Evidence for a Prolate to Oblate Shape Change at High Spin inYb160

The population and the decay properties of the entry states in ${}_{}{}^{160}{}_{}{}^{}\mathrm{Yb}$ have been investigated via the reactions of 136- and 149-MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ with ${}_{}{}^{146}{}_{}{}^{}\mathrm{Nd}$ with use of a $4\pi$ $\gamma$-ray multidetector system gated by a Ge counter for exit channel selection. A sudden decrease in the slope of the entry line accompanied by a change in the structure of the $\gamma$-ray spectrum is observed at multiplicity $M_{\gamma }=25\mathrm{}$. These changes indicate that a sudden transition from a prolate to an oblate shape occurs over a narrow range of spin above $45\hslash$.