etector, thus provides sufficient information to determine the amount of each element and its location within the material being examined. This determination, in turn, leads to an indication as to whether an explosive material is present within the object under examination.
Advantageously, an even more accurate count of the gamma rays emitted from nitrogen (i.e., gamma rays at 10.8 MeV) is made possible in accordance with the present invention by effectively removing the background noise from the gamma ray spectra at 10.8 MeV using the spectral correlation method (SCM) described herein.
The determination of whether an explosive material is present within the object may also be greatly facilitated, further in accordance with the teachings of the present invention, with the probability of detection (PD) being increased, while maintaining the probability of false alarm (PFA) at acceptably low levels, through the use of the ANS system. The ANS system efficiently processes in parallel the recorded data from all of the numerous detectors within the detector array. Further, the ANS system, unlike conventional statistical discriminate analysis (which may also be used to make the explosive/non-explosive decision), does not require complex and labor-intensive calibration procedures. Rather, by using an ANS, the detection system learns to detect explosive materials by simply being presented with representative examples of objects containing explosive materials. Such a system is not only brought up to speed much quicker than prior detection systems, but is also more adaptable to changing circumstances. For example, the system can in theory be used to detect materials other than explosive materials, e.g., contraband.
The present invention includes, in one embodiment, a specific arrangement and type of detectors, positioned relative a source of neutrons, to detect or count the number of gamma rays so as to provide the proper information needed to ascertain the presence and density distribution of nitrogen. The information may then be readily analyzed by the detection system to indicate the possible presence of an explosive threat. A high count rate of detected gamma rays of a particular energy, for example, immediately indicates the presence of a great deal of nitrogen. The ANS can then quickly determine with a high PD whether the presence of the nitrogen indicates the presence of an explosive material, or whether it indicates the presence of some other benign nitrogen-rich material. Advantageously, with the ANS, the system of the present invention is also capable of detecting explosives in unconventional configurations while maintaining the number of false alarms to a relatively low level. The prior art detector systems, in contrast, while providing for the gross detection of explosives, cannot provide for the more sensitive detection of the unconventional explosive configurations concurrent with a relatively low level of false alarms.
A preferred embodiment of the explosive detection system of the present invention includes the use of efficient inorganic scintillators capable of resolving closely spaced high energy gamma ray lines. Specifically, sodium iodide scintillators are used to provide for detection. It is to be appreciated, however, that other inorganic scintillators such as cesium iodide, bismuth germinate and barium fluoride scintillators may also be used. In addition, inorganic solid state detectors such as lithium-drifted germanium, high purity germanium or mercuric iodide may be used.
The inorganic scintillators of the present invention are arranged to form at least one ring of detectors so as to provide for a detection of a plurality of slices or parallel successive planes of the object under inspection as the object is moved continuously through the ring of detectors. In a specific embodiment of the invention, this ring is broken into sets of C-rings; and in order to provide for a better three dimensional representation, two spaced sets of C-ring detectors may be used with the open ends of the C-rings facing each othe