vantageously allows neutron flux to be measured, which neutron flux provides yet another means for obtaining certain density information concerning the contents of the object being examined. That is, by counting the number of neutrons that are able to pass through the object without interacting with atomic nuclei, similar to that which is done in conventional neutron radiography, another reasonably independent measure of the density of the elements within the object is obtained. This additional information further improves the imaging resolution capabilities of the system.
Yet a further embodiment of the present invention includes means for improving the manner in which the gamma ray count signals are processed, and more particularly the manner in which background noise (gamma ray counts due to non-nitrogen elements) is removed from the measured gamma ray spectra. In accordance with this technique, background noise at one point of the gamma ray spectrum is measured, appropriately weighted, and subtracted from the gamma ray spectrum at the point of interest, e.g., 10.8 MeV, in order to remove background noise from the point of interest. This technique of background subtraction is referred to hereafter as the "spectral correlation method" (SCM).
A still further embodiment of the present invention includes the use of an artificial neural system (ANS), a form of a parallel distributed processing technique in order to improve the speed and performance with which the detection system is able to distinguish objects with explosives from objects without explosives.
The present invention may thus be summarized as a TNA-based explosive detection system that includes various combinations of the following improvements: (1) an X-ray system; (2) at least one neutron detector; (3) means for using a SCM, or an equivalent technique, to remove background noise from the gamma ray spectra; and/or (4) an ANS to improve the speed and accuracy with which the decision concerning the presence of explosives is made.
In operation, the present 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 d