Properties of the composite systems formed in the reactions of U238+U238 and Th232+Cf250

Strongly damped reactions of U238+U238, Th232+Cf250 at E_c.m.=680-1880 MeV have been studied based on the improved quantum molecular dynamics model. The formation and properties of the transiently formed composite system during the reaction process are carefully studied. We find that at a certain energy region, the entrance channel potential is weakly repulsive, and the dissipation is very strong after touching configuration. These two effects delay the re-separation time for a composite system. The single-particle potential well of the transiently formed composite system has a Coulomb barrier about 15 20 MeV high at the surface, which causes the excited unbound protons still embedded in the potential well to move in a common mono-single-particle potential for a period of time and thus restrains the quick decay of the composite system. The incident-energy dependence of the lifetime of the composite system and how the lifetime correlates with the shape configuration, the momentum distribution of the composite system, and the angular distribution of primary fragments are carefully investigated. The results indicate that the longest average lifetime for the transient composite system of U+U can reach about 1200 fm/c at properly selected incident energies.