Two-hole structure outsideNi78: Existence of aμsisomer ofCo76andβdecay intoNi76

In the EURICA campaign aimed at exploration of the $^{78}\mathrm{Ni}$ region, an isomeric state of $^{76}\mathrm{Co}$ has been observed via $\ensuremath{\gamma}$-ray spectroscopy. The nuclei were produced by in-flight fission of a $^{238}\mathrm{U}$ beam at the Radioactive Isotope-Beam Factory. Two coincident $\ensuremath{\gamma}$ rays of $192.02(30)$ and $446.4(7)$ keV from the decay of a ${t}_{1/2}=2.96{(}_{25}^{29})\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ 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 $B(E1;{3}^{+}\ensuremath{\rightarrow}{2}^{\ensuremath{-}})=1.79(16)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ W.u. A $\ensuremath{\beta}$-decaying state with spin-parity ${1}^{\ensuremath{-}}$ and a half-life of $16(4)$ 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 ${8}^{\ensuremath{-}}$. Furthermore, the isomer of $^{76}\mathrm{Ni}$ has been remeasured to $547.8(33)$ ns giving a $B(E2;{8}^{+}\ensuremath{\rightarrow}{6}^{+})$ value of $0.786(5)$ W.u. A new excited state at $2994.6(5)$ keV, decaying via a $\ensuremath{\gamma}$ 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 $\ensuremath{\nu}{p}_{1/2}$ and $\ensuremath{\pi}{f}_{7/2}$ orbitals.