SCINTILLATION COUNTER Radiation DetectorsScintillation counters are constructed by coupling a suitable scintillation phosphor to a light-sensitive photomultiplier tube. Figure 25 illustrates an example of a scintillation counter usinga thallium-activated sodium iodide crystal.There are three classes of solid state scintillation phosphors: organic crystals, inorganic crystals,Figure 25 Scintillation Counterand plastic phosphors.Inorganic crystals include lithium iodide (LiI), sodium iodide (NaI), cesium iodide (CsI), and zincsulfide (ZnS). Inorganic crystals are characterized by high density, high atomic number, andpulse decay times of approximately 1 microsecond. Thus, they exhibit high efficiency fordetection of gamma rays and are capable of handling high count rates.Organic scintillation phosphors include naphthalene, stilbene, and anthracene. The decay timeof this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently usedin the detection of beta particles.Plastic phosphors are made by adding scintillation chemicals to a plastic matrix. The decayconstant is the shortest of the three phosphor types, approaching 1 or 2 nanoseconds. The plastichas a high hydrogen content; therefore, it is useful for fast neutron detectors.IC-06 Page 46 Rev. 0
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