Sievert

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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.

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[edit] 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, WR . Q is the same thing as the Relative Biological Effectiveness [RBE]. 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/s2 = 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, 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, should be used. 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 WR. This variation in effect is attributed to the Linear Energy Transfer [LET] of the type of radiation, creating a different relative biological effectiveness for each type of radiation under consideration. Per most government regulations, the RBE [Q] for electron and photon radiation is 1, for neutron radiation it is 10, and for alpha radiation it is 20. There is some controversy that the Q or RBE for alpha radiation is underestimated due to mistaken assumptions in the original work in the 1950s that developed those values. That original work neglected the component of the nucleus recoil radiation for alpha emitters.

[edit] 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 (Röntgen equivalent man); 1 Sv is equal to 100 rem, for a quality factor Q=1. In some fields and countries, rem and mrem continue to be used along with Sv and mSv, causing confusion.

[edit] 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 nausea, 2-5 Sv causes hair loss, hemorrhage and will cause death in many cases. More than 3 Sv will lead to death in 50% of cases within 30 days, and over 6 Sv survival is unlikely. See radiation poisoning for a more complete analysis of effects of various dosage levels.

Given the linear no-threshold model of radiation response , the collective dose that a population is exposed to is measured in "man-sieverts" (man.Sv).

[edit] 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

[edit] N values

Here are some N values for organs and tissues:

And for other organisms, relative to humans:

[edit] Spelling

This SI unit is named after Rolf Maximilian Sievert. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (Sv). When an SI unit is spelled out in English, it should always begin with a lowercase 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.

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