20 .1MeV, 6 = 3 ~ , 35 ----7; h) 19 .5MeV, 6 : 7 ~ 3 5 : 7 .

c~

% 10-:

B R E A K - U P M E C H A N I S M S E T C . 353

in the Ed.~0 spectra, can be associated to the following process: exci tat ion, via direct ~-transfer, of 160 nucleus states, followed by the a-decay f rom such states. However , probably due to the higher statistics and/or to the extension of measurements at more forward 0a angles, the E~.~o spectra seen in the present exper iment are more rich of fine structurel ike peaks than in the previous in- vest igat ion (9).

A detailed analysis of this f ine-structure peaks will be given elsewhere. Here we assume tha t these peaks correspond to well-defined 160 states.

In fig. 7 the d -~ angular correlations for some ~60 states obta ined in the pres- ent exper iment are shown. In accordance with the findings of ref. (9), the data exhibi t strong oscillations and, for a given transi t ion, are ra ther well approx- imated by squared Legendre polinomials of order L = J , i.e.

w ( 1 0 ~ 0~)~ IP~=~[cos ( 0 ~ - 6)]1 ~ .

J is the spin of the 160" decaying state and ($ the angular shift of the max-

imum of the angular correlat ion funct ion with respect to the direction of motion of the 1~0" recoiling nucleus.

t te re , due to the inclusion oi ve ry forward O~ angle data, the angular shifts are ve ry well exper imenta l ly defined.

The best-fi t t ing squared Legendre polinomials are shown as dot ted lines in fig. 7, where are also reported, for each 160 state, the ex t rac ted 0 and L values.

As far as t ransi t ions involving 160 states lying at exci ta t ion energies up to ~ 17 !KeV are concerned, the data are v e ry well reproduced in all the ex- plored angles. On the other side, the angular correlations of the states at higher exci tat ion energy show only a par t ia l agreement and in par t icular one extra oscillation at forward angles. We recall t ha t on the basis of kinemat ical ar- guments , these data do not contain cont r ibut ion f rom the e l i sequential break-up. These features suggest the existence of a common background due to a (~ th i rd ~) react ion mechanism. In the hypothesis t ha t this (~ th i rd ~) react ion mechanism does not appreciably interfere with the second one, the (~ anomalous ~> data for each involved level have been decomposed in two com- ponents : one evaluated f rom IPL] 2 correlat ion funct ion (best f i t ted to the back- ward-angle data) and one obta ined as difference between data and the [PL] 2 contr ibut ion.

These differences have been repor ted in the corresponding no-coincident deuteron energy spectra and fi t ted by mean curves.

The results, some of which are shown as continuous lines in fig. 3, consist in a few ~ e V wide structures, approximate ly centred a t the beam veloci ty

tha t , in the present ly explored angular range, present a narrOw distr ibut ion peaked at about 0~b_ ~ 20 ~

Excluding, to a reasonable confidence level, contr ibut ions to this struc- tu re f rom the I~C(6Li, a)141W* -+ doq-1-'Cg.~, sequential reactions on the basis of

354 A. CUNSOL0, A. FOTI, G. IMM~, G. PAPPALARD0, G. RACITI, F. RIZZO :ETC.

the I~C(6Li, =) angular-dis tr ibut ion shapes and of kinemat ical arguments (when the ~-particles are emi t ted at 0~b-----20 ~ the 14N* nucleus recoils at

- - 0 d -- -- one 0R~ (-- 20-- 34) ~ i.e. at angles definitely higher than ~ b 10 o) could

a t t r ibu te this (( background ~> to the direct (nonsequential) break-up of the ~Li nucleus.

Final ly the present ly found value of A0d. ~ ~ 30 ~ for the angular separat ion at which the nonsequent ia l ~Li break-up events have a max imum in the angular correlation, compares well with the value of A0~.~-----28 ~ obta ined by GEM- ~XEKE et al. (~) f rom the i r invest igat ion on the direct break-up of 22.2 Me ~Li

incident on naSn.

4. - Analysis of the d - ( ~ ~3) data and the inelastic ~Li break-up.

