Solid state chemistry and luminescence of X-ray phosphors

X-ray phosphors are solid state inorganic materials used in medical X-ray imaging applications. The role of these phosphors is to reduce the exposure of the patient to X-rays while maintaining the structural features of the X-ray image. This is done by amplifying every X-ray photon absorbed by a phosphor screen into hundreds of visible- or UV-light photons which are then recorded by a detector, such as a piece of photographic film. Conventional screen/film imaging methods, still the highest quality, most widely used, and most cost effective diagnostic tools in radiology, are emphasized. A good X-ray phosphor must meet challenging prerequisites: good X-ray absorption in the diagnostic medical energy range (15–100 keV), high luminescence efficiency, emission in the green to near-UV region, proper crystallite size and shape, air and water stability, and easy large-scale production. The commercially important X-ray phosphors (e.g., LaOBr:Tm, Gd 2 O 2 S:Tb and M′-YTaO 4 ) are all synthesized by high-temperature reactions (800–1300°C) in the presence of a flux. The chemistry and properties of these phosphor compounds and recent discoveries in reaction chemistry are discussed. Research on new X-ray phosphors is also described.