Unsafe coulomb excitation of 240-244Pu

The high spin states of 240Pu and 244Pu have been investigated with GAMMASPHERE at ATLAS, using Coulomb excitation with a 208Pb beam at energies above the Coulomb barrier. Data on a transfer channel leading to 242Pu were obtained as well. In the case of 244Pu, the yrast band was extended to 34(ℎ/2π), revealing the completed πi13/2 alignment, a ''first'' for actinide nuclei. The yrast sequence of 242Pu was also extended to higher spin and a similar backbend was delineated. In contrast, while the ground state band of 240Pu was measured up to the highest rotational frequencies ever reported in the actinide region (∼300 keV), no sign of particle alignment was observed. In this case, several observables such as the large B(E1)/B(E2) branching ratios in the negative parity band, and the vanishing energy staggering between the negative and positive parity bands suggest that the strength of octupole correlations increases with rotational frequency. These stronger correlations may well be responsible for delaying or suppressing the πi13/2 particle alignment. (c) 1999 American Institute of Physics