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

1.1 The DNS-concept <strong>of</strong> fusion
A special feature <strong>of</strong> deep-inelastic collisions is that they lead to the formation <strong>of</strong> a DNS [28].
During the entire formation and evolution <strong>of</strong> the DNS there is a continuous redistribution <strong>of</strong>
nucleons, excitation energy and angular momentum between the nuclei. An unstable formation
like a dinuclear system ”lives” for the order <strong>of</strong> a few 10 −21 s and decays before it achieves
complete statistical equilibrium with respect to all degrees <strong>of</strong> freedom. The establishment <strong>of</strong>
equilibrium between the proton and neutron numbers [29] occurs practically instantaneously
in light systems and is a monotonic continuous process in heavy systems [30]. Nucleons are
exchanged slowly between the parts <strong>of</strong> the DNS. As it evolves, a DNS can in principle pass with
some definite probability through many macroscopic configurations allowed by the conservation
laws for the particle number, charge and total energy. A DNS decays either by the dominance
<strong>of</strong> forces <strong>of</strong> repulsion between the interacting nuclei or by the coupling <strong>of</strong> modes <strong>of</strong> motion.
Experimental data on the widths <strong>of</strong> the charge (mass) distributions <strong>of</strong> the reaction products
[29], the dependence <strong>of</strong> the yields <strong>of</strong> nuclei on their nucleon composition [31], and data on the
yields <strong>of</strong> light particles in collisions that lead to the formation <strong>of</strong> compound nuclei [32] indicate
that as dinuclear systems evolve, individual features <strong>of</strong> the nuclei are preserved, and shell effects
play an important part. A review on these experimental evidences is given in Ref. [33]. The
considered experimental data confirm the idea that the individuality <strong>of</strong> the interacting nuclei is
preserved even for a relatively high excitation energy <strong>of</strong> the DNS and indicate that shell effects
play a fundamental role in the evolution <strong>of</strong> the DNS. The study <strong>of</strong> the influence <strong>of</strong> shell effects is
topical in connection with planned experiments with radioactive beams, which will significantly
extend the possibilities for investigating the mechanism <strong>of</strong> nuclear reactions [13].
In the model <strong>of</strong> fusion based on the DNS-concept, the fusion process is considered as the
evolution <strong>of</strong> a DNS caused by the transfer <strong>of</strong> nucleons from the light nucleus to the heavy
one. The dynamics <strong>of</strong> the DNS is considered as a combined diffusion in the degrees <strong>of</strong> freedom
<strong>of</strong> the mass asymmetry η =(A1 − A2)/( A1 + A2) ( A1 and A2 are the mass numbers the
DNS nuclei) and <strong>of</strong> the relative distance between the centers <strong>of</strong> nuclei R describing the quasi-
fission process (decay <strong>of</strong> the DNS), respectively (Fig.1-2). The potential barrier B ∗ fus
a hindrance for the fusion. The energy required to overcome the fusion barrier B supplies ∗
fus is
in the DNS excitation energy. As was found in [34], the value <strong>of</strong> B contained ∗ can be much
fus
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in η