Gamma ray

A gamma ray, also known as gamma radiation (symbol γ), is a penetrating form of electromagnetic radiation.

It comes from the radioactive decay of atomic nuclei. Gamma rays have the shortest wavelength electromagnetic waves. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium.

In 1903, Ernest Rutherford called this radiation "gamma rays" because they strongly penetrated matter. In 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel): alpha rays and beta rays in the order of penetrating power. Gamma rays penetrated matter even more strongly.

Gamma rays are ionizing radiation and are hazardous to life. They can damage bone marrow and internal organs. Unlike alpha and beta rays, they easily pass through the body and so they are a formidable radiation protection challenge. They need shielding made from lead or concrete. On Earth, the magnetosphere protects life from most types of lethal cosmic radiation other than gamma rays.

Gamma Ray Media

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  NASA guide to electromagnetic spectrum showing overlap of frequency between X-rays and gamma rays

• This animation tracks several gamma rays through space and time, from their emission in the jet of a distant blazar to their arrival in Fermi's Large Area Telescope (LAT).

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  Radioactive decay scheme of Template:SimpleNuclide

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  Gamma emission spectrum of cobalt-60

• The red dots show some of the ~500 terrestrial gamma-ray flashes daily detected by the Fermi Gamma-ray Space Telescope through 2010. Credit: NASA/Goddard Space Flight Center.

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  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.

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  A hypernova. Artist's illustration showing the life of a massive star as nuclear fusion converts lighter elements into heavier ones. When fusion no longer generates enough pressure to counteract gravity, the star rapidly collapses to form a black hole. Theoretically, energy may be released during the collapse along the axis of rotation to form a long duration gamma-ray burst.

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  Alpha radiation consists of helium nuclei and is readily stopped by a sheet of paper. Beta radiation, consisting of electrons or positrons, is stopped by an aluminium plate, but gamma radiation requires shielding by dense material such as lead or concrete.

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  The total absorption coefficient of aluminium (atomic number 13) for gamma rays, plotted versus gamma energy, and the contributions by the three effects. As is usual, the photoelectric effect is largest at low energies, Compton scattering dominates at intermediate energies, and pair production dominates at high energies.

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  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.