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 ${}^{233,235}\mathrm{U}$ and in the spontaneous fission of ${}^{252}\mathrm{Cf}$. Calculations performed with the cgmf fission event generator are compared with recent experimental data on ${}^{252}\mathrm{Cf}\left(\mathrm{sf}\right)$ and ${}^{235}\mathrm{U}(n,\mathrm{f})$ 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, $\varphi ({\epsilon }_{\gamma },t)$; the increase of the average $\gamma$-ray multiplicity over time, ${\overline{N}}_{\gamma }\left(t\right)$; and its evolution in time as a function of the fission fragment mass, ${\overline{N}}_{\gamma }$($\mathrm{A},t$). The time evolution of isomeric ratios in postneutron 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.

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• Fission
• Nuclear reactions
• Nucleon induced nuclear reactions
• A ≥ 220