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
Isotope left in the periodic table, whereas beta decay emission produced an element one place to the right. Soddy recognized that emission of an alpha particle followed by two beta particles led to the formation of an element chemically identical to the initial element but with a mass four units lighter and with different radioactive properties. Soddy proposed that several types of atoms (differing in radioactive properties) could occupy the same place in the table. For example, the alpha-decay of uranium-235 forms thorium-231, whereas the beta decay of actinium-230 forms thorium-230. The term "isotope", Greek for "at the same place", was