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 as the dose causing 100 ergs
of energy to be absorbed by one gram
of matter. It was restated in SI units in 1970 as the dose causing 0.01 joule
to be absorbed per kilogram
It is now superseded in the SI by the gray; 100 rad are equal to 1 gray.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 author style guide of the US National Institute of Standards and Technology.