What Does an Ionization Chamber Measure?

The ionization chamber is a gas-filled radiation detector that is widely used for the detection and measurement of certain types of ionizing radiation, such as x-rays, gamma rays, and beta particles. Gamma rays can easily penetrate the metal walls of the chamber, making it suitable for detecting gamma radiation and x-rays. 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. Alpha particles are more ionizing than beta particles and gamma rays, so more current is produced in the region of the alpha ionization chamber than beta and gamma.

Gamma rays deposit significantly less energy on the detector than other particles. Noble gas ionization chambers are simple, resistant to radiation, and can be constructed in the 4π geometry used for accurate measurements of gamma-ray source activity. Parallel plate cameras are also used and are the recommended camera geometry for electron beam dosimetry. The output signal in the ionization chamber is a direct current, unlike the Geiger-Muller tube which produces a pulse output.

Regardless of their geometric design, ionization chambers used in diagnostic radiology must be of the ventilated type, that is, their volume of sensitive gas must communicate with the atmosphere. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure. For example, high-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments since their response is consistent over wide temperature ranges (20 to 170 °C). The transmission ionization chamber generally consists of layers of PMMA coated with conductive material.

Two types of amplifiers are used to make the pulse height proportional to the amount of ionization produced by the particle in the chamber. Open-air ionization chambers are the defining instrument of the Roentgen unit and, as such, are fundamentally linked to the absorbed dose. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. The gas amplification curve describes the behavior of an ionization chamber as a function of the applied voltage.

A CT camera is often referred to as a pencil chamber because its active volume comprises a thin cylinder 100 mm in length (sometimes longer). Self-reading pocket dosimeters in the form of a pen, consisting of an ionization chamber that functions as a condenser, fully charged (corresponding to zero dose) before use. Small ventilated air ionization chambers with a volume of 0.01 to 0.3 cm3 are considered suitable for measuring field parameters up to 2 cm × 2 cm.

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