Towards Predictions of Fission Probabilities for Astrophysics Applications

Abstract

Abstract In almost all the evaluations of the neutron-induced fission cross section for nuclei involved in nuclear applications, a double- humped fission barrier described by two independent inverted parabolas is considered. In addition, the continuum transition states are described by a simple Fermi-gas type nuclear level density expression for which the formulation as well as the parametrization remain highly uncertain. For these reasons, these approaches can be considered as having a rather poor predictive power and are not recommended for applications requiring a proper description of fission for nuclei far from stability, such as nuclear astrophysics. Hartree-Fock-Bolgolyubov (HFB) calculation are now available and can provide all the nuclear ingredients required to describe the fission path from the equilibrium deformation up to the nuclear scission point. The aim of this contribution is to apply the basic features of the optical model for fission, using the full microscopic information obtained from HFB models to calculate neutron- induced fission cross sections on selected actinide nuclei. This approach includes not only the details of the energy surface along the fission path, but also the estimate of the nuclear level density derived within the combinatorial approach on the basis of the same HFB single-particle properties, in particular at the fission saddle points. The sensitivity of the calculated fission cross sections to different model approximations is illustrated and the predictive power of such a microscopic approach tested. It is also shown that the various inputs can be tuned to reproduce at best experimental data in one unique coherent framework, so that in a close future it should become possible to make, on the basis of such models, accurate fission cross-section calculations and the corresponding estimates for nuclei, energy ranges or reaction channels for which no data exist. Finally, the same model is applied to the calculation of the fission probabilities, including β-delayed fission, spontaneous fission and neutron-induced fission, for all nuclei potentially produced by the r-process nucleosynthesis. Nucleosynthesis calculations resulting from the decompression of neutron star matter are performed and the sensitivity of the abundance calculation with respect to nuclear fission uncertainties (including fission probabilities and fission fragment distributions) is studied.