Two-neutron transfer in the {sup 6}He+{sup 209}Bi reaction near the Coulomb barrier

The cross section for {alpha}-particle emission in the {sup 6}He+{sup 209}Bi reaction at energies near the Coulomb barrier is remarkably large. Possible reactions that may produce the observed {alpha} particles include two-neutron transfer, one-neutron transfer, and direct projectile breakup. Each of these mechanisms results in a distinctive angular correlation between the {alpha} particle and the outgoing neutron(s). A neutron-{alpha}-particle coincidence experiment was performed to study two-neutron transfer to unbound states of {sup 211}Bi. It is shown that approximately 55% of the observed {alpha}-particle yield at and beyond the grazing angle is because of this process. This is more than 2.5 times the fraction attributable to single-neutron transfer. The corresponding 2n-transfer cross section is 0.4 {+-} 0.1 b.