Barkas effect and effective charge in the theory of stopping power

Giant Barkas effect observed for light ions channeling in Si

Measurements of the electronic energy loss are presented for (4)He and (7)Li ions channeling along the Si main axial directions at intermediate to high projectile energies. The Barkas effect, an energy-loss enhancement proportional to the third power of the projectile charge at high energies, is clearly separated from other processes. It reaches about 50% for Li ions channeling along the Si [110] direction. The observed Barkas contribution from the valence-electron gas is in fair agreement with the Lindhard model.

Energy loss of 70 MeV protons in tissue-substitute materials

Measurements of the energy loss of 70 MeV protons in phantom materials have been made, with a small uncertainty. Average stopping powers relative to water have been determined for the energy range between approximately 70 MeV and 40 MeV. These are expressed in terms of the mean excitation energy I of the Bethe theory. The effect of grains of inhomogeneous materials (e.g. calcium fluoride) has been explored and understood.