On the Search for Nuclear Resonance Fluorescence Signatures of $^{235}{\rm U}$ and $^{238}{\rm U}$ Above 3 MeV

Nuclear resonance fluorescence is a physical process that provides an isotope-specific signature that could be used for the identification and characterization of materials. The technique involves the detection of prompt discrete-energy photons emitted from a sample that is exposed to MeV-energy photons. Potential applications of the technique range from detection of high explosives to characterization of special nuclear materials such as 235 U. We conducted a pair of measurements to search for a nuclear resonance fluorescence response of 235 U above 3 MeV and of 238 U above 5 MeV using an 8 g sample of highly enriched uranium and a 90 g sample of depleted uranium. No new signatures were observed. The minimum detectable integrated cross section for 235 U varies from 4 eV b at 3 MeV up to 120 eV b at 8 MeV.