Photostimulated luminescence
Photostimulated luminescence (PSL) is the release of stored energy within a phosphor by stimulation with visible light, to produce a luminescent signal. Commonly this is used to allow the storage of a weak radioactive signal in a phosphor plate that takes the place of X-ray film, potentially over a prolonged period, which PSL transforms to obtain a highly sensitive image of the original pattern of radiation. Also known as an image plate, a photostimulable phosphor (PSP) plate can be used to record a two-dimensional image of the intensity short-wavelength (typically, X-ray) electromagnetic radiation. 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).
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.
Contents
Basic explanation
After the initial exposure, 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. This metastable state is higher in energy than the original condition, so a lower-frequency light source that is insufficient in energy to create more Eu3+ ions can return the trapped electrons to the conduction band. As these mobilized electrons encounter Eu3+ ions, they release a blue-violet 400 nm luminescence.[1] This light is produced in proportion to the number of trapped electrons, and thus in proportion to the original X-ray signal. It can be collected (often by a photomultiplier tube), enabling the resulting signal to be converted into a digital image.
This process is also known as Photostimulable Luminescence (PSL).
Unlike film, a PSP plate can be reused: plates can be "erased," by exposing the plate to room-intensity white light.
History
Image plates were pioneered for commercial use by Fuji in the 1980s.
Uses
Medical X-ray Imaging
In modern hospitals, a PSP plate is used for X-ray imaging in place of the photographic plate, in a process called computed radiography. The PSP plate can be used over and over again.
X-ray Diffraction Studies
Image plate detectors have been used in numerous crystallography studies.
See also
References
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