How Does a Free Air Ionization Chamber Work?

Charges dissipate through the measuring electrode (M) whose length in the direction of the beam defines the ion collection volume. 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.

For example, in some types of smoke detectors, you can find artificial radionuclides such as americium-241, a source of alpha particles. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. The radioactive source emits alpha particles in both chambers, ionizing some air molecules. The open air chamber allows smoke particles to enter the sensitive volume and changes the attenuation of alpha particles. If any smoke particles enter the open air chamber, some ions will bind to the particles and will not be available to carry current in that chamber.

An electronic circuit detects that a current difference has developed between the open and sealed chambers and sounds the alarm. Ionization chambers have a uniform response to radiation over a wide range of energies and are the preferred means for measuring high levels of gamma radiation. The response of an ionization chamber depends to a large extent on the voltage applied between the outer electrode and the center electrode. Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to the absorbed dose. All types of these devices have a filter in the opening of the chamber to prevent the passage of particulate radioactive materials, such as radon decay products, into the chamber.

Ionization chambers are widely used to assess the activity of artificial radionuclides during processing. For example, high-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments, since their response has been shown to be consistent over wide temperature ranges (20 to 170 °C).Parallel plane, sometimes called a parallel plate, ionization chambers are commonly used in low energy (alpha particle) applications. The alpha particle causes ionization inside the chamber, and the ejected electrons cause additional secondary ionizations. These cameras were manufactured at NIST, but similar cameras are commercially available with a useful range of up to ~300 keV.

The ionization chamber is also used as a means for directly determining absorbed dose. 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. Devices designed for short-term measurements use a short-term electret and a short-term camera that incorporates a spring-loaded mechanism to expose the electret to the entire volume of the chamber at placement time. This fundamental requirement limits outdoor camera use since camera size for higher photon energies is extremely large. The gas amplification curve describes how an ionization chamber behaves as a function of applied voltage.

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