The sievert (symbol: Sv) is the SI derived unit of dose equivalent. Such a dose would be caused by an exposure imparted by ionizing x-ray or gamma radiation undergoing an energy loss of 1 joule per kilogram of body tissue (1 gray). It attempts to reflect the biological effects of radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in grays. It is named after Rolf Sievert, a Swedish medical physicist famous for work on radiation dosage measurement and research into the biological effects of radiation.
The equivalent dose to a tissue is found by multiplying the absorbed dose, in grays, by a dimensionless quality factor Q, dependent upon radiation type, and by another dimensionless factor N, dependent on all other pertinent factors. N depends upon the part of the body irradiated, the time and volume over which the dose was spread, even the species of the subject. Together, Q and N constitute the radiation weighting factor, WR. For an organism composed of multiple tissue types a weighted sum or integral is often used. In terms of SI base units:
Although the sievert has the same dimensions as the gray (i.e. joules per kilogram), it measures a different thing. To avoid confusion between the absorbed dose and the equivalent dose, one should use the corresponding special units, namely the gray instead of the joule per kilogram for absorbed dose and the 'sievert instead of the joule per kilogram for the dose equivalent. For a given amount of radiation (measured in grays), the biological effect (measured in sieverts) can vary considerably due to variations in the radiation weighting factor, WR.
SI multiples and conversions
Frequently used SI multiples are the millisievert (1mSv = 10–3Sv) and microsievert (1?Sv = 10–6Sv).
An older unit of the equivalent dose is the rem (Roentgen equivalent man); 1 Sv is equal to 100 rem. In some fields, rem and mrem continue to be used along with Sv and mSv, causing significant confusion (1 Sv = 100 rem, 10 mSv = 1 rem).
- SI base units and SI derived units, the Physics Laboratory, National Institute of Standards and Technology.