Proton band crossings and electromagnetic transition probabilities in ¹⁶¹Lu

Two new negative-parity and two new positive-parity decay sequences are established to high spin for ¹⁶¹Lu₉₀. The resulting systematics of the routhians, alignments, and B(M1)/B(E2) ratios indicate that ¹⁶¹Lu has both a smaller and less stable quadrupole deformation than the lighter N = 90 isotones and the heavier lutetium isotopes. At low spin, the data are consistent with a sizeable negative γ deformation. Above the band crossing associated with the excitation of a pair of i _{3 2} quasineutrons, the evidence for γ deformation disappears; however, the configuration-dependent B(M1)/B(E2) ratios indicate that the nuclear shape may not be completely stable. The systematics of the band crossings based on the excitation of a pair of h _{1 2} quasiprotons observed at higher spins in ¹⁶¹Lu and other odd- and even-Z, N = 90 isotones provides the first experimental evidence from high-spin data for proton quadrupole pair correlations.