A gas-filled tube that is used to detect ionizing radiation. It is a very versatile radiation detector as it can be used to detect and measure a wide range of energies and intensities of radiation. A typical chamber has a sensitive volume defined using a guard ring, contained within it, and two electrodes with a potential difference maintained across them. When the gas is ionized by a beam of radiation the ions migrate to the electrodes under the influence of the applied voltage.

In conjunction with a suitable external circuit the ionization chamber can be used as a counter – an ionization counter – to count particles, such as alpha or beta particles, a pulse of current being produced by each particle of radiation. The most common application however is as a continuous measuring instrument in which an ionizing current is produced and the size of the current is proportional to the intensity of the ionizing radiation.

The sensitivity of the instrument is proportional to the mass of gas enclosed within the sensitive volume, and the size of instrument depends on the intensity of radiation to be measured. Extremely large chambers have been produced for measuring background radiation levels and very small chambers are used to calibrate high-output beams of X-rays or electrons.

https://www.orau.org/ptp/collection/ionchamber/introionizationchamberr.htm A short introduction to ionization chambers