Ion chambers are the preferred medium for measuring high levels of gamma radiation, and are widely used in the nuclear power industry, research laboratories, radiography, radiobiology and environmental monitoring. They have a good, uniform response to radiation over a wide range of energies. Pressurized well type cylindrical ionization chambers are often used to determine radioactive sample activity. These instruments offer excellent stability over time, ease of implementation and excellent linearity according to activity levels.
A well type ionization chamber consists 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. A diagram of the configuration is shown below. 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 with some inert gas.
After exposure to radiation for a period of time, the ionization produced in the chamber discharges the condenser; the exposure (or air kerma) is proportional to the discharge, which can be read directly against the light through a built-in eyepiece. The electric field allows the ionization chamber to work continuously by cleaning electrons that can cause ion pair recombination, which can result in reduced ion current. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields. A protective electrode is typically provided in the chamber to further reduce chamber leakage and ensure improved field uniformity in the active or sensitive volume of the chamber, with advantages in charge collection. Proportional counters work on successive ionization by collision between ions and gas molecules (charge multiplication); in the proportional region, amplification occurs (approximately 103-104 times) for the primary ions to obtain enough energy in the vicinity of the thin central electrode to cause more ionization in the detector.
They act as solid-state ionization chambers when exposed to radiation and, like scintillation detectors, belong to the class of solid-state detectors. The unique use of CT chambers requires that their active volume response be uniform along its entire axial length, a restriction that is not required in other full immersion cylindrical 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. Multi-channel xenon ionization chambers pressurized to 20 bar were developed in the 1970s and 1980s (Drost and Fenster, 1982, 198) and were successfully used in several clinical computed tomography (CT) scanners. Self-reading pocket dosimeters in the form of a pen, consisting of an ionization chamber that functions as a condenser, fully charged (corresponding to zero dose) before use. When the gas between the electrodes is ionized by the incident ionizing radiation, positive ions and electrons are created under the influence of the electric field.
