Patent Number: 
Section: description

FIG. 1 shows diagrammatically a known arrangement for X-ray analysis with two parabolic multilayer mirrors. This arrangement is notably suitable for X-ray diffraction. The arrangement includes an X-ray source 2 for irradiating a sample 4 to be analyzed by means of the arrangement. In order to parallel as well as possible the radiation 6 incident on the sample, a device for paralleling the radiation beam is arranged in the beam pat between the X-ray source and the sample, said device being a multilayer mirror 8 for X-ray reflection in the present example. The reflecting surface of this multilayer mirror has a parabolic shape as symbolically represented by a dashed line 10. The reflecting layers provided on the surface of the multilayer mirror may have a thickness which is dependent on the location, so that a so-called graded multilayer mirror is obtained. The grading is such that when the mirror is irradiated by a (from a two-dimensional point of view) point-shaped source (being a line-shaped source perpendicular to the plane of drawing when viewed three-dimensionally), the Bragg reflection condition is satisfied in each point of the multilayer mirror, with the result that a large reflecting surface is obtained for the multilayer mirror. After diffraction of the X-rays on the sample 4, a mainly mutually parallel beam of X-rays 12 emanates from the sample. Due to interaction of the X-rays with the sample or the vicinity thereof, however, directions other than the predominant parallel direction may also occur in the beam emanating from the sample. The X-rays having such deviating directions usually affect the accuracy of the measurement; therefore, it will be attempted to eliminate such deviating beam directions from the beam 12. To this end, a further multilayer mirror 14 for X-ray reflection is arranged in the beam path between the sample 4 and an X-ray detector 16. Like the multilayer mirror 8, the multilayer mirror 14 is constructed as a graded multilayer mirror whose surface has a parabolic shape as symbolically denoted by the dashed line 18. Due to the parabolic shape of the multilayer mirrors 8 and 14, the X-ray beam emanating from the X-ray source 2 is converted, before reaching the sample 4, into a substantially parallel beam and after the sample into a focused beam again that has a focus point in the focus 20 of the multilayer mirror 14. The collimator slit 22 is arranged at the area of said focus. FIG. 2 shows diagrammatically a detail of an arrangement for X-ray analysis in accordance with the invention. A number of auxiliary lines 24a, 24b, 26a and 26b in this Figure indicate how substantially the same angular value is observed for the passage width of the collimator from every reflecting point of the multilayer mirror. (For the sake of clarity it is to be noted that said auxiliary lines do not represent rays of the X-ray beam emanating from the multilayer mirror 14, but denote only the boundaries of the angle at which the angular value of the passage width of the collimator slit 28 is seen from the points A and B, respectively.) In the embodiment shown in FIG. 2 the collimator is shaped as a collimator slit that is formed by two knife edges which are situated at different distances from the reflecting points of the multilayer mirror. The distance between the relevant reflecting point (for example, the point B) and the center 32 of the passage width of the collimator 28 can be taken as said distance, for example, as represented by the length of the line segment 30. A situation in which the angular value xcex3 or xcex4 of the passage width is substantially constant for the points of the surface of the multilayer mirror 14 that participate in the reflection can be achieved by a suitable choice of said difference in distances. (For the sake of clarity this reflecting part of the surface in FIG. 2 is shown to be much larger than the value corresponding to a practical situation.) The desired effect of enhanced resolution is achieved only if the angular value (xcex3 or xcex4) of the passage width of the first collimator, viewed from the reflecting points of the multilayer mirror, is smaller than the maximum angular range of the reflection xcex1max. Because the value of the maximum angular range is of the order of magnitude of 0.05xc2x0 for practical multilayer mirrors, it will be evident that the angles xcex3 and xcex4 are significantly exaggerated in FIG. 2. The knife edges of the collimator are displaceable, in a manner not shown in the Figure, relative to one another in a direction transversely of the direction of the beam path through the collimator. The passage width of the collimator, and hence the resolution of the apparatus, is thus controlled without introducing deviations in respect of the angular value at which the collimator slit is viewed from the various points of the reflecting surface. FIG. 3 shows diagrammatically a further embodiment of the invention. Like in FIG. 2, the collimator 28 in this Figure is shaped as a collimator slit that is formed by two knife edges which are situated at different distances from the reflecting points of the multilayer mirror, so that the same angular value of the passage width is observed from every reflecting point of the multilayer mirror. The apparatus shown in FIG. 3 is also provided with a second collimator 34 which is arranged in the beam path between the sample 4 and the X-ray detector 16. The second collimator 34 is adjustable (in a manner not shown in the Figure) in that the knife edges are displaceable relative to one another in the direction of the beam path through the collimator. The detector will always perceive a defined part of the sample when the passage width is adapted to the angle of incidence of the radiation on the sample.