Ionization chamber, radiation detector used to determine the intensity of a radiation beam or to count individual charged particles. An ion chamber is an extremely simple device that uses this principle to detect ionizing radiation. The basic chamber is simply a conductive can, usually metallic, with a wire electrode in the center, well insulated from the walls of the chamber. The chamber is most commonly 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. Typically, 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. The transmission ionization chamber generally consists of layers of PMMA coated with conductive material. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation.
The circuit is similar to previous single transistor detectors and requires only two Darlington transistors, an LED, and one or two resistors along with a battery, a power switch, and a small homemade ionization chamber. When the atoms or gas molecules between the electrodes are ionized by the incident ionizing radiation, ion pairs are created and the resulting positive ions are created and the dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. An ionization chamber and an electrometer require calibration before use and, with a triaxial connection cable, tools are required for calibration of the radiation beam. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields.
The power connector and control are removed and the appropriate holes for the passage of the ionization chamber and the mounting holes are drilled. This unique use of the CT chamber requires that the active volume response be uniform along its entire axial length, a restriction that is not required in other full immersion cylindrical chambers. Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for the detection of fluorescent radiation. Proportional counters work on successive ionization by collision between ions and gas molecules (charge multiplication); in the proportional region, amplification occurs (approximately 103-104 times) for the primary ions to obtain enough energy in the vicinity of the thin central electrode to cause more ionization in the detector.
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. Two types of amplifiers are used to make the pulse height proportional to the amount of ionization produced by the particle in the chamber. Noble gas ionization chambers are simple, resistant to radiation, and are easily constructed in the 4π geometry used for accurate measurements of gamma-ray source activity (Suzuki et al. Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to the absorbed dose.
Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure. They also act as solid-state ionization chambers by applying reverse polarization to detectors and by being exposed to radiation. .