Application of a temperature-dependent liquid-drop model to dynamical Langevin calculations of fission-fragment distributions of excited nuclei

Abstract

A stochastic approach to fission dynamics based on three-dimensional Langevin equations was applied to calculation of the mass-energy and angular distributions of fission fragments. The dependence of the mass-energy distribution parameters on the angular momentum and the anisotropy of the fission-fragment angular distribution on excitation energy have been studied in a wide range of the fissility parameter. A temperature-dependent finite-range liquid-drop model was used in a consistent way to calculate the functional of the Helmholtz free energy and level-density parameter. The modified one-body mechanism of nuclear dissipation (the so-called surface-plus-window dissipation) was used to determine the dissipative forces in Langevin equations. The evaporation of light prescission particles was taken into account on the basis of a statistical model combined with Langevin dynamics. The calculated parameters of the mass-energy distribution and their angular dependencies are in good quantitative agreement with the available experimental data at the value of the reduction coefficient of the contribution from the wall formula equal to $0.25$. Analysis of the anisotropy of the fission-fragment angular distribution performed with the saddle-point transition state model and scission-point transition state model indicates that it is necessary to take into account the dynamical aspects of the fission-fragment angular distribution formation.