The gray (symbol: Gy) is the SI unit of absorbed dose. Look up si, Si, SI in Wiktionary, the free dictionary. ... Absorbed dose is a measure of the energy deposited in a medium by ionising radiation. ...

Definition

One gray is the absorption of one joule of radiation energy by one kilogram of matter. The joule (IPA pronunciation: or ) (symbol: J) is the SI unit of energy. ... The U.S. National Prototype Kilogram, which currently serves as the primary standard for measuring mass in the U.S. It was assigned to the United States in 1889 and is periodically recertified and traceable to the primary international standard, The Kilogram, held at the Bureau International des Poids et... This article or section does not cite any references or sources. ...

Note that, for x and gamma rays, 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. The sievert (symbol: Sv) is the SI derived unit of dose equivalent. ... Absorbed dose is a measure of the energy deposited in a medium by ionising radiation. ... The equivalent dose is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effect of different types of radiation. ...

This SI unit is named after Louis Harold Gray. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (Gy). But when an SI unit is spelled out, it should always be written in lowercase (gray), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

Image File history File links SI_Brochure_Cover. ... Look up si, Si, SI in Wiktionary, the free dictionary. ... Louis Harold Gray (10 November 1905 - 9 July 1965) was a British physicist who worked mainly on the effects of radiation on biological systems, inventing the field of radiobiology as he went. ... Majuscules or capital letters (in the Roman alphabet: A, B, C, ...) are one type of case in a writing system. ... Minuscule, or lower case, is the smaller form (case) of letters (in the Roman alphabet: a, b, c, ...). Originally alphabets were written entirely in majuscule (capital) letters which were spaced between well-defined upper and lower bounds. ... Celsius is, or relates to, the Celsius temperature scale (previously known as the centigrade scale). ...

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. Louis Harold Gray (10 November 1905 - 9 July 1965) was a British physicist who worked mainly on the effects of radiation on biological systems, inventing the field of radiobiology as he went. ... The röntgen or roentgen (symbol R) is a unit of exposure to ionizing radiation (X or gamma rays), and is named after the German physicist Wilhelm Röntgen. ...

Explanation

The gray measures the deposited energy of radiation. The biological effects vary by the type and energy of the radiation and the organism and tissues involved. The sievert attempts to account for these variations. A whole-body dose of 10-20 grays of high-energy radiation, delivered at one time, can be fatal to humans^[1]. This dosage represents 750-1500 joules for a 75kg adult (equivalent to the chemical energy in a few grams of sugar). Since grays are such large amounts of radiation, medical use of radiation is typically measured in milligrays (mGy). The sievert (symbol: Sv) is the SI derived unit of dose equivalent. ...

The average radiation dose from an abdominal x-ray is 1.4 mGy, that from an abdominal CT scan is 8.0 mGy, that from a pelvic CT scan is 25 mGy, and that from a selective spiral CT scan of the abdomen and the pelvis is 30 mGy.^[2]

Conversions

One gray is equivalent to 100 rad. The rad is a unit of radiation dose, with symbol rad. ...

The röntgen is defined as the radiation exposure equal to the quantity of ionizing 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. The röntgen or roentgen (symbol R) is a unit of exposure to ionizing radiation (X or gamma rays), and is named after the German physicist Wilhelm Röntgen. ... The statcoulomb (statC) or franklin (Fr) or electrostatic unit of charge (esu) is the physical unit for electrical charge used in the centimetre-gram-second (cgs) electrostatic system of units. ... A cubic centimetre (cm3) is an SI derived unit of volume, equal to the volume of a cube with side length of 1 centi metre. ... The degree Celsius (°C) is a unit of temperature named after the Swedish astronomer Anders Celsius (1701–1744), who first proposed a similar system in 1742. ... Atmospheric pressure is the pressure caused by the weight of air above any area in the Earths atmosphere. ...

References

1. ^ Fred Solomon and Robert Q. Marston, Editors, The Medical Implications of Nuclear War (1986), National Academies Press, p. 235-236, http://www.nap.edu/catalog/940.html
2. ^ Parungo C, The Pregnant Surgical Patient, ACS Surgery, http://www.acssurgery.com