ionization chamber, device for the detection and measurement of ionizing radiation. It consists basically of a sealed chamber containing a gas and two electrodes between which a voltage is maintained by an external circuit. When ionizing radiation, e.g., a photon, enters the chamber (through a foil-covered window), it ionizes one or more gas molecules. The ions are attracted to the oppositely charged electrodes; their presence causes a momentary drop in the voltage, which is recorded by the external circuit. The observed voltage drop helps identify the radiation because it depends on the degree of ionization, which in turn depends on the charge, mass, and speed of the photon. See radioactivity.