n.
Electromagnetic radiation emitted by radioactive decay and having energies in a range from ten thousand (104) to ten million (107) electron volts.
| Dictionary: gamma ray |
Electromagnetic radiation emitted by radioactive decay and having energies in a range from ten thousand (104) to ten million (107) electron volts.
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| Sci-Tech Encyclopedia: Gamma rays |
Electromagnetic radiation emitted from excited atomic nuclei as an integral part of the process whereby the nucleus rearranges itself into a state of lower excitation (that is, energy content). See also Nuclear structure; Radioactivity.
The gamma ray is an electromagnetic radiation pulse—a photon—of very short wavelength. The electric (E) and magnetic (H) fields associated with the individual radiations oscillate in planes mutually perpendicular to each other and also the direction of propagation with a frequency ν which characterizes the energy of the radiation. The E and H fields exhibit various specified phase-and-amplitude relations, which define the character of the radiation as either electric (EL) or magnetic (ML). The second term in the designation indicates the order of the radiation as 2L-pole, where the orders are monopole (20), dipole (21), quadrupole (22), and so on. The most common radiations are dipole and quadrupole. Gamma rays range in energy from a few kiloelectronvolts to 100 MeV, although most radiations are in the range 50–6000 keV. As such, they lie at the very upper high-frequency end of the family of electromagnetic radiations, which include also radio waves, light rays, and x-rays. See also Electromagnetic radiation; Multipole radiation; Photon.
The dual nature of gamma rays is well understood in terms of the wavelike and particlelike behavior of the radiations. For a gamma ray of intrinsic frequency ν, the wavelength is λ = c/ν, where c is the velocity of light; energy is E = hν, where h is Planck's constant. The photon has no rest mass or electric charge but, following the concept of mass-energy equivalence set forth by Einstein, has associated with it a momentum given by p = hν/c = E/c. See also Light;
Various nuclear species exhibit distinctly different nuclear configurations; the excited states, and thus the γ-rays which they produce, are also different. Precise measurements of the γ-ray energies resulting from nuclear decays may therefore be used to identify the γ-emitting nucleus. This has ramifications for nuclear research and also for a wide variety of more practical applications. One of the most useful studies of the nucleus involves the bombardment of target nuclei by energetic nuclear projectiles to form final nuclei in various excited states. Measurements of the decay γ-rays are routinely used to identify the various final nuclei according to their characteristic γ-rays.
In practical applications, the presence of γ-rays is used to detect the location or presence of radioactive atoms which have been deliberately introduced into the sample. In irradiation studies, for example, the sample is activated by placing it in the neutron flux from a reactor. The resultant γ-rays are identified according to isotope, and thus the composition of the original sample can be inferred. Such studies have been used to identify trace elements found as impurities in industrial production, or in ecological studies of the environment, such as minute quantities of tin or arsenic in plant and animal tissue. See also Activation analysis.
In tracer studies, a small quantity of radioactive atoms is introduced into fluid systems (such as the human blood stream), and the flow rate and diffusion can be mapped out by following the radioactivity. Local concentrations, as in tumors, can also be determined. See also Radioactive tracer.
For the three types of interaction with matter which together are responsible for the observable absorption of γ-rays, namely, Compton scattering, the photoelectric effect, and pair production, See also Compton effect; Electron-positron pair production; Photoemission.
| US Military Dictionary: gamma rays |
High-energy electromagnetic radiation emitted from atomic nuclei during a nuclear reaction. Gamma rays and very-high-energy X rays differ only in origin (X rays do not originate from atomic nuclei).
See the Introduction, Abbreviations and Pronunciation for further details.
| Britannica Concise Encyclopedia: gamma ray |
For more information on gamma ray, visit Britannica.com.
| Electronics Dictionary: gamma rays |
High frequency electromagnetic radiation from radioactive particles.
| Cosmic Lexicon: Gamma ray |
Gamma rays are like the light we see with our eyes, the x-rays used to probe our bodies in search of problems, and the radio waves that our television sets translate into inane shows. The only difference among these types of electromagnetic radiation is the wavelengths of the waves and how fast they vibrate. Gamma rays are the most energetic (and vibrate the most); radio waves are the least energetic and vibrate the least.
| Wikipedia: Gamma ray |
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Gamma rays (denoted as γ) are electromagnetic radiation of high energy (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 therefore energies above 100 keV and wavelength less than 10 picometers, often smaller than an atom. Gamma radioactive decay 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, gamma rays can cause serious damage when absorbed by living tissue, and are therefore a health hazard.
Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900, while studying radiation emitted from radium. Alpha and beta "rays" had already been separated and named by the work of Ernest Rutherford in 1899, and in 1903 Rutherford named Villard's distinct new radiation "gamma rays."
In the past, the distinction between X-rays 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 ("linacs") and astrophysical processes now often have higher energy than gamma rays produced by radioactive gamma decay. (In fact, one of the most common gamma-ray emitting isotopes used in nuclear medicine, technetium-99m 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.) 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 (and when produced by therapeutic linacs are often simply called "photons"), while gamma rays are specifically emitted by the nucleus (that is, produced by gamma decay). In theory, there is no lower limit to the energy of such photons, and thus "ultraviolet gamma rays" have been postulated. 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.
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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 weight of another shielding material such as aluminum, 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 to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of Granite rock, 6 cm (2½") of concrete or 9 cm (3½") of packed soil. However, (again) the mass of this much concrete or soil is only 20-30% larger than that of this amount of lead. Depleted uranium 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.
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 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 cm2 and d the thickness of material in cm.
In passing through matter, gamma radiation ionizes via three main processes: the photoelectric effect, Compton scattering, and pair production.
The secondary electrons (and/or positrons) produced in any of these three processes frequently have enough energy to produce much ionization themselves.
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.[1]
Gamma rays are often produced alongside other forms of radiation such as alpha or beta. When a nucleus emits an α or β particle, the daughter nucleus is sometimes left in an excited state. It can then jump down to a lower level by emitting a gamma ray in much the same way that an atomic electron can jump to a lower level by emitting visible light or ultraviolet radiation.
Gamma rays, x-rays, visible light, and radio waves are all forms of electromagnetic radiation. The only difference is the frequency and hence the energy of the photons. Gamma rays are the most energetic. An example of gamma ray production follows.
First 60Co decays to excited 60Ni by beta decay. Then the 60Ni drops down to the ground state (see nuclear shell model) by emitting two gamma rays in succession (1.17 MeV then 1.33 MeV):
Another example is the alpha decay of 241Am to form 237Np; this alpha decay is accompanied by gamma emission. In some cases, the gamma emission spectrum for a nucleus (daughter nucleus) is quite simple, (eg 60Co/60Ni) while in other cases, such as with (241Am/237Np and 192Ir/192Pt), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels can exist. The fact that an alpha spectrum can have a series of different peaks with different energies reinforces the idea that several nuclear energy levels are possible.
Because a beta decay is accompanied by the emission of a neutrino which also carries energy away, the beta spectrum does not have sharp lines, but instead is a broad peak. Hence from beta decay alone it is not possible to probe the different energy levels found in the nucleus.
In optical spectroscopy, it is well known that an entity which emits light can also absorb light at the same wavelength (photon energy). For instance, a sodium flame can emit yellow light as well as absorb the yellow light from a sodium vapor lamp. In the case of gamma rays, this can be seen in Mössbauer spectroscopy. Here, a correction for the energy lost by the recoil of the nucleus is made and the exact conditions for gamma ray absorption through resonance can be attained.
This is similar to the Franck Condon effects seen in optical spectroscopy.
Gamma rays compete with neutrons as the most dangerous form of ionizing radiation emitted by something such as a nuclear explosion because they are highly penetrating, highly energetic ionizing radiation. Gamma rays have the shortest wavelength of all waves in the electromagnetic spectrum, and therefore have the greatest ability to penetrate through any gap, even a subatomic one, in what might otherwise be an effective shield. The most biological damaging forms of gamma radiation occur in the gamma ray window, between 3 and 10 MeV. See cobalt-60.
