a high nitrogen count rate and coarse three-dimensional imaging capabilities.
To improve upon the invention described in applicants' first patent application, the present invention, in accordance with one embodiment thereof, enhances the spatial resolution of the nitrogen image (and thus improves the measurement of nitrogen density) by correlating the low resolution gamma ray imaging with a high-resolution two-dimensional density image obtained using X-ray radiography. This technique enhances the detection of explosives because explosives are generally high in physical density as well as nitrogen content. Advantageously, the resolution and precision obtained using such well accepted and tested X-ray systems is far beyond that which neutron based systems can provide. Thus, by correlating the coarse (but very nitrogen specific) image obtained from the gamma ray detector with the high spatial resolution (but non-nitrogen specific) image delivered by the X-radiography system, a much more meaningful image of dense nitrogen-bearing materials in the object under examination is obtained.
The combination of gamma and X-ray imaging, or other X-ray methods, advantageously provides enhanced features that neither alone can provide. For example, if an X-ray image of an object under examination indicates the presence of an item that could be an explosive during an on-line operation, the combined image of the object may be presented to the operating personnel responsible for detecting such explosives. The gamma ray image can then be used to "color" the X-ray image so that the explosive can be readily picked out from other items seen in the X-ray image. The form of the explosive can also be determined with sufficient accuracy, thus allowing explosives to be shipped, e.g., as ammunition, if declared, in order to confirm that only explosives in the proper form are present.
Another embodiment of the improved explosive detection system of the present invention includes the use of at least one neutron detector. The neutron detector advantageously 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 prese