Tritium

Tritium (/ˈtrɪtiəm/ or /ˈtrɪʃiəm/) or hydrogen-3 is a rare and radioactive isotope of hydrogen, with symbol T or 3H. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of the common isotope hydrogen-1 ('protium') contains just one proton, and that of hydrogen-2 ('deuterium') contains one proton and one neutron. Tritium (/ˈtrɪtiəm/ or /ˈtrɪʃiəm/) or hydrogen-3 is a rare and radioactive isotope of hydrogen, with symbol T or 3H. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of the common isotope hydrogen-1 ('protium') contains just one proton, and that of hydrogen-2 ('deuterium') contains one proton and one neutron. Naturally occurring tritium is extremely rare on Earth. The atmosphere has only trace amounts, formed by the interaction of its gases with cosmic rays. It can be produced by irradiating lithium metal or lithium-bearing ceramic pebbles in a nuclear reactor. Tritium is used as a radioactive tracer, in radioluminescent light sources for watches and instruments, and, along with deuterium, as a fuel for nuclear fusion reactions with applications in energy generation and weapons. The name of this isotope is derived from Greek τρίτος (trítos), meaning 'third'. While tritium has several different experimentally determined values of its half-life, the National Institute of Standards and Technology lists 4,500 ± 8 days (12.32 ± 0.02 years). It decays into helium-3 by beta decay as in this nuclear equation: and it releases 18.6 keV of energy in the process. The electron's kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the nearly undetectable electron antineutrino. Beta particles from tritium can penetrate only about 6.0 mm of air, and they are incapable of passing through the dead outermost layer of human skin. The unusually low energy released in the tritium beta decay makes the decay (along with that of rhenium-187) appropriate for absolute neutrino mass measurements in the laboratory (the most recent experiment being KATRIN). The low energy of tritium's radiation makes it difficult to detect tritium-labeled compounds except by using liquid scintillation counting. Tritium is produced in nuclear reactors by neutron activation of lithium-6. This is possible with neutrons of any energy, and is an exothermic reaction yielding 4.8 MeV. In comparison, the fusion of deuterium with tritium releases about 17.6 MeV of energy. For applications in proposed fusion energy reactors, such as ITER, pebbles consisting of lithium bearing ceramics including Li2TiO3 and Li4SiO4, are being developed for tritium breeding within a helium cooled pebble bed (HCPB), also known as a breeder blanket. High-energy neutrons can also produce tritium from lithium-7 in an endothermic (net heat consuming) reaction, consuming 2.466 MeV. This was discovered when the 1954 Castle Bravo nuclear test produced an unexpectedly high yield.