Highly-energetic, short-wave electromagnetic radiation emitted from the nucleus of an atom. Gamma radiation energies usually range between 0.01 and 10 MeV. X-rays also occur within this energy range; they originate, however, not from the nucleus, but are generated by electron transfers in the electron sheath or by electron deceleration in matter (bremsstrahlung). In general, alpha and beta decays and always the fission process are accompanied by gamma radiation. Gamma rays are extremely penetrative and may best be weakened by material of high density (lead) and high atomic number.