Stand-off detection of special nuclear materials using neutron imaging methods

Neutrons can travel considerable distances through the air, and can be used for stand-off detection of special nuclear materials. Plutonium emits neutrons by spontaneous fission, while uranium can be induced to fission by active interrogation with either energetic photons or neutrons. Traditional neutron detectors consisting of ³He tubes embedded in polyethylene moderator do not record the directions of incident neutrons. Their efficiency is usually less than 10% for fission spectrum neutrons. We have developed two neutron imaging methods, one for fast neutrons and one for thermal neutrons. The fast neutron directional detector is based on double proton recoil in two layers of plastic scintillators consisting of arrays of paddles with photomultipliers at both ends. This arrangement provides relatively smooth spatial uniformity and allows neutrons to be distinguished from background gammas and muons using time of flight. The thermal neutron imager is a coded aperture camera based on a ³He wire chamber. Neutrons that are thermalized by materials close to the source have a mean free path in air of about 20 meters, and can be imaged at distances up to 60 meters. In this case, no moderation takes place at the detector, and the quantum efficiency of the ³He wire chamber is about 60%. Background neutrons generated by cosmic rays arrive from all directions at a low rate, whereas a manmade source is likely to produce a bright spot in an image. Various methods of image enhancement can be applied, such as auto filtering, but the practical limitation stems from the requirement that the real source must stand out against artifacts in the image.