EP0805967A1

EP0805967A1 - X-ray spectrometer comprising a plurality of fixed measuring channels

Info

Publication number: EP0805967A1
Application number: EP96922175A
Authority: EP
Inventors: Walterus Alphonsus L. A. Van Egeraat; Robertus Jacobus Maria Brinker
Original assignee: Koninklijke Philips Electronics NV; Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1995-07-25
Filing date: 1996-07-17
Publication date: 1997-11-12
Also published as: JPH10507532A; WO1997005474A1

Abstract

An X-ray spectrometer comprising at least one fixed measuring channel can be used to measure a spectral line and possibly a limited vicinity thereof. If this spectrometer is to be arranged for the measurement of a plurality of spectral lines, in accordance with the invention it can be provided with a plurality of analyzer crystals which are associated with one, fixed detector only, and with a changer device for realizing each time a desired configuration in which one of the analyzer crystals is situated in a fixed location, associated with the relevant analyzer crystal in the beam path between the sample and said detector during the measurement. It is then no longer necessary to mount for each spectral line a separate measuring channel (i.e. an analyzer crystal and an associated detector) in a reserved special location on the housing of the spectrometer, so that a substantial cost saving is achieved.

Description

X-ray spectrometer comprising a plurality of fixed measuring channels.

The invention relates to an apparatus for X-ray analysis, comprising a sample location for a sample to be examined, means for generating X-rays in the sample, a detector for detecting the X-rays emanating from the sample, and at least one analyzer crystal which is arranged in the beam path between the sample and the detector, the sample location and the detector being situated at a fixed distance from one another in said apparatus and the analyzer crystal having a fixed location and orientation relative to the sample location as well as the detector during the measurement.

An apparatus of this kind is known from a leaflet published by Applicant and entitled "Simultaneous x-ray spectrometer system, PW2600".

In an X-ray spectrometer X-rays (the fluorescence radiation) are generated in a sample to be studied, which rays are characteristic of the elements and chemical combinations in the sample. The fluorescence radiation can be excited in the sample by electrons or by X-rays which originate, generally speaking, from an X-ray tube. The fluorescence radiation excited in the sample exhibits a more or less broad spectrum of wavelengths which are present in the spectrum with a given intensity distribution (characteristic of the sample). Therefore, because knowledge of the intensity of the fluorescence radiation at one or more wavelengths is desired, the fluorescence radiation must be analyzed as a function of wavelength. This can be realized by making the fluorescence radiation incident on an analyzer crystal. Such analysis is based on the well-known Bragg relation: 2d-sinι9=nλ, in which d is the distance between the X-ray reflecting crystal planes in the analyzer crystal, ϋ is the angle at which the radiation to be analyzed is incident on the analyzer crystal, and λ is the wavelength of the reflected radiation. Thus, by measuring the intensity of the radiation which has been subjected to a given deflection, the wavelength of this intensity becomes known. It is to be noted that herein the term "analyzer crystal" is to be understood to mean not only actual crystals, but also the known multilayer mirrors for X- rays. Such a mirror consists of a stack of several, thin layers having a comparatively small thickness of, for example 2 nm. They are also suitable for the analysis of long-wave X-rays, as opposed to namral crystals which practically always have a grid constant and hence an inherent spacing of the reflecting crystal planes which is too small for reflection of said long- wave X-rays.

Generally speaking, there are two types of X-ray spectrometers: sequential spectrometers and spectrometers comprising one or some fixed measuring channels. In a sequential spectrometer the wavelength composition of the radiation to be analyzed is determined by determining the intensity as a fimction of the incidence and take- off angle ϋ. This is realized by displacing and rotating the analyzer crystal and the detector in such a manner that all angles ϋ are passed through. This type of spectrometer owes its name to the fact that the intensity of the various wavelengths is sequentially measured during a continuous process.

A spectrometer comprising a fixed measuring channel is used if only the determination of the intensity of a limited number of discrete wavelengths (and possibly a very narrow surrounding range) is of interest. This could be, for example an application in given manufacmring processes in which, for example the concentration of one element or combination is of interest. In such a spectrometer, therefore, it is not necessary to pass through a ϋ range, which means that the sample location and the detector may be situated at a fixed distance from one another in such an apparams. Moreover, the analyzer crystal may then also have a fixed location and orientation relative to the sample location as well as the detector during the measurement.

