An ionization chamber is a radiation detector used to measure the intensity of a radiation beam or to count individual charged particles. It is usually portable and can be used to measure the rate of radiation exposure (the amount of radiation exposure received in a specific period of time). The ability of the ion chamber to measure the exposure rate of a radionuclide is based on the emission reaching the active part of the meter and the energy of the emission. Ionization chambers are used when there is measurable exposure or the possibility of measurable exposure to x-rays and gamma rays. When the atoms or gas molecules between the electrodes are ionized by the incident ionizing radiation, ion pairs are created and positive ions and dissociated electrons move to the electrodes of opposite polarity under the influence of an electric field.
All types of these devices have a filter in the opening of the chamber to prevent the passage of particulate radioactive materials, such as radon decay products, into the chamber. There are two basic configurations; an integral unit with the camera and electronics in one housing, and a two-piece instrument that has a separate ion chamber probe attached to the electronics module by a flexible cable. Ionization chambers are widely used in the nuclear industry, as they provide an output that is proportional to the radiation dose. They have a longer useful life than standard Geiger-Müller tubes, which suffer from gas decomposition and are generally limited to a life of approximately 10-11 counting events. A positively charged electret is used together with an ionization chamber made of an electrically conductive plastic. In other words, all the energy of primary electrons produced in the sensitive volume of the chamber must dissipate in the chamber.
The ionization chamber is also the only gas-filled detector that allows direct determination of absorbed dose. Ionization chambers are widely used to assess activity of artificial radionuclides during processing. With reference to an ion pair collection graph, it can be seen that in the operating region of an ion chamber, charge of a collected ion pair is effectively constant over an applied voltage range, since due to its relatively low electric field strength, it has no effect of multiplication. A more recent application of primitive total ionization chambers (such as electroscopes used by Rutherford in early 20th century) is based on use of an electret, which maintains charge for extended period and is discharged by exposure to radiation. Multi-cavity ionization chambers can measure intensity of radiation beam in several different regions, providing information on symmetry and flatness of beam. A simple ionization chamber consists of metal cylinder with thin axial wire enclosed in glass envelope filled with some inert gas. It consists of gas-filled cavity surrounded by two electrodes of opposite polarity and electrometer.
Open-air ionization chambers are defining instrument of Roentgen unit and, as such, are fundamentally linked to absorbed dose. Devices designed for short-term measurements use short-term electret and short-term camera that incorporates spring-loaded mechanism to expose electret to entire volume of chamber at time of placement. When gas between electrodes is ionized by incident ionizing radiation, positive ions and electrons are created under influence of electric field.
