[mattheww]

Close to nothing would happen. The relevant quantity to understand is the stopping power (usually referred to by us physicists as dE/dx). Stopping power drops significantly as energy of the charged particle increases. So for very large energies, very little of that energy is deposited over a given distance (in an absolute sense).

This principle is how some radiation therapies work to treat cancer. Because the stopping power curve for protons is well understood, a proton beam can be tuned to deposit all of its energy is a fairly small area.

Aside/Rant: this question clearly can be answered by someone with expertise in the field. So why did people feel the need to speculate about it instead of just waiting for someone who knows how to answer it?