X-RAY ANALYSIS APPARATUS INCLUDING A ROTATABLE PRIMARY COLLIMATOR

Abstract: An apparatus for performing X-ray fluorescence as well as X-ray diffraction by means of only a single X-ray tube (2) which is preferably constructed as a fluorescence tube. Between the tube and the sample (12) there is arranged a primary collimator (40) (Soller slit unit) which is rotatable about an (imaginary) axis (44) through the X-ray focus of the tube (2), so that the angle of incidence nu on the sample can be varied by rotation of the collimator. Because the diffraction detector channel (50, 52, 56) occupies a fixed position, a nu scan is performed by rotation of the collimator (40). As a result, it is not necessary to equip the diffraction section of the apparatus with a (comparatively heavy and expensive) goniometer with an associated drive for the detector (56) and for the wavelength analysis crystal (52).

Description: X-ray analysis apparatus including a rotatable primary coUimator. The invention relates to an apparatus for the examination of materials by means of X-ray diffraction, including a sample location for accommodating a sample of the material to be examined, an X-ray source for irradiating the sample location by means of X- rays, a parallelizing primary coUimator which is arranged between the X-ray source and the sample location and whose orientation with respect to the sample can be varied, and a detector for detecting the diffracted radiation emanating from the sample. An apparatus of this kind is known from European Patent Application EP 0 597 668. Generally speaking, two analysis techniques are available of X-ray analysis of materials: X-ray fluorescence and X-ray diffraction. In the case of X-ray fluorescence, a sample is exposed to a polychromatic X-ray beam. In order to expose the sample to an as high as possible X-ray power, the X-ray tube for fluorescence is constructed so that the X-ray focus on the anode wherefrom X-rays emanate is comparatively large, the angle of aperture to the sample being chosen so as to be as large as possible. The exposure excites the various elements present in the sample so that they emit X-rays which are characteristic of the constituent elements (fluorescence radiation). The elementary composition of the sample can be determined by detection and analysis of the fluorescence radiation. Generally speaking, in the case of X-ray diffraction the sample is exposed to a monochromatic X-ray beam which, because of the regularity of the crystal structure of the constituents of the sample, is diffracted only at given angles (whose value 2ϋ is measured with respect to the non-diffracted beam). The diffraction angles provide information regarding the crystal structure of the components present in the sample. In a practical set-up the diffraction angles are measured by performing a so-called 2ϋ scan by means of an X-ray detector, i.e. by travelling a (part of) a circle around the sample while measuring the intensity of the X-rays. Until recently each of these techniques was customarily performed by means of an apparams specifically conceived for the relevant technique. For some time X-ray analysis apparatus has been known whereby X-ray fluorescence as well as X-ray diffraction can be performed. Such an apparatus is disclosed in the cited European Patent Application. This apparatus includes only one X-ray source which is constructed as a polychromatic X-ray fluorescence tube, the X-rays emitted thereby emanating from the tube as a diverging beam. At the sample location of the apparatus a polycrystalline sample is introduced into the diverging X-ray beam, emission of fluorescent X-rays and diffraction of the incident beam then taking place therein. Between the exit window of the X-ray tube and the sample a parallelizing primary coUimator is arranged for diffraction purposes. This coUimator is constructed in such a manner that the coUimation obtained is variable in respect of its orientation relative to the sample surface, so that the various crystallographic faces can produce diffraction within the given angular range. Such variation of the coUimation is realized by mechanical adjustment of the orientation of the coUimator. The known apparatus also includes a detector for detecting the diffracted radiation emanating from the sample, said detector forming part of a diffraction channel which also includes a secondary coUimator and a monochromator crystal. The diffraction channel detects the diffracted radiation at a take-off angle which is adjusted by angulation means under the control of instrument control means. Thus, a diffraction pattern of the sample is obtained by the scanning by the angulation means. Angulation means for adjusting a take-off angle for the diffracted X-rays (usually referred to as goniometers) are expensive precision instruments, because accurate measurement of angles imposes severe requirements as regards positioning accuracy and reproducibility of the angular adjustment. Moreover, these accuracy requirements are difficult to satisfy because the assembly formed by the secondary coUimator, the monochromator crystal and the associated angulation means and the detector must be displaced, implying displacement of a comparatively large weight. Moreover, many diffraction measurements for comparatively long wavelengths and fluorescence measurements must be performed in a conditioned space, i.e. a space which is evacuated or filled with a specially chosen gas. In these cases the displacement of a heavy and voluminous diffraction channel within such a space is objectionable, because the vacuum space must be sufficiently large for this purpose and, moreover, the equipment used may not be subject, also in vacuum, to friction problems imposed by moving parts. It is an object of the invention to provide