Radioluminescence
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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.
Mechanism
Radioluminescence occurs when an incoming particle of ionizing radiation 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 that is radioactive. The electron then returns to its ground energy level by emitting the extra energy as a photon of light. A chemical that releases light of a particular color when struck by ionizing radiation is called a phosphor. Radioluminescent light sources usually consist of a radioactive substance mixed with, or in proximity to, a phosphor. Some sufficiently radioactive elements such as actinium or einsteinium have the ability to ionize the air and release a glow without any additives.
Common materials
- Radium: one of the earliest radioluminescent materials, it was commonly used in dial clocks and a variety of quack products. Due to its radiation hazard (it also gives off radon gas which is also harmful) it has been replaced by safer alternatives.
- Promethium: unlike radium, promethium is only a relatively low-energy beta-emitter, which, unlike alpha emitters, does not degrade the phosphor lattice and the luminosity of the material does not degrade so fast. The isotope promethium-147 is commonly used. Unlike radium, its half-life is only 2.62 years, meaning it cannot be used for more than a few years.
- Tritium: The latest generation of the 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. The low-energy 5.7 keV beta particles emitted by tritium cannot pass through the enclosing glass tube and even if they could, they are not able to penetrate human skin.
