An ionization chamber is an electrical device that detects various types of ionizing radiation. It consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. A voltage is applied between the electrodes, and negative charges are attracted by the anode, while positive charges are attracted by the cathode. This creates an electric field that accelerates the ions produced by the radiation to be collected by the electrodes.
The charge is then read by the electrometer and can be converted into absorbed dose. 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 detectors in the ionization region operate at a low electric field strength, so gas multiplication does not occur. The collected load (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 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. 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. All the energy of the primary electrons produced in the sensitive volume of the chamber must dissipate in the chamber. Parallel plane, sometimes called a parallel plate, ionization chambers are commonly used in low energy (<300 keV) applications. This makes open-air ionization chambers the preferred reference dosimeter for Accredited Dosimetry Calibration Laboratories (ADCL), but their large size makes them unsuitable for clinical applications. An ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation.
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 and dissociated electrons move to the electrodes of opposite polarity under the influence of an electric field. Therefore, ionization chambers can be used to detect gamma radiation and x-rays, collectively known as photons, and for this, a windowless tube is used. There are two basic configurations; an integral unit with camera and electronics in one housing, and a two-piece instrument with a separate ion chamber probe attached to an electronics module by a flexible cable. The smoke detector has two ionization chambers, one open to air and a reference chamber that does not allow particles to enter. The gas amplification curve describes the behavior of an ionization chamber as a function of applied voltage. 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. These cameras were manufactured at NIST but similar cameras are commercially available with useful range up to ~300 keV.
Alpha particle causes ionization inside chamber and ejected electrons cause additional secondary ionizations.
