Sievert

The sievert (symbol: Sv) is a derived unit of ionizing radiation dose in the International System of Units (SI) and is a measure of the health effect of low levels of ionizing radiation on the human body. The sievert is of importance in dosimetry and radiation protection, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation. The sievert (symbol: Sv) is a derived unit of ionizing radiation dose in the International System of Units (SI) and is a measure of the health effect of low levels of ionizing radiation on the human body. The sievert is of importance in dosimetry and radiation protection, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dose measurement and research into the biological effects of radiation. 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. One sievert carries with it a 5.5% chance of eventually developing cancer based on the linear no-threshold model. To enable consideration of stochastic health risk, calculations are performed to convert the physical quantity absorbed dose into equivalent dose and effective dose, the details of which depend on the radiation type and biological context. For applications in radiation protection and dosimetry assessment the International Commission on Radiological Protection (ICRP) and International Commission on Radiation Units and Measurements (ICRU) have published recommendations and data which are used to calculate these. These are under continual review, and changes are advised in the formal 'Reports' of those bodies. Conventionally, the sievert is not used for high dose rates of radiation that produce deterministic effects, which is the severity of acute tissue damage that is certain to happen, such as acute radiation syndrome; these effects are compared to the physical quantity absorbed dose measured by the unit gray (Gy). One sievert equals 100 rem. The rem is an older, non-SI unit of measurement. The SI definition given by the International Committee for Weights and Measures (CIPM) says: 'The quantity dose equivalent H is the product of the absorbed dose D of ionizing radiation and the dimensionless factor Q (quality factor) defined as a function of linear energy transfer by the ICRU' The value of Q is not defined further by CIPM, but it requires the use of the relevant ICRU recommendations to provide this value. The CIPM also says that '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'.