Persistence of the $Z=28$ shell gap in $A=75$ isobars: Identification of a possible $(1/2^{-})$ $\hat{I}¼$s isomer in ${}^{75}$Co and ${\hat{I}}^2$ 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 `southwest' 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 ${\hat{I}}^2$ 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 ${\hat{I}}^2$-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 in the RIKEN Nishina Center. Its isomeric and ${\hat{I}}^2$ decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial ${\hat{I}}^2$-decay spectra for ${}^{75}$Ni and ${}^{74}$Ni, and report a new isomeric transition in ${}^{75}$Co. The energy [$E_{\hat{I}}^3=1914(2)$ keV] and half-life [$t_{1/2}=13(6)$ $\hat{I}¼$s] of the delayed ${\hat{I}}^3$ ray lend support for the existence of a $J^{\ddot{I}}€=(1/2^{-})$ isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in ${}^{75}$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 $J^{\ddot{I}}€=(1/2^{-})$ isomer are necessary.

15 pages, 6 figures, 3 tables. Physical Review C