Tritium

Tritium is an isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas a hydrogen nucleus consists of just one proton. Its atomic weight is 3.

It is radioactive (an average 6 keV beta emitter) and has a half-life of 12.3 years. The low-energy beta radiation from tritium cannot penetrate human skin, so tritium is only dangerous if consumed in large quantities.

Tritium occurs naturally due to cosmic rays interacting with deuterium in the atmosphere. It is produced in nuclear reactors by exposing Li⁶ to a neutron source.

Tritium figures prominately in studies of nuclear reactions, especially nuclear fusion due to its favorable reaction cross section and high energy yield. Please explain further.

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