X-RAY SPECTROMETER

Abstract: An X-ray spectrometer comprises an X-ray source (2), an analysis crystal (7) for separating secondary radiation emitted by the sample into discrete wavelengths by diffraction, and an X-ray detector (8). The analysis crystal (7) comprises at least two planar segments (71, 72, 73) arranged such that the transverse profile of the crystal (7) has a shallow U- or V-shape. The analysis crystal (7) preferably comprises three segments: a central segment (71) and two outer segments (72, 73) adjacent the central segment (71) which are tilted out of the plane of the central segment (71).

Description: X-Ray Spectrometer This invention relates to an X-ray spectrometer, and in particular to an analysis crystal for use in such a spectrometer. In X-ray spectrometry an X-ray source is generally used to bombard the sample under investigation with X-rays. This causes the sample to emit secondary X-radiation with wavelengths characteristic of the constituent elements of the sample. A suitable single crystal is used as a diffraction device to separate the secondary radiation into discrete beams corresponding to the different wavelengths in accordance with the Bragg law n. λ = 2.d.sin θ, in which n = an integer (the "order" of the diffraction) , = the wavelength d = the crystal interlayer spacing, and θ = the angle of incidence An appropriately positioned detector is then used to measure the intensity of the emitted radiation of a particular wavelength, thereby yielding a qualitative and/or quantitative determination of the corresponding element. X-ray spectrometers can be subdivided into two categories: •sequential' spectrometers which measure several elements sequentially and 'simultaneous' spectrometers which can be used to measure several elements at the same time. Analysis crystals for use in sequential X-ray spectrometers are conventionally flat, whereas those used in simultaneous spectrometers are curved longitudinally. Flat crystals suffer from the disadvantage that the spectral resolution, especially at higher Bragg angles, tends to be dominated by the vertical divergence allowed by the primary collimator. In effect this results in asymmetric line shapes towards higher 2Θ angles. Crystals have also been proposed which are curved in the transverse direction (see European Patent Application No 0290058A) and, more recently, in both longitudinal and transverse directions (see European Patent Application No 0339713A, published after the priority date of the present invention) . Such curved crystals suffer from the disadvantage, however, that the formation of the necessary curvature may involve heat treatment or the application of mechanical stress to the crystal. This may cause imperfections in the crystal lattice which result in vertical divergence, offsetting at least partially the beneficial effect of the curvature. Also, the optimal degree of curvature may vary depending on the particular element under examination which necessitates the use of several such crystals or else some compromise in the radius of curvature. Furthermore, since the crystal must be thin (to permit it to be curved) , problems may be encountered with the use of high-energy X-rays which penetrate relatively deep into the crystal. We have now devised a crystal arrangement for use in X-ray spectrometers which is superior to the conventional flat crystals in several important respects and also offers advantages, notably simplicity of construction and adjustability, over the curved crystals described above. According to the invention there is provided an X-ray spectrometer comprising an X-ray source for bombarding a sample to be analysed with X-rays, an analysis crystal for separating secondary radiation emitted by the sample into discrete wavelengths by diffraction, and an X-ray detector, characterised in that the analysis crystal comprises at least two planar segments arranged such that the transverse profile of the crystal has a shallow U- or V-shape. The X-ray spectrometer of the present invention is advantageous primarily in that it produces a higher intensity of radiation at the detector than do X-ray spectrometers including known, flat analysis crystals. In addition, the spectral lines obtained may be narrower and more symmetrical which leads to improved resolution. Further, the spectrometer is advantageous over spectrometers including transversely curved crystals in that the analysis crystal is simpler to manufacture and such manufacture does not involve treatment of the crystal by, for example, heating or grinding which may have deleterious effects. Furthermore, the angle of tilt of the outer segments of the crystal can be easily optimised for the particular element or application under analysis; in other words, some 'tuning' is possible. The crystal may comprise any number of segments, eg up to seven segments. However, increasing the number of segments beyond about three increases the cost of manufacture without substantially