The sievert (symbol: Sv) is the International System of Units (SI) derived unit of equivalent radiation dose, effective dose, and committed dose. Quantities that are measured in sieverts are designed to represent the stochastic biological effects of ionizing radiation. The sievert should not be used to express the unmodified absorbed dose of radiation energy, which is a clear physical quantity measured in grays. To enable consideration of biological effects, further calculations must be performed to convert absorbed dose into effective dose, the details of which depend on the biological context. This can be far more complicated than just multiplying by a weighting factor.

The sievert is of fundamental importance in radiation dosimetry, and is named after Rolf Maximilian Sievert, a Swedish medical physicist renowned for work on radiation dosage measurement and research into the biological effects of radiation. One sievert equals 100 rem, an older unit of measurement still in widespread use. One sievert carries with it a 5.5% chance of eventually developing cancer.^[1] Doses greater than 1 sievert received over a short time period are likely to cause radiation poisoning, possibly leading to death within weeks.

Definition[link]

The gray and sievert units are both special names for the SI derived units of joules per kilogram (m²/s² if expressed in base units), though they are not interchangeable.^[2]

1 Sv = 1 J/kg = 1 Gy

The gray is used with quantities of absorbed dose in any material, while the sievert is used with equivalent, effective, and committed dose in biological tissue. The latter quantities are weighted averages of absorbed dose designed to be more representative of the stochastic health effects of radiation, and use of the sievert implies that appropriate regulatory weighting factors have been applied to the original measurement.^[1]

For example, an absorbed dose of 1 mGy of alpha radiation gives an equivalent dose of 20 mSv because alpha has a radiation weighting factor of 20. This can appear to lead to a paradox whereby the energy of the incident radiation field is increased by a factor of 20 when sieverts are used, thereby violating the laws of Conservation of energy. However this is not the case, as it is the biological effect that is being expressed when using sieverts. Sieverts do not represent the actual physical energy imparted by the incident radiation, but an estimation of the biological effect.

Confusion can be caused as there are two different radiation quantities that can both be measured in the same units of J/kg; the sievert and the gray. To differentiate, consider an analogy where a fire can be put out with plain water or fire fighting foam. In either case, the flow rate could be measured in units of litre/sec, but the foam could be 5 or 20 times more effective than plain water, depending on the type of foam. One could choose to use litres per second (analogous to grays) to express the real flow rate of water or foam, and cubic decimetres per second (analogous to sieverts) to describe the equivalent flow of plain water that would produce the same effect as the foam.

The sievert and the gray are different names for the same unit of delivery, but they are used in different contexts.^[2]

SI units[link]

This SI unit is named after Rolf Maximilian Sievert. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Sv). When an SI unit is spelled out in English, it should always begin with a lower case letter (sievert), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. —Based on The International System of Units, section 5.2.

Frequently used SI multiples are the millisievert (1 mSv = 0.001 Sv) and microsievert (1 μSv = 0.000001 Sv). The conventional units for its time derivative is mSv/h. Regulatory limits and chronic doses are often given in units of mSv/yr or Sv/yr, where they are understood to represent an average over the entire year. In many occupational scenarios, the hourly dose rate might fluctuate to levels thousands of times higher for a brief period of time, without infringing on the annual limits. There is no exact conversion from hours to years because of leap years, but approximate conversions are:

1 mSv/h = 8.766 Sv/yr

114.1 μSv/h = 1 Sv/yr

An older unit for the equivalent dose is the rem,^[3] still often used in the United States. One sievert is equal to 100 rem:

Health effects[link]

Ionizing radiation has deterministic and stochastic effects on human health. The deterministic effects that can lead to acute radiation syndrome only occur in the case of high doses (> ~0.1 Gy) and high dose rates (> ~0.1 Gy/h). A model of deterministic risk would require different weighting factors (not yet established) than are used in the calculation of equivalent and effective dose. To avoid confusion, deterministic effects are normally compared to absorbed dose in units of Gy, not Sv.

Stochastic effects are those that occur randomly, such as radiation-induced cancer. The consensus of the nuclear industry, nuclear regulators, and governments, is that the incidence of cancers due to ionizing radiation can be modeled as increasing linearly with effective dose at a rate of 5.5% per sievert.^[1] Individual studies, alternate models, and earlier versions of the industry consensus have produced other risk estimates scattered around this consensus model. There is general agreement that the risk is much higher for infants and fetuses than adults, higher for the middle-aged than for seniors, and higher for women than for men, though there is no quantitative consensus about this.^[4][5] There is much less data, and much more controversy, regarding the possibility of cardiac and teratogenic effects, and the modelling of internal dose.^[6]

The International Commission on Radiological Protection (ICRP) recommends limiting artificial irradiation of the public to an average of 1 mSv (0.001 Sv) of effective dose per year, not including medical and occupational exposures.^[1] For comparison, radiation levels inside the US capitol building are 0.85 mSv/yr, close to the regulatory limit, because of the uranium content of the granite structure.^[7] According to the ICRP model, someone who spent 20 years inside the capitol building would have an extra one in a thousand chance of getting cancer, over and above any other existing risk. (20 yr X 0.85 mSv/yr X 0.001 Sv/mSv X 5.5%/Sv = ~0.1%) That "existing risk" is much higher; an average American would have a one in ten chance of getting cancer during this same 20 year period, even without any exposure to artificial radiation.

Dose examples[link]

Since radiation doses are not frequently encountered in everyday life, the following examples can help illustrate relative magnitudes. These are meant to be examples only, not a comprehensive list of possible radiation doses. An "acute dose" is one that occurs over a short and finite period of time, while a "chronic dose" is a dose that continues for an extended period of time so that it is better described by a dose rate.

Acute dose examples[link]

Chronic dose examples[link]

Notes on examples:

History[link]

The sievert has its origin in the roentgen equivalent man (rem) which was derived from CGS units. The International Commission on Radiation Units and Measurements (ICRU) promoted a switch to coherent SI units in the 1970's,^[27] and announced in 1976 that it planned to formulate a suitable unit for equivalent dose.^[28] The ICRP pre-empted the ICRU by introducing the sievert in 1977.^[29]

The sievert was adopted by the International Committee for Weights and Measures (CIPM) in 1980, five years after adopting the gray. The CIPM then issued an explanation in 1984, recommending when the sievert should be used as opposed to the gray. That explanation was updated in 2002 to bring it closer to the ICRP's definition of equivalent dose, which had changed in 1990. Specifically, the ICRP had renamed the dose equivalent to equivalent dose, renamed the quality factor (Q) to radiation weighting factor (W_R), and dropped another weighting factor 'N' in 1990. In 2002, the CIPM similarly dropped the weighting factor 'N' from their explanation but otherwise kept the old terminology and symbols. This explanation only appears in the appendix to the SI brochure and is not part of the definition of the sievert.^[2]

See also[link]

• Becquerel (disintegrations per second)
• Counts per minute
• Curie (unit)
• Orders of magnitude (radiation) - Example exposure scenarios
• Rad (unit)
• Roentgen (unit)
• Rutherford (unit)
• Sverdrup (a non-SI unit of volume transport with the same symbol Sv as Sievert)
• Background radiation
• Relative Biological Effectiveness
• Radiation poisoning
• Linear Energy Transfer
• Fluence

References[link]

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