Determining the precise mass of the deuteron A proton (1.6726 x 10-27 kg) and a neutron (1.6749 x 10-27 kg) at rest combine to form a deuteron, the nucleus of deuterium or "heavy hydrogen." In this process, a gamma ray (high-energy photon) is emitted, and its energy is measured to be 2.2 MeV (2.2 x 106 eV). Part 1 x Your answer is incorrect. Try again. (a) Keeping all five significant figures, what is the mass of the deuteron? Assume that you can neglect the small kinetic energy of the recoiling deuteron.