First Observation of Backbending in an Actinide Nucleus

The yrast bands in /sup 242/Pu and /sup 244/Pu have been studied up to spin 26/sup +/ by Coulomb excitation by use of /sup 208/Pb beams of 5.1 and 5.3 MeV/u. In the case of /sup 244/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 sequence of both nuclei are due to alignment effects in the i/sub 13/2/ proton shell.