Comprehensive test of the Brink-Axel hypothesis in the energy region of the pygmy dipole resonance

TL;DR

This study rigorously tests the Brink-Axel hypothesis in tin isotopes near the pygmy dipole resonance energy region using three experimental methods, confirming its validity for heavy nuclei.

Contribution

It provides the first comprehensive experimental validation of the Brink-Axel hypothesis in the energy region of the pygmy dipole resonance for heavy nuclei.

Findings

Gamma-ray strength functions are consistent across methods.

GSFs are independent of excitation energy and spin.

Results confirm the hypothesis's applicability in the studied energy region.

Abstract

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The $\gamma$-ray strength functions (GSFs) extracted from primary $\gamma$-decay spectra following charged-particle reactions with the Oslo method and with the Shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance.