Sievert

The sievert (symbol: Sv) is the SI derived unit of dose equivalent. It attempts to reflect the biological effects of radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in grays. It is named after Rolf Sievert, a Swedish medical physicist famous for work on radiation dosage measurement 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 composed of multiple tissue types a weighted sum or integral is often used. (In 2002, the CIPM decided that the distinction between Q and N causes too much confusion and therefore deleted the factor N from the definition of absorbed dose in the SI brochure. [1].)

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 multiples are the millisievert (1 mSv = 10^–3 Sv) and microsievert (1 μSv = 10^–6 Sv).

An older unit of the equivalent dose is the rem (Roentgen equivalent man); 1 Sv is equal to 100 rem. In some fields and countries, 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 tomography and computed tomography). The natural background effective dose varies considerably from place to place, but typically is around 2.4 mSv/year [2] (pdf).

For acute 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 3 Sv will lead to death in less than two months in more than 80% of cases, and much over 4 Sv is more likely than not to cause death. See radiation poisoning for a more complete analysis of effects of various dosage levels.

The collective dose that 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
• Electrons and 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 particles and 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
• Bone surface, 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

This SI unit is named after Rolf Maximilian Sievert. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (Sv). But when an SI unit is spelled out, it should always be written in lowercase (sievert), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

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 Radiation UK National Physical Laboratory

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