Aligned Bands and Nuclear Shapes in the N = 90 Region

The N = 90 transition region is studied by gauge-space systematics for Nd through Yb nuclei and by the trend of transition quadrupole moments for ground-state bands. Lifetime measurements on quasiparticle bands in 160,161Yb indicate a systematic decrease in the collectivity at higher rotational frequencies, indicating triaxial shapes. Spectroscopy measurements on 159Tm are described. A change in the order of the signatures of the h11/2 band after compared to before the back-bend is used to extract qualitative information on shape triaxiality. The systematics of 2 and 3 quasiparticle bands in N = 90 and 91 nuclei is presented and used to deduce the occurrence of a large-interaction crossing in 160Yb and 162Hf due to the alignment of negative-parity neutrons (h9/2 + f7/2).

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