Fission rate and transient time of highly excitednuclei in multi-dimensional stochasticcalculations

摘要

The influence of the dynamical model dimensionality in use on the fission rates studied within the stochastic approach to fission dynamics [1]. Time dependence of the fission rate was calculated with the use of multidimensional Langevin equation. Particle evaporation was not taken into account. One-, two- and three-dimensional cases were considered on the basis of the {c,h,a}-parametrization of the nuclear surface shape. Calculations were performed for the large number of compound nuclei with Z~2/A parameter in the range 20 < Z~2/A < 40 and excitation energy 200 MeV. A considerable increase of the stationary fission rate in the transition from one-dimensional to three-dimensional case was revealed [2]. This increase is about two times for the nuclei around ~(224)Th and about 10~2 for the light nuclei near the Businaro-Gallone point. The influence of the dissipation mechanism on the transient time is studied for multidimensional systems. It was shown that the ratios of the stationary fission rates obtained in the calculations with the different dimensionalities: R~(3D)_(st)/ R~(1D)_(st) and R~(2D)_(st)/R~(1D)_(st) remain almost the same for different dissipation mechanisms. Thus we conclude that the fission rate is mostly determined by the structure of the potential energy surface of the system. For one-body dissipation mechanism it was shown that the transient time τ_(tr) is about 5 or 6 times greater at k_s = 1.0 than at k_s = 0.25. Two-body dissipation mechanism leads to the smaller τ_(tr) values in comparison with the one-body dissipation mechanism at k_s= 0.25. The transient time does not change significantly in the dynamical calculations with the different dimensionality. We also compared the dynamically calculated stationary fission rate to the Kramers rate.