Evolution of collectivity with spin in Y-81

Lifetimes of 34 high-spin levels in ${}^{81}\mathrm{Y}$ were measured using the Doppler-shift attenuation method. The high-spin states were populated using the ${}^{58}\mathrm{Ni}{(}^{32}\mathrm{S},2\ensuremath{\alpha}p)$ reaction at 135 MeV, and the recoils were stopped in a thick Ta backing. Prompt multi-$\ensuremath{\gamma}$ coincidences were detected using GAMMASPHERE, and evaporated charged particles were detected with the MICROBALL. Transition quadrupole moments inferred from the lifetimes in the lowest positive- and negative-parity bands show a relatively gradual decrease with spin, in agreement with Hartree-Fock-Bogolyubov cranking calculations. The calculations also predict a shape change in each band from predominantly prolate at low spin to triaxial above the first quasiparticle alignment. Thus the observed decrease in collectivity with spin is interpreted as the result of competing shapes which tend to lower the amount of deformation. Other negative-parity bands show evidence for structures with large deformation.