The roentgen or röntgen (; symbol R) is a legacy unit of measurement for the exposure of

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

s and

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

s, and is defined as the

electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...

freed by such

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

in a specified

volume Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch) ...

of air divided by the

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

of that air (

statcoulomb The statcoulomb (statC), franklin (Fr), or electrostatic unit of charge (esu) is the unit of measurement for electrical charge used in the centimetre–gram–second electrostatic units variant (CGS-ESU) and Gaussian systems of units. In term ...

per kilogram). In 1928, it was adopted as the first international measurement quantity for

ionizing radiation Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...

to be defined for

radiation protection Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposu ...

, as it was then the most easily replicated method of measuring air ionization by using ion chambers. It is named after the German physicist

Wilhelm Röntgen Wilhelm Conrad Röntgen (; 27 March 1845 – 10 February 1923), sometimes Transliteration, transliterated as Roentgen ( ), was a German physicist who produced and detected electromagnetic radiation in a wavelength range known as X-rays. As ...

, who discovered X-rays and was awarded the first Nobel Prize in Physics for the discovery. However, although this was a major step forward in standardising radiation measurement, the roentgen has the disadvantage that it is only a measure of air ionisation, and not a direct measure of radiation absorption in other materials, such as different forms of human tissue. For instance, one roentgen deposits of

absorbed dose Absorbed dose is a dose quantity which represents the specific energy (energy per unit mass) deposited by ionizing radiation in living matter. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protecti ...

in dry air, or in soft tissue. One roentgen of X-rays may deposit anywhere from in bone depending on the beam energy. As the science of radiation

dosimetry Radiation dosimetry in the fields of health physics and radiation protection is the measurement, calculation and assessment of the ionizing radiation dose absorbed by an object, usually the human body. This applies both internally, due to ingest ...

developed, it was realised that the ionising effect, and hence tissue damage, was linked to the energy absorbed, not just radiation exposure. Consequently new radiometric units for

were defined which took this into account. In 1953 the International Commission on Radiation Units and Measurements (ICRU) recommended the rad, equal to 100 erg/g, as the unit of measure of the new radiation quantity

. The rad was expressed in coherent cgs units. In 1975 the unit gray was named as the SI unit of absorbed dose. One gray is equal to 1 J/kg (i.e. 100 rad). Additionally, a new quantity,

kerma Kerma was the capital city of the Kerma culture, which was founded in present-day Sudan before 3500 BC. Kerma is one of the largest archaeological sites in ancient Nubia. It has produced decades of extensive excavations and research, including t ...

, was defined for air ionisation as the exposure for instrument calibration, and from this the

can be calculated using known coefficients for specific target materials. Today, for radiation protection, the modern units,

for energy absorption and the equivalent dose (

sievert The sievert (symbol: SvPlease note there are two non-SI units that use the same Sv abbreviation: the sverdrup and svedberg.) is a derived unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizin ...

) for stochastic effect, are overwhelmingly used, and the roentgen is rarely used. The

International Committee for Weights and Measures The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre C ...

(CIPM) has never accepted the use of the roentgen. The roentgen has been redefined over the years. It was last defined by the U.S.'s

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...

(NIST) in 1998 as , with a recommendation that the definition be given in every document where the roentgen is used.

History

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."Van Loon, R.; and Van Tiggelen, R.
''Radiation Dosimetry in Medical Exposure: A Short Historical Overview''
, 2004> Using 1 esu ≈ 3.33564 C and the air density of ~1.293 kg/m³ at 0 °C and 101 kPa, this converts to 2.58 × 10⁻⁴ C/kg, which is the modern value given by NIST. 1 × 3.33564 × 10⁻¹⁰ × 1,000,000 ÷ 1.293 = 2.58 × 10⁻⁴ 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 different definition which amounted to 0.444 German R.

ICR definitions

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 76 cm of mercury pressure such a degree of conductivity that 1 esu of charge is measured at saturation current." The stated cubic centimetre (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.

GOST definition

The

USSR The Union of Soviet Socialist Republics. (USSR), commonly known as the Soviet Union, was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 until Dissolution of the Soviet ...

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 ICR roentgen.

ICRP definition

The introduction of the roentgen measurement unit, which relied upon measuring the ionisation of air, replaced earlier less accurate practices that relied on timed exposure, film exposure, or fluorescence. This led the way to setting exposure limits, and the National Council on Radiation Protection and Measurements of the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

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 Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to revea ...

