Importance of Elastic, Non-Resonant Scattering in the Assessment of NRF for NDA

Measurements of the background in the energy range of interest to NRF were performed for depleted uranium (DU) targets to benchmark MCNPX simulation background spectra. The targets were irradiated with a bremsstrahlung photon beam and spectra were recorded with HPGe detectors placed at backward angles with respect to the direction of the interrogating photons. Results show that the contributions of nuclear elastic scatter to the total spectra are larger than represented by the ENDF coherent scattering cross sections alone. The addition of nuclear elastic scatter is needed, either by analytical means or by alteration of the transport codes, in order to properly model the expected elastic scatter background in the analysis of NRF systems for NDA.