The sievert (Sv)
is the SI-derived unit of equivalent radiation dose. An equivalent dose of 1
Sv is received when the actual absorbed dose of ionizing radiation, after
being multiplied by the dimensionless factors Q (the so-called quality
factor) and N (the product of any other multiplying factors), is 1
joule per kilogram. In this scheme, the relationship between the absorbed dose
of radiation D and the dose equivalent H is, therefore, given by
H = QND. Both Q and N are stipulated by the
International Commission on Radiological Protection. Also known as the
relative biological efficiency, Q depends on the nature of the
radiation and has a value of 1 for x rays, gamma rays, and beta particles; 10
for neutrons; and 20 for alpha particles. N is a factor that takes into
account the distribution of energy throughout the dose.
The unit is named
after the pioneering Swedish clinical physicist Rolf Maximillian Sievert
(1896-1960) and superseded the rem (short for rad equivalent mammals or man),
which corresponds to 0.01 Sv for x rays of energy ranging from 200 to 250 keV.
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