States above the known $K^{\pi }=8^{-}$ 6 $\mu$s isomer in ${}^{138}\mathrm{Gd}$ have been populated with the ${}^{106}\mathrm{Cd}$(${}^{36}\mathrm{Ar}$,$2p2n$) reaction at a beam energy of 180 MeV at the University of Jyväskylä, Finland. The recoil-isomer tagging technique was utilized to correlate delayed $\gamma$-ray decays, detected in the GREAT focal plane spectrometer, with prompt decays measured in the JUROGAM II spectrometer at the target position. The lifetime of the $K^{\pi }=8^{-}$ isomeric state has been remeasured as 6.2(2) $\mu$s. Two high-lying strongly coupled bands have been established with $K^{\pi }⩾{12}^{-}$. Potential-energy surface calculations, in conjunction with $g$ factor measurements, reveal that they are built upon four-quasiparticle structures comprising two-quasineutron plus two-quasiproton configurations. The short half-life or lack of hindrance for the decays from these four-quasiparticle band-head states is reasoned to be a consequence of increased triaxial deformation and mixing due to the high density of states relative to the lower two-quasiparticle 6-$\mu$s isomeric state.

• Received 12 November 2010

DOI:https://doi.org/10.1103/PhysRevC.83.034311