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# Gray (unit)

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

## Definition

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

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

Note that, for X-rays 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.

## 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 exposure to 5 or more grays of high-energy radiation at one time usually leads to death within 14 days (see Radiation poisoning for details). This dosage represents 375 joules for a 75kg adult (equivalent to the chemical energy in 20 mg 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.

## 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

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