Reaction cross sections for ⁶He+²⁰⁹Bi below the Coulomb barrier

In a recent experiment, we have for the first time measured near-barrier and sub-barrier transfer and/or breakup yields for an exotic Borromean nucleus ⁶He, on a ²⁰⁹Bi target. An isolated ⁴He group was observed at an effective Q-value of approximately -2.5 MeV whose integrated cross section was exceptionally large, greatly exceeding the fusion yield both above and below the barrier. Simultaneously measured elastic-scattering angular distributions required total reaction cross sections that confirmed this large yield. Preliminary coupled-channels calculations suggested that the reaction mechanism can best be described by direct breakup and neutron transfer to unbound states in ²¹¹Bi. The calculations also predicted an enhancement in the sub-barrier fusion yield due to coupling to the transfer and/or breakup channel, which strongly suggested that this is the doorway state that accounts for the remarkable suppression of the fusion barrier that was observed in a previous experiment. In this work we extend these measurements to even lower energies, down to 14.7 MeV, which is nearly 6 MeV below the barrier. It is found that the ⁴He group completely dominates the total reaction yield at these energies and some evidence is given that a new process might become important at low energies.