View source for Gray (unit)

{{short description|SI derived unit of absorbed dose of ionizing radiation}}
{{redirect-distinguish|Gy (unit)|Gigayear}}
{{Infobox Unit 
|bgcolour =
|name = gray
|image = 
|caption = 
|standard = [[SI]]
|quantity = [[Absorbed dose |absorbed dose of ionizing radiation]]
|symbol = Gy
|namedafter = [[Louis Harold Gray]]
|units1 = [[SI base unit]]s
|inunits1 = [[metre |m]]<sup>2</sup>&sdot;[[second |s]]<sup>−2</sup>
|units2 = [[CGS]] units (non-SI)
|inunits2 = 100 [[Rad (unit) |rad]]
}}

The '''gray''' (symbol: '''Gy''') is the unit of [[ionizing radiation]] dose in the [[International System of Units]] (SI), defined as the absorption of one [[joule]] of radiation [[energy]] per [[kilogram]] of [[matter]].<ref>{{cite web |url = http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |title = The International System of Units (SI) |publisher = Bureau International des Poids et Mesures ([[BIPM]]) |access-date = 2010-01-31 }}</ref>

It is used as a unit of the radiation quantity [[absorbed dose]] that measures the energy deposited by [[ionizing radiation]] in a unit mass of absorbing material, and is used for measuring the delivered dose in [[radiotherapy]], [[food irradiation]] and [[radiation sterilization]]. It is important in predicting likely acute health effects, such as [[acute radiation syndrome]]  and is used to calculate [[equivalent dose]] using the [[sievert]], which is a measure of the stochastic health effect  on the human body.

The gray is also used in radiation metrology as a unit of the radiation quantity [[kerma (physics)|kerma]]; defined as the sum of the initial [[kinetic energy |kinetic energies]] of all the [[charged particle]]s liberated by uncharged [[ionizing radiation]]{{efn |I.e., indirectly ionizing radiation such as [[photon]]s and [[neutron]]s}} in a sample of [[matter]] per unit mass. The unit was named after British physicist [[Louis Harold Gray]], a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue.<ref>{{cite web |url = http://www.lhgraytrust.org/lhgraybiography.html |title = Rays instead of scalpels |publisher = LH Gray Memorial Trust |year = 2002 |access-date = 2012-05-15}}</ref> 

The gray was adopted as part of the International System of Units in 1975. The corresponding [[cgs]] unit to the gray is the [[Rad (unit)|rad]] (equivalent to 0.01&nbsp;Gy), which remains common largely in the United States, though "strongly discouraged" in the style guide for U.S. [[National Institute of Standards and Technology]].<ref>{{cite journal |url=http://physics.nist.gov/Pubs/SP811/sec05.html#5.2 |title=NIST Guide to SI Units – Units temporarily accepted for use with the SI |journal=NIST |date=2 July 2009 |publisher=National Institute of Standards and Technology}}</ref>

==Applications==
[[File:Dose quantities and units.png |thumb |400px |External dose quantities used in radiation protection and dosimetry]]

The gray has a number of fields of application in measuring dose:

===Radiobiology===
The measurement of absorbed dose in tissue is of fundamental importance in [[radiobiology]] and [[radiation therapy]] as it is the measure of the amount of energy the incident radiation deposits in the target tissue. The measurement of absorbed dose is a complex problem due to scattering and absorption, and many specialist dosimeters are available for these measurements, and can cover applications in 1-D, 2-D and 3-D.<ref>{{cite journal |vauthors=Seco J, Clasie B, Partridge M |title=Review on the characteristics of radiation detectors for dosimetry and imaging |journal=Phys Med Biol |volume=59 |issue=20 |pages=R303–47 |year=2014 |pmid=25229250 |doi=10.1088/0031-9155/59/20/R303 |bibcode = 2014PMB....59R.303S |s2cid=4393848 }}</ref><ref>{{cite journal |vauthors=Hill R, Healy B, Holloway L, Kuncic Z, Thwaites D, Baldock C |title=Advances in kilovoltage x-ray beam dosimetry |journal=Phys Med Biol |volume=59 |issue=6 |pages=R183–231 |year=2014 |pmid=24584183 |doi=10.1088/0031-9155/59/6/R183 |bibcode = 2014PMB....59R.183H |s2cid=18082594 }}</ref><ref>{{cite journal |vauthors=Baldock C, De Deene Y, Doran S, Ibbott G, Jirasek A, Lepage M, McAuley KB, Oldham M, Schreiner LJ |title=Polymer gel dosimetry |journal=Phys Med Biol |volume=55 |issue=5 |pages=R1–63 |year=2010 |pmid=20150687 |pmc=3031873 |doi=10.1088/0031-9155/55/5/R01 |bibcode = 2010PMB....55R...1B }}</ref>

