A detector of ionizing radiation. When a fast-moving charged particle traverses a Geiger-Müller counter, an electrical impulse is produced. These impulses can readily be counted by electronic circuits. Geiger-Müller (GM) counters, usually referred to simply as Geiger counters, are widely used to indicate the presence and intensity of nuclear radiations.
Cylindrical external-cathode Geiger-Müller counter, with thin soda glass and central wire of 0.003-in.-diameter (0.008-cm) tungsten. Metal spring keeps central wire taut.
A GM counter consists of a gas between two electrodes (see illustration). One electrode, usually cylindrical and hollow, is the cathode. The other electrode, a fine “wire stretched along the axis of the cylinder, is the anode. A potential of about 1000 volts is placed on the wire. When an atom of the gas between the two electrodes is ionized by collision with a charged particle passing through the gas, the electron produced in the collision is drawn toward the central wire. The electron then collides with the atoms of the gas. Near the central wire the electric field is very intense, and the electron may acquire enough energy between two collisions to allow it to ionize another atom. A second electron is then set free, and by successive collisions, an avalanche of electrons is produced which is then collected as charge on the central wire. This charge produces an electrical impulse which in typical cases may be 50 volts. See also Ionization chamber; Particle detector.
