The gray (symbol: Gy) is the unit of
ionizing radiation dose in the
International System of Units (SI), defined as the absorption of one
joule
The joule ( , ; symbol: J) is the unit of energy in the International System of Units (SI). It is equal to the amount of work done when a force of 1 newton displaces a mass through a distance of 1 metre in the direction of the force applie ...
of radiation
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
per
kilogram of
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
.
It is used as a unit of the radiation quantity
absorbed dose that measures the energy deposited by
ionizing radiation in a unit mass of matter being irradiated, and is used for measuring the delivered dose in
radiotherapy,
food irradiation and
radiation sterilization. It is important in predicting likely acute health effects, such as
acute radiation syndrome and is used to calculate
equivalent dose
Equivalent dose is a dose quantity '' H '' representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the p ...
using the
sievert
The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing rad ...
, which is a measure of the stochastic health effect on the human body.
The gray is also used in radiation metrology as a unit of the radiation quantity
kerma
Kerma was the capital city of the Kerma culture, which was located in present-day Sudan at least 5,500 years ago. Kerma is one of the largest archaeological sites in ancient Nubia. It has produced decades of extensive excavations and research, i ...
; defined as the sum of the initial
kinetic energies of all the
charged particle
In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary pa ...
s liberated by uncharged
ionizing radiation in a sample of
matter
In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
per unit mass. The gray is an important unit in ionising radiation measurement and was named after British physicist
Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue.
The gray was adopted as part of the International System of Units in 1975. The corresponding
cgs unit to the gray is the
rad (equivalent to 0.01 Gy), which remains common largely in the United States, though "strongly discouraged" in the style guide for U.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 physical s ...
.
Applications
![Dose quantities and units](https://upload.wikimedia.org/wikipedia/commons/9/96/Dose_quantities_and_units.png)
The gray has a number of fields of application in measuring dose:
Radiobiology
The measurement of absorbed dose in tissue is of fundamental importance in
radiobiology
Radiobiology (also known as radiation biology, and uncommonly as actinobiology) is a field of clinical and basic medical sciences that involves the study of the action of ionizing radiation on living things, especially health effects of radiation. ...
and
radiation therapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radi ...
as it is the measure of the amount of energy the incident radiation deposits in the target tissue. The measurement of absorbed dose is a complex problem due to scattering and absorption, and many specialist dosimeters are available for these measurements, and can cover applications in 1-D, 2-D and 3-D.
In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid
epithelial
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...
tumor ranges from 60 to 80 Gy, while
lymphoma
Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). In current usage the name usually refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enla ...
s are treated with 20 to 40 Gy. Preventive (adjuvant) doses are typically around 45–60 Gy in 1.8–2 Gy
fractions
A fraction (from la, fractus, "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight ...
(for breast, head, and neck cancers).
The average radiation dose from an abdominal X-ray is 0.7 millisieverts (0.0007 Sv), that from an abdominal
CT scan is 8 mSv, that from a pelvic CT scan is 6 mGy, and that from a selective CT scan of the abdomen and the pelvis is 14 mGy.
Radiation protection
![SI Radiation dose units](https://upload.wikimedia.org/wikipedia/commons/0/03/SI_Radiation_dose_units.png)
The absorbed dose also plays an important role in
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". Expos ...
, as it is the starting point for calculating the stochastic health risk of low levels of radiation, which is defined as the ''probability'' of cancer induction and genetic damage. The gray measures the total absorbed energy of radiation, but the probability of stochastic damage also depends on the type and energy of the radiation and the types of tissues involved. This probability is related to the
equivalent dose
Equivalent dose is a dose quantity '' H '' representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the p ...
in
sievert
The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing rad ...
s (Sv), which has the same dimensions as the gray. It is related to the gray by weighting factors described in the articles on
equivalent dose
Equivalent dose is a dose quantity '' H '' representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the p ...
and
effective dose.
