Helium-2 (diproton)
Helium-2 or 2He, also known as a diproton, is an extremely unstable isotope of helium, which consists of two protons without any neutrons. According to theoretical calculations it would have been much more stable (although still beta decaying to deuterium) had the strong force been 2% greater. Its instability 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.
There may have been observations of 2He. 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 18Ne nucleus 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, correlated in a quasibound 1S configuration, 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.
2He is an intermediate in the first step of the proton-proton chain reaction. The first step of the proton-proton chain reaction is a two-stage process; first, two protons fuse to form a diproton:
-
1
1H+ 1
1H→ 2
2He
followed by the immediate beta-plus decay of the diproton to deuterium:
-
2
2He→ 2
1D+ e+ + ν
e
with the overall formula:
-
1
1H+ 1
1H→ 2
1D+ e+ + ν
e+ 0.42 MeV
Bradford has considered the hypothetical effect of this isotope on Big Bang and stellar nucleosynthesis.
Read more about this topic: Isotopes Of Helium