Roentgen

The röntgen (symbol R) is a unit of dosage of X-rays or gamma radiation, being the dose that liberates positive and negative charges of 2.58×10^-4 coulombs per kilogram of air. It is named after the physicist Wilhelm Röntgen.

Natural background is around 10 microroentgens per hour. 500 roentgens in 5 hours is lethal dose for human.