Decay of a New Isomer inTb154to High-Spin Levels inGd154

Radioactive sources of ${}_{}{}^{154}{}_{}{}^{}\mathrm{Tb}$ made by the ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}(\alpha ,3n)$ and ${}_{}{}^{155}{}_{}{}^{}\mathrm{Gd}\mathrm{}(p,2n)\mathrm{}$ reactions have been studied with Ge(Li) and Si(Li) detectors. A new isomer ($T_{\frac{1}{2}}=22.5\mathrm{}\pm 1.5$ h; spin and parity probably $7^{-}$) has been found in ${}_{}{}^{154}{}_{}{}^{}\mathrm{Tb}$. This state decays primarily to an $I^{\pi }K=7^{-}7$ isomeric state at 2137.8 keV in ${}_{}{}^{154}{}_{}{}^{}\mathrm{Gd}$. Quasiparticle configurations involving the ${\frac{11}{2}}^{-}\mathrm{}\left[505\right]\mathrm{}$ neutron orbital are proposed for both of these isomeric states, and retardation factors are determined for transitions from the 2137.8-keV state.