Patent Publication Number: US-2022214291-A1

Title: X-Ray Spectroscopic Analysis Apparatus and Elemental Analysis Method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an X-ray spectroscopic analysis apparatus and an elemental analysis method. 
     Description of the Background Art 
     An X-ray spectroscopic analysis apparatus has been known as an apparatus that subjects characteristic X-rays (fluorescent X-rays) to spectroscopy and detects the intensity of the characteristic X-rays for each wavelength, the characteristic X-rays being emitted by a sample irradiated with excitation beams such as primary X-rays and electron beams. WO2018/053272 describes as such an X-ray spectroscopic analysis apparatus, an apparatus in which a curved analyzing crystal and a detector are disposed along a circumference of one Rowland circle. In the X-ray spectroscopic analysis apparatus described in WO2018/053272, the curved analyzing crystal collects characteristic X-rays from a light source arranged inside the Rowland circle and simultaneously separates the characteristic X-rays and the detector detects the separated characteristic X-rays. 
     SUMMARY OF THE INVENTION 
     In the X-ray spectroscopic analysis apparatus described in WO2018/053272, in separating a group of characteristic X-rays different in peak wavelength from one another with a spectroscopic element such as a curved analyzing crystal, attention is not paid to in which spectral range characteristic X-rays in the group of characteristic X-rays should be separated. When the spectral range is set to cover all peak wavelengths of the group of characteristic X-rays, the spectroscopic element has a spectral surface whose length, measured along the Rowland circle, becomes long. With the long length, owing to a difference in radius of curvature between the Rowland circle and the spectroscopic element, accuracy in detection of characteristic X-rays by the detector is disadvantageously lowered in an area of the spectroscopic element distant from the Rowland circle. 
     The present disclosure was made in view of such circumstances, and one of objects thereof is to provide an X-ray spectroscopic analysis apparatus capable of accurately detecting characteristic X-rays with a detector. An X-ray spectroscopic analysis apparatus according to one aspect of the present disclosure includes an excitation source, a curved spectroscopic element, a position-sensitive detector, and a computing unit. The excitation source causes generation of a group of characteristic X-rays different in peak wavelength from one another by emitting excitation beams to a sample held by a sample holder. The curved spectroscopic element separates the group of characteristic X-rays. The position-sensitive detector detects at least some of the group of characteristic X-rays separated by the spectroscopic element. The computing unit analyzes an element contained in the sample based on a result of detection by the detector. The spectroscopic element and the detector are disposed along a circumference of one Rowland circle. The spectroscopic element has a spectral surface whose length, measured along the Rowland circle, is shorter than a length in the Rowland circle plane, of an irradiation surface irradiated with the excitation beams emitted to the sample holder. The spectroscopic element and the sample holder are disposed to separate the group of characteristic X-rays within a common spectral range of the spectroscopic element. 
     An elemental analysis method according to another aspect of the present disclosure includes generating a group of characteristic X-rays different in peak wavelength from one another by emitting excitation beams to a sample held by a sample holder, causing the generated group of characteristic X-rays to be incident on a curved spectroscopic element, causing the spectroscopic element to separate the incident group of characteristic X-rays, and causing a position-sensitive detector to detect at least some of the separated group of characteristic X-rays, and analyzing an element contained in the sample based on a result of detection by the detector. The spectroscopic element and the detector are disposed along a circumference of one Rowland circle. The spectroscopic element has a spectral surface whose length, measured along the Rowland circle, is shorter than a length in the Rowland circle plane, of an irradiation surface irradiated with the excitation beams emitted to the sample holder. The spectroscopic element and the sample holder are disposed to separate the group of characteristic X-rays within a common spectral range of the spectroscopic element. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing an overall configuration of an X-ray spectroscopic analysis apparatus. 
         FIG. 2  is a diagram showing relation between the X-ray spectroscopic analysis apparatus and a Rowland circle. 
         FIG. 3  is a diagram showing an exemplary curved spectroscopic element. 
         FIGS. 4A and 4B  are diagrams showing a result of simulation of a projection image of a Kα 1  line of Co viewed from a light reception surface of a detector. 
         FIGS. 5A to 5C  are diagrams showing results of analysis based on a result of detection by the detector. 
         FIGS. 6A and 6B  are diagrams each showing an exemplary collimator. 
         FIG. 7  is a diagram showing relation between an X-ray spectroscopic analysis apparatus and the Rowland circle according to a first modification. 
