arXiv subjects
Publications and source records attributed to F. A. Ivanyuk.
Numerical computations of transport coefficients at low temperatures are presented for shapes typically encountered in nuclear fission. The influence of quantum effects of the nucleonic degrees of freedom is examined, with pair correlations included. Consequences for global collective motion are studied for the case of the decay rate. The range of temperatures is specified above which this motion may be described as a quantal diffusion process.
The linear response approach to nuclear transport has been extended to pair correlations. The latter are treated within a mean field approximation to a pairing interaction with constant matrix elements $G$. The constraint of particle number conservation has been accounted for on a time dependent average, which leads to modified response functions, both in the pairing degree of freedom as well as in the shape variable. The former is expressed by the gap parameter $Δ$ and the latter by a $Q$ which specifies the elongation of a fissioning nucleus. The tensors for friction and inertia corresponding to these two collective coordinates are computed along the fission path of $^{224}Th$ for temperatures around $T=1 \mev$ and less. It is seen that dissipation decreases with decreasing temperature and increasing pairing gap and falls well below the values of common "macroscopic models". Both friction and inertia show a sensible dependence on the configurations of the mean field caused both by shell effects as well as by avoided crossings of single-particle levels.
Previous computations of the potential landscape with the shapes parameterized in terms of Cassini ovaloids are extended to collective dynamics at finite excitations. Taking fission as the most demanding example of large scale collective motion, transport coefficients are evaluated along a fission path. We concentrate on those for average motion, namely stiffness C, friction γand inertia M. Their expressions are formulated within a locally harmonic approximation and the help of linear response theory. Different approximations are examined and comparisons are made both with previous studies, which involved different descriptions of single particle dynamics, as well as with macroscopic models. Special attention is paid to an appropriate definition of the deformation of the nuclear density and its relation to that of the single particle potential. For temperatures above 3 MeV the inertia agrees with that of irrotational flow to less than a factor of two, but shows larger deviations below, in particular in its dependence on the shape. Also friction exhibits large fluctuations along the fission path for small excitations. They get smoothed out above 3 - 4 MeV where γattains values in the range of the wall formula. For T > (or=) 2 MeV the inverse relaxation time β= γ/M turns out to be rather insensitive to the shape and increases with T.