improving the performance of the crystal. It is particularly preferred that the analysis crystal comprise three segments: a central segment and two outer segments adjacent the central segment which are tilted out of the plane of the central segment such that the profile of the crystal has a shallow U-shape. By 'outer segments' is meant segments to the right and left of the central segment as viewed along the line of incidence of the secondary radiation. The central segment of the analysis crystal is preferably the major portion of the crystal. For example, in a preferred embodiment, the crystal comprises three segments with widths in the ratio 1:2:1. The crystal may, for example, have overall dimensions of 60mm x 32mm, the central segment being 16mm wide and the outer segments 8mm wide. The angle of tilt of the outer segments out of the plane of the central segment is generally very small, eg of the order of 1 to 2°. As mentioned above, the angle may be adjusted. The adjustment may, for example, be brought about by simple mechanical means such as by adjustment of screws passing through a mounting plate located below the analysis crystal and bearing on the outer segments. We have found it particularly convenient to affix the crystal segments to a thin metal sheet which acts as a spring. The spring is fixed to the mounting plate, eg by adhesive, in the region of the central segment with the adjustment screws bearing against the spring in the region of the outer segments. The nature and thickness of the metal spring are not critical, provided that the spring has the necessary resilience. The spring may, for example, be of steel with a thickness of 0.15mm. The crystal may be of any suitable material including those conventionally used for this purpose. Such materials include clinochlore, potassium acid phthalate, rubidium acid phthalate, muscovite, gypsum, ammonium dihydrogen phosphate, ethylenediamine D-tartrate, pentaerythritol, α-quartz, graphite, germanium, silicon, sodium chloride, lithium fluoride, topaz, indium antimonide and thallium acid phthalate. The thickness of the crystal segments is not critical but will generally be up to about 5mm, typically of the order of 1mm or 2mm. The crystal segments need not necessarily consist of slabs of single crystal but may consist of a carrier coated with material which reflects X-radiation. Examples of such materials include lead mellisate, lead lignocerate, lead stearate and lead laurate. The elements which may be determined using the X-ray spectrometer of the present invention are those which are conventionally determined by X-ray spectrometry with a high 2Θ angle, eg light elements such as Al, Si, P, S, Cl and Mg and heavy elements like rare earths. Apart from the analysis crystal, the remaining components of the X-ray spectrometer may be of conventional design and will be familiar to those skilled in the art. A particularly preferred embodiment of the present invention will now be described, by way of illustration °nly, with reference to the accompanying drawings, in which Figure 1 is a schematic view of an X-ray spectrometer according to the present invention; Figure 2 is a perspective schematic view showing in more detail the disposition of a sample, an analysis crystal and a detector forming part of the spectrometer of Figure 1 ; Figure 3 shows a perspective view along the line of incidence of the secondary radiation of the analysis crystal; Figure 4 is a view in end elevation of a mounting for the crystal of Figure 2 with the crystal in place; and Figure 5 is a side view in elevation of the mounting of Figure 4. Referring first to Figure 1, an X-ray spectrometer comprises a housing (1) enclosing an X-ray tube (2) , a sample changer assembly (3) which delivers a cassette containing a sample (4) into the instrument and a detection unit comprising a primary collimator (6) , an analysis crystal (7) and a detector (8) fitted with a secondary collimator (9) . Angular movement of the analysis crystal (7) and the detector (8) is controlled by a Moire fringe goniometer (10) . The X-ray tube (1) is of conventional design, eg a rhodium-anode tube with a 0.127mm beryllium end window powered by a 3kW stabilized generator of maximum voltage 60kV and maximum current 100mA. As can be seen more clearly from Figure 2, the primary collimator (6) and secondary collimator (9) comprise sets of parallel blades. The Moire fringe goniometer (10) comprises a fixed radial grating (11) with 18000 lines, reader heads (12,13) for the crystal (7) and detector (8) respectively, and drive motors (not shown) which maintain the 2:1 coupling of the movement between the detector (8) and the analysis crystal (7) . It is of course possible to use more conventional goniometer designs involving reduction gears to maintain the 2:1 coupling of the detector and crystal movements but the software-controlled Moire fringe goniometer offers numerous advantages including better speed and precision of