Energy-loss straggling of helium in elemental solids: The importance of the charge-exchange and bunching effects

In this study, we extend to Helium (He) ions the analysis of the energy-loss straggling of protons in simple materials as presented in NIMB 497 70 (2021). Our findings show that the excess of energy-loss straggling for both protons and He ions is highly correlated to the inhomogeneous distribution of electrons over the unit cell, referred to here as bunching effects, and that charge-exchange straggling has a minor impact, although it can compete with bunching effects for light targets. The Convolution approximation for swift Particles program was used to demonstrate that the bunching effect is a significant contributor to the straggling for both protons and He ions. In addition, we show that packing effects, a special form of bunching is of minor importance. The Yang-O'Connor-Wang formula for He was refined in this work to account for recent experimental data.