Gray (SI unit)
The gray (symbol Gy) is the SI unit of absorbed dose and is defined as the absorption of one joule of energy, in the form of ionising radiation, per kilogram of matter, i.e. one gray = 1 J/kg 2.
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 when the unit is written out in full it is not capitalised, but when shortened to its symbol it is capitalised.
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 2.
The unit is named for the British physicist Louis Harold Gray (1905-1965) 1, who made a major contribution to the field of radiation dosimetry with the Bragg-Gray principle 2.
- 1. Sekiya M, Yamasaki M. Louis Harold Gray (November 10, 1905-July 9, 1965): a pioneer in radiobiology. (2017) Radiological physics and technology. 10 (1): 2-7. doi:10.1007/s12194-016-0379-9 - Pubmed
- 2. Wyckoff HO, Allisy A, Lidén K. The new special names of SI units in the field of ionizing radiations. (1976) Radiology. 118 (1): 233-4. doi:10.1148/118.1.233 - Pubmed
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