gaseous ionization detectors are detectors designed to seek the presence of particles (a particle detector). 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 in different ways. The three basic types of gaseous ionization detectors are:
All of these have the same basic design of two electrodes separated by air or a special counting gas, but each uses a unique method to form the total number of ion-pairs that are collected. The strength of the electric field between the electrodes determines the detector's response to ionizing radiation. Ionization chambers operate at a low voltage, selected such that no avalanche multiplication takes place. Current is generated by the drift of ions to anode or cathode. Proportional counters operate at a slightly higher voltage, selected such that limited avalanche multiplication takes place. The avalanche is localised, rather than spreading widely through the detecting gas, so that an output current pulse is generated which is proportional to the detected particle energy. The term "gas proportional detector" (GPD) is generally used in radiometric practice, and the property of being able to detect particle energy is particular useful when using flat arrays for Alpha and Beta particle detection and discrimination. Geiger-Müller tubes are the primary components of Geiger counters. They operate at an even higher voltage, selected such that avalanche multiplication is total, and the entire gas volume ionizes from a single event. The peak in current is passed through an audio device producing the clicks of the Geiger Mueller counter. Wire chambers are essentially gaseous ionization detectors with many electrodes instead of one. Ionization-type smoke detectors are gaseous ionization detectors. A bit of radioactive americium is placed so that it maintains a current between the two plates. If smoke gets between the plates where the ionization is taking place, the ionized gas can be neutralized leading to a reduced current. The decrease in current triggers a fire alarm.