X-ray or gamma ray systems or devices – Specific application – Absorption
Patent
1996-09-05
1997-11-11
Porta, David P.
X-ray or gamma ray systems or devices
Specific application
Absorption
378 56, G01N 2310
Patent
active
056872100
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a method for determining the attenuation function of an object and a device/apparatus with which to implement this method. In particular though not solely the invention relates to radioscopy. In general, it applies to the field of spectroscopy in the energy range from several Kev to several Mev.
As a rule the attenuation function of an object is not sought, instead only certain functional values at given energies. Thereupon the attenuation function can be reconstituted by interpolating a curve passing through said points. Foremost monochromatic x-ray or .gamma.-ray beams of stable energy are used, which are obtained from isotope sources of which the energy might be lowered by the Compton effect, or from x-ray tubes with a broad spectrum filtered through a Bragg lattice to obtain a spectral-line spectrum.
The object to be measured must be placed in the beam path and a detector picks up the transmitted beam intensities for the various energies.
Letting I.sub.0 (E) be the incident beam intensity, the energy being E, and letting I.sub.t be the transmitted beam intensity on the other side of the object, then the transmission Tr(E) for the energy E is the ratio I.sub.t
These known techniques many incur many drawbacks. The particular attenuations are determined only for a slight number of energies. Consequently if the attenuation function must be ascertained, a curve must be made to fit said points, as a result of which accuracy depends on how the curve was interpolated.
Moreover the determination of the attenuation values by such techniques is time-consuming when an accuracy of about 1% is desired: a large number of measurements must be averaged and about one second is needed to ascertain an attenuation value.
The latter consideration precludes using these techniques for imaging, and even more so to make images in near real-time as is mandatory in luggage and security inspections.
One image comprises about 500 dots per line and about 600 lines; if each measurement requires one second, obtaining an image at a single energy requires about 83 h, which must be multiplied by the desired number of energies.
On the other hand Bragg diffraction and the Compton effect depend on the beam's angle of incidence and allow obtaining a beam of the desired energy only at a given angle of incidence when using a collimated beam. If using such a collimated beam, transmission measurement can take place only at one point in the object. To obtain simultaneously a large number of test points, the beam emission means must be multiplied because, considering the angular dependency, a fanning beam is precluded. It is clear that implementing such an assembly is highly complex and costly.
Isotope sources offer greater simplicity but also incur drawbacks: on account of safety precautions, the handling of such sources is rigorously prescribed, while beam intensity is low.
Accordingly the known procedures for measuring attenuation allow analysis only for specific energies strongly dependent on the equipment in use, but not on the user's wishes or needs. Moreover they are exceedingly time-consuming, costly and temperamental in operation; thus such techniques are hardly applicable in radioscopy or when used in security imaging, which represent important fields of application for object-attenuation determinations.
The present invention palliates these drawbacks. It allows determining the attenuation function very quickly (in tens of milliseconds) using x-ray tubes or standard .gamma.-ray generators. The attenuation function is determined as a variable bijectively related to energy. As shown below, the transmission u of a reference thickness of a reference material is selected as the variable. When it is desired to state the attenuation by the object as an energy function, it will be sufficient to measure once (rather than at each determination) the transmission u for the desired energies, employing prior art techniques for that purpose.
More specifically, the invention concerns a method determining the attenuation fu
REFERENCES:
patent: 4400827 (1983-08-01), Spears
patent: 5044002 (1991-08-01), Stein
patent: 5479023 (1995-12-01), Bartle
Maitrejean Serge
Perion Didier
Bruce David Vernon
Europ Scan
Porta David P.
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