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

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 $\beta$ 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 $\beta$-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 $\beta$ decay are studied exploiting the BigRIPS and EURICA setups. Results: We obtain partial $\beta$-decay spectra for ${}^{75}$Ni and ${}^{74}$Ni, and report a new isomeric transition in ${}^{75}$Co. The energy [$E_{\gamma }=1914(2)$ keV] and half-life [$t_{1/2}=13(6)$ $\mu$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 ${}^{75}$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 $J^{\pi }=(1/2^{-})$ isomer are necessary.

arXiv comment15 pages, 6 figures, 3 tables. Physical Review C

References in corpus (27)

• The Shell Model as Unified View of Nuclear Structure
• Evolution of shell structure in exotic nuclei
• Identification of 45 New Neutron-Rich Isotopes Produced by In-Flight Fission of a 238U Beam at 345 MeV/nucleon
• Identification and Separation of Radioactive Isotope Beams by the BigRIPS Separator at the RIKEN RI Beam Factory
• Quantum Phase Transition in the Shape of Zr isotopes
• Novel shape evolution in exotic Ni isotopes and configuration-dependent shell structure
• ${}^{78}$Ni revealed as a doubly magic stronghold against nuclear deformation
• Half Life of the Doubly-magic r-Process Nucleus 78Ni
• Shape Coexistence in 78 Ni and the new Island of Inversion
• Half-lives and branchings for β-delayed neutron emission for neutron-rich Co-Cu isotopes in the r-process
• Partial conservation of seniority and nuclear isomerism
• Seniority conservation and seniority violation in the g_{9/2} shell
• Underlying structure of collective bands and self-organization in quantum systems
• Beta decay of deformed r-process nuclei near A = 80 and A= 160, including odd-A and odd-odd nuclei, with the Skyrme finite-amplitude method
• Shell evolution of $N=40$ isotones towards ${}^{60}$Ca: First spectroscopy of ${}^{62}$Ti
• Partial conservation of seniority in $j=9/2$ shell: Analytic and numerical studies
• Enhanced collectivity in 74Ni
• Energy expressions for n=3 and 4 systems in a single-j shell
• Is Seniority a Partial Dynamic Symmetry in the First $\nu g_{9/2}$ Shell?
• Simultaneous investigation of the $\mathbf{T}=1({\mathbf{J}}^{\pi }=0^{+})$ and $\mathbf{T}=0({\mathbf{J}}^{\pi }=9^{+})$ $\beta$ decays in ${}^{70}$Br
• Partial conservation of seniority and its unexpected influence on E2 transitions in $g_{9/2}$ nuclei
• Beta decay of the very neutron-deficient ${}^{60}$Ge and ${}^{62}$Ge nuclei
• Beta decay of 71,73Co; probing single particle states approaching doubly magic 78Ni
• Relations Between Coefficients of Fractional Parentage
• Absence of Low-Energy Shape Coexistence in ${}^{80}$Ge: The Nonobservation of a Proposed Excited 0${}_2^{+}$ Level at 639 keV
• The structure of 70Fe: Single-particle and collective degrees of freedom
• Probing the role of proton cross-shell excitations in 70Ni using nucleon knockout reactions

Cited by in corpus (2)

• Seniority and $(\frac{9}{2}{)}^n$ configurations in neutron-rich Nickel isotopes
• Partial conservation of seniority in semi-magic nuclei