Low-energy enhancement and fluctuations of $\gamma$-ray strength functions in $^{56,57}$Fe: test of the Brink-Axel hypothesis

TL;DR

This study confirms a significant low-energy enhancement in the gamma-ray strength functions of $^{56,57}$Fe, supports the Brink-Axel hypothesis, and provides detailed experimental insights into their dipole nature and excitation-energy independence.

Contribution

The paper provides experimental validation of the low-energy gamma-ray strength enhancement and tests the Brink-Axel hypothesis in $^{56,57}$Fe using high-resolution measurements.

Findings

Low-energy gamma-ray strength enhancement up to 30 times theoretical models.

Dipole nature of the low-energy enhancement confirmed by angular distributions.

No significant excitation-energy dependence of gamma-ray strength observed.

Abstract

Nuclear level densities and $\gamma$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$\gamma$ coincidences. A low-energy enhancement in the $\gamma$-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in $^{57}$Fe indicate its dipole nature, in agreement with findings for $^{56}$Fe. The high statistics and the excellent energy resolution of the large-volume LaBr$_{3}$(Ce) detectors allowed for a thorough analysis of $\gamma$ strength as function of excitation energy. Taking into account the presence of strong Porter-Thomas fluctuations, there is no indication of any significant excitation-energy dependence in the $\gamma$-ray strength function, in support of the generalized Brink-Axel hypothesis.