Lifetime measurements of strongly deformed rotational bands in ¹³³Pm

We have measured lifetimes and deduced deformation parameters for rotational bands in ¹³³Pm by the Doppler-shift attenuation method. A strongly coupled band based on the configuration with a hole in the g_9/2 orbital has deformation parameter β₂ = 0.40 (5), which is comparable to or larger than typical superdeformed bands in the A ∼130 region. We observe this band in ¹³³Pm down to its I=K=9/2 bandhead. Calculations with a configuration-dependent shell correction to the cranked Nilsson potential can explain the existence of such low-spin superdeformed structures in the A ∼ 130 region. These shapes, which also occur in ^129,131Pr, can be observed experimentally because of relatively low-lying shell gaps for β₂=0.4 near Z=58, N=72. No high-spin intruder orbitals (vi _13/2) are occupied (over the spin range observed), however the strongly deformation-driving properties of a hole in the extruder πg_9/2 orbital appear to be an essential ingredient in lowering the energy of the superdeformed shape.