Observation of a Prolate-to-Oblate Shape Change Near Spin $40\hslash$ in ${}_{}{}^{158}{}_{}{}^{}\mathrm{Er}$

Discrete $\gamma$-ray transitions have been observed above spin 40 in ${}_{}{}^{158}{}_{}{}^{}\mathrm{Er}$ following the ${}_{}{}^{114}{}_{}{}^{}\mathrm{Cd}({}_{}{}^{48}{}_{}{}^{}\mathrm{Ca},4n)$ reaction. Structure characteristic of the collective rotation of a prolate nucleuś ends at $I^{\pi }={38}^{+}$ in the yrast band and probably at ${41}^{-}$ in the lowest-energy negative-parity band corresponding in both cases to a rotational energy of ∼ 0.53 MeV. It is suggested that the observed phase transition is the prolate-to-oblate shape change which is predicted to occur at higher spin.