an X-ray analysis apparatus which does not require a goniometer for the detector adjustment for displacement of the diffraction channel during the execution of diffraction measurements. To this end, the X-ray analysis apparatus according to the invention is characterized in that it includes control means for continuously varying the orientation of the primary coUimator about an axis extending transversely of the X-ray beam during the execution of a diffraction measurement, and that the detector remains in a fixed position relative to the sample and the X-ray source during the execution of diffraction measurements. The invention is based on the idea that the primary coUimator can be used for traversing a comparatively large range of angular values in order to perform a 2ϋ scan. This is possible in that this primary coUimator is situated in the X-ray beam directly emanating from the X-ray source, i.e. in an X-ray beam coming from the X-ray focus with a comparatively large divergence. In these circumstances the angle of incidence of the X-rays on the sample can be varied through a comparatively large range by rotating the primary coUimator about an axis transversely of the X-ray beam, thus executing the 2t? scan. Because the 2ϋ scan is performed by rotation of the primary coUimator, the diffraction channel and the detector can remain in a fixed position, so that the problems associated with the angular rotation are avoided. In an embodiment of the X-ray analysis apparatus according to the invention the location of the axis of rotation for varying the orientation of the primary coUimator is chosen in such a manner that it extends substantiaUy through the X-ray focus on the anode surface. As a result of this step it is achieved that the swing of the primary coUimator is maximum during the execution of a 2ϋ scan; the primary coUimator thus utilizes the maximum width of the X-ray beam without leaving the beam. In another embodiment of the X-ray analysis apparatus according to the invention there is provided a further detector for detecting the diffracted radiation emanating from the sample, said further detector remaining in a fixed position relative to the sample and the X-ray source during the execution of diffraction measurements. As a result of this step the measuring range for a 2ϋ scan can be increased. This is of importance notably when a comparatively small sample diameter is used. The swing of the primary coUimator could then cause the irradiating beam to leave the sample. By arranging the second detector at an angle which is a given amount (for example, 30°) larger than the angle at which the sample is perceived by the first detector, the additional detection by the second detector during the execution of the 2ϋ scan ensures that two angular ranges are traversed simultaneously, i.e. the angular range of the first detector from the initial value ι9, to the final value ϋ 2 , and the angular range of the second detector from the initial value u\+30° to the final value # 2 +30°. As a result, the swing required for the 2d scan is smaller than when only one detector is used. In accordance with a further step of the invention, the further detector of the X-ray analysis apparatus is arranged in a plane extending through the first detector and the sample and, moreover, is situated to the other side of the sample in comparison with the first detector in this plane. As a result of this step two simultaneous ϋ scans are performed through the same angular range (one ϋ scan taking place in a direction which opposes that of the other ϋ scan). The renditions of the intensity variation of each of the ϋ scans can then be summed so as to obtain an improved signal-to-noise ratio (or a shorter measuring time for the same signal-to-noise ratio), or a different (for example, lower) value can be assigned to the resolution of the second diffraction detection channel in order to obtain the advantage of low resolution as well as that of high resolution during the measurement. Preferably, the X-ray source is constructed as an X-ray fluorescence tube. This offers the advantage that the high X-ray power of an X-ray fluorescence tube can be used for fluorescence measurements in the X-ray analysis apparatus which, however, can also be used for X-ray diffraction by installation of a controllable, rotatable primary coUimator. Moreover, in customary, commercially available diffraction tubes the X-ray focus is situated comparatively deeply within the tube, so that a substantial pan of the X-ray power emanating from the X-ray focus is intercepted by the tube wall and hence does not contribute to the X-ray beam emanating through the exit window. This exit beam, therefore, also has a comparatively small angle of aperture, so that the swing of the primary coUimator through this beam is also comparatively small. A fluorescence tube does not have these drawbacks or only to a much smaller extent. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows diagrammatically an apparatus for combined X-ray diffraction and X-ray fluorescence analysis; Fig. 2a shows diagrammatically a part of the apparatus for combined X- ray diffraction and X-ray fluorescence analysis in order to illustrate the execution of a ϋ scan in the configuration shown in Fig. 1; Fig. 2b shows some beam paths in order to illustrate Fig. 2a; Fig. 3a is a perspective view of the situation of two diffraction detector channels relative to the X-ray tube in order to increase the measuring range; Fig. 3b is a perspective view of the situation of two diffraction detector channels relative to the X-ray tube to both sides of the sample. Fig. 1 shows diagrammatically an apparatus for combined X-ray diffraction and X-ray fluorescence analysis. An X-ray tube 2 includes an anode 4 which produces an X-ray beam 6 from