Alphatron is a type of cold cathode ionization gauge and can also be considered as a radioactive ionization gauge. As the cold cathode and hot cathode types earlier explained, are composition dependent, the transfer characteristics may be obtained relative to air for different gases and the system can be used as a leak detector.
The device uses alpha particles in order to ionize the gas in the vacuum chamber. The number of ions formed in the chamber is directly proportional to the gas pressure, if the chamber dimensions are shorter than the range of alpha particles. The figure below shows the schematic diagram of an alphatron.
The ions produced by the alpha particles are collected by the collector electrode and a current between 10-13 and 10-9 Amperes will flow though the resistor R. The output voltage e0 is measured using a high input impedance output meter. The device has a range between 103 to 10-3 Torr.