Isotopes of helium

Although there are nine known isotopes of helium (₂He) (standard atomic weight: 4.002602(2)), only helium-3 (³
He
) and helium-4 (⁴
He
) are stable.^[4] All radioisotopes are short-lived, the longest-lived being ⁶
He
with a half-life of 806.92(24) milliseconds. The least stable is ¹⁰
He
, with a half-life of 260(40) yoctoseconds (2.6(4)×10⁻²² s), although it is possible that ²
He
may have an even shorter half-life.

In the Earth's atmosphere, the ratio of ³
He
to ⁴
He
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 ³
He
to ⁴
He
is 1.62(29)×10⁻⁴,^[6] which is 121(22) times higher than that of atmospheric helium. 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.^[7] 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 separate into two immiscible phases due to differences in quantum statistics: ⁴
He
atoms are bosons while ³
He
atoms are fermions.^[8] Dilution refrigerators take advantage of the immiscibility of these two isotopes to achieve temperatures of a few millikelvins.