Radioactive nuclei (radionuclides) commonly emit gamma rays in the energy range from a few keV to ~10 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. Such sources typically produce gamma-ray "line spectra" (i.e., many photons emitted at discrete energies), whereas much higher energies (upwards of 1 TeV) may occur in the continuum spectra observed in astrophysics and elementary particle physics. The difference between gamma rays and X-rays is somewhat blurred. Gamma rays arise from transitions between nuclear energy levels and are monoenergetic, whereas X-rays are either related to transitions between atomic energy levels (characteristic X rays, which are monoenergetic), or are electrically generated (X-ray tube, linear accelerator) and have a broad energy range.