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

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.

Figure 1

Three-dimensional cluster plot of the nuclei implanted in WAS3ABi as a function of their charge $Z$ and mass-to-charge ratio $A/Q$. The cluster corresponding to ${}^{75}\mathrm{Co}$ is indicated by the red arrow.

Figure 2

Left: Energy-time matrix showing the $\gamma$ rays detected after implantation of ${}^{75}\mathrm{Co}$. The $\gamma$-ray energy is plotted against the ion-$\gamma$ time difference. Top right: Projection of the matrix on the energy axis for a time window of $\approx 35\mu \mathrm{s}$. Bottom right: Projection of the matrix on the time axis, gated on the 1914-keV transition. The fit to a single exponential function is shown in red. The resulting half-life is indicated. See text for details.

Figure 3

The $\beta$-delayed $\gamma$-ray energy spectrum following implantations of ${}^{75}\mathrm{Co}$ nuclei during a time interval of 135 ms. The panels present two different ranges of the $y$ axis to facilitate the observation of weak $\gamma$ rays. The transitions assigned to the $\beta$ (${}^{75}\mathrm{Ni}$) and ${\beta }_n$ (${}^{74}\mathrm{Ni}$) daughters are marked in bold and italics, respectively. Expanded inset spectra are shown for the $\gamma$ rays marked with a dagger.

Figure 4

From top to bottom, $\beta \left(\gamma \gamma \right)$ coincidence spectra gated on the 232-, 1045-, and 738-keV $\gamma$ transitions.

Figure 5

Experimental isomeric and $\beta$-decay level schemes of ${}^{75}\mathrm{Co}$ extracted from the present work. The energies of the levels are given in keV. The thickness of the arrows is proportional to the intensities of the transitions connecting the states. Spins and parities, apparent $\beta$ feedings and $\log ft$ values are indicated at the sides of the levels. Theoretical states, obtained with the PFSDG-U interaction [14, 55], are indicated in red and blue. The half-life of ${}^{75}\mathrm{Co}$ is reported in Ref. [2] while the $Q_{\beta }$, $Q_{\beta n}$, and $S_n$ values are taken from Ref. [68]. See text for details.

Figure 6

Evolution of the experimental (black) and theoretical (red, blue, and green) levels discussed here for the odd-mass Co isotopes occupying the $\nu g_{9/2}$ shell. The experimental states are taken from [12] and the present work and the theoretical ones are calculated with the PFSDG-U interaction [14, 55]. The left panel shows states with a spherical-like structure and the right one shows the lowest-lying levels of the $K=1/2$ deformed band.