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How is the rad related to the gray?

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The rad and the gray are both units used to measure the absorbed dose of ionizing radiation.12

The gray (Gy) is the standard unit within the International System of Units (SI), defined as the absorption of one joule of energy per kilogram of matter. The rad, which stands for "radiation absorbed dose," was defined as the amount of radiation that releases 100 ergs of energy per gram of matter.21

In 1975, the gray replaced the rad as the official unit because the rad was not coherent with the SI system. The two units are related by the following conversion:12

1 gray=100 rads

While the gray is the internationally recognized standard, the rad remains in use specifically within the United States.1