Astroparticle Physics

6.3 Gamma Astronomy 109All parts of this spectrum have been used for astronomicalobservations. From large wavelengths (radio astronomy),the sub-optical range (infrared astronomy), the classical opticalastronomy, the ultraviolet astronomy, and X-ray astronomyone arrives finally at the gamma-ray astronomy.Gamma-ray astronomers are used to characterize gammaquanta not by their wavelength λ or frequency ν, butratherby their energy,E = hν. (6.57)Planck’s constant in practical units ish = 4.136 × 10 −21 MeV s . (6.58)The frequency ν is measured in Hz = 1/s. The wavelengthλ is obtained to beλ = c/ν , (6.59)where c is the speed of light in vacuum (c = 299 792 458m/s).In atomic and nuclear physics one distinguishes gammarays from X rays by the production mechanism. X rays areemitted in transitions of electrons in the atomic shell whilegamma rays are produced in transformations of the atomicnucleus. This distinction also results naturally in a classificationof X rays and gamma rays according to their energy.X rays typically have energies below 100 keV. Electromagneticradiation with energies in excess of 100 keV is called γrays. There is no upper limit for the energy of γ rays. Evencosmic γ rays with energies of 10 15 eV = 1 PeV have beenobserved.Fig. 6.31Spectral range of electromagneticradiationdistinctionbetween γ and X rays