Magicity of the Ni-68 semidouble-closed-shell nucleus probed by Gamow-Teller decay of the odd-A neighbors

The particle-hole excitations through the N = 40 subshell around Ni-68 have been studied by the beta decay of Co-69 and Ni-69. The half-life of Co-69 was measured to be 0.22(2) s, and a new beta-decaying isomer with a half-li fe of 3.5(5) s was identified in Ni-69. From the decay of the Ni-69 isomer a 9(4)% mixing of the pi p(3/2)(+1)nu p(1/2)(-2)nu g(9/2)(+2) configuration into the ground state of Cu-69 can be deduced. Significant polarization of the Ni-68 core nucleus is observed with the coupling of a single nucleon, which implies a rapid decrease in the stabilizing effect of the N = 40 semi magic shell gap.