a given part of its surface (the X-ray focus). The X-ray beam leaves the X-ray tube via an X-ray window 8 and irradiates a sample 12 which is to be analyzed by means of the apparatus and is arranged in the sample location 10. The X-ray tube 2 of the present embodiment is constructed as an X-ray fluorescence tube, because such a tube can be used for fluorescence measurements as well as (in this example) for diffraction measurements. Such a tube produces polychromatic X-rays from a comparatively large region on the anode surface of the X-ray tube. It is assumed that the sample is a powdery sample or a type which may be considered to be powdery for diffraction purposes, such as a one-piece metal sample consisting of a large number of comparatively small crystallites. When it is exposed to the X-ray beam 6 emanating from the X-ray mbe, fluorescence radiation as well as diffracted radiation is produced in the sample 12. The fluorescence radiation is taken off as a beam 14 from one side of the sample for further analysis in a fluorescence detection channel 18. The beam 14 originates from all parts of the sample irradiated by the beam 6, and in principle is emitted in all directions. The diffracted radiation is taken off as a beam 20 at another side of the sample and is further analyzed in a diffraction detector channel 22. For fluorescence measurements the sample 12 is exposed directly to the radiation emanating from the X-ray tube 2. As opposed to the situation shown in the Figure, in that case no coUimator is arranged between the tube 2 and the sample 12. The fluorescence radiation produced by the radiation emanating from the X-ray tube 2 is applied to a first secondary coUimator 24 in the fluorescence detection channel 18 which is known per se; this secondary coUimator is constructed as a known Soller slit unit which parallelizes the incident beam 14. The beam 26 thus parallelized is incident on 6 an analysis crystal 28 which is constructed as a plane crystal in the present example. The analysis crystal 28 is arranged for a wavelength-selective analysis of the incident beam. The wavelength of the radiation reflected by the analysis crystal 28 is dependent of the angle of incidence of the radiation on the crystal. When the crystal is rotated about an axis 30, under the control of a control unit 32, a fluorescence spectrum of the intensity of the fluorescence radiation is recorded in dependence on the wavelength of the radiation. The radiation 34 reflected by the analysis crystal is detected in a detector 36. The fluorescence spectrogram can be displayed on a monitor 38. For diffraction measurements a parallelizing primary coUimator 40 is arranged between the X-ray tube 2 and the sample 12. This coUimator is called a primary coUimator because it is arranged ahead of the sample, as opposed to a secondary coUimator, such as the coUimator 24, which is arranged in the beam path behind the sample. The coUimator 40 is called a parallelizing coUimator because it ensures that the X-rays emanating from the X-ray tube 2 in all directions are incident on the sample in only one, for the beam mutually parallel direction. The orientation of the coUimator 40 with respect to the sample, i.e. the angle at which the radiation emanating from the coUimator is directed onto the sample, can be continuously varied, under the control of control means 42, during the execution of a diffraction measurement. Such continuous variation takes place about an (imaginary, non-material) axis 44 which extends transversely of a symmetry axis of the X- ray beam, i.e. perpendicularly to the plane of drawing in Fig. 1. The execution of a ϋ scan in this configuration will be described in detail with reference to Fig. 2. The diffracted beam 20 is further processed by the diffraction detector channel 22. This channel includes a second, secondary coUimator 24 which is constructed as a combination of two known Soller slit units which are consecutively arranged in the beam path, the first one of these units being provided for transverse coUimation of the beam and the second for collimating the beam in a direction perpendicular thereto. Because the beam emanating from the sample is polychromatic, there is provided a monochromator crystal 52 which selects the desired wavelength from the beam 20 by the known Bragg reflection (via angular adjustment of this crystal 52 with respect to the beam 20 incident thereon). The radiation 54 reflected by the monochromator crystal is detected by a detector 56. The diffraction diagram obtained by variation of the angle of incidence ϋ can be displayed on a monitor 58. Fig. 2a shows a position of the primary coUimator 40 in which the rays emanating from this coUimator are substantially perpendicularly incident on the surface of the sample 12. In the sample 12 there is shown a crystallographic lattice face 44 wherefrom Bragg reflection takes place in the situation shown. A representative ray 46, forming part of the beam incident on the sample, encloses an angle ϋ with respect to the lattice face 44. A representative ray 48, forming part of the beam diffracted by the sample, also encloses an angle ϋ with respect to the lattice 44 in conformity with the Bragg diffraction theory. It will be apparent from the Figure that the angle between the incident ray 46 and the diffracted ray 48 has the value 2ϋ. A ϋ scan is performed by variation of the angle of incidence ϋ, so that each time a different set of parallel lattice faces is irradiated by the incident beam 46. This situation wUl be described in detail with reference to Fig. 2b. Fig. 2b shows two incident representative rays 46a and 46b which are incident at angles ϋ Λ and ι5 b , respectively, on lattice faces 44a and 44b, respectively, in the sample 12. The Figure shows that in both cases the diffracted