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Tritium is an isotope of the chemical element hydrogen. It has not only a single proton but also two neutrons in the nucleus of its atoms. Although technically it is still the element hydrogen, it has its own chemical symbol, T. Chemically, tritium reacts in exactly the same manner as hydrogen, although slightly slower because of its greater atomic weight. A tritium atom has almost three times the mass of a regular hydrogen atom: the atomic weight of tritium is 3.016 whereas the atomic weight of hydrogen is 1.008. Tritium is radioactive, with a half-life of 12.26 years. Its nucleus emits a low-energy beta particle, leaving behind an isotope of helium, helium-3, that has a single neutron in its atomic nucleus. (The common isotope of helium, helium-4, contains two neutrons in its atomic nucleus.) No gamma rays, which are high-energy electromagnetic radiation, are emitted in the decay of tritium, so the radioactive decay of tritium is of little hazard to humans.

The heavier atomic weight of tritium has an effect on the physical properties of this hydrogen isotope. For example, tritium has a boiling point of 25K (-415°F; -248°C), compared with ordinary hydrogen's boiling point of 20.4K (-423°F; -252.8°C). Molecules containing tritium show similar variances. For example, water made with tritium and having the formula T2O has a melting point of 40°F (4.5°C), compared with 32°F (0°C) for normal water.

Tritium was present in nature at very low levels, about 1 atom every 1018atoms of hydrogen, before atmospheric nuclear bomb testing. It is produced in the upper atmosphere, as highly energetic neutrons in cosmic rays bombard nitrogen atoms, making a tritium atom and an atom of carbon-12.

Industrially, tritium is prepared by bombarding deuterium with other deuterium atoms to make a tritium atom and a regular hydrogen atom:

The resulting two types of hydrogen can be separated by distillation. Another way to make tritium is to bombard lithium-6 atoms (the less-abundant isotope of lithium) with neutrons, which produces a helium atom and a tritium atom:

Due to the testing of nuclear weapons in the atmosphere (before such testing was banned), the tritium content of the atmosphere rose to ~500 atoms per 1018, declining steadily even since the ban due to radioactive decay.

Tritium is used in nuclear fusion processes because it is easier to fuse tritium nuclei than either of the other isotopes of hydrogen. However, because of its scarcity, it is commonly used with deuterium in fusion reactions:

This is the nuclear reaction that occurs in fusion bombs, or hydrogen bombs. Such weapons must be recharged periodically due to the radioactive decay of the tritium. Fusion reactions are also being used in experimental fusion reactors as scientists and engineers try to develop controllable nuclear fusion for peaceful power.

Tritium is used as a tracer because it is relatively easy to detect due to its radioactivity. In groundwater studies, tritium-labeled water can be released into the ground at one point, and the amount of tritium-labeled water that appears at other points can be monitored. In this way, the flow of water through the ground can be mapped. Such information is important when drilling oil fields, for example. Tritium can also be substituted for ordinary hydrogen in organic compounds and used to study biological reactions. Because of its radioactivity, it is easy to follow the tritium as it participates in biochemical reactions. In this way, specific metabolic processes at the cellular level can be monitored. Tritium is also used to make "glow-in-the-dark" objects by mixing tritium-containing compounds with compounds like zinc sulfide, which emit light when struck by alpha or beta particles from nuclear decay.



Evans, E.A. Tritium and its compounds. New York: Wiley, Inc., 1974.

Herman, R. Fusion: The Search for Endless Energy. Oxford: Cambridge University Press, 1990.

Parker, Sybil, ed. McGraw-Hill Encyclopedia of Chemistry. 2nd ed. New York: McGraw Hill, 1999.

Romer, A. Radioactivity and the Discovery of Isotopes. New York: Dover, 1970.

David W. Bal


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Beta particle

—One type of radioactive decay particle emitted from radioactive atomic nuclei. A beta particle is the same thing as an electron.

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