The ionization chamber is a gas-filled radiation detector that is widely used for the detection and measurement of certain types of radiation. It is the simplest type of detector and works by creating an electric field between two electrodes of opposite polarity, which accelerates the ions produced by the radiation to be collected by the electrodes. This charge is then read by an electrometer and can be converted into absorbed dose. The ionization chamber, also known as an ion chamber, operates at a low electric field strength, so gas multiplication does not occur.
This means that the collected load (output signal) is independent of the applied voltage. The detector voltage is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to cause gas amplification (secondary ionization). Individual minimum ionization particles tend to be quite small and generally require special low-noise amplifiers for efficient operating performance. Ionization chambers are preferred for high radiation dose rates because they have no “dead time”, a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates.
This is because there is no inherent signal amplification in the operating medium; therefore, these meters do not require much time to recover from large currents. In addition, because there is no amplification, they provide excellent energy resolution, which is mainly limited by electronic noise. Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to the absorbed dose. Multi-cavity ionization chambers can measure the intensity of the radiation beam in several different regions, providing information on the symmetry and flatness of the beam. 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. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure.
The transmission ionization chamber generally consists of layers of PMMA coated with conductive material. Ionization chambers operate in region II and are an important type of radiation dosimeter as the primary device used for calibration of radiation therapy beams. 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) so that the primary ions obtain enough energy in the vicinity of the thin central electrode to cause additional ionization in the detector. When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and resulting positive ions are created and dissociated electrons move to electrodes of opposite polarity under influence of electric field. High-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments, as their response has been proven to be consistent over wide temperature ranges (20 to 170 °C).In medical physics and radiation therapy, ionization chambers are used to ensure that dose delivered from therapy unit or radiopharmaceutical is as intended. 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.
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