The nuclear car wash: A system to detect nuclear weapons in commercial cargo shipments

A concept for detecting the presence of special nuclear material (235U or 239Pu) concealed in intermodal cargo containers has been developed, studied, and recent performance results are described. It is based on interrogation with a pulsed beam of 3–7 MeV neutrons that produce fission events and subsequent detection of their β-delayed neutron emission or β-delayed high-energy γ-radiation reveals the presence of fissionable material. Fission product β-delayed γ-rays above 3 MeV are nearly 10 times more abundant than β-delayed neutrons and are distinct from natural radioactivity and from nearly all of the induced activity in a normal cargo. Detector backgrounds and potential interferences with the fission signature radiation have been identified and quantified. Their impact on detection sensitivity is relatively minor and can be addressed readily. Components of a simple laboratory prototype have been assembled, tested with the most challenging cargo threat scenarios, and results compared to computer simulations. Preliminary results will be presented.