The level structure of ${}_{}{}^{139}{}_{}{}^{}\mathrm{Pm}$ has been investigated. The yrast band, based on a decoupled $h_{\frac{11}{2}}$ proton orbital, was observed up to $J^{\pi }={\frac{59}{2}}^{-}$. A sharp backbend with a surprisingly large gain in alignment $\Delta i=14\mathrm{}\hslash$ was observed at a frequency $\hslash \omega =0.43\mathrm{}$ MeV. This large increase in alignment, which is twice that observed in the ${}_{}{}^{137}{}_{}{}^{}\mathrm{Pm}$ isotope, is due to the simultaneous alignment of both a pair of $h_{\frac{11}{2}}$ protons and $h_{\frac{11}{2}}$ neutrons. A $\Delta J=1\mathrm{}$ sideband based on the neutron-aligned configuration $\pi h_{\frac{11}{2}}\otimes {\left(\nu h_{\frac{11}{2}}\right)}^2$ with a large negative $\gamma$ deformation ($\gamma \sim -60\mathrm{}°$) was also observed. The observation of both $\pi h_{\frac{11}{2}}$ and $\nu h_{\frac{11}{2}}$ alignments is consistent with cranked-shell model calculations.

• Received 18 May 1987

DOI:https://doi.org/10.1103/PhysRevC.36.1649