The nucleus of the atom of heavy hydrogen, H² (deuterium). The deuteron d is composed of a proton and a neutron. As the simplest multinucleon nucleus, the deuteron has been the subject of extensive study. Its binding energy is 2.227 MeV; that is, this is the amount of energy which must be added to a deuteron for it to dissociate into a proton and a neutron. The accurate determination of this dissociation energy provides the means of calculating the mass of the neutron, the mass of the deuteron (2.014187 amu) and proton being known from other experiments.