Isotopes of helium

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. 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 most common isotope, 4
He, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized
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. 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. 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.[5] 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.