The ionization chamber, also known as an ion chamber, is an electrical device that detects various types of ionizing radiation. It works by adjusting the detector voltage to correspond to the ionization region, where the voltage is insufficient to cause gas amplification (secondary ionization). This means that the collected load (output signal) is independent of the applied voltage. Individual minimum ionization particles tend to be quite small and 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. The electric field allows the ionization chamber to work continuously by cleaning electrons that can cause ion pair recombination, which can result in reduced ion current.
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 approximately 1011 counting events. For example, high-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments, as the response of a detector has been proven to be consistent over wide temperature ranges (20 to 170 °C). This causes the output signal in the ionization chamber to be a direct current, unlike the Geiger-Muller tube which produces a pulse output. 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.
The electric field allows the ionization chamber to detect and measure charge from the number of ion pairs created within a gas caused by incident radiation. If the inner surface of the ionization chamber is coated with a thin layer of boron, the (n, alpha) reaction can occur. The alpha particle causes ionization inside the chamber, and the ejected electrons cause additional secondary ionizations. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter. 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.
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. 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. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation. A proportional counter is a modified ionization chamber, one in which a higher voltage is printed, which makes the electric field near the axial cable strong enough to accelerate approaching electrons to such high energies that their collisions with gas molecules cause further ionization.