Gamma-rays are not stopped by the skin. They can induce DNA alteration by effect of whole-body gamma-irradiation on localized beta-irradiation-induced skin reactions in mice.[2]
This property means that gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilizing medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foods or preventing fruit and vegetables from sprouting to maintain freshness and flavor.
Gamma-rays have the smallest wavelengths and the most energy of any other wave in the electromagnetic spectrum. These waves are generated by radioactive atoms and in nuclear explosions. Gamma-rays can kill living cells, a fact which medicine uses to its advantage, using gamma-rays to kill cancerous cells.
Gamma-rays travel to us across vast distances of the universe, only to be absorbed by the Earth's atmosphere. Different wavelengths of light penetrate the Earth's atmosphere to different depths. Instruments aboard high-altitude balloons and satellites like the Compton Observatory provide our only view of the gamma-ray sky.
Due to their tissue penetrating property, gamma rays/X-rays have a wide variety of medical uses such as in CT Scans and radiation therapy (see X-ray). However, as a form of ionizing radiation they have the ability to effect molecular changes, giving them the potential to cause cancer when DNA is affected. The molecular changes can also be used to alter the properties of semi-precious stones, and is often used to change white topaz into blue topaz.
Despite their cancer-causing properties, gamma rays are also used to treat some types of cancer. In the procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to concentrate the radiation on the growth while minimizing damage to the surrounding tissues. (As an illustration of the radiation origin-process contributing to its name, a similar technique which uses photons from linacs rather than cobalt gamma decay, is called "Cyberknife").
Gamma rays are also used for diagnostic purposes in nuclear medicine. Several gamma-emitting radioisotopes are used, one of which is technetium-99m. When administered to a patient, a gamma camera can be used to form an image of the radioisotope's distribution by detecting the gamma radiation emitted. Such a technique can be employed to diagnose a wide range of conditions (e.g. spread of cancer to the bones).
In the US, gamma ray detectors are beginning to be used as part of the Container Security Initiative (CSI). These US$5 million machines are advertised to scan 30 containers per hour. The objective of this technique is to screen merchant ship containers before they enter US ports.
After gamma-irradiation, and the breaking of DNA double-strands, a cell can repair the damaged genetic material to the limit of its capability. However, a study of Rothkamm and Lobrich has shown that the repairing process works well after high-dose exposure but is much slower in the case of a low-dose exposure.[3]
This could mean that a chronic low-dose exposure cannot be fought by the body[citation needed]. The probability of detecting small alterations or of a detectable defect occurring is most likely small enough that the cell would replicate before initiating a full repair[citation needed]. Some cells cannot detect their own genetic defects[citation needed].
The natural outdoor exposure in Great Britain ranges from 2 × 10–7 to 4 × 10–7 cSv/h (centisieverts per hour).[4] Natural exposure to gamma rays is about 0.1 to 0.2 cSv per year, and the average total amount of radiation received in one year per inhabitant in the USA is 0.36 cSv.[5]
By comparison, the radiation dose from chest radiography is a fraction of the annual naturally occurring background radiation dose,[6] and the dose from fluoroscopy of the stomach is, at most, 5 cSv on the skin of the back.
For acute full-body equivalent dose, 100 cSv causes slight blood changes; 200–350 cSv causes nausea, hair loss, hemorrhaging and will cause death in a sizable number of cases (10%–35%) without medical treatment; 500 cSv is considered approximately the LD50 (lethal dose for 50% of exposed population) for an acute exposure to radiation even with standard medical treatment; more than 500 cSv brings an increasing chance of death; eventually, above 750–1000 cSv, even extraordinary treatment, such as bone-marrow transplants, will not prevent the death of the individual exposed (see Radiation poisoning).[clarification needed][citation needed]
For low dose exposure, for example among nuclear workers, who receive an average yearly radiation dose of 1.9 cSv,[clarification needed] the risk of dying from cancer (excluding leukemia) increases by 2 percent. For a dose of 10 cSv, that risk increase is at 10 percent. By comparison, risk of dying from cancer was increased by 32 percent for the survivors of the atomic bombing of Hiroshima and Nagasaki.[7]
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