s 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 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 ad