Qualitative measurement of spatial shielding isotopics via Compton imaging neutron-induced gamma rays using 3-D CdZnTe detectors

摘要

Characterization of shielding around special nuclear materials (SNMs) in unknown objects is important for homeland security, international safeguards and potential nuclear disarmament verification. Shielding modulates source photons and neutrons, complicating radiation-based SNM characterization, and indicates object purpose and guides response. Previous gamma-ray-based shielding detection algorithms only estimate effective shielding atomic number and areal thickness, complicating the characterization of compound shields comprised of multiple elements. Neutron-induced gamma rays, from inelastic scatter or capture, provide a complementary, isotope-specific shielding signal which can be localized using Compton imaging. Qualitative, angularly-resolved isotopic distributions in a compound shield were measured using polyethylene and polyvinyl chloride targets, excited via a single PuBe neutron source, and a 3-D, position-sensitive CdZnTe imaging spectrometer. Hydrogen capture gamma rays, which predominately originate from the polyethylene target, were well separated in angular space from chlorine capture gamma rays, originating in the polyvinyl chloride target, using Compton imaging. The described qualitative probe of spatial shielding isotopics can be immediately applied on any commercial gamma-ray imaging spectrometer without modification.