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The röntgen or roentgen (symbol R) is a unit of exposure to ionising radiation (X or gamma rays), and is named after the physicist Wilhelm Röntgen. It is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge in 1 cm3 of air at standard temperature and pressure (STP). This corresponds to the generation of approximately 2.08×109 ion pairs.
In SI units, 1 R = 2.58×10-4 C/kg.
Natural background is around 10 μR per hour. A dose of 500 R in 5 h is lethal for humans.
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