The ionization chamber is an integral part of a mass spectrometer, a device used to identify and measure the mass of atoms and molecules. It is a region where atoms of unknown material are excited to cause them to lose electrons. This process is known as ionization. The energy needed to excite atoms can be obtained by heating the sample or by using an electrically heated metal coil.
The ionization chamber consists of two charged electrodes that collect the ions formed within their respective electric fields. It can measure the dose or dose rate because it provides an indirect representation of the energy deposited in the chamber. A filter is placed at the opening of the chamber to prevent the passage of particulate radioactive materials, such as radon decay products, into the chamber. When the atoms or gas molecules between the electrodes are ionized by incident ionizing radiation, ion pairs are created and the resulting positive ions are attracted to the electrodes of opposite polarity under the influence of an electric field.
This process is known as direct determination of absorbed dose. An electret, which maintains a charge for an extended period and is discharged by exposure to radiation, is used together with an ionization chamber made of an electrically conductive plastic. The ICP has the ability to ionize almost all elements, including those with the highest ionization potentials. It is important that the ions produced in the ionization chamber pass freely through the machine without colliding with air molecules. Positively charged ions are repelled by a positively charged ionization chamber and accelerate towards three negatively charged slits with progressively decreasing voltage.
Sometimes two mass spectrometers are paired, so that a gentle ionization method can be followed by a more vigorous ionization of individual fragments. Multi-cavity ionization chambers can measure the intensity of radiation beams in several different regions, providing information on beam symmetry and flatness. The sample is vaporized before passing it to an ionization chamber where it is bombarded by a current of electrons emitted by an electrically heated metal coil. Ionization chambers are widely used in nuclear industry as they provide an output that is proportional to radiation dose. They have a longer useful life than standard Geiger-Müller tubes, which suffer from gas decomposition and are generally limited to a life of approximately 10-11 counting events.
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