Diproton

A diproton (or Helium-2, symbol ²He) is a hypothetical type of helium nucleus consisting of two protons and no neutrons. 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 binding energy greater than zero.^[1]

Observations of unstable He-2

In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once - perhaps a helium-2 nucleus^[2][3] 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 Ne-18, 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 He-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. The best evidence of He-2 was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of Ne-20 ions was collided into a foil of beryllium. In this collision some of the neon ended up as Ne-18 nuclei. These same nuclei then collided with a foil of lead. The second collision had the effect of exciting the Ne-18 nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the Ne-18 nucleus decayed into an O-16 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 Moscow, during productions of H-5 with collisions between a beam of He-6 nuclei and a cryogenic hydrogen target, it was discovered that the He-6 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 He-2 nucleus, which quickly decays into two protons. A similar reaction has also been from He-8 nuclei colliding with hydrogen.

References

1. ^ “Nuclear Physics in a Nutshell”, C.A. Bertulani, Princeton University Press, Princeton, NJ, 2007, Chapter 1, ISBN 978-0-691-12505-3.
2. ^ Physicists discover new kind of radioactivity, in physicsworld.com Oct 24, 2000
3. ^ Decay of a Resonance in ¹⁸Ne by the Simultaneous Emission of Two Protons, Physical Review Online Archive, by del Campo, Galindo-Uribarri et al.

See also

• Dineutron