gamma ray, electromagnetic radiation of the shortest wavelength and highest energy.

Gamma rays are produced in the disintegration of radioactive atomic nuclei and in the decay of certain subatomic particles. The commonly accepted definitions of the gamma-ray and X-rayregions of the electromagnetic spectrum include some wavelength overlap, with gamma-ray radiation having wavelengths that are generally shorter than a few tenths of an angstrom (10−10metre) and gamma-ray photons having energies that are greater than tens of thousands of electron volts (eV). There is no theoretical upper limit to the energies of gamma-ray photons and no lower limit to gamma-ray wavelengths; observed energies presently extend up to a few trillion electron volts—these extremely high-energy photons are produced in astronomical sources through currently unidentified mechanisms.

The term gamma ray was coined by British physicist Ernest Rutherford in 1903 following early studies of the emissions of radioactive nuclei. Just as atoms have discrete energy levels associated with different configurations of the orbiting electrons, atomic nuclei have energy level structures determined by the configurations of the protons and neutrons that constitute the nuclei. While energy differences between atomic energy levels are typically in the 1- to 10-eV range, energy differences in nuclei usually fall in the 1-keV (thousand electron volts) to 10-MeV (million electron volts) range. When a nucleus makes a transition from a high-energy level to a lower-energy level, a photon is emitted to carry off the excess energy; nuclear energy-level differences correspond to photon wavelengths in the gamma-ray region.

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