Isomeric transitions in ⁶⁹Se and the spin of the ground state

Isomeric transitions in ⁶⁹Se are investigated using charged particle-γ and γ-γ delayed coincidence measurements. A precise three-point angular distribution shows that the decay of the 9/2⁺ isomer is nearly isotropic, suggesting either significant dealignment during the lifetime of the isomer or a small E3 admixture in this transition. An improved measurement of the lifetime of the isomer yields a half-life of 960±23 ns. The single particle level structure of negative parity states in ⁶⁹Se below the 9/2⁺ isomer is investigated using a recoil shadowing technique. A 39.4 keV gamma ray with a total-internal conversion coefficient of 28.0±2.5 is observed deexciting the 5/2^- first excited state, which is found to be isomeric with a half-life of 2.0±0.2 μs. The observed lifetime, along with the internal conversion coefficient, suggests E2 multipolarity for this transition and strongly favors a spin and parity assignment of 1/2^- for the ⁶⁹Se ground state.