X-ray or gamma ray systems or devices – Specific application – Diffraction – reflection – or scattering analysis
Patent
1997-10-20
2000-09-19
Church, Craig E.
X-ray or gamma ray systems or devices
Specific application
Diffraction, reflection, or scattering analysis
378 57, G01N 23201
Patent
active
061223441
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of X-ray inspection systems and more particularly those inspection systems which use coherently scattered X-rays to detect the presence of explosives, incendiary materials, prohibited drugs or other illegally transported substances (hereinafter collectively referred to as "target material") in a container, the presence of imperfections or impurities in a material or object or the non-destructive, spatially-resolved, chemical analysis of an object.
2. Discussion of Prior Art
As the problems discussed in relation to known inspection systems below are equally valid for container, object and material inspection systems they will be discussed only in relation to container inspection systems. The term "container" shall hereinafter be taken to refer to both the container and its contents.
Many systems which utilise X-rays have been developed for the inspection of containers such as baggage and parcels. These include dual energy transmission imagers, backscatter imagers and computer-aided tomography (CAT) scanners all of which essentially yield effective atomic number and density information in two or three dimensions. However, these systems are inherently chemically non-specific and rely heavily on operator vigilance and interpretation during visual inspection of the spatial distribution of the contents of the containers and on other indicators such as the presence of initiating devices in the case of explosive target materials. A problem here is that many kinds of target material are not easily identifiable by their shape since they can be easily moulded or packed inside other items within the container to mask their presence. Furthermore, initiating devices are often masked by adjacent clutter within the container. These materials, and more specifically explosive devices, are therefore not readily detectable using these systems.
One known system which overcomes this problem is described in the U.S. Pat. No. 4,751,722 (EP 0,153,786). This discloses a system which analyses the intensities of X-rays which are coherently scattered through different small angles when passing through a container. This information is then used to generate an angle dispersive X-ray spectrum which is characteristic of the ordered molecular structure of the material responsible for the scattering. The X-ray spectrum may then be passed to a data processing system where an automatic comparison is made with stored spectra characteristic of the target materials of interest in order to determine whether or not these target materials are present within the container.
However a problem with this system is that the contents of a container can only be resolved in the two dimensions of the scattering voxel (volume element) in the plane perpendicular to the direction of travel of the unscattered X-rays, ignoring the third dimension in the direction of the unscattered beam, through the depth of the container. Where either the container or the target material has a third dimension greater than a few centimeters then depth resolution is required in order to satisfactorily identify the presence of a target material. This is because coherent scattering over a range of scattering angles from non-target materials at different depths within a container cannot be differentiated from the scattering caused by localised target material at any given angle of interest thereby causing the generated X-ray spectrum to become confused and identification of target material difficult.
A further problem with this earlier system is that the low X-ray flux generated by presently available monochromatic sources results in too great an inspection time, making it impractical to use in situations where a large number of objects of large size need to be rapidly inspected eg. such as for containers in airports. However, the use of a high intensity polychromatic source would degrade the material discrimination capability of the system.
UK patent No. 1,463,054 provides a means of examining
REFERENCES:
patent: 4956856 (1990-09-01), Harding
patent: 5007072 (1991-04-01), Jenkins et al.
patent: 5231652 (1993-07-01), Harding
patent: 5428657 (1995-06-01), Pananicolopoulos et al.
Church Craig E.
The Secretary of State for Defence in Her Brittanic Majesty's Go
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