The
International Committee for Weights and Measures
The General Conference on Weights and Measures (GCWM; french: Conférence générale des poids et mesures, CGPM) is the supreme authority of the International Bureau of Weights and Measures (BIPM), the intergovernmental organization established i ...
states: "In order to avoid any risk of confusion between the absorbed dose ''D'' and the
dose equivalent
Equivalent dose is a dose quantity '' H '' representing the stochastic health effects of low levels of ionizing radiation on the human body which represents the probability of radiation-induced cancer and genetic damage. It is derived from the ...
''H'', the special names for the respective units should be used, that is, the name gray should be used instead of joules per kilogram for the unit of absorbed dose ''D'' and the name
sievert
The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing rad ...
instead of joules per kilogram for the unit of dose equivalent ''H''."
:
The accompanying diagrams show how absorbed dose (in grays) is first obtained by computational techniques, and from this value the equivalent doses are derived. For X-rays and gamma rays the gray is numerically the same value when expressed in sieverts, but for
alpha particles
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...
one gray is equivalent to 20 sieverts, and a radiation weighting factor is applied accordingly.
Radiation poisoning
Radiation poisoning: The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation. These are effects that are ''certain'' to happen, as opposed to the uncertain effects of low levels of radiation that have a ''probability'' of causing damage. A whole-body acute exposure to 5 grays or more of high-energy radiation usually leads to death within 14 days.
LD1 is 2.5 Gy, LD
50 is 5 Gy and LD
99 is 8 Gy. The LD
50 dose represents 375 joules for a 75 kg adult.
Absorbed dose in matter
The gray is used to measure absorbed dose rates in non-tissue materials for processes such as
radiation hardening
Radiation hardening is the process of making electronic components and circuits resistant to damage or malfunction caused by high levels of ionizing radiation ( particle radiation and high-energy electromagnetic radiation), especially for environ ...
,
food irradiation and
electron irradiation
Electron-beam processing or electron irradiation (EBI) is a process that involves using electrons, usually of high energy, to treat an object for a variety of purposes. This may take place under elevated temperatures and nitrogen atmosphere. Poss ...
. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes.
Kerma
Kerma ("kinetic energy released per unit mass") is used in radiation metrology as a measure of the liberated energy of ionisation due to irradiation, and is expressed in grays. Importantly, kerma dose is different from absorbed dose, depending on the radiation energies involved, partially because ionization energy is not accounted for. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, because some energy escapes from the absorbing volume in the form of
bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
(X-rays) or fast-moving electrons.
Kerma, when applied to air, is equivalent to the legacy
roentgen unit of radiation exposure, but there is a difference in the definition of these two units. The gray is defined independently of any target material, however, the roentgen was defined specifically by the ionisation effect in dry air, which did not necessarily represent the effect on other media.
Development of the absorbed dose concept and the gray
Wilhelm Röntgen
Wilhelm Conrad Röntgen (; ; 27 March 184510 February 1923) was a German mechanical engineer and physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range known as X-rays or Röntgen rays, an achie ...
first discovered
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
s on November 8, 1895, and their use spread very quickly for medical diagnostics, particularly broken bones and embedded foreign objects where they were a revolutionary improvement over previous techniques.
Due to the wide use of X-rays and the growing realisation of the dangers of ionizing radiation, measurement standards became necessary for radiation intensity and various countries developed their own, but using differing definitions and methods. Eventually, in order to promote international standardisation, the first International Congress of Radiology (ICR) meeting in London in 1925, proposed a separate body to consider units of measure. This was called the
International Commission on Radiation Units and Measurements The International Commission on Radiation Units and Measurements (ICRU) is a standardization body set up in 1925 by the International Congress of Radiology, originally as the X-Ray Unit Committee until 1950. Its objective "is to develop concepts, de ...
, or ICRU, and came into being at the Second ICR in Stockholm in 1928, under the chairmanship of
Manne Siegbahn
Karl Manne Georg Siegbahn FRS(For) HFRSE (3 December 1886 – 26 September 1978) was a Swedish physicist who was awarded the Nobel Prize in Physics in 1924 "for his discoveries and research in the field of X-ray spectroscopy".
Biography
Siegbahn ...
.
