Two-hole structure outside 78Ni: Existence of a μ s isomer of 76Co and β decay into 76Ni

In the EURICA campaign aimed at exploration of the ⁷⁸Ni region, an isomeric state of ⁷⁶Co has been observed via γ -ray spectroscopy. The nuclei were produced by in-flight fission of a ²³⁸U beam at the Radioactive Isotope-Beam Factory. Two coincident γ rays of 192.02 (30 ) and 446.4 (7 ) keV from the decay of a t_{1 /2}=2.96 (25₂₉) μ s isomeric state of ⁷⁶Co have been observed. The decay of the isomer was assigned to an E 1 transition with a reduced transition probability of B (E 1 ;3⁺→2^-) =1.79 (16 ) ×10^-8 W.u. A β -decaying state with spin-parity 1^- and a half-life of 16 (4 ) ms was also observed in the data, and the known state with a half-life of 22 (5₇) ms was assigned to have a spin-parity of 8^-. Furthermore, the isomer of ⁷⁶Ni has been remeasured to 547.8 (33 ) ns giving a B (E 2 ;8⁺→6⁺) value of 0.786 (5 ) W.u. A new excited state at 2994.6 (5 ) keV, decaying via a γ ray of 2004.5(4) keV, has also been observed. This is in agreement with either of the predicted 0₂⁺ or 2₂⁺ states. These results are discussed in terms of the shell model and the interaction of the ν p_{1 /2} and π f_{7 /2} orbitals.