Isotopes of helium

Helium (₂He) has nine known isotopes, but only helium-3 (³He) and helium-4 (⁴He) are stable. All radioisotopes are short-lived; the only particle-bound ones are ⁶He and ⁸He with half-lives 806.9 and 119.5 milliseconds.

In Earth's atmosphere, the ratio of ³He to ⁴He is 1.37×10⁻⁶. However, the isotopic abundance of helium varies greatly depending on its origin, though helium-4 is always in great preponderance. In the Local Interstellar Cloud, the proportion of ³He to ⁴He is 1.62(29)×10⁻⁴, which is about 120 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. 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. Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures as low as a few millikelvin.

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