An ionization chamber is an electrical device used to detect and measure the intensity of a radiation beam or to count individual charged particles. It consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. When charged particles, such as radiation, pass through the gas, gas molecules ionize to produce ions and electrons. 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. 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. Parallel plane, sometimes called a parallel plate, ionization chambers are commonly used in low energy (0 to 1 kV) applications. This causes the output signal in the ionization chamber to be a direct current, unlike the Geiger-Muller tube which produces a pulse output.
A proportional counter is one in which the voltage in the ionization chamber increases above a certain level. 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. 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). Therefore, ionization chambers can be used to detect gamma radiation and x-rays, collectively known as photons. The gas amplification curve describes the behavior of an ionization chamber as a function of the applied voltage. Ionization chambers have a uniform response to radiation over a wide range of energies and are the preferred means for measuring high levels of gamma radiation.
