The roentgen or röntgen is a legacy unit of measurement for the exposure of X-rays and gamma rays, and is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. Wikipedia
1 R in: is equal to
Named after: Wilhelm Röntgen
SI base units: 2.58×10−4 A⋅s/kg
Symbol: R
Unit system: Legacy unit