Wiktionary
n. (context isotope English) The rare isotope of helium, (nuclide 3 2 He), that has a single neutron
Wikipedia
Helium-3 (He-3, also written as He) is a light, non- radioactive isotope of helium with two protons and 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 thought 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. Other than H, helium-3 is the only stable isotope of any element with more protons than 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, which can be 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. Most of this comes from the decay of tritium (hydrogen-3), which decays into helium-3 with a half life of 12.3 years. Furthermore, some nuclear reactors (landbound or shipbound) periodically release some helium-3 and tritium into the atmosphere. The nuclear reactor disaster at Chernobyl released a huge amount of radioactive tritium into the atmosphere, and smaller accidents have caused 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 abundance of helium-3 is thought to be greater on the Moon than on Earth, having been embedded in the upper layer of regolith by the solar wind over billions of years, though still lower in quantity than in the solar system's gas giants.
Usage examples of "helium-3".
It is argued that the ultimate solution to world energy problems is to strip-mine the Moon, return the solar-wind-implanted helium-3 back to Earth, and use it in fusion reactors.
The Japanese helium-3 fusion process, by contrast, produced charged protons, which could be kept away from reactor walls with magnetic fields.
A series of microexplosions—fusion of deuterium and helium-3 probably—set off behind a pusher plate.
If the Gaijin were after helium-3, did that mean they used fusion processes similar to—perhaps no more advanced than—those already known to humans?
On the lunar surface, the lights of the spreading Japanese colonies and helium-3 mines glittered.
The newest Lunar Japanese helium-3 fusion drives were, Madeleine learned, much more effective.
I know your people point to deep-field camera images going all the way back to some wonky hubble-bubble scrying mirror from the late twentieth, but we've got no evidence except some theories about the Casimir effect and pair production and spinning beakers of helium-3 – much less proof that whole bunch of alien galactic civilizations are trying to collapse the false vacuum and destroy the universe!