A phosphor is a material that luminesces when exposed to a stimulating radiation. Often there is a matrix, for example ZnS and an activator. Examples of activators are silver in ZnS:Ag or copper in ZnS:Cu. The activator concentration in the matrix can be less than 0.01% in some systems up to well over several percent in others. The properties of the luminescence depend strongly on the activator selected. Sometimes the matrix compound luminesces without an activator, e.g. calcium tungstate CaWO4. This phosphor is the mineral scheelite and like some others was characterized and used from its mineral form and later synthesized. Luminescence in many phosphors can be excited by ionizing radiation like X-rays or UV-light.
Some companies have concentrated on development and supply of phosphors for application in lighting where fluorescence dominates and in television where excitation is with energetic electrons (cathode-rays).
Screens are thin layers of granular phosphor held together with a transparent binder. This two dimensional distribution of the phosphor is useful for covering large areas and for imaging. They may be made of phosphor grains mixed in the binder and painted or allowed to settle onto cardboard, plastic or glass plates. The binder may also allow fabrication of self supporting films. If the exciting radiation is ultraviolet light the device is a fluorescent screen. If the excitation is with X-rays we have a radio luminescent screen more commonly called an X-ray screen. If this screen is used next to and with the purpose of exposing neighbouring photographic plates or film it is an (X-ray) intensifying screen.
In late 1896 Rutherford discovered and immediately applied X-rays to medical imaging and others quickly followed. The images were recorded on photographic plates and the required exposure time was dramatically reduced by placing a phosphor screen in close contact. Separately Edison had found that calcium tungstate was a very bright phosphor. By 1916 photographic film with calcium tungstate Intensifying screens, on both sides, was the preferred system for general medical images which tolerate some phosphorescence or afterglow. This phosphor was generally preferred for this application until the 1970s.
For fluoroscopy the favoured phosphor changed in the 1930s to ZnS when Levy West eliminated phosphorescence/afterglow by doping with ppm levels of nickel. But even more signal was needed and image intensifiers were developed. That effort led to columnar Thallium doped Caesium Iodide (CsI(Tl)) cathodes and gadolinium oxysulphide (Gadox) ones which were also used outside the image intensifier. Today Scintacor offers both scintillators.
Scintacor’s X-ray screens are based on Gadox doped with either Terbium or Praseodymium (Gadox:Tb; Gadox:Pr). The columnar structured CsI(Tl) X-ray imaging products are usually not referred to as screens because they are not granular.
X-Ray detectors are rated on effectiveness at converting X-rays to light which in turn depends on absorption and on the efficiency of converting absorbed energy to light. Further if imaging is involved the system also depends strongly on how the emitted light is distributed. These
two are referred to as light output and resolution or more specifically, pulse height and MTF. In some cases, such as dynamic imaging, the overall performance is also related to how rapidly the light is produced and whether or not the glow persists.
Let’s look at how screen design incorporates these performance parameters. Absorption depends strongly on the detecting material’s atomic number and effective density. Light production and timing are set by the choice of dopant and optimization of its concentration. These parameters are established first by selecting the phosphor. But the light output and distribution require selection of grain size, particle size distribution, and on screen thickness. X-ray absorption also changes with these values in ways that require making trade-offs. For example, for a given grain size, X-ray absorption increases with thickness but increased light scatter and eventually light output falls with thickness. Similarly decreasing grain size improves resolution by increasing scatter. These trade-offs in grain size and screen thickness are complex and further influenced by surface, reflector and so forth. Fortunately, over the years, screen performance has been optimized for the various applications and a series of screens are offered as shown in Table 1 below
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Light Output (1)
MTF % @ 2lp/mm (2)
MTF % @ 5lp/mm (2)
Attenuation % (2)
Decay to 10% μs