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. In semiconductor detectors, also known as solid-state detectors, charge carriers are produced and collected by electrodes, similar to ionization chambers. 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. An ionization chamber is described having separate drift and detection regions electrically isolated from each other by a fine wire grid.
This can also be achieved chemically with a cooling gas, such as halogen, that absorbs the additional photons created by an ionization avalanche without ionizing itself. 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, the ion chamber has no effect of multiplication. There are two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument having a separate ion chamber probe attached to the electronics module by a flexible cable. At the lower end of the voltage scale for gas-filled detectors are ionization chambers or ion chambers.
A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation. However, charge carriers are electrons and holes and not electrons and ions as in ionization chambers. Typically, ionization chambers are used in the current mode, while proportional and Geiger-Muller meters use pulse mode to measure radiation. Since ionizing radiation is not easily detected and it also has a high ionizing power and penetrating force, it constitutes a risk to human health when it is outside its acceptable limits.
The electrons from primary ionization acquire enough energy between collisions to produce additional ionizations due to the strong electric field. 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. Alpha particles are more ionizing than beta particles and gamma rays, so alpha produces more current in the ion chamber region than beta and gamma, but particles cannot differentiate. Ionization chambers are widely used in the nuclear industry, as they provide an output proportional to the radiation dose.
They find wide use in situations where a constant high dose rate is measured, since they have a longer useful life than standard Geiger-Müller tubes, which suffer from gas breakage, and are generally limited to a life of approximately 1011 counting events.
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