Patent Number: 
Section: description

The invention will be described with reference to an embodiment in which the apparatus for radiation analysis is formed by an X-ray analysis apparatus, more particularly, an X-ray diffraction apparatus. Therein, the analysing radiation has the form of X-ray radiation. However, there should be pointed out that the invention is applicable to all further apparatus for radiation analysis in which a collimator is used for the analysing radiation beam. FIG. 1 is a diagrammatic representation of a known X-ray analysis apparatus, in this case being an X-ray diffraction apparatus. In this apparatus a goniometer 4 is mounted on a frame 2. This goniometer 4 can be provided with an angular encoder for measuring the angular rotation of the X-ray source 7 which is mounted thereon and of the detector device 9 which is also mounted thereon. The goniometer is furthermore provided with a sample holder 8 on which a sample 10 is arranged. An angular encoder may be provided on the sample holder for the cases where measurement of the angular rotation of the sample is important. The X-ray source 7 includes a holder 12 for an X-ray tube (not shown in the Figure) which is secured in the holder by way of a fixing ring 20. This X-ray tube includes a high-voltage connector 15 for applying the high voltage and the filament current to the X-ray tube via high-voltage cable 18. On the same side of the X-ray tube, supply and discharge ducts 22 and 24 for the cooling water of the X-ray tube are provided. The tube holder 12 further includes an exit window for X-rays 44 and a unit 16 for parallelization of the X-ray beam (a Soller-slit collimator). The plates of the Soller-slit collimator 16 are parallel to the plane of drawing in such a way that the X-ray beam generated by the X-ray source 7 illuminates the sample 10 with a divergent beam. The detector device 9 comprises a holder 26 for a Soller-slit collimator, a holder 28 for a monochromator crystal, and a detector 30. The plates of the Soller-slit collimator in holder 26 are also parallel to the plane of drawing. If the X-ray source and the detector are both rotatable around the sample, it is not necessary for the sample to be mounted so as to be rotatable. However, it is alternatively possible to mount the X-ray source so as to be stationary as this may sometimes be necessary in the case of voluminous and heavy X-ray sources. In that case, the sample holder as well as the detector should be rotatable. The X-ray diffraction apparatus as shown in FIG. 1 also includes a processing device for processing the various measured data. This processing device comprises a central processing unit 32 with a memory unit 36 and a monitor 34 for the presentation of the various data and for the display of the measured and calculated result. The X-ray source 7, the detector device 9 and the sample holder 8, mounted on the goniometer 4, are all provided with a unit (not shown) for determining the angular position of the respective element relative to the scaled graduation of the goniometer. A signal representing this angular position is transferred to the central processing unit 32 via connection leads 38-1, 38-2 and 38-3. FIG. 1 shows a so-called Bragg-Brentano arrangement, which means that the X-rays emanating from a single point are again focused at one point after reflection by the sample 10, provided that the surface of the sample is tangent to a circle through the point of origin and the focal point. The sample 10 is irradiated by means of X-rays originating from the X-ray source 7. An anode 40, which forms part of the X-ray tube that is not further shown in this Figure, is diagrammatically represented in this X-ray source. In anode 40 the X-rays are generated in a customary manner by exposing this anode to high-energetic electrons. As a result, X-rays 42 emanating from X-ray window 44 are generated in the anode. The said point of origin in the arrangement shown in FIG. 1 is not formed by a single point, but by a line focus 41 on the anode which line focus is perpendicular to the plane of drawing. Said focal point is formed by the point of union 43 of the beam 45 leaving the sample at the area of the entrance of the detector 30. Consequently, this arrangement has a focusing effect only in the plane of drawing. FIG. 2 shows a respective view of an embodiment of a variable Soller-slit collimator in which the plates of the collimator have a rectangular shape. The collimator shown comprises a stack of collimator plates 46 with spacings 48. All the plates in this collimator have the same dimensions. A radiation beam 45 whose aperture angle is bounded by the collimator is incident in parallel with the plane of the collimator plates 46. The angle of aperture xcex1 of the radiation beam is given by twice the ratio of spacing d between the plates 46 to the collimating element length L exposed to the radiation beam (see also FIG. 2b), so that the following holds xcex1=2d/L. The value of the magnitude L may be varied by rotating the collimator plates around a shaft 50 that is perpendicular to the plane of the plates 46. For this purpose, a movement mechanism is provided, in this embodiment formed by a shaft 50 and a drive unit 52 in which the shaft 50 is carried in bearings and which is fixedly connected to the analysis apparatus the collimator forms part of. The drive unit comprises, for example, a motor for rotating the shaft, which motor is controlled by a control unit 54 which may form part of a computer belonging to the analysis apparatus. When the measurements to be carried out by the analysis apparatus so require, the collimator plates 46 are rotated around the shaft 50 until the correct aperture angle is reached, that is, until the relationship xcex1=2d/L, where xcex1 is a prescribed value, has been satisfied. FIG. 3 shows a perspective view of a second embodiment of a variable Soller-slit collimator according to the invention. This embodiment is pre-eminently suitable for the devices in which the radiation beam is strongly diverging or converging in a plane parallel to the collimator plates. This situation may occur, for example, in a spectrometer of the Bragg-Brentano type. With a beam so divergent, the value of L (i.e. the collimator plate length L exposed to the radiation beam 45) is not the same for all the rays in the radiation beam. This may be a disadvantage for measurements that require a high degree of accuracy. It can be demonstrated that for such measurements a Soller-slit collimator having elliptically shaped plates eliminates this disadvantage entirely or to a large extent. Just like in the collimator as shown in FIG. 2, the collimator in FIG. 3 is driven via shaft 50 in the same way as has already been described with reference to FIG. 2. FIG. 4 shows a perspective view of an embodiment of a variable Soller-slit collimator with X-ray optical fibres according to the invention. Such fibres are known per se for influencing radiation beams of X-rays. With such fibres, a high degree of collimation, i.e. a very small aperture angle of the radiation beam, may be obtained. The collimator shown in this Figure comprises a two-dimensional stack of X-ray fibres 60. The X-ray fibres 60 have the same cross-section, but a length that depends on their height in the stack. Parallel with the axial direction of the X-ray fibres 60, a radiation beam 45 is incident whose aperture angle is bounded by the stack of X-ray fibres. The aperture angle of the radiation beam is determined by the ratio of the internal cross section and the length of the hollow fibre. The aperture angle may thus be varied by reciprocating the collimator. For this purpose, a movement mechanism is provided in this embodiment, which is formed by a holder for the stacking of X-ray fibres, which holder comprises two guides 62 which may be reciprocated by a driving rod 64, the guides 62 being led along parts 56 of the arrangement of the analysis apparatus. The driving of said movement is performed by a drive unit 52 in which the driving rod 64 is carried in bearings and which is also fixedly connected to the analysis apparatus. The drive unit comprises, for example, a motor for reciprocating the driving rod, which motor is controlled by a control unit 54 which may form part of a computer belonging to the analysis apparatus. When the measurements to be performed by the analysis apparatus so require, the collimator is reciprocated until the correct aperture angle is reached.