Radioluminescence luminosity over their working life. The latest generation of radioluminescent materials is based on tritium, a radioactive isotope of hydrogen with half-life of 12.32 years that emits very low-energy beta radiation. It is used on wristwatch faces, gun sights, and emergency exit signs. The tritium gas is contained in a small glass tube, coated with a phosphor on the inside. Beta particles emitted by the tritium strike the phosphor coating and cause it to fluoresce, emitting light, usually yellow-green. Tritium is used because it is believed to pose a negligible threat to human health, in contrast to the previous radioluminescent
Tritium in nuclear reactors. Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural production, a fast neutron (which must have energy greater than 4.0 MeV) interacts with atmospheric nitrogen: Worldwide, the production of tritium from natural sources is 148 petabecquerels per year. The global equilibrium inventory of tritium created by natural sources remains approximately constant at 2,590 petabecquerels. This is due to a fixed production rate and losses proportional to the inventory. According to a 1996 report from Institute for Energy and Environmental Research on the US Department of Energy, only of