Two-hole structure outside ${}^{78}\mathrm{Ni}$: Existence of a $\mu \mathrm{s}$ isomer of ${}^{76}\mathrm{Co}$ and $\beta$ decay into ${}^{76}\mathrm{Ni}$

In the EURICA campaign aimed at exploration of the ${}^{78}\mathrm{Ni}$ region, an isomeric state of ${}^{76}\mathrm{Co}$ has been observed via $\gamma$-ray spectroscopy. The nuclei were produced by in-flight fission of a ${}^{238}\mathrm{U}$ beam at the Radioactive Isotope-Beam Factory. Two coincident $\gamma$ rays of $192.02\left(30\right)$ and $446.4\left(7\right)$ keV from the decay of a $t_{1/2}=2.96{(}_{25}^{29})\mu \mathrm{s}$ isomeric state of ${}^{76}\mathrm{Co}$ have been observed. The decay of the isomer was assigned to an $E1$ transition with a reduced transition probability of $B(E1;3^{+}\to 2^{-})=1.79\left(16\right)\times {10}^{-8}$ W.u. A $\beta$-decaying state with spin-parity $1^{-}$ and a half-life of $16\left(4\right)$ ms was also observed in the data, and the known state with a half-life of $22{(}_5^7)$ ms was assigned to have a spin-parity of $8^{-}$. Furthermore, the isomer of ${}^{76}\mathrm{Ni}$ has been remeasured to $547.8\left(33\right)$ ns giving a $B(E2;8^{+}\to 6^{+})$ value of $0.786\left(5\right)$ W.u. A new excited state at $2994.6\left(5\right)$ keV, decaying via a $\gamma$ ray of 2004.5(4) keV, has also been observed. This is in agreement with either of the predicted $0_2^{+}$ or $2_2^{+}$ states. These results are discussed in terms of the shell model and the interaction of the $\nu p_{1/2}$ and $\pi f_{7/2}$ orbitals.

Figure 1

Isomer-decay spectrum and the decay time histogram of the 990 keV $\gamma$-ray (inset) of ${}^{76}\mathrm{Ni}$. Measured energies of the observed transitions are shown over each $\gamma$-ray peak. The red line shows the fit of an exponential decay convoluted with a Gaussian time resolution.

Figure 2

Singles (top) and $\gamma \gamma$-gated (middle, bottom) isomer $\gamma$-ray spectra of ${}^{76}\mathrm{Co}$. The time distribution summed over the 192 keV and the 446 keV transitions in ${}^{76}\mathrm{Co}$ is shown in the inset. The red lines shows the fit of an exponential decay convoluted with a Gaussian time resolution. Measured energies of the observed transitions are shown over each $\gamma$-ray peak.

Figure 3

Spectra of $\gamma$-ray transitions in ${}^{76}\mathrm{Ni}$ following $\beta$ decay of ${}^{76}\mathrm{Co}$. The top panel shows the prompt component of the $\gamma$-ray spectrum and the middle panel shows the delayed component. The insets show the $\beta$-decay time-distribution gated on the prompt 990 keV (top) and delayed 355 keV (middle) $\gamma$ rays, respectively. Blue curves represent the exponential decay and magenta lines the constant background component of the total decay function, shown in red. Bottom panel shows the total $\gamma$-ray spectrum correlated with at least one of the two isomeric $\gamma$-ray transitions in ${}^{76}\mathrm{Co}$.

Figure 4

Proposed experimental level scheme of ${}^{76}\mathrm{Ni}$ (bottom left) and ${}^{76}\mathrm{Co}$ (top left) with two levels at $446.4\left(7\right)$ and $638.4\left(8\right)$ keV, relative to the lowest $1^{-}$ state, compared to Monte Carlo shell-model calculations [25] (bottom right) and shell-model calculations using a modified LNPS interaction (top right).