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Helium-3 (He-3) is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research. The abundance of helium-3 is thought to be greater on the Moon (embedded in the upper layer of regolith by the solar wind over billions of years) and the solar system's gas giants (left over from the original solar nebula), though still low in quantity (28 ppm of lunar regolith is helium-4 and from 0.01 ppm to 0.05 ppm is helium-3).[1] [2]

The helion, the nucleus of a helium-3 atom, consists of two protons but only one neutron, in contrast to two neutrons in ordinary helium. Its existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while based at Cambridge University's Cavendish Laboratory, in an experiment in which fast deuterons were reacted with other deuteron targets (the first demonstration of nuclear fusion).[3] Helium-3, as an isotope, was postulated to be radioactive, until helions from it were accidentally identified as a trace "contaminant" in a sample of natural helium (which is mostly helium-4) from a gas well, by Luis W. Alvarez and Robert Cornog in a cyclotron experiment at the Lawrence Berkeley National Laboratory, in 1939.[4] The presence of helium-3 in underground gas deposits implied that it either did not decay or had an extremely long half-life compatible with a primordial isotope.

Helium-3 is proposed as a second-generation fusion fuel for fusion power uses. Tritium, with a 12-year half-life, decays into helium-3, which can be recovered. Irradiation of lithium in a nuclear reactor — either a fusion or fission reactor — can also produce tritium, and thus (after decay) helium-3.


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