Moon glows brighter than sun in images from NASA's Fermi

If our eyes could see high-energy radiation called gamma rays, the Moon would appear brighter than the Sun! That's how NASA's Fermi Gamma-ray Space Telescope has seen our neighbor in space for the past decade.

The mechanism for gamma-ray bursts from space is decoded

Gamma-ray bursts, short and intense flashes of energetic radiation coming from outer space, are the brightest explosions in the universe. As gamma rays are blocked by the atmosphere, the bursts were discovered accidentally ...

Powering the extreme jets of active galaxies

An active galaxy nucleus (AGN) contains a supermassive black hole that is vigorously accreting material. It typically ejects jets of particles that move at close to the speed of light, radiating across many wavelengths, in ...

Researchers connect lightning with gamma-ray phenomena in clouds

University of Tokyo graduate student Yuuki Wada with colleagues from Japan have discovered a connection between lightning strikes and two kinds of gamma-ray phenomena in thunderclouds. The research suggests that in certain ...

Fermi mission reveals its highest-energy gamma-ray bursts

For 10 years, NASA's Fermi Gamma-ray Space Telescope has scanned the sky for gamma-ray bursts (GRBs), the universe's most luminous explosions. A new catalog of the highest-energy blasts provides scientists with fresh insights ...

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Gamma ray

Gamma rays (denoted as γ) are electromagnetic radiation of high energy. 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.

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."

Hard X-rays produced for by linear accelerators ("linacs") and astrophysical processes 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.

In the past, distinction between the X-rays and gamma rays was arbitrarily based on energy (or equivalently frequency or wavelength), but because of the wide overlap and increasing use of megavoltage X-ray sources, now the two types of radiation are 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.

As a form of ionizing radiation, gamma rays can cause serious damage when absorbed by living tissue, and they are therefore a health hazard.

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