First Observation of Backbending in an Actinide Nucleus

The yrast bands in ${}_{}{}^{242}{}_{}{}^{}\mathrm{Pu}$ and ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}$ have been studied up to spin ${26}^{+}$ by Coulomb excitation using ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ beams of 5.1 and 5.3 MeV/u. In the case of ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}$ a pronounced backbending has been observed for the first time in an actinide nucleus. Microscopic calculations are presented which indicate that the observed anomalies in the yrast sequences of both nuclei are due to alignment effects in the $i_{\frac{13}{2}}$ proton shell.