FIELD OF THE INVENTION

The invention relates to an X-ray diffractometer. More particularly, it relates to a diffractometer for analyzing the structure of a material from the scattering pattern produced when an X-ray beam interacts with it.

BACKGROUND ART

An X-ray diffractometer is a measuring instrument for analyzing the structure of a material from the scattering pattern produced when an X-ray beam interacts with it. Patent application EP0497406 describes such an X-ray diffractometer device. An X-ray source emits a divergent X-ray beam that irradiates the surface of a sample under investigation. By X-ray diffraction this beam is scattered in directions obeying Braggs law. The scattered beams are subsequently detected by a detector rotatable around a first rotation point. This rotation ensures that all, or at least a large fraction of all scattered beams can be detected sequentially. To enlarge the accessible number of scattered beams the angle at which the X-ray beam hits the sample surface can be varied. To accomplish this variation the sample is fixed by means of a sample holder on a first arm, which is in turn connected to a second arm at the first rotation point. The second arm is rotatably connected to the X-ray source. The center of rotation for the detector coincides with the first rotation point. When the sample is dragged along a main axis of the X-ray beam by means of a spindle, the connections between the first and second arm force the sample to rotate as well and thereby vary the angle between incident beam and sample surface.

When scattered beams at low scattering angles need to be measured (as is often the case), the divergent X-ray beam may hit the detector directly, without first being scattered by the sample. This leads to a seriously increased background. SUMMARY OF THE INVENTION

One of the objects of the invention is to eliminate this unwanted background which may occur using the X-ray diffractometer of the state of the art.

A first aspect of the invention provides an X-ray diffractometer comprising a first arm and a second arm. The first arm is rotatably connected to the second arm at a first rotation point. The diffractometer also comprises a sample holder for holding a sample under investigation, the sample holder being mounted onto the first arm at an outer end of the first arm away from the first rotation point. An X-ray source is configured to emit a divergent X-ray beam so as to irradiate a surface of the sample, wherein the second arm is rotatably connected to the X-ray source at a second rotation point. A detector is configured to detect scattered beams coming from the sample, the detector being rotatably arranged around the first rotation point. The diffracto meter also comprises guiding means configured to guide the sample holder along a main axis of the X-ray beam thereby varying an angle between the first and second arm. The connection between the first and second arm forces the sample holder to rotate relative to the main axis of the X-ray beam so that an angle between the main axis of the X-ray beam and the surface of the sample is varied.

The diffracto meter is characterized in that it comprises a shield rotatably arranged around a third rotation point which rotation point is fixed relative to the detector, wherein the shield is configured and arranged to shield the detector from the divergent X-ray beam coming from the X-ray source but not obstructing the scattered beams coming from the sample so as to allow the scattered beams to reach the detector.

When scattered beams at low scattering angles need to be measured, the detector may be moved into the divergent X-ray beam thereby measuring unwanted background. By adding a shield that moves together with the detector in the direction of the X-ray beam, the detector can be shielded in such a way that the divergent X-ray beam will not reach the detector but still reaches the sample. Furthermore, by rotatably arranging the shield, the shield can always be oriented in such a way that it will not obstruct scattered beams coming from the sample.

In an embodiment, the detector is coupled to the first rotation point by way of a detector arm, wherein the third rotation point is located on the detector arm next to the detector. By placing this third rotation point next to the detector the shield can be easily oriented not to obstruct scattered beams from the sample for high and low scattering angles, while the detector is shielded from the divergent X-ray beam.

In an embodiment, the shield comprises a outer end away from the third rotation point, wherein the diffractometer further comprises an orientation mechanism arranged to orientate the outer end of the shield so that while varying the angle between the main axis of the X-ray beam and the surface of the sample, the outer end points to a side edge of the sample which is nearest to the X-ray source. This construction guarantees that the shield is not obstructing the scattered X-ray beams coming from the sample. In an embodiment, the outer end of the shield is tapered. The tapered end ensures that the shield functions in the same way irrespective of the orientation relative to the detector.

