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

The cross section for $\alpha $-particle emission in the $^{6}$He + $^{209}$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 $^{211}$Bi. It is shown that approximately 55{\%} of the observed $\alpha $-particle yield at and beyond the grazing angle is due to this process. This is more than 2.5 times the fraction attributable to single-neutron transfer. This work has been published: Phys. Rev. C 71, 051601 (R) (2005)