Geiger counter, or Geiger-Müller tube, instrument for detecting the presence of and measuring radiation, such as alpha particles, and beta, gamma, and X rays. It can count individual particles at rates up to about 10,000/sec and is used widely in medicine, in industries that use radioactive materials, and in prospecting for radioactive ores. A fine wire anode runs along the axis of a metal cylinder that has sealed insulating ends, contains a mixture of argon or neon and methane at low pressure, and acts as the cathode, the potential between them being about 1 kV. Particles entering through a thin window cause ionization in the gas; electrons build up around the anode, and there is a momentary drop in the interelectrode potential, which appears as a voltage pulse in an associated counting circuit.
See also: Radiation.