The orientation mechanism may comprise a guiding block and a rod slidably arranged in the guiding block wherein the guiding block is rotatably arranged around the third rotation point and wherein the rod extends from the guiding block in the direction of the end of shield nearest to the sample.

In an embodiment, the shield is arranged at an outer end of the rod and wherein the shield is spring biased by means of a spring arranged between the shield and the guiding block. With this mechanism it can be avoided that the shield runs into the X-ray source at extremely low values for both the angle between the divergent X- ray beam and the sample surface and the distance between the detector and the sample surface.

In an embodiment, the orientation mechanism comprises a cable coupled between the detector arm near or at the third rotation point and a connection point on the first arm at the outer end of the first arm away from the first rotation point.

The flexibility of the cable enables the shield to be rotated away when the detector comes close to the sample surface. This way the shield will never obstruct the divergent X-ray beam before it intersects with the sample surface.

In an embodiment, the diffractometer comprises a spring loaded roll which pulls at an outer end of the cable. The use of a spring loaded roll minimizes the space needed at the rear end of the shield when the detector is moved close to the sample. Therefore, the construction can be made very compact.

In an embodiment, the diffractometer comprises blocking means which forces the shield to rotate around the third rotation point in a situation wherein the shield abuts against the blocking means so as to avoid the shield from shielding the sample under investigation. This ensures that the shield will never obstruct the divergent X-ray beam before it intersects with the sample surface, while the detector is still effectively shielded from the divergent X-ray beam.

In an embodiment, the shield comprises an extension extending in a direction perpendicular to a plane in which the first and second arm are able to rotate, the blocking means comprising a bar extending from the X-ray source towards the sample holder and being slightly angled relative to the main axis of the X-ray beam, wherein if the extension of the shield is forced against a side of the bar, the shield will rotate out of the X-ray beam. In an embodiment, the cable runs along a guidance point arranged on a bottom side of the shield or of the guiding block, wherein the cable bends at the guiding point due to forces originating from the shield being forced against the blocking means. This guidance point ensures that the shield is always oriented correctly.

In an embodiment, the diffractometer comprises a further shield arranged close to diaphragm, the further shield extending substantially parallel to the guidance bar, wherein the further shield prevents radiation scattered from the edge of diaphragm to enter the detector. This second shield reduces background radiation that

circumvents obstruction from the first shield, because it was scattered from the diaphragm.

In an embodiment, the further shield can be rotated away from its original position. This rotation prevents collision between the first and the second shield at extremely low values for both the angle between the divergent X-ray beam and the sample surface and the distance between the detector and the sample surface.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings, Figure 1 shows a top view of the X-ray diffractometer 1 according to an embodiment of the invention;

Figure 2 shows a top view of the X-ray diffractometer according to the embodiment of Figure 1 but in a different state;

Figure 3 is a perspective view of an embodiment of the X-ray diffractometer; Figure 4 is a perspective view of the embodiment of the X-ray diffractometer of Figure 3 seen from a different perspective;

Figure 5 shows the divergent X-ray beam coming from the source and hitting the sample;

Figure 6a schematically shows a top view of the shield and the bar;

Figure 6b schematically shows a top view of the shield and the cable;

Figure 7 shows a top view of the embodiment of Figures 3 and 4 in which the pin abuts against the bar;

Figure 8 shows a perspective view of the situation of Figure 7;

Figure 9 shows a perspective of an embodiment with a second screen close to the X-ray source to block X-rays scattered from the divergence slit; Figure 10 shows a top view of an embodiment at very low settings for both detector angle and sample position;

Figure 1 1 shows a detail of the same embodiment in the same state as in figure 10 with a provision to rotate the second screen to avoid collisions with the first screen, and

Figures 12-14 show details of part of the X-ray diffractometer according to an embodiment.

It should be noted that items which have the same reference numbers in different Figures, have the same structural features and the same functions, or are the same signals. Where the function and/or structure of such an item has been explained, there is no necessity for repeated explanation thereof in the detailed description.