         FIG. 8  is a diagram showing relation between an X-ray spectroscopic analysis apparatus and the Rowland circle according to a second modification. 
         FIG. 9  is a diagram showing relation between an X-ray spectroscopic analysis apparatus and the Rowland circle according to a third modification. 
         FIG. 10  is a diagram showing a sample holder and a rotation mechanism according to a fourth modification. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Each embodiment will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. 
     [X-Ray Spectroscopic Analysis Apparatus  10 ] 
       FIG. 1  is a diagram schematically showing an overall configuration of an X-ray spectroscopic analysis apparatus  10 . 
     As shown in  FIG. 1 , X-ray spectroscopic analysis apparatus  10  includes an X-ray tube  11  as an excitation source, a curved spectroscopic element  12 , a position-sensitive detector  14 , and a computing unit  15 . 
     Computing unit  15  is configured to control operations of X-ray spectroscopic analysis apparatus  10  and to analyze an element contained in a sample based on a result of detection by detector  14 . Computing unit  15  is implemented by a processor, a memory, and the like. These components are connected to communicate with one another through a bus. 
     The processor is typically a computing processing unit such as a central processing unit (CPU) or a micro processing unit (MPU). The processor controls operations of each component of X-ray spectroscopic analysis apparatus  10  by reading and executing a program stored in the memory. The memory is implemented by a non-volatile memory such as a random access memory (RAM), a read only memory (ROM), and a flash memory. A program executed by the processor or data used by the processor is stored in the memory. 
     X-ray tube  11  causes generation of a group of characteristic X-rays different in peak wavelength from one another (a plurality of characteristic X-rays different in wavelength range from one another) by emitting excitation X-rays (which are also simply referred to as “excitation beams”) to a sample held by a sample holder  108 . 
     Specifically, sample holder  108  includes a rectangular irradiation surface  108   a  with each side having a length of L 2 . Irradiation surface  108   a  is an opening in sample holder  108  and the entire irradiation surface  108   a  is irradiated with excitation beams. Since the sample is held within the entire irradiation surface  108   a , a group of characteristic X-rays is generated from irradiation surface  108   a.    
     Curved spectroscopic element  12  separates the group of characteristic X-rays that comes from irradiation surface  108   a . Position-sensitive detector  14  detects at least some of the group of characteristic X-rays separated by spectroscopic element  12 . In the present embodiment, characteristic X-rays generated by excitation with X-rays are also referred to as “fluorescent X-rays” below. 
     Position-sensitive detector  14  may be a one-dimensional detector. The one-dimensional detector is, for example, a silicon strip detector. By employing the one-dimensional detector as position-sensitive detector  14 , reduction in cost for the apparatus can be expected as compared with a charge coupled device (CCD) camera and a complementary metal oxide semiconductor (CMOS) camera which are two-dimensional detectors. Furthermore, time and efforts for reconfiguring two-dimensional data into one-dimensional data are not required. 
     Computing unit  15  controls X-ray tube  11  to emit excitation beams, obtains a result of detection of the group of characteristic X-rays detected by detector  14 , and analyzes an element contained in a sample. Valence (average valence) of the element in the sample can thus be analyzed. In analyzing valence, based on peak energy of characteristic X-rays (peak energy of each X-ray in a group of characteristic X-rays) emitted from a plurality of standard samples different in valence from one another (such a standard sample that an element therein has already been known and valence of the element has already been known), a standard curve that shows peak energy with respect to the valence (for example, a curve that expresses relation between energy and valence with a linear function) is created. The valence is obtained by measuring a sample with X-ray spectroscopic analysis apparatus  10  and applying a value of energy of each X-ray in the group of characteristic X-rays obtained based on the result of detection by detector  14  to the standard curve. X-ray spectroscopic analysis apparatus  10  may include a rotation mechanism  110 . Computing unit  15  can control rotation mechanism  110  to rotate sample holder  108 . Rotation mechanism  110  will be described later with reference to  FIG. 10 . 
     L 2  represents a length in a plane of Rowland circle  104  ( FIG. 2 ), of irradiation surface  108   a  irradiated with excitation beams emitted to sample holder  108 . L 1  represents a length of a spectral surface of spectroscopic element  12  measured along Rowland circle  104 . In the present embodiment, spectroscopic element  12  and detector  14  are disposed along a circumference of one Rowland circle  104 . Specific description will be given below with reference to  FIG. 2 . 
