Cluster transfer in the reaction 16O+208Pb at energies well below the fusion barrier: A possible doorway to energy dissipation

The reaction O-16 + Pb-208 is a benchmark in nuclear reaction studies as it involves two doubly magic nuclei. New measurements of back-scattered projectile-like fragments at sub-barrier energies show that the probability of two-proton (2p) transfer is much larger than that of alpha-particle transfer. At energies around the fusion barrier the probability for 2p transfer is similar to 10%, similar to that for one-proton transfer. The 2p transfer probabilities are enhanced by up to an order of magnitude compared to calculations based on an independent particle picture as simulated by the fully microscopic time-dependent Hartree-Fock model (TDHF). Since beyond mean-field correlations like nucleon pairing are not included in the TDHF model, the enhancement indicates strong pairing correlations between the transferred protons. 2p transfer leads to excitation energies (most likely in the target-like nucleus) up to similar to 13 MeV, indicating that it may represent an effective doorway for the dissipation of energy and thus provide a microscopic mechanism toward understanding the inhibition of fusion and energies both above and below the barrier.