Advancements in Active Photon Interrogation through Photo-neutron Spectrometry: Unveiling Nitrogen Signatures for Illicit Materials Detection

Active photon interrogation (API) has been used for a long time in the detection of special nuclear materials and the characterization of nuclear waste, primarily relying on neutron counting from photo-fission reactions. However, recent advancements have opened new ways for extending the capabilities of API techniques. In this study, we introduce a novel approach aimed at broadening the scope of API by incorporating neutron spectrometry with an organic scintillator to unveil the signatures of light elements like nitrogen, oxygen, and carbon. These elements are commonly associated with conventional explosives, narcotics, and chemical weapons and can be identified through their photo-neutron spectra. This endeavor presents a significant challenge due to the intense and pulsed nature of the photon flux when using electron LINACs as interrogation sources. Our method entailed the utilization of a 22-MV electron linear accelerator (LINAC) in conjunction with a BC501A liquid scintillator, coupled with pulse shape discrimination and pile-up rejection algorithms. To assess the efficacy of our approach, we subjected to irradiation melamine powder (C3N6H6), a simulant for conventional explosives from the point of view of nitrogen content. The results of our investigation are very promising. We successfully detected fast neutrons originating from (γ,Xn) reactions, despite mixed short-pulse fields of photon and neutron radiation characterized by intense photon flashes. Moreover, we achieved the unfolding of energy deposited in the detector, enabling the extraction of photo-neutron spectra that highlight distinct nitrogen signatures from (γ,pn) reactions. This pioneering work represents the first experimental proof-of-concept for leveraging photo-neutron spectrometry within API techniques, offering a promising alternative for the enhanced detection of illicit materials.