A device for detecting ionizing radiations, whether corpuscular
(α-, β-particles), or electromagnetic (X- and
gamma-rays). It consists essentially of a fine wire anode (e.g.,
tungsten) surrounded by a coaxial cylindrical metal cathode, mounted
in a glass envelope containing gas at low pressure. A large potential
difference (800 to 2000 volts) is maintained between the anode and the
cathode. The ionizing particle can enter through a thin glass or mica
window. The particle produces ionization of gas molecules. The ions
are accelerated by the electric field and produce more ions by
collisions, causing the ionization current to build up rapidly. The
current, however, decays quickly since the circuit has a small time
constant. There is therefore a momentary potential surge which may be
amplified and made to actuate a relay to advance a mechanical counter,
or to produce a click in a loudspeaker. Same as Geiger-Mulle counter.
See also: Named after Hans Geiger (1882-1945), the German physicist, who invented the instrument.
He is also known for his work on atomic theory and cosmic rays;
→ counter.