       FIG. 2  is a diagram showing relation between X-ray spectroscopic analysis apparatus  10  and Rowland circle  104 . As shown in  FIG. 2 , sample holder  108  and X-ray tube  11  are disposed within Rowland circle  104  having a radius R. A spectral surface of spectroscopic element  12  has a shape and arrangement along the circumference of Rowland circle  104 . In the present embodiment, detector  14  is arranged such that a detection surface thereof is in contact with the circumference of Rowland circle  104  not at one point but intersects with the circumference at two points (focuses  134  and  138  in  FIG. 2 ). In general, X-ray spectroscopic analysis apparatus  10  may be required to detect X-rays of a plurality of types of energy (for example, a Kα line and a Kβ line) emitted from a single element and X-rays of different types of energy emitted from a plurality of elements (for example, a Kα line from Mn and a Kα line from Ni). By arranging detector  14  as in the present embodiment, such a requirement can be met. By doing so, X-ray spectroscopic analysis apparatus  10  can detect X-rays of different types of energy at high resolution. Spectroscopic element  12  and detector  14  may have a shape and arrangement different from the above, so long as functions and effects of the present invention are achieved. 
     Initially, excitation X-rays from X-ray tube  11  are emitted to sample holder  108  within irradiation surface  108   a , and fluorescent X-rays specific to an element contained in a sample are generated. Then, the fluorescent X-rays generated from the sample are reflected based on Bragg reflection by spectroscopic element (analyzing crystal)  12  disposed along the circumference of Rowland circle  104  and detected by detector  14  disposed such that the surface thereof intersects with Rowland circle  104  at two points (focuses  134  and  138 ). Sample holder  108  is desirably disposed such that irradiation surface  108   a  is perpendicular to a direction of incidence of characteristic X-rays incident on spectroscopic element  12  from irradiation surface  108   a , and in the present embodiment, L 2  represents a length in an example where irradiation surface  108   a  is perpendicular. Detector  14  may be disposed along the circumference of Rowland circle  104 . 
     Any sample containing a metal material that generates characteristic X-rays by irradiation with excitation beams, such as a battery or a catalyst, can be employed as the sample. For example, the sample may contain three elements of manganese (Mn), cobalt (Co), and nickel (Ni). In a specific example, the sample may be a lithium ion battery (LIB) containing Li(Mn 1/3 Co 1/3 Ni 1/3 ) as a positive electrode material. The sample may contain iron (Fe). 
     In this case, initially, a component of fluorescent X-rays having a first peak wavelength (which is also referred to as a “first wavelength range”) and generated from Mn representing one of contained elements, the component being emitted from a first area  140  of a sample surface, reaches a spectral range  173  of spectroscopic element  12  and is reflected based on Bragg reflection, passes through an area shown with optical paths  116  and  118 , and is collected at focus  138  on Rowland circle  104 . 
     A virtual focus on the Rowland circle defined by connecting first area  140  and spectral range  173  to each other is shown with a focus  128 . Geometrically, fluorescent X-rays can be regarded as being emitted from focus  128  (an area shown with optical paths  125  and  127 ), passing through the area shown with optical paths  116  and  118 , and being collected at focus  138 . 
     A component of fluorescent X-rays having a second peak wavelength (which is also referred to as a “second wavelength range”) and generated from Co, the component being emitted from a second area  142  of the sample surface, reaches spectral range  173  of spectroscopic element  12 , is reflected based on Bragg reflection, and passes through an area shown with optical paths  120  and  122 , and a focus  136  on the Rowland circle is a position of collection of light. 
     A virtual focus on the Rowland circle defined by connecting second area  142  and spectral range  173  to each other is shown with a focus  130 . Geometrically, fluorescent X-rays can be regarded as being emitted from focus  130  (an area shown with optical paths  121  and  123 ), passing through the area shown with optical paths  120  and  122 , and being collected at focus  136 . 
     A component of fluorescent X-rays having a third peak wavelength (which is also referred to as a “third wavelength range”) and generated from Ni, the component being emitted from a third area  144  of the sample surface, reaches spectral range  173  of spectroscopic element  12  and is reflected based on Bragg reflection, passes through an area shown with optical paths  124  and  126 , and is collected at focus  134  on the Rowland circle. 
