β decaying T_1/2=3.4 s isomer in ⁶⁹Ni

A radioactive beam of ⁶⁹Ni was produced by the fragmentation of a 70 MeV/nucleon ⁷⁶Ge beam in a Be target. β-delayed γ-ray studies were performed using two thin plastic scintillators and two large-volume Ge detectors following implantation of the ⁶⁹Ni nuclei into a foil of a collection wheel apparatus. A 1296-keV γ-ray transition with a half-life of 3.4(7) s was identified and has been attributed to the decay of the νp_1/2^-1 isomeric state in ⁶⁹Ni. The relative population of the low-spin J^π=1/2^- isomer to the known high-spin J^π=(17/2^-) isomer was determined to be 6:1 for the production of ⁶⁹Ni via fragmentation of ⁷⁶Ge, based on an upper limit of 36% extracted for the ⁶⁹Ni^m1 β-decay branch to the 3/2^- ground state of ⁶⁹Cu. The half-life and branching of the ⁶⁹Ni^m1 β decay is discussed in light of possible two particle–two hole excitations in the low-energy structure of ⁶⁹Cu.