Radius of curvature of the S factor maximum in sub-barrier fusion hindrance

A maximum of the S(E) factor is evidence for an onset of sub-barrier fusion hindrance and it can be well described by a radius-of-curvature expression near the maximum. The systematics of this radius of curvature has been studied over a wide range of projectile-target combinations. It follows a tentative general trend as a function of the parameter zeta=Z(1)Z(2)root mu, and is strongly affected by effects associated with the nuclear structure of the nuclei in the entrance channel. It also explains the reason why the S factor maximum is not easily recognized visually for lighter, astrophysically interesting fusion systems.