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 $\ensuremath{\gamma}$ multiplicities have been analyzed for the ${}^{16}\mathrm{O}{+}^{208}\mathrm{Pb}$-induced fission of ${}^{224}\mathrm{Th}.$ 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 ${k}_{s}=0.25--0.5$ for compound nuclei ${}^{172}\mathrm{Yb},$ ${}^{205}\mathrm{Fr},$ ${}^{215}\mathrm{Fr},$ and ${}^{224}\mathrm{Th}.$ 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 ${}^{256}\mathrm{Fm}$ and ${}^{252}\mathrm{Fm}.$