Isomeric transitions inSe69and the spin of the ground state

Isomeric transitions in ${}_{}{}^{69}{}_{}{}^{}\mathrm{Se}$ 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 ${}_{}{}^{69}{}_{}{}^{}\mathrm{Se}$ 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/$2^{\mathrm{-}}$ 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/$2^{\mathrm{-}}$ for the ${}_{}{}^{69}{}_{}{}^{}\mathrm{Se}$ ground state.