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Sievert

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For other uses, see Sievert (disambiguation).

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

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

The sievert is named after Rolf Maximilian Sievert. As with every SI unit named for a person, its symbol starts with an upper case letter (Sv), but when written in full, it follows the rules for capitalisation of a common noun; i.e., sievert becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

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:

100.0000 rem = 100000.0 mrem = 1 Sv = 1.000000 Sv = 1000.000 mSv = 1000000 µSv
1.0000 rem = 1000.0 mrem = 1 rem = 0.010000 Sv = 10.000 mSv = 10000 µSv
0.1000 rem = 100.0 mrem = 1 mSv = 0.001000 Sv = 1.000 mSv = 1000 µSv
0.0010 rem = 1.0 mrem = 1 mrem = 0.000010 Sv = 0.010 mSv = 10 µSv
0.0001 rem = 0.1 mrem = 1 µSv = 0.000001 Sv = 0.001 mSv = 1 µSv

Health effects

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

Main article: Orders of magnitude (radiation)

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

98 nSv -banana equivalent dose, a whimsical unit of radiation[8]
0.25 μSv -U.S. limit on effective dose from a single airport security screening[9]
5 to 10 μSv -one set of dental radiographs[10]
80 μSv -average dose to people living within 16 km of Three Mile Island accident[11]
0.4 to 0.6 mSv -two-view mammogram, using weighting factors updated in 2007[12]
2 to 7 mSv -barium fluoroscopy, e.g. Barium meal, up to 2 minutes, 4-24 spot images[13]
10 to 30 mSv -single full-body CT scan[14][15]
68 mSv -estimated dose to evacuees from the Fukushima I nuclear accidents[16]
0.67 Sv -highest dose received by a worker responding to the Fukushima emergency[17][note 1]
4.5 to 6 Sv -fatal doses during Goiânia accident
21 Sv -fatal dose to Louis Slotin in 1946 criticality accident[18]

Chronic dose examples

<1 mSv/yr <0.1 μSv/h dose rates below 1 mSv/yr are difficult to measure and should viewed skeptically
1 mSv/yr (0.1 μSv/h avg) ICRP recommended maximum for artificial irradiation of the public,
excluding medical and occupational exposures.
2.4 mSv/yr (0.27 μSv/h avg) Natural background radiation, global average
24 mSv/yr (2.7 μSv/h avg) Natural background radiation at airline cruise altitude[19] [note 2]
0.64 Sv/yr (73 μSv/h avg) natural radiation in worst case house in Ramsar, Iran[20]
9 Sv/yr 1 mSv/h NRC definition of a high radiation area in a nuclear power plant, warranting a chain-link fence[21]
0.24 kSv/yr 27 mSv/h close proximity to a 100W radioisotope thermal generator[22]
1.4 kSv/yr 0.16 Sv/h Peak dose rate to a worker during a close call at Perry Nuclear Power Plant[23]
>90 kSv/yr >10 Sv/h most radioactive hotspot found in Fukushima I in areas normally accessible to workers[24]
2.3 MSv/yr 270 Sv/h typical PWR spent fuel bundle, after 10 year cooldown, no shielding[25]
90 MSv/yr 10 kSv/h waste components from fusion reactors[26]


Notes on examples:

  1. ^ The Fukushima worker dose is dominated by a committed dose which gradually turns into effective dose over an extended period of time. Therefore the true acute dose to the worker must be lower, but standard dosimetry practice is to account committed doses as acute in the year they are received.
  2. ^ The dose rate received by air crews is highly dependent on the radiation weighting factors chosen for protons and neutrons, which have changed over time and remain controversial.

History

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 (WR), 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

