The proportional counter is a type of gaseous ionization detector device used to count particles of ionizing radiation. A key feature is its ability to measure the energy of incident radiation, and it is widely used where discrimination between radiation types is required, such as between alpha and beta particles.
A proportional counter uses a combination of the mechanisms of a Geiger-Muller tube and an ionisation chamber, and operates in an intermediate voltage region between these. Considering a gas-filled chamber with a wire anode, if the field strength everywhere in the volume is below a critical value, Townsend avalanches do not occur at all, and the detector operates as an ionization chamber. If the applied voltage is too high, complete ionisation of the fill gas occurs with almost each ion pair and the detector operates as a Geiger-Müller counter, with the consequent loss of incident particle energy information. The accompanying plot shows the proportional operating region for a co-axial cylinder arrangement.
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