High spin states in /sup 137/Pr

The level structure of the /sup 137/Pr nucleus has been investigated via in-beam γ-ray spectroscopy using an 81-MeV /sup 122/Sn(/sup 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/sub 11/2/ protons, was seen at a rotational frequency (h/2π)ω = 0.34 MeV. Such a high crossing frequency is taken as evidence for a triaxial shape (γ/similar to/300) for the ground-state band of /sup 137/Pr. Above the crossing, both signatures (π,α) = +, +- (1/2) were observed up to I/sup π/ = 37/2+ and 43/2+ (for α = +1/2 and α = -1/2), respectively. A negative-parity band, based on an h/sub 11/2/ proton orbital, was also observed showing a blocked h/sub 11/2/ proton crossing frequency of h/2πω = 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 πh/sub 11/2/x(νh/sub 11/2/)2 configuration. The alignment of the h/sub 11/2/ neutron pair drives the nuclear core towards γ = -600, namely, the collective oblate shape, while the alignment of the h/sub 11/2/ proton pair drives the nuclear core towards γ = 00, the collective prolate shape. These results are consistent with cranked shell model calculations