In order to achieve a suitable resolution and sensitivity, and hence a suitable measuring accuracy, in both types of spectrometer, it is known to provide such apparams with a so-called focusing optical system; this means that a point or line at the area of the sample (the "object point") is imaged by the analyzer crystal on a point or line at the area of the detector (the "image point"). This imaging effect is achieved by imparting a curved shape to the analyzer crystal, for example a spherical or circular-cylindrical surface, so that the analyzer crystal not only has an analyzing function but also an imaging function. In order to satisfy the focusing condition nevertheless, for all desired values of ϋ the object point as well as the image point (i.e. the detector) should remain situated on a given circle, being the so-called Rowland circle. Moreover, the analyzer crystal should remain tangent to the Rowland circle. The diameter of the Rowland circle is determined by the radius of curvamre of the analyzer crystal, which radius is constant during the measurement. This configuration of components is referred to as the "Rowland geometry".

In a sequential spectrometer with a focusing optical system the position of the Rowland circle is variable during the measurement, be it that the object point and the image point should always remain simated on the Rowland circle and that the analyzer crystal should always remain tangent to the Rowland circle. During the execution of the measurement (performing a so-called t?-scan), the analyzer crystal is moved along a straight line, during which movement it is rotated in such a manner that it remains tangent to the Rowland circle. This straight line passes through the object point. The detector should then accurately describe a complex lemniscate path. This requires a complex displacement mechanism which must satisfy very severe requirements as regards accuracy and reproducibility.

In a spectrometer comprising a fixed measuring channel the position of the Rowland circle is fixed during the measurement, it again being necessary to satisfy the requirement that the object point and the image point should be simated on the Rowland circle and that the analyzer crystal should be tangent to this circle.

If it is desirable to use a spectrometer comprising a fixed measurmg channel to smdy more than one spectral line, known spectrometers of this kind can do so only after addition of additional channels, i.e. an additional analyzer crystal and an additional detector should be provided for each wavelength (each spectral line) to be studied, it being necessary to arrange said additional components in such a manner that they are simated on the Rowland circle associated with the radius of curvamre of the relevant analyzer crystal. This configuration (i.e. a configuration with an accurately defined position of the detector and the analyzer crystal relative to the sample) achieves optimum accuracy of measurement. Such additional measuring channels are commercially available as self-contained units and can be installed at a prepared location on the apparams. Applicant's cited leaflet shows a spectrometer in which a maximum of 28 fixed channels can be in operation in this manner. This type of spectrometer is referred to as a simultaneous spectrometer; it owes its name to the fact that the intensity of the various wavelengths can be simultaneously measured when a plurality of measuring channels are used. The sequential spectrometer and the simultaneous spectrometer both are expensive apparams. In the case of the sequential spectrometer the high costs are due to the need for the complex movement mechanism whereby nevertheless a high positioning accuracy must be achieved. In the simultaneous spectrometer the high costs are due to the fact that an own, separate analyzer crystal and an own, separate detector are required for each measuring channel, both components being very expensive. Moreover, some additional mechanical components are then also required, for example housings and connection flanges on the additional channel unit as well as on the location for attachment to the housing of the spectrometer.

It is an object of the invention to provide a spectrometer which comprises fixed measuring channels and can be substantially less expensive than the known simultaneous spectrometers. To this end, the invention is characterized in that the apparams comprises a plurality of analyzer crystals which are associated with one fixed detector only, and a changer device for realizing each time a desired configuration in which one of the analyzer crystals is simated in a fixed location, associated with the relevant analyzer crystal, in the beam path between the sample and said detector during the measurement.

The invention is based on the recognition of the fact that it is not necessary to equip each fixed measuring channel with its own detector, provided that the analyzer crystal is arranged in the beam path at the correct angle (associated with the desired wavelength to be selected) and at the correct distance from the sample and the detector.

In an embodiment of the invention, the fixed location of the analyzer crystal during the measurement is determined by the Rowland geometry. The Rowland circle is then defined by a suitable choice of the radius of curvamre of the analyzer crystal (since this radius defines the dimension of the Rowland circle). The angle ϋ, and hence the wavelength to be selected, is determined by the material of the crystal (defining the variable d) and by the situation of the crystal on the Rowland circle.

