The gas-solid density effect for MeV/u ions: Charge changing cross sections and the effective charge

Abstract

It is commonplace to describe projectile ion charge-state fluctuations within gaseous and solid targets in terms of “charge-changing cross sections”. However the physical meaning of such cross section has been unclear in solids, where a significant proportion of the ions may be in excited states. For the case of a charge-state distribution dominated by fully stripped and one-electron ions, formulae for the charge-changing cross sections are derived in terms of more fundamental cross sections for projectile excitation, ionisation and electron capture. Density effects in the electron loss and capture cross sections, described in an informal way by Bohr and Lindhard, are presented in simple closed form. Quantitative estimates are given for the density effect in the loss cross section for one-electron ions. The results are supported by experimental data for 3 MeV/u C ions in gaseous and solid C targets. The corresponding density effect in the effective stopping-power charge q eff is considered, both for nearly fully stripped ions and for many-electron ions; and is compared with available experimental data for light and heavy ions at MeV/u energies.