In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid [[epithelium |epithelial]] tumor ranges from 60 to 80&nbsp;Gy, while [[lymphoma]]s are treated with 20 to 40&nbsp;Gy. Preventive (adjuvant) doses are typically around 45–60&nbsp;Gy in 1.8–2&nbsp;Gy [[Dose fractionation#Radiation fractionation as cancer treatment |fractions]] (for breast, head, and neck cancers).

The average radiation dose from an abdominal X-ray is 0.7 millisieverts (0.0007 Sv), that from an abdominal [[X-ray computed tomography |CT scan]] is 8&nbsp;mSv, that from a pelvic CT scan is 6&nbsp;mGy, and that from a selective CT scan of the abdomen and the pelvis is 14&nbsp;mGy.<ref>{{cite web |url=http://www.xrayrisk.com/calculator/calculator.php |title=X-Ray Risk |website=www.xrayrisk.com}}</ref>

===Radiation protection===
[[File:SI Radiation dose units.png |thumb |400px |Relationship of ICRU/ICRP computed Protection dose quantities and units]]
The absorbed dose also plays an important role in [[radiation protection]], as it is the starting point for calculating the stochastic health risk of low levels of radiation, which is defined as the ''probability'' of cancer induction and genetic damage.<ref>{{cite journal |at=paragraph 64 |id=ICRP publication 103 |title=The 2007 Recommendations of the International Commission on Radiological Protection |journal=Ann ICRP |volume=37 |issue=2–4 |year=2007 |pmid=18082557 |doi=10.1016/j.icrp.2007.10.003 |s2cid=73326646 |doi-access=free }}</ref> The gray measures the total absorbed energy of radiation, but the probability of stochastic damage also depends on the type and energy of the radiation and the types of tissues involved. This probability is related to the [[equivalent dose]] in [[sievert]]s (Sv), which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles on [[equivalent dose]] and [[Effective dose (radiation) |effective dose]].

The [[International Committee for Weights and Measures]] states: "In order to avoid any risk of confusion between the absorbed dose ''D'' and the [[dose equivalent]] ''H'', the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose ''D'' and the name [[sievert]] instead of joules per kilogram for the unit of dose equivalent ''H''."<ref>{{cite web |title=CIPM, 2002: Recommendation 2 |publisher=BIPM |url=http://www.bipm.org/en/CIPM/db/2002/2/}}</ref>
:<math>1 \ \mathrm{Gy} = 1\ \frac{\mathrm{J}}{\mathrm{kg}}= 1\ \frac{\mathrm{m}^2}{\mathrm{s}^{2}}</math>

The accompanying diagrams show how absorbed dose (in grays) is first obtained by computational techniques, and from this value the equivalent doses are derived. For X-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for [[alpha particles]] one gray is equivalent to 20 sieverts, and a radiation weighting factor is applied accordingly.

===Radiation poisoning===
{{main|Acute radiation syndrome}}
The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation. These are effects that are ''certain'' to happen, as opposed to the uncertain effects of low levels of radiation that have a ''probability'' of causing damage. A whole-body acute exposure to 5 grays or more of high-energy radiation usually leads to death within 14 days. [[Lethal dose|LD]]<sub>1</sub> is 2.5 Gy, LD<sub>50</sub> is 5 Gy and LD<sub>99</sub> is 8 Gy.<ref>{{cite web |url=https://www.euronuclear.org/glossary/lethal-dose/ |title=Lethal dose |date=5 June 2019 |publisher=[[European Nuclear Society]]}}</ref> The LD<sub>50</sub> dose represents 375 joules for a 75 kg adult.

===Absorbed dose in matter===
The gray is used to measure absorbed dose rates in non-tissue materials for processes such as [[radiation hardening]], [[food irradiation]] and [[electron irradiation]]. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes.

