Detection of special nuclear material with a transportable active interrogation system

Special nuclear materials hidden in shipping containers are extremely difficult to detect through 12 their faint spontaneous emission of neutrons and photons. R&D efforts focus on active interrogation (AI) 13 techniques, employing external beams of neutrons or high-energy X-rays to first trigger fission reactions and 14 then detect prompt or delayed neutrons and/or photons. Our group created a complete active interrogation 15 system based on detectors developed by the universities of Pisa and Yale and on an ultra-compact linear 16 accelerator (LINAC). The detectors contain liquid droplets that vaporize when exposed to fast neutrons 17 but are insensitive to X-rays. The X-ray generator is based on 9MeV electron LINAC developed by 18 S.I.T. Sordina S.p.A. for intraoperative radiotherapy. The latter is a standing-wave design that does not 19 require external solenoids for electron radial focusing. Copper is used both as X-ray production target and 20 as collimator, which prevents the production of photo-neutrons. In our first tests, we detected depleted 21 uranium, while excluding significant production of contaminant photo-neutrons.