The stable and neutron-rich isotopes ${}^{168}\mathrm{Er}$, ${}^{170}\mathrm{Er}$, and ${}^{172}\mathrm{Er}$ have been studied with Gammasphere using inelastic excitation with energetic ${}^{136}\mathrm{Xe}$ beams. The previously assigned structures based on the proposed $K^{\pi }=4^{-}$ isomeric intrinsic states in both ${}^{168}\mathrm{Er}$ and ${}^{170}\mathrm{Er}$ have been re-evaluated and an equivalent band identified in ${}^{172}\mathrm{Er}$. In ${}^{170}\mathrm{Er}$, the identification of a $K^{\pi }=6^{-}$ band with transitions close in energy to those of the $4^{-}$ band leads to a modified interpretation, since the overlap would have compromised previous analyses. The $g_K-g_R$ values for the $4^{-}$ bands deduced from the in-band $\gamma$-ray intensities for the sequence of isotopes suggest a predominantly two-neutron configuration in ${}^{168}\mathrm{Er}$, an equally mixed two-neutron, two-proton configuration in ${}^{170}\mathrm{Er}$, and a two-proton configuration in ${}^{172}\mathrm{Er}$. A comprehensive decay scheme for the previously proposed $6^{+}$ isomer in ${}^{172}\mathrm{Er}$ has also been established, as well as band structures built on this isomer that closely resemble the $6^{+}$ and $7^{-}$ two-neutron structures known in the isotone ${}^{174}\mathrm{Yb}$. The implied $K$ hindrances are discussed. The main decay path of the $6^{+}$ isomer occurs through the newly identified $4^{-}$ isomer. The measured lifetimes of the $4^{-}$ and $6^{+}$ isomers in ${}^{172}\mathrm{Er}$ are 57(3) and 822(90) ns, respectively. Multiquasiparticle calculations support the suggested configuration changes across the isotopic chain.

• Received 2 March 2010

DOI:https://doi.org/10.1103/PhysRevC.81.054313