Implementation of a multi-chance fission model in FIFRELIN for 235U(n,f) reaction

Neutron emission is a physical process that can occur at various stages of nuclear fission. The main source of neutron emission comes from primary fission fragments after full acceleration. Another source of neutron emission comes from the de-excitation of the nucleus prior to scission (scission neutrons are neglected in this work). This type of event can occur if the energy of the incoming neutron is above a threshold energy dependent on nuclear properties (neutron binding energy Sn and fission barrier height Bf). This type of de-excitation initiates a competition between fission and neutron emission. This physical mechanism is referred to as multi-chance fission and becomes significant for incident neutron kinetic energies higher than the fission barrier height of the residual nucleus (approximately 5 MeV). Until now, this phenomenon has not been considered in the FIFRELIN code, limiting its scope of application. In this context, we propose a method to model multi-chance fission for the 235U(n,f) reaction. It is based on evaluated microscopic partial fission cross-sections used to calculate multi-chance fission probabilities.