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The theoretical description of nuclear fission remains one of the major challenges of quantum many-body dynamics. The motion through the fission barrier is followed by a fast, nonadiabatic descent of the potential between the fragments. The latter stage is crucial as it generates most of the excitation energy in the fragments. The superfluid dynamics in the latter stage of fission is obtained from the time-dependent Hartree-Fock theory including BCS dynamical pairing correlations. The fission modes of the 258Fm nucleus are studied. The resulting fission fragment characteristics show good agreement with experimental data. Quantum shell effects are shown to play a crucial role in the dynamics and formation of the fragments. The importance of quantum fluctuations beyond the independent particle and quasiparticle picture is emphasized and qualitatively studied. [ABSTRACT FROM AUTHOR]

Using multidimensional Langevin equations, we have calculated the angular distribution of fission fragments with nonconstant values for the dissipation coefficient of the K coordinate (the projection of the total angular momentum I onto the symmetry axis of the fissioning nuclear system) for symmetry and asymmetry fission. We investigated the relaxation time of the K coordinate, τK, as a function of scission criteria and the asymmetry parameter. Calculations are done for 16O + 232Th → 248Cf, 16O + 238U → 254Fm, and 16O + 248Cm → 264Rf reactions and obtained results are compared with available experimental data. Our calculations with nonconstant values of the K coordinate show that the agreement between theoretical calculations and experimental data for asymmetry fission is better than that for symmetry fission. [ABSTRACT FROM AUTHOR]

Motivated by new experimental spectra in transfermium mass region, we have investigated broken-pair high-K multiparticle excited states and their rotational bands for the N = 150 isotones around Z = 100, by using the configuration-constrained pairing-deformation-frequency self-consistent total-Routhian-surface (TRS) model. In order to avoid the spurious phase transition encountered in the Bardeen-Copper-Schrieffer (BCS) pairing, the particle-number-conserving method has been employed for pairing calculations. Pairing correlations are remarkably reduced for the rotations of broken-pair multiparticle states. The present configuration-constrained TRS calculations reproduce reasonably existing experimental data. The abnormal feature in the recently observed Kπ = 8- bands of 252No and 250Fm would be associated with configuration mixing. [ABSTRACT FROM AUTHOR]

The systematic of the spontaneous fission half-lives for the nuclei 242-262Fm and 250-260No is analyzed, within a least action scheme, with the parametrization D1M of the Gogny energy density functional. The properties of the dynamic (least action) fission paths are analyzed and compared to those of the static (minimal energy) ones. The constrained Hartree-Fock-Bogoliubov approximation is used to compute deformed mean-field configurations, zero-point quantum corrections, and collective inertias. It is shown that a cumbersome full variational search of the least action fission path, within the space of Hartree-Fock-Bogoliubov (HFB) states, might not be required if the relevant degrees of freedom are taken into account in the minimization of the Wentzel-Kramers-Brillouin action. The action is minimized in terms of pairing fluctuations that explore the pairing content of the HFB states along the fission paths of the considered nuclei. It is found that, for a given shape, the minimum of the action in fermium and nobelium nuclei corresponds to a value of the pairing fluctuations larger than the one associated with the minimal energy solution for the same shape. The reduction of the action, via larger pairing correlations, has a significant impact on the predicted spontaneous fission half-lives, improving their comparison with experiments by several orders of magnitude. [ABSTRACT FROM AUTHOR]

Fission fragment mass distributions in the reactions 11B+232Th and 11B+243Am were measured in an energy range around the barrier. No sudden change in the width of the mass distribution as a function of center-of-mass energy was observed at near-barrier energies, indicating no quasifission transition in the near-barrier energies. Interestingly, the previous measurements of fission fragment angular anisotropies for the same systems showed significant departure from the statistical saddle-point model predictions at near-barrier energies, indicating the presence of nonequilibrium fission processes. [ABSTRACT FROM AUTHOR]