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an instrument for detecting and measuring the intensity of ionizing radiation. It consists of a gas-filled tube containing a fine wire anode along the axis of a cylindrical cathode with a potential difference of several hundred volts. Any particle or photon which ionizes any number of gas molecules in the tube causes a discharge which is registered by electronic equipment. The magnitude of the discharge does not depend upon the nature or the energy of the ionizing particle Compare proportional counter

An electronic instrument that detects and measures nuclear radiation, such as x-rays or gamma rays. The Geiger counter consists of a gas-filled tube with a charged electrode connected to a counter. As radiation passes through the gas it ionizes atoms along its path. The ions are attracted to the charged electrode, creating pulses of electric current that are registered by the counter.