Impact of the Brink Axel Hypothesis on Unique First Forbidden \b{eta} transitions for r process nuclei

TL;DR

This study evaluates how the Brink Axel hypothesis influences the calculation of beta decay and electron capture rates in neutron-rich nuclei relevant to the r process, highlighting potential impacts on nucleosynthesis modeling accuracy.

Contribution

It provides a microscopic assessment of the Brink Axel hypothesis's validity for unique first forbidden transitions in r process nuclei, using pn QRPA calculations.

Findings

BA hypothesis significantly affects calculated weak rates.

Differences observed between BA-based and microscopically calculated rates.

Implications for r process nucleosynthesis modeling accuracy.

Abstract

Key nuclear inputs for the astrophysical r process simulations are the weak interaction rates. Consequently, the accuracy of these inputs directly affects the reliability of nucleosynthesis modeling. Majority of the stellar rates, used in simulation studies, are calculated invoking the Brink Axel (BA) hypothesis. The BA hypothesis assumes that the strength functions of all parent excited states are the same as for the ground state, only shifted in energies. However, BA hypothesis has to be tested against microscopically calculated state by state rates. In this project we study the impact of the BA hypothesis on calculated stellar \b{eta} decay and electron capture rates. Our investigation include both Unique First Forbidden (U1F) and allowed transitions for 106 neutron rich trans iron nuclei ([27, 77] less than equal to [Z, A] less than equal to [82, 208]). The calculations were performed using the deformed proton-neutron quasiparticle randomphase approximation (pn QRPA) model with a simple plus quadrupole separable and schematic interaction. Waiting-point and several key r process nuclei lie within the considered mass region of the nuclear chart.