Ultradipole photon production in 40 and 50 MeV α-nucleus collisions

γ-ray yields in the energy range ∼5--35 MeV have been measured in the reactions α+197Au and 159Tb at Eα=40 MeV and α+197Au, 116Sn, natAg, and 59Co at Eα=50 MeV at θγ=90 degree. Angular distribution measurements have been carried out for the heaviest and the lightest target at Eα=50 MeV. A reasonably good agreement with the statistical model calculations is obtained for photon energies up to the giant dipole resonance region. A nonstatistical component is observed at higher γ-ray energies for all the systems studied. An attempt to understand the production mechanism of the nonstatistical part is made in terms of the potential bremsstrahlung and the incoherent nucleon-nucleon bremsstrahlung processes. While the calculations give the right order of magnitude of the measured cross sections for the highest γ-ray energies, they fail to reproduce the measured angular distributions and the cross sections in the entire γ-ray energy range