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

elements from Z=104 to Z=112 in the cold fusion reactions, the experimentalists observed a
rapid fall-<strong>of</strong>f <strong>of</strong> the evaporation residue cross sections (about four orders <strong>of</strong> magnitude) with
increasing charge number <strong>of</strong> the compound nucleus [7, 8]. In the model <strong>of</strong> fusion based on the
DNS-concept, the fusion cross sections and excitation functions for the cold fusion reactions
leading to the formation <strong>of</strong> superheavy elements are obtained in good agreement with the
experimental data [20].
The structure <strong>of</strong> the mass (charge) distribution <strong>of</strong> the quasi-fission products well corresponds
to the peculiarities <strong>of</strong> the driving potential (the DNS potential energy as a function <strong>of</strong> mass
(charge) asymmetry) shown in chapter 4. In order to illustrate the connection between the
quasi-fission product yield and the driving potential, Fig. 7-4 shows the quasi-fission charge dis-
tribution for the reactions 110 Pd+ 136 Xe (η=0.1), 86 Kr+ 160 Gd (η=0.3) and 76 Ge+ 170 Er (η=0.4),
which lead to the formation <strong>of</strong> the same compound nucleus 246 Fm. The distributions reveal
large widths and one can observe a notable drift in charge (mass) away from the initial charge
(mass) asymmetry <strong>of</strong> the interacting nuclei. The product masses are substantially different
from the target—projectile masses. It is seen that the symmetric fragments can be formed
in the quasi-fission process if the entrance DNS is an asymmetric one. We can observe that
the main peak <strong>of</strong> the distributions is around the initial configuration <strong>of</strong> the DNS. Compar-
ing Fig. 7-4 with Fig. 4-7a), we observe that there is a correlation between the peaks <strong>of</strong> the
quasi-fission distribution and the minima <strong>of</strong> the driving potential. The inner fusion barrier Bη
increases with decreasing mass asymmetry η (Figs. 4-1to 4-7b)), while the quasi-fission bar-
rier B qf becomes smaller because the Coulomb repulsion increases with decreasing η and leads
to very shallow pockets in the nucleus-nucleus potential for nearly symmetric configurations.
Therefore, the decay <strong>of</strong> the initial DNS dominates in symmetric or nearly symmetric collisions.
The experimental fusion probability becomes larger with the increase <strong>of</strong> the mass asymmetry
η in the entrance channel [118]. This experimental evidence for a hindrance <strong>of</strong> fusion has been
mainly concluded from the impossibility to produce fermium evaporation residues with nearly
symmetric projectile-target combinations [118].
The main conclusions are: 1) The quasi-fission products <strong>of</strong> fusion reactions are correctly
described within the DNS model: diffusion in charge (mass) asymmetry and in relative distance
(the DNS decay) coordinates contributes to the yield <strong>of</strong> quasi-fission products. 2) The quasi-
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