Gray (unit)

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

Contents 1 Definition 2 SI multiples 3 Origin 4 Explanation 5 Conversions 6 References

[edit] 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.

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.

[edit] SI multiples

Multiple	Name	Symbol		Multiple	Name	Symbol
100	gray	Gy
101	decagray	daGy		10–1	decigray	dGy
102	hectogray	hGy		10–2	centigray	cGy
103	kilogray	kGy		10–3	milligray	mGy
106	megagray	MGy		10–6	microgray	µGy
109	gigagray	GGy		10–9	nanogray	nGy
1012	teragray	TGy		10–12	picogray	pGy
1015	petagray	PGy		10–15	femtogray	fGy
1018	exagray	EGy		10–18	attogray	aGy
1021	zettagray	ZGy		10–21	zeptogray	zGy
1024	yottagray	YGy		10–24	yoctogray	yGy

[edit] 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.

[edit] 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. A whole-body dose of approximately 10-20 grays, delivered at one time, can be fatal to humans^{[citation needed][1]}. This dosage represents 750-1500 joules for a 75kg adult. This small amount of energy is equivalent to the food energy in 2 to 4 grams of sugar.

[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.

[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