Neutron resonance radiography for security applications

Abstract

Fast Neutron Resonance Radiography (NRR) has been devised as an elemental imaging method, with applications such as contraband detection and mineral analysis. In the NRR method, a 2-D elemental mapping of hydrogen, carbon, nitrogen, oxygen and the sum of other elements is obtained from fast neutron radiographic images taken at different neutron energies chosen to cover the resonance cross section features of one or more elements. Images are formed using a lens-coupled plastic scintillator-CCD combination. In preliminary experiments, we have produced NRR images of various simulants using a variable energy neutron beam based on the Li(p,n)Be reaction and a variable energy proton beam. In order to overcome practical limitations to this method, we have studied NRR imaging using the D-D reaction at a fixed incident D energy and scanning through various neutron energies by using the angular variation in neutron energy. The object-detector assembly rotates around the neutron source and different energy (2-6 MeV) neutrons can be obtained at different angles from a D-D neutron source. The radiographic image provides a 2-D mapping of the sum of elemental contents (weighted by the attenuation coefficients). Transmission measurements taken at different neutron energies (angles) form a set of linear equations, which can then be solved to map individual elemental contents.