Gray (unit)

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

Contents 1 Definition 2 Origin 3 Explanation 4 Conversions 5 References

[edit] Definition

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

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 every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase (Gy). When an SI unit is spelled out in English, it should always begin with a lowercase letter (gray), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. — Based on The International System of Units, section 5.2.

[edit] 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.

[edit] 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]

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[edit] 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.

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 multiples are in bold face.

[edit] 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