The Hauser-Feshbach fission-fragment decay $\left(\mathrm{HF}{}^3\mathrm{D}\right)$ model is extended to calculate the prompt fission neutron spectrum (PFNS) for the thermal-neutron-induced fission on ${}^{235}\mathrm{U}$, where the evaporated neutrons from all possible fission-fragment pairs are aggregated. By studying model parameter sensitivities on the calculated PFNS as well as nonstatistical 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.

• Received 2 April 2021
• Accepted 25 June 2021

DOI:https://doi.org/10.1103/PhysRevC.104.014611