Rad (unit)

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..."[1] 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. [2]

The Système International has introduced as a rival unit, the gray (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. National Institute of Standards and Technology.[3] Nevertheless, use of the rad remains widespread and is still an industry standard.[4]

Radiation poisoning

To gauge biological effects, the dose in rads is multiplied by a 'quality factor' or 'radiation weighting factor' which is dependent on the type of ionizing radiation. The modified dose is now measured in rems (roentgen equivalent mammal, or man)[5]. The number of rems is equal to Q times the number of rads, where Q is the quality factor. Q varies between 1 and 20, but is typically 1 for x-rays and gamma rays and 10 for neutrons. A dose of under 100 rems is subclinical and will produce nothing other than blood changes. 100 to 200 rems will cause illness, but will rarely be fatal. Doses of 200 to 1,000 rems will probably cause serious illness with poor outlook at the upper end of the range. Doses of more than 1,000 rems are almost invariably fatal[6].

See also


  1. ^ "Russ S, Archives of Radiology and Electrotherapy 23 (1918) 226-32.". http://lists.radlab.nl/pipermail/radsafe/2007-October/008087.html. 
  2. ^ "Converting rad to rem, Health Physics Society .". http://www.hps.org/publicinformation/ate/q3142.html. 
  3. ^ "NIST Guide to SI Units - Units temporarily accepted for use with the SI". http://physics.nist.gov/Pubs/SP811/sec05.html#5.2. 
  4. ^ Handbook of Radiation Effects, 2nd edition, 2002, Andrew Holmes-Siedle and Len Adams
  5. ^ The Effects of Nuclear Weapons, Revised ed., US DOD 1962, p. 579
  6. ^ The Effects of Nuclear Weapons, Revised ed., US DOD 1962, pp. 592–593