Abstract:
An x-ray mirror provides focusing and monochromatization while maintaining a high degree of reflectivity. The mirror has at least two mirror portions, one with a multilayer surface that provides the desired monochromating, and the other with a total external reflection surface. The multiple surfaces combine to provide the desired focusing of the x-rays from a source to a focus point. A variety of configurations may be used, each of which does the desired focusing and monochromatization with minimal energy loss. Relative positioning of the mirror portions may also allow for adjustment of the focus length.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application takes priority from U.S. Provisional Patent Application Ser. No. 60/613,734, filed Sep. 28, 2004. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of x-ray diffraction analysis and, more particularly, to the imaging of an x-ray beam with a structure that provides both focusing and monochromating of the beam. 
     BACKGROUND OF THE INVENTION 
     In an x-ray diffraction system the function of x-ray optics is to condition the primary x-ray beam into the required wavelength, beam focus size, beam profile and divergence. One type of x-ray optics device is the total external reflection mirror. Total external reflection happens when x-rays strike on a polished surface at a small grazing incident angle. The reflected x-rays from the surface take off at the same angle as the incident angle. The polished surface behaves similarly to a mirror reflecting visible light. Therefore such a mirror is referred as an x-ray mirror. The reflecting mirror is made of materials with refractive index less than unity. The total external reflection can only be observed at an incident angle less than the critical incident angle θ C . The value of the critical angle is dependent on the wavelength of the x-ray radiation and the reflecting materials. For a typical laboratory x-ray source, the wavelength is in the range of a fraction of nanometer, and the critical angle is in the range of a fraction of a degree to several degrees. 
     Another type of x-ray optics device is the multilayer mirror. A multilayer mirror consists of alternating layers of heavy materials as reflection layers and light materials as spacer layers. A multilayer mirror works on the same principle as Bragg diffraction from a natural crystal, selectively reflecting certain wavelengths based on the spacing between the mirror layers. In this way, multilayer mirrors can be used as monochromators. In contrast to a natural crystal, a multilayer mirror typically has larger d-spacing so that the incident angle and the diffracted angle are typically only a few degrees. In these mirrors, the number of layers, the d-spacing of the layers, and the distribution of the layer thickness can be varied to modify the mirror performance. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a hybrid mirror for x-ray diffraction systems is provided that provides monochromatization, but maintains a higher overall reflectivity than conventional multilayer mirrors. The hybrid mirror can take a number of different forms, but in each case includes a multilayer mirror portion and a total external reflection mirror portion. The multilayer mirror portion provides the desired monochromatization and, together, the multilayer mirror portion and the total external reflection mirror portion provide the desired focusing of the x-ray energy, which may originate at substantially a point source, toward a focal point. 
     In one embodiment, the mirror uses two mirror surfaces side-by-side, one surface being a multilayer material and the other being a total external reflection material. In another embodiment, the multilayer mirror portion and the total reflection mirror portion are arranged in a Kirkpatrick-Baez configuration. In still another embodiment, a side-by-side single bounce mirror component is coupled with a double side reflection mirror component, where one of the components comprises a multilayer, while the other comprises a total reflection material. In a different embodiment, the multilayer mirror portion and the total external reflection portion are located on two partially cylindrical surfaces that lie opposite each other. One partially cylindrical portion has its reflective surface on an inner surface that faces a reflective outer surface of the other partially cylindrical portion, which lies adjacent to it. Still another embodiment uses double cross-coupled hybrid mirror sections, that is, two side-by-side mirror sections, one a multilayer and the other a total reflection material. Yet another embodiment of the invention is configured so that one mirror surface completely encompasses another mirror surface circumferentially. On the outer portion is an inner surface that faces an outer surface of the inner portion. One of these surfaces comprises a multilayer while the other comprises a total reflection surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic view of an embodiment of the present invention in which two mirror surfaces are side-by-side; 
         FIG. 2  is a graphical view showing the general reflectivity characteristics of multilayer mirror surfaces and total external reflection surfaces; 
         FIG. 3  is a schematic view of an embodiment of the present invention in which two mirror surfaces are arranged in a Kirkpatrick-Baez configuration; 
         FIG. 4  is a graphical view showing the experimental reflectivity difference between a conventional multilayer mirror and a hybrid mirror according to the present invention; 
         FIG. 5  is a schematic view showing an embodiment of the invention in which a side-by-side single bounce mirror component is coupled with a double side reflection mirror component; 
         FIG. 6  is a schematic view of an embodiment of the invention in which two mirror surfaces are opposing partially cylindrical surfaces; 
         FIG. 7  is a schematic view of an embodiment of the invention that uses double cross-coupled hybrid mirror sections; and 
         FIG. 8  is a schematic view of an embodiment of the invention in which one mirror surface circumferentially encompasses another mirror surface. 
     
