Simulation of an active interrogation system for the interdiction of special nuclear materials

Special nuclear materials hidden in shipping containers are extremely difficult to detect through their faint spontaneous emission of neutrons and photons. R&D efforts focus on active interrogation techniques, employing external beams of neutrons or high-energy X-rays to first trigger fission reactions and then detect prompt or delayed neutrons and/or photons. Our group created an active interrogation system based on detectors developed by Yale University and the University of Pisa. These detectors contain liquid droplets that vaporize when exposed to fast neutrons, but are insensitive to X-rays. The system was tested with an X-ray generator based on a 9 MeV electron LINAC available at an active interrogation facility. Copper is used as an X-ray production target at this facility, which prevents the production of photo-neutrons. With this system, we detected a sample of natural uranium either uncovered or shielded under heavy loads of wood or steel pipes. In order to interpret the experimental results, the response of our detector systems was assessed using Monte Carlo simulations with the code PHITS. Computational results are in good agreement with the experimental ones and open the way to simulations of real-world scenarios of interest to nonproliferation and homeland security, namely active interrogation at a standoff.