Gamma ray, also known as gamma radiation, is a form of electromagnetic radiation with a high frequency and very short wavelength. Gamma rays are the most energetic form of electromagnetic radiation, and are produced by the most energetic events in the universe, such as supernova explosions and black hole accretion discs.
Gamma rays were first discovered in 1900 by Paul Villard, who was studying radioactivity. In 1932, gamma rays were identified as a distinct type of electromagnetic radiation by Carl David Anderson while he was studying cosmic rays.
Gamma rays are emitted by all nuclei withatomic numbers greater than 83. The emission of gamma rays from nuclear decay is called gamma decay. When a nucleus emits a gamma ray, it transitions to a lower energy state (decays) and often changes its atomic number (undergoes radioactive decay).
The most common process that produces gamma rays is electron-positron annihilation, where an electron collides with its antiparticle (a positron) and they both annihilate each other, producing two photons of gamma ray energy. This process occurs frequently in Nature: when matter falls into a black hole (due to the high gravitational field), electrons and positrons are constantly being created and destroyed near the event horizon; when stars explode as supernovae; during active periods of some types of pulsars; etc.
The Earth’s atmosphere absorbs almost all incoming gammarays so we do not receive any natural backgroundgamma radiation on the ground except for a few veryweak sources like radon gas. However, space is filledwith a diffuse flux of gammarays coming from allover the sky that has been dubbed “the isotropicgamma-ray background” or IGRB