Abstract
Special nuclear materials (SNM) hidden in shipping containers are extremely difficult to detect through their faint radioactive signature. Therefore, R&D efforts concentrate on active interrogation (AI) techniques, using external beams of neutrons or high-energy X-rays to trigger fission reactions. Our team has created a complete AI system based on an ultracompact linear accelerator (LINAC) and on detectors developed in collaboration between the universities of Pisa and Yale. The detectors contain liquid droplets that vaporize when exposed to fast neutrons but are insensitive to X-rays. Their signal can be recorded and then immediately reset. Our 9 MeV electron LINAC, based on the well-established technology developed by S.I.T. S.p.A. for intraoperative radiotherapy (IORT), 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 photoneutrons. This design minimizes power requirements and the mass of shielding needed for a safe operation. The entire apparatus is so light and compact that its implementation appears possible in fixed and mobile (terrestrial, maritime, and even aerial) interrogation systems. In our first tests, we detected depleted uranium while excluding significant production of contaminant photoneutrons.
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