DETAILED DESCRIPTION OF EMBODIMENTS

Figure 1 shows a top view of the X-ray diffractometer 1 according to an embodiment of the invention. The diffractometer comprises an X-ray source 2 which emits a divergent X-ray beam (not visible in Figure 1 ) that irradiates the surface of a sample 5 under investigation. In Figure 1 a main axis 3 of the divergent X-ray beam is indicated by a line 3. In the following the divergent X-ray beam is also referred to as the direct beam 50, see also Figure 5. A diaphragm 4 is positioned close to the X-ray source 2 to limit the beam divergence to ensure that only the sample surface is irradiated. However, because the direct beam 50 is also scattered by air the edges of the direct beam are rather diffuse. This results in a significant part of the direct beam hitting the detector without interacting with the sample.

Due to X-ray diffraction the direct beam 50 is scattered in directions obeying Braggs law. In Figure 1 , an arrow 6 indicated a main axis of one of the scattered beams. The scattered beams are subsequently detected by a rotating detector 7. This rotation ensures that all, or at least a large fraction of all scattered beams can be detected sequentially. In an embodiment the detector 7 comprises an array of sensors. By using an array of sensors rather than a single detector the sensitivity and the speed of the measurements is improved.

To enlarge the accessible number of scattered beams an angle a (hereafter: angle of incidence a) at which the direct beam 50 hits the sample surface can be varied. To accomplish this variation the sample 5 is fixed on a first arm 8 (also referred to as sample arm), which is in turn connected to a second arm 9 (also referred to as tube arm) at rotation point 10. The tube arm 9 is rotatably connected to the X-ray source 2 at a rotation point 1 1 . So both connections of tube arm 9 are rotation points. The center of rotation for a detection surface of the detector 7 coincides with the rotation point 10.

As shown in the top view of Figure 1 , the X-ray diffractometer 1 also comprises a spindle 12 and a coupling body 13 slidably arranged onto the spindle 12. The coupling body 13 is arranged to rotatably couple the sample arm 8 to the spindle 12. The coupling body 13 is moved along the spindle 12 by means of a motor (not shown). In this way the sample 5 being positioned in a sample holder at the outer end of the sample arm 8, can be dragged along the main axis 3 of the direct beam 50 by means of the spindle 12, the coupling body 13 and the motor. The spindle 12, the coupling body 13 and the motor are further referred to as the guiding means for guiding the sample 5.

When the sample 5 is dragged along a straight line parallel to the spindle 12, the connections between the sample arm 8, the tube arm 9 and the coupling body 13 force the sample 5 to rotate as well and thereby vary the angle a between the incident beam (i.e. main axis 3) and sample surface. As can be seen from Figure 1 , an angle between the sample arm 8 and the tube arm 9 is two times a.

The diffractometer 1 further comprises a detector arm 15 which is rotatably coupled to the tube arm 9 at the rotation point 10. An angle between the main axis of the tube arm 9 and the detector arm 15 is referred to as angle β.

It is noted that the X-ray diffractometer 1 in Figure 1 is in a position wherein the diffracted beams 6 do not reach the detector 7. Once the angle β is increased, as will be shown in Figure 2, the diffracted beams 6 will reach the detector 7. But when scattered beams at low scattering angles need to be measured, the direct beam 50 might hit the detector 7 directly, without first being scattered by the sample 5 (as will explained in detail with reference to Figure 5, 6a and 6b). This leads to a seriously increased background.

To eliminate this unwanted background, a shield 16 is moved between the X-ray source 2 and the detector 7. This shield 16 is positioned to block radiation coming directly from the source 2, while radiation travelling via the sample 5 (i.e. the diffracted beams 6) can enter the detector 7 freely. The shield 16 is rotatable around a rotation point 17 near the detector 7. In the embodiment of Figure 1 , the rotation point 17 is arranged on the detector arm 15. The shield 16 may have a tapered outer end as can be seen from the top view shown in Figure 1. It is noted that all rotation points mentioned above are actually rotation axis which run parallel relative to each other and perpendicular to the paper plane of Figure 1.

