When a high-energy proton collides with an atom in the Earth’s atmosphere, it can break apart that atom to produce (still high-energy) secondary radiation in the form of neutrons, protons, and other subatomic particles.
In practice, we are not going to be able to tell the difference between a rock which has reached 99.9% of this maximum and one which has reached 99.99%.
Consequently, the practical limit for the use of cosmogenic surface dating seems to be about 10 million years; after that, one old rock looks much like another.
Super high energy particles—mostly protons— are produced by our Sun, supernovae, and probably other extraterrestrial sources.
These particles continuously enter the Earth system at incredible rates and are often, but misleadingly, called cosmic rays.
Upon entering the Earth’s surface, these high-energy particles quickly collide with the relatively dense assortment of atoms around, breaking apart more atoms in the upper ~2 meters of a rock (or other) surface but failing to reach much deeper than that.