Ionization: (4.4.1) General Description of the Phenomenon: The avalanche is a cascade reaction involving electrons in a region with a sufficiently high electric field in a gaseous medium that can be ionized, such as air. Following an original ionisation event, due to such as ionising radiation, the positive ion drifts towards the cathode, while the free electron drifts towards the anode of the device. If the electric field is strong enough, the free electron gains sufficient energy to liberate a further electron when it next collides with another molecule. The two free electrons then travel towards the anode and gain sufficient energy from the electric field to cause impact ionisation when the next collisions occur; and so on. This is effectively a chain reaction of electron generation, and is dependent on the free electrons gaining sufficient energy between collisions to sustain the avalanche. The total number of electrons reaching the anode is equal to the number of collisions, plus the single initiating free electron. The limit to the multiplication in an electron avalanche is known as the Raether limit. The Townsend avalanche can have a large range of current densities. In common gas filled tubes, such as those used as gaseous ionization detectors; magnitudes of currents flowing during this process can range from about 10¹⁸ amperes to about 10⁵ amperes. Any subsequent transitions to the ionisation processes of dark discharge, glow discharge, and finally to arc discharge are driven by increasing current densities. However in all these discharge regimes, the basic mechanism of conduction is avalanche breakdown