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

Evaporation-residue residue cross sections: role of the entrance channel
IPNO Participation: N. Rowley
Collaboration: Nabila Saffi dine-Grar (Department of Physics, University Ferhat Abbas of Sétif,
19000 Algérie)
Sections efficaces de résidus d’évaporation : rôle de la voie d’entrée
Pour une réaction entre deux ions lourds, la création des résidus d’évaporation est un processus très
complexe, surtout pour les systèmes symétriques où la quasi-fission peut intervenir très fortement. L’étude
d’une gamme de réactions menant au même noyau composé, permet une séparation approximative des
effets de la voie d’entrée (choix du projectile et de la cible) et les effets parvenant des propriétés du noyau
composé lui-même. Nous définissons deux distributions de barrière ; une pour la capture (étape initial da la
réaction) et, en exploitant la section efficace des résidus d’évaporation, une deuxième pour la création du
noyau composé. Une comparaison des deux permet de voir l’importance de la quasi-fission dans les
différents systèmes.
For heavy systems, the formation of high-Z evaporation
residues via heavy-ion capture is a very
complex process. In the early stages of the reaction,
the colliding partners encounter an external
Coulomb barrier which they must surmount or penetrate
for the reaction to proceed. Once the barrier
has been crossed, the composite system must
evolve to form an equilibrated compound nucleus,
though of course there may be pre-equilibrium fission
before this state is reached; a systemdependent
process referred to as quasifission. Furthermore,
if and when a compound nucleus is formed,
it may decay by true fission, as well as becoming
a longer-lived evaporation residue through
the emission of low-mass particles (neutrons, protons,
alpha-particles...). At sufficiently high angular
momentum all of the compound nuclei will fission
and their contribution to the evaporation residue
cross section will fall to zero.
Even in its first stage, the reaction may be further
complicated by the presence of a “distribution of
Coulomb barriers” arising from the internal structures
of the reaction partners (principally their highly
collective states). This phenomenon will modify the
coefficents for crossing the external barriers and
entering into the composite system.
Well above the highest barrier, all of the low partial
waves that are capable of surviving fission will be
fully transmitted, and, in the absence of quasifission,
the reduced cross section for evaporationresidue
formation at a given compound-nucleus
excitation energy, becomes independent of the
reaction partners [1]. This significantly simplifies
the reaction dynamics. However, when attempting
to optimize compound-nucleus formation (for example
in superheavy-element creation) one might
choose a lower energy, where transmission is not
complete but survival is much greater. We investigate
the differences that arise from different
choices of target and projectile in this lower-energy
Fig. 1. A comparison of the experimental barrier
distribution ( 96 Zr + 124 Sn reaction) for fusion
(compound nucleus creation) with the theoretical
capture distribution shows the dominant role of
quasi-fission for all but the highest barriers. Both
curves are normalized to 1 for comparison, but the
true fusion probability is around 8%.
regime. They essentially come from:
1) the compound nucleus excitation energy at the
corresponding barrier,
2) the “inertial parameter” 2MR 2 which determines
the energy range over which the evaporationresidue
cross section saturates, and
3) the entrance-channel barrier distribution due to
the intrinsic structures of the reactants.
A good understanding of these effects will facilitate
the study of the role of quasi-fission which is also
system dependent, being significantly more important
for mass-symmetric systems. It may also possess
a deformation dependence (the survival probability
being greatest for the most “compact” configurations
at contact), and may be further influenced
by closed shells in the target and/or projectile
(with the fragment-mass drift favouring the
closed-shell structures).
We derive the correct structural form of the evaporation-residue
cross section and demonstrate our
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