Not to be confused with roentgen equivalent man or roentgen equivalent
physical The roentgen 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 in 1998 as 2.58×10−4 C/kg, with a recommendation that the
definition be given in every document where the roentgen is used. One roentgen
of air kerma deposits 0.00877 grays of absorbed dose in dry air, or 0.0096 Gy
in soft tissue. One roentgen of X-rays may deposit anywhere from 0.01 to 0.04
Gy 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. 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." Using 1 esu ≈ 3.33564×10−10 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 esu/cm3 × 3.33564 × 10−10 C/esu ×
1,000,000 cm3/m3 ÷ 1.293 kg/m3 = 2.58 × 10−4 C/kg
This definition was used under different names 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 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 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 all-union committee of standards 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 of tissue. They named this derivative quantity the roentgen
equivalent physical to distinguish it from the IRC roentgen.
= ICRP definition= 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 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 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 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. = European Union=
In 1971 the European Economic Community, in Directive 71EEC, 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 80EEC, published in December 1979, which
replaced directive 71EEC, 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. = NIST definition=
Today the roentgen is rarely used, and the International Committee for Weights
and Measures 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 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. Significance
As roentgens describe radiation, their relation to absorbed dose is not
straightforward and depends on different absorption of radiated particles. Though
as a rule of thumb: 1 roentgen is approximately 10mSv.
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 a human
due to exposure, units of absorbed dose are used, or, with consideration of
biological effects from differing radiation types, units of equivalent
dose, such as the related rem or the SI sievert.
Radiation-related quantities The following table shows radiation
quantities in SI and non-SI units. See also
Wilhelm Conrad Röntgen Rad—c.g.s. unit of absorbed dose
Gray—SI unit of absorbed dose Roentgen equivalent man, or rem, a unit
of radiation dose equivalent Sievert—The sievert is the SI derived
unit of dose equivalent. Orders of magnitude
References External links
NIST: Units outside the SI Health Physics Society information page
on radiation dose units