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  Geiger counter is a portable device which is used for the detection and counting of ionized particles and radiation
It consists of a hollow metallic cylinder, one end of which is closed by an insulating cap. At the center of the cap is fixed a stiff straight wire along the axis of cylinder .A thin mica or glass disc closes the other end which also serves as the entrance window for ionizing particles. The tube contains a special mixture of air, argon, alcohol at a pressure of 50-100 mmHg.


A potential difference of the range of 1000V is maintained between the metal cylinder and the axial wire through a suitable series resistor of 109 ohms. When an ionizing particle enters the tube through the window, it ionizes some gas molecules in it. These ions are accelerated by the strong radial electric field producing more ions by collision which produces ionization current so a momentary current flows between the wire and the cylinder and also through the resistor R. The ends of “R” are connected to a loud speaker or an electronic counter.
 Each time a  particle enters the counter and ionization current pulse is created which gives a click in the loud speaker or a count in a counter.
In the case of ionizing radiation, the numbers of counts register by the counter measures the intensity or ionizing power of incident radiation.


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