High-spin level structure ofPt190andPt192

The level structures of the shape transitional nuclei ${}_{}{}^{190}{}_{}{}^{}\mathrm{Pt}$ and ${}_{}{}^{192}{}_{}{}^{}\mathrm{Pt}$ have been studied by ($\alpha ,xn\gamma$) reactions on enriched Os targets. The measurements included $\gamma$-ray singles, prompt and delayed $\gamma -\gamma$ conincidences, half-life determinations in the range 1-500 ns, and $\gamma$-ray angular distributions. Detailed level schemes for ${}_{}{}^{190}{}_{}{}^{}\mathrm{Pt}$ and ${}_{}{}^{192}{}_{}{}^{}\mathrm{Pt}$, incorporating much new spectroscopic information, are reported. Acute backbending observed at about spin 10 in the positive parity yrast sequences of the two nuclei is attributed to intersection of the ground bands by rotation aligned bands of both ($\nu i_{\frac{13}{2}}^{}{}_{}{}^{-2\mathrm{}}$) and ($\pi h_{\frac{11}{2}}^{}{}_{}{}^{-2\mathrm{}}$) character. A description of well-developed $5^{-}$ bands observed in both nuclei as semidecoupled ($\nu i_{\frac{13}{2}}, \nu$) bands is briefly discussed. ${10}^{-}$ isomers with half-lives of 47±6 ns in ${}_{}{}^{190}{}_{}{}^{}\mathrm{Pt}$ and 250±30 ns in ${}_{}{}^{192}{}_{}{}^{}\mathrm{Pt}$ are reported. The nature of these isomers is discussed in light of our related finding that the neighboring nuclei ${}_{}{}^{189,191,193}{}_{}{}^{}\mathrm{Pt}$ have triaxial shapes ($\gamma \sim 30°$), and it is concluded that the ${10}^{-}$ states are predominantly of ($\nu i_{\frac{13}{2}}, \nu h_{\frac{9}{2}}$) two-quasiparticle composition.