As shown by arrows in fig. 4 and 5, one observes again double-peaked structures and, due to the exper imenta l E~ threshold, single peaks respectively in Ed.~l and Ed.~, spectra t aken at small re la t ive angles A0d. ~. These peaks kinemat ical ly correspond to the (~ inelastic >> sequential break-up of the 6Li always th rough the 3 + state at 2.18 ]VIeV. This mutua l exci ta t ion process, up to now unrepor ted for the el i , would be analogous to the ones seen with ~-par-

t i d e s (~3) and 7Li (~4) beams. In the same lines as for the (~ elastic ~> sequential sLi break-up, the angular

distr ibutions of the double inelastic scattering 12C(eLi~ 6Li*(3+))~C* have been ex t rac ted and are shown in fig. 6. In addi t ion to the mutua l exci ta t ion peaks, the Ed.~l and Ed.~, spectra show a few other peaks t h a t can be associated with the decay of the corresponding ~60 states to the ~C first (~5) or th i rd excited states superimposed on a cont inuum. As can be inferred from fig. 4 and 5, such a cont inuum becomes more apparen t when the observat ion angle 0~ is increased. Again, as for the (( inelastic ~) sequential e l i break-up, these events are indicat ive of (~inelastic ~) break-up process, where par t of the in ternal kinetic energy is t ransfer red to the in ternal degrees of freedom of the colliding system.

Most likely, this continuous spectrum could be associated to a direct

(( inelastic >) break-up process like in the (aHe, dp) case (~). However , in the

lack of appropr ia te theoret ical f ramework t h a t could allow the detailed analysis

(12) H. GEMMEKE, B. DELUIGI, D. SCHOLZ and L. LASSEN: Phys. Lett. B, 96, 47 (1980). (13) j . V .~ DRIWL, M. N. H~AKEN, R. KAM~R~ANS and R. J. DE MEIJ~R: Phys. Rev. •ett., 46, 525 (1981). (14) j . COOK, ~. M. CLARKE, J. COOP]~RS~ITH and R.J . GRIFFITHS: ]~Uvl. Phys. A, 386, 346 (1982). (15) A. CUNSOLO, A. FOTI, G. PAPPALARDO, G. RACITI, N. SAUNIER and E. F. DA SIL- VEIRA: Nuovo Cimento A, 40, 293 (1977). (is) E .H.L. AA~TS, R. MALFLIET, S.Y. VAN DER WERF and R.J. DE MEIJER: N~cl. Phys. A, 380, 465 (1982).

BREAK-UP MECHANIS'MS ETC. 355

12 000

6000

2O0O

~b o

2000

a)

400 ~ I

05 10 15 20

I b)

c)

215 Ed(MeV)

Fig. 8. - a) Single energy spec t rum of deute ron emi t t ed in the 6Li + 12 C collision at 0 ~ b = - - 10 ~ and 34 MeV inc iden t energy; b) angle in tegra ted Ed_(~o+.,+a,). coincidences spec t rum; c) as b) for d-~ o coincidences; d) as b) for d-~ 1 coincidences; e) as b) for d-~ ~ coincidences.

356 A. CUNSOLO, A. FOTI, G. IMM~, G. PAPPALARDO, G. RACITI, i". RIZZO :ETC.

of such a continuous contribution, we have tr ied to est imate the global con- t r ibut ion of d-:r and d-g3 coincidences in the forward angular region with respect to the d-go ones. The Ed.~,, Ed.~ and Ed.~, spectra integrated over the forward 0~ angles (0~ < 30 ~ are shown in fig. 8, together with the Ea.(~0+~,+~) spectrum and the inclusive deuteron spectrum detected at , ~ b 0 a - - - - 10 ~ In- tegrat ing these spectra on the (5 - -16)~eV common deuteron energy interval, and normalizing to the tota l d-(go+gl+ga) yield one obtains the following values for the relative d-go, d-g~, and d-ga populations: 64%, 21~o ~nd 15 %, respectively.

As far as the Ed.(~o+~+~) angle integrated spectrum is similar to the single deuteron spectrum (see fig. 8), the found relative population values est imate the corresponding cross-section for the elastic and inelastic 6Li break-up, neglecting the single-level contribution. These findings could indicate the rather high degree of inelasticity of the break-up process even when a so loosely pro- jectile is involved.

5 . - C o n c l u s i o n s .

In order to investigate on the reaction mechanisms leading to the e l i -~ g ~ d break-up process at moderate energy, the deuteron--~-particle angular corre- lations have been measured for the 1~C~-eli system at 34 )SeV incident energy. Deuterons have been detected at 0~b-- ~ - - 1 0 ~ and coincident g-particles in the reaction plane.

The experimental energy resolution allowed us to distinguish between d-g events in which the 1~C nucleus is left or in the ground or in excited states. Accordingly, in the analysis of those coincidence events tha t can be a t t r ibuted to the e l i break-up process, we distinguish between elastic and inelastic 6Li break-up. Two distinct contributions to the elastic 6Li break-up have been seen: one coming from the known (2,5) sequential mechanism in which the 5Li nucleus is inelastically excited to its 2.18 )/[eV (3 +) level from which it decays into the ~ - d channel, and the other coming from a nonsequential (direct) mechanism.

The elastic sequential break-up events, assuming the 6Li*(3 +) - + g + d decay isotropic in its own frame, have been integrated and expressed as the angular distribution of the inelastic scattering 12C(6Li, eLi*(3+))l-~C. This angular distribution is well reproduced by an EFR- DWBA calculation.

