Collectivity of160,161Yb at high spin

Lifetimes of high-spin states in ${}_{}{}^{160}{}_{}{}^{}$,161Yb have been measured by the recoil-distance technique and, from these, changes in the nuclear shape due to the alignment of quasiparticles have been deduced. It is observed that the addition of the favored $i_{13/2}$ quasineutron causes a reduction in collectivity at large rotational frequencies. This reduction is interpreted as due to induced triaxiality and is much larger than predicted by theory.