Neutron mass energy equivalent

Subtract the proton's 938.272 MeV and you get 1.293 MeV — the single number that drives beta decay, powers the proton-proton chain in reverse, and decides whether a given isotope is stable. Of that 1.293 MeV, 0.511 MeV is spent creating the electron, leaving 0.782 MeV shared as kinetic energy between the electron and the antineutrino, which is exactly the endpoint of the free-neutron beta spectrum.

Neutron energies span an enormous range in practice: "thermal" neutrons at 0.025 eV, the ones that fission U-235 most readily, are moving at 2200 m/s, while fission itself emits them near 2 MeV and they must be moderated down. All of these are minuscule compared with the 939.565 MeV of rest energy, so reactor neutronics is entirely non-relativistic even though its accounting is pure E = mc².