Patent Number: 049875825
Section: description

DETAILED DESCRIPTION Referring now to the drawings, FIG. 1 shows a system 10 in accordance with the principles of the present invention. The system is adapted to detect the presence of materials 12 concealed within a container 14 such as a suitcase, for example. The system 10 comprises a gamma ray source 16, or alternatively an X-ray source. The gamma ray source 16 may comprise an Americium 241 radioactive source, or may comprise a compact linear electronic accelerator and tungsten target, or a small compact betatron, or an X-ray tube, for example. The source 16 emits a conical beam of gamma rays indicated by arrows 18 which impinge upon and penetrate the container 14 and the materials 12 therein. As is well known, the gamma rays produce coherent scattering of X-rays by the atoms of the elements of which the container 14 and materials 12 are comprised. These X-rays result from gamma ray collisions with the K and L band electrons of the container 14 and materials 12. It is further well known that the X-rays emitted by an element have an energy or wavelength that is uniquely characteristic of the element. These X-rays, indicated by waved lines 20, will vary in wavelength or energy from relatively low energy to relatively high energy. For example, the fluorescent X-rays from plastic explosives and drugs have relatively low energy, in the range of several keV, whereas nuclear explosives have relatively high energy, on the order of 100 keV. These X-rays are detected by an X-ray detector arrangement 22 and described at more detail below. Referring now to FIG. 2, the detector arrangement 22 comprises an elongated signal dislocation-free crystal 23a made of a material such as silicon for example. In accordance with now well-known principles, X-rays incident upon the end or input surface 24 of the crystals 23a are absorbed or reflected. However, X-rays 20a, 20b of a specific wavelength incident upon input surface 24 at an angle defined as the Bragg angle will pass through the length of the crystal 23a with very little attenuation. The Bragg angle for transmission is given generally by the equation .lambda.=2d since .sigma., where .lambda. is the wavelength of the X-rays, d is the lattice spacing of the crystals atoms, and .sigma. is the Bragg angle relative to the axis 27 of the crystal 23a. X-rays 20a, 20b having the specific wavelength incident at the bragg angle are therefore transmitted by the crystal 23a. The transmitted X-rays 20a, 20b are then emitted from an output or transmission surface 26 of the crystal 23a in two equal parts or rays, each directed at the Bragg angle with respect to transmission surface 26. The crystal 23 a functions much as the slit of a spectrometer with the slit spacing being the lattice spacing of the atoms in the crystal 23a. The transmitted X-rays 20a, 20b may be subsequently detected by a suitable X-ray detector 29 such as a scintillation counter, proportional counter, charge coupled detector array, or the like. The crystal 23a is positioned such that its input surface 24 faces the container 12. For a specific element of interest such as a primary constituent of explosives or a drug, for example, those X-rays 20a impacting the input surface 24 at the appropriate Bragg angle produce X-rays 20a transmitted by the crystal 23a. These transmitted X-rays 20a, since they have the correct Bragg angle and wavelength, are indicative of the presence of the specific explosives or a drug located within the container 14. X-rays emitted by various elements of and in the container 14 and located at different places in the container 14 will impact the input surface 24 at different angles. Some X-rays 20b will be incident at the Bragg angle for that material 12 and these X-rays 20b will also be transmitted through the crystal 23a. It is possible that a plurality of materials disposed in the container 14 have wavelength and Bragg angle combinations that cause X-rays 20a, 20b to impinge at the same location on the detector 29. In order to eliminate ambiguity arising from the above possible occurrence, and to discriminate between the different materials, a second crystal 23b may be disposed at an axially offset position relative to the first crystal 23. The offset spacing permits only those X-rays 20a having a selected Bragg angle to be transmitted by the second crystal 23b. Hence, the use of the second crystal 23b eliminates ambiguity. Operation of the system can be further enhanced by providing a linear array 28 of crystals 23. If the container 14 then passes the array 28 in a direction generally perpendicular to the array 28 as indicated by arrow 25 of FIG. 1, scanning the container 14 is effected. Accumulation of data by a data processing system 30 enables generation of an image of a specific material 12 including its size, shape, and location. Alternatively the system 10 may be adapted in various ways to scan the container 14 to generate the image. This images 32 may be displayed, or recorded on a strip chart recorder 34. Since it is anticipated that more than one material would be of interest when scanning an article such as luggage prior to loading aircraft or importation of packages, referring to FIG. 3, a plurality of crystals 23 may be provided in a linear array 28. In addition, each of the crystals 23 of the array are oriented at slightly different angles with respect to the container 14, in order to simultaneously detect different elements arriving at different Bragg angles. Again, well-known data processing techniques allow accumulation of this data and the generation of images showing the size, shape, and location of materials 12 within the container 14 as will as providing detection of the presence of such materials 12. From the above description, it will now be seen that the present invention provides a unique means for detecting the presence of low density materials 12 within a container 14 by the use of gamma ray induced X-ray fluorescence. The system 10 provides a uniquely sensitive X-ray detector capable of detecting X-rays emitted by low density materials such as explosives or drugs. Orientation of the highly sensitive silicon crystal 23a and detector 29 enables the system 10 to detect a plurality of specific materials 12. The system 10 is readily adaptable for use with data processing techniques to generate images of such materials 12 within the container 14. The system 10, which can be used in conjunction with conventional X-ray scanners to detect the presence of dangerous or contraband materials, or the presence of metallic containers that are normally cannot be scanned, to precipitate careful hand searching of such articles. Thus there has been described a new and improved system for detecting the presence of predetermined materials in a body or container. It is to be understood that the above-described embodiment is merely illustrative of some of the many specific embodiments which represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.