In 1940, Frisch revolutionized the field of alpha spectrometry with the invention of the grid ionization chamber. This type of gas-filled detector still has limited applications today. In the late 1940s, a third type of gas-filled detector, the proportional counter, was introduced. This device amplified the load originating from the gas.
An ion chamber is a simple device that uses the principle of ionization to detect radiation. It consists of a pair of charged electrodes that collect the ions formed within their respective electric fields. The basic chamber is usually made of a conductive metal can with a wire electrode in the center, insulated from the walls. It is usually 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. The power connector and control are removed and holes are drilled for the passage of the ionization chamber and for mounting. The outer can has most of the potential relative to ground, so the circuitry is close to ground potential. The center wire is kept close to zero volts and the resulting current in the center wire is measured.
Ionization chambers respond to any ionizing radiation from 100 nm ultraviolet light through X-rays and gamma rays. Operation as an ionization chamber involves using 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 absorbed dose.
This apparatus was later adapted by Pierre Curie to allow accurate quantification of tiny leakage currents produced in an ionization chamber by samples of radioactive material. Multi-cavity ionization chambers can measure intensity of radiation beam in several different regions, providing information on symmetry and flatness of beam. P-10 gas was used as 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 partner universities of Oak Ridge (then Oak Ridge Institute for Nuclear Studies).
In medical physics and radiation therapy, ionization chambers are used to ensure dose delivered from therapy unit or radiopharmaceutical is as intended. Alternatively, voltage increase can simply be detected with comparator and then restored as done in ionization chamber of Prospector. A positively charged electret is used together with an ionization chamber made of electrically conductive plastic. Ionization chambers are widely used to assess activity of artificial radionuclides during processing.
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