Multinucleon transfer in ¹⁶,¹⁸ O, ¹⁹F + ²⁰⁸Pb reactions at energies near the fusion barrier

Abstract

Background: Nuclear reactions are complex, involving collisions between composite systems where many-body dynamics determines outcomes. Successful models have been developed to explain particular reaction outcomes in distinct energy and mass regimes, but a unifying picture remains elusive. The irreversible transfer of kinetic energy from the relative motion of the collision partners to their internal states, as is known to occur in deep inelastic collisions, has yet to be successfully incorporated explicitly into fully quantal reaction models. The influence of these processes on fusion is not yet quantitatively understood.

Purpose: To investigate the population of high excitation energies in transfer reactions at sub-barrier energies, which are precursors to deep inelastic processes, and their dependence on the internuclear separation.

Methods: Transfer probabilities and excitation energy spectra have been measured in collisions of O16,18,F19+Pb208, at various energies below and around the fusion barrier, by detecting the backscattered projectile-like fragments in a ΔE−E telescope.

Results: The relative yields of different transfer outcomes are strongly driven by Q values, but change with the internuclear separation. In O16+Pb208, single nucleon transfer dominates, with a strong contribution from −2p transfer close to the Coulomb barrier, though this channel becomes less significant in relation to the −2p2n transfer channel at larger separations. For O18+Pb208, the −2p2n channel is the dominant charge transfer mode at all separations. In the reactions with F19,−3p2n transfer is significant close to the barrier, but falls off rapidly with energy. Multinucleon transfer processes are shown to lead to high excitation energies (up to ∼15 MeV), which is distinct from single nucleon transfer modes which predominantly populate states at low excitation energy.

Conclusions: Kinetic energy is transferred into internal excitations following transfer, with this energy being distributed over a larger number of states and to higher excitations with increasing numbers of transferred nucleons. Multinucleon transfer is thus a mechanism by which energy can be dissipated from the relative motion before reaching the fusion barrier radius.