What Does an Ionization Chamber Do?

An ionization chamber is a type of radiation detector used to measure the intensity of a radiation beam or to count individual charged particles. It is widely used in the nuclear power industry, research laboratories, radiography, radiobiology and environmental monitoring. Ionization chambers consist of a pair of charged electrodes that collect ions formed within their respective electric fields. The ionization chamber can measure dose or dose rate because it provides an indirect representation of the energy deposited in the chamber.

The detector voltage is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to cause gas amplification (secondary ionization). Detectors in the ionization region operate at a low electric field strength, so gas multiplication does not occur. The collected load (output signal) is independent of the applied voltage. Ionization chambers are preferred for high radiation dose rates because they have no “dead time”, a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates.

This is because there is no inherent signal amplification in the operating medium; therefore, these meters do not require much time to recover from large currents. In addition, because there is no amplification, they provide excellent energy resolution, which is mainly limited by electronic noise. An ionization chamber consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. The electric field established between the electrodes accelerates the ions produced by the radiation to be collected by the electrodes.

This charge is read by the electrometer and can be converted into absorbed dose. Individual minimum ionization particles tend to be quite small and generally require special low-noise amplifiers for efficient operating performance. A more recent application of primitive total ionization chambers (such as the electroscopes used, for example, by Rutherford in the early 20th century), is based on the use of an electret, which maintains a charge for an extended period and is discharged by exposure to radiation. In an ionization chamber, two opposing electrodes are placed in a gas-filled container and a high voltage is applied. As charged particles (radiation) pass through the gas, gas molecules ionize to produce ions and electrons.

All types of these devices have a filter in the opening of the chamber to prevent the passage of particulate radioactive materials, such as radon decay products, into the chamber. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. The response of an ionization chamber depends to a large extent on the voltage applied between the outer electrode and the center electrode. 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. There are two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument that has a separate ion chamber probe attached to the electronics module by a flexible cable. When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and dissociated electrons move to electrodes of opposite polarity under influence of electric field.

This makes output signal in ionization chamber direct current unlike Geiger-Muller tube which produces pulse output. This fundamental requirement limits use of outdoor cameras since camera size for higher photon energies is extremely large. With reference to attached ion pair collection graph it can be seen that in operating region of ion chamber charge of collected ion pair is effectively constant over applied voltage range since due to its relatively low electric field strength, ion chamber has no effect of multiplication.

Jada Urquiza
Jada Urquiza

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