sievert

SI coherent derived unit with special name and symbol
Name Symbol Derived quantity Expressed in terms of SI base units
sievert Sv dose equivalent m2 s−2

Definition

The sievert, symbol Sv, is the SI derived unit of ionising radiation dose equivalent.

One sievert is equal to the dose equivalent to the absorption of one joule of ionising radiation energy per kilogram of matter. It is a measure of the absorption of ionising radiation energy by human tissue, and includes a qualitative scaling factor to account for the type of radiation involved, and the tissue type affected.

The sievert is named after the Swedish medical physicist Rolf Maximilian Sievert (1896 – 1966).

Dose equivalent

The dose equivalent H is the product of the absorbed dose D of ionising radiation and a dimensionless quality factor Q:

$H=Q \times D$

The absorbed dose D, in J/kg, is given the special name gray, symbol Gy. To avoid confusion with the unit gray, the dose equivalent H, in J/kg, is given a separate special name in the SI – the seivert, symbol Sv.

The sievert is a measure of the health effect of low levels of ionising radiation on the human body. It is used widely in dosimetry and radiation protection. At high levels of radiation, the gray is used.

The sievert is used for radiation dose quantities such as equivalent dose and effective dose, which represent the risk of external radiation from sources outside the body, and committed dose which represents the risk of internal irradiation due to inhaled or ingested radioactive substances.

The sievert is intended to represent the stochastic health risk, which for radiation dose assessment is defined as the probability of radiation-induced cancer and genetic damage.

Examples of low radiation dose equivalents
 Description Dose equivalent in SI units Estimated annual dose for an airline flight attendant 1.5 – 1.7 mSv Barium fluoroscopy – barium meal 2 – 7 mSv Full-body CT scan 10 – 30 mSv Estimated maximum dose to evacuees near Fukushima incident, in 2011 68 mSv 6-month stay on the International Space Station 80 mSv Estimated exposure to cosmic rays during a 6-month trip to Mars 250 mSv
Examples of high radiation dose equivalents
 Description Dose equivalent in SI units Highest dose to a worker responding to the Fukushima incident, in 2011 0.67 Sv Maximum allowed radiation exposure for a NASA astronaut over career 1 Sv Human LD50/30 dose – 50% risk of death within 30 days (LD50/30) 4 – 5 Sv Fatal acute doses during the Goiânia accident, in 1987 4.5 – 6 Sv Fatal acute doses during Tokaimura nuclear accident, in 1997 10 – 17 Sv Fatal acute dose to Louis Slotin in criticality accident, in 1946 21 Sv Fatal acute dose to Cecil Kelley, in 1958 36 Sv Fatal acute dose to Boris Korchilov in K-19 submarine, while working on unshielded reactor, in 1961 54 Sv