Röntgen (unit)

Roentgen
Display of quartz fiber dosimeter, in units of roentgen.
Unit information
Unit system	Legacy unit
Unit of	Exposure to ionizing radiation
Symbol	R
Unit conversions
1 R in ...	... is equal to ...
SI base units	2.58×10−4A⋅s/kg

The roentgen or röntgen (/ˈrɛntɡən, -dʒən/) (symbol R) 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 was the first international measurement quantity of ionising radiation to be defined for radiation protection as it was a convenient method of measuring air ionization 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 2.58×10⁻⁴C/kg, (i.e. 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 0.00877 grays (0.877 rads) of absorbed dose in dry air, or 0.0096 Gy (0.96 rad) in soft tissue. One roentgen (air kerma) of X-rays may deposit anywhere from 0.01 to 0.04 Gy (1.0 to 4.0 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.