Influence of the asymmetry parameter and dissipation coefficient of the K coordinate on different aspects of fission of excited compound nuclei

The dynamics of fission of the excited compound nuclei Fm-256, Fr-215, Ir-187, Yb-172, Yb-162, and Ce-142 produced in fusion reactions with 158.8 MeV O-18 has been studied by solving three- and four-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula. The constant dissipation coefficients of K, gamma(K) = 0.077 (MeV zs)(-1/2), gamma(K) = 0.2 (MeV zs)(-1/2) and a nonconstant dissipation coefficient of K have been used to reproduce the experimental data for both symmetric and asymmetric splitting of the fissioning systems. The average kinetic energies of fission fragments, the pre-scission neutron multiplicities, the fission time, and the variances of the mass and kinetic energy of fission fragments are calculated for the excited compound nuclei Fm-256, Fr-215, Ir-187, Yb-172, Yb-162, Ce-142, and results of the calculations are compared with each other and with the experimental data. Comparison of the theoretical results with the experimental data calculated by using different values of gamma(K) shows that the difference is small between the results of calculations for symmetric and asymmetric simulations of the fission process of excited intermediate nuclei, whereas for heavy compound nuclei the difference is slightly high. In other words, the effect of the asymmetry parameter on the fission process of intermediate nuclei is smaller than the effect on heavy nuclei. Furthermore, we show that the pre-scission neutron multiplicity decreases rapidly with increasing fragment asymmetry.