Photostimulated luminescence

Photostimulated luminescence (PSL) is the release of stored energy within a phosphor by stimulation with visible light, to produce a luminescent signal. X-rays may induce such an energy storage. A plate based on this mechanism is called a photostimulable phosphor (PSP) plate and is one type of X-ray detector used in projectional radiography. Creating an image requires illuminating the plate twice: the first exposure, to the radiation of interest, "writes" the image, and a later, second illumination (typically by a visible-wavelength laser) "reads" the image. The device to read such a plate is known as a phosphorimager (occasionally spelled phosphoimager, perhaps reflecting its common application in molecular biology for detecting radiolabeled phosphorylated proteins and nucleic acids). Projectional radiography using a photostimulable phosphor plate as an X-ray detector can be called "phosphor plate radiography" or "computed radiography" (not to be confused with computed tomography which uses computer processing to convert multiple projectional radiographies to a 3D image). On photostimulable phosphor (PSP) plates, the phosphor layer is typically 0.1 to 0.3 mm thick. After the initial exposure by short-wavelength (typically, X-ray) electromagnetic radiation, excited electrons in the phosphor material remain 'trapped' in 'colour centres' ("F-centers") in the crystal lattice until stimulated by the second illumination. For example, Fuji's photostimulable phosphor is deposited on a flexible polyester film support with grain size about 5 micrometers, and is described as "barium fluorobromide containing a trace amount of bivalent europium as a luminescence center". Europium is a divalent cation that replaces barium to create a solid solution. When Eu2+ ions are struck by ionizing radiation, they lose an additional electron to become Eu3+ ions. These electrons enter the conduction band of the crystal and become trapped in the bromine ion empty lattice of the crystal, resulting in a metastable state that is higher in energy than the original condition.