High Quality Content by WIKIPEDIA articles! The roentgen (R, also rontgen) is a legacy unit of measurement for the kerma of X-rays and gamma rays up to 3 MeV. The unit is still used in the United States Navy nuclear propulsion program. It is named after the German physicist Wilhelm Rontgen, the man who discovered X-rays. Originating in 1908, this unit has been redefined and renamed over the years. It was last defined by the US National Institute of Standards and Technology (NIST) in 1998 as 2.58?10?4C/kg, (1 C/kg = 3876 R,) with a recommendation that the definition be given in every document where the roentgen is used. One roentgen of air kerma deposits 0.00877 gray (0.877 rad) of absorbed dose in dry air, or 0.0096 gray (0.96 rad) in soft tissue. One roentgen (air kerma) of X-rays may deposit anywhere from 0.01 to more than 0.04 gray (1 to 4 rad) in bone depending on the beam energy. This tissue-dependent conversion from kerma to absorbed dose is called the F-factor in radiotherapy contexts. The conversion depends on the ionizing energy of a reference medium, which is ambiguous in the latest NIST definition. Even where the reference medium is fully defined, the ionizing energy of the calibration and target mediums are often not precisely known.