3He

Helium-3, denoted 3He, is a stable isotope of helium with two protons and one neutron, giving it a mass number of 3. It is a fermion with nuclear spin 1/2 and a magnetic moment of about −2.13 μN. Chemically it behaves like other helium isotopes, but its nuclear properties give it distinct applications in science and technology. On Earth, 3He is rare. Most of it is produced cosmogenically in the atmosphere and also arises from the beta decay of tritium. It occurs in trace amounts in natural helium and can be concentrated from helium-rich gas reservoirs.

Physically, 3He shares the noble-gas chemistry of helium but differs in quantum behavior at low temperatures. It has a very low boiling point and, when used with 4He, enables dilution refrigeration that can reach temperatures in the millikelvin and sub-millikelvin range. This makes 3He crucial in low-temperature physics experiments and in cryogenics.

Applications of 3He are diverse. It is used in neutron detection through the 3He(n,p)3H reaction, which releases energy and charged particles that can be measured by detectors. In medicine and imaging, hyperpolarized 3He gas has been used as a contrast agent for magnetic resonance imaging of the lungs. 3He is also discussed as a potential fusion fuel when paired with deuterium (D-3He fusion), which could produce fewer neutrons than the standard D-T fusion, though it requires higher temperatures and remains commercially unproven.

Safety and handling are generally straightforward since 3He is inert and non-toxic, though it is an asphyxiation hazard in enclosed spaces when used in gas form.