Fusion excitation function revisited, P. Eudes, SUBATECH, FRANCE − We study the energy dependence of fusion cross section with the objective to understand the evolution of the fusion mechanism as a function of input channel parameters and the reasons for its (non-) disappearance.  By a careful scrutiny of published data on measured fusion cross sections we have collected some 168 experimental points relative to 57 systems.  This allows us to cover a span in system mass between 26 and 246, a system mass asymmetry in the range from 0 to 0.886 and a range of incident energy that lies between 5 and 155 MeV/A.  The fusion cross sections normalized by reaction cross sections and plotted as a function of centre-of-mass energy per nucleon (ECM), strongly suggests the existence of two branches corresponding to light symmetric systems and heavy asymmetric ones.

 

The light symmetric branch shows a rather universal behavior leading to a disappearance of the fusion around ECM=10 MeV/A.  The second branch indicates the persistence of (incomplete) fusion and seems to tend towards a constant value at high incident energies that can be explained in the frame of a simple geometrical approach.

 

From a theoretical viewpoint, this excitation function is a strong constraint that the dynamical models used below Fermi energy have to reproduce and this is a real challenge.  For instance, the Landau-Vlasov model with the momentum-dependent interaction due to Gogny is able to explain the behavior of the fusion in the high energy part, but overestimates the cross sections below about the Fermi energy.