1. Field of the Invention
The present invention relates to a collimator with an adjustable focal length, particularly in x-ray inspection systems.
2. Description of the Background Art
Inspection methods with the use of x-rays are employed particularly in the detection of critical substances and objects in luggage or other freight. For this purpose, multi-stage systems are known whose first stage is based on the absorption of x-radiation. To detect certain critical substances such as, for example, explosives, a second stage is employed to which objects from the first stage are selectively supplied. Systems whose operating principle is based on diffraction phenomena are used as the second stage. In this case, the diffraction angle at which an incident x-ray is diffracted depends on the atomic lattice distance of the material to be analyzed and the energy and thereby the wavelength of the incident radiation. By analyzing the diffraction phenomena by means of x-ray detectors, conclusions can be reached about the lattice distance and thereby about the material. This type of two-stage system is disclosed, for example, in the German patent application 103305211.
Because x-ray inspection systems work with extremely low radiation intensities, highly sensitive detectors are employed. To avoid measurement inaccuracies, it must therefore be achieved that only the radiation produced by the testing device strikes the detector. In addition, care must be taken that radiation diffracted only at a single point is detected, because otherwise localization within the object to be examined is not possible. Spatial filtering is therefore necessary, which is performed by a so-called collimator.
Because it is technically very costly to generate monochromatic x-radiation, the highly limited x-ray used for testing, the so-called pencil beam, has an energy spectrum known, for example, from measurements. It follows from the Bragg equation that the incident radiation is diffracted at each point at an angle that depends on the radiation energy. Radiation with an energy spectrum is therefore diffracted within an angle range, and thereby the diffraction is rotation symmetric about the incident pencil beam. In an x-ray inspection, it is desirable to detect only radiation diffracted at a certain angle. This is also achieved with the use of a collimator. The passband of the collimator corresponds substantially to the generated surface of a cone whose tip coincides with the point whose diffraction properties are to be analyzed. To examine an area within an object, a plurality of points must be focused.
German patent application 103305211 discloses a method for the examination of an object area in which the setup comprising a detector and collimator can be moved in the direction of the incident x-ray. The disadvantage of this method is that, on the one hand, a highly precise traveling unit is required and, on the other, the entire device must have an overall height more than twice the height of the object to be examined.
A second possibility is the use of a collimator that has a plurality of parallel apertures of the same aperture angle and with which therefore a plurality of points on the rotation axis can be focused simultaneously. The use of a non-segmented detector, which is not position-sensing and therefore provides a common output signal for all focused points, however, results in the disadvantage that the evaluation and clear assignment of the detected radiation to a diffraction point are difficult. With use of a segmented detector, which, for example, is divided into separately evaluable circular rings, this disadvantage in fact does not arise, but this type of detector is laborious and costly.