Abstract:
A device for identifying materials on a conveyor belt ( 101 ) by means of X-ray fluorescence comprising an X-ray source ( 102 ), from which X-ray radiation ( 103 ) is guided onto material parts ( 104 ), comprising a detector head ( 107 ) containing an X-ray detector array ( 108 ) having a multiplicity of detector elements ( 113, 114, 115 ) arranged in a planar fashion for receiving X-ray radiation ( 105 ) and for converting said X-ray radiation into electrical charge signals, and also an electronic unit ( 109 ) for reading out and processing the charge signals, which comprises for each individual detector element a signal channel ( 120 ) having in each case:
       a discriminator unit ( 117 ) having at least two adjustable discriminator thresholds ( 116, 122 ) for detecting all Gaussian curve-like signals ( 119 ) whose amplitude is greater than one of the two or simultaneously greater than both discriminator thresholds,   and also one counting unit ( 118, 121 ) per discriminator threshold for converting the signals into digital counting events,
 
wherein the individual detector elements of the X-ray detector array have a spatial resolution of 50 μm to 500 μm with a sensitivity to X-ray radiation in an energy range of between 500 eV and 30 keV, with an energy resolution of less than 0.5 keV at counting rates of up to 100 kcps and relative to an energy of 8.04 keV, the electronic unit comprises a signal channel for each individual detector element of the X-ray detector array,
 
and each discriminator unit for a specific detector element is in each case electrically connected to the discriminator units of the detector elements that are spatially directly adjacent to said detector element, wherein all the discriminator units are interconnected with one another via a digital and/or analog circuit in such a way that simultaneous occurrence of signals on more than one detector element can be identified and treated electrically separately.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    This invention relates generally to the field of X-ray fluorescence and, more particularly, to the use of X-ray fluorescence for sorting materials. 
       Description of the Related Art 
       [0002]    The invention relates to a device for identifying materials, in particular metal parts such as scrap particles, on a stationary or moving conveyor belt by means of X-ray fluorescence comprising an X-ray source, from which X-ray radiation is guided onto the material parts to be examined on the conveyor belt, comprising a detector head containing an X-ray detector array having a multiplicity of detector elements arranged in a planar fashion for receiving X-ray radiation scattered at the material parts or excited by the latter and for converting said X-ray radiation into electrical charge signals, and also an electronic unit for reading out and processing the charge signals from the detector elements, which comprises for each individual detector element of the X-ray detector array a signal channel having in each case: a discriminator unit having at least two adjustable discriminator thresholds for detecting all Gaussian curve-like signals whose amplitude is greater than one of the two or simultaneously greater than both discriminator thresholds; and also one counting unit per discriminator threshold for converting the analog Gaussian curve-like signals into digital counting events. A sorting device having these features is known from U.S. Pat. No. 8,855,809 B2 (Reference [1]). 
         [0003]    Measurements using X-ray fluorescence are powerful methods of instrumental analysis which can be used to detect and characterize metallic materials, in particular. Various measurement geometries are used depending on the analytical objective. 
         [0004]    Normally an X-ray source irradiates material lying on a stationary or moving conveyor belt with X-ray radiation. The X-ray fluorescence radiation generated in the process is usually guided via a collimation unit and subsequently impinges on an individual X-ray radiation detector or on an X-ray detector array. Such an array may be composed of a plurality of discrete X-ray radiation detectors or is formed by a multiplicity of individual detector elements that are usually arranged on a monolithic semiconductor substrate. The simultaneous combination of discrete and monolithic detectors is also possible. Generally, such a detection system has the task of determining the chemical composition of the materials moved on a moving conveyor belt and of monitoring them, or of determining the exact spatial position of materials on the conveyor belt which have a predefined chemical composition. 
         [0005]    Reference [ 1 ] describes a generic material sorting device on the basis of X-ray fluorescence measurements using a variety of different types of detector elements of the X-ray detector array: Reference [ 1 ] mentions X-ray detector arrays based on the sensor materials CdT (cadmium telluride) and CZT (cadmium zinc telluride) or the detector technologies SiLi (lithium-drifted silicon detectors), SDD (silicon drift detectors) and phosphor-on-silicon. In particular, the detector types Si-Pin and XR-100T-CZT from Amptek and the so-called NEXIS system from Nova R&amp;D are mentioned. 
         [0006]    The detection technologies mentioned in Reference [ 1 ] are distinguished by individual strengths. In this regard, by way of example, the NEXIS system has a high spatial resolution and can determine the spatial position of individual materials relatively precisely. This is achieved by virtue of the small spacing of its individual detector elements of 1 mm. In addition, this detector has a very high dynamic range. Each individual detector element can process radiation intensities of the order of magnitude of up to 10 6  X-ray photons per second. However, the NEXIS system has a relatively poor energy resolution of several 1000 eV (FWHM) and is therefore not able to analytically separate chemical elements whose fluorescence spectra are energetically close together. Moreover, the NEXIS system in particular in conjunction with the detector materials CdT and CZT is suitable only to a very limited extent for an X-ray fluorescence analysis in the energy range of less than 20 keV. 
