Persistence of the ${Z=28}$ shell gap in ${A=75}$ isobars: Identification of a possible ${(1/2^-)}$ $μ$s isomer in ${^{75}}$Co and $β$ decay to ${^{75}}$Ni

Abstract

Background: The evolution of shell structure around doubly-magic exotic nuclei is of great interest in nuclear physics and astrophysics. In the 'south-west' region of 78 Ni, 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 β decay of 75 Co, with one proton-and two neutron-holes relative to 78 Ni, to access new nuclear structure information in 75 Co and its β-decay daughters 75 Ni and 74 Ni.Methods: The nucleus 75 Co is produced in relativistic in-flight fission reactions of 238 U at the Radioactive Ion Beam Factory (RIBF) in the RIKEN Nishina Center. Its isomeric and β decay are studied exploiting the BigRIPS and EURICA setups.Results: We obtain partial β-decay spectra for 75 Ni and 74 Ni, and report a new isomeric transition in 75 Co. The energy (Eγ = 1914(2) keV) and half-life (t 1/2 = 13(6) µs) of the delayed γ ray lend support for the existence of a J π = (1/2 − ) isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides good account for the observed states in 75 Ni, but the first calculated 1/2 − level in 75 Co, a prolate K = 1/2 state, is predicted about 1 MeV below the observed (1/2 − ) level.Conclusions: The spherical-like structure of the lowest-lying excited states in 75 Ni is proved. In the case of 75 Co, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below 78 Ni. Further experimental efforts to discern the nature of the J π = (1/2 − ) isomer are necessary.