Late-time emission of prompt fission γ rays

The emission of prompt fission 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 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 beta decay, is analyzed. The time evolution of the average total gamma-ray energy, the average total gamma-ray multiplicity, and the fragment-specific gamma-ray spectra is presented in the case of neutron-induced fission reactions of U-235 and Pu-239, as well as spontaneous fission of Cf-252. The production of specific isomeric states is calculated and compared to available experimental data. About 7% of all prompt fission. rays are predicted to be emitted between 10 ns and 5 mu s following fission, in the case of U-235 and Pu-239 (nth, f) reactions, and up to 3% in the case of Cf-252 spontaneous fission. The cumulative average total 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.