Ionization chambers are dosimetry devices used to measure the output of X-ray tubes. They act as photo-timers in automatic exposure controls and in nuclear medicine, as dose calibrators. Pocket ionization chambers are a portable form of dosimetry devices, taking the form of large pens. 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 absorption within an ionization chamber can be controlled by selecting the make-up gas composition and pressure. Ionization chambers are widely used in the nuclear industry, as they provide an output proportional to the radiation dose. They find wide use in situations where a constant high dose rate is measured, since they have a longer useful life than standard Geiger-Müller tubes, which suffer from gas breakage, and are generally limited to a life of approximately 1011 counting events. The 3- and 5-field ionization chamber is used in X-ray diagnostics in automatic exposure control systems as a measuring detector.
The gas amplification curve describes the behavior of an ionization chamber as a function of the applied voltage. A proportional counter is a modified ionization chamber, one in which a higher voltage is printed, which makes the electric field near the axial cable strong enough to accelerate approaching electrons to such high energies that their collisions with gas molecules cause further ionization. In the administration of static IMRT, the symmetry and flatness of the field can be checked with an ionization chamber matrix or films. The dose calibrator is an ionization chamber used to assess the amount of activity in vials and syringes. 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 opposite polarity under the influence of the electric field. Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for detecting fluorescent radiation.
Calorimetry measures the heat released into water; ionization chambers record the number of ion pairs produced in air. All types of these devices have a filter in the opening of the chamber to prevent passage of particulate radioactive materials, such as radon decay products, into the chamber. Parallel plane, sometimes called parallel plate, ionization chambers are commonly used in low energy (. Open-air ionization chambers are defining instruments of Roentgen units and, as such, are fundamentally linked to absorbed dose. 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. This makes open-air ionization chambers preferred reference dosimeter for Accredited Dosimetry Calibration Laboratories (ADCL), but their large size makes them unsuitable for clinical applications.
