A four-dimensional dynamical model based on Langevin equations was developed and applied to calculate a wide set of experimental observables for the reactions 16 O + 208 Pb → 224 Th and 16 O + 232 Th → 248 Cf over a wide range of excitation energy. The fusion-fission and evaporation residue cross sections, fission fragment mass-energy distribution parameters, prescission neutron multiplicities, and anisotropy of angular distribution of fission fragments could be reasonably reproduced using a modified one-body mechanism for nuclear friction with a reduction coefficient of the contribution from a wall formula k s 0.25 and a dissipation coefficient for the orientation degree of freedom (K coordinate) γ K 0.077 (MeV zs) −1/2 . Inclusion of the K coordinate into calculation of potential energy changes the stiffness of the nucleus with respect to mass asymmetry coordinate for the values of K = 0 and results in a shift of the Businaro-Gallone point towards larger Z 2 /A values. The experimental data on the fission fragment mass-energy distribution parameters together with mean prescission neutron multiplicity for heavy fissioning nuclei are reproduced through the four-dimensional Langevin calculations more accurately than through three-dimensional calculations.

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A four-dimensional stochastic approach to dynamics of nuclear fission induced by heavy ions was applied to calculations of the fission rate and time of highly excited compound nuclei. The research took into account not only three shape collective coordinates introduced on the basis of the {c, h, α}-parametrization but also orientation degree of freedom (K-state) -spin about the symmetry axis. Overdamped Langevin equation was used to describe the evolution of the K-state. Impact of orientation degree of freedom on the fission rate and time of the compound nuclei was studied in detail for the reactions with high-energy projectiles 14 N and 16 O on target nuclei 197 Au, 208 Pb, 232 Th and 238 U. It was revealed that inclusion of the K-state in the dynamical model produces considerable increase in the mean fission time and decrease in the stationary fission rate. The K-state impact on the fission rate and time almost fully canceled the opposite effect produced by inclusion of nuclear neck and mass-asymmetry coordinates in the 1D Langevin calculations. The difference of 5-25% between 4D and 1D calculations was found as the result of this research.

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