Encyclopedia
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

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
Centimetre gram second system of units
The centimetre–gram–second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time...

in 1953 as the dose causing 100 erg
Erg
An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work"....

s of energy to be absorbed by one gram
Gram
The gram is a metric system unit of mass....

of matter. It was restated in SI units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...

in 1970 as the dose causing 0.01 joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...

of energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

to be absorbed per kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...

of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...

.

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
Röntgen equivalent man
Named after Wilhelm Röntgen , the roentgen equivalent in man or rem is a unit of radiation dose equivalent...

," 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
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....

(Gy); the rad is equal to the centigray and 100 rads are equal to 1 Gy. The continued use of the rad is "strongly discouraged" by the author style guide of the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

National Institute of Standards and Technology
National Institute of Standards and Technology
The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

. Nevertheless, use of the rad remains widespread and is still an industry standard.