Alpha-particle emission from 6 He + 209 Bi

In a recent experiment, we have for the first time studied near-barrier and sub-barrier fusion of the exotic 'Borromean' nucleus 6 He with 209 Bi and found that the sub-barrier fusion of this system is exceptionally enhanced, implying a 20% reduction in the nominal fusion barrier. It was suggested that this striking effect might be due to coupling to positive Q-value neutron transfer channels, leading to 'neutron flow' and consequent neck formation between the projectile and target. The results of a new experiment using the radioactive nuclear beam facility at the University of Notre Dame to measure fast c-particle emission from 6 He + 209 Bi are discussed. A exceptionally strong transfer/breakup group was observed at near-barrier and sub-barrier energies; this is very likely to be the 'doorway state' that explains the enhanced sub-barrier fusion. (Author) 12 refs., 2 tabs., 4 figs