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 works on the principle of ionization. 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 usually filled with ordinary dry air, but other gases such as carbon dioxide or pressurized air can be used for greater sensitivity.A DC voltage is applied between the outer can and the center electrode to create an electric field that sweeps ions towards 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.
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 atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and the resulting positive ions are created and dissociated electrons move to the electrodes of opposite polarity under the influence of electric field.An ionization chamber and an electrometer require calibration before use and, with a triaxial connection cable, tools are required for calibration of 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 appropriate holes for passage of ionization chamber and mounting holes are drilled.
This unique use of CT chamber requires that 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 detection of fluorescent radiation. Proportional counters work on successive ionization by collision between ions and gas molecules (charge multiplication); in proportional region, amplification occurs (approximately 103-104 times) for primary ions to obtain enough energy in vicinity of thin central electrode to cause more ionization in detector.Regardless of their geometric design, ionization chambers used in diagnostic radiology must be ventilated type, that is their volume of sensitive gas must communicate with atmosphere. Two types of amplifiers are used to make pulse height proportional to amount of ionization produced by particle in chamber.
Noble gas ionization chambers are simple, resistant to radiation, and easily constructed in 4π geometry used for accurate measurements of gamma-ray source activity (Suzuki et al.). Open-air ionization chambers are defining instrument of Roentgen unit and, as such, fundamentally linked to 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.
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