Photonuclear reaction mechanisms at intermediate energies

The goal of the present experiment was to study the reaction mechanisms involved in the absorption of photons by nuclei in the tagged photon energy range E$gamma; = 60 to 140 MeV. For the photon energy of 64 MeV the 16O(ã, p)15Ng.s. reaction was measured. A description of the process in terms of a direct knockout formalism was inadequate to explain the magnitude of the measured cross section. The inclusion of two-body meson-exchange-currents in the calculation was found to give a much better fit to the data. Of particular interest for this experiment was to study the absorption of a photon on a correlated proton-neutron pair for the photon energy range Eã = 100 to 140 MeV. The two nucleon absorption mechanism is dominant in this photon energy range. The reaction studied was 16O(ã, pn)14N0,1,2... Having sufficient experimental energy resolution to determine the population to discrete states in the residual 14N nucleus provided information on the quantum numbers involved in the two nucleon absorption mechanism. Also the large continuum yield at excitation energies larger than 20 MeV compared to the yield below 20 MeV is seen as evidence that short range nucleon-nucleon correlations play a significant role for the two body reaction mechanism in the photon energy region studied