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

S. Escrig , A.I. Morales , S. Nishimura , M. Niikura , A. Poves , Z.Y. Xu , G. Lorusso , F. Browne , P. Doornenbal , G. Gey (1, 2) , H.-S. Jung , Z. Li , P.-A. Söderström , T. Sumikama , J. Taprogge , Zs. Vajta , H. Watanabe , J. Wu , A. Yagi , K. Yoshinaga , H. Baba , S. Franchoo (3) , T. Isobe , P.R. John , I. Kojouharov , S. Kubono , N. Kurz , I. Matea (3) , K. Matsui , D. Mengoni , P. Morfouace (3) , D.R. Napoli , F. Naqvi , H. Nishibata , A. Odahara , E. Sahin , H. Sakurai , H. Schaffner , I.G. Stefan (3) , D. Suzuki (3) , R. Taniuchi , V. Werner , D. Sohler

1 LPSC - Laboratoire de Physique Subatomique et de Cosmologie
2 ILL - Institut Laue-Langevin
3 IJCLab - Laboratoire de Physique des 2 Infinis Irène Joliot-Curie

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 ${}^{78}\mathrm{Ni}$, 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 $\beta$ decay of ${}^{75}\mathrm{Co}$, with one-proton and two-neutron holes relative to ${}^{78}\mathrm{Ni}$, to access new nuclear structure information in ${}^{75}\mathrm{Co}$ and its $\beta$-decay daughters ${}^{75}\mathrm{Ni}$ and ${}^{74}\mathrm{Ni}$. Methods: The nucleus ${}^{75}\mathrm{Co}$ is produced in relativistic in-flight fission reactions of ${}^{238}\mathrm{U}$ at the Radioactive Ion Beam Factory in the RIKEN Nishina Center. Its isomeric and $\beta$ decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial $\beta$-decay spectra for ${}^{75}\mathrm{Ni}$ and ${}^{74}\mathrm{Ni}$, and report a new isomeric transition in ${}^{75}\mathrm{Co}$. The energy $[E_{\gamma }=1914\left(2\right)\mathrm{keV}]$ and half-life $[t_{1/2}=13\left(6\right)\mu \mathrm{s}]$ of the delayed $\gamma$ ray lend support for the existence of a $J^{\pi }=(1/2^{-})$ isomeric state at 1914(2) keV. A comparison with PFSDG-U shell-model calculations provides a good account for the observed states in ${}^{75}\mathrm{Ni}$, but the first calculated $1/2^{-}$ level in ${}^{75}\mathrm{Co}$, 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 ${}^{75}\mathrm{Ni}$ is proved. In the case of ${}^{75}\mathrm{Co}$, the results suggest that the dominance of the spherical configurations over the deformed ones might be stronger than expected below ${}^{78}\mathrm{Ni}$. Further experimental efforts to discern the nature of the $J^{\pi }=(1/2^{-})$ isomer are necessary.

Domaines: Physique Nucléaire Expérimentale [nucl-ex]

S. Escrig, A.I. Morales, S. Nishimura, M. Niikura, A. Poves, et al.. 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. Phys.Rev.C, 2021, 103 (6), pp.064328. ⟨10.1103/PhysRevC.103.064328⟩. ⟨hal-03136679⟩