Structure of the N=Z+1 nucleus ⁶⁹Se

Charged-particle-γ and charged-particle-neutron-γ coincidences have been used to identify in-beam γ-ray transitions in ⁶⁹Se produced in the reaction ⁴⁰Ca(³²S,2pn)⁶⁹Se. Charged-particle-γ-γ coincidences, neutron-γ-γ coincidences, and neutron-gated γ-ray angular distributions were used to deduce the level scheme. A long-lived isomer attributed to the g_9/2 state was found; its half-life was measured with charged-particle-γ timing as 1023±110 ns. The presence of strong ΔJ=1 and ΔJ=2 transitions in the band built on the νg_9/2 neutron single-particle state and the measured sign of the E2/M1 mixing ratio for the (11/2⁺→(9/2⁺ transition are consistent with a theoretically predicted oblate shape for this nucleus.