movement. The motor drive of the detector (8) and the analysis crystal (7) are not driven through reduction gears, but directly: one DC-motor rotates the axis of the analysis crystal drive, another DC-motor the axis of the detector drive. The angular positions of crystal (7) and detector (8) are measured directly by the respective reader heads (12,13) which count the Moire fringes produced during any movement. In various embodiments, the instrument may be equipped with more than one detector and more than one analysis crystal. In such cases, the detectors and crystals may be mounted on rotatable drums, the selection of the appropriate detector and/or crystal being made by rotation of the drum under microprocessor control. Furthermore, several independent goniometers may be mounted in a single instrument. In addition, the instrument includes other conventional items which are not shown, eg the various electronic control elements and vacuum pumping equipment and/or means for flushing the instrument with gas, eg helium or dry nitrogen, for the analysis of liquids. Also, a primary beam filter may be positioned between the X-ray tube (2) and the sample (4) ; the filter may be, for example, copper, aluminium or iron. Referring now to Figure 3, the analysis crystal (7) comprises three planar segments: a central segment (71) of dimensions 16mm x 60mm and two outer segments (72,73) of dimensions 8mm x 60mm. The outer segments (72,73) are tilted out of the plane of the central segment (71) by approximately 1° to 2' so that the transverse profile of the crystal (7) forms a shallow U-shape (the angle is exagerrated in the drawings) . The arrow in Figure 3 shows the line of incidence of the secondary radiation. The tilt of the outer segments (72,73) is shown more clearly in Figures 4 and 5 which also show that the crystal segments are mounted on a thin steel spring (74) which is affixed (by adhesive) in the region of the central segment (71) to a mounting plate (75) . Four adjustment screws (81) are held in threaded bores in the mounting plate (75) , two being located beneath each outer segment (72,73). The adjustment screws (81) bear against the underside of the steel spring in the region of the outer segments (72,73). The angle of tilt of the outer segments (72,73) can be adjusted by rotation of the adjustment screws (81) . In use, the X-ray source (2) bombards the sample (4) with X-radiation such that the sample (4) emits secondary X-radiation characteristic of its constituent elements. The beam of secondary radiation is collimated by the primary collimator (6) and is incident upon the analysis crystal (7) . When the Bragg condition is satisfied ie when the angle of incidence θ is such that sin θ - n. X/2.d the secondary radiation is diffracted by the crystal (7) . The secondary collimator (9) and the detector (8) are automatically positioned by the goniometer (10) to measure the intensity of the diffracted radiation, the wavelength of which is characteristic of an element in the sample.

Claims: / - 11 - Claims 1. An X-ray spectrometer comprising an X-ray source for bombarding a sample to be analysed with X-rays, an analysis crystal for separating secondary radiation emitted by the 5 sample into discrete wavelengths by diffraction, and an X-ray detector, characterised in that the analysis crystal comprises at least two planar segments arranged such that the transverse profile of the crystal has a shallow U- or 0 V-shape. 2. An X-ray spectrometer according to Claim 1, wherein the analysis crystal comprises from two to seven planar segments. 3. An X-ray spectrometer according to Claim 1 or Claim 2, 25 wherein the analysis crystal comprises three segments: a central segment and two outer segments adjacent the central segment which are tilted out of the plane of the central segment such that the profile of the crystal has a shallow U-shape. 20 4. An X-ray spectrometer according to Claim 3, wherein the central segment of the analysis crystal is the major portion of the crystal. 5. An X-ray spectrometer according to Claim 4, wherein the three segments have widths in the ratio 1:2:1. 25 6. An X-ray spectrometer according to Claim 3, wherein the angle of tilt of the outer segments is adjustable. 7. An X-ray spectrometer according to Claim 6, wherein the adjustment of the tilt of the outer segments is made by means of adjustment screws extending through a mounting plate to which the central segment is affixed. 8. An X-ray spectrometer according to Claim 1, wherein the planar segments are affixed to a thin metal sheet acting as a spring. 9. An analysis crystal for use in an X-ray spectrometer, comprising at least two planar segments arranged such that the transverse profile of the crystal has a shallow U- or V-shape.

Inventors: NEGRO PIERRE-YVES, VOGEL WILFRIED, YELLEPEDDI RAVISEKHAR, NEGRO, PIERRE-YVES, VOGEL, WILFRIED, YELLEPEDDI, RAVISEKHAR
Applicants: FISONS PLC
Application year: 1990
Main classification: G21K1
IPC codes: G01N23, G21K1