     A virtual focus on the Rowland circle defined by connecting third area  144  and spectral range  173  to each other is shown with a focus  132 . Geometrically, fluorescent X-rays can be regarded as being emitted from focus  132  (an area shown with optical paths  117  and  119 ), passing through the area shown with optical paths  124  and  126 , and being collected at focus  134 . 
     Thus, characteristic X-rays (fluorescent X-rays) include a group of characteristic X-rays different in peak wavelength from one another (a plurality of characteristic X-rays different in wavelength range; in this case, characteristic X-rays generated from Mn, Co, and Ni). In the present embodiment, spectroscopic element  12  and sample holder  108  are disposed such that the group of characteristic X-rays is separated within a common spectral range of spectroscopic element  12 . 
     Specifically, as described above, characteristic X-rays generated from any of Mn, Co, and Ni are separated within common “spectral range  173 .” Spectroscopic element  12  and sample holder  108  are disposed such that the characteristic X-rays are separated within spectral range  173 . In the present embodiment, spectroscopic element  12  and sample holder  108  are disposed such that length L 1  of the spectral surface of spectroscopic element  12  measured along Rowland circle  104  is shorter than length L 2  in the plane of Rowland circle  104 , of the irradiation surface irradiated with the excitation beams emitted to sample holder  108 . 
     When the spectral ranges of the group of characteristic X-rays are not common but different, length L 1  of the spectral surface of spectroscopic element  12  measured along Rowland circle  104  becomes longer. When spectroscopic element  12  is a Johann type spectroscopic element, a radius of curvature thereof is  2 R. In this case, when spectroscopic element  12  has a size large relative to Rowland circle  104  having radius R, displacement from Rowland circle  104  is large in a peripheral portion of spectroscopic element  12 , and consequently, defocus due to optical aberration occurs. Accuracy in detection by detector  14  is thus deteriorated. 
     When the spectral ranges of the group of characteristic X-rays are completely separate from one another and when there is a crystal defect, position displacement of fluorescent X-rays incident on detector  14  occurs, which results in peak shift. Therefore, valence may not correctly be assessed. For example, in WO2018/053272, spectral ranges for a plurality of X-rays are shown with references  171 ,  173 , and  175 , respectively ( FIG. 2 ) and they are completely separate from one another. The present embodiment is configured such that the group of characteristic X-rays is separated only within common spectral range  173  and the spectral range of spectroscopic element  12  is small relative to the size of the sample. Effective spectral range  173  of curved spectroscopic element  12  can thus be limited to the vicinity of a region in contact with the circumference of Rowland circle  104  and hence lowering in accuracy in detection of the characteristic X-rays due to difference in radius of curvature between Rowland circle  104  and spectroscopic element  12  can be prevented. Detector  14  can thus accurately detect fluorescent X-rays. 
     Three advantages as below are thus expected. Firstly, by spectroscopy in common spectral range  173 , spectroscopic element  12  can be compact and hence cost for manufacturing spectroscopic element  12  can be reduced. Secondly, in the event that spectroscopic element  12  is defective, the defect can be inspected only by conducting inspection for one element. Thirdly, even when spectroscopic element  12  is defective, there is no peak shift because spectral range  173  is common. 
       FIG. 3  is a diagram showing exemplary curved spectroscopic element  12 . In the present embodiment, as shown in  FIG. 3 , spectroscopic element  12  is a doubly-curved analyzing crystal. 
     Spectroscopic element  12  is made by bonding a thin plate  1022  to serve as the analyzing crystal to a base  1020  polished to be curved. A metal such as SUS or low-expansion glass is employed as a material for the base. A single crystal of Si, Ge, LiF, or quartz is suitable for the material of the analyzing crystal. 
     Length L 1  described with reference to  FIGS. 1 and 2  refers to a length of the spectral surface of spectroscopic element  12  measured along Rowland circle  104 . An x direction in  FIG. 3  is a direction along Rowland circle  104  and also referred to as a “direction of spectroscopy” below. A y direction refers to a direction along spectroscopic element  12  perpendicular to the x direction and also referred to as a “direction of collection” below. 
     Spectroscopic element  12  has a size Wx in the direction of spectroscopy and Wy in the direction of collection. In the example shown in  FIGS. 1 and 2 , a condition of Wx=L 1  and Wy=L 1 ×6 is set, without being limited thereto. 
