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Geiger Muller Counter



The common radiation detectors make use of the electric conductivity of a gas resulting from the ionization produced in it. The gas counter consists of a suitable chamber containing usually an inert gas such as argon at atmospheric pressure, a central electrode in the form of a wire and a thin end-window. The central electrode is kept at a positive potential (anode) and the chamber at negative potential (cathode). When a potential is applied between the electrodes the gas is ionized. The positively charged heavy ions move towards the walls of the container. The electrons move towards the anode. The ionization current can be measured by a suitable electronic circuit or can be be digitalized and measured as counts per minute. As we increase the applied voltage across the electrodes initially there will be no current till the threshold voltage for ionization is reached. There on the current starts increasing slowly at the early stage as the ions have a tendency to recombine till a sufficiently high potential is applied. The negatively charged electrons move swiftly towards the central anode and the heavy positive ions move relatively at a slower speed to the cathode. This does not seriously impair measurements as the applied potential relatively low. Further increase in the potential shows a plateau over a region of about 100 volts. In this region the output current remains steady as all atoms interacting with radioactive emissions get ionized. This region is known as the “saturation current region”. The current produced is a direct measure of the rate of production of charged ions. Among the three radiations α, β, and γ, the alpha particle has a short range as it has large mass and charge. Consequently this region is most sensitive to α particles and other radiations contribute very little. As the applied potential is quite low the current produced is small and requires good amplification. To eliminate any absorption of radiation by air and window material the radioactive source is kept inside the chamber itself.


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