Tritium: The Radioactive Heavy Isotope of Hydrogen

Tritium (symbol ³H or T), also called hydrogen-3, is the heaviest naturally occurring isotope of hydrogen. Its nucleus holds one proton and two neutrons, giving it roughly three times the mass of ordinary hydrogen. Unlike protium and deuterium, tritium is radioactive: it undergoes beta decay, emitting a low-energy electron and an antineutrino as one of its neutrons converts to a proton, transforming the atom into stable helium-3. The total decay energy is only about 18.6 keV, and the emitted beta particle averages around 5.7 keV — so weak it travels barely six millimetres in air and cannot pierce human skin, though tritium is hazardous if inhaled or swallowed. Tritium's half-life is 12.32 years, short enough that any given stock decays away within a human lifetime and must be replenished. Only trace amounts occur naturally, formed when cosmic rays strike atmospheric nitrogen. For practical quantities, tritium is bred by neutron activation of lithium-6 inside nuclear reactors, and it also arises as a minor byproduct of nuclear fission. Its applications follow directly from its gentle but persistent radiation. In self-powered lighting, sealed glass tubes filled with tritium gas coat their walls with phosphor; the beta particles make the phosphor glow for years without batteries, a trick used in watch dials, gun sights, and emergency exit signs. As fusion fuel, tritium reacts with deuterium to release 17.6 MeV per event, making the deuterium-tritium reaction the primary fuel pathway for experimental reactors such as ITER. In the life sciences it serves as a radiolabeling tracer, substituting for ordinary hydrogen so scientists can follow a molecule's path through a reaction or organism. Tritium can also bond with oxygen to form tritiated water (HTO), a radioactive form of water that behaves chemically like normal H₂O and resists separation by distillation. See Can Pure Water Be Radioactive? Tritiated Water and Distillation for why that property matters.