I see an error in my last message.I wrote "electron" but was thinking about
a proton. That is why I said "E is pointing out away" from the charged
particle. But that is not essential, both electrons and protons ionize atoms.
Is it not true that an increase in the density of ionization versus energy,
above some point, is due to the relativistic growth of the impact
parameters for which ionization is possible? I am talking about the dE/dx
curves for any charged particle.