Ionization chambers are a type of radiation detector used in medical physics and radiation therapy to ensure that the dose delivered from a therapy unit or radiopharmaceutical is as intended. These chambers come in a variety of shapes and sizes, and can be used for a range of applications. In this article, we'll explore the different types of ionization chambers, their uses, and how they work. A gas ionization chamber is the most common type of ionization chamber. It measures charge from the number of ion pairs created within a gas caused by incident radiation.
This type of chamber is simple, resistant to radiation, and can be easily constructed in the 4π geometry used for precise measurements of gamma ray sources. Ventilated air ionization chambers are also used in diagnostic radiology. These chambers have a volume of 0.01 to 0.3 cm3 and are suitable for measuring field parameters up to 2 cm × 2 cm. They must be connected to the atmosphere in order for them to work properly.
After exposure to radiation, the ionization produced in the chamber discharges the condenser; the exposure (or air kerma) is proportional to the discharge, which can be read directly against the light through a built-in eyepiece. A proportional counter is a modified ionization chamber in which a higher voltage is applied, creating an electric field near the axial cable that is strong enough to accelerate approaching electrons to high energies. This causes further ionization when they collide with gas molecules. Proportional meters are more sensitive than regular ionization chambers and are suitable for measurements in low-intensity radiation fields. Transmission ionization chambers consist of layers of PMMA coated with conductive material. They act as solid-state ionization chambers when exposed to radiation and, like scintillation detectors, belong to the class of solid-state detectors.
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. Multi-channel xenon ionization chambers pressurized to 20 bar were developed in the 1970s and 1980s and were successfully used in several clinical computed tomography (CT) scanners. A protective electrode is typically provided in this type of chamber to further reduce leakage from the chamber and ensure improved field uniformity in the active or sensitive volume. Self-reading pocket dosimeters in the form of a pen also use an ionization chamber that functions as a condenser, fully charged (corresponding to zero dose) before use. In conclusion, there are several types of ionization chambers available for different applications. Gas ionization chambers are simple and resistant to radiation, while ventilated air ionization chambers are suitable for measuring field parameters up to 2 cm × 2 cm. Proportional counters are more sensitive than regular ionization chambers, while transmission ionization chambers act as solid-state detectors when exposed to radiation.
Multi-channel xenon ionization chambers pressurized to 20 bar have been used in clinical CT scanners, while self-reading pocket dosimeters come in the form of a pen.
