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Concept
Gaseous ionization detector
Summary
Gaseous ionization detectors are radiation detection instruments used in particle physics to detect the presence of ionizing particles, and in radiation protection applications to measure ionizing radiation.
They use the ionising effect of radiation upon a gas-filled sensor. If a particle has enough energy to ionize a gas atom or molecule, the resulting electrons and ions cause a current flow which can be measured.
Gaseous ionisation detectors form an important group of instruments used for radiation detection and measurement. This article gives a quick overview of the principal types, and more detailed information can be found in the articles on each instrument. The accompanying plot shows the variation of ion pair generation with varying applied voltage for constant incident radiation. There are three main practical operating regions, one of which each type utilises.
The three basic types of gaseous ionization detectors are 1) ionization chambers, 2) proportional counters, and 3) Geiger–Müller tubes
All of these have the same basic design of two electrodes separated by air or a special fill gas, but each uses a different method to measure the total number of ion-pairs that are collected. The strength of the electric field between the electrodes and the type and pressure of the fill gas determines the detector's response to ionizing radiation.
Ionization chambers operate at a low electric field strength, selected such that no gas multiplication takes place. The ion current is generated by the creation of "ion pairs", consisting of an ion and an electron. The ions drift to the cathode while free electrons drift to the anode under the influence of the electric field. This current is independent of the applied voltage if the device is being operated in the "ion chamber region". Ion chambers are preferred for high radiation dose rates because they have no "dead time"; a phenomenon which affects the accuracy of the Geiger–Müller tube at high dose rates.
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