Outside the 6Li sequential break-up cone, the nonsequential elastic break-up contribution presents a maximnm at a relative angle A0d. ~ --~ 30 ~

Two contributions are also found to the inelastic e l i break-up: one as- sociated with a mutua l excitation process, in which the incoming 6Li is ine- lastically excited to the (3 +) state at 2.18 ~ e V and the target nucleus to the (2 +) state at 4.43 MeV or to the (3-) at 9.64 MeV; the other one associated with

B R E A K - U P MECHANISMS ETC. 357

large s t ruc tu res i n the d e u t e r o n ene rgy spect ra c o i n c i d e n t w i t h alpha-part ic les~

cor respond ing to r e s idua l x~C nuc leus in t h e (2 +) or (3-) exc i t ed s ta tes . F i n a l l y ,

we have e s t i m a t e d f rom the da t a the r e l a t ive p o p u l a t i o n s for the c h a n n e l

d-~0('~Cg...); d -~ (~C*(2+ ; 4.43 ~ e V ) ) a n d d-~a(~C*(3-; 9.64 ~r as the 6 4 % ,

2 1 % a n d 15 %, respec t ive ly . These f ind ings show t h e r a t h e r s t rong (( ine las t i c ~)

e h a m c t e r of t he ~Li b r e a k - u p process even a t m o d e r a t e i n c i d e n t energy .

�9 R I A S S U N T O

Sono state misurate le correlazioni angolari deutoni-alfa per la reazione ~C(0Li, d~) a E(~Li )= 34 1V[eV e 0 ~ = - 10 ~ L'anal is i ha mostrato l 'esistenza dei meccanismi di break-up el~stico e inelastico, a seconda che, nel processo di break-up del ~Li, il nucleo 6 lasciato hello stato fondamentale o in stati eccitati. Con questa definizione, il noto meccanismo sequenziale che coinvolge lo stato 3 + del nucleo di ~Li ad una cnergia di eccitazione di 2.18 MeV e un meccanismo diretto (non sequenziale) con un massimo ad un angolo relativo A0,~.~ ~_ 30 ~ contribuiscono al processo di break-up elastico. Sono stati anche trovati due contributi per il processo di break-up inelastico del a l l : l 'uno dovuto al nieccanismo sequenziale del ~ +) e l 'altro dovuto ad un meccanismo non sequenziale. I1 rapporto fra la resa del break-up inelastico e quella del break-up elastico 6 stato stimato essere circa 0.5.

MexaHtl3MbI paena~a a peardlaa 12C(6Li, d~x) llpH 3nepran na~lammax qacrm~ 34 M3B.

PeaIoMe(*). - - I/I~Mepermi yrYlOBble roppen~Hrt Me~)/y ~efiTOr[OM 1~ anbqba-,tacTrmefi,

o6pa3oBaHHbIMri B peaKttrri~ 12C(SLi, da) rxpi~ E(0Li)-- - 34 M3B I4 6~a6~ - - 10 ~ AHan•3 d-~ coBrta)xeHi~ noKa3bmaeT cyt~eCTBOBarme yrIpyroro rt rteynpyroro MexaH~3MOB pacrIa~a, B 3aBrlCI4MOCTI~ OT TOFO B OCI-IOBHOM H.rl~ B B O 3 6 y g ~ e H I ~ O M COCTO,qHI4ffX o6pa3yIOTC~t ocTaTOqrmm a~pa. Xopomo H3BeCTrmI/~ nocne~oBaTe~bn~i~ MexaHrI3M pacna~a, BrmO- �9 lammm~ 3 + COCTOZHHe s~pa ~ npI~ 3aeprrm 2.18 M3B ~ npgMo~ (He nocae~oBaTeaJ, rmna) Mexanli3M c MaKc~MyMOM npIJI OTHOC!,fTeJ/bHOM yrne A0d.~ ~ 30 ~ ~amT BKYIa~bI B ynpyrr~ npottecc pacrm~a. O6HapygeH~,I Tai~ge ~Ba BK~a~a B neynpyrm~ ripottecc pacna~a 6Li: o ~ a o6ycnoa~eH noc~e~oBaTe~,H~iM MexaH~3MOMOLi(3+), a ~pyro~ o6yc~oBneI~ ae rtOCYle:IoaaTeYtbIiblM MexaHrI3MOM. Fpy6as oneida OTUOnIenrLq neynpyroro ~ ynpyroMy pacrIa)Iy ~Li ~aeT Be~Hqnny OrO~IO 0.5.

(*) H e p e e e b e n o pebam/ue( t .

25 - II Nuovo Cimento A.