Detectors (GM) tubes typically identify gamma and beta radiation. Some GM tubes
also detect alpha radiation sources. The GM tube is filled with a low-pressure
inert gas (He or Ar). It has two electrodes, the walls of the tube form the cathode,
and the anode is a wire in the center of the tube. The voltage across the two
electrodes creates a strong electric field.
radiation strikes the tube, some of the gas molecules are ionized. The positive
ions are accelerated towards the cathode and the electrons towards the anode.
Close to the anode in the "avalanche region" the electrons gain
sufficient energy to ionize additional gas molecules and create a large number
of electron avalanches which spread along the anode and effectively multiply
the effect. The GM tube counts the number of gamma rays or beta particles
entering the detector per second. They do not provide identification
information unless coupled with alternate detectors.