High energy gamma rays in intermediate energy nucleus-nucleus collisions

During an experiment to measure charged pion production for 14N + Cu collisions at E/A = 40 MeV using an Enge split-pole magnetic spectrograph, a much larger than expected background of high energy electrons and positrons was observed. By comparison of the target-in to target-out spectra, it was deduced that most of the leptons were produced by gamma rays which were produced by gamma rays which were converted to electron-positron pairs in the material of the entrance aperture of the spectrograph. An experiment was constructed and carried out to determine the photon, electron, and positron yield. The beam was 14N accelerated by the National Superconducting Cyclotron Laboratory K500 cyclotron to an energy E/A = 40 MeV. Use of three converter elements with very different conversion efficiencies permitted the separation of the yield of electrons and positrons due to the pair conversion of gamma rays from that originating in the collision. The direct yield of e+e- pairs is less than 2% of the gamma yield. An improved experiment using a telescope of active converter backed by a stack of Cherenkov counters was then used to measure the high energy gamma rays for 14N + Pb, Zn, and C at E/A = 40 MeV, and 14N + Pb at E/A = 30 MeV. The cross sections are nearly isotropic. For gamma rays of energy E/sub γ/ > 20 MeV, σ/sub 14N + Pb/ = 0.3 mb. The gamma ray cross section is compared to the neutral pion cross section allowing for the difference in mass and phase space; the two appear quite similar. Several theoretical models are compared to the data