Lifetime measurements and terminating structures in ⁸⁷Nb

Two experiments have been performed using the ⁵⁸Ni(³²S,3p) reaction at 135 MeV with Gammasphere and the Microball to study the high-spin structure of the transitional nucleus ⁸⁷Nb. The first experiment using a thin target provided a considerable extension and refinement of the level scheme, as well as firm spin assignments from directional correlation of oriented nuclei. Sub-picosecond lifetimes were measured in the second backed-target experiment using the Doppler-shift attenuation method. The lifetimes imply a rather modest average deformation of β₂∼0.1, but with considerable variation from state to state. Strong alternations were observed in the B(M1) strengths of transitions between some pairs of bands. The experimental results were compared with calculations performed within the cranked Nilsson-Strutinsky approach. The calculations generally reproduce the irregularities in the structure of the yrast lines and plunging transition quadrupole moments Q_t within the bands explaining them as reflections of configuration changes and an approach of band termination, respectively.