         [0007]    Other detectors mentioned in Reference [1] are based on the technologies Si-Pin, SiLi and SDD and have best energy resolutions of less than 150 eV @ 5.9 keV (FWHW). They are able to precisely analyze the X-ray fluorescence spectra of different chemical elements in the energy range of from 1 keV up to normally 30 keV. However, the usable global counting rate of such an X-ray radiation detector is limited and much lower than in the NEXIS system. In practice, detector arrays based on the technologies Si-Pin, SiLi or SDD are generally composed of discrete individual detectors. In this case, each individual detector has an active area of several square millimeters. Therefore, the spatial resolution of materials that is achievable in the case of such configurations is one or more orders of magnitude worse than in the case of the NEXIS system. On the basis of these detector technologies, there are also monolithic solutions having a plurality of detector elements on a single semiconductor substrate. However, here as well the individual detector elements are at a distance of at least several millimeters from one another. Moreover, the technologies Si-Pin, SiLi and SDD require very costly cooling of the sensor to temperatures of significantly less than −10° C. 
         [0008]    For applications that simultaneously require a high energy resolution and also a high dynamic range and a high spatial resolution, Reference [ 1 ] proposes the formation of hybrid X-ray detector arrays composed of detectors of different types of detector technologies. In particular, the patent mentions hybrid detector arrays composed of combinations based on CdT, CZT, Si(Li) and SDD detectors. However, such hybrid detector arrays solve only few of the inadequacies mentioned above for the individual detector technologies. 
         [0009]    The following disadvantages apply to all the X-ray detection arrangements mentioned in Reference [1]:
       Materials of different chemical compositions whose X-ray fluorescence radiation has been detected with a high energy resolution and whose chemical composition has been precisely identified can be localized on the conveyor belt only with a relatively large spatial uncertainty.   Materials of different chemical compositions whose position on the conveyor belt has been localized relatively precisely on the conveyor belt by a detector having good spatial resolution cannot be identified, or can be identified only with limited accuracy, with regard to their chemical composition, since the detector used for this purpose has an excessively poor energy resolution.   Since the detectors mentioned which have a high energy resolution have only a relatively limited dynamic range of the local and global counting rate, the intensity of the X-ray fluorescence radiation has to be limited to relatively low values in order to avoid saturation in the radiation detectors in the case of greatly varying material compositions. This substantive matter directly influences the achievable counting statistics and the maximum possible conveyor belt speeds.   For the energy range of 0 to 20 keV, Reference [ 1 ] does not mention any suitable detector technology having a high spatial resolution capability. The detectors Si-Pin, SiLi and SDD proposed for this energy range have a very good energy resolution, but can only coarsely detect the spatial position of materials.   The detectors mentioned having a high energy resolution (Si-Pin, Si—Li and SDD) require costly cooling of the detector elements, which in real operation is possibly susceptible.       
 
         [0015]    The technical literature discloses numerous detector array arrangements which have a very good spatial resolution and additionally are suitable for photon energies of less than 20 keV. Inter alia, such detectors are described in “Semiconductor Radiation Detectors”, Gerhard Lutz (Reference [2]). 
         [0016]    One example of a commercially used product is the MYTHEN detector, which was developed by the PSI Paul Scherrer Institut, Switzerland (“Mythen detector system”, B. Schmitt et al., Nuclear Instruments and Methods in Physics Research A 501 (2003) 267-272) (Reference [ 3 ]). The spacing of the individual detector elements is 50 μm, for example, in the MYTHEN detector. 
         [0017]    However, these detector array arrangements mentioned are usable only to a very limited extent for use in a device for identifying materials. Firstly, these detector array arrangements normally have only an energy resolution of the order of magnitude of 1000 eV (FWHM) or higher. This energy resolution does not suffice for many technically relevant material analysis tasks. 
         [0018]    Secondly, the so-called charge sharing effect occurs in such detectors—in a manner governed by the sensor construction. The effect describes the circumstance that, potentially, charge carriers generated by an individual X-ray photon are not picked up in their entirety by a single detector element, but rather are detected in subsets by adjacent detector elements. The charge sharing effect is all the greater, the smaller the individual detector elements are structured. If a material sample to be examined contains only a single chemical element, then charge sharing has the effect that the energy distribution of the detected X-ray fluorescence radiation is no longer Gaussian curve-like, but rather becomes highly asymmetrical and has low-energy components. If materials contain a plurality of chemical elements, as is always the case normally in devices for material identification, charge sharing then has the effect that the individual X-ray fluorescence spectra are energetically superimposed on one another and greatly influence one another. The actual strength of the components generated by charge sharing in the energy spectrum depends on many influencing variables. In particular, severe fluctuations of the energy components generated by charge sharing occur if a material sample contains a plurality of chemical elements whose concentrations moreover change spatially and temporally. An energetically highly resolved and also precise and reproducible material analysis by means of X-ray fluorescence radiation is therefore virtually impossible with such detectors. 
         [0019]    By contrast, “Position sensitive and energy dispersive x-ray detector based on silicon strip detector technology”, P. Wiqcek et al. 2015 JINST 10 P04002 (Reference [4]) discloses a detection arrangement that was developed for applications in X-ray diffractometry. As a commercial product the arrangement is known by the trade name “LYNXEYE XE-T”. Whereas in devices for material analysis the X-ray fluorescence spectra of a multiplicity of chemical elements are normally superimposed on one another simultaneously, a quasi-monochromatic X-ray radiation is present at the location of a detector element in X-ray diffractometry. For diffractometry applications it is of great importance that K-Alpha and K-Beta X-ray radiation components can be reliably separated. The X-ray detector described in Reference [ 4 ] makes it possible to completely separate these radiation components solely by means of electronic discrimination. By way of example, X-ray tubes having a Cu anode are used very often in diffractometry. In such systems, the detector is able to precisely differentiate Cu—K-Alpha X-ray radiation (8.04 keV) from Cu—K-Beta X-ray radiation (8.9 keV), specifically without the assistance of a K-Beta beam path filter (Ni filter) or so-called secondary monochromators. 
