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

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 Ni78, 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 Co75, with one-proton and two-neutron holes relative to Ni78, to access new nuclear structure information in Co75 and its β-decay daughters Ni75 and Ni74. Methods: The nucleus Co75 is produced in relativistic in-flight fission reactions of U238 at the Radioactive Ion Beam Factory 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 Ni75 and Ni74, and report a new isomeric transition in Co75. The energy [Eγ=1914(2)keV] and half-life [t1/2=13(6)μs] of the delayed γ ray lend support for the existence of a Jπ=(1/2-) isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in Ni75, but the first calculated 1/2- level in Co75, 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 Ni75 is proved. In the case of Co75, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below Ni78. Further experimental efforts to discern the nature of the Jπ=(1/2-) isomer are necessary.