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sievert

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A special unit in SI, the unit of “dose equivalent,” used by health physicists in protecting people from overexposure to radiation such as X rays and gamma rays. Symbol, Sv. The 16th CGPM added the sievert to SI in 1979 (Resolution 5). It is named for the Swedish physicist Rolf Sievert (1896-1966).

Photograph of Rold Sievert

Rolf Sievert in 1929

The sievert is measured as joules per kilogram, or, in SI base units,

$a fraction, meters squared over seconds squared$

and thus has the dimensions L²/T².

When anything absorbs radiation, energy is deposited in it, and the amount of energy deposited can be measured. The amount of energy deposited in tissue by an exposure to ionizing radiation (“a dose”) can be expressed in joules per kilogram. Health physicists give 1 joule per kilogram a special name, a gray.

Simply measuring the amount of energy tissue has absorbed from a dose of ionizing radiation is not enough to predict the amount of harm done. There are different kinds of ionizing radiation, including alpha, beta and gamma rays. Experience has shown that a 1-gray dose of alpha rays, for example, is about 10 to 20 times more harmful than a 1-gray dose of gamma rays, depending on the energy of the gamma ray. Beta rays and X rays are about as harmful as gamma rays. Slow neutrons are about 5 times as harmful, and fast neutrons 10 times as harmful.

To express the size of an exposure in terms of biological damage, which is what health physicists need to do, a measurement of the absorbed dose in joules per kilogram (hence in grays) is multiplied by a “quality factor” for that kind of radiation. The quality factor is in part determined experimentally and in part based on expert judgment. These dimensionless quality factors are maintained by the International Commission for Radiation Protection.

The quality factors are chosen so that 1 sievert of radiation is the amount of any kind of radiation which would cause the same amount of biological damage in a human being as would result from absorbing 1 gray of X rays. The sievert is said to measure “dose equivalent” because it indicates, for a dose of any kind of radiation, what dose of X rays or gamma rays would produce the same amount of damage. It is intended to be used at the sort of radiation levels encountered in medicine or the workplace; it “should not be used in assessing the effects of high-level, accidental exposures.”

The sievert was recommended by the ICRU and the International Commission on Radiation Protection in 1977, and adopted by the 16th General Conference on Weights and Measures in October 1979 (Resolution 5). It replaced the rem, although continued use of the rem was sanctioned for the time being. 100 rem = 1 Sv.

1984 Clarification

Because of continued confusion over the meaning of the sievert, in 1984 the CIPM voted to add the following explanation to the official SI booklet.

The quantity dose equivalent H is the product of the absorbed dose D of ionizing radiation and the dimensionless factors Q (quality factor) and N (product of any other multiplying factors) stipulated by the International Commission on Radiological Protection:

H = Q · N · D

Thus, for a given radiation, the numerical value of H in joules per kilogram may differ from that of D in joules per kilogram depending on the values of Q and N. In order to avoid any risk of confusion between the absorbed dose D and the dose equivalent H, the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose D and the name sievert instead of joules per kilogram for the unit of dose equivalent H.

International Commission on Radiation Units and Measurements.
Radiation Quantities and Units, ICRU Report 33.
1980.

CIPM, Recommendation 1
Procès-Verbaux des Sèances du Comite International des Poids et Mesures, volume 52, page 31.

Metrologia, volume 21, page 90 (1985).

resources

The Karolinska Institutet has a short biography and explanation (in English) of Sieverts' work: https://ki.se/en/onkpat/about-medical-radiation-physics.