An ionization chamber is a type of radiation detection device used to measure the intensity of a radiation beam or count individual charged particles. It consists of two opposing electrodes 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. This creates an electric current which can be measured and used to determine the intensity of the radiation beam. Multi-channel xenon ionization chambers pressurized to 20 bar were developed in the 1970s and 1980s and were successfully used in several clinical computed tomography (CT) scanners.
A protective electrode is typically provided in the chamber to reduce leakage and ensure improved field uniformity in the active or sensitive volume of the chamber. Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for the detection of fluorescent radiation. 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 medical physics and radiation therapy, ionization chambers are used to ensure that the dose delivered from a therapy unit or radiopharmaceutical is as intended. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure.
When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and the resulting positive ions move to the electrodes of opposite polarity under the influence of an electric field. Ionization chambers operate in region II and are an important type of radiation dosimeter as they are used for calibration of radiation therapy beams. 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.
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