Beta-decay of nuclei around Se-90. Search for signatures of a N=56 sub-shell closure relevant the r-process

TL;DR

This study measured beta-decay half-lives of nuclei around Se-90 to investigate the existence of an N=56 sub-shell closure and its implications for the r-process nucleosynthesis, finding no evidence for a doubly-magic Se-90.

Contribution

First measurements of half-lives for nuclei near Se-90, providing insights into nuclear structure and the role of N=56 sub-shell in the r-process.

Findings

Se-90 does not exhibit a closed N=56 sub-shell.

Measured half-lives align with QRPA model predictions.

Impact on understanding heavy element synthesis in astrophysics.

Abstract

Nuclear structure plays a significant role on the rapid neutron capture process (r-process) since shapes evolve with the emergence of shells and sub-shells. There was some indication in neighboring nuclei that we might find examples of a new N=56 sub-shell, which may give rise to a doubly magic Se-90 nucleus. Beta-decay half lives of nuclei around Se-90 have been measured to determine if this nucleus has in fact a doubly-magic character. The fragmentation of Xe-136 beam at the National Superconducting Cyclotron Laboratory at Michigan State University was used to create a cocktail of nuclei in the A=90 region. We have measured the half lives of twenty-two nuclei near the r-process path in the A=90 region. The half lives of As-88 and Se-90 have been measured for the first time. The values were compared with theoretical predictions in the search for nuclear-deformation signatures of a N=56 sub-shell, and its possible role in the emergence of a potential doubly-magic Se-90. The impact of such hypothesis on the synthesis of heavy nuclei, particularly in the production of Sr, Y and Zr elements was investigated with a weak r-process network. The new half lives agree with results obtained from a standard global QRPA model used in r-process calculations, indicating that Se-90 has a quadrupole shape incompatible with a closed N=56 sub-shell in this region. The impact of the measured Se-90 half-life in comparison with a former theoretical predication associated with a spherical half-life on the weak-r-process is shown to be strong.