STOPPING POWER OF LIGHT-IONS NEAR THE MAXIMUM

Some of the most interesting recent results are reviewed: in the regime of high ion velocities, there is large progress in the understanding of the stopping process beyond the first Born approximation (Bethe theory) due to theoretical and experimental investigations of the Barkas effect. Both investigations show that close collisions contribute to the Z31 correction, the importance of which becomes larger at lower ion velocities. For stopping measurements in metals and in semiconductors, the cooperation of two labs using different techniques has increased the attainable precision. The data may be compared to BEA calculations. The physical and chemical state of the material is of importance for ion stopping near the maximum on targets whose valence electrons dominate the stopping process. Ziegler has recently given a systematic treatment of the stopping of He and Li ions in hydrocarbons, using a core-and-bonds model. We have shown that, also for heavier compounds (Al2O3 and SiO2), large chemical effects are found. The stopping properties of these compounds can be well described by BEA calculations. Sabin et al. (1985) have predicted large phase effects for alkalis: for lithium and sodium targets and protons as projectiles they find a stopping ratio (vapor to solid) which exceeds a factor of 2 near the stopping maximum. The concept of an "effective charge" is useful to predict stopping powers for heavier ions from proton stopping data. For low ion velocities (near the velocity proportional region) and for some substances, the value of the He effective charge may exceed the value suggested by Ziegler et al. considerably. © 1990.