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.

European Union

In 1971 the

European Economic Community The European Economic Community (EEC) was a regional organisation created by the Treaty of Rome of 1957,Today the largely rewritten treaty continues in force as the ''Treaty on the functioning of the European Union'', as renamed by the Lisbo ...

, in Directive 71/354/EEC, catalogued the units of measure that could be used "for ... public health ... purposes". The directive included the

curie Curie may refer to: *Curie family, a family of distinguished scientists: :* Jacques Curie (1856–1941), French physicist, Pierre's brother :* Pierre Curie (1859–1906), French physicist and Nobel Prize winner, Marie's husband :* Marie Curi ...

, 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 As of 2009, the European Union had issued two units of measurement directives. In 1971, it issued Directive 71/354/EEC, which required EU member states to standardise on the International System of Units (SI) rather than use a variety of CGS ...

, published in December 1979, which replaced directive 71/354/EEC, explicitly catalogued the gray,

becquerel The becquerel (; symbol: Bq) is the unit of radioactivity in the International System of Units (SI). One becquerel is defined as an activity of one per second, on average, for aperiodic activity events referred to a radionuclide. For applicatio ...

, and

for this purpose and required that the curie, rad, rem and roentgen be phased out by 31 December 1985.

NIST definition

Today the roentgen is rarely used, and the

(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

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.

Development of replacement radiometric quantities

Although a convenient quantity to measure with an air ion chamber, the roentgen had the disadvantage that it was not a direct measure of either the intensity of X-rays or their absorption, but rather was a measurement of the ionising effect of X-rays in a specific circumstance; which was dry air at 0 °C and 1 standard atmosphere of pressure. Because of this the roentgen had a variable relationship to the amount of energy absorbed dose per unit mass in the target material, as different materials have different absorption characteristics. As the science of radiation dosimetry developed, this was seen as a serious shortcoming. In 1940,

Louis Harold Gray Louis Harold Gray FRS (10 November 1905 – 9 July 1965) was an English physicist who worked mainly on the effects of radiation on biological systems. He was one of the earliest contributors of the field of radiobiology. Amongst many other ach ...

, who had been studying the effect of neutron damage on human tissue, together with William Valentine Mayneord and the radiobiologist John Read, published a paper in which a unit of measure, dubbed the "''gram roentgen''" (symbol: gr) defined as "that amount of neutron radiation which produces an increment in energy in unit volume of tissue equal to the increment of energy produced in unit volume of water by one roentgen of radiation" was proposed. This unit was found to be equivalent to 88 ergs in air. In 1953 the ICRU recommended the rad, equal to 100 erg/g, as the new unit of measure of absorbed radiation. The rad was expressed in coherent cgs units. In the late 1950s the

General Conference on Weights and Measures The General Conference on Weights and Measures (abbreviated CGPM from the ) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established in 1875 under the terms of the Metre C ...

(CGPM) invited the ICRU to join other scientific bodies to work with the

(CIPM) in the development of a system of units that could be used consistently over many disciplines. This body, initially known as the "Commission for the System of Units", renamed in 1964 as the "Consultative Committee for Units" (CCU), was responsible for overseeing the development of the

International System of Units The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...

(SI). At the same time it was becoming increasingly obvious that the definition of the roentgen was unsound, and in 1962 it was redefined. The CCU decided to define the SI unit of absorbed radiation in terms of energy per unit mass, which in MKS units was J/kg. This was confirmed in 1975 by the 15th CGPM, and the unit was named the "gray" in honour of Louis Harold Gray, who had died in 1965. The gray was equal to 100 rad. The definition of the roentgen had had the attraction of being relatively simple to define for photons in air, but the gray is independent of the primary ionizing radiation type, and can be used for both kerma and absorbed dose in a wide range of matter. When measuring

in a human due to external exposure, the SI unit the gray, or the related non-SI rad are used. From these can be developed the dose equivalents to consider biological effects from differing radiation types and target materials. These are equivalent dose, and effective dose for which the SI unit

or the non-SI rem are used.

Radiation-related quantities

The following table shows radiation quantities in SI and non-SI units:

References

External links

NIST: Units outside the SI
– Health Physics Society {{DEFAULTSORT:Roentgen Units of radiation dose Non-SI metric units