In an embodiment of the apparams in accordance with the invention the changer device is a slide which is linearly displaceable relative to the detector and in which the analyzer crystals are mounted. An alternative embodiment of the changer device consists of a wheel which is rotatable relative to the detector and on which the analyzer crystals are mounted.

In a further embodiment of the apparams in accordance with the invention the changer device comprises at least one detector collimation slit which is associated with one of the analyzer crystals. Because the various analyzer crystals may have different shapes and dimensions and could notably be arranged at different angles relative to the detector, generally speaking the beam between the relevant crystal and the detector will have a different appearance for each analyzer crystal. This makes it desirable to adapt the entrance collimator of the detector always to the instantaneously used analyzer crystal; this can be suitably realized by providing the detector collimation slit associated with the relevant analyzer crystal on the changer device instead of connecting it permanently to the detector so that when a given analyzer crystal is arranged in the beam path, the associated collimation slit is automatically moved in front of the detector.

In conformity with a further step of the invention, the detector collimation slit is adjustable. As a result of this step, the adjustment of the detector slit can be readily adapted to any new crystal to be provided. This holds notably if a multilayer mirror is chosen for the analyzer crystal. A known drawback of multilayer mirrors consists in that the period of the layers of these mirrors is of the desired order of magmmde, but between individual multilayer mirrors it exhibits a large spread, for example of the order of magmmde of 4%. This means that a deviation of the same order of magmmde may also occur for the value of ϋ, which deviation must be compensated for by a correspondmg variation of ϋ in conformity with said Bragg relation, i.e. by a rotation of the analyzer crystal. However, rotation of the analyzer crystal would rotate the direction of the emanating beam (i.e. the X-ray beam extending from the analyzer crystal in the direction of the detector) through double the correction angle, so that the (focused) beam would no longer enter the exit slit. This can be corrected by shifting the detector-collimation slit, i.e. by making it adjustable. In an alternative embodiment of the apparams in accordance with the invention the analyzer crystals are rigidly arranged relative to the sample location, and in order to change the analyzer crystal involved in a measurement the changer device is arranged to move the detector along a circular path around the sample location. In given circumstances it may be advantageous to arrange the crystals rigidly in the apparams and to rotate the detector around the assembly formed by the sample and the crystals; it is then assumed that the fluorescence radiation emanating from the sample does not have a preferred orientation, so that in this sense the sample may be considered to be circular-symmetrical.

In a further embodiment of the apparams in accordance with the invention, the changer device comprises a plurality of identical analyzer crystals, each of which has its own fixed location and orientation during the measurement. Because analyzer crystals are expensive components, with a view to the cost of inventory control and range of products it is attractive to use an as small as possible series of different crystals. This purpose is also served by the insight that a given crystal (i.e. a crystal having a given composition and a given radius of curvamre), if irradiated at a different angle and arranged in a different location, can serve for the analysis of another wavelength and hence can yield another measuring channel.

The changer device in a further embodiment of the invention is arranged so that the displacement of the analyzer crystals and the detector relative to one another is such that the displacement direction at the area of the fixed location in the beam path is perpendicular to the diffraction plane of the analyzer crystal. Any play in the displacement direction of the crystals or in that of the detector, consequently, hardly affects the accuracy of the apparams in the diffraction direction (i.e. the value of ϋ).

In a further embodiment of the apparams of the invention the sample, the analyzer crystal and the detector collimator are accommodated in a measuring space which can be hermetically sealed, the sample being simated in a sample space which can be separated from the measuring space by way of a valve which is slidable in its own plane.

Generally speaking, in order to achieve an as high as possible intensity of the fluorescence radiation the aim is to arrange the sample as near to the X-ray source as possible. Another requirement is that the changing of the sample should be easy and fast, so that it is objectionable to evacuate or fill a measuring space, evacuated or filled with a special gas, each time when the sample is changed. To this end, it is desirable to make the sample space separable from the remainder of the measuring space in a gastight manner, so that only the (small) sample space need be vented and subsequently conditioned again when the sample is changed. In order to ensure that the valve required for this purpose does not impose a larger distance between the X-ray mbe and the sample, the valve is constructed as described above.

