Detection of special nuclear material with a transportable active interrogation system

.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 (AI) 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 a complete active interrogation system based on detectors developed by the universities of Pisa and Yale and on an ultra-compact linear accelerator (LINAC). The detectors contain liquid droplets that vaporize when exposed to fast neutrons but are insensitive to X-rays. The X-ray generator is based on 9MeV electron LINAC developed by S.I.T. Sordina S.p.A. for intraoperative radiotherapy. The latter is a standing-wave design that does not require external solenoids for electron radial focusing. Copper is used both as X-ray production target and as collimator, which prevents the production of photo-neutrons. In our first tests, we detected depleted uranium, while excluding significant production of contaminant photo-neutrons.