Sub-barrier Fusion of ${}_{}{}^6{}_{}{}^{}He$ with ${}_{}{}^{209}{}_{}{}^{}Bi$

The fusion of ${}^6\mathrm{He}$ with a ${}^{209}\mathrm{Bi}$ target has been studied at energies near to and below the Coulomb barrier. Despite the weak binding of the valence neutrons in ${}^6\mathrm{He}$, little evidence is found for suppression of fusion due to projectile breakup. Instead, a large enhancement of sub-barrier fusion is observed. It is suggested that this enhancement may arise from coupling to positive $Q$ value neutron transfer channels, resulting in “neutron flow” between the projectile and the target.