Ionization cameras are dosimetry devices used to measure the output of x-ray tubes. They act as photo-timers in automatic exposure controls and in nuclear medicine, as dose calibrators. Pocket ionization chambers are a portable form of dosimetry devices, taking the form of large pens. An ionization chamber consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. 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. 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 are created and the dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. The gas amplification curve describes the behavior of an ionization chamber as a function of the applied voltage. All types of these devices have a filter in the opening of the chamber to prevent the passage of particulate radioactive materials, such as radon decay products, into the chamber. These cameras were manufactured at NIST, but similar cameras are commercially available with a useful range of up to ~300 keV. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation.
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. With reference to the attached ion pair collection graph, it can be seen that in the operating region of the ion chamber, the charge of a collected ion pair is effectively constant over an applied voltage range, since due to its relatively low electric field strength, it has no multiplication effect. Parallel plane, sometimes called a parallel plate, ionization chambers are commonly used in low energy (. Devices that are designed for short-term measurements use a short-term electret and a short-term camera that incorporates a spring-loaded mechanism to expose the electret to the entire volume of the chamber at the time of placement. A positively charged electret is used together with an ionization chamber made of an electrically conductive plastic. The 3- and 5-field ionization chamber is used in X-ray diagnostics in automatic exposure control systems as a measuring detector.
The response of an ionization chamber depends to a large extent on the voltage applied between its outer electrode and its center electrode. Operation as an ionization chamber involves using an applied voltage that is large enough to collect all ion pairs (positive ions and removed electrons) produced in the gas by a radioactive source, but not large enough to cause any amplification of said gas. Ionization chambers are widely used to assess activity levels from artificial radionuclides during processing. This fundamental requirement limits their use outdoors, since camera size for higher photon energies is extremely large.