     A concave surface of spectroscopic element  12  has a curvature of a radius Rx in the direction of spectroscopy (the x direction) and a radius Ry in the direction of collection (the y direction). An exact solution that gives an optimal curvature is expressed by Rx= 2 R and Ry= 2 R×sin 2 θ B  where R represents a radius of the Rowland circle and OB represents a Bragg reflection angle determined by a lattice spacing of the analyzing crystal (spectroscopic element  12 ) and a wavelength of incident X-rays. 
     An Si strip detector (SSD) is employed as detector  14 . By way of example of the SSD, a one-dimensional semiconductor array detector Mythen 2  manufactured by Dectris AG (Switzerland) can be employed. Mythen 2  has a size of one pixel of 50 μm (the x direction; the direction along Rowland circle  104 )×8 mm (the y direction: the direction along the detector perpendicular to the x direction) and has 1280 pixels (channels) integrated in the x direction. A field of view has a total size of 64 mm (the×direction)×8 mm (the y direction). 
     [Result of Simulation of Projection Image of Kα 1  Line of Co] 
       FIGS. 4A and 4B  are diagrams showing a result of simulation of a projection image of a Kα 1  line of Co viewed from a light reception surface of detector  14 . 
     A direction along Rowland circle  104 , of irradiation surface  108   a  irradiated with excitation beams emitted to sample holder  108  is defined as the x direction and a direction along irradiation surface  108   a  perpendicular to the x direction is defined as the y direction. L 2  described in the example in  FIG. 1  represents a length in the x direction of irradiation surface  108   a  in sample holder  108 . The length in the y direction of irradiation surface  108   a  is also assumed as L 2 . 
       FIGS. 4A and 4B  each show a result of calculation by Monte Carlo simulation (ray tracing) of what kind of projection image is formed by a CoKα 1  line (6.9303 keV) on the detection surface of the detector when it is assumed that the lines are evenly emitted from the irradiation surface (irradiation surface  108   a  in  FIG. 1 ) of the sample having a size of L 2  (the x direction)×L 2  (the y direction). A single crystal of Ge( 220 ) is assumed as the analyzing crystal (spectroscopic element  12 ). 
       FIG. 4A  shows a result of simulation when spectroscopic element  12  has a size of Wx=L 1  and Wy=L 1 ×6 and  FIG. 4B  shows a result of simulation when spectroscopic element  12  has a size of Wx=L 1 ×6 and Wy=L×6. In the present embodiment, it is assumed that relation of L 2 =L 1 ×4 is satisfied. 
     In  FIG. 4B , trailing in the x direction due to aberration is observed in the detection surface of detector  14 . In  FIG. 4A , on the other hand, such trailing is not observed, and it can be seen that good spectral characteristics are obtained. 
     Thus, the example in which spectroscopic element  12  has a size in the x direction of Wx=L 1  ( FIG. 4B ) which is as small as ⅙ is better in spectral characteristics than the example in which spectroscopic element  12  has a size in the x direction of Wx=L 1 ×6 (the example in  FIG. 4B ). 
     From the result of simulation, good detection characteristics are obtained by reducing the size Wx (=L 1 =L 2 /4) of spectroscopic element  12  to at most ½ or preferably at most ¼ of length L 2  in the x direction of irradiation surface  108   a  in sample holder  108 . 
     In the present embodiment described with reference to  FIGS. 1 and 2  as well, L 1  (the length of the spectral surface within the spectral range, in the spectral surface of spectroscopic element  12  measured along Rowland circle  104 ) is designed to be equal to or smaller than ½ of L 2  (the length in the plane of Rowland circle  104 , of the irradiation surface). 
     Peak center energy, a half-value breadth, and a peak height of a waveform of fluorescent X-rays of a contained element detected by detector  14  are calculated by data processing with software, and any value thereof is associated with physical characteristics of a sample. For example, peak center energy correlates with a valence electron state of the contained element, and variation in valence of the sample can be known from slight variation in peak center energy. Details are shown in the literature A below. 
     [Literature A] K. Sato, T. Yoneda, T. Izumi, T. Omori, S. Tokuda, S. Adachi, M. Kobayashi, T. Mukai, H. Tanaka and M. Yanagida, “Evaluation of Analytical Precision of Polychromatic Simultaneous WDXRF Spectrometer and Application to Valence Analysis of Cathode Materials of Lithium-Ion Batteries,” Analytical Chemistry, Vol. 92(1), pp. 758-765, 2020. 
