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Tritium (symbol T or 3H) is an isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas a normal hydrogen nucleus consists of just one proton. Its atomic weight is 3.016049. It is a gas (T2 or 3H2) at standard temperature and pressure.

It is radioactive (an average 6.5 keV energy Emax=18.6 keV pure beta emitter) and has a half-life of 12.43years. The low-energy beta radiation from tritium cannot penetrate human skin, so tritium is only dangerous if consumed in large quantities. Its low energy also makes it difficult to detect tritium labelled compounds except by using liquid scintillation counting.

Tritium occurs naturally due to cosmic rays interacting with deuterium in the atmosphere. It is produced in nuclear reactors by activation of Heavy water moderator by D(n,gamma)T and 10B(n,t)8Be or by nuclear reactors by exposing Li6 to a neutron source.

Tritium figures prominently in studies of nuclear reactions, especially nuclear fusion due to its favorable reaction cross section and high energy yield (17.6 keV for T(d,n)4He reaction). 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 (as is the case in the core of the Sun, for example), than their random motions can overcome such electrical repulsion, and they can be be jammed together into new atoms. Since tritium has the same charge as ordinary hydrogen, it experiences the same electrical repulsive force. 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't burn hydrogen, but do indeed burn deuterium.

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.

Small amounts are used with phosphors for self-illuminating devices such as watches and exit signs.

Tritium was first produced in 1934 from deuterium, another isotope of hydrogen, by Ernest Rutherford, working with Marcus L. 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. W. F. Libby discovered that tritium could be used for dating water, and therefore geological samples and vintage wines.

Tritium combines with oxygen to form a liquid called tritiated water (T2O).