Z3 effect in the ionization cross section of argon by 1.2- and 2.0-MeV α particles

The Z3 correction to the differential cross section for ejection of electrons from argon by α particles is estimated by adapting the theory of Ashley, Ritchie, and Brandt for the Z3 effect on stopping power. Results are compared with the difference between cross sections for ionization by α particles and equivalent-velocity protons. For ejected electron energies between 40 and 190 eV, good agreement is obtained between the model and experiment. The model predicts a significant enhancement of the cross section for ejection of low-energy electrons by α particles which is not observed