The level structure of the ${}_{}{}^{137}{}_{}{}^{}\mathrm{Pr}$ nucleus has been investigated via in-beam γ-ray spectroscopy using an 81-MeV ${}_{}{}^{122}{}_{}{}^{}\mathrm{Sn}$${(}^{19}$F, 4n) reaction. A strongly populated positive-parity band was observed based on a proton orbital with (π,α)=(+,+(1/2); its first backbend, due to the alignment of a pair of $h_{11/2}$ protons, was seen at a rotational frequency ħω=0.34 MeV. Such a high crossing frequency is taken as evidence for a triaxial shape (γ∼30°) for the ground-state band of ${}_{}{}^{137}{}_{}{}^{}\mathrm{Pr}$. Above the crossing, both signatures (π,α)=(+,±(1/2) were observed up to $I^{\pi }$${=(37/2\mathrm{}}^{+}$ and ${(43/2\mathrm{}}^{+}$ (for α=+1/2 and α=-1/2), respectively. A negative-parity band, based on an $h_{11/2}$ proton orbital, was also observed showing a blocked $h_{11/2}$ proton crossing frequency of ħω=0.44 MeV. In addition, a ΔI=1 band with strong M1 transitions and a small signature splitting was observed, which is believed to be based on the π$h_{11/2}$⊗(ν$h_{11/2}$${)}^2$ configuration. The alignment of the $h_{11/2}$ neutron pair drives the nuclear core towards γ=-60°, namely, the collective oblate shape, while the alignment of the $h_{11/2}$ proton pair drives the nuclear core towards γ=0°, the collective prolate shape. These results are consistent with cranked shell model calculations.

• Received 14 December 1988

DOI:https://doi.org/10.1103/PhysRevC.39.1799