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The BIPM maintains primary standards for the determination of the quantities air kerma and absorbed dose to water in 60Co  -radiation, and air kerma in 137Cs -radiation. For these standards, key comparisons are made with the NMI standards at the reference distance of 1 m and with a radiation field size in the reference plane of 10 cm by 10 cm (10 cm diameter for the 137Cs beam). Primary standards are also maintained for ambient dose equivalent in both 60Co and 137Cs beams. These are at a reference distance of 3.5 m and 3 m respectively with radiation field diameters of approximately 75 cm and 60 cm for the respective beams.
The BIPM also maintains a standard for absorbed dose to graphite, which is a cavity ionization chamber of similar design housed in a cylindrical graphite phantom.
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Summary