References

  1. ^ a b c d "The 2007 Recommendations of the International Commission on Radiological Protection". Annals of the ICRP. ICRP publication 103. 37 (2–4). 2007. ISBN 978-0-7020-3048-2. Retrieved 17 May 2012.
  2. ^ a b c The International System of Units (PDF) (9th ed.), International Bureau of Weights and Measures, December 2022, ISBN 978-92-822-2272-0
  3. ^ Office of Air and Radiation (2007). "Radiation: Risks and Realities" (PDF). Radiation: Risks and Realities. U.S. Environmental Protection Agency. p. 2. Retrieved 19 March 2011. {{cite web}}: More than one of |pages= and |page= specified (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  4. ^ Peck, Donald J. "How to Understand and Communicate Radiation Risk". Image Wisely. Retrieved 18 May 2012. {{cite web}}: More than one of |author= and |last= specified (help)
  5. ^ Effects of ionizing radiation : UNSCEAR 2006 report to the General Assembly, with scientific annexes. New York: United Nations. 2008. ISBN 978-92-1-142263-4. Retrieved 18 May 2012. {{cite book}}: |first= missing |last= (help)
  6. ^ European Committee on Radiation Risk (2010). Busby, Chris; et al. (eds.). 2010 recommendations of the ECRR : the health effects of exposure to low doses of ionizing radiation (PDF) (Regulators' ed. ed.). Aberystwyth: Green Audit. ISBN 978-1-897761-16-8. Retrieved 18 May 2012. {{cite book}}: |edition= has extra text (help); Explicit use of et al. in: |editor= (help)
  7. ^ Formerly Utilized Sites Remedial Action Program. "Radiation in the Environment" (PDF). US Army Corps of Engineers. Retrieved 18 May 2012.
  8. ^ RadSafe mailing list: original posting and follow up thread. FGR11 discussed.
  9. ^ American National Standards Institute (2009). Radiation Safety for Personnel Security  Screening Systems Using X‐Rays or Gamma Radiation (PDF). ANSI/HPS N43.17. Retrieved 31 May 2012. {{cite book}}: no-break space character in |id= at position 9 (help); no-break space character in |title= at position 10 (help)
  10. ^ Hart, D. (2002). Radiation Exposure of the UK Population from Medical and Dental X-ray Examinations (PDF). National Radiological Protection Board. p. 9. ISBN 0 85951 468 4. Retrieved 18 May 2012. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ "What Happened and What Didn't in the TMI-2 Accident". American Nuclear Society. Retrieved 16 March 2011.
  12. ^ Hendrick, R. Edward (2010). "Radiation Doses and Cancer Risks from Breast Imaging Studies". Radiology. 257: 246–253. Retrieved 18 May 2012. {{cite journal}}: Unknown parameter |month= ignored (help)
  13. ^ Wall, B.F. (1997). "Revised Radiation Doses for Typical X-Ray Examinations" (PDF). The British Journal of Radiology. 70: 437–439. Retrieved 18 May 2012. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) (5,000 patient dose measurements from 375 hospitals)
  14. ^ Brenner, David J.; Hall, Eric J. (2007). "Computed Tomography — an Increasing Source of Radiation Exposure". New England Journal of Medicine. 357 (22): 2277–2284. doi:10.1056/NEJMra072149. PMID 18046031.
  15. ^ Van Unnik, JG; Broerse, JJ; Geleijns, J; Jansen, JT; Zoetelief, J; Zweers, D (1997). "Survey of CT techniques and absorbed dose in various Dutch hospitals". The British journal of radiology. 70 (832): 367–71. PMID 9166072. (3000 examinations from 18 hospitals)
  16. ^ Hosoda, Masahiro (2011). "The time variation of dose rate artificially increased by the Fukushima nuclear crisis". Scientific Reports. 1. doi:10.1038/srep00087. Retrieved 19 May 2012.
  17. ^ American Nuclear Society (2012). "Appendix B". In Klein, Dale; and Corradini, Michael (ed.). Fukushima Daiichi: ANS Committee Report (PDF). Retrieved 19 May 2012. {{cite book}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: editors list (link)
  18. ^ McLaughlin, Thomas P. (2000). Los Alamos National Laboratory. p. 75. LA-13638 http://www.orau.org/ptp/Library/accidents/la-13638.pdf. Retrieved 21 May 2012. {{cite book}}: Missing or empty |title= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |month= ignored (help)
  19. ^ Bailey, Susan (January 2000). "Air crew radiation exposure—An overview" (PDF). Nuclear News. Retrieved 19 May 2012.
  20. ^ Babapouran, Mozhgan (2005), "New public dose assessment from internal and external exposures in low- and elevated-level natural radiation areas of Ramsar, Iran", International Congress Series, 1276: 169–174, doi:10.1016/j.ics.2004.11.102
  21. ^ US Nuclear Regulatory Commission (2006). REGULATORY GUIDE 8.38: CONTROL OF ACCESS TO HIGH AND VERY HIGH RADIATION AREAS IN NUCLEAR POWER PLANTS (PDF).
  22. ^ Summerer, Leopold (2009). Technology-Based Design and Scaling Laws for RTG's for Space Exploration in the 100 W Range (PDF). 60th International Astronautical Congress. Daejeon, South Korea. Retrieved 19 May 2012. {{cite conference}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Reynolds, Steven A. (2011). REPORT 05000440/2011013 (PDF). US Nuclear Regulatory Commission, Region 3. Retrieved 19 May 2012.
  24. ^ "Fukushima radiation hotspot". World Nuclear News. 2 August 2011. Retrieved 19 May 2012.
  25. ^ Su, S. (2006). TAD Source Term and Dose Rate Evaluation (PDF). Bechtel Saic. 000-30R-GGDE-00100-000-00A. Retrieved 20 May 2012. {{cite book}}: Unknown parameter |month= ignored (help)
  26. ^ Di Pace, Luigi (2012). "Ch. 14: Radioactive Waste Management of Fusion Power Plants". In Rehab Abdel Rahman (ed.). Radioactive Waste (PDF). InTech. p. 318. ISBN 978-953-51-0551-0. Retrieved 19 May 2012. {{cite book}}: More than one of |author= and |last= specified (help)
  27. ^ Wyckoff, H. O. (1977). Round table on SI units: ICRU Activities (PDF). International Congress of the International Radiation Protection Association. Paris, France. Retrieved 18 May 2012. {{cite conference}}: Unknown parameter |month= ignored (help)
  28. ^ Wyckoff, H. O. (1976). "The New Special Names of SI Units in the Field of Ionizing Radiations" (PDF). British Journal of Radiology. 49: 476–477. ISSN 1748-880X. Retrieved 18 May 2012. {{cite journal}}: Unknown parameter |month= ignored (help)
  29. ^ "Recommendations of the ICRP". Annals of the ICRP. ICRP publication 26. Vol. 1 (3). 1977. Retrieved 17 May 2012. {{cite journal}}: |volume= has extra text (help)

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