Sievert

The sievert (symbol Sv) is the International System of Units (SI) for measuring the health impact of ionizing radiation. It is used for two related dose quantities: the equivalent dose and the effective dose. The unit is named after Rolf Maximilian Sievert, a Swedish physicist noted for his work on radiation dose and biological effects.

Equivalent dose expresses the risk from different types of radiation by weighting the absorbed dose in a tissue with a radiation weighting factor. Absorbed dose is measured in grays (Gy), where 1 Gy equals 1 joule deposited per kilogram of matter. The equivalent dose in a tissue T is the absorbed dose D_T multiplied by a radiation weighting factor W_R that accounts for the biological effectiveness of the radiation. The effective dose further accounts for the varying sensitivity of different tissues by summing the tissue-weighted equivalent doses across all tissues, using tissue weighting factors W_T.

Common weighting factors give X-rays, gamma rays, and beta particles a W_R of about 1, while alpha particles have a much larger factor (about 20) and neutrons vary with energy. Thus, the sievert is a unit of risk, not a direct measure of energy deposited.

Typical context: natural background radiation amounts to a few millisieverts per year. Medical exposures vary; simple X-ray tests are typically tens of microsieverts to a fraction of a millisievert, while CT scans often yield several millisieverts. Regulatory guidance often places occupational exposure limits around 20 mSv per year (averaged over several years) and public exposure limits around 1 mSv per year, with country-specific variations.