THE INFLUENCE OF TARGET COMPOSITION ON THE SPECIFIC ENERGY-LOSS MEASURED IN TRANSMISSION GEOMETRY

At low projectile velocities, stopping cross section measurements by t ransmission methods with high directional correlation between incoming beam and detected projectiles and with a detector acceptance apprecia bly smaller than 4pi, may suffer from experimental artifacts. Thicknes s inhomogeneities make the measured specific energy loss appear angle dependent. The influence of the target areal density may be traced to the impact parameter dependence of both, the elastic and the inelastic stopping cross section. In this contribution we deal with a mixture o f gases. A ''true'' deviation from Bragg's rule may be due to charge c hanging processes. Here, we discuss an additional but only apparent vi olation of Bragg's rule in gas mixtures: due to impact parameter selec tion by the experimental geometry, target atoms with different atomic numbers may contribute differently to the measured specific energy los s. Our arguments are based on experimental results and on Monte Carlo simulations.