Rad (unit)

The rad is a unit of absorbed radiation dose. 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 of energy to be absorbed per kilogram of matter.

The United States Nuclear Regulatory Commission requires the use of the units curie, rad and rem as part of the Code of Federal Regulations 10CFR20.

The older quantity and unit of radiation exposure (ionization in dry air) is the "roentgen" (R), where 1 R is equal to 2.58 × 10^-4 C/kg. The older quantity and unit of absorbed dose is the "rad," where 1 rad = 0.01 J/kg. The material absorbing the radiation can be tissue or any other medium (for example, air, water, lead shielding, etc.). To convert absorbed dose to dose equivalent, or "rem," the biological effects in man are now considered, which is done by modifying with a quality factor. For practical scenarios, with low "linear energy transfer" (LET) radiation such as gamma or x rays, 1 R = 1 rad = 1 rem.

The Système International has introduced as a rival unit, the gray (Gy); the rad is equal to the centi and 100 rads are equal to 1 Gy. The continued use of the rad is...
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