Neutron and γ multiplicities as a function of incident neutron energy for the Np237(n,f) reaction

Abstract

In the nuclear fission process, the excitation energy sharing mechanism between the fission fragments, the fragment angular momentum generation process and the interdependence between the two are still poorly known. The study of the neutron and $\ensuremath{\gamma}$ emission characteristics as a function of fragment and compound nucleus properties brings valuable information on these mechanisms since they decrease respectively the fragment excitation energy and angular momentum. In the 1980s, Naqvi and M\uller experimentally highlighted, with $^{237}\mathrm{Np}(n,f)$ direct fission reactions, that only the prompt neutron multiplicity for the heavy fragment increases with the incident neutron energy. This means that the additional excitation energy goes into the heavy fragment. This data set allows one to test the different energy sharing models. This paper investigates, using the fifrelin fission fragment deexcitation code, the impact of the models used to assign the fragment initial state on fission observables. It focuses on the impact of a constant or an energy dependent spin cutoff model, whose parameters define the initial total angular momentum distribution, coupled with an energy sharing model based on an empirical temperature ratio law ${R}_{T}(A)$ in which fission fragments are considered to behave as a Fermi gas. It shows that both spin cutoff models succeed in reproducing the experimental neutron multiplicities which are mainly driven by the ${R}_{T}(A)$ law. However, they predict different $\ensuremath{\gamma}$ observables and neutron-$\ensuremath{\gamma}$ multiplicity correlations. Therefore, to infer about the validity of the two models, it is necessary to measure neutron and $\ensuremath{\gamma}$ observables in correlation in order to have a deeper understanding of the fission mechanism.