     Kα lines which are fluorescent X-rays of Mn, Co, and Ni are composed of two lines of a Kα 1  line and a Kα 2  line. Mn has energy of 5898.7 eV of the Kα 1  line and 5887.6 EV of Kα 2 . Co has energy of 6930.3 eV of the Kα 1  line and 6915.3 eV of Kα 2 . Ni has energy of 7478.1 eV of the Kα 1  line and 7460.9 eV of Kα 2 . 
     In general, measured waveforms of the Kα 1  line and the Kα 2  line have a peak having a certain finite width under the influence by quantum mechanical fluctuation and noise in a detection system, and tails thereof are detected as overlapping with each other. Therefore, peak separation by curve fitting is preferred, and in order to do so, data on a prescribed energy range (a wavelength range) including both of these lines should continuously be obtained. 
     At least two times or preferably at least three times as large as an energy difference between the Kα 1  line and the Kα 2  line is one guideline of an energy range (a wavelength range) suitable for waveform analysis. Though a signal of the Kα line is obtained in the present embodiment, without being limited as such, a signal of a Kβ line may be obtained. In that case, a suitable energy range (wavelength range) can be calculated by replacing the Kα 1  line with a Kβ 1 , 3  line and replacing the Kβ 2  line with a Kβ′ line. 
       FIGS. 5A to 5C  are diagrams showing results of analysis based on a result of detection by detector  14 .  FIG. 5A  shows a result in connection with Fe,  FIG. 5B  shows a result in connection with Co, and  FIG. 5C  shows a result in connection with Ni. As shown in  FIG. 5A , spectral peaks of Kα 1  and Kα 2  of Fe are suitably detected at respective prescribed positions in the x direction of detector  14 . Spectral peaks of 
     Kα 1  and Kα 2  are suitably detected also similarly for Co and Ni. 
     [Modification] 
     A modification of the present embodiment will be described below. 
     &lt;First Modification&gt; 
     In the present embodiment, as shown in  FIG. 2 , spectral range  173  which is the common spectral range is configured to be delimited by the length (L 1 ) of the spectral surface of spectroscopic element  12  measured along Rowland circle  104 . 
     Without being limited as such, a collimator (collimators  180  to  183 ) may be provided and the collimator may delimit (determine) the common spectral range. The collimator is disposed on paths of the group of characteristic X-rays from sample holder  108  via spectroscopic element  12  until detector  14 . 
       FIGS. 6A and 6B  are diagrams each showing an exemplary collimator (collimators  180  and  183 ).  FIG. 6A  is a diagram showing a single-aperture collimator  180  and  FIG. 6B  shows a multiple-aperture collimator  183 . Single-aperture collimator  180  includes a single aperture  181   a  as shown in FIG. 
       6 A. In contrast, multiple-aperture collimator  183  includes three apertures (apertures  184   a  to  184   c ) as shown in  FIG. 6B . 
       FIG. 7  is a diagram showing relation between an X-ray spectroscopic analysis apparatus  10   a  and Rowland circle  104  according to a first modification. Since X-ray spectroscopic analysis apparatus  10   a  is basically the same in configuration as X-ray spectroscopic analysis apparatus  10  except that collimator  180  is further disposed, detailed description will not be provided. 
     As shown in  FIG. 7 , collimator  180  is disposed on paths of the group of characteristic X-rays from sample holder  108  via spectroscopic element  12  until detector  14 . Specifically, as the collimator is disposed in the vicinity of spectroscopic element  12 , common spectral range  173  is narrowed by aperture  181   a  of collimator  180  and the spectral range is thus delimited. 
     In other words, common spectral range  173  is delimited depending not on the size of spectroscopic element  12  but on the size of aperture  181   a  of collimator  180 . 
     Impingement of X-rays as a signal on an end of spectroscopic element  12  may cause generation of unintended scattered radiation. Therefore, by disposing collimator  180  as above, generation of unintended scattered radiation can be prevented. 
     &lt;Second Modification&gt; 
       FIG. 8  is a diagram showing relation between an X-ray spectroscopic analysis apparatus  10   b  and Rowland circle  104  according to a second modification. Since X-ray spectroscopic analysis apparatus  10   b  is basically the same in configuration as X-ray spectroscopic analysis apparatus  10  except that collimators  181  and  182  are further disposed, detailed description will not be provided. Collimators  181  and  182  are each a single-aperture collimator similarly to collimator  180 . 
