Cross Sections for Ion and Electron Production in Gases by Fast Helium Ions (0.133-1.0 MeV). II. Comparison with Theory

The theory of ionization of gases by fast, but nonrelativistic, ions is briefly reviewed, with emphasis on the correspondences expected at high energies among the cross sections for various projectile ions incident on a given target gas. The problem of deducing cross sections for simple ionization from experimental gross ion and electron production cross sections is discussed. The methods developed are applied to the experimental data presented in the preceding paper. Estimates are thus obtained of the simple ionization cross sections for ${\mathrm{He}}^{+}$ ions incident on ${\mathrm{H}}_2$, He, Ar, and ${\mathrm{N}}_2$ in the energy range 0.133 to 1.0 MeV, and ${\mathrm{He}}^{+}$ on Ne, ${\mathrm{O}}_2$, and CO in the energy range 0.6 to 1.0 MeV. Similar results are obtained for ${\mathrm{He}}^{++}$ ions incident on ${\mathrm{H}}_2$ and He in the energy range 0.5 to 1.0 MeV. These results are compared with semitheoretical predictions based upon empirical values of target-gas matrix elements determined from previous measurements of the ionization cross sections for protons incident on the same targets. It is shown that the ionization cross sections for ${\mathrm{He}}^{++}$ conform rather well to the predictions, and that the ionization cross sections for ${\mathrm{He}}^{+}$ ions are in good agreement with those expected for a point-charge ion with the helium mass and an "effective charge" of $Z^{\prime }e=+1.2e$. The ionization cross sections are also compared with explicit detailed calculations in the full Born approximation for the cases where such calculations are available. The agreement obtained is quite good in general.