       SUMMARY OF THE INVENTION 
       [0020]    In contrast with the prior art, the present invention provides a device similar to that described above that can be used to precisely identify individual particles of a material that are situated on a stationary or moving conveyor belt, by means of X-ray fluorescence and with an energy resolution of better than 500 eV (FWHM, relative to an energy of 8.04 keV). If the energy spectra of a plurality of individual chemical elements are superimposed on one another in this case, then they must be able to be precisely separated from the detection unit and the individual spectra are not permitted to influence one another. In addition to the chemical identification, the device is intended at the same time to be able to make possible a spatially high resolved position determination for the individual material particles. In particular, individual material particles are intended to be able to be localized with a spatial resolution of up to better than 50 μm. This spatial resolution is intended to be achieved both in the conveyor belt direction and orthogonally with respect to the conveyor belt direction. The device may also have a large counting rate dynamic range in order to be able to identify and localize material samples whose chemical compositions change spatially and temporally to a great extent. 
         [0021]    This is achieved in a manner that is surprisingly simple and effective in equal measure using readily available technical means by virtue of the fact that the individual detector elements of the X-ray detector array have a spatial resolution of 50 μm to 500 μm with a sensitivity to X-ray radiation in an energy range of between 500 eV and 30 keV, with an energy resolution of less than 0.5 keV (FWHM) at counting rates of up to 100 kcps and relative to an energy of 8.04 keV, that the electronic unit comprises a signal channel (which may have in each case a baseline restorer for maintaining the 0 V baseline even in the case of high and moreover temporally changing radiation intensities) for each individual detector element of the X-ray detector array, and that each discriminator unit for a specific detector element is in each case electrically connected to the discriminator units of the detector elements that are spatially directly adjacent to said detector element, wherein all the discriminator units are interconnected with one another via a digital and/or analog circuit in such a way that simultaneous occurrence of signals on more than one detector element can be identified and treated electrically separately. 
         [0022]    In particular, the detection arrangement used according to an exemplary embodiment of the invention has the following properties:
       The individual detector elements have a spacing of 75 μm.   X-ray photons in the range of 1 keV to 30 keV can be detected.   The energy resolution of an individual detector element is better than 400 eV (FWHM) relative to an energy of 8.04 keV and for counting rates of 0 to 20 kcps per detector element.   The energy resolution of an individual detector element is better than 500 eV (FWHM) relative to an energy of 8.04 keV and for counting rates of 0 to 100 kcps per detector element. Given a somewhat reduced energy resolution, X-ray photons of up to 1.5 Mcps can be processed. The detector arrangement can thus process global photon counting rates of from 8 Mcps up to more than 200 Mcps depending on the required energy resolution.   The charge sharing effect is virtually completely eliminated.   The discriminator thresholds can be set with a resolution of 10 eV and allow a precise selection of X-ray fluorescence spectra of individual chemical elements.   The sensor substrate need not be cooled in a costly way.       
 
         [0030]    Thus, considerable functional advantages can be afforded by such a detection arrangement for the device for material identification according to the invention. To summarize, it is possible to construct detection systems in which each individual detector element simultaneously has a very large dynamic range and also a very high energy resolution and a very high spatial resolution. Since such a detection system additionally has the ability to eliminate the charge sharing effect, even material samples which contain a plurality of chemical elements can be identified precisely and without mutual spectral influencing with the aid of the X-ray fluorescence analysis. 
         [0031]    In one particular embodiment of the invention, individual detector elements are disposed in a planar way and arranged in a strip-shaped fashion, in one or more groups parallel or collinear with respect to one another, and within a group the individual strips are aligned parallel to one another. 
         [0032]    Advantageous developments of these embodiments are characterized by the fact that first groups of detector elements may be arranged orthogonally with respect to the conveyor belt direction and the individual strips of said group are arranged parallel to the conveyor belt direction and/or second groups of detector elements are arranged parallel to the conveyor belt direction and the individual strips of said group are arranged orthogonally with respect to the conveyor belt direction. With the aid of such detector element arrangements it is possible to determine the spatial position of material pieces which are situated on a conveyor belt. These arrangements enable a high-resolution spatial determination in the conveyor belt direction or in an orthogonal direction with respect to the conveyor belt direction. The achievable spatial resolution is determined by the spacing of the individual detector elements. 
         [0033]    Other variations of the invention are distinguished by the fact that the X-ray detector array is constructed from a combination of first and second groups of detector elements. These detector element arrangements simultaneously enable a high-resolution spatial determination both in the conveyor belt direction and in an orthogonal direction with respect to the conveyor belt direction. 
         [0034]    Further advantageous embodiments of the invention are characterized by the fact that at least some of the detector elements are constructed from a silicon strip sensor having a substrate with a thickness of more than 300 μm, in particular 300 μm to 2 mm. Such silicon strip sensors are able to detect X-ray radiation in the energy range of from 0.5 keV up to normally 30 keV and to convert it directly into electrical charge carriers. The construction and the functioning of silicon strip sensors per se are described inter alia in Reference [2]. 
