Straggling in energy loss of energetic hydrogen and helium ions

The straggling in energy loss for protons and alpha particles is discussed, with special emphasis on correlation effects, stemming from the bunching of electrons in atoms and molecules and from charge-state fluctuations. It is shown that for gaseous targets, the correlation effects give rise to an increased straggling relative to that calculated from an electron-gas picture. For an amorphous, uniform solid, the contribution from correlation is much reduced, and the total energy straggling should approximately be given by the Lindhard-Scharff electron-gas expression. The theoretical is in good agreement with accurate measurements of energy straggling for hydrogen and helium ions on a variety of atomic and molecular gases. Measurements in solids are often obscured by the influence of target non-uniformity and texture. A simple method to check for such effects is discussed and demonstrated by an example. (orig.)