Helium-3 (He-3, sometimes called tralphium) is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on the Earth, and it 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), though still low in quantity (28 ppm of lunar regolith is helium-4 and from one ppb to 50 ppb is helium-3), and the solar system's gas giants (left over from the original solar nebula).
The helion, the nucleus of a helium-3 atom, consists of two protons but only one neutron, in contrast with two neutrons in common helium. Its hypothetical existence was first proposed in 1934 by the Australian nuclear physicist Mark Oliphant while he was working at the University of Cambridge Cavendish Laboratory. Oliphant had performed experiments in which fast deuterons collided with deuteron targets (incidentally, the first demonstration of nuclear fusion).
Helium-3 was hypothesized to be a radioactive isotope until helions were also found in samples of natural helium, which is mostly helium-4, taken both from the terrestrial atmosphere and from natural gas wells. This was done by Luis W. Alvarez and Robert Cornog in cyclotron experiments at the Lawrence Berkeley National Laboratory in California in 1939.
Although helium-3 was found to be about 10,000 times rarer than helium-4 in the helium from the gas wells, its significant presence in underground gas deposits implied that either it did not decay, or else it had a very long half-life - billions of years. Hydrogen-1 and helium-3 are the only stable nuclides in existence that contain more protons than they have neutrons.
Helium-3 occurs as a primordial nuclide, escaping from the Earth's crust into the atmosphere and into outer space over millions of years. Helium-3 is also thought to be a natural nucleogenic and cosmogenic nuclide, one produced when lithium is bombarded by natural neutrons. Those are released by spontaneous fission and by nuclear reactions with cosmic rays. Some of the helium-3 found in the terrestrial atmosphere is also a relic of atmospheric and underwater nuclear weapons testing, conducted by the three big nuclear powers before 1963. Most of this comes from the decay of tritium (hydrogen-3), which decays into helium-3 with a half life of about 12.3 years. Furthermore, some nuclear reactors (landbound or shipbound) periodically release some helium-3 and tritium into the atmosphere, especially whenever something goes wrong. The nuclear reactor disaster at Chernobyl released a huge amount of radioactive tritium into the atmosphere, and smaller problems cause smaller releases. Furthermore, significant amounts of tritium and helium-3 have been deliberately produced in national arsenal nuclear reactors by the irradiation of lithium-6. The tritium is used to “boost” nuclear weapons, and some of this inevitably escapes during its production, transportation, and storage. Hence, helium-3 enters the atmosphere both through its direct release and through the radioactive decay of tritium. The vast majority of these two gases have been produced and leaked by the former Soviet Union, Russia, the United Kingdom, and the Republic of France.
Helium-3 is proposed as a second-generation fuel for nuclear fusion in hypothetical fusion power plants, but such plants are still very early in their development -- especially since the first generation has not entered service anywhere, yet. Helium-3 can be used in instruments for the detection of free neutrons, such as neutrons leaking from nuclear reactors.
Read more about Helium-3: Physical Properties, Fusion Reactions, Neutron Detection, Cryogenics, Medical Lung Imaging, Production, Extraterrestrial Abundance, Power Generation, In Popular Culture