Ion chambers are a type of electrical device that can detect various forms of ionizing radiation. They have a uniform response to radiation over a wide range of energies, making them the preferred choice for measuring high levels of gamma radiation. Ion chambers are used in many industries, such as 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. Cylindrical cameras are most commonly used in MT photon and electron reference dosimetry applications above about 6 MeV. Cylindrical chambers, especially farmers' chambers, are well characterized and are considered the gold standard of clinical reference dosimetry.
Due to its axial design, the effective measuring point is upstream of the central axis of the camera at 0.6 rcav for photons and 0.5 rcav for electrons. An ionization chamber (or ion chamber) is usually portable and used to measure the rate of radiation exposure (how much radiation exposure is received in a specific period of time). The ability of the ion chamber to measure the exposure rate of a radionuclide is based on the ability of the emission to reach the active part of the meter and the energy of the emission. Ion cameras are used when there is measurable exposure or the possibility of measurable exposure to x-rays and gamma rays. Ionization chambers operate in region II and are an important type of radiation dosimeter as the primary device used for calibration of radiation therapy beams. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter.
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. 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. They also act as solid-state ionization chambers by applying reverse polarization to detectors and by being exposed to radiation. This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output. 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). Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for the detection of fluorescent radiation.
Self-reading pocket pen-shaped dosimeters, consisting of an ionization chamber that functions as a condenser, fully charged (corresponding to zero dose) before use. When ionization chambers are not the most suitable detectors for side profile measurements, an alternative is to use 2D detectors, such as scintillation detectors14,15 and Gafchromic films. Proportional counters work on successive ionization by collision between ions and gas molecules (charge multiplication); in the proportional region, amplification occurs (approximately 103-104 times) for the primary ions to obtain enough energy in the vicinity of the thin central electrode to cause more ionization in the detector. An ionization chamber consists of a gas-filled cavity surrounded by two electrodes of opposite polarity and an electrometer. The response of an ionization chamber depends to a large extent on the voltage applied between the outer electrode and the center electrode. Parallel plate cameras are also used and are the recommended camera geometry for electron beam dosimetry (Figure 6-27, B).
For example, high-pressure xenon ionization (HPXe) chambers are ideal for use in uncontrolled environments, as their response has been proven to be consistent over wide temperature ranges (20°C - 170°C).
