Backbending and Forking in the Yrast States of Even Os Isotopes

The yrast levels have been determined to spin 16 and 18 in ${}_{}{}^{182,184,186}{}_{}{}^{}\mathrm{Os}$ and to spin 12 in ${}_{}{}^{188}{}_{}{}^{}\mathrm{Os}$ from $\gamma$-ray excitation functions, anisotropies, and coincidence data from the reactions ${}_{}{}^{182,184,186}{}_{}{}^{}\mathrm{W}(\alpha , 4n)$ and ${}_{}{}^{186}{}_{}{}^{}\mathrm{W}(\alpha , 2n)$. A fork is observed in the ${}_{}{}^{186}{}_{}{}^{}\mathrm{Os}$ yrast band at spin 12. Above spin 12 the $\mathcal{I}$-versus-${\omega }^2$ curves for ${}_{}{}^{182,184,186}{}_{}{}^{}\mathrm{Os}$ display "backbending" behavior. Such behavior has not been reported for any other nuclei with more than 96 neutrons.