Electret ion chamber types use the surface voltage drop in a dielectric material, usually Teflon, that is almost permanently charged. This material is known as an electret and typically has the shape of a disc that is around 1 mm thick and 10 mm in diameter. Electrets are created by heating them and exposing them to an electric field at the same time. This process causes many dipoles in the material to be oriented in a preferred direction.
After the heating process, 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 that is filled with some inert gas. Ionization chambers are used to assess the activity of artificial radionuclides during processing, as well as to ensure that the dose delivered from a therapy unit or radiopharmaceutical is as intended. When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and the resulting positive ions and dissociated electrons move to the electrodes of opposite polarity under the influence of an 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 electroscopes used by Rutherford in the early 20th century) is based on using 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 radiation beams in several different regions, providing information on its symmetry and flatness. Ionization chambers consist of two 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 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 the gas.
The ionization chamber is the only gas-filled detector that allows direct determination of absorbed dose. They are widely used in nuclear industry since they provide an output proportional to radiation dose, and they have a longer useful life than standard Geiger-Müller tubes, which suffer from gas breakage and generally have a life of approximately 1011 counting events. A positively charged electret can be used together with an ionization chamber made from electrically conductive plastic. This type of chamber is used in situations where a constant high dose rate is measured.
