Validity of the generalized Brink-Axel hypothesis in ²³⁸Np

الأصل · EN

We have analyzed primary γ-ray spectra of the odd-odd ²³⁸Np nucleus extracted from ²³⁷Np(d,pγ)²³⁸Np coincidence data measured at the Oslo Cyclotron Laboratory. The primary γ spectra cover an excitation-energy region of 0 ≤ Eᵢ ≤ 5.4 MeV, and allowed us to perform a detailed study of the γ-ray strength as function of excitation energy. Hence, we could test the validity of the generalized Brink-Axel hypothesis, which, in its strictest form, claims no excitation-energy dependence on the γ strength. In this work, using the available high-quality ²³⁸Np data, we show that the γ-ray strength function is to a very large extent independent on the initial and final states. Thus, for the first time, the generalized Brink-Axel hypothesis has been experimentally verified for γ transitions between states in the quasi-continuum region, not only for specific collective resonances, but also for the full strength below the neutron separation energy. Based on our findings, the necessary criteria for the generalized Brink-Axel hypothesis to be fulfilled are outlined.