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Gray (unit)

The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per kilogram of matter.{{cite web | url = http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf | title = The International System of Units (SI) | publisher = Bureau International des Poids et Mesures ( BIPM) | accessdate = 2010-01-31 }} It is used as a measure of absorbed dose, specific energy (imparted), and kerma (an acronym for kinetic energy released per unit mass). It is a physical quantity, and does not take into account any biological context. Unlike the pre-1971 non-SI roentgen unit of radiation exposure, the gray when used for absorbed dose is defined independently of any target material. However, when measuring kerma the reference target material must be defined explicitly, usually as dry air at standard temperature and pressure. The corresponding cgs unit, the rad (equivalent to 0.01 Gy), remains common 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 British physicist Louis Harold Gray, a pioneer in the measurement of X-ray and radium radiation and their effects on living tissue.{{cite web |url = http://www.lhgraytrust.org/lhgraybiography.html |title = Rays instead of scalpels |publisher = LH Gray Memorial Trust |year = 2002 |accessdate = 2012-05-15}} It was adopted as part of the International System of Units in 1975.


One gray is the absorption of one joule of energy, in the form of ionizing radiation, per kilogram of matter. 1 \ \mathrm{Gy} = 1\ \frac{\mathrm{J}}{\mathrm{kg}}= 1\ \frac{\mathrm{m}^2}{\mathrm{s}^{2}} The CIPM states: "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:

Absorbed dose in matter

The gray is used to measure absorbed dose rates in non-tissue materials for processes such as radiation hardening, food irradiation and electron irradiation. Measuring and controlling the value of absorbed dose is vital to ensuring correct operation of these processes.


Kerma ("kinetic energy released per unit mass") is 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 (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. The measurement of absorbed dose is a complex problem, and so many different dosimeters are available for these measurements. These dosimeters cover measurements that can be done 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 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 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.http://www.xrayrisk.com/calculator/calculator.php The absorbed dose also plays an important role in radiation protection, as it is the starting point for calculating the effect of low levels of radiation. In radiation protection dose assessment, the "stochastic health risk" is defined as the probability of cancer induction and genetic damage. The gray measures the total absorbed energy of radiation, but the probability of damage also depends on the type and energy of the individual particles or photons of which the radiation consists, and on the tissues involved. This probability is related to the equivalent dose in sieverts (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 and effective dose. To avoid any risk of confusion between the absorbed dose and the equivalent dose, the absorbed dose is stated in grays and the equivalent dose is stated in sieverts. 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 one gray is equivalent to 20 sieverts, and a radiation weighting factor is applied accordingly. 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 which are certain to happen, as opposed to the uncertain effects of low levels of radiation which 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. This dose represents 375 joules for a 75 kg adult.

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 ionizing 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 ionizing 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 |url =https://books.google.com/books?id=lK48AAAAIAAJ&pg=PA56&lpg=PA56&dq=roentgen+defined#v=onepage&q=roentgen%20defined&f=false |title = An introduction to Radiation Dosimetry |first1 = S |last1 = Lovell |year = 1979 |chapter = 3. The effects of ionizing radiation on matter in bulk |pages = 43–51 |isbn = 0 521 22436 5 |publisher = Cambridge University Press |accessdate = 2012-05-15}} 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.

Radiation-related quantities

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

See also



External links

  • — An account of chronological differences between USA and ICRP dosimetry systems.
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This article based upon the http://en.wikipedia.org/wiki/Gray_(unit), the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=Gray_(unit)&action=history
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