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In particle physics, '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, the resulting electrons emitted can cause a current flow which can be measured in different ways. Examples are ionization chambers, proportional counters and Geiger-Müller tubes, all of which have the same basic design of two electrodes separated by air or a special counting gas. The strength of the electric field between the electrodes determines the detector's response to ionizing radiation.
Smoke detectors work on this principle; a bit of radioactive americium is placed so that it maintains a current between the two plates. If smoke gets in 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.
Geiger-Müller tubes are the primary components of Geiger counters; as the gas ionizes a current is able to pass through a potential difference between two plates. The peak in current is passed through an audio device producing the clicks of the Geiger counter.
Proportional counters work on a similar principle, but by using a smaller potential difference between the electrodes, the determination of ionization energy is allowed for.
Wire chambers are essentially gaseous ionization detectors with many electrodes instead of one.