Cluster transfer in the reaction 16O+208Pb and its role in understanding the suppression of fusion, M. Evers, The Australian National University, Australia − In nuclear reactions of heavy nuclei, the phenomenon of fusion suppression seen both at deep sub-barrier and at above-barrier energies has been the source of great dispute [1–4].  A consistent understanding of the mechanisms leading to this suppression of fusion is crucial for e.g. reliable predictions of reaction rates in astrophysical scenarios.  It has been argued [1, 5] that the suppression of fusion above the barrier can be associated with deep inelastic collisions (DIC), leaving the residual nuclei in highly excited states. Measurements of these DIC products [6, 7] show that both energy dissipation of kinetic energy into nucleonic degrees of freedom and nucleon transfer are important and related to each other.  We expect in reality a smooth transition from nucleon transfer to low-lying discrete states in sub-barrier quasi-elastic scattering on one end, to (multi-)nucleon transfer leading to the dissipation in DIC at energies above the barrier on the other end.  Detailed measurements of the backscattered flux in the reaction 16O+208 Pb, performed at the Heavy-Ion accelerator facility of the Australian National University will be presented.  They suggest that the concept of energy dissipation may play a significant role already at energies below the fusion barrier [8]. The systematic analysis to determine the transfer probabilities of the detected projectile-like fragments will be described. A detailed comparison with calculations based on the coupled reaction channels framework as well as the fully microscopic time-dependent Hartree-Fock (TDHF) model will be presented.  Results indicate that (i) the transfer of two protons (2p) occurs with probabilities 10% at energies near the fusion barrier, (ii) the 2p transfer probabilities are significantly enhanced compared to TDHF calculations, and (iii) 2p transfer leads to excitation energies as high as 13 MeV in the residual nuclei.  These results show that experimental and theoretical work on multi-nucleon transfer, particularly cluster transfer, may be a key towards developing a complete understanding of both fusion and scattering in low energy heavy-ion collisions, how these processes may be linked to the suppression of fusion at sub-barrier energies, and how processes leading to large excitation energies in the residual nuclei may be included in future nuclear reaction models.


[1] J. O. Newton, R. D. Butt, M. Dasgupta, D. J. Hinde, I. I. Gontchar, and C. R. Morton, Phys. Rev. C 70, 024605 (2004).