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

considered, the diabatic potential has a minimum as a function <strong>of</strong> ε around ε =0.65 − 0.85.
Since the mass parameter in ε has been shown to be large [43], it strongly hinders the growth
<strong>of</strong> the neck even for a small energy gain from the diabatic contributions during a variation <strong>of</strong>
the neck coordinate.
Fig. 3-9 shows diabatic potentials as a function <strong>of</strong> λ for the asymmetric systems 110 Pd+ 136
Xe, 86 Kr + 160 Gd and 48 Ca+ 198 Hg, which lead to the same compound nucleus 246 Fm. These
diabatic potentials are also strongly repulsive for smaller elongations and the quasi-fission bar-
rier <strong>of</strong> the pocket becomes larger with the increase <strong>of</strong> the mass asymmetry η. In the upper
part <strong>of</strong> Fig. 3-10, the diabatic contributions for the system 220 U( 110 Pd+ 110 Pd) are shown for
η =0and0.5 atε =0.74. For an asymmetric clusterization <strong>of</strong> 220 U, the diabatic hindrance
for the motion to smaller values <strong>of</strong> λ is smaller than for the symmetric configuration. This
means that the evolution <strong>of</strong> the asymmetric DNS to the compound nucleus is more favored,
which is supported by the experimental data. Since the diabatic effects are small near the
touching <strong>of</strong> the nuclei, they are not important for the potential <strong>of</strong> the DNS as a function <strong>of</strong> the
mass asymmetry at the touching configuration <strong>of</strong> the nuclei. The difussion process in the mass
asymmetry η at the touching configuration <strong>of</strong> the nuclei starts after the formation <strong>of</strong> the initial
DNS at the pocket <strong>of</strong> the potential in λ for a large fixed value <strong>of</strong> ε, e.g., ε =0.74. This slow
difussion process is studied using an adiabatic potential which will be presented in chapter 4.
The dependence <strong>of</strong> the diabatic contribution on the temperature is presented in the lower
part <strong>of</strong> Fig. 3-10 for the system 220 U( 110 Pd+ 110 Pd). For this calculation, we take the occupa-
probabilities n tion diab
α
given by the Fermi-distribution at finite temperature for the touching
configuration <strong>of</strong> the nuclei. This temperature could be related to the excitation <strong>of</strong> the nuclei
in the approaching phase <strong>of</strong> the collision. The initial excitation energy <strong>of</strong> the system decreases
the repulsive character <strong>of</strong> the diabatic contribution due to smaller occupation numbers <strong>of</strong> the
diabatic states under the Fermi-level. If these states are occupied and cross the Fermi-level from
below, they increase the repulsive contributions <strong>of</strong> the potential, while occupied states above
the Fermi-level, crossing it from above with decreasing λ, diminish the diabatic contributions.
The diabatic contributions increase with prolate deformations and decrease for oblate defor-
mations (Fig. 3-11). However, in order to conclude about an advantageous fusion with oblate
nuclei, the corresponding mass parameters should first be analysed.
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