The most commonly used ionization chamber for radiotherapy measurements is a cylindrical or thimble-shaped camera. This type of camera has an active volume housed within a thimble-shaped cavity with an internal conductive surface (cathode) and a central anode. Dosimetry protocols recommend that ionization chambers used in radiation therapy be pre-irradiated until they “settle”, meaning that the response of the chamber is stable. Previous reports have suggested that the lack of prior irradiation could lead to errors of up to several percent.
Recently, data collected for a large number of commonly used ion chambers at the Institute for National Measurement Standards, NRC, Canada, has been collected and analyzed, with additional data provided by the National Physics Laboratory of the United Kingdom. With this data set, it was possible to relate the behavior patterns of the ion chamber to the design parameters. Several mechanisms appear to contribute to this behavior, with the most obvious correlations involving the type of insulator surrounding the center collector electrode, the extent of the collector electrode shield, and possibly the area of the insulator exposed at the base of the active air volume. The results show that ion chambers with electrode connections protected up to the volume of active air settle quickly (approximately 9 minutes) and the change in response is small (less than approximately 0.2%).
For ion chambers where the protective connection surrounding the central manifold does not extend to the volume of active air, settling times of 15 to 20 minutes and an associated change in response of up to 1% are typical. For some ion chamber models, irradiation rate may also play a role in sedimentation behavior. The sedimentation times for the ion chambers studied here were found to be independent of beam quality. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure. Because these chambers are long, the flow of electrons through them may not be uniform (there may be no temporal uniformity in the chamber volume).
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. The CC13 ionization chamber is designed for absolute and relative dosimetry of photon, electron and proton beams in radiation therapy. 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). 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. Noble gas ionization chambers are simple, resistant to radiation, and are easily constructed in the 4π geometry used for accurate measurements of gamma-ray source activity. They also act as solid-state ionization chambers by applying reverse polarization to detectors and by being exposed to radiation.
Since the sensitivity of the camera is proportional to the volume, the effect of leakage on the measured load is relatively greater for small cameras. 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. This makes the output signal in the ionization chamber a direct current, unlike the Geiger-Muller tube which produces a pulse output. The ionization chamber is a radiation detector that is used to detect and measure charge from the number of ion pairs created within a gas caused by incident radiation. 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 generally 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. Radiation indicators are considered, whereas ionization chambers are used for more quantitative measurements.
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 primary ions to obtain enough energy in vicinity of thin central electrode to cause more ionization in detector.
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