Helium-3 (He-3, sometimes called tralphium[1]) is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare onEarth, 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)[citation needed] 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 1 ppb to 50 ppb is helium-3).[2][3]
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).[4]
Helium-3 was postulated to be a radioactive isotope until helions from it were identified in a sample of natural helium (which is mostly helium-4) from both the atmosphere and gas well sources, by Luis W. Alvarez and Robert Cornog in a cyclotron experiment at the Lawrence Berkeley National Laboratory, in 1939.[5] Although helium-3 was found to be about 10,000 times more rare with respect to helium-4 in helium from wells, its significant presence in underground gas deposits implied that it either did not decay, or else had an extremely long half-life compatible with a primordial isotope.
Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into space, over time. It is also a naturalnucleogenic and cosmogenic nuclide, produced when lithium is bombarded by natural neutrons generated by spontaneous fission, and by cosmic rays. Some helium-3 in the atmosphere is also a relic of open air thermonuclear weapon testing. Tritium, with a roughly 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.[citation needed]
Helium-3 is proposed as a second-generation fusion fuel for fusion power uses, but such systems are still in very early experimental development phases. Helium-3 is used in neutron detection, and extremely low temperature cryogenics. It has been used as a magnetizable (hyperpolarized) gas to carry out nuclear magnetic imaging studies of the lungs.
