nt invention functions as follows: neutrons from a neutron source are partially thermalized creating a cloud or bath of neutrons that interrogate the object under examination. The neutrons can easily penetrate into the object being examined, e.g., through the walls or sides of the parcels or luggage. These neutrons are absorbed by or interact with the atomic nuclei of the elements that make up the materials carried within the object being examined. Different elements emit gamma rays at different characteristic energy levels after such neutron absorption or interaction (much like fluorescence). For example, gamma rays emitted from nitrogen have a different characteristic energy level than do gamma rays emitted from hydrogen or carbon for the same neutron exposure. The emitted gamma rays have a high enough energy so that they also easily penetrate through the walls of the object being examined. A suitable detector array is positioned to effectively surround the object as it is examined. This detector array catches or captures the emitted gamma rays and records the number of gamma-rays (i.e., counts the gamma rays) observed at a selected characteristic energy (for example 10.8 MeV, corresponding to nitrogen.) The number of gamma rays of a particular characteristic energy (the gamma ray count) detected by a particular detector depends upon the amount of the element present within the material being examined, the location of the element within the parcel under investigation, the number of neutrons present (the neutron concentration), and the probability that the element will capture a neutron and emit that gamma ray. Because the number of neutrons present is roughly known (or can be roughly measured, which measurement is facilitated by use of one or more neutron detectors), and the probability that the element will capture a neutron and emit a gamma ray is a known constant for any particular element, an analysis of the recorded gamma rays of the selected energy, as well as the neutron flux as measured by the neutron detector, 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 