===Kerma===
Kerma ("'''k'''inetic '''e'''nergy '''r'''eleased per unit '''ma'''ss") is used in radiation metrology as a measure of the liberated energy of ionisation due to irradiation, and is expressed in grays. Importantly, kerma dose is different from absorbed dose, depending on the radiation energies involved, partially because ionization energy is not accounted for. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, because some energy escapes from the absorbing volume in the form of [[bremsstrahlung]] (X-rays) or fast-moving electrons. 

Kerma, when applied to air, is equivalent to the legacy [[roentgen (unit)|roentgen]] unit of radiation exposure, but there is a difference in the definition of these two units. The gray is defined independently of any target material, however, the roentgen was defined specifically by the ionisation effect in dry air, which did not necessarily represent the effect on other media.

==Development of the absorbed dose concept and the gray==
[[File:Crookes tube xray experiment.jpg |thumb |Using early [[Crookes tube]] X-Ray apparatus in 1896. One man is viewing his hand with a [[fluoroscope]] to optimise tube emissions, the other has his head close to the tube. No precautions are being taken.]]
[[File:Ehrenmal der Radiologie (Hamburg-St. Georg).1.ajb.jpg |thumb |[[Monument to the X-ray and Radium Martyrs of All Nations]] erected 1936 at St. Georg hospital in Hamburg, commemorating 359 early radiology workers.]]

[[Wilhelm Röntgen]] discovered [[X-ray]]s on November 8, 1895, and their use spread very quickly for medical diagnostics, particularly broken bones and embedded foreign objects where they were a revolutionary improvement over previous techniques.

Due to the wide use of X-rays and the growing realisation of the dangers of ionizing radiation, measurement standards became necessary for radiation intensity and various countries developed their own, but using differing definitions and methods. Eventually, in order to promote international standardisation, the first International Congress of Radiology (ICR) meeting in London in 1925, proposed a separate body to consider units of measure. This was called the [[International Commission on Radiation Units and Measurements]], or ICRU,{{efn|Originally known as the International X-ray Unit Committee}} and came into being at the Second ICR in Stockholm in 1928, under the chairmanship of [[Manne Siegbahn]].<ref>{{cite journal |last=Siegbahn |first=Manne |title=Recommendations of the International X-ray Unit Committee |journal=Radiology |date=October 1929 |volume=13 |issue=4 |pages=372–3 |doi=10.1148/13.4.372 |s2cid=74656044 |display-authors=etal}}
</ref><ref>
{{cite web |title = About ICRU - History |url = http://www.icru.org/index.php?option=com_content&task=view&id=25&Itemid=63 |publisher = International Commission on Radiation Units & Measures |access-date = 2012-05-20}}
</ref>{{efn |The host country nominated the chairman of the early ICRU meetings.}}

One of the earliest techniques of measuring the intensity of X-rays was to measure their ionising effect in air by means of an air-filled [[ion chamber]]. At the first ICRU meeting it was proposed that one unit of X-ray dose should be defined as the quantity of X-rays that would produce one [[Statcoulomb |esu]] of charge in one [[cubic centimetre]] of dry air at 0&nbsp;[[°C]] and 1 [[standard pressure|standard atmosphere]] of pressure. This unit of [[radiation exposure]] was named the roentgen in honour of Wilhelm Röntgen, who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to [[gamma radiation]].<ref name=GM>{{cite conference |title = Dosimetry in Europe and the USSR |first1 = JH |last1 = Guill |first2 =John |last2 = Moteff |page = 64 |url = https://books.google.com/books?id=czTi4G6-Hq8C&q=roentgen+redefinition&pg=PA63 |conference = Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959 |publisher = ASTM International |book-title = Third Pacific Area Meeting Papers — Materials in Nuclear Applications |series=American Society Technical Publication |volume=276 |lccn = 60014734 |date = June 1960 |access-date = 2012-05-15}}</ref> This approach, although a great step forward in standardisation, had the disadvantage of not being a direct measure of the absorption of radiation, and thereby the ionisation effect, in various types of matter including human tissue, and was a measurement only of the effect of the X-rays in a specific circumstance; the ionisation effect in dry air.<ref name=Lovell4>{{cite book |chapter-url =https://books.google.com/books?id=lK48AAAAIAAJ&q=roentgen+defined&pg=PA56 |title = An introduction to Radiation Dosimetry |first1 = S |last1 = Lovell |year = 1979 |chapter = 4: Dosimetric quantities and units |pages = 52–64 |isbn = 0-521-22436-5 |publisher = Cambridge University Press |access-date = 2012-05-15}}</ref>