    
    
     DETAILED DESCRIPTION 
     Shown in  FIG. 1  is a hybrid mirror  10  according to a first embodiment of the invention. The mirror  10  includes a total reflection portion  12  and a multilayer portion  14 . The x-ray radiation energy from x-ray source  16  diverges toward both portions of the mirror. Part of the x-ray energy is first reflected by the total external reflection portion  12  and then reflected by the multilayer mirror portion before reaching a focus point  18 . Similarly, another part of the x-ray radiation energy from the same x-ray source  16  is first reflected by the multilayer portion  12  and then reflected by the total external reflection portion en route to the same focus point  18 . Thus, all of the x-ray energy from the source is reflected by the total external reflection mirror once and by the multilayer portion once. All of the initial x-ray beam is therefore monochromatized by the multilayer portion prior to reaching focus point  18 . However, compared to conventional side-by-side multilayer mirrors, the output x-ray beam from this device has a higher intensity, since the high reflectively portion is relatively low loss. In this embodiment, both mirror portions in the hybrid mirror may be either flat or curved depending on the desired performance. Various curved shapes and multilayer features available to the existing multilayer mirrors and total reflection mirrors are applicable to this hybrid mirror. 
       FIG. 2  is an illustration showing the relation between reflectivity and 2θ angles for the total external reflection portion and the multilayer portion of the hybrid mirror  10 . The use of 2θ as a measurement is commonplace in x-ray diffraction analysis, where the resulting direction of a reflected or refracted x-ray beam is twice the angle of incidence θ. In  FIG. 2 , θ C  is the critical angle for total external reflection, that is, the maximum incident angle that allows total external reflection. It depends on the reflection material and the energy (or wavelength) of the radiation. θ B  is the Bragg angle of the multilayer mirror which depends on the d-spacing of the multilayer. The reflectivity of a typical total reflection surface is better than 90%, while the reflectivity of a multilayer surface is typically lower, 70% for instance. The graph in  FIG. 2  also indicates the wavelength selectivity of the multilayer surface as compared to the total reflection surface. 
     In a double bounce mirror, the overall reflectivity is a multiple of the first bounce and the second bounce. For the hybrid mirror shown in  FIG. 1 , in which x-rays encounter the multilayer portion and the total reflection portion, the overall reflectivity R OH  can be given as:
 
 R   OH   =R   T   ·R   M  
 
where R T  is the reflectivity of the total reflection portion and R M  is the reflectivity of the multilayer portion. For a conventional side-by-side multilayer mirrors, x-rays are bounced twice by multilayer mirrors, so the overall reflectivity R OC  can be given as:
 
 R   OC   =R   M   ·R   M  
 
Because of the relative higher reflectivity from a total reflection mirror:
 
R T &gt;R M , or
 
R OH &gt;R OC  
 
The intensity gain with the hybrid mirror relative to a side-by-side multilayer mirror is therefore R T /R M .
 