         [0035]    In certain embodiments of the invention, the discriminator thresholds of each individual detector element group can be set independently of one another, and as a result each group can be allocated one or a plurality of individual energy windows for the detection of the impinging X-ray radiation. This device is thus able to identify X-ray photons whose photon energy corresponds to a predefined energy range. At the same time the device can determine the spatial position of the detected X-ray photons with high resolution, specifically separately for each of the predefined energy ranges. 
         [0036]    A further class of embodiments of the invention is distinguished by the fact that the discriminator units of adjacent channels are electrically interconnected with one another via an elimination unit for shared charges signals, wherein each of said elimination units has a parameterizable VETO discriminator threshold, and wherein the elimination unit has the effect that X-ray photons arriving in the X-ray detector array are in each case not counted as an event if they have the consequence that a VETO threshold is exceeded by the electrical output signals of two adjacent signal channels simultaneously. The device is thus able to identify whether the charge sharing effect occurred for an individual X-ray photon, and in such a case can prevent this photon from being registered by the discriminator units as an event. If a material sample contains a plurality of chemical elements, then the individual energy spectra in the resulting overall energy spectrum appear separated from one another as a consequence and no longer energetically influence one another. The term shared charges signals is understood to mean Gaussian curve-like output signals which were generated for X-ray photons for which the charge sharing effect occurred. 
         [0037]    Alternatively, in a further class of advantageous embodiments of the invention, the discriminator units of adjacent channels are electrically interconnected with one another via a correction unit for shared charges signals, wherein said correction unit identifies simultaneously occurring Gaussian curve-like output signals in adjacent channels, in such a case forms the sum of the output signal amplitudes, allocates the summation signal determined as a single event to that channel which exhibited the highest signal amplitude, and suppresses the registration of the output signal in the other channel. A device configured in this way can recognize whether the charge sharing effect occurred for an individual X-ray photon, and in such a case can reconstruct the original energy and spatial position of the X-ray photon. If a material sample contains a plurality of chemical elements, then the individual energy spectra in the resulting overall energy spectrum appear separated from one another as a consequence and no longer energetically influence one another. 
         [0038]    Other embodiments of the invention are distinguished by the fact that the X-ray detector array is constructed from n&gt;2 groups having in each case an identical strip spacing, the strips are arranged offset from one group to the next group by the distance of the strip spacing divided by n. For the variants of the invention as described further above, the minimum achievable spatial resolution is limited to the spacing of the individual detector elements. The above-mentioned variant of the device according to the invention makes it possible to determine individual materials with a spatial resolution that is smaller than the spacing of two detector elements. 
         [0039]    It may also be expedient for the use of the invention if individual groups of detector elements are operated in the TDI (“Time Delay Integration”) mode synchronously with the instantaneous speed of the conveyor belt. The counting statistics for the detected X-ray photons are significantly improved by applying the TDI method. As a consequence thereof, the spatial position and also the material composition of a material sample can be determined significantly more precisely in comparison with operation without the TDI method. For some measurement tasks, the improved counting statistics make it possible that the conveyor belt can be operated at higher speeds. 
         [0040]    In yet another variation, the sub-channel method is applied to individual groups of detector elements, said method dividing the counting events of each individual physical detector element into more than two virtual signal channels. The achievable spatial resolution of an X-ray detector is normally limited by the spacing of the individual detector elements. By applying the sub-channel method, by contrast, it is possible still to identify changes in the X-ray photon intensity even within a detector element. As a result, the spatial position of material samples moved in the conveyor belt direction and their material structures can be determined even more finely. Moreover, the sub-channel method generates significantly more data points than when this method is not applied. The more data points are present, the more precisely it is possible to reconstruct the spatial position of material pieces with the aid of suitable mathematical algorithms. 
         [0041]    A further advantageous embodiment of the device according to the invention is distinguished by the fact that for reading out the digital signals from the counting units, an FPGA (“Field-programmable gate array”) or an ASIC (“Application-specific integrated circuit”) or a freely programmable controller is provided, in each case in a manner preferably integrated in the detector head. The detector properties described above necessitate a high miniaturization and, moreover, that large volumes of data appertaining to counting events must be read out and processed synchronously with the instantaneous speed of the conveyor belt. This can normally only be realized if the counting events are read out and processed with the aid of FPGA, ASIC or programmable controller components which are arranged in the detector head. 
         [0042]    Variants of the invention in a further class of embodiments are distinguished by the fact that one or a plurality of groups of detector elements is (are) not operated as a spatially resolving 1D detector, but rather as a 0D detector arranged in a planar fashion, by virtue of the fact that the counting events of all the individual detector elements within said group(s) are added together and output as a single measurement value. An individual 0D detector is able to detect X-ray fluorescence radiation that occurs in a manner distributed over a comparatively large conveyor belt region. With the exception of the spatial resolution, all the other detector properties mentioned in the previous claims are achieved. 
         [0043]    A further advantageous embodiment of the device according to the invention is characterized in that each individual detector element of a group of detector elements can be deactivated. As a result, diverse electronic diaphragms can be defined for the X-ray detector array. With said diaphragms, the X-ray detector array can be optimally adapted to the geometry of different beam paths and individual spatial characteristics. 
         [0044]    Finally, the scope of the present invention also includes embodiments in which the X-ray detector is configured such that it can carry out continuously at least 2000 individual measurements per second. Specifically, a high spatial resolution at high belt speeds can be achieved only if the X-ray detector array is able to carry out a high number of individual measurements per second. 
