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I've already learnt alpha and beta emissions but I really don't understand gamma emission. How come an element come to the excited state and is it only the energy which is emitted by the element, which is called "gamma" radiation?!

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  • $\begingroup$ Gamma radiation is defined as a particular (very high frequency) portion of the electromagnetic spectrum. As far as an element becoming in an excited nuclear state, if it only emits gamma, its probably because the transition from the excited state is a higher order one (rather than a simple dipole transition), thus it can have a significant half life. $\endgroup$
    – R. Rankin
    Jul 14, 2017 at 2:57

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Gamma ray (also called gamma radiation), denoted by the lower-case Greek letter gamma (γ or $ \gamma $), 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.

Gamma rays typically have frequencies above 10 exahertz (or >1019 Hz), and therefore have energies above 100 keV and wavelengths less than 10 picometers (10−11 m), which is less than the diameter of an atom.However, this is not a strict definition, but rather only a rule-of-thumb description for natural processes. Electromagnetic radiation from radioactive decay of atomic nuclei is referred to as "gamma rays" no matter its energy, so that there is no lower limit to gamma energy derived from radioactive decay. This radiation commonly has energy of a few hundred keV, and almost always less than 10 MeV. In astronomy, gamma rays are defined by their energy, and no production process needs to be specified. The energies of gamma rays from astronomical sources range to over 10 TeV, an energy far too large to result from radioactive decay. A notable example is the extremely powerful bursts of high-energy radiation referred to as long duration gamma-ray bursts, of energies higher than can be produced by radioactive decay. These bursts of gamma rays are thought to be due to the collapse of stars called hypernovae.

In short, when alpha and beta particles are emitted, then the nucleus may hold excess energy. This energy is given off from the excited nucleus as gamma rays.

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    $\begingroup$ I have added the link and quote highlight. imo it is ok to quote from wiki, to repeat well known stufff in own words is tiring, but the link should be given. $\endgroup$
    – anna v
    Jul 14, 2017 at 4:45

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