Cluster transfer in the reaction16O+208Pb at energies well below the fusion barrier: A possible doorway to energy diss

Abstract

The reaction 16 O + 208 Pb 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 α-particle transfer. At energies around the fusion barrier the probability for 2p transfer is ∼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 ∼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.

Investigating energy dissipation through nucleon transfer reactions

“…Here contributions from correlated 2p transfer and alpha transfer are both evident, though with probabilities lower than the observed 1p transfer, and with the 2p channel dominant over the ΔZ = 2 channel. 16 O has traditionally been thought to be a good candidate for alpha clustering structures, being a light N = Z nucleus, though this result, as well as those reported in [15], contradict this picture. However, as seen in figure 11, at the largest internuclear separations the 2p and α transfer channels appear to become almost equivalent.…”

“…The physical origin of this phenomenon has been extensively sought [11][12][13], but no convincing explanation capable of consistently describing fusion above and below the Coulomb barrier has been forthcoming. Several theoretical explanations have been put forward-Amongst other mechanisms, energy dissipation [14,15] and transfer processes [16] in heavy ion collisions have been suggested to play a part in the observed suppression of fusion cross sections in many systems below the Coulomb barrier. The inclusion of dissipative couplings in fusion models has been shown to potentially account for the hindrance effect [14].…”

Microscopic study ofCa40+Ni58,64fusion reactions

“…In particular, transfer could favour neck formation and, thus, diffusion towards a compound nucleus. It has also been argued that transfer could be a doorway to dissipation hindering fusion [17].…”

“…Nucleon transfer channels have also been shown to affect the fusion process [15][16][17][18][19]. However, a deep understanding of the interplay between fusion and transfer is still needed.…”

Nucleus-nucleus potential with shell-correction contribution and deep sub-barrier fusion of heavy nuclei

“…The fusion cross sections evaluated by using the full (the sum of macroscopic and shell-correction energy parts) nucleus-nucleus potential are compared with the experimental data for reaction 16 O+ 208 Pb [28] in Fig. 1.…”