Results for gamma ray
On this page:
 
Dictionary:

gamma ray


n.

Electromagnetic radiation emitted by radioactive decay and having energies in a range from ten thousand (104) to ten million (107) electron volts.


 
 
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; Quantum mechanics; Relativity.

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

Penetrating very short-wavelength electromagnetic radiation, similar to an X-ray but of higher energy, that is emitted spontaneously by some radioactive substances (see gamma decay; radioactivity). Gamma radiation also originates in the decay of certain subatomic particles and in particle-antiparticle annihilation (see also antimatter). Gamma rays can initiate nuclear fission, can be absorbed by ejection of an electron (see photoelectric effect), and can be scattered by free electrons (see Compton effect).

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
For the music band, see Gamma Ray (band).

Gamma rays or gamma-ray (denoted as γ) are forms of electromagnetic radiation (EMR) or light emissions of a specific frequency produced from sub-atomic particle interaction, such as electron-positron annihilation and radioactive decay; most are generated from nuclear reactions occurring within the interstellar medium of space. Gamma rays are generally characterized as electromagnetic radiation, having the highest frequency and energy, and also the shortest wavelength, within the electromagnetic spectrum, i.e. high energy photons. Due to their high energy content, they are able to cause serious damage when absorbed by living cells.

Properties

Shielding

Artist's impression of an emission of a gamma ray (γ) from an atomic nucleus
Enlarge
Artist's impression of an emission of a gamma ray (γ) from an atomic nucleus

Shielding for gamma rays requires large amounts of mass. The material used for shielding takes into account that gamma rays are better absorbed by materials with high atomic number and high density. Also, the higher the energy of the gamma rays, the thicker the shielding required. Materials for shielding gamma rays are typically illustrated 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 inches) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 6 cm (2½ inches) of concrete or 9 cm (3½ inches) of packed dirt.


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.
Enlarge
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 photo effect dominates at low energy. Above 5 MeV, pair production starts to dominate
Enlarge
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 photo 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 of intensity with thickness.

I(d) = I_0 \cdot e ^{-\mu d}

Here, μ = n×σ 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.

  • 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 (megaelectronvolts), an energy spectrum which includes most gamma radiation present in a nuclear explosion. Compton scattering is relatively independent of the atomic number of the absorbing material.
  • Pair Production: By interaction via the Coulomb force, in the vicinity of the nucleus, the energy of the incident photon is spontaneously converted into the mass of an electron-positron pair. A positron is the anti-matter equivalent of an electron; it has the same mass as an electron, but it has a positive charge equal in strength to the negative charge of an electron. 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. The positron has a very short lifetime (if immersed in matter) (about 10-8 seconds). At the end of its range, it combines with a free electron. The entire mass of these two particles is then converted into two gamma photons of 0.51 MeV energy each.

The secondary electrons (or positrons) produced in any of these three processes frequently have enough energy to produce many ionizations up to the end of range.

The exponential absorption described above holds, strictly speaking, 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.

Gamma decay

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.

Decay scheme of 60Co
Enlarge
Decay scheme of 60Co

Gamma rays, x-rays, visible light, and UV rays 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:

{}^{60}\hbox{Co}\;\to\;^{60}\hbox{Ni*}\;+\;e^-\;+\;\overline{\nu}_e.

Then the 60Ni drops down to the ground state (see nuclear shell model) by emitting two gamma rays in succession:

{}^{60}\hbox{Ni*}\;\to\;^{60}\hbox{Ni}\;+\;\gamma.

Gamma rays of 1.17 MeV and 1.33 MeV are produced.

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 nucleu) 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.

Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the CGRO spacecraft. Bright spots within the galactic plane are pulsars while those above and below the plane are thought to be quasars.
Enlarge
Image of entire sky in 100 MeV or greater gamma rays as seen by the EGRET instrument aboard the CGRO spacecraft. Bright spots within the galactic plane are pulsars while those above and below the plane are thought to be quasars.

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 vapour 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.

Uses

Gamma-ray Image of a Truck taken with a VACIS (Vehicle and Container Imaging System)
Enlarge
Gamma-ray Image of a Truck taken with a VACIS (Vehicle and Container Imaging System)

Because the wavelength of gamma radiation is so short, a single incident photon can impart significant damage to a living cell. This property means that gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilising medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foodstuffs or preventing fruit and vegetables from sprouting to maintain freshness and flavour.

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 focus the radiation on the growth while minimizing damage to the surrounding tissues.

The Moon as seen in gamma rays by the Compton Gamma Ray Observatory. Surprisingly, the Moon is actually brighter than the Sun at gamma ray wavelengths.
Enlarge
The Moon as seen in gamma rays by the Compton Gamma Ray Observatory. Surprisingly, the Moon is actually brighter than the Sun at gamma ray wavelengths.

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).

Gamma ray detectors are also starting to be used in Pakistan 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 pre-screen merchant ship containers before they enter U.S. ports.

Health effect

The gamma rays are the most dangerous form of radiation emitted by a nuclear explosion because of the difficulty in stopping them. Gamma-rays are not stopped by the skin.

They can induce DNA alteration by interfering with the genetic material of the cell. DNA double-strand breaks are generally accepted to be the most biologically significant lesion by which ionizing radiation causes cancer and hereditary disease.[1].

