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1896, in Roentgen rays "X-rays," in recognition of German physicist Wilhem Conrad Röntgen (1845-1923), who discovered X-rays in 1895. As a unit of exposure to radiation, it is attested from 1922, proposed in French in 1921.
roentgen roent·gen or rönt·gen (rěnt'gən, -jən, rŭnt'-)
Abbr. R, r
A unit of radiation exposure that is equal to the quantity of ionizing radiation that will produce one electrostatic unit of electricity in one cubic centimeter of dry air at 0°C and standard atmospheric pressure.
Roentgen Roent·gen (rěnt'gən, -jən, rŭnt'-) or Rönt·gen (rɶnt'gən), Wilhelm Konrad. 1845-1923.
German physicist who discovered x-rays and developed x-ray photography, revolutionizing medical diagnosis. He won a 1901 Nobel Prize.