Fissile Isotope Detection as a Complement to Zero Knowledge Neutron Radiography

High energy neutron transmission radiography is not sensitive to the isotopic composition of uranium or plutonium. However, the production of fission neutrons, even from 14 MeV neutrons, is isotope dependent. We have begun experiments to demonstrate that this is detectable, using a modification of the EXCALIBUR neutron collimator/moderator designed in collaboration with Pacific Northwest National Laboratory. The motivation for the modification was that the measurement requires placing a detection system to the side of the imaging neutron beam axis, looking at the test object along a line perpendicular to that axis. The detectors must be well shielded from source neutrons scattering in the system. To achieve this, steel bars are placed such that the detectors sit in their shadow. Additional borated polyethylene is placed in the space around the steel bars and the detectors themselves. We completed the setup of this system with a BTI N-Probe neutron spectrometer as a side detector. We conducted initial experiments with a 14 MeV neutron beam impinging on depleted uranium, stainless steel, aluminum, lead and polyethylene. We also tested the same materials with neutrons moderated to below 1 MeV. We found that depleted uranium had the highest signal in the energy range of fission neutrons, in good agreement with MCNP calculations. Based on the MCNP calculation, the difference between DU and HALEU is greater with moderated source neutrons than 14 MeV neutrons. We plan to continue tests including the use of 16% enriched HALEU metallic blocks.