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Isotopes of helium
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<p><a href="/facts/Helium/yRul93TG">Helium</a> (2He) (<a href="/facts/Standard_atomic_weight/8arqbQsq">standard atomic weight</a>: 4.002602(2)) has nine known <a href="/facts/Isotope/KD9B1y6o">isotopes</a>, but only <a href="/facts/Helium-3/SmPC5oT3">helium-3</a> (3He) and <a href="/facts/Helium-4/vNsL5qhC">helium-4</a> (4He) are <a href="/facts/Stable_isotope/vgcdxgjA">stable</a>. All <a href="/facts/Radioisotope/5WpCfbdq">radioisotopes</a> are short-lived; the longest-lived is 6He with <a href="/facts/Half-life/kGVH8BAB">half-life</a> 806.92(24) milliseconds. The least stable is 10He, with half-life 260(40) <a href="/facts/Yoctoseconds/bEovB6Tp">yoctoseconds</a> (2.6(4)×10−22 s), though 2He may have an even shorter half-life. </p><p>In Earth's atmosphere, the ratio of 3He to 4He is 1.343(13)×10−6. However, the isotopic abundance of helium varies greatly depending on its origin. In the <a href="/facts/Local_Interstellar_Cloud/xm3wKNrN">Local Interstellar Cloud</a>, the proportion of 3He to 4He is 1.62(29)×10−4, which is ~121 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 <a href="/facts/Geology/WGvVRIYF">geology</a> to investigate the origin of rocks and the composition of the Earth's <a href="/facts/Mantle_(geology)/RpDdtjCr">mantle</a>. The different formation processes of the two stable isotopes of helium produce the differing isotope abundances. </p><p>Equal mixtures of liquid 3He and 4He below 0.8 K separate into two <a href="/facts/Immiscible/HwQaolbQ">immiscible</a> phases due to differences in <a href="/facts/Quantum_statistics/hJaSBe1l">quantum statistics</a>: 4He atoms are <a href="/facts/Boson/XtPFcjb5">bosons</a> while 3He atoms are <a href="/facts/Fermion/LEckdDsR">fermions</a>. <a href="/facts/Dilution_refrigerator/s0RYbDAY">Dilution refrigerators</a> take advantage of the immiscibility of these two isotopes to achieve temperatures of a few milli<a href="/facts/Kelvin/E3trzC4C">kelvin</a>. </p><p>A mix of the two isotopes spontaneously separates into 3He-rich and 4He-rich regions. Phase separation also exists in <a href="/facts/Ultracold_atom/LdM0IeD7">ultracold gas</a> systems. It has been shown experimentally in a two-component ultracold <a href="/facts/Fermi_gas/D00lFTqW">Fermi gas</a> case. The phase separation can compete with other phenomena as <a href="/facts/Quantum_vortex/0Tzjncgk">vortex lattice</a> formation or an exotic <a href="/facts/Fulde%25E2%2580%2593Ferrell%25E2%2580%2593Larkin%25E2%2580%2593Ovchinnikov_phase/hqzoCSch">Fulde–Ferrell–Larkin–Ovchinnikov phase</a>. </p>