High-Spin States inIr183andIr185Nuclei: Is the Breaking of a Few Pairs of Nucleons Responsible for the Backbending Effect in the Osmium Region?

High-spin levels of the rotation-aligned bands built on the $h_{\frac{9}{2}}$ subshell in ${}_{}{}^{183}{}_{}{}^{}\mathrm{Ir}$ and ${}_{}{}^{185}{}_{}{}^{}\mathrm{Ir}$ have been populated by ($\alpha , 6n\gamma$) and ($\alpha , 4n\gamma$) reactions. In ${}_{}{}^{183}{}_{}{}^{}\mathrm{Ir}$ a rapid increase in the apparent moment of inertia has been observed at high spins. The implication of this result for the interpretation of backbending phenomena in even nuclei in this mass region is discussed.