Tritium

From CPFWiki

Tritium is an isotope of Hydrogen which is typically designated H3. It is radioactive, but not in the sense most people think of (Gamma radiation); Tritium radioactive decay is via "Beta" radiation which are electrons (of low average energy, around 6 keV Tritium, thus it is completely safe if enclosed in a sealed container. Breaking open a container of tritium and breathing it in is, of course, a very different matter.

Tritium light sources can be best described as a strong glow. They have a clear advantage over conventional luminous light sources in that they do not need to be "charged up."

Tritium does not in itself emit light but excites phosphors, thereby generating light. It is therefore most often encountered in glass capsules with an internal phosphor coating. These inserts are used in gun sights, archery sights, self luminous exit signs, watches, compasses, and the Traser Glowring keychain. Due to U.S. regulations regarding radioactive substances, all of the above items can be legally sold in the U.S. except the glow rings. (Note: restrictions are on the sale and not necessarily on the possession)

The half life of tritium is 12.3 years which means that a tritium based light source will be half as bright after 12.3 years.