Paradoxical features of small angle multiple scattering in matter and their implications in depth profiling with IBA

Energy loss and small angle multiple scattering (MS) of fast ions both arise from the cumulative effect of successive independent binary encounters, the corresponding energy transfers or deflections being additive random variables. These processes are hence governed by statistical laws of the same type. Nevertheless their evolution with thickness of matter crossed is radically different, MS presenting a number of seemingly paradoxical features. These peculiar aspects are emphasized and illustrated. The strong statistical dependence between angular spread and lateral displacement of the projectiles is also stressed. These special features play a basic role in ion beam handling and in the interpretation of IBA depth profiling measurements