, all of these explosive types contain a relatively high amount of nitrogen ranging from nine to thirty five percent by weight. The nominal density of these explosives is typically 1.6 g/cm3, with ranges between 1 to 2 g/cm3 or more. These physical properties demonstrate that the most unique signature of explosives is the high concentration and density of the nitrogen content. Also the presence of other elements in combination with the presence of nitrogen can be considered. Physical factors may also help identify explosives. One physical factor, for example, is a minimum propagation thickness or diameter in order for most explosives to be effective. This minimum propagation thickness requires a sizeable contiguous body of explosives in the other two dimensions. This information is useful to the detection of explosives without making a specific assumption of the actual shape of the explosive.
It can be seen, therefore, that a nuclear detection technique can provide for the detection of the nitrogen content, which can provide an indication as to the presence of an explosive. However, the frequent occurrence of nitrogen in non-explosive materials limits the level of detection sensitivity and merely detecting the presence of absence of nitrogen alone is generally not sufficient. Therefore, additional information is required beyond simply sensing the presence of the nitrogen. The present invention provides for this additional information.
The present invention provides a nuclear-based detection system and method that efficiently detects explosives in checked airline baggage or other parcels with a high probability of detection (PD) and a low probability of false alarms (PFA). The detection system advantageously combines several detection criteria to improve the speed and performance with which the detection system is able to discriminate between objects (parcels, suitcases, packages, etc.) with explosives and objects without explosives.
The first and foremost criteria used by the present invention to detect explosives is to ascertain a high content and concentration of nitrogen in the object under examination. The content and concentration of other elements in combination with nitrogen is also selectively used. To this end, the present invention uses thermal neutron activation (TNA) to immerse the object in a bath of thermal neutrons, which neutrons interact with the atomic nuclei causing prompt gamma rays to be emitted. If nitrogen is present, the gamma rays have a unique energy (10.8 MeV). If other prescribed elements are present, such as hydrogen and chlorine, the gamma rays have other known energies. The presence of such nitrogen-specific (or other element-specific or combinations of element-specific) gamma rays thus comprises one "signature" of an explosive. A main function of the present invention is thus to detect such nitrogen-specific (or other element-specific) gamma rays.
In order to achieve the highest possible sensitivity to nitrogen, the detection scheme employed by the invention provides the highest possible count-rate of gamma rays due to nitrogen with as low a background (of gamma rays not due to nitrogen) as possible. This is accomplished by bathing the object under examination in thermal neutrons and utilizing a properly designed detector array that surrounds the object as it is submersed in the thermal neutron bath, and appropriate signal processing techniques to perform the gamma ray counts. The 10.8 Mev gamma ray count-rate from each detector within the array is then combined with a suitable reconstruction algorithm to yield a rough two and/or three-dimensional image of the nitrogen bearing material within the object. This rough image helps identify explosives because it provides a coarse measure of the distribution and level of nitrogen density. (This density measure is inherently coarse due to the practical sizes of the gamma detectors.) The invention described in applicant's first application, Ser. No. 07/053,950, was the first known disclosure of a TNA based system that advantageously provided both 