Low-energy enhancement and fluctuations of γ -ray strength functions in 56,57 Fe: test of the Brink–Axel hypothesis

Nuclear level densities and γ -ray strength functions of 56,57 Fe have been extracted from proton- γ coincidences. A low-energy enhancement in the γ -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 γ 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 γ -ray strength function, in support of the generalized Brink–Axel hypothesis.