Radiation DetectorsSCINTILLATION COUNTERSCINTILLATION COUNTERThe scintillation counter is a solid state radiation detector.EO 2.7 DESCRIBE the operation of a scintillation counter toinclude:a. Radiation detectionb. Three classes of phosphorsc. Photomultiplier tube operationThe scintillation counter is a solid state radiation detector which uses a scintillation crystal(phosphor) to detect radiation and produce light pulses. Figure 24 is important in the explanationof scintillation counter operation.As radiation interacts in theFigure 24 Electronic Energy Band of an Ionic Crystalscintillation crystal, energy istransferred to bound electrons of thecrystal’s atoms. If the energy that istransferred is greater than theionization energy, the electron entersthe conduction band and is free fromthe binding forces of the parent atom.This leaves a vacancy in the valenceband and is termed a hole. If theenergy transferred is less than thebinding energy, the electron remainsattached, but exists in an excitedenergy state. Once again, a hole iscreated in the valence band. Byadding impurities during the growth ofthe scintillation crystal, themanufacturer is able to produceactivator centers with energy levelslocated within the forbidden energygap. The activator center can trap amobile electron, which raises the activator center from its ground state, G, to an excited state,E. When the center de-excites, a photon is emitted. The activator centers in a scintillationcrystal are referred to as luminescence centers. The emitted photons are in the visible region ofthe electromagnetic spectrum.Rev. 0 Page 45 IC-06
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