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Pulses of light emitted by the scintillating material can be detected by a sensitive light detector, usually a photomultiplier tube (PMT). The photocathode of the PMT, which is situated on the backside of the entrance window, converts the light (photons) into so-called photoelectrons. The photoelectrons are then accelerated by an electric field towards the dynodes of the PMT where the multiplication process takes place. The result is that each light pulse (scintillation) produces a charge pulse on the anode of the PMT that can subsequently be detected by other electronic equipment, analyzed or counted with a scaler or a rate meter. The combination of a scintillator and a light detector is called a scintillation detector. Since the intensity of the light pulse emitted by a scintillator is proportional to the energy of the absorbed radiation, the latter can be determined by measuring the pulse height spectrum. This is called spectroscopy. To detect nuclear radiation with a certain efficiency, the dimension of the scintillator should be chosen such that the desired fraction of the radiation is absorbed. For penetrating radiation, such as g-rays, a material with a high density is required. Furthermore, the light pulses produced somewhere in the scintillator must pass the material to reach the light detector. This imposes constraints on the optical transparency of the scintillation material. When increasing the diameter of the scintillator, the solid angle under which the detector "sees" the source increases. This increases detection efficiency. Ultimate detection efficiency is obtained with so-called "well counters" where the sample is placed inside a well in the actual scintillation crystal. The thickness of the scintillator is the other important factor that determines detection efficiency. For electromagnetic radiation, the required thickness to stop say 90 % of the incoming radiation depends on the X-ray or g -ray energy. For electrons (e.g. b-particles) the same is true but different dependencies apply. For larger particles (e.g. a-particles or heavy ions) a very thin layer of material already stops 100 % of the radiation. The thickness of a scintillator can be used to create a selected sensitivity of the detector for a distinct type or energy of radiation. Thin (e.g. 1 mm thick) scintillation crystals have a good sensitivity for low energy X-rays but are almost insensitive to higher energy background radiation. Large volume scintillation crystals with relatively thick entrance windows do not detect low energy X-rays but measure high energy gamma rays efficiently. |
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a link for more information: Interactions in Scintillation Materials Scintillation Response to g-rays Scintillator Interaction with Charged Particles; a - and b-particle detection
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