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Ionization chambers measure exposure by detecting liberated electron charge when x-ray photons ionize the gas within the chamber. The chambers need a high positive voltage applied at the collecting anode to attract the liberated electrons. The electron charge is collected and used to determine the radiographic exposure expressed in coulombs per kilogram (C/kg), the preferred SI unit, or the legacy unit, roentgens (R).
There are different types of ionization chambers:
- ionization chambers are dosimetry devices used to measure the output of x-ray tubes. They function as photo timers in automatic exposure controls and in nuclear medicine, as dose calibrators.
- pocket ionization chambers are a portable form of dosimetry devices in the shape of large pens. They provide immediate readings, are easily recharged and reused but are not very accurate due to their mechanical fragility.
- Geiger counters have a high voltage across the chamber. Due to the high voltage and positive potential, liberated electrons are accelerated and gain energy, resulting in more liberated electrons from the chamber and amplification of liberated charge (electron avalanche). Therefore, they are highly sensitive and are used to detect radioactive contamination (low level of radiation).
- 1. Walter Huda, Richard M. Slone. Review of Radiologic Physics. (2003) ISBN: 9780781736756