Near-barrier Fusion Evaporation and Fission of 28Si+174Yb and 32S+170Er

Fusion evaporation residues and fission fragments have been measured, respectively, at energies around the Coulomb barrier for the 28 Si+ 174 Yb and 32 S+ 170 Er systems forming the same compound nucleus 202 Po. The excitation function of fusion evaporation, fission as well as capture reactions were deduced. Coupled-channels analyses reveal that couplings to the deformations of targets and the two-phonon states of projectiles contribute much to the enhancement of capture cross sections at sub-barrier energies. The mass and total kinetic energy of fission fragments were deduced by the time-difference method assuming full momentum transfer in a two-body kinematics. The mass-energy and mass-angle distributions were obtained and no obvious quasi-fission components were observed in this bombarding energy range. Further, mass distributions of fission fragments were fitted to extract their widths. Results show that the mass widths decrease monotonically with decreasing energy, but might start to increase when E c.m. / V B < 0.95 for both systems.