One of the earliest techniques of measuring the intensity of X-rays was to measure their ionising effect in air by means of an air-filled
ion chamber
The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...
. At the first ICRU meeting it was proposed that one unit of X-ray dose should be defined as the quantity of X-rays that would produce one
esu of charge in one
cubic centimetre
A cubic centimetre (or cubic centimeter in US English) (SI unit symbol: cm3; non-SI abbreviations: cc and ccm) is a commonly used unit of volume that corresponds to the volume of a cube that measures 1 cm × 1 cm × 1 cm. One cub ...
of dry air at 0
°C
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
and 1
standard atmosphere of pressure. This unit of
radiation exposure was named the
roentgen in honour of Wilhelm Röntgen, who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to
gamma radiation.
This approach, although a great step forward in standardisation, had the disadvantage of not being a direct measure of the absorption of radiation, and thereby the ionisation effect, in various types of matter including human tissue, and was a measurement only of the effect of the X-rays in a specific circumstance; the ionisation effect in dry air.
In 1940, Louis Harold Gray, who had been studying the effect of neutron damage on human tissue, together with
William Valentine Mayneord
William Valentine Mayneord, CBE FRS (14 February 1902 – 10 August 1988) was a British physicist and pioneer in the field of medical physics.
Early life and education
He was born in Redditch, Worcestershire to Walter and Elizabeth Mayneord b ...
and the radiobiologist John Read, published a paper in which a new unit of measure, dubbed the ''"gram roentgen"'' (symbol: gr) was proposed, and 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".
This unit was found to be equivalent to 88 ergs in air, and made the absorbed dose, as it subsequently became known, dependent on the interaction of the radiation with the irradiated material, not just an expression of radiation exposure or intensity, which the roentgen represented. 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 CGPM invited the ICRU to join other scientific bodies to work on the development of the International System of Units, or SI. The CCU decided to define the SI unit of absorbed radiation as energy deposited by reabsorbed charged particles per unit mass of absorbent material, which is how the rad had been defined, but in MKS units it would be equivalent to the joule per kilogram. 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 thus equal to 100 rad. Notably, the centigray (numerically equivalent to the rad) is still widely used to describe absolute absorbed doses in radiotherapy.
The adoption of the gray by the 15th General Conference on Weights and Measures as the unit of measure of the absorption of ionizing radiation, specific energy absorption, and of ]kerma
Kerma was the capital city of the Kerma culture, which was located in present-day Sudan at least 5,500 years ago. Kerma is one of the largest archaeological sites in ancient Nubia. It has produced decades of extensive excavations and research, i ...
in 1975 was the culmination of over half a century of work, both in the understanding of the nature of ionizing radiation and in the creation of coherent radiation quantities and units.
Radiation-related quantities
The following table shows radiation quantities in SI and non-SI units.
See also
* Dose area product Dose area product (DAP) is a quantity used in assessing the radiation risk from diagnostic X-ray examinations and interventional procedures. It is defined as the absorbed dose multiplied by the area irradiated, expressed in gray- centimetres squared ...
(Gy·cm2)
* International System of Units base units
* Orders of magnitude (radiation)
Recognized effects of higher acute radiation doses are described in more detail in the article on radiation poisoning. Although the International System of Units (SI) defines the sievert (Sv) as the unit of radiation dose equivalent, chronic radi ...
* Order of magnitude (unit)
* Rad (unit)
The rad is a unit of absorbed radiation dose, defined as 1 rad = 0.01 Gy = 0.01 J/kg. It was originally defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one gram of matter. The material absorbing the radiat ...
* Roentgen equivalent man
* SI derived unit
* Sievert
The sievert (symbol: SvNot be confused with the sverdrup or the svedberg, two non-SI units that sometimes use the same symbol.) is a unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizing rad ...
, SI derived unit of dose equivalent radiation
Notes
References
External links
* An account of chronological differences between USA and ICRP dosimetry systems.
{{DEFAULTSORT:Gray (Unit)
Nuclear physics
Radiation protection
Radioactivity
SI derived units
Units of radiation dose