Tritium

[Rennhack]

A radioactive isotope of hydrogen (one proton, two neutrons). Because it is chemically identical to natural hydrogen, tritium can easily be taken into the body by any ingestion path. It decays by beta emission. It has a radioactive half-life of about 12.5 years.

[Rennhack]

See Also:

http://www.nukeworker.com/study/radiation_faqs/h3.shtml
http://blast02.lns.mit.edu/targets/ion/manuals/tritium_hdbk1079.pdf
http://www.eh.doe.gov/techstds/standard/hdbk1105/HDBK11052002.pdf