Geiger-Muller 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. When 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.