Fine structure in the 208Pb photoneutron cross section between 9.9 and 11.2 MeV

The partial differential photoneutron cross sections for 208Pb were measured in the energy region from 9.9 to 11.2 MeV with an energy resolution of about 100 keV. Neutrons leaving 207Pb in its ground state and first two excited states were resolved, and angular distributions were determined from measurements at seven angles from 450 to 1350. These data provide more information about the fine structure observed in earlier, poorer resolution measurements of the total photoneutron cross section and strongly support the interpretation of this structure as being due to electric dipole excitations. The measured partial cross sections provide a sensitive test for models attempting to describe the photon interaction cross section and the coupling of the excited states to the continuum. Calculations using a statistical model with transmission coefficients obtained from a conventional optical model are unable to explain even the average behavior of the branching ratios with energy. The measured angular distributions show clear evidence for interference of either E2 or M1 processes with the predominant E1 interaction. The interference terms have a magnitude approximately equal to that predicted by a simple direct-semidirect calculation including the electric dipole and isoscalar electric quadrupole giant resonance. However, this agreement is puzzling because the average of interferences between the many compound nuclear levels in this region of excitation would be expected to reduce anisotropies about 900 toward zero