Said valve is preferably provided with a material which does not transmit X-rays. In addition to its function as a vacuum valve, it can then also serve as a beam stop for the X-rays in the case of a change of sample; in many countries this function is prescribed by law for reasons of safety.

In a further embodiment of the apparams in accordance with the invention, the slidable valve contains a magnetizable material and an electric winding is provided around the valve seat. If the valve is made of, for example iron, in the case of a sample change the iron can be magnetized by excitation of the electric winding so that the valve is firmly pulled against the valve seat. (Automatic) locking of the valve can thus be simply realized.

These and otiier aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings:

Fig. 1 is a partly sectional view of a part of an X-ray spectrometer in accordance with the invention; Fig. 2 is a diagrammatic representation of an alternative changer device for use in the X-ray spectrometer in accordance with the invention;

Fig. 3 is a diagrammatic representation of a changer device in the form of a linearly displaceable slide for use in the X-ray spectrometer in accordance with the invention; Fig. 4 shows a device comprising a vacuum valve for separating the sample space from the remainder of the measuring space, which valve can also be used as a beam stop.

Fig. 1 shows the part of an X-ray spectrometer which is of relevance to the invention. The X-ray spectrometer comprises an X-ray mbe 2 for generating the X-rays whereby a sample 4 to be examined is irradiated. In the sample X-ray fluorescence radiation is generated, which radiation is reflected to an X-ray detector 10 by an analyzer crystal 6-1. It is not an essential aspect of the present invention that the X-ray fluorescence radiation is generated in the sample by means of X-rays emanating from the X-ray mbe; it is also feasible for the sample to be irradiated, for example by means of electrons so that X-rays are generated in the sample. The X-rays reflected by the analyzer crystal 6-1 reach the detector 10 via a detector-collimation slit 12 which may be adjustable, if desired, in respect of location as well as cross-section. The beam path from the X-ray mbe to the detector extends in a measuring space 16 which can be hermetically sealed; the sample 4 may be arranged in a sample space 20 which is separate from the measuring space and can be isolated therefrom by means of a valve 24. The space 16 is enclosed by a housing 18. The separate sample space is provided with its own access 22. The measuring space can thus be conditioned (for example, evacuated, filled with a desired gas, or adjusted to a desired temperamre) in a manner imposed by the measurement to be executed. If the sample is to be changed, the valve 24 is closed so that not the entire measuring space need come into contact with the ambient atmosphere. The sample is changed via the access 22, after which only the (much smaller) sample space need be adapted to the measuring conditions again. o

The analyzer crystal 6-1 is accommodated, together with other analyzer crystals, on a changer device in the form of a wheel 26. Only one of the other analyzer crystals, i.e. the crystal 6-2, is shown in the Figure. Each of said other analyzer crystals can be arranged in the beam of X-rays emanating from the sample 4 in the correct orientation and in the correct location. The wheel 26 is arranged so as to be rotatable about an axis 28, so that it can be driven, externally of the measuring space, so as to be moved to a desired position. If desired, the correct position can also be defined by an abutment (not shown). It is also possible to provide the detector-collimation slits 12, 14, associated with the analyzer crystals 6-1 and 6-2, respectively, on the changer device formed by the wheel. As a result, in the case of a change of analyzer crystal the detector is automatically given the correct collimation slit.

Fig. 2 shows an alternative embodiment of the changer device for use in the X-ray analysis apparams in accordance with the invention. Like in Fig. 1, in Fig. 2 the X-ray mbe 2 which irradiates the sample 4 by means of X-rays is accommodated in the housing 18. Around the sample 4 there is arranged a number of analyzer crystals 6-1, 6-2, ... 6-n, ..., which have a fixed location and a fixed orientation relative to the sample 4. In this embodiment the detector 10 is arranged so as to be rotatable about the sample 4, so that it can occupy a number of positions associated with the permanently arranged analyzer crystals. These positions are denoted by the references 10-1, 10-2, 10-3, ..., 10-n in Fig. 2. This arrangement can be used in cases where the location and orientation of the analyzer crystal relative to the sample must satisfy very severe accuracy requirements, whereas this does not apply to the detector, for example because the detector has a very large detector surface.

