More detailed study of fission dynamics in fusion-fission reactions within a stochastic approach

Abstract

A stochastic approach based on three-dimensional Langevin equations was applied to a more detailed study of fission dynamics in fusion-fission reactions. The dynamical model has been developed and extended to investigate fission characteristics of light fissioning nuclei at low excitation energies. The energy dependences of an anisotropy of the fission fragment angular distribution, an evaporation residue cross section, a fission cross section, mean prescission neutron, and giant dipole ␥ multiplicities have been analyzed for the 16 O ϩ 208 Pb-induced fission of 224 Th. Also, dependence of the prescission neutron multiplicity on the fragment mass asymmetry and total kinetic energy have been calculated. Analysis of the results shows that not only characteristics of the mass-energy distribution of fission fragments, but also the mass and kinetic-energy dependence of the prescission neutron multiplicity, the angular anisotropy, and fission probability can be reproduced using a modified one-body mechanism for nuclear dissipation with a reduction coefficient of the contribution from a wall formula k s ϭ0.25-0.5 for compound nuclei 172 Yb, 205 Fr, 215 Fr, and 224 Th. Decrease of the prescission neutron multiplicities with fragment mass asymmetry is due to a decrease of the fission time. The results obtained show that prescission neutrons are evaporated predominantly from the nearly spherical compound nucleus at an early stage of fission process before the saddle point is reached. From performed analysis one can conclude that coordinate-independent reduction coefficient k s is not compatible with simultaneous description of the main fission characteristics for heavy fissioning systems 256 Fm and 252 Fm.

Mass-energy distribution of fragments in Langevin dynamics of fission induced by heavy ions

“…The results of this systematic investigation were previously published in [7,8]. The formation of compound nuclei that have rather high excitation energies and high angular momenta is a feature peculiar to the fission process induced by heavy ions.…”

“…The Langevin equation for the coordinate K permits simulating the relaxation of K states with allowance for instantaneous physical properties of the fissioning system, such as its temperature and moment of inertia, instead of treating the respective relaxation time as a free parameter [9,10]; moreover, it describes the evolution of all collective degrees of freedom of the nucleus within a unified conceptual framework. Thus, the Langevin dynamics of fission induced by heavy ions must include at least four collective coordinates-three for the evolution of the nuclear shape [7,8] and one for the evolution of the K mode [11][12][13][14][15][16].…”

“…A stochastic approach to fission dynamics on the basis of three-dimensional Langevin equations [7,8] makes it possible to perform a comprehensive study of mass-energy distributions of fission fragments and mean multiplicities of prescission neutrons. The results of this systematic investigation were previously published in [7,8].…”

Dissipation strength of the tilting degree of freedom in fusion-fission reactions

“…In the present study we used recently developed 4D dynamical model for the description of fissioning nucleus shape evolution based on a stochastic approach [1,2,4,7,8]. The detailed description of the model could be found in Refs.…”

“…A modified one-body mechanism of nuclear dissipation [9,10] was employed to determine the dissipative part of the driving forces with reduction coefficient from the "wall" formula k s . The value k s = 1.0 corresponds to the "wall" and "wall-plus-window" formulas, whereas values 0.2 < k s < 0.5 allow to reproduce different features of the experimental fission fragment MED and particle multiplicities in multidimensional Langevin calculations [2,8,11].…”