Role of neutron flow in the enhancement of heavy ion fusion at energies below the Coulomb barrier and the systematics of threshold barriers

By integrating the classical formula for fusion of two charged spheres with a flat distribution of barriers extending down to a threshold barrier the fusion cross sections of heavy nuclei at energies near and below the Coulomb barrier have been successfully parametrized by P. H. Stelson. The systematics of the threshold barriers derived from the data are examined in terms of a simple picture in which the onset of free neutron flow between the collision partners promotes fusion at a large distance. A consistent correlation between observed barrier shifts and the conditions necessary to maintain free neutron flow has emerged.