# Gamma ray

This article describes the electromagnetic radiative phenomena of Gamma rays, for other uses of the word "Gamma ray", see Gamma ray (disambiguation)
Nuclear processes

Gamma rays or gamma-ray photons (denoted as γ) are forms of electromagnetic radiation or light emissions of a specific frequency produced from sub-atomic particle interaction, such as electron-positron annihilation and radioactive decay; most usually generated from nuclear reactions occurring within the interstellar medium of space.

They are often characterised as being light with the highest frequency and energy within the light spectrum. Due to their high energy content, they are able to cause serious damage when absorbed by living cells. Gamma rays are also able to penetrate dense materials.

##  History

Gamma rays were discovered by the French chemist and physicist, Paul Ulrich Villard in 1900 while he was studying uranium. Working in the chemistry department of the École Normale in rue d'Ulm, Paris with self-constructed equipment, he found that the rays were not bent by a magnetic field.

For a time, it was assumed that gamma rays were particles. The fact that they could be described as rays was demonstrated by the British Physicist, William Henry Bragg in 1910 when he showed that the rays ionized gas in a similar way to X-rays.

In 1914, Ernest Rutherford and Edward Andrade showed that gamma rays were a form of electromagnetic radiation by measuring their wavelengths using crystal diffraction. The wavelengths are similar to those of X-rays and are very short, in the range 10-11m to 10-14m. It was Rutherford who coined the name 'gamma rays', after naming 'alpha' and 'beta' rays; the natures of the different rays were unknown at that time.

Gamma-ray astronomy did not develop until it was possible to get our detectors above all or most of the atmosphere, using balloons or spacecraft. The first gamma-ray telescope, carried into orbit on the Explorer XI satellite in 1961, picked up fewer than 100 cosmic gamma-ray photons. Perhaps the most spectacular discovery in gamma-ray astronomy came in the late 1960s and early 1970s. Detectors on board the Vela satellite series, originally military satellites, began to record bursts of these rays, not from Earth, but from deep space. <ref>NASA EM spectrum infopage – http://imagers.gsfc.nasa.gov/ems/gamma.html</ref>

## Properties

### Shielding

Shielding for γ 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

Image:Gamma Abs Al.png
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, Compton effect dominates.
Image:Gamma Abs Pb.png
The total absorption coefficient of lead (atomic number 82) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. Here, 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 imersed 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 in from the sides reduces the absorption.

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.

Image:Cobalt 60.png
Decay schema 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 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:Egret all sky gamma ray map from CGRO spacecraft.gif
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 away energy, the beta spectrum does not have sharp lines, but instead it 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 Frank Condon effects seen in optical spectroscopy.

##  Uses

The powerful nature of gamma rays have made them useful in the sterilization of medical equipment by killing bacteria. They are also used to kill bacteria and insects in foodstuffs, particularly meat, marshmallows, pie, eggs, and vegetables, to maintain freshness.

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.

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 minimising damage to the surrounding tissues.

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.

###  In fiction

• Exposure to gamma rays transformed the scientist Bruce Banner into the Incredible Hulk in the Marvel comic of the same name; many of the Hulk's villains and allies also attained their superpowers through this method.
• In both Gundam Seed and Seed Destiny gamma ray technology is incorporated in the space cannon G.E.N.E.S.I.S.
• In David Weber's Honorverse, grasers are powerful gamma-radiation-powered energy weapons.
• Metroids, creatures in the popular series of the same name, go through a large metamorphosis when exposed to gamma-radiation.
• EVE Online, a space-based MMORPG, has a group of weapons technology that uses various EM radiations as lasers, some of which are gamma lasers.

##  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

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##  External Links

 Visible (optical) spectrum: The Electromagnetic Spectrum(Sorted by wavelength, short to long) Gamma ray | X-ray | Ultraviolet | Visible spectrum | Infrared | Terahertz radiation | Microwave | Radio waves Violet | Blue | Green | Yellow | Orange | Red W band | V band | K band: Ka band, Ku band | X band | C band | S band | L band EHF | SHF | UHF | VHF | HF | MF | LF | VLF | ULF | SLF | ELF Microwave | Shortwave | Mediumwave | Longwave
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