How Does an Ionization Chamber Work?

Learn how an ion chamber works and why it's preferred for measuring high levels of gamma radiation. Find out how it's used in nuclear power industry and research laboratories.

How Does an Ionization Chamber Work?

A voltage is applied between the electrodes of an ionization chamber, also known as an ion chamber. Negative charges are attracted to the anode, while positive charges are attracted to the cathode. This type of detector has a uniform response to radiation over a wide range of energies, making it the preferred choice for measuring high levels of gamma radiation. It is widely used in nuclear power, research laboratories, radiography, radiobiology, and environmental monitoring.

The detector voltage is adjusted so that the conditions correspond to the ionization region, and the voltage is insufficient to cause gas amplification (secondary ionization). This means that detectors in the ionization region operate at a low electric field strength, so gas multiplication does not occur. The collected load (output signal) is independent of the applied voltage. Individual minimum ionization particles tend to be quite small and generally require special low-noise amplifiers for efficient operating performance.

Ionization chambers are preferred for high radiation dose rates because they have no “dead time”, a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates. This is because there is no inherent signal amplification in the operating medium; therefore, these meters do not require much time to recover from large currents. In addition, because there is no amplification, they provide excellent energy resolution, which is mainly limited by electronic noise. 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).

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. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. The transmission ionization chamber generally consists of layers of PMMA coated with conductive material.There are two basic configurations; the integral unit with the camera and electronics in the same housing, and the two-piece instrument that has a separate ion chamber probe attached to the electronics module by a flexible cable. When the gas between the electrodes is ionized by the incident ionizing radiation, positive ions and electrons are created under the influence of the electric field.These detectors act as solid-state ionization chambers when exposed to radiation and, like scintillation detectors, belong to the class of solid-state detectors.

Ionization chambers are widely used in the nuclear industry as they provide an output proportional to the radiation dose. They find wide use in situations where a constant high dose rate is measured, as they have a longer service life than standard Geiger-Müller tubes which suffer from gas breakage and are generally limited to a lifetime of around 1011 counting events.The electric field allows the ionization chamber to operate continuously by cleaning electrons which can cause ion pair recombination which can result in reduction of ion current. Therefore, ionization chambers can be used to detect gamma radiation and x-rays, collectively known as photons, and for this, the windowless tube is used. An ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation.This unique use of the CT chamber requires that the active volume response be uniform along its entire axial length, a restriction that is not required in other full immersion cylindrical chambers.

When ionization chambers are not the most suitable detectors for side profile measurements, an alternative is to use 2D detectors such as scintillation detectors and Gafchromic films. Radiation indicators are considered whereas ionization chambers are used for more quantitative measurements.The alpha particle causes ionization inside the chamber and the ejected electrons cause additional secondary ionizations.

Tami Arunachalam
Tami Arunachalam

Amateur travel trailblazer. Hipster-friendly food enthusiast. Proud twitter geek. Passionate beer guru. Evil twitter geek. Lifelong coffee maven.

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