Characterization of Shielded Special Nuclear Materials through Neutron Spectroscopy

The detection and characterization of shielded special nuclear material (SNM) presents an open challenge in the security and non-proliferation fields. SNM-emitted decay gamma rays exhibit low energies that can be easily shielded. Shielding materials can modify the shape and energy spectrum emitted by fissile or fissionable nuclides. In this work, we have used a deuterated trans-stilbene organic scintillator to detect fast neutrons emitted by a shielded plutonium source. We used Bayesian inference unfolding algorithms, associated with unmixing algorithms, to reconstruct the detected neutron spectrum from the detector light output response and identify the type of shielding materials, respectively. In order to achieve this, we measured shielded SNM spectra using a dueterated stilbene detector at the Device Assembly Facility in the Nevada National Security Site. We took measurements of the BeRP ball, a 4.5 kg sphere of alpha-phase plutonium encased in steel cladding, shielded by lead, polyethylene, and mixed configurations to test the efficacy of our unmixing algorithm. The superior spectroscopic capability of deuterated trans-stilbene, associated with advanced statistical algorithms, allowed us to identify the type of shielding material in all the cases accurately.