Sub- barrier transfer in 16O+208Pb and 32S+208Pb and its role in understanding the suppression of fusion

A detailed analysis of the projectile-like fragments detected at backward angles in the reactions O-16, S-32 + Pb-208 at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with Delta Z = 1 and Delta Z = 2 were extracted, indicating surprisingly large absolute probabilities at sub-barrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments up to similar to 15MeV for the O-16 and similar to 25MeV for S-32-induced reactions demonstrate the population of highly excited states in the residual nuclei, indicating substantial dissipation of kinetic energy. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.