Helium (₂He) (standard atomic weight: 4.002602(2)) has nine known isotopes, but only helium-3 (³He) and helium-4 (⁴He) are stable.^[4] All radioisotopes are short-lived; the longest-lived is ⁶He with half-life 806.92(24) milliseconds. The least stable is ¹⁰He, with half-life 260(40) yoctoseconds (2.6(4)×10⁻²² s), though ²He may have an even shorter half-life.

In Earth's atmosphere, the ratio of ³He to ⁴He is 1.343(13)×10⁻⁶.^[5] However, the isotopic abundance of helium varies greatly depending on its origin. In the Local Interstellar Cloud, the proportion of ³He to ⁴He is 1.62(29)×10⁻⁴,^[6] which is ~121 times higher than in Earth's atmosphere. Rocks from 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.^[7] 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 separate into two immiscible phases due to differences in quantum statistics: ⁴He atoms are bosons while ³He atoms are fermions.^[8] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvin.

A mix of the two isotopes spontaneously separates into ³He-rich and ⁴He-rich regions.^[9] Phase separation also exists in ultracold gas systems.^[10] It has been shown experimentally in a two-component ultracold Fermi gas case.^[11][12] The phase separation can compete with other phenomena as vortex lattice formation or an exotic Fulde–Ferrell–Larkin–Ovchinnikov phase.^[13]

Helium-2, ²He, is extremely unstable. Its nucleus, a diproton, consists of two protons with no neutrons. According to theoretical calculations, it would be much more stable (but still β⁺ decay to deuterium) if the strong force were 2% greater.^[18] Its instability is due to spin–spin interactions in the nuclear force and the Pauli exclusion principle, which states that within a given quantum system two or more identical particles with the same half-integer spins (that is, fermions) cannot simultaneously occupy the same quantum state; so ²He's two protons have opposite-aligned spins and the diproton itself has negative binding energy.^[19]

²He may have been observed. In 2000, physicists first observed a new type of radioactive decay in which a nucleus emits two protons at once—perhaps ²He.^[20][21] The team led by Alfredo Galindo-Uribarri of Oak Ridge National Laboratory announced that the discovery will help understand the strong nuclear force and provide fresh insights into stellar nucleosynthesis. 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 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. The two-proton emission may proceed in two ways: the neon might eject a diproton, which then decays into separate protons, or the protons may be emitted separately but simultaneously in a "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 Istituto Nazionale di Fisica Nucleare, in Italy.^[15][22] 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 excited 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 Joint Institute for Nuclear Research in Dubna, Russia, where beams of ⁶He nuclei were directed at a cryogenic hydrogen target to produce ⁵H. It was discovered that the ⁶He can donate all four of its neutrons to the hydrogen.^{[citation needed]} The two remaining protons could be simultaneously ejected from the target as a diproton, which quickly decayed into two protons. A similar reaction has also been observed from ⁸He nuclei colliding with hydrogen.^[23]

Under the influence of electromagnetic interactions, the Jaffe-Low primitives^[24] may leave the unitary cut, creating narrow two-nucleon resonances, like a diproton resonance with a mass of 2000 MeV and a width of a few hundred keV.^[25] To search for this resonance, a beam of protons with kinetic energy 250 MeV, and an energy spread below 100 keV, is required, which is feasible considering the electron cooling of the beam.

²He is an intermediate in the first step of the proton–proton chain. The first step of the proton-proton chain is a two-stage process: first, two protons fuse to form a diproton:

¹H + ¹H + 1.25 MeV → ²He;

then the diproton immediately beta-plus decays into deuterium:

²He → ²H + e⁺ + ν_e + 1.67 MeV;

with the overall formula

¹H + ¹H → ²H + e⁺ + ν_e 0.42 MeV.

The hypothetical effect of a bound diproton on Big Bang and stellar nucleosynthesis, has been investigated.^[18] Some models suggest that variations in the strong force allowing a bound diproton would enable the conversion of all primordial hydrogen to helium in the Big Bang, which would be catastrophic for the development of stars and life. This notion is an example of the anthropic principle. However, a 2009 study suggests that such a conclusion can't be drawn, as the formed diproton would still decay to deuterium, whose binding energy would also increase. In some scenarios, it is postulated that hydrogen (in the form of ²H) could still survive in large amounts, rebutting arguments that the strong force is tuned within a precise anthropic limit.^[26]

³He is the only stable isotope other than ¹H with more protons than neutrons. (There are many such unstable isotopes; the lightest are ⁷Be and ⁸B.) There is only a trace (~2ppm)^[16] of ³He on Earth, mainly present since the formation of the Earth, although some falls to Earth trapped in cosmic dust.^[7] Trace amounts are also produced by the beta decay of tritium.^[27] In stars, however, ³He is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, has traces of ³He from solar wind bombardment.

To become superfluid, ³He must be cooled to 2.5 millikelvin, ~900 times lower than ⁴He (2.17 K). This difference is explained by quantum statistics: ³He atoms are fermions, while ⁴He atoms are bosons, which condense to a superfluid more easily.

The most common isotope, ⁴He, is produced on Earth by alpha decay of heavier elements; the alpha particles that emerge are fully ionized ⁴He nuclei. ⁴He is an unusually stable nucleus because it is doubly magic. It was formed in enormous quantities in Big Bang nucleosynthesis.

Terrestrial helium consists almost exclusively (all but ~2ppm)^[16] of ⁴He. ⁴He's boiling point of 4.2 K is the lowest of all known substances except ³He. When cooled further to 2.17 K, it becomes a unique superfluid with zero viscosity. It solidifies only at pressures above 25 atmospheres, where it melts at 0.95 K.

Though all heavier helium isotopes decay with a half-life of <1 second, particle accelerator collisions have been used, to create unusual nuclei of elements such as helium, lithium and nitrogen. The unusual nuclear structures of such isotopes may offer insights into the isolated properties of neutrons and physics beyond the Standard Model.^[28][29]

The shortest-lived isotope is ¹⁰He with half-life ~260 yoctoseconds. ⁶He beta decays with half-life 807 milliseconds. The most widely studied heavy helium isotope is ⁸He. ⁸He and ⁶He are thought to consist of a normal ⁴He nucleus surrounded by a neutron "halo" (of two neutrons in ⁶He and four neutrons in ⁸He). Halo nuclei have become an area of intense research. Isotopes up to ¹⁰He, with two protons and eight neutrons, have been confirmed. ¹⁰He, despite being a doubly magic isotope, is not particle-bound and near-instantly drips out two neutrons.^[30]

• General Tables — abstracts for helium and other exotic light nuclei