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Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as beta particles. Radioluminescence is used as a low level light source for night illumination of instruments or signage or other applications where light must be produced for long periods without external energy sources. Radioluminescent paint used to be used for clock hands and instrument dials, enabling them to be read in the dark. Radioluminescence is also sometimes seen around high-power radiation sources, such as nuclear reactors and radioisotopes.
Radioluminescence occurs when an incoming radiation particle collides with an atom or molecule, exciting an orbital electron to a higher energy level. The particle usually comes from the radioactive decay of an atom of a radioisotope, an isotope of an element which is radioactive. The electron then returns to its ground energy level by emitting the extra energy as a photon of light. The emitted photon is often in the ultraviolet energy range or higher and so invisible to the human eye. Therefore, in radioluminescent light sources, the radioactive substance is mixed with a phosphor, a chemical that releases light of a particular color when struck by the particle.
Currently, tritium is virtually the only radioisotope permitted to be used commercially as a radioluminescent light source. 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 molecules and cause them 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 source, radium (below), which proved to be a significant radiological hazard. The low-energy 5.7 keV beta particles emitted by tritium cannot pass through the enclosing glass tube. Even if they could, they are not able to penetrate human skin. Tritium is only a health threat if ingested. Since tritium is a gas, if a tritium tube breaks, the gas dissipates in the air and is diluted to safe concentrations.
Tritium has a half-life of 12.3 years, so the brightness of a tritium light source will decline to half its initial value in that time.
Historically, a mixture of radium and copper-doped zinc sulfide was used to paint instrument dials giving a greenish glow. Phosphors containing copper-doped zinc sulfide (ZnS:Cu) yield blue-green light; copper and manganese-doped zinc sulfide (ZnS:Cu,Mn), yielding yellow-orange light, are also used. Radium-based luminescent paint is no longer used due to the radiation hazard posed to those manufacturing the dials. These phosphors are not suitable for use in layers thicker than 25 mg/cm2, as the self-absorption of the light then becomes a problem. Furthermore, zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium.
The case of the "Radium Girls", workers in watch factories in the early 1920s who painted watch faces with radium paint and later contracted fatal cancer through ingesting radium when they pointed their brushes with their lips, increased public awareness of the hazards of radioluminescent materials, and radioactivity in general.