Photon-based Imaging in Active Interrogation of Special Nuclear Material

In cargo scanning for special nuclear materials, beam source and detector response influence output image quality, which ultimately determines whether special nuclear material (SNM) can be detected. While bremsstrahlung beams are industry standard, the spectrum is continuous and highly biased towards low energies resulting in low penetration capabilities and increased scan time and dose to ensure adequate detection statistics and image quality. Use of monoenergetic interrogation beams could lead to decreased dose and scan time and improved image quality and material determination. Low-energy nuclear reactions result in quasi-monoenergetic beams, for example ¹¹B(d,n-γ)¹²C produces prominent gamma peaks at 4.4 and 15.1 MeV. Inverse Compton Scattering (ICS) is another technique which produces quasi-monoenergetic photons and has the advantage of being tunable, allowing the user to select the beam energy. In this presentation, we discuss a novel active interrogation system leveraging highly penetrating, discrete energy photon sources and custom designed Cherenkov detectors.  The investigation starts with a detailed investigation of the ¹¹B(d,n-γ)¹²C source to characterize the exact energies of the observed gamma rays and probable origins.  Once the source is understood, we are able to employ an array of custom-designed Cherenkov detectors to measure the differential attenuation of the most prominent reaction products and relate the relative transmission of the two main energies to an approximate Z_eff.  We demonstrate imaging using the source – detector combination to evaluate the spatial resolution and material discrimination capabilities of this novel active interrogation system. Finally, we compare performance of the monoenergetic photon imaging systems with conventional bremsstrahlung interrogation based on image quality and dose due to primary interrogating photons as well as secondary radiation.