β decay of nuclei around 90Se: Search for signatures of a N=56 subshell closure relevant to the r process

Background: Nuclear structure plays a significant role on the rapid neutron capture process (r process) since shapes evolve with the emergence of shells and subshells. There was some indication in neighboring nuclei that we might find examples of a new N = 56 subshell, which may give rise to a doubly magic Se-90(34)56 nucleus. Purpose: beta-decay half-lives of nuclei around Se-90 have been measured to determine if this nucleus has in fact a doubly magic character. Method: The fragmentation of a 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. Results: We have measured the half-lives of 22 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 subshell, 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. Conclusions: 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 subshell 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.