The gray (symbolized Gy) is the standard unit of absorbed ionizing-radiation dose, equivalent to one joule per kilogram (1 J · kg -1 ). Reduced to base units in the International System of Units ( SI ), 1 Gy is equivalent to one meter squared per second squared (1 m 2 · s -2 ).
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In general, ionizing-radiation dose is the amount of X-ray, gamma-ray, or high-speed-particle energy absorbed per unit mass . The gray supersedes the rad , an older unit of radiation dose. In some documents the rad is still used; the gray is 100 times larger. To convert rads to grays, multiply by 0.01. To convert grays to rads, multiply by 100.