The symbol for radian.
A unit of absorbed radiation dose (the name comes from radiation absorbed dose). Symbol, rad, but “wherever confusion with the symbol for the radian (angular measure) appears possible,”¹ rd.
The rad was defined by the International Commission on Radiological Units as the amount of radiation that leads to the absorption of 100 ergs of energy per gram (0.01 joule per kilogram) of whatever substance is being used. The value 0.01 joule was chosen so that absorption of 1 roentgen of X rays or gamma radiation (of energies typically used in medicine) in water or soft tissue would produce an absorbed dose of 1 rad. For soft tissue, a measurement in rads of any dose will closely resemble its measurement in reps; this is not true of exposures to bone.
Unlike the roentgen, the rad can be applied to radiation of any type. Unlike the rep, the amount of energy absorbed is defined, not determined experimentally. However, since different types of radiation differ in the damage they do, another unit, the rem, is required to express a quantity of radiation in terms of biological damage done.
The rad has been replaced by an SI unit, the gray. One rad = 0.01 gray.
1. Quantities, Units and Symbols. A Report by the Symbols Committee of the Royal Society representing the Royal Society, the Chemical Society, the Faraday Society, the Institute of Physics. 1971.
London: The Royal Society, 1971.
Page 27, footnote 4.
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Last revised: 11 July 2002.