A study done on Russian nuclear workers exposed to external whole-body gamma radiation at high cumulative doses shows the link between radiation exposure and death from leukemia, lung, liver, skeletal and other solid cancers.[2].

Alongside radiation, gamma-rays also produce thermal burn injuries and induce an immunosuppressive effect.[3][4]

Body response

After gamma-irradiation, and the breaking of DNA double-strands, a cell can repair the damaged genetic material to the limit of its capability [citation needed]. 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. [5] 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 can not detect their own genetic defects [citation needed].

Risk assessment

The natural outdoor exposure in Great Britain is in the range 20-40 nSv/h.[6] Natural exposure to gamma rays is about 1 to 2 mSv a year, and the average total amount of radiation received in one year per inhabitant in the USA is 3.6 mSv.[7]

By comparison, the radiation dose from chest radiography is a fraction of the annual naturally occurring background radiation dose,[8] and the dose from fluoroscopy of the stomach is, at most, 0.05 Sv on the skin of the back.

For acute full-body equivalent dose, 1 Sv causes slight blood changes, 2-5 Sv causes nausea, hair loss, hemorrhaging and will cause death in many cases. More than 3 Sv will lead to death in less than two months in more than 80% of cases, and much over 4 Sv is more likely than not to cause death (see Sievert).

For low dose exposure, for example among nuclear workers, who receive an average radiation dose of 19mSv, the risk of dying from cancer (excluding leukemia) increases by 2 percent. For a dose of 100mSv, that risk increase is at 10 percent. By comparison, it was 32% for the Atom Bomb survivors.[9].

References

  1. Kelly, K. (2005). Radiation may have positive effects on health: study -- Low, chronic doses of gamma radiation had beneficial effects on meadow voles University of Toronto
  1. ^ Rothkamm K. – Evidence for a lack of DNA double-strand break repair in human cells exposed to very low x-ray doses – Proceedings of the National Academy of Science of the USA, 2003; 100 (9) : 5057-5062.
  2. ^ Shilnikova D.L. et al. – Cancer mortality risk among workers at the Mayak nuclear complex – Radiation Research, 2003; 159 (6): 787-798
  3. ^ Ran X.Z. et al. – Effects of combined radiation and thermal burn injury on the survival of skin allograft and immune function in – Chinese Medical Journal, 1998; 111 (7): 634-637
  4. ^ Randall K. et al. – The effect of whole-body gamma-irradiation on localized beta-irradiation-induced skin reactions in mice – International Journal of Radiation Biology, 1992; 62 (6): 729-733.
  5. ^ Rothkamm K. – Evidence for a lack of DNA double-strand break repair in human cells exposed to very low x-ray doses – Proceedings of the National Academy of Science of the USA, 2003; 100 (9) : 5057-5062.
  6. ^ Department for Environment, Food and Rural Affairs (Defra) UK – Keys facts about radioactivity – 2003, http://www.defra.gov.uk/environment/statistics/radioact/kf/rakf03.htm
  7. ^ United Nations Scientific Committee on the Effects of Atomic Radiation Annex E: Medical radiation exposures – Sources and Effects of Ionizing – 1993, p. 249, New York, UN
  8. ^ US National Council on Radiation Protection and Measurements – NCRP Report No. 93 – pp 53-55, 1987. Bethesda, Maryland, USA, NCRP
  9. ^ IARC – Cancer risk following low doses of ionising radiation - a 15 country study – http://www.iarc.fr/ENG/Units/RCAa1.html

See also

External links

The Electromagnetic Spectrum
(Sorted by wavelength, short to long)
Gamma ray • X-ray • Ultraviolet • Visible spectrum • Infrared • Terahertz radiation • Microwave • Radio waves
Visible (optical) spectrum Violet • Blue • Green • Yellow • Orange • Red
Microwave spectrum W band • V band • Ka band • K band • Ku band • X band • C band • S band • L band
Radio spectrum EHF • SHF • UHF • VHF • HF • MF • LF • VLF • ULF • SLF • ELF
Wavelength designations Microwave • Shortwave • Mediumwave • Longwave
Nuclear processes
Radioactive decay Alpha decay · Beta decay · Gamma radiation · Cluster decay · Double beta decay · Double electron capture · Internal conversion · Isomeric transition · Spontaneous fission
Other processes Emission processes: Neutron emission · Positron emission · Proton emission
Capturing: Electron capture · Neutron capture
Stellar nucleosynthesis pp-Chain · CNO cycle · α process · Triple-α · Carbon burning · Ne burning · O burning · Si burning · R-process · S-process · P-process · Rp-process

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

 
 

Join the WikiAnswers Q&A community. Post a question or answer questions about "gamma ray" at WikiAnswers.

 

Copyrights:

Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved.  Read more
US Military Dictionary. The Oxford Essential Dictionary of the U.S. Military. Copyright © 2001, 2002 by Oxford University Press, Inc. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Electronics Dictionary. Copyright 2001 by Twysted Pair. All rights reserved.  Read more
Cosmic Lexicon. Copyright 1996 Planetary Science Research Discoveries.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gamma ray".  Read more

Search for answers directly from your browser with the FREE Answers.com Toolbar!  
Click here to download now. 

Get Answers your way! Check out all our free tools and products.

On this page:   E-mail   print Print  Link  

 

Keep Reading

Mentioned In:

 |  More >More >