COMPETITION BETWEEN FUSION-FISSION AND QUASI-FISSION IN THE REACTION SI-28-208(PB)

It has recently been shown that collisions of O-16 with the tips of pr olate deformed U-238 nuclei result in quasi-fission, whilst collisions with the sides result in fusion-fission. Detailed measurements have n ow been made of the fusion barrier distribution and fission anisotropi es for the reaction Si-28 + Pb-208, forming a compound nucleus of simi lar fissility, to investigate whether Si-28 displays analogous effects of deformation to those of U-238, in both fusion and fission. Althoug h the fusion barrier distribution shows a large influence due to the S i-28, not inconsistent with anoblate deformation, the fission anisotro pies decrease steadily as the bombarding energy falls through the fusi on barrier region. In contrast, the O-16 + (238) U reaction shows a dr amatic rise at energies below the average barrier. This difference is interpreted in terms of the transition from the sudden (fusion) potent ial to the adiabatic (fission) potential.