Definition
One gray is the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter.
For x-rays and gamma rays, these are the same units as the sievert (Sv). For alpha particles one gray is twenty sievert. 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 dose equivalent. The word "gray" is both the singular and plural spelling.
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".
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.
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