Why does a plateau exist in the Geiger-Muller region?

Answer 1

In a Geiger-Müller (GM) tube, there is a central anode and a "case" that is the cathode. A voltage is applied across these two elements, and an ionizing particle passing through the GM tube will cause current flow. But how much? Let's step through things and check it out. At low voltage, any electrons released by the cathode will eventually be collected by the anode, but there is no appreciable "current" per se in this, the ionization region. Things are still pretty "tame" in the GM tube through this range of voltages. By applying more voltage, an ionizing event will generate more current flow, and this current flow will be proportional to the voltage in what is (naturally) the proportional region. And as we apply more voltage, gas amplification, or Townsend avalanche, which appeared at the beginning of this region, is increasing across the area of the anode. As we apply even more voltage, it will only make for limited additional current flow in an ionizing event because the limits of the geometry of the GM tube and of the gas media to ionize and "conduct more" with the increasing voltage are being reached. This is the limited-proportional region. As voltage is increased even more, we enter the Geiger-Müller region. In this region, the current avalanche in an ionizing event is so great that is causes a "shield" of positive ions around the anode. The high current "sucks up" all the electrons and blankets the anode in a positive field that prevents additional current flow even with an increase in voltage. This is the Geiger plateau. It's the operating region where additional differential voltage will not cause higher current flow in an ionizing event.