Editing Sievert (section)

==External dose quantities==
[[File:Dose quantities and units.png|thumb|upright=1.8|External radiation dose quantities used in radiological protection]]
When the sievert is used to represent the stochastic effects of external ionizing radiation on human tissue, the radiation doses received are measured in practice by radiometric instruments and [[dosimeters]] and are called operational quantities. To relate these actual received doses to likely health effects, protection quantities have been developed to predict the likely health effects using the results of large epidemiological studies. Consequently, this has required the creation of a number of different dose quantities within a coherent system developed by the ICRU working with the ICRP.

The external dose quantities and their relationships are shown in the accompanying diagram. The ICRU is primarily responsible for the operational dose quantities, based upon the application of ionising radiation metrology, and the ICRP is primarily responsible for the protection quantities, based upon modelling of dose uptake and biological sensitivity of the human body.

===Naming conventions===
The ICRU/ICRP dose quantities have specific purposes and meanings, but some use common words in a different order. There can be confusion between, for instance, ''equivalent dose'' and ''dose equivalent''.

Although the CIPM definition states that the linear energy transfer function (Q) of the ICRU is used in calculating the biological effect, the ICRP in 1990<ref>ICRP publication 60 published in 1991</ref> developed the "protection" dose quantities ''effective'' and ''equivalent'' dose which are calculated from more complex computational models and are distinguished by not having the phrase ''dose equivalent'' in their name. Only the operational dose quantities which still use Q for calculation retain the phrase ''dose equivalent''. However, there are joint ICRU/ICRP proposals to simplify this system by changes to the operational dose definitions to harmonise with those of protection quantities. These were outlined at the 3rd International Symposium on Radiological Protection in October 2015, and if implemented would make the naming of operational quantities more logical by introducing "dose to lens of eye" and "dose to local skin" as ''equivalent doses''.<ref name="endo" />

In the [[United States|USA]] there are differently named dose quantities which are not part of the ICRP nomenclature.<ref>[http://www.wmsym.org/archives/2009/pdfs/9444.pdf "The confusing world of radiation dosimetry"] - M.A. Boyd, U.S. Environmental Protection Agency 2009. An account of chronological differences between US and ICRP dosimetry systems.</ref>

===Physical quantities===
These are directly measurable physical quantities in which no allowance has been made for biological effects. Radiation [[fluence]] is the number of radiation particles impinging per unit area per unit time, [[Kerma (physics)|kerma]] is the ionising effect on air of [[gamma rays]] and [[X-rays]] and is used for instrument calibration, and absorbed dose is the amount of radiation energy deposited per unit mass in the matter or tissue under consideration.

===Operational quantities===
Operational quantities are measured in practice, and are the means of directly measuring dose uptake due to exposure, or predicting dose uptake in a measured environment. In this way they are used for practical dose control, by providing an estimate or upper limit for the value of the protection quantities related to an exposure. They are also used in practical regulations and guidance.<ref>ICRP publication 103, paragraph B147</ref>

The calibration of individual and area dosimeters in photon fields is performed by measuring the collision "air kerma free in air" under conditions of secondary electron equilibrium. Then the appropriate operational quantity is derived applying a conversion coefficient that relates the air kerma to the appropriate operational quantity. The conversion coefficients for photon radiation are published by the ICRU.<ref>''Measurement of H*(10) and Hp(10) in Mixed High-Energy Electron and Photon Fields.'' E. Gargioni, L. Büermann and H.-M. Kramer Physikalisch-Technische Bundesanstalt (PTB), D-38116 Braunschweig, Germany</ref>

Simple (non-anthropomorphic) "phantoms" are used to relate operational quantities to measured free-air irradiation. The ICRU sphere phantom is based on the definition of an ICRU 4-element tissue-equivalent material which does not really exist and cannot be fabricated.<ref>"Operational Quantities for External Radiation Exposure, Actual Shortcomings and Alternative Options", G. Dietze, D.T. Bartlett, N.E. Hertel, given at IRPA 2012, Glasgow, Scotland. May 2012</ref> The ICRU sphere is a theoretical 30&nbsp;cm diameter "tissue equivalent" sphere consisting of a material with a density of 1 g·cm<sup>−3</sup> and a mass composition of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen and 2.6% nitrogen. This material is specified to most closely approximate human tissue in its absorption properties. According to the ICRP, the ICRU "sphere phantom" in most cases adequately approximates the human body as regards the scattering and attenuation of penetrating radiation fields under consideration.<ref>ICRP publication 103, paragraph B159</ref> Thus radiation of a particular energy fluence will have roughly the same energy deposition within the sphere as it would in the equivalent mass of human tissue.<ref name="IAEACalibration" />

To allow for back-scattering and absorption of the human body, the "slab phantom" is used to represent the human torso for practical calibration of whole body dosimeters. The slab phantom is {{nowrap|300 mm × 300 mm × 150 mm}} depth to represent the human torso.<ref name="IAEACalibration" />

The joint ICRU/ICRP proposals outlined at the 3rd International Symposium on Radiological Protection in October 2015 to change the definition of operational quantities would not change the present use of calibration phantoms or reference radiation fields.<ref name="endo" />

===Protection quantities===
Protection quantities are calculated models, and are used as "limiting quantities" to specify exposure limits to ensure, in the words of ICRP, "that the occurrence of stochastic health effects is kept below unacceptable levels and that tissue reactions are avoided".<ref>ICRP publication 103, paragraph 112</ref><ref>ICRP publication 103, paragraph B50</ref><ref name="IAEACalibration">{{citation |title=Calibration of Radiation Protection Monitoring Instruments |series=Safety Reports Series 16 |quote=In 1991, the International Commission on Radiological Protection (ICRP) [7] recommended a revised system of dose limitation, including specification of primary ''limiting quantities'' for radiation protection purposes. These protection quantities are essentially unmeasurable |url=http://www-pub.iaea.org/MTCD/Publications/PDF/P074_scr.pdf |year=2000 |publisher=IAEA |isbn=978-92-0-100100-9}}</ref> These quantities cannot be measured in practice but their values are derived using models of external dose to internal organs of the human body, using [[Computational human phantom|anthropomorphic phantom]]s. These are 3D computational models of the body which take into account a number of complex effects such as body self-shielding and internal scattering of radiation. The calculation starts with organ absorbed dose, and then applies radiation and tissue weighting factors.<ref>ICRP publication 103, paragraph B64</ref>

As protection quantities cannot practically be measured, operational quantities must be used to relate them to practical radiation instrument and dosimeter responses.<ref>ICRP publication 103, paragraph B146</ref>

===Instrument and dosimetry response===
This is an actual reading obtained from such as an ambient dose [[gamma ray|gamma]] monitor, or a personal [[dosimeter]]. Such instruments are calibrated using radiation metrology techniques which will trace them to a national radiation standard, and thereby relate them to an operational quantity. The readings of instruments and dosimeters are used to prevent the uptake of excessive dose and to provide records of dose uptake to satisfy radiation safety legislation; such as in the [[UK]], the [[Ionising Radiations Regulations 1999]].