Fast neutron resonance radiography for elemental imaging: theory and applications

Fast neutron resonance radiography (NRR) has been devised as an elemental imaging method with applications such as contraband detection and mineral analysis. In NRR, a two-dimensional (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 and chosen to cover the resonance cross-section features of one or more elements. Images are formed using a lens-coupled plastic scintillator charged coupled device (CCD) combination. In preliminary experiments, we have produced NRR images of various simulants using a variable energy beam based on the Li(p,n)Be reaction and a variable energy proton beam. As an alternative to this method, we have studied NRR imaging using the D-D reaction, d(d,He)n, at fixed incident D energy and scanning through various neutron energies by using the angular variation in neutron energy. The object and detector rotate together around the neutron source; different energy (2-6 MeV) neutrons can be obtained at different angles from the target. The radiographic transmission 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) then form a set of linear equations, which can then be solved to map individual elemental contents.