Effective charge and related/unrelated quantities in heavy-ion stopping

Stopping cross sections for swift heavy ions are commonly parameterized in terms of a velocity-dependent effective charge defined with reference to pr oton or helium stopping. This assumes projectile screening by core electron s to be a dominating factor governing the velocity dependence of the stoppi ng cross section. In this note Mle demonstrate? on the basis of improved th eoretical estimates of stopping cross sections, that projectile screening o nly affects the quantitative details of the velocity dependence of the effe ctive charge while the general behavior reflects the transition from the Bo rn to the classical regime. This is consistent with the experience that eff ective charges derived from measured stopping cross sections show dependenc ies on projectile and target atomic number which differ distinctly from tho se of the equilibrium projectile charge. It follows that there is no theore tical basis for the effective-charge concept as it is commonly used in heav y-ion stopping. (C) 2001 Elsevier Science B.V. All rights reserved.