An ionization chamber is a radiation detector used to measure the intensity of a radiation beam or to count individual charged particles. It is an extremely simple device that uses an electric field to detect ionizing radiation. The basic chamber consists of a conductive can, usually metallic, with a wire electrode in the center, well insulated from the walls of the chamber. The chamber is typically filled with ordinary dry air, but other gases such as carbon dioxide or pressurized air can give greater sensitivity. A DC voltage is applied between the outer can and the center electrode to create an electric field that sweeps ions toward the oppositely charged electrodes.
The outer can has most of the potential relative to ground, so the circuitry is close to the ground potential. The center wire is kept close to zero volts and the resulting current in the center wire is measured. 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.
Regardless of their geometric design, ionization chambers used in diagnostic radiology must be of the ventilated type, that is, their volume of sensitive gas must communicate with the atmosphere. The power connector and control are removed and the appropriate holes for the passage of the ionization chamber and the mounting holes are drilled. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure. Alternatively, the voltage increase can simply be detected with a comparator and then restored as is done in the ionization chamber of the Prospector.
Multi-cavity ionization chambers can measure the intensity of the radiation beam in several different regions, providing information on its symmetry and flatness. There are two basic configurations; an integral unit with camera and electronics in one housing, and a two-piece instrument that has a separate ion chamber probe attached to an electronics module by a flexible cable. A simple ionization chamber consists of a metal cylinder with a thin axial wire enclosed in a glass envelope filled with some inert gas. A protective electrode is typically provided in the chamber to further reduce leakage and ensure improved field uniformity in its active or sensitive volume, with advantages in charge collection. The response of an ionization chamber depends on the voltage applied between its outer electrode and center electrode. The transmission ionization chamber generally consists of layers of PMMA coated with conductive material.
Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields. This makes open-air ionization chambers ideal for Accredited Dosimetry Calibration Laboratories (ADCL), but their large size makes them unsuitable for clinical applications. Open-air ionization chambers are also used to define Roentgen units and measure absorbed dose. They also act as solid-state ionization chambers by applying reverse polarization to detectors and being exposed to radiation. The method of connecting a shield to a camera is not critical; however, low-frequency line-related electric fields and changing electrostatic fields due to movement near the camera should be avoided.
