Gray
Contents
Definition
The gray (symbol: Gy) is the unit of absorbed dose in the International System of Units (SI). One gray is the absorption of one joule of radiation energy by one kilogram of matter.
The gray has the same units as the sievert. To avoid confusion between the absorbed dose and the equivalent dose, one should 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 unit gray can be used for any type of radiation, but it does not describe the biological effects of the different radiations. Absorbed dose is often expressed in terms of hundredths of a gray, or centi-grays. One gray is equivalent to 100 rads. 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. 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.
| Biological Effects of Acute, Total Body Irradiation | |
|---|---|
| Amount of Exposure | Effect |
| * 50 mGy (5 rads) | No detectable injury or symptoms |
| * 1 Gy (100 rads) | May cause nausea and vomiting for 1-2 days and temporary drop in production of new blood cells |
| * 3.5 Gy (350 rads) | Nausea and vomiting initially, followed by a period of apparent wellness. At 3-4 weeks, there is a potential for deficiency of white blood cells and platelets. Medical care is required. |
| * Higher levels of exposure can be fatal. Medical care is required. | |
|
Source: Radiation Emergency Assistance Center/Training Site, Oak Ridge Institute for Science and Education, Oak Ridge National Laboratory. | |
History
The unit is named for the British physician L. Harold Gray (1905-1965), an authority on the use of radiation in the treatment of cancer. Gray 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.
Further reading
- Radiation Related Terms, Idaho State University, Radiation Information Network
- How Many? A Dictionary of Units of Measurement, Russ Rowlett, Center for Mathematics and Science Education, University of North Carolina at Chapel Hill
- SI base units and SI derived units, the Physics Laboratory, National Institute of Standards and Technology
