On the inter-relation between ionization of light atoms by virtual and real photons

The correspondence between the ionization of light atoms by charged particle impact (virtual photons) and by photoabsorption (real photons) is analysed. It is shown that the `genuine' photo-like mechanism in the impact ionization of light targets can already be observed at collision energies corresponding to the Lorentz factor γ≳10, but that it starts to dominate electron emission only at γ≳1000. It is demonstrated that the non-relativistic Bethe-Born and Weizsäcker-Williams approximations represent extremely different limits of the impact ionization but that `by chance' their results are formally close for electron emission in non-relativistic fast collisions.