helium3

Helium-3, denoted 3He, is a stable isotope of helium that contains two protons and one neutron. Its nucleus has spin 1/2, making it a fermion, and its atomic mass is about 3.016 u. Like ordinary helium, a 3He atom has two electrons, but its nuclear composition gives it distinct physical and nuclear properties.

On Earth, 3He is extremely rare. It occurs in trace amounts in natural gas and in the atmosphere as a product of natural processes such as tritium decay and cosmic-ray interactions. Because it is scarce and valuable, 3He commands higher prices than many other gases. In contrast, the Moon’s surface contains higher concentrations of 3He trapped in the regolith by long-term exposure to the solar wind, a factor that has attracted interest in proposed lunar resource extraction.

3He has several important applications. In low-temperature physics, it is used in dilution refrigerators with 3He-4He mixtures to reach temperatures well below a kelvin. In neutron detection, helium-3 gas is used in proportional counters because it captures neutrons via the reaction 3He(n,p)3H, producing charged particles that can be detected. In fusion research, deuterium–helium-3 (D–3He) fusion is of interest because it is considered aneutronic, producing mostly charged particles, but it requires extremely high temperatures and faces supply and engineering challenges.

Safety and handling of 3He are similar to other inert gases: it is non-toxic and chemically inert, but in enclosed spaces it can displace oxygen.