Not to be confused with roentgen equivalent man or roentgen equivalent physical

The roentgen (R, also röntgen) is a legacy unit of measurement for the exposure of X-rays and gamma rays up to several megaelectronvolts. It is a measure of the ionization produced in air by X-rays or gamma radiation and it is used because air ionization can be measured directly. It is named after the German physicist Wilhelm Röntgen, 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^-4 C/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 (kinetic energy released per unit mass) deposits of absorbed dose in dry air, or in soft tissue. One roentgen (air kerma) of X-rays may deposit anywhere from 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.