Modelling fission with microscopic input: excitation-energy-dependent fission paths for neutron-induced reactions

Abstract

To calculate transmission coefficients, current Hauser-Feshbach reaction codes assume a one-dimensional fission path that is both independent of excitation energy and universal across different fission channels. In contrast, microscopic fission studies typically explore multiple collective variables. Predicting observable fission quantities such as cross sections based on microscopic input thus requires a way to reduce the dimensionality of the latter. For spontaneous fission, the least action path (LAP) -- which minimizes the semiclassical action and maximizes the transmission coefficient -- is the solution. We demonstrate that the LAP does not generalize to other fission modes such as neutron-induced or $β$-delayed fission: at finite excitation energy, the LAP generally does not maximize the transmission. A microscopic PES can have multiple coexisting stationary-action paths. We show that these (i) can switch their action ordering as a function of excitation energy and (ii) yield different neutron-induced fission cross sections predictions, yet match known spontaneous fission lifetimes equally well. For the BSkG3 Skyrme-type energy density functional model, the transition from the axially symmetric LAP at zero excitation energy to a path exploiting the triaxial degree of freedom occurs at an excitation energy that is smaller than the neutron separation energy for essentially all actinides and the majority of unknown exotic nuclei.