Persistence of the ${Z=28}$ shell gap in ${A=75}$ isobars: Identification of a possible ${(1/2^-)}$ ${\mu}$s isomer in ${^{75}}$Co and ${\beta}$ decay to ${^{75}}$Ni
Résumé
Background: The evolution of shell structure around doubly magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the ‘southwest’ region of , the development of deformation might trigger a major shift in our understanding of explosive nucleosynthesis. To this end, new spectroscopic information on key close-lying nuclei is very valuable.
Purpose: We intend to measure the isomeric and decay of , with one-proton and two-neutron holes relative to , to access new nuclear structure information in and its -decay daughters and .
Methods: The nucleus is produced in relativistic in-flight fission reactions of at the Radioactive Ion Beam Factory in the RIKEN Nishina Center. Its isomeric and decay are studied exploiting the BigRIPS and EURICA setups.
Results: We obtain partial -decay spectra for and , and report a new isomeric transition in . The energy and half-life of the delayed ray lend support for the existence of a isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in , but the first calculated level in , a prolate state, is predicted about 1 MeV below the observed level.
Conclusions: The spherical-like structure of the lowest-lying excited states in is proved. In the case of , the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below . Further experimental efforts to discern the nature of the isomer are necessary.