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.

The ionization chamber is a radiation detector that is used to detect and measure charge from the number of ion pairs created within a gas caused by incident radiation. 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. 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. In other words, all the energy of the primary electrons produced in the sensitive volume of the chamber must dissipate in the chamber.

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. Ionization chambers consist of a pair of charged electrodes that collect ions formed within their respective electric fields. This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output. Devices that are 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 the time of placement.

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. A more recent application of primitive total ionization chambers (such as the electroscopes used, for example, by Rutherford in the early 20th century), is based on the use of an electret, which maintains a charge for an extended period and is discharged by exposure to radiation. The response of an ionization chamber depends to a large extent on the voltage applied between the outer electrode and the center electrode. A positively charged electret is used together with an ionization chamber made of an electrically conductive plastic.

Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to the absorbed dose. This makes open-air ionization chambers the preferred reference dosimeter for Accredited Dosimetry Calibration Laboratories (ADCL), but their large size makes them unsuitable for clinical applications. These cameras were manufactured at NIST, but similar cameras are commercially available with a useful range of up to ~300 keV. There are two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument having a separate ion chamber probe attached to the electronics module by a flexible cable.

Jada Urquiza
Jada Urquiza

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