An ionization chamber is an electrical device used to detect and measure various types of ionizing radiation. It consists of a pair of charged electrodes that collect ions formed within their respective electric fields. The detector voltage is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to cause gas amplification (secondary ionization). The collected load (output signal) is independent of the applied voltage.
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. Pressurized well type cylindrical ionization chambers are widely used for the determination of the activity of radioactive samples.
They are used as secondary measuring instruments, in particular for transferring standards and, thanks to their stability over time, periodically check the consistency of the measurement results of the primary activity over several years. The fields of application of these instruments are varied, they are used in research, industry and nuclear medicine services. A well type ionization chamber is composed of a cylinder containing the gas (nitrogen, argon or gas mixture) under a given pressure and electrodes that will be used to collect electrical charges. The unit is connected to an electrometer that will supply high voltage to the camera, acquire the current signal given by the camera and transmit it to the acquisition program.
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. 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. A more recent application of primitive total ionization chambers (such as the electroscopes used, for example, by Rutherford in the early 20th century), is based on the use of an electret, which maintains a charge for an extended period and is discharged by exposure to radiation. In other words, all the energy of the primary electrons produced in the sensitive volume of the chamber must dissipate in the chamber. The electric field allows the ionization chamber to operate continuously by cleaning electrons, which can cause ion pair recombination, which can result in reduction of ion current. 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.
This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output. The ionization chamber is also the only gas-filled detector that allows direct determination of absorbed dose. Multi-cavity ionization chambers can measure intensity of radiation beam in several different regions, providing information on symmetry and flatness of beam. When atoms or gas molecules between electrodes are ionized by incident ionizing radiation, ion pairs are created and resulting positive ions created and dissociated electrons move to electrodes of opposite polarity under influence of electric field.
