Patent Number: 051620960
Section: abstract

A detector structure adjacent an opening in a cavity structure for detecting explosives in objects within such opening including a plurality of thin nonchlorinated hydrogenous wall members forming a passageway to define such opening. A source of high energy neutrons located adjacent the passageway to direct neutrons into the passageway. A premoderator formed by nonchlorinated hydrocarbon material surrounding the source of high energy neutrons to reduce the energy in at least a portion of the neutrons. A first moderator formed by heavy water surrounding the source of neutrons and the premoderator to reduce the energy in at least a further portion of the neutrons. A second moderator formed by carbonaceous material at least partially surrounding the plurality of thin hydrogenous wall members and the first moderator to reduce the energy in at least an additional portion of the neutrons and reflect back the additional portion of neutrons into the opening. The plurality of thin hydrogenous wall members additionally forming a thin moderator to additionally slow down high energy neutrons within the opening. The combination of the premoderator and the first, second and third moderators moderating the neutrons produced by the neutron source to form a spectrum of neutrons in the opening with a peak value in the thermal energy portion of the spectrum to maximize the detection of nitrogen by interacting the thermal neutrons with the nitrogen to produce gamma rays for detection by the detector structure.