Gamma rays (denoted as γ Gamma is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. It was derived from the Phoenician letter Gimel . Letters that arose from Gamma include the Roman C and G and the Cyrillic letters Ge Г and Ghe Ґ) are electromagnetic radiation Electromagnetic radiation is a phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It comprises electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified into several types according to of high frequency (very short wavelength). They are produced by sub-atomic The atom is a basic unit of matter that consists of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons . The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain particle interactions such as electron-positron annihilation Electron–positron annihilation occurs when an electron and a positron collide. The result of the collision is the conversion of the electron and positron and the creation of gamma ray photons or, less often, other particles. The process must satisfy a number of conservation laws, including:, neutral pion decay In particle physics, a pion is any of three subatomic particles: π0, π+, and π−. Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force, radioactive decay Radioactive decay is the process by which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom of one type, called the parent nuclide transforming to an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom emits radiation, fusion In nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple atomic nuclei join together to form a single heavier nucleus. It is accompanied by the release or absorption of energy. Large scale fusion processes, involving many atoms fusing at once, must occur in matter which is at very high densities, fission or inverse Compton scattering In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic[citation needed] scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect. Part of the energy of the X/gamma ray is transferred to a scattering electron, which recoils in astrophysical processes. Gamma rays typically have frequencies above 10¹⁹ Hz, and therefore have energies above 100 keV In physics, the electron volt is a unit of energy equal to approximately 1.602×10−19 J. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb) multiplied by the electron charge (1 e, or 1.60217 and wavelength less than 10 picometers A picometre is a unit of length in the metric system, equal to one trillionth, i.e. (1/1,000,000,000,000) of a metre, which is the current SI base unit of length. It can be written in scientific notation as 1×10−12 m, or as 1 E−12 m in engineering notation — both meaning 1 m / 1,000,000,000,000, often smaller than an atom The atom is a basic unit of matter that consists of a dense, central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons . The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain. Gamma radioactive decay Gamma rays are electromagnetic radiation of high frequency (very short wavelength). They are produced by sub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz, and photons commonly have energies of a few hundred keV, and are almost always less than 10 MeV in energy.

Because they are a form of ionizing radiation Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number. An intense flood of particles or waves will not cause, gamma rays can cause serious damage when absorbed by living tissue and, are therefore a health hazard.

Paul Villard Paul Ulrich Villard was a French chemist and physicist, born in Saint-Germain-au-Mont-d'Or 28th of September 1860, France. He discovered gamma rays in 1900 while studying the radiation from radium, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium Radium is a radioactive chemical element which has the symbol Ra and atomic number 88. Its appearance is almost pure white, but it readily oxidizes on exposure to air, turning black. Radium is an alkaline earth metal that is found in trace amounts in uranium ores. Its most stable isotope, 226Ra, has a half-life of 1601 years and decays into radon. Alpha and beta "rays" had already been separated and named by the work of Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS was a British-New Zealand chemist and physicist who became known as the father of nuclear physics. In early work he discovered the concept of radioactive half life, proved that radioactivity involved the transmutation of one chemical element to another, and also differentiated and named in 1899, and in 1903 Rutherford named Villard's distinct new radiation "gamma rays."

In the past, the distinction between X-rays X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × 1016 Hz to 3 × 1019 Hz) and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma rays. In many and gamma rays was based on energy (or equivalently frequency or wavelength), the latter being considered a higher-energy version of the former. However, high-energy X-rays produced by linear accelerators A linear particle accelerator is a type of particle accelerator that greatly increases the velocity of charged subatomic particles or ions by subjecting the charged particles to a series of oscillating electric potentials along a linear beamline; this method of particle acceleration was invented in 1928 by Rolf Widerøe ("linacs") and astrophysical processes now often have higher energy than gamma rays produced by radioactive gamma decay Gamma rays are electromagnetic radiation of high frequency (very short wavelength). They are produced by sub-atomic particle interactions such as electron-positron annihilation, neutral pion decay, radioactive decay, fusion, fission or inverse Compton scattering in astrophysical processes. Gamma rays typically have frequencies above 1019 Hz, and. In fact, one of the most common gamma-ray emitting isotopes used in nuclear medicine Nuclear medicine is a branch or specialty of medicine and medical imaging that uses radioactive isotopes and relies on the process of radioactive decay in the diagnosis and treatment of disease, technetium-99m Technetium-99m is a metastable nuclear isomer of technetium-99, symbolized as 99mTc. The "m" indicates that this is a metastable nuclear isomer, i.e., that its half life is considerably longer than most nuclear isomers which undergo gamma decay. As in all gamma decay reactions, a metastable nuclear isomer does not change into another, produces gamma radiation of about the same energy (140 KeV) as produced by a diagnostic X-ray machine, and significantly lower energy than the therapeutic treatment X-rays produced by linac machines in cancer radiotherapy Radiation therapy , or radiotherapy (in the UK and Australia) also called radiation oncology, and sometimes abbreviated to XRT, is the medical use of ionizing radiation as part of cancer treatment to control malignant cells (not to be confused with radiology, the use of radiation in medical imaging and diagnosis). Radiotherapy may be used for. Because of this overlap in energy ranges, the two types of electromagnetic radiation are now usually defined by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus (that is, produced by gamma decay), or from other particle decays or annihilation events. There is no lower limit to the energy of photons produced by nuclear reactions, and thus ultraviolet Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3eV to 124 eV. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet and even lower energy photons produced by these processes would also be defined as "gamma rays".^[1] In certain fields such as astronomy, gamma rays and X-rays are still sometimes defined by energy, as the processes which produce them may be uncertain.