Fig. 3 is a diagrammatic representation of a changer device in the form of a linearly displaceable slide for use in the X-ray spectrometer in accordance with the invention. The X-ray mbe 2 for irradiating the sample 4 is rigidly arranged relative to the sample, like the detectors 10-1 and 10-2. (It is not an essential aspect of the invention that two detectors are shown in this Figure.) The analyzer crystals 6-1, 6-2 to 6-n are mounted on a slide by way of respective supports 5-1, 5-2 to 5-n, each analyzer crystal again having the correct desired location and orientation relative to the sample and the detector. The slide is slidable in the direction of the arrow 9, so that the desired analyzer crystal can be moved into the beam to be analyzed.

Fig. 4 is a more detailed representation of the junction between the sample space 20 and the measuring space 16. The housing 18 comprises a holder 36 in which the X-ray mbe 2 (see Fig. 1) can be arranged. In this Figure the holder is closed by a blind flange 38. In conjunction with the intermediate piece 34, the valve 24 hermetically seals, if desired, the connection between the measuring space 16 and the sample space 20. This valve is rotatable in its own plane, i.e. perpendicularly to the plane of drawing. A gastight seal is realized by means of the rubber rings 30 and 32. If the valve is made of a magnetizable material, for example iron, it can be maintained in position by means of an electric winding 42 which is arranged around the valve seat. This valve may notably be provided with a layer of an X-ray absorbing material (not shown), so that in the case of a sample change a beam stop is automatically created for the X-ray beam because the valve is then automatically closed and locked. The assembly formed by the sample space 20 and the holder 36 can be mounted on the remainder of the housing of the measuring space by way of a flange 40.

Claims

CLAIMS:

1. An apparams for X-ray analysis, comprising

* a sample location for a sample to be examined,

* means for generating X-rays in the sample,

* a detector for detecting the X-rays emanating from the sample, * and at least one analyzer crystal which is arranged in the beam path between the sample and the detector, the sample location and the detector being simated at a fixed distance from one another in said apparams and the analyzer crystal having a fixed location and orientation relative to the sample location as well as the detector during the measurement, characterized in that the apparams comprises a plurality of analyzer crystals which are associated with one fixed detector only, and a changer device for realizing each time a desired configuration in which one of the analyzer crystals is simated in a fixed location, associated with the relevant analyzer crystal, in the beam path between the sample and said detector during the measurement.

2. An apparams as claimed in Claim 1, in which the fixed location of the analyzer crystal during the measurement is determined by the Rowland geometry.

3. An apparams as claimed in Claim 1, in which the changer device comprises a slide which is linearly displaceable relative to the detector and in which the analyzer crystals are mounted.

4. An apparams as claimed in Claim 1 in which the changer device comprises a wheel which is rotatable relative to the detector and on which the analyzer crystals are mounted.

5. An apparams as claimed in Claim 3 or 4, in which the slide or the wheel comprises at least one detector collimation slit which is associated with one of the analyzer crystals.

6. An apparams as claimed in Claim 5, in which the detector collimation slit is adjustable.

7. An apparams as claimed in Claim 1, in which the analyzer crystals are rigidly arranged relative to the sample location and, in order to change the analyzer crystal involved in a measurement, the changer device is arranged to move the detector along a circular path around the sample location.

8. An apparams as claimed in Claim 1, in which the changer device comprises a plurality of identical analyzer crystals, each of which has its own fixed location and orientation during the measurement.

9. An apparams as claimed in Claim 1, in which the changer device is arranged so that the displacement of the analyzer crystals and the detector relative to one another is such that the displacement direction at the area of the fixed location in the beam path is peφendicular to the diffraction plane of the analyzer crystal.

10. An apparams as claimed in Claim 1, in which the sample, the analyzer crystal and the detector collimator are accommodated in a measuring space which can be hermetically sealed, the sample being simated in a sample space which can be separated from the measuring space by way of a valve which is slidable in its own plane.

11. An apparams as claimed in Claim 10, in which the slidable valve is provided with a material which does not transmit X-rays.

12. An apparams as claimed in Claim 10, in which the slidable valve contains a magnetizable material and an electric winding is provided around the valve seat.