The Rigorous Test of the Generalized Brink–Axel Hypothesis in the \(A=138\) Nuclear Mass Region

The generalized Brink–Axel (gBA) hypothesis suggests that the γ -ray strength function ( γ SF) of a nucleus only depends on the γ -ray energy, and not on the properties of the initial and final excitation energy levels between which the nucleus decays. This hypothesis has been tested in various studies and it is still controversial. In this study, the gBA hypothesis was tested in the A = 138 nuclear mass region by rigorously investigating the dependence of the γ SF of 138 La on both initial and final excitation energies. The results showed that the shape and absolute value of the γ SF are independent of the initial and final excitation energy. Therefore, the results of this work are in support of the generalized Brink–Axel hypothesis.