Revisiting the role of the $(n,\gamma f)$ process in the low-energy fission of $^{235}$U and $^{239}$Pu

TL;DR

This paper reviews the $(n, ext{}\gamma f)$ process in low-energy fission of $^{235}$U and $^{239}$Pu, explaining observed fluctuations and the role of M1 transitions, with implications for nuclear reaction modeling.

Contribution

It provides a modern analysis of the $(n, ext{ }\gamma f)$ process, highlighting the significance of M1 transitions and low-lying modes in fission, extending understanding to fast neutron energies.

Findings

M1 transitions significantly contribute to pre-fission gamma spectra.

Fluctuations in neutron multiplicity and gamma energy are explained by transition state probabilities.

$(n, ext{ }\gamma f)$ corrections impact up to 3 ext% of total fission cross section.

Abstract

The $(n,\gamma f)$ process is reviewed in light of modern nuclear reaction calculations in both slow and fast neutron-induced fission reactions on $^{235}$U and $^{239}$Pu. Observed fluctuations of the average prompt fission neutron multiplicity and average total $\gamma$-ray energy below 100 eV incident neutron energy are interpreted in this framework. The surprisingly large contribution of the M1 transitions to the pre-fission $\gamma$-ray spectrum of $^{239}$Pu is explained by the dominant fission probabilities of 0$^+$ and $2^+$ transition states, which can only be accessed from compound nucleus states formed by the interaction of $s$-wave neutrons with the target nucleus in its ground state, and decaying through M1 transitions. The impact of an additional low-lying M1 scissors mode in the photon strength function is analyzed. We review experimental evidence for fission fragment mass and kinetic energy fluctuations in the resonance region and their importance in the interpretation of experimental data on prompt neutron data in this region. Finally, calculations are extended to the fast energy range where $(n,\gamma f)$ corrections can account for up to 3\% of the total fission cross section and about 20\% of the capture cross section.