Patent Number: 049875825
Section: summary

BACKGROUND The present invention relates to systems for detecting the presence of concealed materials, and in particular to such a system which uses gamma-ray induced X-ray fluorescence and a scanning spectroscope to detect the presence of specific material in concealed locations such as luggage or other containers. The use of X-rays to detect or examine the contents of luggage, packages and the like is well known. Such systems, however, are only capable of detecting the presence of high density elements such as steel, lead and the like. Many materials cannot be detected with current X-ray systems. It has been proposed to use fast neutron activation of concealed materials for the detection of low density elements. It is known that some elements like nitrogen produce an isotope that emits a beta decay positron which in turn reacts with electron producing a 511 keV annihilation gamma ray. However, the resulting nucleus that produces these positrons has a half life of about ten minutes. This creates problems since even after one hour the radioactivity of the sample has dropped by only a factor of 5. Further, even using high-flux fast neutron sources, the resultant count rate of the emitted gamma rays is sufficient only to determine the presence of a particular element in a sample and not to get is spatial distribution. To produce a spatial distribution picture requires a long period of time, typically, in excess of one hour. Such systems require the use of complex technology and are expensive. Therefore, there has been a need for a system that detects low density materials concealed in articles such as luggage and containers and the like, which is capable of producing a real-time detection of such materials and is capable of providing a spatial distribution or picture of these materials. SUMMARY OF THE INVENTION The invention is a system for detecting the presence of known materials in a body or container, or the like. The system incorporates a gamma or X-ray source such as an X-ray tube for irradiating the body with gamma rays to produce X-ray fluorescence of materials contained therein. A directionally discriminate X-ray detector arrangement is positioned to intercept X-rays emitted from the body and which passes only those X-rays having a predetermined wavelength characteristic of a material of interest and from a specific direction. The detector arrangement comprises a single crystal material that passes X-rays in accordance with the Borrmann effect. A typical crystal comprises single-crystal dislocation-free silicon, for example. A second crystal may be disposed in an axially offset position relative to the first crystal to receive the X-rays transmitted by the first crystal for discriminating X-rays emanating from different materials and hence remove ambiguity. A specific embodiment of the invention, a plurality of the crystals are arranged in a linear array and the container is either scanned with the array of the container passed linearly pas the array. The detected X-rays are measured in intensity and the data is processed by suitable data processor to generate a video image indicative of the presence and shape of the specific material for which the system is adapted to detect. In yet another specific embodiment of the invention, a plurality of such arrays may be arranged with the input surfaces of the crystals disposed at different Bragg angles to simultaneously detect the presence of a selected group of materials. It is therefore a feature of the invention to provide a system for detecting the presence of specific low density materials concealed within a container in real time. It is another feature of the invention to provide such a system for detecting gamma ray induced X-ray fluorescence of low density materials. Still another feature of the invention is to provide a system that incorporates elongated dislocation-free single crystals for selectively transmitting X-rays of predetermined wavelength to a detector, the wavelengths being characteristic of specific materials of interest. Yet another feature of the invention is to provide a system that incorporates an array of crystals arranged in a linear array and adapted to scan a container to provide a visual image indicative of the shape, size, and location of specific materials within the container in real time. Yet another feature of the invention to to provide a system that is relatively inexpensive and which can be operated without creating unnecessary radiation risks.