Special article: Di-proton

A diproton (or helium-2, symbol 2He) is a hypothetical isotope of helium nucleus consisting of two protons and no neutrons, and is predicted to be less stable than ⁵He. Diprotons are not stable; this is due to spin-spin interactions in the nuclear force, and the Pauli exclusion principle, which forces the two protons to have anti-aligned spins and gives the diproton a negative binding energy.

In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once - perhaps a 2He nucleus.The team led by Alfredo Galindo-Uribarri of the Oak Ridge National Laboratory announced that the discovery will help scientists understand the strong nuclear force and provide fresh insights into the creation of elements inside stars. Galindo-Uribarri and co-workers chose an isotope of neon with an energy structure that prevents it from emitting protons one at a time. This means that the two protons are ejected simultaneously. The team fired a beam of fluorine ions at a proton-rich target to produce 18Ne, which then decays into oxygen and two protons. Any protons ejected from the target itself were identified by their characteristic energies. There are two ways in which the two-proton emission may proceed. The neon nucleus might eject a 'diproton' - a pair of protons bound together as a 2He nucleus - which then decays into separate protons. Alternatively, the protons may be emitted separately but at the same time - so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place.

The best evidence of 2He was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20Ne ions was collided into a foil of beryllium. In this collision some of the neon ended up as 18Ne nuclei. These same nuclei then collided with a foil of lead. The second collision had the effect of exciting the 18Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18Nenucleus decayed into an 16O nucleus, plus two protons detected exiting from the same direction. The new experiment showed that the two protons were initially ejected together before decaying into separate protons much less than a billionth of a second later.

Also, at RIKEN in Japan and JINR in Dubna, Russia, during productions of 5He with collisions between a beam of 6He nuclei and a cryogenic hydrogen target, it was discovered that the 6He nucleus can donate all four of its neutrons to the hydrogen. This leaves two spare protons that may be simultaneously ejected from the target as a 2He nucleus, which quickly decays into two protons. A similar reaction has also been observed from 8He nuclei colliding with hydrogen.