β decay of 81Zn and migrations of states observed near the N=50 closed shell

The beta decay of the N = 51 nucleus Zn-81 was studied by means of beta-gamma spectroscopy and from isotopically pure beams produced at the Holifield Radioactive Ion Beam Facility (HRIBF). We observe several competing beta transitions populating Ga-81 that are interpreted as allowed Gamow-Teller decays to positive parity, core excited states, and first-forbidden decays to negative parity states. The measured beta-decay pattern suggests an assignment of I-pi = 5/2(+) for the Zn-81 ground state. The systematics of core excited states in N = 50 isotones indicate a strengthening of the N = 40 subshell gap near Ni-78.