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

Abstract

We investigate the time evolution of prompt fission ${\hat{I}}^3$ 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 ${\hat{I}}^3$-ray energy spectrum as a function of time since scission, $\ddot{I}•(\hat{I}{\mu }_{\hat{I}}^3,t)$, the increase of the average ${\hat{I}}^3$-ray multiplicity over time, $N_{\hat{I}}^3(t)$, and its evolution in time as a function of the fission fragment mass, $N_{\hat{I}}^3(A,t)$. 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.