New isotopes ${}_{}{}^{186,188}{}_{}{}^{}\mathrm{Tl}$ (${T_{\frac{1}{2}}}^{186}={4.5}_{-1.5\mathrm{}}^{+1.0\mathrm{}} \mathrm{and} 28\pm 2$ sec and ${T_{\frac{1}{2}}}^{188}=71\mathrm{}\pm 1$ sec, respectively) have been identified. Conversion-electron, $\gamma$, $\gamma -\gamma$, and $e-\gamma$ studies helped establish in ${}_{}{}^{186,188}{}_{}{}^{}\mathrm{Hg}$ two bands, one built on a slightly deformed ground state and one on a more strongly deformed shape with $0^{+}$ band heads at 522 and 824.5 keV, respectively. Their crossing explains the anomalous behavior of $\mathcal{I}$ in the yrast cascades. States above and below the crossing in both bands support the existence of a well-defined second minimum in ${}_{}{}^{188}{}_{}{}^{}\mathrm{Hg}$.

• Received 12 May 1975

DOI:https://doi.org/10.1103/PhysRevLett.35.562