         [0045]    Further advantages of the invention are evident from the description and the drawings. Likewise, the features mentioned above and those that will be explained even further can be used according to the invention in each case individually by themselves or as a plurality in arbitrary combinations. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather have exemplary character for portraying the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0046]      FIGS. 1-1 to 1-5  schematically illustrate a first embodiment of the device according to the invention. 
           [0047]      FIGS. 2-1 to 2-3  schematically illustrate an alternative embodiment of the device according to the invention. 
           [0048]      FIG. 3-1  to  FIG. 3-2  show another alternative embodiment of the device according to the invention. 
           [0049]      FIG. 4-1  shows an embodiment of the device according to the invention which is characterized by the fact that the X-ray detector array is constructed from a combination of first  406 - 407  and second  403 - 405  groups of detector elements. 
           [0050]      FIG. 5-1  shows one embodiment of the device according to the invention in which the detector elements  501  are embodied as a silicon strip sensor. 
           [0051]      FIG. 6-1  and  FIG. 6-2  show one embodiment of the device according to the invention which is characterized by the fact that the discriminator thresholds of each individual detector element group  603  to  607  can be chosen independently of one another. 
           [0052]      FIG. 7-1  to  FIG. 7-3  illustrate the functioning of one embodiment of the device according to the invention which is characterized by the fact that the discriminator units of adjacent channels are electrically interconnected with one another via an elimination unit for shared charges signals. 
           [0053]      FIG. 8-1  to  FIG. 8-3  illustrate the functioning of one embodiment of the device according to the invention which is characterized by the fact that the discriminator units of adjacent channels are electrically interconnected with one another via a correction unit for shared charges signals. 
           [0054]      FIG. 9-1  to  FIG. 9-5  illustrate the functioning of one embodiment of the device according to the invention which is characterized by the fact that the X-ray detector array is constructed from n&gt;2 groups having in each case an identical strip spacing, and the strips are arranged offset from one group to the next group by the distance of the strip spacing divided by n. 
           [0055]      FIG. 10-1  to  FIG. 10-3  illustrate the functioning of one embodiment of the device which is characterized by the fact that individual groups of detector elements are operated in the TDI (“Time Delay Integration”) mode synchronously with the instantaneous speed of the conveyor belt. 
           [0056]      FIG. 11-1  to  FIG. 11-3  illustrate the functioning of one development which is characterized by the fact that the sub-channel method is applied to individual groups of detector elements, said method dividing the counting events of each individual physical detector element into at least two virtual signal channels. 
           [0057]      FIG. 12-1  shows one embodiment of the device which is characterized by the fact that the detector interface unit  1200  is preferably situated within the detector head  107 . 
           [0058]      FIG. 13-1  shows one embodiment of the device according to the invention which is characterized by the fact that one or a plurality of groups of detector elements is (are) not operated as a spatially resolving 1D detector, but rather as a 0D detector arranged in a planar fashion. 
           [0059]      FIG. 14-1  shows one embodiment of the invention which is characterized by the fact that each individual detector element of a group of detector elements can be deactivated. 
       
    
    
     DETAILED DESCRIPTION 
       [0060]      FIGS. 1-1 to 1-5  schematically illustrate a first embodiment of the device according to the invention. 
         [0061]      FIG. 1-1  shows an X-ray source  102 , which emits X-ray radiation  103  and irradiates an individual material piece  104  with said X-ray radiation, which material piece can be moved by means of a conveyor belt  101 . The X-ray fluorescence radiation  105  excited by the material particle is usually guided through a collimation unit  106  and impinges on an X-ray detector head  107 . The latter contains the X-ray detector array  108  and an electronic unit  109  for reading out and processing the signals from the X-ray detector array. 
         [0062]      FIG. 1-2  shows the individual functional units within the detector head  107 . Each individual detector element  114  of the X-ray detector array  108  is interconnected with an individual electronic sub-unit  112  consisting of a signal channel  120 , a discrimination unit  117  and a plurality of counting units. The signal processing for detector element #N is illustrated by way of example. The signal channel converts the analog output signals of the detector element into Gaussian curve-like signals  119 , which are converted into digital counting events in the discriminator unit. Each discriminator unit has at least two adjustable discriminator thresholds and each discriminator threshold is assigned a respective counting unit. In this example, the counting unit  118  counts all Gaussian curve-like signal events which exceed the discriminator threshold- 1   116 ; counting unit  121  adds up all signal events which exceed the discriminator threshold- 2   122 . 
         [0063]      FIG. 1-3  shows by way of example a histogram  126  having a typical frequency distribution of the amplitudes of the analog Gaussian curve-like time signals  119 . The number (Counts) of the individual time signals which have a specific amplitude is plotted. Since the amplitude of the Gaussian curve-like signals is proportional to the energy (Energy) of the detected X-ray photons, this histogram is likewise an image of the energy distribution of the individual X-ray photons which have been detected in the detector element #N  114 . Discriminator threshold- 1  is normally chosen to the left of the maximum of the Gaussian curve-like histogram curve, for example at position  123 . Discriminator threshold- 2  is usually set to the right thereof, that is to say for example at position  125 . In general, the aim is to detect all radiation events which lie in an energy window  124  defined thereby. It was assumed in this representation that no charge sharing occurs in the detector element. 
