The gray (symbol Gy) is the SI unit of absorbed dose. It is defined as the absorption of one joule of energy originating from ionising radiation per kilogram of matter 1,2, i.e. Gy = 1 J/kg.
Terminology
One gray is a large unit and is usually used with a prefix, e.g. milligray (mGy), microgray (μGy).
As per all other eponymous SI units, the unit is not capitalised when written out in full, but it is capitalised when shortened to its symbol.
History and etymology
In July 1974, the International Commission on Radiation Units and Measurements (ICRU) recommended that a specific name, the gray, be adopted for the SI unit 'joule per kilogram' when used for absorbed radiation dose. This recommendation was expedited and passed by the International Committee of Weights and Measures (CIPM) at the 15th General Conference of Weights and Measures (CPGM) in May-June 1975 3.
The unit is named for the British physicist Louis Harold Gray (1905-1965) 2, who made a major contribution to radiation dosimetry with the Bragg-Gray principle 3.
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