Geiger counter or Geiger-Müller (G-M) counter [key], instrument for the detection and quantitative determination of ionizing radiation such as the alpha and beta rays given off by radioactive minerals and cosmic rays. It was first developed by Hans (J. W.) Geiger and later improved by Geiger and A. Müller. Variously designed for different uses, it consists commonly of a gas-filled metal cylinder that acts as one electrode, and a needle or thin taut wire along the axis of the cylinder that acts as the other electrode. Glass caps used to seal the ends of the tube serve as insulators. A voltage applied to the device is so adjusted that it is almost strong enough to cause a current to pass through the gas from one electrode to the other. The gas becomes ionized whenever the counter is brought near radioactive substances, however little the quantity and however faint the emanations. The resulting ionized particles of gas are able to carry the current from one electrode to the other, thus completing a circuit. Once established, the current is amplified by an electronic device so that it can indicate by an audible click the presence of ionized particles. The gas quickly returns to its normal nonionized state, permitting each new particle or ray to register, making counting possible. The instrument can also register ionization by a pointer and scale called a rate meter. The Geiger counter is used in the detection of cosmic rays and for locating radioactive minerals. Counters enable radioactive tracers to be followed as they make their way through complex organisms such as the human body; in medicine Geiger counters have found several successful uses in the location of malignancies. They are used also to follow radioactive isotopes in chemical reactions. For a number of research applications the Geiger counter has been largely replaced by scintillometers and other more complex devices.
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