Isotopes of helium

Although there are nine known isotopes of helium (2He) (standard atomic weight: 4.002602(2)), only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived, the longest-lived being 6He with a half-life of 806.7 milliseconds. The least stable is 5He, with a half-life of 7.6×10−22 s, although it is possible that 2He has an even shorter half-life. Although there are nine known isotopes of helium (2He) (standard atomic weight: 4.002602(2)), only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived, the longest-lived being 6He with a half-life of 806.7 milliseconds. The least stable is 5He, with a half-life of 7.6×10−22 s, although it is possible that 2He has an even shorter half-life. In the Earth's atmosphere, there is one 3He atom for every million 4He atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3He is around a hundred times higher. Rocks from the Earth's crust have isotope ratios varying by as much as a factor of ten; this is used in geology to investigate the origin of rocks and the composition of the Earth's mantle. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. Equal mixtures of liquid 3He and 4He below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: 4He atoms are bosons while 3He atoms are fermions). Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins. Helium-2 or 2He, also known as a diproton, is an extremely unstable isotope of helium that consists of two protons with no neutrons. According to theoretical calculations, it would have been much more stable (although still undergoing beta-decay to deuterium) if the strong force had 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 decayed 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 simultaneously—so-called 'democratic decay'. The experiment was not sensitive enough to establish which of these two processes was taking place. More evidence of 2He was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy. A beam of 20Ne ions was directed at a target of beryllium foil. This collision converted some of the heavier neon nuclei in the beam into 18Ne nuclei. These 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 nanosecond later. Further evidence comes from RIKEN in Japan and JINR in Dubna, Russia, where beams of 6He nuclei were directed at a cryogenic hydrogen target to produce 5He. It was discovered that the 6He nucleus can donate all four of its neutrons to the hydrogen. The two remaining protons could be simultaneously ejected from the target as a 2He nucleus, which quickly decayed 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: followed by the immediate beta-plus decay of the diproton to deuterium: