An ionization chamber is a type of radiation detection device that is used to detect and measure charge from the number of ion pairs created within a gas caused by incident radiation. It consists of two opposing electrodes placed in a gas-filled container, with a high voltage applied. As charged particles (radiation) pass through the gas, gas molecules ionize to produce ions and electrons. A positively charged electret is used together with an ionization chamber made of an electrically conductive plastic.
The ionization chamber provides an indirect representation of the energy deposited in the chamber, allowing it to measure dose or dose rate. Operation as an ionization chamber involves the use of an applied voltage that is large enough to collect all ion pairs (positive ion and removed electron) produced in the gas by a radioactive source, but not large enough to cause any amplification of the gas. Devices that are designed for short-term measurements use a short-term electret and a short-term camera that incorporates a spring-loaded mechanism to expose the electret to the entire volume of the chamber at the time of placement. These cameras were manufactured at NIST, but similar cameras are commercially available with a useful range of up to ~300 keV.
There are two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument having a separate ion chamber probe attached to the electronics module by a flexible cable. When the gas between the electrodes is ionized by the incident ionizing radiation, positive ions and electrons are created under the influence of the electric field. This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output. This fundamental requirement limits the use of outdoor cameras, since the camera size for higher photon energies is extremely large. A proportional counter is one in which the voltage in the ionization chamber increases above a certain level.
Multi-cavity ionization chambers can measure the intensity of the radiation beam in several different regions, providing information on the symmetry and flatness of the beam. In summary, an ionization chamber is an instrument that detects and measures ionizing radiation by measuring the electrical current that flows when radiation ionizes gas in a chamber, making it a conductor of electricity.
