Novel insights into transfer processes in the reaction 16O+208Pb at sub-barrier energies

The collision of the doubly-magic nuclei ${}^{16}$O+${}^{208}$Pb is a benchmark in nuclear reaction studies. Our new measurements of back-scattered projectile-like fragments at sub-barrier energies show show that transfer of 2 protons ($2p$) is much more probable than $\alpha$-particle transfer. $2p$ transfer probabilities are strongly enhanced compared to expectations for the sequential transfer of two uncorrelated protons; at energies around the fusion barrier absolute probabilities for two proton transfer are similar to those for one proton transfer. This strong enhancement indicates strong $2p$ pairing correlations in ${}^{16}$O, and suggests evidence for the occurrence of a nuclear supercurrent of two-proton Cooper pairs in this reaction, already at energies well below the fusion barrier.