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Exposure

(Science: radiobiology) a quantitative measure of x or gamma radiation at a certain place, based on its ability to produce ionisation in air. The former special unit of exposure was the roentgen (R). 1R = 2.58 x 10-4 C/kg. In the international system (si unit), the special unit is coulomb per kilogram. (exposure also is frequently used as a synonym for irradiation).