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

Abstract

In the EURICA campaign aimed at exploration of the Ni-78 region, an isomeric state of Co-76 has been observed via gamma-ray spectroscopy. The nuclei were produced by in-flight fission of a U-238 beam at the Radioactive Isotope-Beam Factory. Two coincident gamma rays of 192.02(30) and 446.4(7) keV from the decay of a t(1/2) = 2.96((29)(25)) mu s isomeric state of Co-76 have been observed. The decay of the isomer was assigned to an E1 transition with a reduced transition probability of B(E1; 3(+) -˃ 2(-)) = 1.79(16) x 10(-8) W.u. A beta-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((7)(5)) ms was assigned to have a spin-parity of 8(-). Furthermore, the isomer of Ni-76 has been remeasured to 547.8(33) ns giving a B(E2; 8(+) -˃ 6(+)) value of 0.786(5) W.u. A new excited state at 2994.6(5) 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.