Large area imaging detector for long-range, passive detection of fissile material

Recent events highlight the increased risk of a terrorist attack using either a nuclear or a radiological weapon. One of the key needs to counter such a threat is long-range detection of nuclear material. Theoretically, gamma-ray emissions from such material should allow passive detection to distances greater than 100 m. However, detection at this range has long been thought impractical due to fluctuating levels of natural background radiation. These fluctuations are the major source of uncertainty in detection and mean that sensitivity cannot be increased simply by increasing detector size. Recent work has shown that this problem can be overcome through the use of imaging techniques. In this paper we describe the background problems, the advantages of imaging and the construction of a prototype, large-area (0.57 m/sup 2/) gamma-ray imager to detect nuclear materials at distances of /spl sim/100 m.