In 1940, Frisch invented the grid ionization chamber, which still has limited application in alpha spectrometry. In the late 1940s, a third type of gas-filled detector, the proportional counter, was introduced, which amplified the load originating from the gas. The ionization chambers consist of a pair of charged electrodes that collect the ions formed within their respective electric fields. Ionization chambers can measure the dose or dose rate because they provide an indirect representation of the energy deposited in the chamber.
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 metallic, 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 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. They respond to any ionizing radiation that may enter the chamber from 100 nm ultraviolet light through X-rays and gamma rays. The power connector and control are removed and the appropriate holes for the passage of the ionization chamber and the mounting holes are drilled.
Operation as an ionization chamber involves the use of an applied voltage that is large enough to collect all of the ion pairs (positive ion and electron removed) produced in the gas by a radioactive source, but not large enough to cause any amplification of the gas. The radiation ionizes the air inside the chamber and the 50 volts attract the resulting free electrons and negative ions to the can, and conduct the positive ions to the inner plate. The ionization chamber is the only gas-filled detector that allows direct determination of the absorbed dose. The circuit is similar to previous single transistor detectors and requires only two Darlington transistors, an LED, and one or two resistors along with a battery, a power switch, and a small homemade ionization chamber.
This apparatus was later adapted by Pierre Curie to allow accurate quantification of the tiny leakage currents produced in an ionization chamber by samples of radioactive material. 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. 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. P-10 gas was used as the detector fill gas, although this gas is normally used with proportional meters instead of ionization chambers.
This example of a Frisch Grid ionization chamber was built in 1959 at the partner universities of Oak Ridge (then the Oak Ridge Institute for Nuclear Studies). 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. Alternatively, the voltage increase can simply be detected with a comparator and then restored as is done in the ionization chamber of the Prospector. A positively charged electret is used together with an ionization chamber made of an electrically conductive plastic.
Ionization chambers are widely used to assess the activity of artificial radionuclides during processing. .