# Is the generalized Brink-Axel hypothesis valid?

## TL;DR

This paper experimentally verifies the generalized Brink-Axel hypothesis for gamma transitions in $^{238}$Np, showing it holds for the full dipole strength below the neutron separation energy, and discusses its validity in lighter systems with large fluctuations.

## Contribution

It provides experimental evidence supporting the generalized Brink-Axel hypothesis across a broad energy range and explores its applicability in lighter nuclei with significant fluctuations.

## Key findings

- gBA hypothesis verified for $^{238}$Np transitions
- Valid for full dipole strength below neutron separation energy
- Discusses applicability in lighter systems like $^{92}$Zr

## Abstract

Experimental results of the $^{237}$Np($d, p \gamma)^{238}$Np reaction are presented, which verifies the generalized Brink-Axel (gBA) hypothesis for $\gamma$ transitions between states in the quasi-continuum. The gBA hypothesis holds not only for specific collective resonances, but for the full dipole strength below the neutron separation energy. We discuss the validity of the gBA hypothesis also for lighter systems like $^{92}$Zr where the concept of a unique $\gamma$-ray strength function ($\gamma$SF) is problematic due to large Porter-Thomas fluctuations. Methods for studying the $\gamma$SF and the fluctuations as function of excitation energy are presented.