Noun
diproton (plural diprotons)
A hypothetical particle consisting of two protons and no neutrons; a helium-2 nucleus. Presumably an intermediate step in the fusion of hydrogen into helium.
A diproton is not stable, so neutrons must also be involved, ideally in such a way that a helium nucleus, with its extremely tight binding, is one of the products. Source: Internet
Unique among all stable isotopes, it has no neutrons (see diproton for a discussion of why others do not exist). Source: Internet