In 1940, Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with [[William Valentine Mayneord]] and the radiobiologist John Read, published a paper in which a new unit of measure, dubbed the ''gram roentgen'' (symbol: gr) was proposed, and defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation".<ref name="Gupta2009">{{cite book |last=Gupta |first=S. V. |title=Units of Measurement: Past, Present and Future : International System of Units |chapter-url=https://books.google.com/books?id=pHiKycrLmEQC&pg=PA144 |access-date=2012-05-14 |date=2009-11-19 |publisher=Springer |isbn=978-3-642-00737-8 |page=144 |chapter=Louis Harold Gray}}</ref>  This unit was found to be equivalent to 88&nbsp;ergs in air, and made the absorbed dose, as it subsequently became known, dependent on the interaction of the radiation with the irradiated material, not just an expression of radiation exposure or intensity, which the roentgen represented. In 1953 the ICRU recommended the [[rad (unit) |rad]], equal to 100&nbsp;erg/g, as the new unit of measure of absorbed radiation. The rad was expressed in coherent [[Cgs system |cgs]] units.<ref name=GM/> 

In the late 1950s, the CGPM invited the ICRU to join other scientific bodies to work on the development of the [[International System of Units]], or SI.<ref>{{cite web |url = http://www.bipm.org/en/committees/cc/ccu/ |title = CCU: Consultative Committee for Units |publisher = [[International Bureau of Weights and Measures]] (BIPM) |access-date = 2012-05-18}}</ref> The CCU decided to define the SI unit of absorbed radiation as energy deposited by reabsorbed charged particles per unit mass of absorbent material, which is how the rad had been defined, but in [[MKS system of units |MKS units]] it would be equivalent to the joule per kilogram. This was confirmed in 1975 by the 15th CGPM, and the unit was named the "gray" in honour of Louis Harold Gray, who had died in 1965. The gray was thus equal to 100&nbsp;rad. Notably, the centigray (numerically equivalent to the rad) is still widely used to describe absolute absorbed doses in radiotherapy.

The adoption of the gray by the 15th [[General Conference on Weights and Measures]] as the unit of measure of the absorption of [[ionizing radiation]], [[Specific energy |specific energy absorption]], and of [[Kerma (physics) |kerma]] in 1975<ref>{{SIBrochure8th |page = 157}}</ref> was the culmination of over half a century of work, both in the understanding of the nature of ionizing radiation and in the creation of coherent radiation quantities and units.

==Radiation-related quantities==
[[File:Radioactivity and radiation.png |thumb |400px |Graphic showing relationships between radioactivity and detected ionizing radiation at a point.]]
The following table shows radiation quantities in SI and non-SI units.
{{Radiation related quantities}}

==See also==

* {{Annotated link |Dose area product}} (Gy·cm<sup>2</sup>)
* {{Annotated link|International System of Units}} (SI base units)
* {{Annotated link |Orders of magnitude (radiation)}}
* {{Annotated link |Order of magnitude |Order of magnitude (unit)}}
* {{Annotated link |Rad (radiation unit)}}
* {{Annotated link |Roentgen (unit)}}
** {{Annotated link |Roentgen equivalent man}}
* [[Sievert]], SI derived unit of dose equivalent radiation
** {{Annotated link |SI derived unit}}

==Notes==
{{noteslist}}

==References==
{{Reflist |2}}


==External links==
* {{cite conference |first=M.A. |last=Boyd |date=March 1–5, 2009 |title=The Confusing World of Radiation Dosimetry—9444 |url=http://www.wmsym.org/archives/2009/pdfs/9444.pdf |conference=WM2009 Conference (Waste Management Symposium) |location=Phoenix, AZ |access-date=2014-07-07 |archive-url=https://web.archive.org/web/20161221205248/http://www.wmsym.org/archives/2009/pdfs/9444.pdf |archive-date=2016-12-21 |url-status=dead }} An account of chronological differences between USA and ICRP dosimetry systems.

{{Radiation protection |state=uncollapsed}}
{{SI units}}
{{Ionising radiation related quantities}}

{{DEFAULTSORT:Gray (Unit)}}
[[Category:Nuclear physics]]
[[Category:Radiation protection]]
[[Category:Radioactivity]]
[[Category:SI derived units]]
[[Category:Units of radiation dose]]