حسین رعنایی

A stochastic approach based on four-dimensional Langevin equations has been used to simulate the fission dynamics of the excited compound nuclei 256Fm, 254Fm, 224Th, 215Fr, 213Fr, 200Pb, 187Ir, 172Yb, 170Yb, 162Yb, 142Ce, and 125Cs produced in 16,18O+X reactions with projectile energies 158.8 and 288 MeV. The effect of nuclear dissipation was investigated in the dynamical simulation of fission process of these nuclei. The average prescission neutron multiplicity, the fission time, the average total kinetic energy of fission fragments, the mass and energy distribution of fission fragments, and the variation of the prescission neutron multiplicity with total kinetic energy of fission fragments have been calculated for these nuclei. The dynamical calculations have taken into account not only three shape collective coordinates introduced on the basis of the “funny hills” shape parameters but also orientation degree of freedom (spin about the symmetry axis), and in the dynamical calculations dissipation generated through the chaos-weighted wall and window friction formula and also the sum of the chaos-weighted wall and window friction formula with the two-body nuclear friction. Furthermore, the effect of dissipation coefficient of K, γK, were also considered in the dynamical calculations. Comparison of the theoretical results with the experimental data shows that the experimental data for compound nuclei produced with the projectile energy 158.8 MeV can be reproduced satisfactorily with the results of calculations by using the chaos-weighted wall and window friction formula, although the results of calculations of the average prescission neutron multiplicity, the fission time, and the variation of the pre-scission neutron multiplicity with total kinetic energy of fission fragments lie somewhat below the experimental data for heavy nuclei produced in reactions with projectile energy Elab=158.8MeV and also for compound nuclei produced with the projectile energy 288 MeV.