Effects of diabaticity on fusion of heavy nuclei in the dinuclear model ...

Chapter 3
The synthesis <strong>of</strong> transuranium and superheavy elements ([4], [7], [8]), the production <strong>of</strong> superde-
formed nuclei and nuclei far from the line <strong>of</strong> stability stimulate the study <strong>of</strong> fusion processes
in heavy ion collisions at low energies (< 15 MeV/u). Existing theoretical models can be
distinguished by the choice <strong>of</strong> the relevant collective variables in which the fusion occurs. The
relative distance between the centers <strong>of</strong> nuclei R (or elongation <strong>of</strong> system) and the neck degree
<strong>of</strong> freedom play an essential role in the macroscopic dynamical model (MDM) [16], which is
based on an adiabatic approach to fusion. Without regarding the competition between complete
fusion and quasi-fission, this model overestimates the fusion cross section <strong>of</strong> heavy nuclei [18].
Attempts were made to improve this model by including thermal fluctuations and the competi-
tion between complete fusion and quasifission in [62], but an agreement with the experimental
data was not achieved for the reactions treated in this study. Corrections to these results could
arise from nuclear structure effects in collisions with energies slightly above the Coulomb bar-
rier. In a recent paper [42], the shell effects were incorporated by using the two-center shell
model (TCSM) [40]. The study <strong>of</strong> the dynamics <strong>of</strong> fusion within the adiabatic TCSM overes-
timates the fusion probabilities for most symmetric and near symmetric reactions. Moreover,
the isotopic dependence <strong>of</strong> the fusion probability resulted incorrectly. Therefore, hindrances
for the growth <strong>of</strong> the neck and for the motion to smaller values <strong>of</strong> R should be assumed to
explain the experimental data. This hindrance allows the dinuclear system (DNS) to keep a
relatively small neck over a time comparable with the reaction time. Then the fusion proceeds
as a motion <strong>of</strong> the DNS in mass asymmetry. Such a hindrance <strong>of</strong> the motion to smaller R is
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