         [0064]      FIG. 1-4  shows by way of example a histogram for the signal amplitudes at the location  119  and for the case where the X-ray radiation registered in detector element #N  114  is composed of X-ray photons of the energies E 1 , E 2  and E 3 . If no charge sharing occurred, then the histogram would correspond to the profile  151 . However, since charge sharing is normally unavoidable and generated charge carriers are consequently divided between adjacent detector elements, the real histogram shows a distribution as indicated in profile  150 . Owing to charge sharing, the energy distribution of the X-ray photons having the energy E 3  becomes asymmetrical and has low-energy components. The latter are superimposed with the energy distributions of the X-ray photons of the energies E 1  and E 2  and change the energy spectra thereof. The same applies to the energy distributions for the X-ray photons of the energies E 2  and E 1 . These also become asymmetrical and in turn have low-energy components. The low-energy components of the photons of the energy E 2  are superimposed with the energy distribution of the photons of the energy E 1 . 
         [0065]      FIG. 1-5  schematically illustrates how the individual discriminator units of a first embodiment of the device according to the invention are coupled to one another. The discriminator units of spatially directly adjacent detector elements are interconnected with one another via electrical circuits such that simultaneous occurrence of signals on more than one detector element is identified, treated electrically separately and registered separately in the counting units. By way of example, the electrical circuit #N  142  is able to identify simultaneously occurring events in the detector elements #N- 1  and #N ( 131  and  132 ) and to treat them separately. As a result of this electronic correction, the energy spectra registered by the individual discriminator units no longer contain any low-energy charge sharing components. If it is assumed by way of example that the energy distribution at the location  138  has a profile corresponding to  150  in  FIG. 1-4 , then the energy spectrum registered by the discriminator unit #N  143  after electronic correction has a profile in accordance with  151  in  FIG. 1-4 . The histograms for the individual photon energies appear completely separated from one another and no longer influence one another energetically. 
         [0066]    One embodiment of the device according to the invention is illustrated in  FIGS. 2-1 to 2-3 . Accordingly, the individual detector elements  202  arranged in a planar fashion are strip-shaped and can be arranged in a single group  201  or in a plurality of groups  203 ,  204  and  205  parallel to one another. The individual detector element groups can also be arranged collinearly, as illustrated for example by  206  and  207 . Within a group, the individual strips are preferably aligned parallel to one another. 
         [0067]      FIG. 3-1  to  FIG. 3-2  show one embodiment of the device according to the invention. By way of example, a material piece  302  is situated on a conveyor belt  301  and moves toward the detector elements at the speed v. 
         [0068]    First groups of detector elements  303 ,  304 ,  305  and  306  are aligned orthogonally with respect to the conveyor belt direction. The individual strips of said groups are oriented parallel to the conveyor belt direction. Second groups of detector elements  307 ,  308  and  309  are arranged parallel to the conveyor belt direction. The individual strips of said second group are orthogonal with respect to the conveyor belt direction. 
         [0069]      FIG. 4-1  shows one embodiment of the device according to the invention which is characterized by the fact that the X-ray detector array is constructed from a combination of first  406 - 407  and second  403 - 405  groups of detector elements. 
         [0070]      FIG. 5-1  shows one embodiment of the device according to the invention in which the detector elements  501  are embodied as a silicon strip sensor whose semiconductor substrate  502  has a thickness D. The spacing of the individual detector elements has the distance p. 
         [0071]      FIG. 6-1  and  FIG. 6-2  show one embodiment of the device according to the invention which is characterized by the fact that the discriminator thresholds of each individual detector element group  603  to  607  can be chosen independently of one another, and that as a result each group can be allocated individual energy windows for the detection of the impinging X-ray radiation. It is assumed in this example that each of the groups mentioned has two globally adjustable discriminator thresholds  611  and  612  and thus an energy window  613 . Consequently, a dedicated energy window E 1 , E 2 , E 4  or E 5  can be defined for each of the groups mentioned. 
         [0072]      FIG. 7-1  to  FIG. 7-3  illustrate the functioning of one embodiment of the device according to the invention which is characterized by the fact that the discriminator units of adjacent channels are electrically interconnected with one another via an elimination unit for shared charges signals. 
         [0073]      FIG. 7-1  shows by way of example an X-ray photon  701  having the energy E 0 , which photon generates electrical charge carriers in the sensor substrate. On account of charge sharing, the charge carriers generated are distributed between two adjacent detector elements. A first subset  702  of said charge carriers is received by detector element  704 , and a second subset  703  by detector element  705 . The time signal  706  is generated at the output of the signal channel of detector element  704 , and the time signal  707  is generated at the output of the signal channel of detector element  705 . The sum of the signal amplitudes corresponds to the original photon energy E 0 . This embodiment of the device prevents the registration of an individual X-ray photon if the latter has the consequence that the preset VETO threshold is exceeded by the output signals of two adjacent detector elements simultaneously. Such a photon is accordingly not counted as an event in any of the individual counting units. 
         [0074]      FIG. 7-2  shows the particular construction of the electronic interface unit  710  for this embodiment of the device according to the invention. A respective elimination unit is interposed between the discriminator units of two adjacent channels. In this example, discriminator unit #N is linked with the elimination units #N and #N+1. Each elimination unit additionally receives as input signals the two analog output signals of the adjacent signal channels and also a freely parameterizable VETO discriminator threshold. In the example, the output signals  719  and  720  and also the VETO threshold #N are passed to the elimination unit #N. The same correspondingly applies to the elimination unit #N+1. The output signal  720  is not registered and counted as a result if at the same time, in the case of output signal  719 , an event occurs which exceeds the VETO threshold #N just like the output signal  720 . The same correspondingly holds true if the output signals  720  and  721  simultaneously exceed the VETO threshold #N+1. 
