The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is a measure of the absorbed dose and is defined as the absorption of one joule of radiation energy by one kilogram of matter. It is a physical quantity, and does not take into account any biological context, and unlike the pre-1971 non-SI roentgen unit of dose, the gray is defined independently for any target material. The gray is also used to measure specific energy (imparted), and kerma. When measuring kerma, the reference target material must be defined explicitly, usually dry air at standard temperature and pressure.
The equivalent cgs unit, the rad (equivalent to 0.01 Gy), remains common in industry in the United States, though "strongly discouraged" in the style guide for U.S. National Institute of Standards and Technology authors.
The gray was named after the British physicist Louis Harold Gray, a pioneer in the field of measurement of radium radiation and X-rays and their effects on living tissue, and was adopted as part of International System of Units in 1975.
The CIPM says that "in order to avoid any risk of confusion between the absorbed dose D and the dose equivalent 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 instead of joules per kilogram for the unit of dose equivalent H".
The gray has a number of fields of application in measuring dose:
Processing of matter
The gray is used to measure absorbed dose rates for processes such as radiation hardening, food irradiation and electron irradiation. Controlling the value of absorbed dose is vital to ensuring correct operation of these processes.
Kerma is a measure 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 (X-rays) or fast-moving electrons.
Absorbed dose in tissue
The measurement of absorbed dose in tissue is of fundamental importance in radiobiology and radiation therapy as it is the measure of the amount of energy the incident radiation is imparting to the target tissue. In radiation therapy, the amount of radiation varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, while lymphomas are treated with 20 to 40 Gy. Preventive (adjuvant) doses are typically around 45–60 Gy in 1.8–2 Gy fractions (for breast, head, and neck cancers).
The average radiation dose from an abdominal X-ray is 0.7 mGy, that from an abdominal CT scan is 8.0 mGy, 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. 
The absorbed dose also plays an important role in radiation protection as it is the starting point for calculating the stochastic effect of low levels of radiation. The stochastic health risk for radiation protection dose assessment is defined as the probability of cancer induction and genetic damage. The gray measures the absorbed energy of radiation, but the stochastic biological effects vary by the type and energy of the radiation and the tissues involved. This stochastic risk is expressed by the sievert, which has the same dimensions as the gray. It is related to the gray by weighting factors which are fully described in the articles on equivalent dose and effective dose. To avoid any risk of confusion between the absorbed dose and the equivalent dose the gray is used instead of the joule per kilogram for absorbed dose and the sievert instead of the joule per kilogram for the equivalent dose.
The accompanying diagrams shows 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 as the sievert (Sv), but for alpha particles one gray is equivalent to twenty sieverts because of the radiation weighting factor that is applied.
Radiation poisoning - The gray is conventionally used to to express the severity of "tissue effects" from doses received in acute exposure to high levels of radiation. These are effects which are certain to happen, and the gray is used as a measure of the severity. A whole-body exposure to 5 or more gray of high-energy radiation at one time usually leads to death within 14 days. This dosage represents 375 joules for a 75 kg adult (equivalent to the chemical energy in 20 mg of sugar).
Leading up to the gray
The adoption of the gray by the 15th General Conference on Weights and Measures as the unit of measure of the absorption of ionising radiation, specific energy absorption, and of kerma in 1975 was the culmination of over half a century of work, both in the understanding of the nature of ionising radiation and in the refinement of measuring techniques.
Wilhelm Röntgen first discovered X-rays on November 8, 1895 and within a few years they were being used to examine broken bones. One of the earliest techniques of measuring the intensity of X-rays was to measure their ionisation potential in air. Initially various countries developed their own standards, but in order to promote international cooperation, the First International Congress of Radiology (ICR) which met in London in 1925 proposed a separate body to consider units of measure. This body, the International Commission on Radiation Units and Measurements (ICRU), came into being at the Second ICR in Stockholm in 1928 under the chairmanship of Manne Siegbahn At their first meeting it was proposed that one unit X-ray dose should be defined as the quantity of X-rays that would produce one esu of charge in one cubic centimetre of dry air at 0 °C and a standard atmosphere. This unit was named the roentgen in honour of Röntgen who had died five years previously. At the 1937 meeting of the ICRU, this definition was extended to apply to gamma radiation as well as X-rays. This technique, although appropriate for the technology of the day, had the disadvantage that it was not a direct measure of either the intensity of X-rays or of their absorption, but rather was a measurement of the effect of the X-rays in a specific circumstance.