Contents 1 Units of measure and exposure 2 Properties 2.1 Shielding 2.2 Matter interaction 2.3 Light interaction 2.4 Gamma ray production 3 Health effects 4 Uses 4.1 Body response 4.2 Risk assessment 5 See also 6 References 7 External links

Units of measure and exposure

The measure of gamma rays' ionizing Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation ability is called the exposure:

• The coulomb The coulomb is the SI derived unit of electric charge, and is approximately equal to the charge of 6.24151 × 1018 protons or −6.24151 × 1018 electrons. It is named after Charles-Augustin de Coulomb per kilogram The kilogram is the base unit of mass in the International System of Units (SI, from the French Le Système International d’Unités),[Note 2] which is the modern standard governing the metric system. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK),[Note 3] which is almost exactly equal to the mass (C/kg) is the SI unit of ionizing radiation Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the impinging individual particles or waves, and not on their number. An intense flood of particles or waves will not cause exposure, and is the amount of radiation required to create 1 coulomb of charge of each polarity in 1 kilogram of matter.
• The röntgen The röntgen or roentgen is a unit of measurement for exposure to ionizing radiation (such as X-ray and gamma rays), and is named after the German physicist Wilhelm Röntgen. Adopted in 1928, 1 R is the amount of radiation required to liberate positive and negative charges of one electrostatic unit of charge (esu) in 1 cm³ of dry air at standard (R) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create 1 esu of charge of each polarity in 1 cubic centimeter of dry air. 1 röntgen = 2.58×10⁻⁴ C/kg

However, the effect of gamma and other ionizing radiation on living tissue is more closely related to the amount of energy In physics, energy is a quantity that is often understood as the ability to perform work. This quantity can be assigned to any particle, object, or system of objects as a consequence of its physical state deposited rather than the charge Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. Electrically charged matter is influenced by, and produces, electromagnetic fields. The interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the. This is called the absorbed dose Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of medium, and so has the unit J/kg, which is given the special name Gray (Gy):

• The gray The gray is the SI unit of absorbed radiation dose due to ionizing radiation (for example, X-rays) (Gy), which has units of (J/kg), is the SI unit of absorbed dose Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of medium, and so has the unit J/kg, which is given the special name Gray (Gy), and is the amount of radiation required to deposit 1 joule The joule , named after James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy expended in applying a force of one Newton through a distance of one metre (1 Newton·metre or N·m). In terms of dimensions: of energy in 1 kilogram The kilogram is the base unit of mass in the International System of Units (SI, from the French Le Système International d’Unités),[Note 2] which is the modern standard governing the metric system. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK),[Note 3] which is almost exactly equal to the mass of any kind of matter.
• The rad The rad is a largely obsolete unit of absorbed radiation dose, with symbol rad. The rad was first proposed in 1918 as "that quantity of X rays which when absorbed will cause the destruction of the [malignant mammalian] cells in question..." It was defined in CGS units in 1953 as the dose causing 100 ergs of energy to be absorbed by one is the (obsolete) corresponding traditional unit, equal to 0.01 J deposited per kg. 100 rad = 1 Gy.

The equivalent dose The equivalent dose is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effects of different types of ionizing radiation. Equivalent dose is therefore a less fundamental quantity than radiation absorbed dose, but is more biologically significant. Equivalent dose has units of is the measure of the biological effect of radiation on human tissue. For gamma rays it is equal to the absorbed dose Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation. It is equal to the energy deposited per unit mass of medium, and so has the unit J/kg, which is given the special name Gray (Gy).

• The sievert The sievert is the SI derived unit of dose equivalent. It attempts to reflect the biological effects of radiation as opposed to the physical aspects, which are characterised by the absorbed dose, measured in gray. It is named after Rolf Sievert, a Swedish medical physicist famous for work on radiation dosage measurement and research into the (Sv) is the SI unit of equivalent dose, which for gamma rays is numerically equal to the gray The gray is the SI unit of absorbed radiation dose due to ionizing radiation (for example, X-rays) (Gy).
• The rem is the traditional unit of equivalent dose. For gamma rays it is equal to the rad or 0.01 J of energy deposited per kg. 1 Sv = 100 rem.

