Observation of high energy gamma rays in intermediate energy nucleus-nucleus collisions

High energy electrons and positrons observed in medium energy nucleus-nucleus collisions are shown to be primarily due to the external conversion of high energy gamma rays. The reaction $^{14}\mathrm{N}$+Cu was studied at E/A=40 MeV, and a magnetic spectrograph was used with a specially constructed multiwire proportional counter plus Cerenkov detectors. A stopping target yield of 1.8\ifmmode\pm\else\textpm\fi{}0.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ photons with ${E}_{\ensuremath{\gamma}}$>25 MeV was measured per incident beam particle. The direct yield of ${e}^{+}$${e}^{\mathrm{\ensuremath{-}}}$ pairs is less than 2% of the gamma yield.