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