- 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.