High-spin states in ${}^{187}\mathrm{Pt}$ have been studied experimentally using the ${}^{173}\mathrm{Yb}$(${}^{18}\mathrm{O}$, $4n$) reaction at beam energies of 78 and 85 MeV. The previously known bands based on the $\nu i_{13/2},\nu 7/2^{-}[503]$, and $\nu i_{13/2}^2\nu j$ configurations have been extended to high-spin states, and new rotational bands associated with the $\nu 3/2^{-}[512]$ and $\nu 1/2^{-}[521]$ Nilsson orbits have been identified. The total Routhian surface calculations indicate that the transitional nucleus ${}^{187}\mathrm{Pt}$ is very soft with respect to $\beta$ and $\gamma$ deformations. The band properties, such as level spacings, band crossing frequencies, alignment gains, and signature splittings, have been compared with the systematics observed in neighboring nuclei and have been interpreted within the framework of the cranked shell model. The rotational bands show different band crossing frequencies, which can be explained by the alignment either of $i_{13/2}$ neutrons or of $h_{9/2}$ protons. Importantly, evidence is presented for a $\pi h_{9/2}$ alignment at very low frequency in the $\nu 7/2^{-}[503]$ band. The proton nature of the band crossing is strongly suggested by comparing the measured $B(M1;I\to I-1)/B(E2;I\to I-2)$ ratios with the theoretical values from the semiclassical Dönau and Frauendof approach.

• Received 11 December 2006

DOI:https://doi.org/10.1103/PhysRevC.75.034314