Fusion probabilities in the reactions 40,48Ca+238U at energies around the Coulomb barrier

Fission cross sections and fission fragment mass distributions were measured in the reactions of Ca-40+U-238 and Ca-48+U-238 at energies around the Coulomb barrier. Fusion probabilities were calculated based on the fluctuation dissipation model. The measured mass distributions for both reactions showed an asymmetric shape at low incident energies, whereas the distribution changed to a flat shape at higher energies. The variation of the mass distribution is explained by a change of the ratio between fusion and quasifission with nuclear orientation. The calculation reproduced the mass distributions and their energy dependence. The trajectories for fusion-fission were used to determine the fusion probability. Fusion probabilities for both reactions are identical as function of the center-of-mass energy (E-c.m.), but they differ when plotted as function of the excitation energy (E*). Evaporation residue cross sections were calculated for the reaction Ca-48+U-238 using a statistical model and the obtained fusion cross sections as input values. The results are compared to experimental data.