Proton mass energy equivalent

938.272 MeV is the reference mass of nuclear and particle physics. A proton beam is called relativistic once its kinetic energy approaches this figure; at the LHC each proton carries 6.8 TeV, roughly 7250 times its own rest energy, so γ ≈ 7250 and the beam travels within 3 m/s of c. Proton therapy beams, by contrast, run at 70–250 MeV — still under a third of the rest energy, yet enough to place a Bragg peak inside a tumour.

The nucleon mass also fixes the scale of everything heavier: a nucleus of mass number A weighs roughly A × 939 MeV/c² minus about 8 MeV per nucleon of binding. That 8-in-939 shortfall — under 1 % — is the entire energy supply of both fission reactors and stars, which is a fair measure of how much energy c² packs into a tiny fractional mass change.