Modification of the Brink-Axel Hypothesis for High Temperature Nuclear Weak Interactions

TL;DR

This paper investigates the validity of the Brink-Axel hypothesis for electron capture strength distributions in nuclei at high temperatures, revealing its limitations at low to moderate excitation energies and potential implications for supernova modeling.

Contribution

It demonstrates the failure of the Brink-Axel hypothesis at certain excitation energies and suggests a modified approach could improve supernova collapse simulations.

Findings

Brink-Axel hypothesis fails at low/moderate excitation energies in A=28 nuclei.

At high excitation, the hypothesis may be valid.

Electron capture strength may shift to lower energies, increasing capture rates.

Abstract

We present shell model calculations of electron capture strength distributions in A=28 nuclei and computations of the corresponding capture rates in supernova core conditions. We find that in these nuclei the Brink-Axel hypothesis for the distribution of Gamow-Teller strength fails at low and moderate initial excitation energy, but may be a valid tool at high excitation. The redistribution of GT strength at high initial excitation may affect capture rates during collapse. If these trends which we have found in lighter nuclei also apply for the heavier nuclei which provide the principal channels for neutronization during stellar collapse, then there could be two implications for supernova core electron capture physics. First, a modified Brink-Axel hypothesis could be a valid approximation for use in collapse codes. Second, the electron capture strength may be moved down significantly in transition energy, which would likely have the effect of increasing the overall electron capture rate during stellar collapse.