         [0075]      FIG. 7-3  shows the histogram profile for the output signal  720  of detector element #N. A typical energy distribution  730  is shown by way of example for the case where X-ray photons of the energies E 1 , E 2  and E 3  act simultaneously on the detector element #N ( 712 ) and the charge sharing effect occurs. The elimination units #N and #N+1 have the effect that the energy spectrum registered by the discriminator unit #N has a profile in accordance with  731  in  FIG. 7-3 . The histograms for the individual photon energies appear completely separated from one another and no longer influence one another energetically. 
         [0076]      FIG. 8-1  to  FIG. 8-3  illustrate the functioning of one embodiment of the device according to the invention which is characterized by the fact that the discriminator units of adjacent channels are electrically interconnected with one another via a correction unit for shared charges signals. 
         [0077]      FIG. 8-1  shows by way of example an X-ray photon  801  having the energy E 0 , which photon generates the time signal  806  at the output of the signal channel of detector element  804  and the time signal  807  at the output of the signal channel of detector element  805  on account of charge sharing. This embodiment recognizes if charge carriers of an individual X-ray photon are distributed between two adjacent detector elements, and reconstructs the original photon energy by addition of the time signals  806  and  807 . The reconstructed event is assigned to that detector element whose time signal had the highest amplitude. In the example, the reconstructed summation signal  808  is allocated to the detector element  805 . 
         [0078]      FIG. 8-2  shows the particular construction of the electronic interface unit for this embodiment of the device according to the invention. A respective correction unit is interposed between the discriminator units of two adjacent channels. In this example, discriminator unit #N is linked with the correction units #N and #N+1. Each correction unit additionally receives as input signals the two analog output signals of the adjacent signal channels. In the example, the signals  817  and  818  are passed to the correction unit #N. The same correspondingly applies to the correction unit #N+1. A signal  818  is not registered and counted as an event by discriminator unit #N if at the same time, in the case of  817 , an event occurs whose amplitude is higher than that of the signal  818 . In this case, the correction unit #N forms the sum of the amplitudes of the signals  817  and  818  and allocates this summation event to the discriminator unit #N- 1 . It can thus be registered by the counting units #N−1. The summation event is correspondingly allocated to the discriminator unit #N if the signal amplitude 818 is greater than that of signal  817 . 
         [0079]      FIG. 8-3  shows the histogram profile for the output signal  818  of detector element #N. A typical energy distribution  830  is shown by way of example for the case where X-ray photons of the energies E 1 , E 2  and E 3  act simultaneously on the detector element #N ( 812 ) and the charge sharing effect occurs. The correction units #N and #N+1 have the effect that the energy spectrum registered by the discriminator unit #N has a profile in accordance with  831  in  FIG. 8-3 . The histograms for the individual photon energies appear completely separated from one another and no longer influence one another energetically. 
         [0080]      FIG. 9-1  to  FIG. 9-5  illustrate the functioning of one embodiment of the device according to the invention which is characterized by the fact that the X-ray detector array is constructed from n&gt;2 groups having in each case an identical strip spacing, and the strips are arranged offset from one group to the next group by the distance of the strip spacing divided by n. 
         [0081]      FIG. 9-1  shows by way of example a material piece  902  which moves toward a detector element group  903  at the speed v. The spacing of the detector elements amounts to the distance p. 
         [0082]      FIG. 9-2  shows in row B 1  the detector elements of the group  903  which have the spacing p and which detect the X-ray fluorescence radiation emitted by the material piece  902 . In this example, the material piece generates N X-ray photons per unit time. Each detector element is assigned, as illustrated in row C, a storage cell in which the number of registered X-ray photons is entered. In rows A 1  to A 6 , the letter x denotes different possible positions of the material piece  902 . For each of the positions A 1  to A 6 , the detector element detects N counting events as illustrated. In this example, the achievable spatial resolution thus corresponds to the distance p 1 , which is equal to the distance p. 
         [0083]      FIG. 9-3  shows by way of example a material piece  902  which moves toward three detector element groups  915 ,  916  and  917 , which are parallel to one another, at the speed v. The spacing of the detector elements within each group amounts to the distance p. The detector elements of the groups  915 ,  916  and  917  are arranged offset by ⅓ p in each case. 
         [0084]      FIG. 9-4  shows in row B 1  the detector elements of the group  915 , in row B 2  the detector elements of the group  916  and in row B 3  the detector elements of the group  917 . Each of the detector elements is assigned, as illustrated in row C, a storage cell in which the number of registered X-ray photons is entered. If the material piece  902  is situated at the positions A 1  or A 2 , then in the three detector elements as illustrated in each case N counting events are registered and assigned to the corresponding storage cells. In this example, the spatial resolution achievable following application of suitable interpolation methods corresponds to the distance p 3 , which is significantly smaller than the distance p. 
         [0085]      FIG. 9-5  shows that this functional principle can also be applied to the high-resolution spatial determination in the conveyor belt direction. The material piece  902  moves toward the groups  922 ,  923  and  924 , which are arranged offset by the spacing p/3. 