When the detector 7 is rotated away from the sample 5, the shield 16 will be rotated to prevent it to block radiation scattered from the sample surface. This is realized by mounting the shield 16 on a ruler or guidance 18. The ruler or guidance 18 may be a rod 18 which is arranged between the detector arm 15 and the outer end of the sample arm 8 where the sample 5 is held. In an embodiment, the guidance 18 ensures that the shield 16 is directed towards the edge of the sample nearest to the X- ray source. Preferably the shield 16 is sharp at its outer end away from rotation point 17. The shield 16 rotates with the angle of incidence and functions in the same way for all angles of incidence.

Figure 2 shows a top view of the X-ray diffractometer 1 according to the embodiment of Figure 1 but in a different state. As compared to the situation of Figure 1 , the angle of incidence a is decreased. The angle β is set so as to be able to detect the diffracted beams. In Figure 2, the angle β is set to about 90°. The guiding rod 18 is fixed close to the sample and slidably mounted at the detector end in a guiding block 19. As the detector 7 moves closer to the sample, the rod 18 slides through the guiding block 19 and thus extends further at the far end of the detector 7.

Figure 3 is a perspective view of an embodiment of the X-ray diffractometer 1 . The diffractometer 1 comprises a bottom plate 30 on which an X-ray source 2 is mounted. A spindle 12 is also arranged on the bottom plate 30. A coupling body 13 is slidably arranged on the spindle 12. A motor 31 is arranged on the bottom plate 30 and configured to displace the coupling body 13 along the spindle 12. A sample holder 14 is arranged on the coupling body 13. The sample holder 14 holds the sample 5. Next to the bottom plate 30 a slider 32 is arranged which interacts with the coupling body so as to eliminate mechanical play thus ensuring an exact linear movement of the sample holder 14.

Figure 3 further shows a detector 7 arranged on the detector arm 15, and a shield 16 rotatably arranged next to the detector 7. The shield 16 is rotatable around a rotation point 17. The shield 16 comprises a main blocking part extending in a direction parallel to the bottom plate 30 and an upright extension extending in a direction perpendicular to the bottom plate. On top of the upright extension, a pin 33 is arranged. A cable 34 is arranged between the rotation point 17 and a connection point 35 near the sample holder 14. In this embodiment the cable 34 functions as guidance 18.

Figure 3 further shows a bar 36 which is connected to the X-ray source 2 and extends along the X-ray beam (not shown) coming from the X-ray source 2. Finally, Figure 3 also shows a motor 38 arranged and configured to determine the angle β of the detector arm 15.

Figure 4 is a perspective view of the embodiment of the X-ray diffracto meter of Figure 3 seen from a different perspective. In Figure 4 the tube arm 9 is visible. Furthermore, the diaphragm 4 is visible through which the divergent X-ray beam leaves the X-ray source 2. It is noted that the X-ray source 2 can be a point source or a line source.

Figure 5 shows a top view of the detector 7, the sample 5 and the shield 16 according to an embodiment. Figure 5 also shows the divergent X-ray beam 50 (i.e. the direct beam) coming from the source 2 and hitting the sample 5. The shield 16 is configured to prevent the direct beam 50 from directly hitting the detector 7. Due to the dimensions and the positioning of the shield 16, the direct beam 50 will not hit the detector 7 and only diffracted beams 6 will reach the detector 7. This means that due to the shield 16, so-called background is prevented. Please note that the dimensions of the sample 5 relative to the detector 7 are exaggerated in Figure 5.

In the embodiment shown in Figure 5, the shield 16 comprises a tapered outer end away from the rotation point 17. The diffracto meter comprises an orientation mechanism arranged to orientate the tapered outer end of the shield 16 so that the tapered outer end points to a side edge of the sample 5 which is nearest to the X-ray source 2. Due to the combination of the tapered outer end (i.e. edge) of the shield 16 and the pointing to the side edge, an optimal part of the X-ray beam can be used to radiate the sample also in these situations with large a and β.

