Ionization chambers are preferred for high radiation dose rates because they have no “downtime”, a phenomenon that affects the accuracy of the Geiger-Mueller tube at high dose rates. 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. An ionization chamber and an electrometer require calibration before use and, with a triaxial connection cable, tools are required for calibration of the radiation. 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.
Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure. When ionization chambers are not the most appropriate detectors for side profile measurements, an alternative is to use 2D detectors such as scintillation detectors14,15 and Gafchromic films. The alpha particle causes ionization inside the chamber, and the ejected electrons cause additional secondary ionizations. When the atoms or gas molecules between the electrodes are ionized by the incident ionizing radiation, ion pairs are created and the resulting positive ions are created and the dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field.
They are considered radiation indicators, whereas ionization chambers are used for more quantitative measurements. 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. Parallel plate cameras are also used and are the recommended camera geometry for electron beam dosimetry (Figure 6-27, B). They also act as solid-state ionization chambers by applying reverse polarization to detectors and by being exposed to radiation.
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. 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. 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. The smoke detector has two ionization chambers, one open to the air and a reference chamber that does not allow particles to enter.