Roentgen (unit)

legacy unit of measurement for the kerma of X-rays and gamma rays up to 3 MeV; equals 0.258 mC/kg

The roentgen or röntgen is a unit of measurement for the exposure of X-rays and gamma rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays.