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The roentgen (R, also röntgen) is a legacy unit of measurement for the exposure of X-rays and gamma rays up to 3 MeV. 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 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.
The roentgen has its roots in the Villard unit defined in 1908 by the American Roentgen Ray Society as "the quantity of radiation which liberates by ionisation one esu of electricity per cm3 of air under normal conditions of temperature and pressure." Using 1 esu ≈ 3.33564×10−10 C and the air density of ~1.293 kg/m³ at 0°C and 101kPa, this converts to 2.58×10−4 C/kg, which is the modern value given by NIST.
1 esu/ × 3.33564×10−10 C/ × 1,000,000 cm3/ ÷ 1.293 kg/ = 2.58×10−4 C/
This definition was used under different names (e, R, and German unit of radiation) for the next 20 years. In the meantime, the French Roentgen was given a completely different definition which amounted to 0.444 German R.
In 1928, the International Congress of Radiology (ICR) defined the roentgen as "the quantity of X-radiation which, when the secondary electrons are fully utilised and the wall effect of the chamber is avoided, produce in 1 cc of atmospheric air at 0°C and 76cm of mercury pressure such a degree of conductivity that 1 esu of charge is measured at saturation current."  The stated 1 cc of air would have a mass of 1.293 mg at the conditions given, so in 1937 the ICR rewrote this definition in terms of this mass of air instead of volume, temperature and pressure. The 1937 definition was also extended to gamma rays, but later capped at 3 MeV in 1950.
The USSR all-union committee of standards (GOST) had meanwhile adopted a significantly different definition of the roentgen in 1934. GOST standard 7623 defined it as "the physical dose of X-rays which produces charges each of one electrostatic unit in magnitude per cm3 of irradiated volume in air at 0°C and normal atmospheric pressure when ionization is complete." The distinction of physical dose from dose caused confusion, some of which may have led Cantrill and Parker report that the roentgen had become shorthand for 83 ergs per gram (0.0083 Gy) of tissue. They named this derivative quantity the roentgen equivalent physical (rep) to distinguish it from the IRC roentgen.
Towards the middle of the 20th century, roentgens were used for the purpose of radiation protection. This replaced earlier practices that relied on time, film exposure, or fluorescence. The National Council on Radiation Protection established the first formal dose limit in 1931 as 0.1 roentgen per day. The International X-ray and Radium Protection Committee, now known as the International Commission on Radiological Protection (ICRP) soon followed with a limit of 0.2 roentgen per day in 1934. In 1950, the ICRP reduced their recommended limit to 0.3 roentgen per week for whole-body exposure. The International Commission on Radiation Units and Measurements (ICRU) took over the definition of the roentgen in 1950, defining it as "the quantity of X or γ-radiation such that the associated corpuscular emission per 0.001293 gram of air produces, in air, ions carrying 1 electrostatic unit of quantity of electricity of either sign." The 3 MeV cap was no longer part of the definition, but the degraded usefulness of this unit at high beam energies was mentioned in the accompanying text. In the meantime, the new concept of roentgen equivalent man (rem) had been developed. Starting in 1957, the ICRP began to publish their recommendations in terms of rem, and the roentgen fell into disuse. The medical imaging community still has a need for ionization measurements, but they gradually converted to using C/kg as legacy equipment was replaced. The ICRU recommended redefining the roentgen to be exactly 2.58×10−4 C/kg in 1971.
In 1971 the European Economic Community, in Directive 71/354/EEC, catalogued the units of measure that could be used "for ... public health ... purposes". The directive included the curie, rad, rem and roentgen as permissible units, but required that the use of the rad, rem and roentgen be reviewed before 31 December 1977. This document defined the roentgen as exactly 2.58×10−4 C/kg, as per the ICRU recommendation. Directive 80/181/EEC, published in December 1979, which replaced directive 71/354/EEC, explicitly catalogued the gray, becquerel and sievert for this purpose and required that the curie, rad, rem and roentgen be phased out by 31 December 1985.
Today the roentgen is rarely used, and the International Committee for Weights and Measures (CIPM) never accepted the use of the roentgen. From 1977 to 1998, the US NIST's translations of the SI brochure stated that the CIPM temporarily accepted the use of the roentgen (and other radiology units) with SI units since 1969. However, the only related CIPM decision shown in the appendix are with regards to the curie in 1964. The NIST brochures defined the roentgen as 2.58×10−4 C/kg, to be employed with exposures of x or γ radiation, but did not state the medium to be ionized. The CIPM's current SI brochure excludes the roentgen from the tables of non-SI units accepted for use with the SI. The US NIST clarified in 1998 that it was providing its own interpretations of the SI system, whereby it accepted the roentgen for use in the US with the SI, while recognizing that the CIPM did not. By then, the limitation to x and γ radiation had been dropped. NIST recommends defining the roentgen in every document where this unit is used. The continued use of the roentgen is strongly discouraged by the NIST.
An exposure of 500 roentgens in five hours is usually lethal for human beings.
The typical exposure to normal background radiation for a human being is about 200 milliroentgens per year, or about 23 microroentgens per hour.
When measuring dose absorbed in man due to exposure, units of absorbed dose are used (the related rad or SI gray), or, with consideration of biological effects from differing radiation types, units of equivalent dose, such as the related rem or the SI sievert.