GEIGERMULLER COUNTER (I) Department of （盖革缪勒计数器(I)部门）.pdf

GEIGER-MULLER COUNTER (I) Introduction A typical G.M. tube consists essentially of a cylindrical cathode in the form of a graphite coating on the inner wall of a glass envelope and an anode in the form of fine tungsten wire which stretches within and along the axis of the tube. Usually it is filled with a mixture of an inert gas (argon or neon) at a partial pressure of about 100 torr and a quenching gas (halogens or organic vapours) at about 10 torr. To allow 1onis1ng particles to enter the tube, a window covered with a thin sheet of mica is provided at one end of a tube. In operation, a sufficiently large potential difference i.e. applied across the anode and cathode of the tube so that a high radial electric field near the central wire is obtained. Under this condition, electrons produced by ionizing collisions between a high-speed particle entering the tube and the inert gas atoms are accelerated towards the anode wire by the strong electric field and acquire within a very short distance a high speed of their own. Because of this speed, they too can ionize other atoms and free more electrons. This multiplication of charges repeats itself in rapid succession producing within a very short interval of time an avalanche of electrons. The electron avalanche is concentrated near the central wire while the positive ions, being much heavier, drift slowly toward the cathode. For a G.M. tube with a cathode of radius 1cm, the time of flight of the positive ions is roughly about 100 microseconds, which is about 100 times longer than the time necessary to build up the electron avalanche. The consequence of this is that after the initiation of an electron avalanche by an entering particle the slowly moving positive ion sheath around the anode wire increases the effective radius of the anodes. The electric field round the wire therefore drops to