Electret ion chamber types use the surface voltage drop in a plastic material. The plastic sample is a dielectric material, usually Teflon, that is almost permanently charged. It is called an electret and generally has the shape of a disc approximately 1 mm thick and 10 mm in diameter. Electrets are prepared by simultaneously heating and exposing them to an electric field.
Because of this process, many dipoles in the material are oriented in a preferred direction. “After heating, the material “" freezes "” and is able to maintain the position of its electrical dipoles for a long period of time.” A voltage gradient of several hundred volts can be maintained between the surfaces of the electret disk. A simple ionization chamber consists of a metal cylinder with a thin axial wire enclosed in a glass envelope in which some inert gas is filled. Ionization chambers are widely used to assess the activity of artificial radionuclides during processing.
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. 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. Two types of amplifiers are used to make the pulse height proportional to the amount of ionization produced by the particle in 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.
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. Ionization chambers consist of a pair of charged electrodes that collect ions formed within their respective electric fields. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation. Operation as an ionization chamber involves the use of an applied voltage that is large enough to collect all ion pairs (positive ion and removed electron) produced in the gas by a radioactive source, but not large enough to cause any amplification of the gas.
The ionization chamber is the only gas-filled detector that allows direct determination of the absorbed dose. 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. A positively charged electret is used together with an ionization chamber made of an electrically conductive plastic.