Sievert

The sievert (symbol: Sv) is the SI derived unitof dose equivalent. It attempts to reflect the biological effects of radiationas opposed to the physical aspects, which are characterised by the absorbed dose, measured in grays. It is named after Rolf Sievert, a Swedishmedical physicist famous for work on radiation dosagemeasurement and research into the biological effects of radiation.

Definition

The equivalent dose to a tissue is found by multiplying the absorbed dose, in grays, by a dimensionless"quality factor" Q, dependent upon radiation type, and by another dimensionless factor N, dependent on all other pertinent factors. N depends upon the part of the body irradiated, the time and volume over which the dose was spread, even the species of the subject. Together, Q and N constitute the radiation weighting factor, rW. For an organism comprised of multiple tissue types a weighted sumor integralis often used. In terms of SI base units:

1 Sv = 1 J/kg = 1 m²·s^?2

Although the sievert has the same dimensions as the gray (i.e. joules per kilogram), it measures a different thing. To avoid any risk of confusion between the absorbed dose and the equivalent dose, one must use the corresponding special units, namely the gray instead of the joule per kilogram for absorbed dose and the sievert instead of the joule per kilogram for the dose equivalent. For a given amount of radiation (measured in grays), the biological effect (measured in sieverts) can vary considerably as a result of the radiation weighting factor rW.

SI multiples and conversions

Frequently used SI multiplesare the millisievert (1mSv = 10^-3Sv) and microsievert (1μSv = 10^-6Sv).

An older unit of the equivalent dose is the rem(Roentgen equivalent man); 1 Sv is equal to 100 rem. In some fields, rem and mrem continue to be used along with Sv and mSv, unavoidably causing confusion (1 Sv = 100 rem, 10 mSv = 1 rem: it is hard to memorize, when to use which conversion factor).

Explanation

Various terms are used with this unit:

• Dose equivalent
• Ambient dose equivalent
• Directional dose equivalent
• Personal dose equivalent
• Organ equivalent dose

The millisievert (mSv) is commonly used to measure the effective dose in diagnostic medical procedures (e.g. X-rays, nuclear medicine, positron emission tomographyand computed tomography). The natural backgroundeffective dose rate varies considerably from place to place, but typically is around 3.5 mSv/year.

For a full body equivalent dose, 1 Sv causes slight blood changes, 2-5 Sv causes nausea, hair loss, hemorrhage and will cause death in many cases. More than 6 Sv will lead to death in less than two months in more than 80% of cases, and much over 4 is more likely than not to cause death. See radiation poisoningfor a more complete analysis of effects of various dosage levels.

The collective dosethat a population is exposed to is measured in "man-sieverts" (man.Sv).

Q values

Here are some quality factor values:

• Photons, all energies : Q = 1
• Electronsand muons, all energies : Q = 1
• Neutrons,
  • energy < 10 keV : Q = 5
  • 10 keV < energy < 100 keV : Q = 10
  • 100 keV < energy < 2 MeV : Q = 20
  • 2 MeV < energy < 20 MeV : Q = 10
  • energy > 20 MeV : Q = 5
• Protons, energy > 2 MeV : Q = 5
• Alpha particlesand other atomic nuclei : Q = 20

N values

Here are some N values for organs and tissues:

• Gonads: N = 0.20
• Bone marrow, colon, lung, stomach: N = 0.12
• Bladder, brain, breast, kidney, liver, muscles, oesophagus, pancreas, small intestine, spleen, thyroid, uterus: N = 0.05
• Bonesurface, skin: N = 0.01

And for other organisms, relative to humans:

• Viruses, bacteria, protozoans: N ≈ 0.03 – 0.0003
• Insects: N ≈ 0.1 – 0.002
• Molluscs: N ≈ 0.06 – 0.006
• Plants: N ≈ 2 – 0.02
• Fish: N ≈ 0.75 – 0.03
• Amphibians: N ≈ 0.4 – 0.14
• Reptiles: N ≈ 1 – 0.075
• Birds: N ≈ 0.6 – 0.15
• Humans: N = 1

Sources

• Comité international des poids et mesures(CIPM) 1984, Recommendation 1 (PV, 52, 31 and Metrologia, 1985, 21, 90)
• Abdeljelil Bakri, Neil Heather, Jorge Hendrichs, and Ian Ferris; Fifty Years of Radiation Biology in Entomology: Lessons Learned from IDIDAS, Annals of the Entomological Society of America, 98(1): 1-12 (2005)
• Introduction to Quantities and Units for Ionising RadiationUK National Physical Laboratoryde:Sievert (Einheit)

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It uses material from the http://en.wikipedia.org/wiki/Sievert Wikipedia article Sievert.