Langevin description of charge fluctuations in fission of highly excited nuclei

A stochastic approach to fission dynamics based on three-dimensional Langevin equations is used to study charge fluctuations in fission of highly excited nuclei. Elongation, neck and charge asymmetry parameters are chosen as relevant collective coordinates. Using as an example $\rm ^{4}He+^{232}Th$ induced fission of $\rm ^{236}U$ in a broad range of excitation energies (from $60$ to 160 \mbox{ MeV}), the isobaric charge variance is investigated to obtain information on nuclear dissipation. The friction parameter of the charge mode is calculated under the assumption of both the one-body and two-body mechanisms of nuclear viscosity. The results obtained for the variances of the charge distribution within the applied approach reveal that the optimal reproduction of the available data is achieved with the value of the two-body viscosity coefficient (0.6\le\nu_0\le 1.8)\times 10^{-23}\mbox{MeV s fm}^{-3} that is close to those deduced earlier from the description of the fission fragment mass-energy distribution. The expression for the friction parameter of the charge mode is derived within the one-body mechanism of nuclear dissipation. The one-body mechanism of nuclear dissipation also provides a good reproduction of data on the isobaric charge variance without any adjustable strength parameter

Influence of orientation degree of freedom on fission dynamics of higly excited nuclei

Four-dimensional dynamical model was developed and employed for study fission characteristics in a wide range of fissility parameter. The three collective shape coordinates plus the K coordinate, which is the spin of the nucleus with respect to the symmetry (fission) axis, were considered dynamically from the ground state deformation till the scission into fission fragments. A modified one-body mechanism for nuclear dissipation with a reduction coefficient ks of the contribution from a “wall” formula have been used in the study. The four-dimensional calculations for heavy nuclei could describe the fission fragment mass-energy distribution (MED) parameters and prescission neutron multiplicity with almost single ks value, in contrast with 3D dynamical calculations, where a consistent description of all observables with the same ks is not possible for heavy nuclei. The estimation of a dissipation coefficient for the orientation degree of freedom γK = 0.077(MeV zs)−1/2 is good for heavy nuclei and lower value of γK = 0.05(MeV zs)−1/2 is needed for nuclei with mass A ≃ 200. The results of 4D and 3D Langevin dynamical calculations for light nuclei near the Businaro-Gallone point predict close results for the fission fragment MED parameters and prescission particles multiplicities