Active Gamma Spectroscopy with NMIS

The Nuclear Materials Identification System (NMIS) is used to identify materials by measuring neutron transmission and scattering with the associated particle fast-neutron imaging technique. The fieldable version of this system (FNMIS), being developed by the U.S. Office of Nuclear Verification, will eventually incorporate gamma-ray spectrometry. Many materials, such as aluminum and polyethylene, have very similar neutron transmission properties and thus cannot be distinguished by neutron transmission measurements alone. Differentiation between these materials, in some cases, can be achieved by measuring gamma rays resulting from nonelastic interactions involving 14.1 MeV neutrons. These measurements demonstrate the feasibility of using active gamma spectroscopy measurements with a deuterium–tritium (DT) generator to aid in materials identification. These measurements allowed differentiation between samples of aluminum and polyethylene and graphite by their measured gamma spectra. MCNP-PoliMi simulations of identical configurations agree with these measurements. The presence of nitrogen in a sample of G-10 epoxy was also detected. Full implementation of active gamma spectroscopy in the NMIS system will enable the identification of a larger range of materials, including explosives and possibly drugs. This work is part of a broader study (bachelor’s thesis project) to evaluate reactions that supplement transmission imaging for material identification.