Late-time emission of prompt fission γ rays

Abstract

The emission of prompt fission $\ensuremath{\gamma}$ rays within a few nanoseconds to a few microseconds following the scission point is studied in the Hauser-Feshbach formalism applied to the deexcitation of primary excited fission fragments. Neutron and $\ensuremath{\gamma}$-ray evaporations from fully accelerated fission fragments are calculated in competition at each stage of the decay, and the role of isomers in the fission products, before $\ensuremath{\beta}$ decay, is analyzed. The time evolution of the average total $\ensuremath{\gamma}$-ray energy, the average total $\ensuremath{\gamma}$-ray multiplicity, and the fragment-specific $\ensuremath{\gamma}$-ray spectra is presented in the case of neutron-induced fission reactions of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$, as well as spontaneous fission of $^{252}\mathrm{Cf}$. The production of specific isomeric states is calculated and compared to available experimental data. About 7% of all prompt fission $\ensuremath{\gamma}$ rays are predicted to be emitted between 10 ns and 5 $\ensuremath{\mu}\mathrm{s}$ following fission, in the case of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}\phantom{\rule{4pt}{0ex}}({n}_{\mathrm{th}},f)$ reactions, and up to 3% in the case of $^{252}\mathrm{Cf}$ spontaneous fission. The cumulative average total $\ensuremath{\gamma}$-ray energy increases by 2% to 5% in the same time interval. Finally, those results are shown to be robust against significant changes in the model input parameters.