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±0.6×${10}^{\mathrm{-}6}$ photons with $E_{\gamma }$>25 MeV was measured per incident beam particle. The direct yield of $e^{+}$ $e^{\mathrm{-}}$ pairs is less than 2% of the gamma yield.