Transfer and surface vibration couplings in the fusion of ⁴⁰Ca+^46,48,50Ti at near-barrier energies

Fusion and quasielastic excitation functions were measured for ⁴⁰Ca+^46,48,50Ti at energies close to the Coulomb barrier. The interest in these systems lies in the opposite target-mass dependence of surface vibration couplings and neutron-transfer couplings. The cross sections were measured at small energy steps and with high precision, so that the structure in the excitation functions (barrier distributions) could be revealed through a differentiation process. The subbarrier fusion data show a larger enhancement with increasing target mass number, which clearly indicates the importance of transfer couplings. The agreement between the fusion and quasielastic barrier distributions is quite good. Simplified coupled-channel calculations were performed to reproduce the fusion data. A very good agreement is obtained by coupling to low lying excited states (2⁺ and 3^-) and to the most positive neutron-transfer Q value (two-neutron projectile pickup channel).