Operation
When a charged particle strikes the scintillator, its atoms are excited and photons are emitted. These are directed at the photomultiplier tube's photocathode, which emits electrons by the photoelectric effect. These electrons are electrostatically accelerated and focused by an electrical potential so that they strike the first dynode of the tube. The impact of a single electron on the dynode releases a number of secondary electrons which are in turn accelerated to strike the second dynode. Each subsequent dynode impact releases further electrons, and so there is a current amplifying effect at each dynode stage. Each stage is at a higher potential than the previous to provide the accelerating field. The resultant output signal at the anode is in the form of a measurable pulse for each photon detected at the photocathode, and is passed to the processing electronics. The pulse carries information about the energy of the original incident radiation on the scintillator. Thus both intensity and energy of the radiation can be measured.
The scintillator must be in complete darkness so that visible light photons do not swamp the individual photon events caused by incident ionising radiation. To achieve this a thin opaque foil, such as aluminised mylar, is often used, though it must have a low enough mass to prevent undue attenuation of the incident radiation being measured.
The article on the photomultiplier tube carries a detailed description of the tube's operation.
Read more about this topic: Scintillation Counter
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