Patent Number: 055090435
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an X-ray analysis apparatus with an X-ray source 1, a monochromator 3, a goniometer 5 and a detector 7 which are only diagrammatically shown. The X-ray source 1 comprises an anode 14 which is accommodated in a housing 10 provided with a radiation window 12, which anode consists of, for example copper, chromium, scandium or another customary anode material. An electron beam generates an X-ray beam 15 in the anode. The monochromator comprises two crystal pairs 18 and 20 with crystals 21, 23, 25 and 27. In the crystal pair 18 crystal end faces 22 and 24 serve as operative crystal faces. Similarly, in the crystal pair 20 crystal end faces 26 and 28 act as operative crystal faces. The first crystal pair can be arranged so as to be rotatable about an axis 30 extending perpendicularly to the plane of drawing, and the second crystal pair can be arranged similarly so as to be rotatable about an axis 32. The end faces 22, 24 and 26, 28 remain mutually parallel in any rotary position. Preferably, the crystals have, for each pair, a U-shape cut from a single monocrystal, the connecting portion of the U being used, for example for mounting the crystals. The inner faces of the limbs of the U then form the operative crystal end faces. Alter cutting and possibly grinding or polishing, a surface layer has been removed from these surfaces, for example by etching, in order to remove material in which stresses may have developed due to mechanical working. The carrier plate 34 for the monochromator has a comparatively rigid construction so that, for example its lower side can be used to support mechanical components, for example for the crystal orientation motions, without risking deformation of the plate. In the present embodiment, the length of one of the crystals of each of the crystal pairs is reduced so that more freedom is obtained in respect of a beam path. The attractive property of the 4-crystal monochromator as regards the angle of aperture for the incoming beam enables the X-ray source, i.e. actually a target spot on the anode 14, to be situated at a minimum distance from the first crystal pair, which minimum distance is determined by the construction of the source. An attractive intensity is thus achieved already for the ultimate analyzing X-ray beam 35. In the present embodiment the first crystal pair 18 is rotatable about the axis 30 of a shaft on which a first friction wheel 40 which is situated beneath the mounting plate is mounted so as to engage a second friction wheel 42 which is mounted on the shaft with the axis 32 about which the second crystal pair 20 is rotatable. However, the two crystal pairs may alternatively be mutually independently adjustable or the adjustment can be performed by means of a drive motor with, for example programmed settings adapted to the anode material to be used or to specimens to be analyzed. The crystals are preferably made of germanium having operative end faces which extend parallel to the (440) crystal faces of a germanium monocrystal which is relatively free from dislocations. By diffraction from the (440) crystal faces an extremely well monochromatized beam having, for example a relative wavelength width of 2.3.times.10.sup.-5, a divergence of, for example 5 arc seconds, and an intensity of up to, for example 3.times.10.sup.4 quants per second per cm.sup.2 can be formed. Such a sharply defined beam enables measurement of errors in lattice spacings of up to 1 to 10.sup.5 can be measured and high-precision absolute crystal lattice measurements can also be performed thereby. The monochromatization of the X-my beam is realized in the monochromator by the central two reflections, i.e. the reflections from the crystal faces 24 and 28. The two reflections from the end faces 22 and 26 do influence the beam parameters, but they guide the beam 35 in the desired direction coincident with the prolongation of the incoming beam 15. Wavelength adjustment is achieved by rotating the two crystal pairs in mutually opposite directions; during this motion, therefore, the position of the emergent beam 35 does not change. An intensity which is, for example 30 times higher can be achieved by utilizing reflections from (220) crystal faces, in which case a larger spread in wavelength and a larger divergence occur. The monochromator is non-rotatably connected to the goniometer 5 in which a specimen 46 to be analyzed is accommodated in a specimen holder 44. For the detection of radiation emerging from the specimen 46 there is provided a detector 7 which is rotatable along a goniometer circle 48 in known manner. The detector enables measurements to be made throughout a larger angular range and for different orientations of the specimen. For exact determination of the position and possible repositioning of the specimen, the goniometer may include an optical encoder which is not shown in the drawing. FIG. 2b shows an example of an asymmetrical system of crystals in accordance with the invention, compared with a similar symmetrical system as shown in FIG. 2a, comprising notably germanium crystals with (440) and (220) lattice planes, respectively. FIG. 2a shows the symmetrical system comprising crystals 21, 23, 25 and 27 in which the lattice planes extend parallel to crystal end faces 22, 24, 26 and 28, respectively. FIG. 2b shows an asymmetrical crystal system in which the lattice planes are chosen to extend parallel to the outwards facing end faces 40, 42, 44 and 46 of the crystals 23, 21, 27 and 25, respectively; however, the inwards facing crystal end faces 22, 24, 26 and 28 no longer extend parallel to the lattice planes in this Figure. Each crystal exhibits (220) as well as (440) lattice planes; in the upper crystal pairs of the FIGS. 2a and 2b the (440) lattice planes are used, whereas in the lower crystal pairs of the FIGS. 2a and 2b the (220) lattice planes are used. An incoming X-ray beam 15 emerges from the crystal system as a beam 35 which is collinear with the incident beam in all situations. A comparison of the beam diameter of the FIGS. 2a and 2b already demonstrates that the difference between the symmetrical and the non-symmetrical system is comparatively small for the (440) crystal planes, whereas it is substantial for the (220) crystal planes. The same holds for the resolution.