Time Evolution of Prompt Gamma-Ray Emission in $^{252}$Cf(sf) and $^{233,235}$U($n$,f) Reactions

TL;DR

This study analyzes the time-dependent prompt gamma-ray emission in fission reactions of uranium and californium isotopes, comparing model calculations with experimental data to understand fragment isomers and gamma-ray characteristics.

Contribution

It introduces a detailed analysis of gamma-ray emission timing and spectra in fission, highlighting the role of isomers and testing nuclear level models with new data.

Findings

Gamma-ray spectra evolve significantly over time after fission.

The average gamma-ray multiplicity increases with time post-scission.

Discrepancies in isomeric ratios reveal gaps in nuclear level knowledge.

Abstract

We investigate the time evolution of prompt fission $\gamma$ emission due to the presence of ns to ms isomers in the fragments produced in the neutron-induced fission of $^{233,235}$U and in the spontaneous fission of $^{252}$Cf. Calculations performed with the CGMF fission event generator are compared with recent experimental data on $^{252}$Cf(sf) and $^{235}$U($n$,f) obtained with the DANCE+NEUANCE setup at the Los Alamos Neutron Science Center. Of particular interest are the average $\gamma$-ray energy spectrum as a function of time since scission, $\phi(\epsilon_\gamma,t)$, the increase of the average $\gamma$-ray multiplicity over time, $N_\gamma(t)$, and its evolution in time as a function of the fission fragment mass, $N_\gamma(A,t)$. The time evolution of isomeric ratios in post-neutron emission fission fragments can be defined and used to test and reveal some deficiencies in our knowledge of the low-lying levels of neutron-rich nuclei produced in fission reactions.