Wiki said:
The emitted electrons from the radioactive decay of small amounts of tritium cause phosphors to glow so as to make self-powered lighting devices called betalights, which are now used in firearms night sights, watches (See Luminox for example), exit signs, map lights, and a variety of other devices. This takes the place of radium, which can cause bone cancer and has been banned in most countries for decades. Commercial demand for tritium is 400 grams per year[3] and costs approximately $US30,000 per gram[18]
