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Concept
Ionization chamber
Summary
The ionization chamber is the simplest type of gaseous ionisation detector, and is widely used for the detection and measurement of many types of ionizing radiation, including X-rays, gamma rays, alpha particles and beta particles. Conventionally, the term "ionization chamber" refers exclusively to those detectors which collect all the charges created by direct ionization within the gas through the application of an electric field. It uses the discrete charges created by each interaction between the incident radiation and the gas to produce an output in the form of a small direct current. This means individual ionising events cannot be measured, so the energy of different types of radiation cannot be differentiated, but it gives a very good measurement of overall ionising effect.
It has a good uniform response to radiation over a wide range of energies and is the preferred means of measuring high levels of gamma radiation, such as in a radiation hot cell as they can tolerate prolonged periods in high radiation fields without degradation.
They are widely used in the nuclear power industry, research labs, fire detection, radiation protection, and environmental monitoring.
A gas ionization chamber measures the charge from the number of ion pairs created within a gas caused by incident radiation. It consists of a gas-filled chamber with two electrodes; known as anode and cathode. The electrodes may be in the form of parallel plates (Parallel Plate Ionization Chambers: PPIC), or a cylinder with a coaxially located internal anode wire.
A voltage potential is applied between the electrodes to create an electric field in the fill gas. When gas atoms or molecules between the electrodes are ionized by incident ionizing radiation, ion-pairs are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. This generates an ionization current which is measured by an electrometer circuit in the region of femtoamperes to picoamperes, depending on the chamber design, and is proportional to the radiation dose.
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