A critical analysis of the modelling of dissipation in fission

We examine the modelling of the effects of dissipation in fission by analysing experimental data where fission is induced by peripheral heavy-ion collisions at relativistic energies. Our results are based on the analysis of available data of the total nuclear fission cross sections of U at 1⋅A GeV on Au and U targets with an abrasion-evaporation code that contains an intermediate break-up stage. We have incorporated in the code three different approximations for the time-dependent fission-decay width that describe the hindrance of fission due to dissipation. One is a highly realistic description based on the analytical solution of the Fokker-Planck equation when the nuclear potential is approximated by a parabola. The two others correspond to the most widely used approximations, a step function and an exponential-like in-growth function. The experimental data are only reproduced when dissipation effects are considered. However, the deduced value of the reduced dissipation coefficient β depends strongly on the approximation applied. A careful analysis sheds severe doubts on the use of the exponential-like in-growth function for describing the time dependence of the fission width. PACS: 24.10.-i; 24.75.+i; 25.75.-q