Tritium

Tritium (symbol T or ³H) is a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called triton) contains one proton and two neutrons, whereas the nucleus of protium (the most abundant hydrogen isotope) contains no neutrons. Its atomic weight is 3.0160492. It is a gas (T₂ or ³H₂) at standard temperature and pressure. Tritium combines with oxygen to form a liquid called tritiated water T₂O or partially tritiated THO.

Tritium is radioactive with a half-life of 12.32 years. It decays into helium-3 by the reaction

{}^3_1\hbox{T}\to{}^3_2\hbox{He}^++\hbox{e}^-+\overline{\nu}_{\hbox{e}}

releasing 18.6 keV of energy. The electron has an average kinetic energy of 6.5 keV, while the remaining energy is carried off undetectably by the electron antineutrino. The low-energy beta radiation from tritium cannot penetrate human skin, so tritium is only dangerous if inhaled or ingested. Its low energy also makes it difficult to detect tritium labelled compounds except by using liquid scintillation counting.

Sources

Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural tritium production, a fast neutron interacts with atmospheric nitrogen:

{}^{14}_7\hbox{N}+{}^1\hbox{n}\to{}^{12}_6\hbox{C}+{}^3_1\hbox{T}

Because of tritium's relatively short half-life, however, tritium produced in this manner does not accumulate over geological timescales, and its natural abundance is negligible. Industrially, tritium is produced in nuclear reactors by neutron activation of lithium-6.

{}^6_3\hbox{Li}+{}^1\hbox{n}\to{}^4_2\hbox{He}+{}^3_1\hbox{T}

Tritium is also produced in heavy water-moderated reactors when deuterium captures a neutron; however, this reaction has a much smaller cross section and is only a useful tritium source for a reactor with a very high neutron flux. It can also be produced from boron-10 through neutron capture.

Properties

Tritium figures prominently in studies of nuclear fusion due to its favorable reaction cross section and the high energy yield of 17.6 MeV for its reaction with deuterium:

{}^3_1\hbox{T}+{}^2_1\hbox{D}\to{}^4_2\hbox{He}+\hbox{n}

All atomic nuclei, being composed of protons and neutrons, repel one another because of their positive charge. However, if the atoms have a high enough temperature and pressure (as is the case in the core of the Sun, for example), then their random motions can overcome such electrical repulsion (called the Coulomb force), and they can come close enough for the strong nuclear force to take effect, fusing them into heavier atoms. Since tritium has the same charge as ordinary hydrogen, it experiences the same electrostatic repulsive force (see Coulomb's law). However, due to its higher mass, it is less responsive to such forces, and thus can more easily fuse with other atoms. The same is also true, albeit to a lesser extent, of deuterium, and that is why brown dwarfs (so called failed stars) can not burn hydrogen, but do indeed burn deuterium.

Before the onset of atmospheric nuclear weapons tests, the global equilibrium tritium inventory was estimated at about 80 megacuries (MCi).

Like hydrogen, tritium is difficult to confine; rubber, plastic, and some kinds of steel are all somewhat permeable. This has raised concerns that if tritium is used in quantity, in particular for fusion reactors, it may contribute to radioactive contamination, although its short half-life should prevent any significant accumulation in the atmosphere.

Atmospheric nuclear testing (prior to the Partial Test Ban Treaty) proved unexpectedly useful to oceanographers, as the sharp spike in surface tritium levels could be used over the years to measure the rate at which the lower and upper ocean levels mixed.

Usage

The emitted electrons from small amounts of tritium cause phosphors to glow so as to make self-illuminating devices called trasers which are now used in watches and exit signs. It is also used in certain countries to make glowing keychains, and compasses. In recent years, the same process has been used to make self-illuminating gun sights for firearms. These take the place of radium, which causes bone cancer, and so has been banned (in America(?)) for decades.

Tritiated thymidine is used in cell proliferation assays. The molecule, a nucleoside, is incorporated into the DNA of cells as they are replicated during cell division. The extent of cell proliferation may then be determined by liquid scintillation counting.

Tritium is used in nuclear weapons to obtain higher yields through nuclear fusion. However, as it quickly decays and is difficult to contain, many nuclear weapons contain lithium instead, since the high neutron fluxes will produce tritium from the lithium when the bomb detonates; see nuclear weapon design. Injection of a variable amount of tritium into the fission core pit just before initiation is one of the techniques to achieve variable yield.

History

Tritium was first predicted in the late 1920s by Walter Russell, using his "spiral" periodic table, then produced in 1934 from deuterium, another isotope of hydrogen, by Ernest Rutherford, working with Mark Oliphant and Paul Harteck. Rutherford was unable to isolate the tritium, a job that was left to Luis Alvarez, who correctly deduced that the substance was radioactive. Willard F. Libby discovered that tritium could be used for dating water, and therefore wine.

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Isotopes of hydrogen | Environmental isotopes | Nuclear materials

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