Photoneutron emission process on nuclei around A = 200 for giant dipole resonance energies based on neutron energy and angular distribution

Photoneutron emission processes in nuclei at approximately A = 200, including 181Ta, natW, 197Au, natPb, and 209Bi, were identified based on experimental data of the photoneutron energy and angular distribution measured at a photon energy of 12.7 MeV, in addition to previously obtained data at 16.6 MeV. Based on these data, parameters describing the angular distributions obtained as a function of neutron energy for these targets were used to characterize the neutron emission processes. The parameters for high-energy photoneutrons are consistent with those of the resonance direct process related to the outermost nuclear shell, as proposed by Wilkinson in the mid-20th century. Conversely, parameters for low-energy photoneutrons, which exhibit an isotropic distribution, are indicative of the statistical decay of compound nuclei. For intermediate the energy range between these two extremes, an increasing angular dependence on the emitted photoneutron energy was observed for 197Au, natPb, and 209Bi, suggesting a combination of the resonance direct process, the statistical decay process, and the pre-equilibrium process. For 181Ta and natW, the parameters exhibiting a peak in the intermediate range imply the contribution of the resonance direct process from transitions of another inner shell to the photoneutron emission.