Ionization chambers are electrical devices that detect various types of ionizing radiation. They consist of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. The material used to make the cylindrical body of the chamber is usually aluminum or plastic, a few millimeters thick, and is selected to have an atomic number similar to that of air. This allows the wall to be equivalent to air over a range of energies of the radiation beam.
Electret ion chamber types make use of 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, which causes many dipoles in the material to be 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. An ionization chamber consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. The electric field established between the electrodes accelerates the ions produced by the radiation to be collected by the electrodes. This charge is read by the electrometer and can be converted into absorbed dose.
The ionization produced within the volume of air in the thimble cover is collected by the center electrode. Aluminum stem transmits bias voltage to thimble cover, while a protective ring minimizes load leakage between the outer (HT) and inner (signal) elements of the interconnect cable. Individual minimum ionization particles tend to be quite small and generally require special low-noise amplifiers for efficient operating performance. 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. An ion chamber is an extremely simple device that uses this principle to detect ionizing radiation. The basic chamber is simply a conductive can, usually made of metal, with a wire electrode in the center, well insulated from the walls of the chamber. The chamber is most commonly filled with ordinary dry air, but other gases such as carbon dioxide or pressurized air can give greater sensitivity. A DC voltage is applied between the outer can and the center electrode to create an electric field that sweeps ions toward the oppositely charged electrodes. Typically, the outer can has most of the potential relative to ground, so that the circuitry is close to the ground potential.
The center wire is kept close to zero volts and the resulting current in the center wire is measured. 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. For example, if the inner surface of the ionization chamber is coated with a thin layer of boron, then (n, alpha) reaction can occur. Ionization chambers have a uniform response to radiation over a wide range of energies and are preferred for measuring high levels of gamma radiation. This little trick allows you to unload the camera without any additional components that could pose leakage problems; note that only element connected to camera cable is JFET gate. Operation as an ionization chamber involves use of applied voltage that is large enough to collect all ion pairs (positive ion and electron removed) produced in gas by radioactive source, but not large enough to cause any amplification of gas.
This fundamental requirement limits use of outdoor cameras since camera size for higher photon energies is extremely large. Devices designed for short-term measurements use short-term electret and short-term camera that incorporates spring-loaded mechanism to expose electret to entire volume of chamber at time of placement. Response of an ionization chamber depends largely on voltage applied between outer electrode and center electrode. For example, high-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments as response of detector has been proven consistent over wide temperature ranges (20°C - 170°C).Radiation ionizes air inside chamber and 50 volts attract resulting free electrons and negative ions to can, conduct positive ions to inner plate. They respond to any ionizing radiation that may enter chamber from 100 nm ultraviolet light through X-rays and gamma rays.
Proportional counter is modified ionization chamber where higher voltage applied making electric field near axial cable strong enough to accelerate approaching electrons to such high energies their collisions with gas molecules cause further ionization. These cameras were manufactured at NIST but similar cameras commercially available with useful range up to ~300 keV.
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