exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Isospin effects on fragment and particle multiplicities
in multifragmentation
IPNO Participation: F. Gagnon-Moisan, B. Borderie, E. Galichet, M. F. Rivet
Collaboration : INDRA
LPC Caen, ENSICAEN, Université de Caen, CNRS/IN2P3, Caen, France
GANIL, CEA et CNRS/IN2P3, Caen, France
IPN Lyon, Université Claude Bernard Lyon1, CNRS/IN2P3, Villeurbanne, France
IRFU/SPhN, CEA/Saclay Gif-sur-Yvette, France
Laboratoire de Physique Nucléaire, Université Laval, Québec, Canada
Dpt de Scienze Fisiche e Sez. INFN, Università di Napoli « Federico II », Napoli, Italy
NIPNE, Bucharest Magurele, Romania
Les effets d’isospin sont étudiés dans les collisions centrales conduisant à la multifragmentation de sources
de quasi-fusion de projectiles de xénon ( 124,136 Xe) accélérés à 32 et 45 MeV/nucléon et de noyaux cibles
d’étain ( 112,124 Sn). Les mesures montrent que les multiplicités de produits chargés dépendent du rapport
N/Z du système total (projectile+cible), mais pas de la configuration particulière de la voie d’entrée
(pour un même N/Z). A une énergie incidente donnée on observe que, lorsque N/Z augmente, la multiplicité
de particules légères (H, He) décroît alors que la multiplicité de fragments croît. Cet effet peut être attribué
à l’effet stabilisateur d’un plus grand nombre de neutrons disponibles pour former des fragments. Des simulations
de collisions à l’aide d’un code de transport reproduisent la tendance observée expérimentalement.
60°, in order to eliminate those resulting from collisions
keeping a strong binary character.
System
N/Z sys
124 Xe+ 112 Sn 1.269
124 Xe+ 124 Sn 1.385
129 Xe+ nat Sn 1.385
136 Xe+ 112 Sn 1.385
136 Xe+ 124 Sn 1.5
With the advent of rare ions accelerators expected
in a near future, nuclear physicists hope to get
information on the symmetry energy term of the
Equation of State of nuclei, particularly on its density
dependence. In the mean time stable isotopes
which cover a large enough domain in isospin (N/
Z) can provide precious information. In this aim the
INDRA collaboration studied collisions between
124,136 Xe projectiles, accelerated to 32 and 45 MeV/
nucleon by the GANIL facility, and 112,124 Sn targets.
The effects of a variation of the N/Z of the composite
system, N/Z sys , and those of different entrance
channels leading to the same composite system
were explored. Previous results were added to enrich
this study.
Experimental data
The charged reaction products were collected
with the INDRA array. Quasi-fusion reactions were
selected by requiring a high quality of detection in
charge and linear momentum: the sum of the
charges of the detected products should be at
least equal to 80 (the total system charge is 104),
and that of their pseudo-impulsion (ZxV) should be
greater than 75% of the projectile one. Moreover
we selected events with a flow angle larger than
Figure 1: Total charged product multiplicities
(triangles and full lines) for the different Xe+Sn
systems at 32 (open symbols) and 45 (close
symbols) MeV/nucleon vs the system N/Z.
Each color characterizes a given projectile.
See table for the systems. All statistical errors
amount to 0.04.
The evolution of different multiplicities of charged
products is followed as a function of the incident
energy and of the N/Z of the composite system.
Figure 1 and 2 show the results for the total multiplicity,
M tot , and the light charged particle multiplicity,
M lcp . Both increase with the incident energy,
they are in all cases about 30% higher at 45 MeV/
nucleon. Conversely, M tot and M lcp decrease when
the composite system gets more neutron-rich. M lcp
is 15% smaller for the neutron-richer system than
for the poorer while M tot is only smaller by 9%. This
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