Possible evidence for an unusual interference between refraction and diffraction in carbon-carbon elastic angular distributions

Bohlen et al. have recently suggested the occurrence of a new interference phenomenon in their angular distributions for carbon-carbon elastic scattering at 20 MeV/A, namely an interference between refractive (deeply penetrating) and diffractive (peripheral) trajectories. We provide a detailed analysis of the farside cross section of the optical potential they employ, and confirm that indeed this cross section does contain such an interference. It is rather weak because refraction dominates over diffraction for their potential, which is unusually transparent for heavy ions. If its absorptive strength W is increased by 30%, the refractive and diffractive contributions become comparable, and the interference minimum in dσ/dΩ is much deeper. However, if W is doubled, to the value employed by Brandan for similar data, the interference vanishes altogether and diffraction dominates over refraction. (orig.)