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 certainly ejected simultaneously. The team fired a beam of radioactive fluorine ions at a proton-rich target to produce neon-18, which then decays into oxygen and two protons. Any 'rogue' protons ejected from the target itself can be 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 helium-2 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.

A long search by scientists for two-proton emission has produced some evidence that beryllium can 'democratically' decay into an alpha particle and two protons, but other investigations have been inconclusive until now. Paddy Regan, a nuclear physicist at the University of Surrey, UK, firmly believes that Galindo-Uribarri's group has now made the crucial breakthrough.

"The results appear to give the first indication that the diproton exists within the nucleus", Regan told PhysicsWeb. "Confirmation will come with more extensive experiments. We are now waiting with bated breath". The Oak Ridge team is currently planning a more sophisticated experiment that will establish the mechanism of the decay.