Late Time Emission of Prompt Fission Gamma Rays

TL;DR

This study models the late-time emission of prompt fission gamma rays using the Hauser-Feshbach formalism, revealing detailed time evolution and the role of isomers in neutron-induced and spontaneous fission of actinides.

Contribution

It introduces a comprehensive theoretical framework for analyzing late-time prompt gamma-ray emission and isomer production in fission fragments, validated against experimental data.

Findings

7% of prompt gamma rays emitted between 10 ns and 5 μs for $^{235}$U and $^{239}$Pu.

Up to 3% of prompt gamma rays emitted in the same interval for $^{252}$Cf.

Total gamma-ray energy increases by 2-5% during the late emission period.

Abstract

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, average total $\gamma$-ray multiplicity, and fragment-specific $\gamma$-ray spectra, is presented in the case of neutron-induced fission reactions of $^{235}$U and $^{239}$Pu, as well as spontaneous fission of $^{252}$Cf. The production of specific isomeric states is calculated and compared to available experimental data. About 7% of all prompt fission $\gamma$ rays are predicted to be emitted between 10 nsec and 5 $\mu$sec following fission, in the case of $^{235}$U and $^{239}$Pu $(n_{\rm th},f)$ reactions, and up to 3% in the case of $^{252}$Cf 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.