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# Gray (unit)

The gray (symbol: Gy) is the SI unit of absorbed dose.

## Definition

SI multiples for gray (Gy)
Submultiples Multiples
Value Symbol Name Value Symbol Name
10–1 Gy dGy decigray 101 Gy daGy decagray
10–2 Gy cGy centigray 102 Gy hGy hectogray
10–3 Gy mGy milligray 103 Gy kGy kilogray
10–6 Gy µGy microgray 106 Gy MGy megagray
10–9 Gy nGy nanogray 109 Gy GGy gigagray
10–12 Gy pGy picogray 1012 Gy TGy teragray
10–15 Gy fGy femtogray 1015 Gy PGy petagray
10–18 Gy aGy attogray 1018 Gy EGy exagray
10–21 Gy zGy zeptogray 1021 Gy ZGy zettagray
10–24 Gy yGy yoctogray 1024 Gy YGy yottagray
Common multples are in bold face.

One gray is the absorption of one joule of radiation energy by one kilogram of matter.

$1 \ \mathrm{Gy} = 1\ \frac{\mathrm{J}}{\mathrm{kg}} = 1\ \mathrm{m}^2\cdot\mathrm{s}^{-2}$

Note that, for x and gamma rays, these are the same units as the sievert (Sv). 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.

 This SI unit is named after Louis Harold Gray. As with 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.

## Origin

The gray was defined in 1975 in honour 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 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 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 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.

## 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