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DEFINITION One ''gray'' is the absorption of one Joule of radiation Energy by one Kilogram of Matter . :1 Gy = 1 J/kg = 1 m2·s–2 Note that these are the same units as the Sievert . To avoid any risk of confusion between the Absorbed Dose and the Equivalent Dose , one must use the corresponding special units, namely the ''gray'' instead of the joule per kilogram for absorbed dose and the ''sievert'' instead of the joule per kilogram for the dose equivalent. SI MULTIPLES ORIGIN The gray was defined in 1975 in honor of Louis Harold Gray (1905-1965), who used 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,” in 1940. EXPLANATION The gray measures the physical effects of radiation. The biological effects can vary by the type and energy of the radiation and the organism and tissues involved. The separate unit Sievert attempts to account for these variations. Typically, a whole-body dose of approximately 10-20 grays is fatal to humans, which equates to around 1000 joules for an adult. This is a tiny amount of energy, equivalent to the food energy in 1/4 of a gram of sugar. CONVERSIONS One gray is equivalent to 100 Rad . The Röntgen is defined as the radiation exposure equal to the quantity of ionising radiation that will produce one Esu of electricity in one Cubic Centimetre of dry air at 0 °C and a Standard Atmosphere , and is conventionally taken to be worth 0.258 mC/kg (using a conventional air density of about 1.293 kg/m³). Using an air ionisation energy of about 36.161 J/C, we have 1 Gy ≈ 107.185 R. |
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