Isotopes of helium

Helium-2 (diproton)

Helium-2, ²He, is unbound. The only bound atom with a mass number of 2 is deuterium. The nucleus of ²He, a diproton, consists of two protons with no neutrons. 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 (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.

A rare form of radioactivity is diproton emission, where a nucleus emits two protons in a quasi-bound ¹S₀ configuration, which then separate. In 2000, Oak Ridge National Laboratory detected two-proton emission from ¹⁸
₁₀Ne, produced by a ¹⁷
₉F ion beam onto a proton-rich target. But the experiment didn't have the sensitivity to distinguish if the emission was a decay by two separate protons, or by a diproton. In 2008, the Istituto Nazionale di Fisica Nucleare confirmed ¹⁸Ne decayed to a diproton with a 31% branching ratio. Several experiments have since detected diproton emission from other isotopes.

The lack of a bound diproton has been used to argue for fine-tuning for the development of life due to its effect on Big Bang nucleosynthesis and stellar evolution. Hypothetical models suggest that if the strong force was 2% greater, then diprotons would be bound (but still β⁺ decay to deuterium). Recent studies have found that a universe with bound diprotons doesn’t preclude the development of stars and life.

One impact of a hypothetical bound diproton is a change to the early steps of the proton-proton chain. In our universe, the first step of the proton-proton chain proceeds via the weak force,

p + p → ²
₁D + e⁺
+ ν
_e + 0.42 MeV

In the hypothetical, instead a diproton could form without the weak force,

p + p → ²
₂He

The diproton would then beta-plus decay into deuterium:

²
₂He → ²
₁D + e⁺
+ ν
_e.

With the overall formula,

p + p → ²
₁D + e⁺
+ ν
_e.

Under the influence of electromagnetic interactions, the Jaffe-Low primitives 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. 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.

Helium-3

³He is the only stable isotope other than ¹H with more protons than neutrons. There are many such unstable isotopes, such as ⁷Be and ⁸B. There is only a trace (~2ppm) of ³He on Earth, mainly present since the formation of the Earth, although some falls to Earth trapped in cosmic dust. Trace amounts are also produced by the beta decay of tritium. 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.

Helium-4

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) 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.

Helium-5

Helium-5 is extremely unstable, decaying to helium-4 with a half-life of 602 yoctoseconds by neutron emission. It is briefly produced in the favorable fusion reaction:

${\displaystyle {}^2\mathrm{H}+{}^3\mathrm{H}⟶{}^5{\mathrm{H}\mathrm{e}}^{\ast }⟶{}^4\mathrm{H}\mathrm{e}+n+17.6\mathrm{M}\mathrm{e}\mathrm{V}}$

The reaction is greatly enhanced by the existence of a resonance. Helium-5, which has a natural spin state of 3/2− at the ground state, has a 3/2+ excited spin state at 16.84 MeV. The reaction creates more frequently helium-5 nuclei in the excited state, since its energy is close to the energy released by the reaction. This was discovered by Egon Bretscher, who was investigating weaponization of fusion reactions for the Manhattan Project.

The DT reaction specifically is 100 times more likely than the DD reaction at relevant energies, but would be similar without the resonance. The ²H-³He reaction benefits from a similar resonance in lithium-5, but is Coulomb-suppressed, i.e., the +2 helium nucleus charge increases the electrostatic repulsion for fusing nuclei.

Helium-6 and helium-8

These are the long-lived radioactive isotopes of helium; helium-6 beta decays with a half-life of 806.9 milliseconds, and helium-8 with a half-life of 119.5 milliseconds, though additional particle emission is possible and significant for the latter. ⁶He and ⁸He are thought to consist of a normal ⁴He nucleus surrounded by a neutron "halo" (of two neutrons in ⁶He and four neutrons in ⁸He). The unusual structures of halo nuclei may offer insights into the isolated properties of neutrons and physics beyond the Standard Model.