CORRELATED PAIRS OF α PARTICLES DUE TO PROJECTILE DISSOCIATION

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CORRELATED PAIRS OF α PARTICLES DUE TO PROJECTILE DISSOCIATION

J. Uckert, M. Bürgel, H. Fuchs, H. Homeyer, A. Budzanowski

To cite this version:

J. Uckert, M. Bürgel, H. Fuchs, H. Homeyer, A. Budzanowski. CORRELATED PAIRS OFα PAR- TICLES DUE TO PROJECTILE DISSOCIATION. Journal de Physique Colloques, 1986, 47 (C4), pp.C4-95-C4-97. �10.1051/jphyscol:1986411�. �jpa-00225774�

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JOURNAL DE PHYSIQUE

Colloque C4, supplbment au n o 8, Tome 47, aoOt 1986

CORRELATED PAIRS OF a PARTICLES DUE TO PROJECTILE DISSOCIATION

J. UCKERT, M. B~RGEL, H. FUCHS, H. HOMEYER and A. BUDZANOWSKI*

Hahn-Meitner-Institut fiir Kernforschung, 0-1000 Berlin 39.

F. R. G.

" ~ n s t i t u t e for Nuclear physics, P-Krakow, Poland

Re'sum6

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Les corre'lations entre pairs de part' ules ' 8 e s (p, d, t, a )

&ires dans les rlactions du syst6me 390 MeV "Ne + IggAu ont 6ttC m6sur6es.

Pour le cas exemplaire des corr6lations a - a les contributions des divers me'canismes bien-connus (la fusion complSte, le transfert massif, l'bission s6quentiel le par les r4sidus du rojectile aprSs les collisions quasi-6las- tiques ou fortement in4lastiquesP ont k C calcul&s. Leur s o m e est trss proche de la section efficace m6sur6e. Des conclusions concernantes une 6ventuelle &mission par des zones chaudes sont fortement empEch6es 1 cause de 1 a comp6tition dominante des m6canismes cite's ci-dessus.

Abstract

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Correlations have been measured betygen pairs 1 ight particles

m,

a) emitted in collisions of 390 M e V , He with '"Au. Treating the example of a

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^acorrelations, the contributions of various well-established processes (complete fusion, massive transfer, sequenti a1 decay of the projec- tile after quasi-elastic or deep-inelastic collisions) have been calculated and found to virtually exhaust the measured coincidence cross sections. Con- clusions on possible emission from hot subsystems are hampered by the over- whelming competition of the processes mentioned above.

Correlations measured between two light particles often are analyzed 111 by attribu- ting them entirely to emission from a thermal source.which may be smaller than the compound nucleus and exists prior to equilibration within the collisions complex.

Suspecting that quasi -direct dissociation of the projectile, too, yields c o r ~ e l ated particle pairs, for instance 8 ~ e g.s. + a + a as frequently as the fragmget Be

we have studied light-particle correlations of the system 390 MeV Ne + l g 7 ~ u

?$.&ich previous investigations provide us with ample information on projectile dissociation processes and their cross sections 12-71.

For demonstration we consider the example of correlations between two a particles.

.The experiment was carried out at the VICKSI accelerator. Light particles were de- tected in two solid-state detector telescopes and 10 phoswich BGO/pilote U detec- tors, both in narrow geometry and for large relative angles. Pairs of a particles observed in close geometry at 6 = 16.5' have a spectrum of relative energies domina- ted by the 8 ~ e g.s. peak at E12 = 92 keV (see fig. 1). Angular corrleations for four emission angles of the triggering a particle are shown in fig. 2.

For their interpretation the contributions of the following processes effectively had to be considered and were calculated by Monte Carlo simulations using as input angular distributions and distributions of Q-values and excitation energies from previous investigations /2,3/.

1. Complete fusion with evaporation of two a particles. This yields a quasi-isotro- pic angular correlation for all triggerangles. Its strength is fixed by the isotro- pic component at large trigger angle (16(P), giving 650 mb cross section to be com-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986411

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JOURNAL DE PHYSIQUE

1 9 7 ~ u ( 2 0 ~ e ; 4 ~ e , 4 ~ e )

A

^{0 ,}^E^0,^, ^{= 390}⁼⁼^16-50^16.5O^MeV^{0 ,}^0,^{= O0}^{= 9.7O}

Fig. 1

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Measured spectrum o f r e l a t i v e energies between two a p a r t i c l e s detected a t c l o s e - l y i n g forward angles, t o g e t h e r w i t h Monta-Carlo simulated c o n t r i b u t i o n s o f processes 2 and 4 (see t e x t ) .

pared t o 800 mb complete fusion, 1500 mb incomplete f u s i o n and t o 2700 mb evaporated a p a r t i c l e s a l t o g e t h e r /2,4/.

