X-ray or gamma ray systems or devices – Specific application – Absorption
Patent
1996-07-12
1998-06-16
Porta, David P.
X-ray or gamma ray systems or devices
Specific application
Absorption
378 57, G01N 2306
Patent
active
057683344
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method of identifying specific materials in an objects composition/constitution and a device/apparatus to implement this method. The invention applies in particular to detecting specific materials such as explosives or drugs.
The patent document WO 92 02892 describes several procedures and apparatus for inspecting luggage and for the detection of specific materials. The object being inspected is exposed to an x-ray beam providing either of two different (high and low) energies. The beam attenuation in the object is measured. Accordingly a pair of attenuations are obtained at each image point, one of the high-energy range and the other for the low-energy range.
Moreover there are preliminary measurement& These cost in determining pairs of attenuations (one for the high, the other for the low energy range) for a large number of samples representative of the specific material(s) being looked for in the object. With respect to each type of specific material, the samples are in the form of different specific-material thicknesses in turn covered with different thicknesses of a coating material. All these pairs of reference attenuations are classified in a stored table. They are also related to a parameter P equal to the thickness of the specific material for the sample being considered.
When the object being examined is exposed to the x-ray beam, then the measured pair of attenuations at one image point will be compared with the pairs of attenuations recorded in the table.
A value of the parameter P is deduced by interpolation. By correlating at least one of the attenuations of the measured pair, for instance that in the high-energy range, with the value of P, it is possible to infer whether the object does contain some amount of the specific material at the test point. Detection errors and hence false alarms are reduced by comparing the different image points with their neighbors.
This procedure incurs major drawbacks:
Two measurements art carried out for each image point, one for the high energies, the other for the low ones. These two measurements are substantially independent and allow deriving two independent data at each point. These two independent data are used in inferences on the chemical constitution of the object being inspected.
These two data are independent because two predominant physical effects, namely the photoelectric and the Compton effects may take place when an X photon interacts with the material. However other independent data also are accessible because other independent physical effects occur during this sort of interaction
Accordingly the procedure advocated in WO 92 02892 does not fully utilize all the latent information in an interaction between an X photon and a material.
In order to compensate for the restriction to two independent data in securing information on the inspected object's chemical constitution, the prior art recommends using correlation between the attenuated measurements and a parameter P corresponding to tabulated attenuation values. These tabulated attenuation values must be stored in very large numbers in order to preclude spurious interpretation. Nevertheless, however large the number of tabulated pairs of attenuations, this highly indirect method will not eliminate the danger of false alarms on one hand nor on the other will it preclude detection failure when the target material is present.
The x-ray beam's characteristics (intensity, spectrum, energy . . . ) vary rapidly and markedly with time.
The attenuation pairs recorded in the table and produced by calibrating with a beam of specified properties depend on these properties and no longer will be applicable as references with a beam of different properties.
As a result, it may happen, on account of beam modification, that specific-material identification in fact will be a false alarm and, even more seriously, that on the other hand the system may overlook a specified, that is a target material inside the object The latter case may entail fatal consequences if such a specified
REFERENCES:
patent: 3673394 (1972-06-01), Hartmann
patent: 4031545 (1977-06-01), Stein et al.
patent: 5335260 (1994-08-01), Arnold
Maitrejean Serge
Perion Didier
Bruce David Vernon
Europ Scan
Porta David P.
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