Systematics of Heavy-Ion Charge-Exchange Straggling

The dependence of heavy-ion charge-exchange straggling on the beam energy has been studied theoretically for several ion-target combinations. Our previous work addressed ions up to krypton, while the present study focuses on heavier ions, especially uranium. Particular attention has been paid to a multiple-peak structure which has been predicted theoretically in our previous work. For high-Z ₁ and high-Z ₂ systems, exemplified by U in Au, we identify three maxima in the energy dependence of charge-exchange straggling, while the overall magnitude is comparable with that of collisional straggling. Conversely, for U in C, charge-exchange straggling dominates, but only two peaks lie in the energy range where we presently are able to produce credible predictions. For U–Al we find good agreement with experiment in the energy range around the high-energy maximum. The position of the high-energy peak – which is related to processes in the projectile K shell – is found to scale as Z ₁ ², in contrast to the semi-empirical Z ₁ ^3/2 dependence proposed by Yang et al. Measurements for heavy ions in heavy targets are suggested in order to reconcile a major discrepancy between the present calculations and the frequently-used formula by Yang et al.