As the angle of incidence a increases the shield 16 may block the direct X- ray travelling to the sample surface. To prevent this, the shield 16 according to an embodiment is turned away with increasing angle of incidence. This is achieved by arranging a pin 33 (see also Figure 8) on the shield 16 which pin 33 will eventually meet a side of the bar 36. Such a situation is depicted in Figure 6b. Figure 6b schematically shows a top view of the shield 16 and the bar 36. An arrow 60 indicates in which direction the shield 16 is turned in order to avoid intercepting the direct beam 50 before it hits the sample 5. The shield 16 is guided by the flexible cable 34. When pin 33 abuts with bar 36 the cable 34 will be bent at the guidance point 42. The spring loaded roll 41 prevents the cable from hanging loose at all times so that the shield 16 will automatically turn back in case the pin 33 leaves the bar 36 again. In this way the shield 16 will return to its position determined by the cable 34 which tends to orientate the sharp side of the shield towards the side edge 51 of the sample 5, see Figure 5, through connection point 35. Because pin 33 is connected to the shield 16, the shield 16 is pushed out of the direct beam 50. This ensures the whole sample is illuminated at all times.

In the embodiment with the stiff guidance 18 shown in Figure 1 and 2, the flexible cable 34 is replaced by the stiff guidance 18. In this embodiment, the shield 16 is pushed along the stiff guidance 18 in its correct position by a spring 61 as shown in Figure 6a. As the sample 5 is moved away from the X-ray source 2, the bar 36 prevents the shield 16 to intercept the beam traveling to the sample surface by blocking pin 33. Because the pin 33 is connected to the shield 16, the shield 16 is pushed back along the stiff guidance 18 by the bar 36, thus ensuring the sample 5 is completely illuminated at all times. This translation is indicated by double sided arrow 70 in Figure 6a. At the same time spring 61 is loaded.

When the sample 5 is moved towards the X-ray source 2, or when the detector 7 is rotated away from the sample 5, the spring 61 will relax and push back the shield 16 in its original position. This ensures the shield 16 is functioning optimally at all times. The embodiment with the flexible cable is easier to realize as compared to the other embodiment.

Figure 7 shows a top view of the embodiment of Figures 3 and 4 in which the pin 33 abuts against the bar 36 in order to turn the shield 16 out of the direct beam 50 so as to avoid the shield 16 from blocking the sample under investigation.

Figure 8 shows a perspective view of the situation of Figure 7. As can be seen from Figure 8, the bar 36 forces the shield 16 to rotate around rotation point 17 in the direction of arrow 60 thus preventing it from intercepting the direct beam before it hits the sample.

To reduce the contribution of the air-scattered beam 50 sufficiently, the shield 16 must be have sufficient length. As a consequence, especially in the embodiment shown in Figure 6b, the tip of the shield 16 may collide with the X-ray tube 2 when both the spindle 12 and the detector 7 are driven to very low positions wherein the sample 5 is as close as possible to the X-ray tube 2 and the detector 7 is as close as possible to the sample 5. To prevent such a collision, the movement of the shield 16 shown in Figure 6a and the rotation of the shield 16 shown in Figure 6b could be combined in one mechanism.

Figure 9 shows an embodiment of the diffractometer 1 with such a combined mechanism. The shield 16 is mounted at an outer end of a guiding rod 91 , which is slidable in a guiding block 90. The guiding block 90 is rotatable around rotation point 17. Details of a possible embodiment of the guiding block 90 are described with reference to Figures 12-14. The shield 16 is spring loaded by means of a spring 92 arranged between the guiding block 90 and the shield 16. The assembly of the guiding block 90, the guiding rod 91 and the shield 16 is fixed to the spring loaded guidance 34 as was implemented in Figure 6b so that it rotates in the same way as the screen 16.

