Rad (unit)

The rad is a largely obsolete unit of absorbed radiation dose, with symbol rad. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the [malignant mammalian] cells in question..." It was defined in CGS units in 1953 and restated in SI in 1970. It is the dose causing 0.01 joule of energy to be absorbed per kilogram of matter.

It is now superseded in the SI by the gray; 100 rad are equal to 1 gray. The continued use of the rad is "strongly discouraged" by the National Institute of Standards and Technology.