 3H is known as tritium and contains one proton and two neutrons in its nucleus . It is radioactive , decaying into helium @-@ 3 through beta decay with a half @-@ life of 12 @.@ 32 years . It is so radioactive that it can be used in luminous paint , making it useful in such things as watches . The glass prevents the small amount of radiation from getting out . Small amounts of tritium are produced naturally by the interaction of cosmic rays with atmospheric gases ; tritium has also been released during nuclear weapons tests . It is used in nuclear fusion reactions , as a tracer in isotope geochemistry , and in specialized self @-@ powered lighting devices . Tritium has also been used in chemical and biological labeling experiments as a <unk> . 