Properties

Shielding

Shielding from gamma rays requires large amounts of mass. They are better absorbed by materials with high atomic numbers and high density, although neither effect is important compared to the total mass per area in the path of the gamma ray. For this reason, a lead shield is only modestly better (20-30%) as a gamma shield than an equal mass of another shielding material such as aluminium, concrete, or soil; the lead's major advantage is in its compactness.

The higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically measured by the thickness required to reduce the intensity of the gamma rays by one half (the half value layer or HVL). For example gamma rays that require 1 cm (0.4") of lead Lead is a main-group element with symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed to air. Lead has a shiny chrome-silver luster when it is melted into a to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of Granite Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granites usually have a medium to coarse grained texture. Occasionally some individual crystals (phenocrysts) are larger than the groundmass in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a porphyry rock, 6 cm (2½") of concrete Concrete is a construction material composed of cement and other cementitious materials such as fly ash and slag cement, aggregate (generally a coarse aggregate made of gravels or crushed rocks such as limestone, or granite, plus a fine aggregate such as sand), water, and chemical admixtures, or 9 cm (3½") of packed soil Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics. It is composed of particles of broken rock that have been altered by chemical and environmental processes that include weathering and. However, the mass of this much concrete or soil is only 20-30% larger than that of this amount of lead. Depleted uranium Depleted uranium is uranium primarily composed of the isotope uranium-238 (U-238). Natural uranium is about 99.27 percent U-238, 0.72 percent U-235, and 0.0055 percent U-234. U-235 is used for fission in nuclear reactors and nuclear weapons. Uranium is enriched in U-235 by separating the isotopes by mass. The byproduct of enrichment, called is used for shielding in portable gamma ray sources, but again the savings in weight over lead is modest, and the main effect is to reduce shielding bulk.

Matter interaction

The total absorption coefficient of aluminium (atomic number 13) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. Over most of the energy region shown, the Compton effect dominates. The total absorption coefficient of lead (atomic number 82) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. Here, the photoelectric effect dominates at low energy. Above 5 MeV, pair production starts to dominate

When a gamma ray passes through matter, the probability for absorption in a thin layer is proportional to the thickness of that layer. This leads to an exponential decrease A quantity is said to be subject to exponential decay if it decreases at a rate proportional to its value. Symbolically, this can be expressed as the following differential equation, where N is the quantity and λ is a positive number called the decay constant. Such a process is modeled by the following differential equation: of intensity with thickness. The exponential absorption holds only for a narrow beam of gamma rays. If a wide beam of gamma rays passes through a thick slab of concrete the scattering from the sides reduces the absorption.

Here μ = nσ is the absorption coefficient, measured in cm⁻¹, n the number of atoms per cm³ in the material, σ the absorption cross section In nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles in cm² and d the thickness of material in cm.

In passing through matter, gamma radiation ionizes via three main processes: the photoelectric effect In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons". First observed by Heinrich Hertz in 1887, the phenomenon is, Compton scattering In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic[citation needed] scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect. Part of the energy of the X/gamma ray is transferred to a scattering electron, which recoils, and pair production Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . This is allowed, provided there is enough energy available to create the pair – at least the total rest mass energy of the two particles – and that the situation allows both energy and momentum to be conserved (though not necessarily.

• Photoelectric effect: This describes the case in which a gamma photon interacts with and transfers its energy to an atomic electron, ejecting that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the binding energy of the electron. The photoelectric effect is the dominant energy transfer mechanism for x-ray and gamma ray photons with energies below 50 keV (thousand electron volts), but it is much less important at higher energies.
• Compton scattering: This is an interaction in which an incident gamma photon loses enough energy to an atomic electron to cause its ejection, with the remainder of the original photon's energy being emitted as a new, lower energy gamma photon with an emission direction different from that of the incident gamma photon. The probability of Compton scatter decreases with increasing photon energy. Compton scattering is thought to be the principal absorption mechanism for gamma rays in the intermediate energy range 100 keV to 10 MeV. Compton scattering is relatively independent of the atomic number of the absorbing material, which is why very dense metals like lead are only modestly better shields, on a per weight basis, than are less dense materials.
• Pair production: This becomes possible with gamma energies exceeding 1.02 MeV, and becomes important as an absorption mechanism at energies over about 5 MeV (see illustration at right, for lead). By interaction with the electric field of a nucleus, the energy of the incident photon is converted into the mass of an electron-positron pair. Any gamma energy in excess of the equivalent rest mass of the two particles (1.02 MeV) appears as the kinetic energy of the pair and the recoil nucleus. At the end of the positron's range, it combines with a free electron. The entire mass of these two particles is then converted into two gamma photons of at least 0.51 MeV energy each (or higher according to the kinetic energy of the annihilated particles).

The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much ionization themselves.

Light interaction

High-energy (from 80 to 500 GeV) gamma rays arriving from far far-distant quasars are used to estimate the extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus their density may be estimated by analyzing the incoming gamma-ray spectrums.^[2]

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