Ionization chambers 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.
SpekTrap, Ltd.
offers ionization chambers with preamplifiers. These devices are intended for use in automatic x-ray exposure systems of diagnostic x-ray apparatus, providing automatic adaptation of their sensitivity to the sensitivity of modern intensifier screens.The preamplifier provides direct integration of ionization chamber current, allowing it to operate within the full dose rate range of the radiographic mode and meeting the requirements of modern diagnostic X-ray detectors for the dose range. Special techniques for increasing the noise immunity of preamplifiers are also described. Ionization chambers come in two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument that has a separate ion chamber probe attached to the electronics module by a flexible cable. 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 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. In static IMRT management, field symmetry and flatness can be checked with an ionization chamber matrix or films.
The dose calibrator is an ionization chamber used to assess the amount of activity in vials and syringes. 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. Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to absorbed dose. Parallel plate cameras are also used and are recommended for electron beam dosimetry. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure.
Ionization chambers are widely used in the nuclear industry, as they provide an output proportional to radiation dose. They have a longer service life than standard Geiger-Müller tubes, which suffer from gas breakage and generally have a lifetime of approximately 1011 counting events. PIN diodes are capable of excellent linearity, but X-ray thickness is not as easy to control experimentally as it is for ionization chambers. An ionization chamber and an electrometer require calibration before use and, with a triaxial connection cable, tools are required for calibration of the radiation beam. The 3- and 5-field ionization chamber is used in X-ray diagnostics in automatic exposure control systems as a measurement detector.
A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation.
