What are the Limitations of Ionization Chambers?

Ionization chambers are widely used in the nuclear power industry, research laboratories, radiography, radiobiology and environmental monitoring due to their good uniform response to radiation over a wide range of energies. They operate at a low electric field strength, so gas multiplication does not occur and the collected load (output signal) is independent of the applied voltage. A gas ionization chamber measures charge from the number of ion pairs created within a gas caused by incident radiation. Proportional meters are more sensitive than ionization chambers and are suitable for measurements in low-intensity radiation fields.

Noble gas ionization chambers are simple, resistant to radiation, and can be easily constructed in the 4π geometry used for accurate measurements of gamma-ray source activity. Absorption within an ionization chamber can be controlled by selection of make-up gas composition and pressure. Ionization chambers with transparent X-ray plates made of aluminized plastic or thin metal mesh are used for the detection of fluorescent radiation. 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.

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 the primary ions to obtain enough energy in the vicinity of the thin central electrode to cause more ionization in the detector. Two types of amplifiers are used to make the pulse height proportional to the amount of ionization produced by the particle in the chamber.The radiobiological data available so far on the FLASH effect indicate that better preservation of normal tissues is obtained by delivering a lower number of pulses and, therefore, a higher dose per pulse, which would lower the collection efficiency in the ionization chambers. A simple ionization chamber consists of a metal cylinder with a thin axial wire enclosed in a glass envelope in which some inert gas is filled. Multi-cavity ionization chambers can measure the intensity of the radiation beam in several different regions, providing information on the symmetry and flatness of the beam.

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.The transmission ionization chamber generally consists of layers of PMMA coated with conductive material. When atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and resulting positive ions are created and dissociated electrons move to electrodes of opposite polarity under influence of electric field. Small ventilated air ionization chambers with volume of 0.01 to 0.3 cm3 are considered suitable for measuring field parameters up to 2 cm × 2 cm. They are considered radiation indicators whereas ionization chambers are used for more quantitative measurements.

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