Ionization chambers are electrical devices that detect various types of ionizing radiation. They 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 operation of an ionization chamber involves the use of an applied voltage that is large enough to collect all of the ion pairs (positive ion and electron removed) produced in the gas by a radioactive source, but not large enough to cause any amplification of the gas.
A voltage is applied between the electrodes, with negative charges attracted by the anode and positive charges by the cathode. 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).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. 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.
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. 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 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. 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 below and are generally limited to a lifetime of around 1011 counting events. 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. 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. The alpha particle causes ionization inside the chamber, and the ejected electrons cause additional secondary ionizations. For example, if the inner surface of the ionization chamber is coated with a thin layer of boron, the (n, alpha) reaction can occur. 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. This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output.
All the energy of the primary electrons produced in the sensitive volume of the chamber must dissipate in the chamber. In conclusion, an understanding of how an ionization chamber works can help us better understand how radiation detectors work and how they can be used in various applications.
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