In 1940, Louis Harold Gray, 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 ICRU was invited by the CGPM to join other scientific bodies to work with the International Committee for Weights and Measures (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 (SI). At the same time it was becoming increasingly obvious that the definition of the roentgen was unsound and many calls were made for its redefinition, and in 1962 it was redefined. The definition of the roentgen had the advantage over the gray of being simpler to measure, but the gray is independent of the primary ionising radiation
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 15th GCPM and the unit was named the "gray" in honour of Hal Gray who had died in 1965. The gray was equal to 100 rad.
The following table shows radiation quantities in SI and non-SI units.
|Exposure (X)||röntgen||R||esu / 0.001293 g of air||1928||non-SI|
|Absorbed dose (D)||erg•g−1||1950||non-SI|
|Activity (A)||curie||c||3.7 × 1010 s−1||1953||non-SI|
|Dose equivalent (H)||röntgen equivalent man||rem||100 erg•g−1||1971||non-SI|
|Fluence (Φ)||(reciprocal area)||cm−2 or m−2||1962||SI (m-2)|
- Dose area product (Gy·cm2)
- International System of Units base units
- Orders of magnitude (radiation)
- Rad (unit)
- Roentgen equivalent man
- SI derived unit
- Sievert, SI derived unit of dose equivalent radiation
-  - "The confusing world of radiation dosimetry" - M.A. Boyd, U.S. Environmental Protection Agency 2009. An account of chronological differences between USA and ICRP dosimetry systems.
Notes and references
- "The International System of Units (SI)". Bureau International des Poids et Mesures (BIPM). Retrieved 2010-01-31.
- "NIST Guide to SI Units - Units temporarily accepted for use with the SI".
- "Rays instead of scalpels". LH Gray Memorial Trust. 2002. Retrieved 2012-05-15.
- CIPM, 2002: Recommendation 2
- ICRP publication 103 paragraph 64
- International Bureau of Weights and Measures (2006), The International System of Units (SI) (8th ed.), p. 157, ISBN 92-822-2213-6
- Originally known as the International X-ray Unit Committee
- Siegbahn, Manne; et al (October 1929). "Recommendations of the International X-ray Unit Committee". Radiology 13 (4): 372–373. doi:10.1148/13.4.372 (inactive 2014-05-23). Retrieved 2012-05-20.
- "About ICRU - History". International Commission on Radiation Units & Measures. Retrieved 2012-05-20.
- The host country nominated the chairman of the early ICRU meetings
- Guill, JH; Moteff, John (June 1960). "Dosimetry in Europe and the USSR". Third Pacific Area Meeting Papers - Materials in Nuclear Applications - American Society Technical Publication No 276. Symposium on Radiation Effects and Dosimetry - Third Pacific Area Meeting American Society for Testing Materials, October 1959, San Francisco, 12–16 October 1959. Baltimore: ASTM International. p. 64. LCCN 60-14734 Check
|lccn=value (help). Retrieved 2012-05-15.
- Lovell, S (1979). "4 - Dosimetric quantities and units". An introduction to Radiation Dosimetry. Cambridge, United Kingdom: Cambridge University Press. pp. 52–64. ISBN 0 521 22436 5. Retrieved 2012-05-15.
- Gupta, S. V. (2009-11-19). "Louis Harold Gray". Units of Measurement: Past, Present and Future : International System of Units. Springer. p. 144. ISBN 978-3-642-00737-8. Retrieved 2012-05-14.
- "CCU: Consultative Committee for Units". International Bureau of Weights and Measures (BIPM). Retrieved 2012-05-18.
- Anderson, Pauline C; Pendleton, Alice E (2000). "14 Dental Radiography". The Dental Assistant (7th ed.). Albany, NY: Delmar. p. 554. ISBN 0-7668-1113-1.
- Lovell, S (1979). "3. The effects of ionizing radiation on matter in bulk". An introduction to Radiation Dosimetry. Cambridge, United Kingdom: Cambridge University Press. pp. 43–51. ISBN 0 521 22436 5. Retrieved 2012-05-15.