     As shown in  FIG. 8 , collimators  181  and  182  are disposed on paths of a group of characteristic X-rays from sample holder  108  via spectroscopic element  12  until detector  14 . Specifically, collimator  181  is disposed on the paths of the group of characteristic X-rays from sample holder  108  until spectroscopic element  12  and collimator  182  is disposed on the paths of the group of characteristic X-rays from spectroscopic element  12  until detector  14 . Common spectral range  173  is thus delimited. 
     &lt;Third Modification&gt; 
       FIG. 9  is a diagram showing relation between an X-ray spectroscopic analysis apparatus  10   c  and Rowland circle  104  according to a third modification. Since X-ray spectroscopic analysis apparatus  10   c  is basically the same in configuration as X-ray spectroscopic analysis apparatus  10  except that collimators  180  and  183  are further disposed, detailed description will not be provided. 
     As shown in  FIG. 9 , collimators  180  and  183  are disposed on the paths of the group of characteristic X-rays from sample holder  108  via spectroscopic element  12  until detector  14 . Specifically, collimator  180  is disposed in the vicinity of spectroscopic element  12  and collimator  183  is disposed in the vicinity of sample holder  108 . 
     Fluorescent X-rays generated from Mn pass through aperture  184   a  of collimator  183  (an area shown with optical paths  125  and  127 ), fluorescent X-rays generated from Co pass through aperture  184   b  of collimator  183  (an area shown with optical paths  121  and  123 ), and fluorescent X-rays generated from Ni pass through aperture  184   c  of collimator  183  (an area shown with optical paths  117  and  119 ). Furthermore, common spectral range  173  is narrowed by aperture  181   a  of collimator  180  to delimit the spectral range. 
     The collimator may thus allow passage of light beams in each wavelength range alone (collimator  183 ). By cutting off X-rays out of the wavelength range of interest, lowering in SN ratio due to scattered radiation can be avoided. 
     &lt;Fourth Modification&gt; 
       FIG. 10  is a diagram showing a sample holder  109  and rotation mechanism  110  according to a fourth modification. In the present embodiment, X-ray spectroscopic analysis apparatus  10  includes rotation mechanism  110 . Computing unit  15  can control rotation mechanism  110  to rotate sample holder  108 . In this case, sample holder  108  and irradiation surface  108   a  are configured as being rectangular. Without being limited as such, rotation mechanism  110  that rotates annular sample holder  109  and an irradiation surface  109   a  may be provided. Irradiation surface  109   a  has a diameter L 2 . 
     The sample in sample holder  109  is not necessarily held in a uniform state due to a defect or unevenness. When the sample is thus non-uniform, peak intensity calculated based on a result of detection by detector  14  may vary. By rotating sample holder  108  that holds the sample as above, variation in peak intensity can be avoided and high reproducibility in a result of analysis can be expected. X-ray spectroscopic analysis apparatus  10  does not have to include rotation mechanism  110 . 
     &lt;Other Modifications&gt; 
     Spectroscopic element  12  may be composed of a single crystal of Si, Ge, LiF, or quartz, or synthetic multi-layers may be used for soft X-rays not higher than 2 keV. Diffraction grating as effective as curved crystal may be employed. Curving of spectroscopic element  12  may be a Johann type or a Johansson type. 
     Spectroscopic element  12  may be curved like a spherical surface or a toroidal surface. So long as a central portion is close to the spherical surface, spectroscopic element  12  may be in another shape such as an elliptical surface or a parabolic surface. Though a curvature in the direction of spectroscopy (the x direction) and the direction of collection (the y direction) is preferably determined to satisfy Rx= 2 R and Ry= 2 R×sin 2 θ B  as described above, it does not have to strictly match with that. In particular, the direction of collection may be the same in curvature as the direction of spectroscopy in consideration of ease in manufacturing. 
     Excitation beams may be X-rays, or may be electron beams, neutron lines, or proton beams. Position-sensitive detector  14  may be implemented by a CCD or a 
     CMOS camera that is a two-dimensional detector. 
     [Aspects] 
     An illustrative embodiment described above is understood by a person skilled in the art as specific examples of aspects below. 
