Gamma rays
Gamma ray (also called gamma radiation), denoted by the lower-case Greek letter gamma (γ or ), is penetrating electromagnetic radiation of a kind arising from the radioactive decay of atomic nuclei. It consists of photons in the highest observed range of photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays. Rutherford had previously discovered two other types of radioactive decay, which he named alpha and beta rays.
Gamma rays are able to ionize other atoms (ionizing radiation), and are thus biologically hazardous. The decay of an atomic nucleus from a high energy state to a lower energy state, a process called gamma decay, produces gamma radiation.
Natural sources of gamma rays on Earth are observed in the gamma decay of radionuclides and secondary radiation from atmospheric interactions with cosmic ray particles. There are rare terrestrial natural sources, such as lightning strikes and terrestrial gamma-ray flashes, that produce gamma rays not of a nuclear origin. Additionally, gamma rays are produced by a number of astronomical processes in which very high-energy electrons are produced, that in turn cause secondary gamma rays via bremsstrahlung, inverse Compton scattering, and synchrotron radiation. However, a large fraction of such astronomical gamma rays are screened by Earth's atmosphere and can only be detected by spacecraft. Gamma rays are produced by nuclear fusion in stars including the Sun (such as the CNO cycle), but are absorbed or inelastically scattered by the stellar material, reducing their energy, before escaping and are not observable from Earth as gamma rays.
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