Radiation unit

Several different internationally agreed units (called SI units) are used to measure ionizing radiation. For example, the roentgen (R) measures the amount of radiation in the air, and the becquerel is the SI unit of spontaneous activity of a radioactive source such as uranium. For medical purposes, the most commonly used units are the gray (Gy) and the sievert (Sv). The gray is the SI unit of radiation that is actually absorbed by any tissue or substance as a result of exposure to radiation. 1 Gy is the absorption of 1 joule of energy (from gamma radiation or X-rays) per kilogram of irradiated matter. The gray supersedes an older unit called the rad (1 Gy = 100 rads). Because some types of radiation affect biological organisms more than others, the sievert is used as a measure of the impact of an absorbed dose. It uses additional factors, such as the kind of radiation and its energy, to quantify the effects on the body of equivalent amounts of different types of absorbed energy. The sievert replaces an older unit, the rem (1 Sv=100 rems).


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