Dissipation and diffusion in nuclear fission

The dynamical descent of nuclei from the fission saddle point onwards was calculated for two extreme dissipation mechanisms: wall-and-window one-body dissipation and ordinary two-body viscosity. This calculation was done by solving numerically the classical generalized Hamilton equations of motion for a fissioning nucleus, with the nuclear potential energy of deformation calculated in terms of a Coulomb energy and a double volume integral of a Yukawa-plus-exponential folding function, the collective kinetic energy calculated by means of the Werner-Wheeler approximation to incompressible irrotational flow. The most probable dynamical trajectories calculated for the two types of dissipation are shown for the fission of /sup 236/U. Although one-body dissipation leads to compact scission shapes and two-body viscosity leads to elongated scission shapes, both types of dissipation reproduce equally well experimental most probable fission-fragment kinetic energies for the fission of nuclei throughout the periodic table. The relative insensitivity of the final kinetic energies to the dissipation mechanism arises because the reduced postscission kinetic energy resulting from an elongated scission shape with two-body viscosity is compensated by a larger prescission kinetic energy. 3 figures. (RWR)