     (Clause 1) An X-ray spectroscopic analysis apparatus according to one aspect includes an excitation source, a curved spectroscopic element, a position-sensitive detector, and a computing unit. The excitation source causes generation of a group of characteristic X-rays different in peak wavelength from one another by emitting excitation beams to a sample held by a sample holder. The curved spectroscopic element separates the group of characteristic X-rays. The position-sensitive detector detects at least some of the group of characteristic X-rays separated by the spectroscopic element. The computing unit analyzes an element contained in the sample based on a result of detection by the detector. The spectroscopic element and the detector are disposed along a circumference of one Rowland circle. The spectroscopic element has a spectral surface whose length, measured along the Rowland circle, is shorter than a length in the Rowland circle plane, of an irradiation surface irradiated with the excitation beams emitted to the sample holder. The spectroscopic element and the sample holder are disposed to separate the group of characteristic X-rays within a common spectral range of the spectroscopic element. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 1, the effective spectral range of the curved spectroscopic element can be limited to the vicinity of an area in contact with the circumference of the Rowland circle. Therefore, lowering in accuracy in detection of characteristic X-rays due to difference in radius of curvature between the Rowland circle and the spectroscopic element can be prevented. The detector can thus accurately detect fluorescent X-rays. 
     (Clause 2) In the X-ray spectroscopic analysis apparatus described in Clause 2, the detector is arranged such that a surface of the detector intersects with the Rowland circle at two points. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 2, X-rays of different energy can be detected at high resolution. 
     (Clause 3) The X-ray spectroscopic analysis apparatus described in Clause 1 further includes a collimator that delimits the common spectral range. The collimator is disposed on paths of the group of characteristic X-rays from the sample holder via the spectroscopic element until the detector. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 3, unintended generation of scattered radiation from an end of the spectroscopic element can be prevented. 
     (Clause 4) In the X-ray spectroscopic analysis apparatus described in Clause 1 or 2, the collimator includes a plurality of apertures in correspondence with the group of characteristic X-rays. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 4, by cutting off characteristic X-rays other than a peak wavelength (wavelength range) of interest, lowering in SN ratio due to scattered radiation can be avoided. 
     (Clause 5) In the X-ray spectroscopic analysis apparatus described in any one of Clauses 1 to 4, the spectroscopic element has the spectral surface within the spectral range whose length, measured along the circumference of the Rowland circle, is equal to or shorter than ½ the length in the Rowland circle plane, of the irradiation surface. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 5, the effective spectral range of the curved spectroscopic element can be limited to the vicinity of an area in contact with the circumference of the Rowland circle. Therefore, lowering in accuracy in detection of characteristic X-rays due to difference in radius of curvature between the Rowland circle and the spectroscopic element can be prevented. The detector can thus accurately detect fluorescent X-rays. 
     (Clause 6) In the X-ray spectroscopic analysis apparatus described in Clauses 1 to 5, the detector is a one-dimensional detector. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 6, reduction in cost for the apparatus can be expected. Furthermore, time and efforts for reconfiguring two-dimensional data as in a two-dimensional detector into one-dimensional data are not required. 
     (Clause 7) The X-ray spectroscopic analysis apparatus described in any one of Clauses 1 to 6 further includes a rotation mechanism that rotates the sample holder. 
     According to the X-ray spectroscopic analysis apparatus described in Clause 7, variation in peak intensity can be avoided and high reproducibility of a result of analysis can be expected. 
     An elemental analysis method described in Clause 8 includes generating a group of characteristic X-rays different in peak wavelength from one another by emitting excitation beams to a sample held by a sample holder, causing the generated group of characteristic X-rays to be incident on a curved spectroscopic element, causing the spectroscopic element to separate the incident group of characteristic X-rays, and causing a position-sensitive detector to detect at least some of the separated group of characteristic X-rays, and analyzing an element contained in the sample based on a result of detection by the detector. The spectroscopic element and the detector are disposed along a circumference of one Rowland circle. The spectroscopic element has a spectral surface whose length, measured along the Rowland circle, is shorter than a length in the Rowland circle plane, of an irradiation surface irradiated with the excitation beams emitted to the sample holder. The spectroscopic element and the sample holder are disposed to separate the group of characteristic X-rays within a common spectral range of the spectroscopic element. 
     According to the elemental analysis method described in Clause 8, the effective spectral range of the curved spectroscopic element can be limited to the vicinity of an area in contact with the circumference of the Rowland circle. Therefore, lowering in accuracy in detection of characteristic X-rays due to difference in radius of curvature between the Rowland circle and the spectroscopic element can be prevented. The detector can thus accurately detect fluorescent X-rays. 
     Though an embodiment of the present invention has been described, it should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.