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Although there are eight known isotopes of helium (He) (standard atomic mass: 4.002602(2) u), only helium-3 (3He) and helium-4 (4He) are stable. In the Earth's atmosphere, there is one 3He atom for every million 4He atoms.[1] 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.[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, 4He, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized 4He nuclei. 4He 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 3He and 4He below 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).[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 3He 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, 3He is more abundant, a product of nuclear fusion. Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of 3He from being bombarded by solar winds. Exotic helium isotopes A subset of exotic light nuclei, the exotic helium isotopes have larger atomic masses than helium's natural isotopes. Although all exotic helium isotopes decay with a half-life of less than one second, researchers have eagerly created exotic light isotopes through particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium, and nitrogen. The bizarre nuclear structures of such isotopes may offer insight into the isolated properties of neutrons. The shortest-lived isotope is helium-5 with a half-life of 7.6×10−22 second. Helium-6 decays by emitting a beta particle and has a half life of 0.8 second. Helium-7 also emits a beta particle as well as a gamma ray. The most widely-studied exotic helium isotope is helium-8. This isotope, as well as helium-6, are thought to consist of a normal helium-4 nucleus surrounded by a neutron "halo" (two for 6He and four for 8He). Halo nuclei have become an area of intense research. Isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. Helium-7 and helium-8 are hyperfragments that are created in certain nuclear reactions.[6] Helium-2 is a hypothetical isotope of helium which according to theoretical calculations would have existed if the strong force had been 2% greater. This atom would have two protons without any neutrons.
* The isotopic composition refers to that in air. See also Isotopes of lithium References * Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003). 1. ^ Emsley, John. Nature's Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2001. Page 178. ISBN 0-19-850340-7 External links * General Tables — abstracts for helium and other exotic light nuclei
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