Electromagnetic radiation with a wavelength shorter than 10 picometres (10-11 m), i.e. a frequency higher than 30 exahertz (3.1019 Hz). This type of radiation is emitted by radioactive nuclei when they undergo de-excitation. Electron-positron annihilation also produces two gamma photons. This phenomenon is exploited in medical imaging (positron emission tomography, PET). Gamma rays are detected in cosmic radiation resulting from sources in which very energetic phenomena have occurred.
