Influence of non-statistical properties in nuclear structure on emission of prompt fission neutrons

TL;DR

This paper extends the HF$^3$D model to better understand prompt fission neutron spectra in thermal neutron induced fission of uranium-235, highlighting the influence of nuclear structure properties beyond statistical assumptions.

Contribution

The study introduces a modified model incorporating non-statistical nuclear structure effects, improving the understanding of PFNS discrepancies at higher energies.

Findings

Discrepancies at high neutron energies are linked to primary fragment yields and high spin states.

Non-statistical properties significantly influence prompt neutron emission spectra.

Model sensitivity analysis identifies key nuclear structure factors affecting PFNS.

Abstract

The Hauser-Feshbach Fission Fragment Decay (HF$^3$D) model is extended to calculate the prompt fission neutron spectrum (PFNS) for the thermal neutron induced fission on $^{235}$U, where the evaporated neutrons from all possible fission fragment pairs are aggregated. By studying model parameter sensitivities on the calculated PFNS, as well as non-statistical behavior of low-lying discrete level spin distribution, we conclude that discrepancies between the aggregation calculation and the experimental PFNS seen at higher neutron emission energies can be attributed to both the primary fission fragment yield distribution and the possible high spin states that are not predicted by the statistical theory of nuclear structure.