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Geigermdas;Müller counter (Geiger counter) An instrument for detecting ionizing radiation which is used in general geologic prospecting. It consists of a cylindrical metal cathode with a wire anode along its axis, the whole being enclosed in a thin-walled tube filled with low-pressure inert gas. In operation the cathode carries a charge of about 1000 volts, which is just short of that level needed to produce an electrical discharge across the cathode-anode space. A charged particle or gamma ray traversing this space collides with atoms of the inert gas, producing positive ions and negative electrons. Under the high voltage these are rapidly accelerated towards the cathode and anode, colliding on the way with other gas atoms and producing many more charged particles in a chain reaction. This avalanche arriving at the anode and cathode is registered as a pulse which is amplified to produce a click in a headphone set, or a succession of such pulses which can be expressed as a meter reading in milliroentgens per hour or counts per second. For more accurate surveys (especially from the air) a scintillation counter is required, which is a more sensitive instrument.