An ionization chamber, also known as an ion chamber, is an electrical device that detects various types of ionizing radiation. It works by applying a voltage between two electrodes, with negative charges attracted to the anode and positive charges to the cathode. This creates an electric field in the chamber, which causes ionization when exposed to radiation. The collected charge is then measured and used to determine the amount of radiation present.
Ion chambers have a good uniform response to radiation over a wide range of energies and are the preferred means for measuring high levels of gamma radiation. They are widely used in the nuclear power industry, research laboratories, radiography, radiobiology and environmental monitoring. 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). This means that the output signal is independent of the applied voltage.
Individual minimum ionization particles tend to be quite small and generally require special low-noise amplifiers for efficient operating performance. Ionization chambers 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. 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.
After exposure to radiation for a period of time, the ionization produced in the chamber discharges the condenser; the exposure (or air kerma) is proportional to the discharge, which can be read directly against the light through a built-in eyepiece. In medical physics and radiation therapy, ionization chambers are used to ensure that the dose delivered from a therapy unit or radiopharmaceutical is as intended. This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. They act as solid-state ionization chambers when exposed to radiation and, like scintillation detectors, belong to the class of solid-state detectors. 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. Multi-cavity ionization chambers can measure intensity of radiation beam in several different regions providing information on symmetry and flatness of beam.
Parallel plate cameras are also used and are recommended camera geometry for electron beam dosimetry. An ionization chamber measures charge from number of ion pairs created within gas caused by incident radiation. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields. Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for detection of fluorescent radiation. Alpha particle causes ionization inside chamber and ejected electrons cause additional secondary ionizations.
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