         [0086]      FIG. 10-1  to  FIG. 10-3  illustrate the functioning of one embodiment of the device which is characterized by the fact that individual groups of detector elements are operated in the TDI (“Time Delay Integration”) mode synchronously with the instantaneous speed of the conveyor belt. 
         [0087]      FIG. 10-1  shows by way of example a material piece  1002  which moves toward a detector element group  1003  at the speed v. The group consists of M detector elements. The spacing of the individual detector elements amounts to the distance p. 
         [0088]      FIG. 10-2  illustrates by way of example the functioning of the TDI method. The illustration shows the spatial position of the material piece  1002  relative to the individual detector elements #1 to #M (row A) and at the points in time t 0 , t 1  and tn. At the point in time t 0 , the detector element #1 registers the number of N11 X-ray photons, and the detector element #2 registers the number of N12 photons. N11 is added to the existing content of storage cell #j+1 (row D), and N12 to the content of storage cell #j+2. Exactly the same procedure is adopted at the subsequent points in time. The X-ray photons emitted by the material piece  1002  are in turn added to the storage cells #j+1 and #j+2. At the point in time t 1 , therefore, the counting rates N21 and N22 are added to the storage cells #j+1 and #j+2, and at the point in time tn, the counting rates Nn1 and Nn2 are added to the storage cells #j+1 and #j+2. 
         [0089]      FIG. 10-3  shows by way of example an intensity profile plotted against the conveyor belt direction, which intensity profile was measured by the detector element group  1003 . The measurement was carried out without the TDI method in case A and with application of the TDI method in case B. In case B, the intensity fluctuations e1B for the detected material piece  1002  and the intensity fluctuations for the measured background radiation e2B, on account of the TDI method, are significantly smaller than those in case A. The spatial position and the material composition of the material piece  1002  can thus be determined significantly better by applying the TDI method. 
         [0090]      FIG. 11-1  to  FIG. 11-3  illustrate the functioning of one development which is characterized by the fact that the sub-channel method is applied to individual groups of detector elements, said method dividing the counting events of each individual physical detector element into at least two virtual signal channels. The method is carried out synchronously with the instantaneous speed of the conveyor belt and can be applied with or without the TDI method. 
         [0091]      FIG. 11-1  shows by way of example a material piece  1102  which moves toward a detector element group  1103  at the speed v. The group consists of M detector elements. The spacing of the individual detector elements amounts to the distance p. 
         [0092]      FIG. 11-2  illustrates by way of example the functioning of the sub-channel method with simultaneous application of the TDI method. The illustration shows the spatial position of the material piece  1102  relative to the individual detector elements #1 to #M (rows A 1  to A 4 ) and at the points in time t 1 , t 2 , t K  and t K+1 . In this example, each real detector element is divided into K virtual detector elements having a virtual spacing of p/K. X-ray photons are in each case measured for a time duration T and allocated to the individual storage cells as follows. At the point in time t 0 , the detector element #1 registers the number of N11 X-ray photons, and all other detector elements register the counting rate 0. N11 is added to the existing content of storage cell #j+1 (row D 1 ). At the point in time t 2 , the counting rate N12 is detected and counted toward the content of storage cell #j+2 (row D 2 ). At the point in time t K , the last sub-channel of detector element #1 is detected, the counting rate N 1K  is determined and allocated to storage cell #j+K (row D 3 ). At the point in time t K+1 , the first sub-channel of detector element #2 is reached. The counting rate N21 is added to storage cell #j+1 (row D 4 ). 
         [0093]      FIG. 11-3  illustrates the mode of action of the sub-channel method for the case where the individual detector elements are divided for example into 5 virtual signal channels. The illustration shows in rows D 1 , D 2 , D 3  and D 4  the intensities summed in the storage cells for four different spatial positions A 1 , A 2 , A 3  and A 4  of the material piece  1102  that are assumed at the point in time t 0 . The exposure duration corresponds to the time T. If the spatial position of the material piece  1102  differs in steps by the distance p/5 toward the right, as illustrated in cases A 1 , A 2 , A 3  and A 4 , then in the data memory the measured intensities are likewise offset toward the right by one storage cell. With this method, therefore, it is possible to register changes in the X-ray photon intensity even actually within an individual detector element. 
         [0094]      FIG. 12-1  shows one embodiment of the device which is characterized by the fact that the detector interface unit  1200  is preferably situated within the detector head  107 . In a customary manner, an FPGA (“Field-programmable gate array”) or an ASIC (“Application-specific integrated circuit”) or a freely programmable controller is used for reading out the digital signals from the counting units which are situated in the electronic unit  109 . 
         [0095]      FIG. 13-1  shows one embodiment of the device according to the invention which is characterized by the fact that one or a plurality of groups of detector elements is (are) not operated as a spatially resolving 1D detector, but rather as a 0D detector arranged in a planar fashion. In the example illustrated, the material pieces  1302  and  1303  are moved by the conveyor belt. The X-ray detector arrays  1308  to  1315  are operated in each case as a 0D detector. Within an X-ray detector array, for this purpose the counting events of the individual detector elements are added together and output as a single measurement value. If a plurality of energy windows were defined, such a measurement value is determined and output for each energy window. 
         [0096]      FIG. 14-1  shows one embodiment of the invention which is characterized by the fact that each individual detector element of a group of detector elements can be deactivated. By way of example, in the groups  1403  and  1410  the detector elements in the regions A 1 , A 3 , B 1  and B 3  were deactivated. The active area thus comprises only the regions A 2  and B 2 .