Evidence for Strongly Deformed Shapes inHg186

Abstract

By means of $\gamma$-ray spectroscopy following (heavy ion, $\mathrm{xn}$) reactions, we have established a level scheme for the yrast states in ${}_{}{}^{186}{}_{}{}^{}\mathrm{Hg}$ up to the ${14}^{+}$ state. From the spectrum we conclude that ${}_{}{}^{186}{}_{}{}^{}\mathrm{Hg}$ makes a rather sudden angular-momentum-induced change in deformation from small values at low spins to rather large values at higher spins. This is in qualitative agreement with several potential-energy surface calculations, and leads to an interpretation of previously measured large isotopic shifts in rms radii from ${}_{}{}^{187}{}_{}{}^{}\mathrm{Hg}$ to ${}_{}{}^{183,185}{}_{}{}^{}\mathrm{Hg}$ as a sudden onset of stable quadrupole deformation.