Low-energy enhancement and fluctuations of ${\hat{I}}^3$-ray strength functions in ${}^{56,57}$Fe: test of the Brink-Axel hypothesis

Abstract

Nuclear level densities and ${\hat{I}}^3$-ray strength functions of ${}^{56,57}$Fe have been extracted from proton-${\hat{I}}^3$ coincidences. A low-energy enhancement in the ${\hat{I}}^3$-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 ${\hat{I}}^3$ 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 ${\hat{I}}^3$-ray strength function, in support of the generalized Brink-Axel hypothesis.

24 pages, 17 figures, accepted for publication in J. Phys. G: Nucl. Phys; Special Issue