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.