ray 48 has the same direction and is further processed by the diffraction detector channel 22 (diagrammatically shown as a cylinder). Therefore, during the execution of diffraction measurements this channel can remain in a fixed position relative to the sample 12 and the X-ray source 2. Fig. 3a is a perspective view of the situation of two diffraction detector channels 50a and 50b relative to the X-ray tube 2. In this Figure the polychromatic X-rays emanating from the X-ray tube 2 are incident on the sample 10 via the coUimator 40. The variability of the angle of incidence of the radiation on the sample is symbolically represented by an axis 60 in this Figure. Whereas the diffraction detector channel 22a covers a first angular range, the diffraction detector channel 22b covers a second angular range which has been shifted through a given value of, for example, 30° with respect to the first angular range (for example, for the channel 22a the angular range extends from the initial value ϋ { to the final value ύ 2 , and for the channel 22b_ the angular range extends from the initial value c^+30 0 to the final value t? 2 +30°). Thus, by simultaneous application of two diffraction detector channels a larger angular range can be obtained for the diffraction measurement. Fig. 3b is a perspective view of the situation of the two diffraction detector channels 22a and 22b_, relative to the X-ray tube 2, to both sides of the sample 10. As a result of this arrangement of the diffraction detector channels it is achieved that two simultaneous ϋ scans are performed through the same angular range (one ϋ scan extending in a direction which opposes that of the other ϋ scan). The renditions of the intensity variation of each of the ϋ scans can then be summed so as to obtain an improved signal-to-noise ratio (or a shorter measuring time for the same signal-to-noise ratio), or a different value, for example a smaller value, can be imparted to the resolution of the second diffraction detection channel in order to obtain the advantage of a low resolution (and an associated higher sensitivity) as well as that of a high resolution (with a lower sensitivity) during a measurement. This can be explained as follows. It is assumed that a diffraction measurement is performed on a material which exhibits reflections which do not have other reflections in their direct vicinity but exhibit a low intensity, as well as reflections which do have other reflections in their direct vicinity but exhibit a high intensity. Via one diffraction detection channel (for example 22a with a low resolution) said former reflections can then be measured in an adequately noise-free manner, whereas the latter reflections can be measured with adequate resolution in the same ϋ scan by means of the other diffraction detection channel 22b, The situation of the diffraction detector channels 22a and 22b shown in Fig. 3b can also be advantageously used in situations involving automatic input feeding of a large number of samples into the diffraction apparatus. These samples may be of a type requiring a high resolution or a type for which a lower resolution (with higher sensitivity) suffices. When use is made of the two diffraction detector channels shown in Fig. 3b, both requirements can be satisfied without necessitating a time-consuming readjustment of the apparatus. It is then merely necessary to inform the apparatus, prior to the measurements, which of the samples to be introduced are to be measured by one channel and which samples by the other channel. However, it is alternatively possible to provide the sample holders with a code which can be recognized by the apparatus and specifies the channel to be used.

Claims: CLAIMS: 1. An apparatus for the examination of materials by means of X-ray diffraction, including * a sample location (12) for accommodating a sample (10) of the material to be examined, * an X-ray source (2) for irradiating the sample location by means of X- rays, * a parallelizing primary coUimator (40) which is arranged between the X- ray source and the sample location and whose orientation with respect to the sample can be varied, and * a detector (22) for detecting the diffracted radiation (20) emanating from the sample, characterized in that * the apparatus includes control means (42) for continuously varying the orientation of the primary coUimator (40) about an axis (44) extending transversely of the X- ray beam during the execution of a diffraction measurement, and that * the detector (22) remains in a fixed position relative to the sample (10) and the X-ray source (2) during the execution of diffraction measurements. 2. An apparatus as claimed in Claim 1, in which the axis of rotation (44) for varying the orientation of the primary coUimator (40) extends substantially through the X- ray focus on the anode surface (4). 3. An apparatus as claimed in Claim 1 or 2, including a further detector (22b for detecting the diffracted radiation emanating from the sample, said further detector remaining in a fixed position relative to the sample and the X-ray source during the execution of diffraction measurements. 4. An apparatus as claimed in Claim 3, in which the further detector (22b) is arranged in a plane extending through the first detector (22a) and the sample (10) and to a side of the sample other than the first detector. 5. An apparatus as claimed in any one of the preceding Claims, in which the X-ray source (2) is constructed as an X-ray fluorescence tube.

Inventors: DE BOER DIRK KORNELIS GERHARDU, DE LANGE ROELOF, VAN DEN HOOGENHOF WALTHERUS WI, VAN DER SLUIS PAUL, DE BOER, DIRK, KORNELIS, GERHARDUS, DE LANGE, ROELOF, VAN DEN HOOGENHOF, WALTHERUS, WILHELMUS, VAN DER SLUIS, PAUL
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: G01N23
IPC codes: G01N23, G21K1