Radioluminescence

Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as alpha particles, beta particles, or gamma rays. 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.

Since radioactivity was discovered around the turn of the 20th century, the main application of radioluminescence has been in radioluminescent paint, used on watch and compass dials, gunsights, aircraft flight instrument faces, and other instruments, to allow them to be seen in the dark. Radioluminescent paint consists of a mixture of a chemical containing a radioisotope with a radioluminescent chemical (phosphor). The continuous radioactive decay of the isotope's atoms releases radiation particles which strike the molecules of the phosphor, causing them to give off light. The constant bombardment by radioactive particles causes the chemical breakdown of many types of phosphor, so radioluminescent paints lose some of their luminosity over their working life.

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