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The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is a measure of the absorbed dose and is defined as the absorption of one joule of radiation energy by one kilogram of matter. It is a physical quantity, and does not take into account any biological context, and unlike the pre-1971 non-SI roentgen unit of dose, the gray is defined independently for any target material. The gray is also used to measure specific energy (imparted), and kerma. When measuring kerma, the reference target material must be defined explicitly, usually dry air at standard temperature and pressure.
The gray was named after the British physicist Louis Harold Gray, a pioneer in the field of measurement of radium radiation and X-rays and their effects on living tissue, and was adopted as part of SI by the 15th CGPM in 1975. The SI unit is similar to the traditional cgs unit, the rad (equivalent to 0.01 Gy), which remains common in industry in the United States, while "strongly discouraged" in the style guide for U.S. National Institute of Standards and Technology authors.
The gray was defined in 1975 in honour of Louis Harold Gray (1905–1965) who, in 1940, first proposed a similar concept, "that amount of neutron radiation which produces an increment of energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one röntgen of radiation".
The CIPM says that "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".
This SI unit is named after Louis Harold Gray. As with every International System of Units (SI) unit whose name is derived from the proper name of a person, the first letter of its symbol is upper case (Gy). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (gray), except in a situation where any word in that position 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. the SI unit description should read '...the first letter of its symbol is upper case (Gy) after the number or value or by itself, not in a sentence.'
The gray measures the absorbed energy of radiation, but the biological effects vary by the type and energy of the radiation and the organism and tissues involved. This is expressed by the sievert, which has the same dimensions as the gray, but is a measure of the potential for damage to human tissue. It is related to the gray by weighting factors which are fully described in the articles on equivalent dose and effective dose.
For X-rays and gamma rays the gray is numerically the same value when expressed as the sievert (Sv), but for alpha particles one gray is equivalent to twenty sieverts because of the radiation weighting factor that is applied. To avoid any risk of confusion between the absorbed dose (by matter) and the equivalent dose (by biological tissues), one must use the corresponding special units; gray is used instead of the joule per kilogram for absorbed dose and the sievert instead of the joule per kilogram for the equivalent dose.
A whole-body exposure to 5 or more gray of high-energy radiation at one time usually leads to death within 14 days. This dosage represents 375 joules for a 75 kg adult (equivalent to the chemical energy in 20 mg of sugar). Since gray are such large amounts of radiation, diagnostic medical use of radiation is typically measured in milligray (mGy).
As experienced from follow-up after radiation therapy, epilation may occur on any hair-bearing skin exposed to doses above 1 Gy. Hair loss may be permanent with a single dose of 10 Gy, but if the dose is fractionated permanent hair loss may not occur until dose exceeds 45 Gy. The salivary glands and tear glands have a radiation tolerance of about 30 Gy in 2 Gy fractions, a dose which is exceeded by most radical head and neck cancer treatments, potentially causing dryness. Dry mouth (xerostomia) and dry eyes (xerophthalmia) can become irritating long-term problems and severely reduce the patient's quality of life. Similarly, sweat glands in treated skin (such as the armpit) tend to stop working, and the naturally moist vaginal mucosa is often dry following pelvic irradiation.
In radiation therapy, the amount of radiation varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, while lymphomas are treated with 20 to 40 Gy. Preventive (adjuvant) doses are typically around 45–60 Gy in 1.8–2 Gy fractions (for breast, head, and neck cancers).
The average radiation dose from an abdominal X-ray is 0.7 mGy, that from an abdominal CT scan is 8.0 mGy, that from a pelvic CT scan is 6 mGy, and that from a selective CT scan of the abdomen and the pelvis is 14 mGy. 
The adoption of the gray by the 15th CGPM in 1975 as the unit of measure of the absorption of ionising radiation, specific energy absorption, and of kerma in 1975 was the culmination of over half a century of work, both in the understanding of the nature of ionising radiation and in the refinement of measuring techniques.
Wilhelm Röntgen first discovered X-rays on November 8, 1895 and within a few years they were being used to examine broken bones. One of the earliest techniques of measuring the intensity of X-rays was to measure their ionisation potential in air. Initially various countries developed their own standards, but in order to promote international cooperation, the First International Congress of Radiology (ICR) which met in London in 1925 proposed a separate body to consider units of measure. This body, the International Commission on Radiation Units and Measurements (ICRU), came into being at the Second ICR in Stockholm in 1928 under the chairmanship Manne Siegbahn and at their first meeting proposed that one unit X-ray dose should be defined as the quantity of X-rays that would produce one esu of charge in one cubic centimetre of dry air at 0 °C and a standard atmosphere. This unit was named the roentgen in honour of Röntgen who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to gamma radiation as well as X-rays. This technique, although appropriate for the technology of the day, had the disadvantage that it was not a direct measure of either the intensity of X-rays or of their absorption, but rather was a measurement of the effect of the X-rays in a specific circumstance.
In 1940, Gray, who had been studying the effect of neutron damage on human tissue, together with Mayneord and Read published a paper in which a unit of measure, dubbed the "gram roentgen" (symbol: gr) 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" was proposed. This unit was found to be equivalent to 88 ergs in air. In 1953 the ICRU recommended the rad, equal to 100 erg/g as the new unit of measure of absorbed radiation. The rad was expressed in coherent cgs units.
In the late 1950s the ICRU was invited by the CGPM to join other scientific bodies to work with the International Committee for Weights and Measures (CIPM) in the development of a system of units that could be used consistently over many disciplines. This body, initially known as the "Commission for the System of Units" (renamed in 1964 as the "Consultative Committee for Units") was responsible overseeing the development of the International System of Units (SI). At the same time it was becoming increasingly obvious that the definition of the roentgen was unsound and many calls were made for its redefinition. In 1962 it was redefined. The definition of the roentgen had the advantage over the gray of being simpler to measure, but the gray is independent of the primary ionising radiation
The CCU decided to define the SI unit of absorbed radiation in terms of energy per unit mass, which in MKS units was J/kg. This was confirmed in 1975 by 15th GCPM and the unit was named the "gray" in honour of Hal Gray who had died in 1965. The gray was exactly equal to 100 rad.
The following table shows radiation quantities in SI and non-SI units.
|Exposure (X)||röntgen||R||esu / 0.001293 g of air||1928||non-SI|
|Absorbed dose (D)||erg•g−1||1950||non-SI|
|Activity (A)||curie||c||3.7 × 1010 s−1||1953||non-SI|
|Dose equivalent (H)||röntgen equivalent man||rem||100 erg•g−1||1971||non-SI|
|Fluence (Φ)||(reciprocal area)||cm−2 or m−2||1962||SI (m-2)|
|lccn=value (help). Retrieved 2012-05-15.