A high resolution photon tagging system in the GeV region

Abstract A photon tagging system which measures the energy of photons in a bremsstrahlung spectrum is described. The apparatus covers a photon energy range of 2 GeV starting 160 MeV below the energy of the incident electron beam. 192 counters provide a nominal resolution between 6 MeV for a photon energy 160 MeV below the incident electron energy and 11 MeV for a photon energy 2160 MeV below the incident electron energy.