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. 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. Self-reading pocket dosimeters in the form of a pen, consisting of an ionization chamber that functions as a condenser, fully charged (corresponding to zero dose) before use. 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.
Noble gas ionization chambers are simple, resistant to radiation and are easily constructed in the 4π geometry used for precise measurements of the activity of gamma ray sources (Suzuki et al. An ionization chamber and an electrometer require calibration before use and, with a triaxial connection cable, tools are required for calibration of the radiation beam. 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 (see Figure 6-26, B) and are an important type of radiation dosimeter as the primary device used for calibration of radiation therapy beams.
Regardless of their geometric design, ionization chambers used in diagnostic radiology must be of the ventilated type, that is, their volume of sensitive gas must communicate with the atmosphere. After exposure to radiation for a period of time, 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. The transmission ionization chamber generally consists of layers of PMMA coated with conductive material. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields.
A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation. They also act as solid-state ionization chambers by applying reverse polarization to detectors and by being exposed to radiation. Multi-channel xenon ionization chambers pressurized to 20 bar were developed in the 1970s and 1980s (Drost and Fenster, 1982, 198) and were successfully used in several clinical computed tomography (CT) scanners, such as the Philips 768-channel LX CT, the General Electric Model CT 90000 Series II, and the Siemens Model Somatom CR. A protective electrode is typically provided in the chamber to further reduce leakage from the chamber and ensure improved field uniformity in the active or sensitive volume of the chamber, with advantages in charge collection.