Radiative strength functions from the energy-localized Brink-Axel hypothesis

TL;DR

This paper introduces a shell model-based method for calculating radiative strength functions (RSFs) in nuclei, verifying its consistency and applying it to $^{56}$Fe to reveal energy-dependent behaviors relevant for nuclear reactions.

Contribution

It presents a novel energy-localized Brink-Axel hypothesis-based shell model approach for efficient RSF computation across all energies, including new results for $^{56}$Fe.

Findings

RSF shape in $^{56}$Fe evolves smoothly with excitation energy

Both M1 and E1 transitions significantly contribute below the photo-absorption threshold

Observed strength below 3 MeV cannot be fully reproduced within the sdpf model space

Abstract

Radiative strength functions (RSFs) model the bulk electromagnetic response of highly-excited nuclei and are critical inputs for statistical reaction codes. In this paper, we present a definition of the RSF that is consistent with Hauser-Feshbach reaction codes and that can be efficiently computed with the shell model using the Lanczos strength-function (LSF) method. We introduce a variant of the shell model LSF method that exploits the energy-localized Brink-Axel hypothesis, which makes it possible to compute both electric and magnetic RSFs across all energies relevant to capture reactions. We verify agreement with the conventional definition of RSFs with benchmark calculations of $^{24}$Mg, then present novel results for $^{56}$Fe. For $^{56}$Fe we find that: (i) the M1 RSF shape evolves smoothly with excitation energy, consistent with the energy-localized Brinkl-Axel hypothesis, (ii) both M1 and E1 transitions contribute significantly to the radiative strength below the photo-absorption threshold, and (iii) within the sdpf model space, the strength below 3 MeV observed in Oslo-type experiments cannot be fully reproduced. These results pave the way for a coherent microscopic description of the RSFs and further motivate the use of energy-dependent RSFs in modern reaction codes.