2. Incom l e t e f u s i o (massive t r a n s f e r ) w i t h forward emission o f one f a s t a p a r t i c - l e s and :apture o f 'GO by t h e t a r g e t nucleus w i t h subsequent evaporation o f one a p a r t i c l e . The c o r r e l a t i o n i s q u a s i - i s o t r o p i c i f t r i g g e r e d a t forward angle (by t h e f i r s t , forward e m i t t e d a p a r t i c l e ) and forward-peaked i f t r i g g e r e d by t h e second a p a r t i c l e ( a t back angles). The l a t t e r component was used t o f i x i t s strength. The r e s u l t i s 400 mb, i n comparison t o 800 mb incomplete-fusion a p a r t i c l e s a l t o g e t h e r

I C I I J I .

3. I n c o m p l e t e f u s i o n massive t r a n s f e r ) w i t h forward emission o f ' ~ e

Q

decaying i n t o two a p a r t i c l e s . The Be g.s. c o n t r i b u t e s o n l y by a narrow spike around t h e t r i g g e r angle n o t v i s i b l e w i t h t h e in-plane d e t e c t o r i n t e r v a l s used f o r t h e c o r r e l a t i o n s shown, t h e f i r s t e x c i t e d s t a t e has 60 a t most.

4. The q u a s i - e l a s t i c r e a c t i o n 2 0 ~ e + lbAu + X + 12c* w ~ t h 12c* decaying v i a ' ~ e i n - t o t h r e e a p a r t i c l e s (60 mb).

5. Dee - i n e l a s t i c c o l l i s i o n s w i t h emission o f two o r t h r e e a p a r t i c l e s from t h e o u t - going F r o j e c t i l e - 1 i ke fragment (about 180 mb). Cross s e c t i o n s o f p r i m a r y decaying fragments r e l e v a n t f o r processes 3 t o 5 may be estimated from t h e primary-fragment y i e l d s delgrmined by v a r i o u s c o i l c i d e n c e measurements /3,6,7/ 13For instance, &he y i e l d o f C* decaying i n t o a + Be was s e t equal t o t h a t o f C* found i n a- Be co-

incidences.

6. Forward emission o f an a p a r t i c l e from 2 0 ~ e i n t h e f i r s t r e a c t i o n phase, t h e 160 remainder e m i t t i n g a second s e q u e n t i a l a p a r t i c l e a f t e r a d e e p - i n e l a s t i c c o l l i s i o n . This process s t i l l must be q u a n t i f i e d .

Processes 3 t o 6 y i e l d c o r r e l a t e d p a i r s w i t h both p a r t n e r s going i n t o forward d i r e c - t i o n , and a c t u a l l y come up f o r almost a l l o f t h e measured forward-forward cross section. The r e l a t i v e angles extend t o 8@ o r more. T h i s corresponds t o r a t h e r l a r g e r e l a t i v e energies, and consequently t h e processes mentioned c o n t r i b u t e e q u a l l y a t t h e l a r g e r e l a t i v e energies i n t h e narrow-geometry d i s t r i b u t i o n s l i k e t h a t of

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Fig. 2

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Possible reaction mechanisms (left) and their contribution to the experi- mental angular correlations (right) of a particles coincident with an a particle detected at BM. The histograms are the Monte Carlo simulations of the processes with two or more outgoing a particles (see text).

fig. 1. Thus they distort precisely that protion of the spectra decisive f ~ r a possible determination of source temperature exploiting the intensity of the Be (2+) resonance at El2 = 3.0 MeV relative to that of the g.s.. Moreover, also the source radii become uncertain, because they are extracted from the value o f the correlation function at small relative energy, the absolute normalization of this function being fixed at large relative energy, where the contributions of the projectile dissoci- at ion processes considered is dominant

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REFERENCES

/I/ C.B. Chitwood et al., Phys. Rev. Lett. 3 (1985) 302 and references quoted therein

/2/ Ch. Egelhaaf et al., Nucl. Phys. A 405 (1983) 397 and references quoted therein

/3/ H. Homeyer et al., Phys. Rev. C 26 (1983) 1335 141 J.R. Huizinga et al., Phys. Rev.T 28 (1983) 1835 /5/ H. Fuchs et al., Phys. Rev. C 31 (1P85) 465 161 U. Jahnke et al., Phys. Rev. ~ z t . ⁵⁰(1983) 1246 171 S. Wald et al., Phys. Rev. C

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