Ground state photoneutron angular distributions in the giant resonance of 16O

Photoneutron spectra for transitions to the ground state in the giant resonance of 16O were measured with high neutron energy resolution (ΔE ~ 60 keV) at angles of 50, 66, 82, 98, 114, 130, and 146 degrees relative to the direction of the incident bremsstrahlung beam. The spectra at the various angles were measured during successive experimental runs using a single 50 m time-of-flight spectrometer. The different angles were obtained by bending the electron beam relative to the direction of the flight path. Angular normalization was achieved by using a fast neutron monitor located at the end of a short flight path which was always normal to the reaction plane. Legendre coefficients for the angular distributions were extracted as functions of the excitation energy. From the energy dependence of the coefficients, interference effects are evident in the ground state (γ,n) cross section in the region from 17 to 30 MeV.