Isotopes of helium

Although there are nine known isotopes of helium (He) (relative atomic mass: 4.002602(2)), only helium-3 (3He) and helium-4 (4He) are stable. All radioisotopes are short-lived, the longest-lived being 6He with a half-life of 7002806700000000000♠806.7 milliseconds. The least stable is 5He, with a half-life of 6978760000000000000♠7.6×10⁻²² s, although it is possible that 2He has an even shorter half-life.

In the Earth's atmosphere, there is one 3He atom for every million 4He atoms. However, helium is unusual in that its isotopic abundance varies greatly depending on its origin. In the interstellar medium, the proportion of 3He is around a hundred times higher. 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. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances.

Equal mixtures of liquid 3He and 4He below 6999800000000000000♠0.8 K will separate into two immiscible phases due to their dissimilarity (they follow different 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 of a few millikelvins.