Refractive or diffractive interpretation of heavy-ion elastic scattering

A characteristic pattern frequently observed in the angular distribution of heavy-ion elastic scattering at moderate energies above the Coulomb barrier has been attributed either to a Fresnel diffraction or to a rainbow effect. We propose a comparison between the strong absorption radius $R_{\mathrm{sa}}$ and the rainbow radius $R_r$ at different energies, which may have some relevance on deciding whether the absorptive or the refractive interpretation is to be preferred. Despite the difficulties in the exact determination of these two distances, an analytical expression for their energy dependence is found empirically by analyzing data of fifteen pairs of heavy ions. According to this criterion the center-of-mass energy, above which refraction prevails over absorption, is proportional to $Z_1{Z}_2$.