Two-quasiparticle structures and isomers in Er168, Er170, and Er172

The stable and neutron-rich isotopes Er168, Er170, and Er172 have been studied with Gammasphere using inelastic excitation with energetic Xe136 beams. The previously assigned structures based on the proposed Kπ=4^- isomeric intrinsic states in both Er168 and Er170 have been re-evaluated and an equivalent band identified in Er172. In Er170, the identification of a Kπ=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 gK-gR values for the 4^- bands deduced from the in-band γ-ray intensities for the sequence of isotopes suggest a predominantly two-neutron configuration in Er168, an equally mixed two-neutron, two-proton configuration in Er170, and a two-proton configuration in Er172. A comprehensive decay scheme for the previously proposed 6⁺ isomer in Er172 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 Yb174. 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 Er172 are 57(3) and 822(90) ns, respectively. Multiquasiparticle calculations support the suggested configuration changes across the isotopic chain.