In the example of Figure 9, the guidance bar 36 now has a tapered end and is slightly bent at the end furthest away from the X-ray source 2. The sole purpose of this modification is to enable sample loading without opening the diffractometer. To change/load the sample 5, the sample holder 14 can be moved up by a user or automatically. Subsequently the sample 5 is exchanged/loaded and the sample holder 14 is moved back to the measurement position. This up-down movement is only possible if the guidance bar 36 is not in the way. Therefore, it is bent at the far end. Please note that at this far end the bar 36 cannot be used for the purpose of guiding the shield 16 properly. The impact of this to the accessible measurement range is however very limited.

Figure 10 shows a detail of the embodiment of Figure 9. As can be seen from Figure 10, the diffractometer 1 also comprises an extension 62 arranged close to the diaphragm 4 and extending from the guidance bar 36 at a side of the guidance bar 36 where the pin touches the guidance bar 36. When the pin 33 touches the guidance bar 36, the shield 16 is rotated away as was described with reference to Figure 6b. However, when the sample 5 is very close to the X-ray tube 2 and the detector 7 is turned to very low angles, the shield 16 may collide with the X-ray tube 2. To prevent this collision, the pin 33 is blocked by the extension 62. Consequently, the shield 16 is pushed back and the spring 92 is loaded. As soon as the detector 7 is moved away from this extreme position, the spring 92 is relaxed and the shield 16 is pushed back by the spring 92 into its original position.

The embodiment of Figure 9 and 10 comprises a second shield 63 arranged close to diaphragm 4. This second shield 63 extends substantially parallel to the guidance bar 36. The second shield 63 prevents radiation scattered from the edge of diaphragm 4 to enter the detector 7. As the second shield 63 extends into this scattered beam 50 (see also Figure 6b), it may collide with the first shield 16 at very low sample position and detector angle. To avoid this collision, the second shield 63 may either be positioned further away from the diaphragm 4 (and longer), or it may be rotatably mounted so that it can pushed away by a mechanism connected to the detector arm 15 as it is close to the position where the shield 16 would run into the second shield 63. The rotatable second shield 63 could be held substantially parallel to bar 36 by means of a spring 66, so that it will return back to a stationary position when not pushed against.

One possible solution is sketched in Figure 1 1 . Fixed on the shield 63 is a triangular block 64 at a height so that the sample arm 8 and the sample holder 14 can run free underneath. On the detector arm 15 a pin 65 is fixed, which pushes against the block 64 before the shield 16 can collide with the second shield 63. Because the block 64 is triangular shaped, the second shield 63 will be rotated away, thus avoiding a conflicting situation. A spring 66 is loaded, which pushes the second shield 63 back to its original position as soon as the detector arm 15 is moved away. The spring 66 in Figure 1 1 is just an example, it may be any type of spring such as a helical spring or leaf spring. Please note that the spring 66 doesn't need to be very strong.

Instead of using the block 64 and the pin 65, the rotation of the second shield could be activated by the first shield 16 itself.

It is noted that in the situation of Figure 10 and 1 1 the X-rays scattered by the diaphragm 4 will not hit the detector 7, so the turning away of the second shield 63 will not affect the measurement.

Figures 12-14 show details of part of the X-ray diffractometer according to an embodiment. The figures show the first shield 16, which is biased by a spring 92 arranged between the shield 16 and a surface of the guiding block 90. The guiding block is rotatable around the rotation point 17. The cable 34 is biased by means of the spring loaded roll 41 arranged in an associated housing. Coming out of the spring loaded roll 41 , the cable 34 runs along a pulley 95 and then along the bottom side of the shield 16. Once the shield 16 (in fact the pin 33 at the top of the field) is blocked by the bar 36, the cable 34 is bent at the guidance point 42 which may be embodied by two pins extending from the bottom of the guiding block 90 or of the shield 16 depending on the embodiment.

Figure 13 shows a sliced view of the figure 12, so as to better show the arrangement of the spring 92. As can be seen from figure 13, the bar 91 is mounted into a recess of the shield 16 and fixed by way of a bolt 96. Figure 14 shows the embodiment of Figures 12-13 but in the situation wherein the shield 16 is pressed towards the guiding block 90, see also Figure 10.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.