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from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A unit expressing the rate of decay of radioactive material, equal to one million disintegrations per second.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. An obsolete unit of radioactivity (symbol: rd), defined as the activity of a quantity of radioactive material in which one million nuclei decay per second.


After Ernest Rutherford.
(American Heritage® Dictionary of the English Language, Fourth Edition)
After Ernest Rutherford. (Wiktionary)



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