Although there are eight known isotopes of helium (He) (standard atomic mass: 4.002602(2) u), only helium-3 (3
He) and helium-4 (4
He) 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 seconds. In the Earth's atmosphere, there is one 3
He atom for every million 4
He atoms.[1] However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3
He is around a hundred times higher.[2] 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.[3] The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.
Equal mixtures of liquid 3
He and 4
He below 0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different quantum statistics: 4
He atoms are bosons while 3
He atoms are fermions).[4] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.
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Helium-2 is a hypothetical isotope of helium which according to theoretical calculations would have existed if the strong force had been 2% greater. This atom would have two protons without any neutrons.
A diproton (or helium-2, symbol 2
He) 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 negative binding energy.[5]
There may have been observations of unstable 2
He. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once - perhaps a 2
He nucleus.[6][7] 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 18
Ne, 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 2
He 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 2
He was found in 2008 at the Istituto Nazionale di Fisica Nucleare, in Italy[8][9]. A beam of 20
Ne ions was collided into a foil of beryllium. In this collision some of the neon ended up as 18
Ne nuclei. These same nuclei then collided with a foil of lead. The second collision had the effect of exciting the 18
Ne nucleus into a highly unstable condition. As in the earlier experiment at Oak Ridge, the 18
Ne nucleus decayed into an 16
O 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 5
He with collisions between a beam of 6
He nuclei and a cryogenic hydrogen target, it was discovered that the 6
He 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 2
He nucleus, which quickly decays into two protons. A similar reaction has also been observed from 8
He nuclei colliding with hydrogen.
There is only a trace amount of 3
He on Earth, primarily present since the formation of the Earth, although some falls to Earth trapped in cosmic dust.[3] Trace amounts are also produced by the beta decay of tritium.[10] In stars, however, 3
He is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of 3
He from being bombarded by solar winds.
The most common isotope, 4
He, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized 4
He nuclei. 4
He is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.
Although all heavier helium isotopes decay with a half-life of less than one second, researchers have created new isotopes through particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium, and nitrogen. The unusual nuclear structures of such isotopes may offer insight into the isolated properties of neutrons.
The shortest-lived isotope is helium-5 with a half-life of 7.6×10−22 second. Helium-6 decays by emitting a beta particle and has a half-life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. The most widely-studied heavy helium isotope is helium-8. This isotope, as well as helium-6, are thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (two for 6
He and four for 8
He). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.[11]
| nuclide symbol |
Z(p) | N(n) | isotopic mass (u) | half-life | decay mode(s)[12] |
daughter isotope(s)[n 1] |
nuclear spin |
representative isotopic composition (mole fraction) |
range of natural variation (mole fraction) |
|---|---|---|---|---|---|---|---|---|---|
| 3 He[n 2] |
2 | 1 | 3.0160293191(26) | Stable[n 3] | 1/2+ | 1.34(3)×10−6 | 4.6×10−10-4.1×10−5 | ||
| 4 He[n 2] |
2 | 2 | 4.00260325415(6) | Stable | 0+ | 0.99999866(3) | 0.999959-1 | ||
| 5 He |
2 | 3 | 5.01222(5) | 700(30)×10−24 s [0.60(2) MeV] |
n | 4 He |
3/2- | ||
| 6 He[n 4] |
2 | 4 | 6.0188891(8) | 806.7(15) ms | β- (99.99%) | 6 Li |
0+ | ||
| β+, fission (2.8×10−4%) | 4 He, 2 H |
||||||||
| 7 He |
2 | 5 | 7.028021(18) | 2.9(5)×10−21 s [159(28) keV] |
n | 6 He |
(3/2)- | ||
| 8 He[n 5] |
2 | 6 | 8.033922(7) | 119.0(15) ms | β- (83.1%) | 8 Li |
0+ | ||
| β-,n (16.0%) | 7 Li |
||||||||
| β+, fission (.09%) | 5 He, 3 H |
||||||||
| 9 He |
2 | 7 | 9.04395(3) | 7(4)×10−21 s [100(60) keV] |
n | 8 He |
1/2(-#) | ||
| 10 He |
2 | 8 | 10.05240(8) | 2.7(18)×10−21 s [0.17(11) MeV] |
2n | 8 He |
0+ | ||
| Book: Helium | |
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| Isotopes of hydrogen | Isotopes of helium | Isotopes of lithium |
| Index to isotope pages · Table of nuclides | ||