       FIG. 3  shows another embodiment of a hybrid mirror  20  according to the present invention. In this embodiment, two mirror portions are arranged in a so-called “Kirpatrick-Baez” configuration. x-rays from a source  16  are bounced by a multilayer portion  22  and then bounced by a total reflection portion  24  further down in the beam path. The surface of the total reflection portion  24  is rotated 90 degrees relative to the multilayer portion  22 . This arrangement may also use the two mirror portions in a reverse configuration, where the total reflection portion is upstream from the multilayer portion in the beam path. 
     The reflectivity of the mirror group in a Kirpatrick-Baez configuration can also be calculated in the manner described above.  FIG. 4  is an experimental result comparing hybrid mirrors according to the present invention (for which the peaks are shown in broken lines) and conventional multilayer mirrors (for which the peaks are shown in solid lines), each in a Kirpatrick-Baez configuration. The diffraction peaks with multilayer mirrors are shifted to the right for easy comparison. As shown in  FIG. 4 , the hybrid mirrors have intensity gains of 38% to 42% relative to multilayer mirrors in a Kirpatrick-Baez configuration. The side-by-side hybrid mirrors should have about the same intensity gain advantage. 
       FIG. 5  shows another embodiment of the present invention, in which a side-by-side single bounce mirror  26  is coupled with a double side reflection mirror  28 . The two adjacent sides of the mirror  26  are multilayer surfaces. The two adjacent sides of the mirror  28 , however, are total external reflection surfaces. A part of the x-ray radiation energy from the x-ray source  16  is first reflected by side-by-side mirror  26 , and then reflected by the double side reflection mirror  28  to reach the focus  18 . The mirrors  26  and  28  face each other. Another part of the x-ray radiation energy from the same x-ray source  16  is first reflected by the double side reflection mirror  28 , and then reflected by the side-by-side mirror  26  to reach the same focus  18 . Both parts of the x-rays are reflected by both multilayer surfaces and by total external reflection surfaces. 
     In the embodiment of  FIG. 5 , the position of the block  28  with the two side reflection mirrors can be adjusted to change the focus distance between the hybrid mirrors and the focus spot  18 . The block can also function as a beamstop to prevent the unconditioned x-rays from the source  16  from reaching the focus  18 . Those skilled in the art will also recognize that, as with the aforementioned embodiments, the relative position of the multilayer surfaces and the total reflection surfaces may be reversed. 
       FIG. 6  shows still another embodiment of the present invention, in which a configuration of partially cylindrical surfaces is used. This configuration is a variation from the face-by-face surfaces discussed above. Instead of two side-by-side surfaces for each mirror section, a single curved surface is used for each. In the configuration shown in  FIG. 6 , an outer partially cylindrical mirror portion  30  may have a concave multilayer mirror surface. An adjacent, inner partially cylindrical mirror portion  32  may have a convex total external reflection surface facing the mirror portion  30 . The x-ray radiation energy from the x-ray source  16  is first reflected by the multilayer mirror portion  30 , and then reflected by the total external reflection mirror portion  32  to reach the focus  18 . The convex mirror portion  30  may be tilted to allow for a variable focus distance. Meanwhile, the convex mirror  32  may act as a beam stop, blocking the direct x-ray energy from reaching the focus  18 . 
       FIG. 7  shows an embodiment that uses a configuration of double cross-coupled hybrid mirror portions. This configuration is a variation of the aforementioned hybrid mirrors that use a Kirpatrick-Baez arrangement. Instead of using a single surface for each mirror surface of a given section, a side-by-side multilayer mirror portion  34  is coupled with a side-by-side total external reflection mirror portion  36 . The x-ray radiation energy from the x-ray source  16  is first reflected by the multilayer mirror portion and then reflected by the total external reflection mirror portion to reach the focus  18 . The total external reflection mirror portion  36  may be tilted relative to the multilayer mirror portion  34  to allow the focus distance to be varied. 
       FIG. 8  shows yet another embodiment of the present invention. In this configuration, two mirror portions are used that are arranged with one encompassing the other. This configuration is a variation of the aforementioned coupled partial cylinder hybrid mirrors, but instead of a partially cylindrical surface for each mirror portion, fully circumferential surfaces are used. An outer mirror portion  38  has an inner surface that is a multilayer mirror surface. Although this mirror portion  38  is shown in cross-section in the figure, those skilled in the art will recognize that it fully encompasses the inner portion  40 . The inner mirror portion  40  has an outer surface that is a total external reflection mirror surface. The outer mirror portion  38  and the inner mirror portion  40  have a common longitudinal axis. The x-ray radiation energy from x-ray source  16  is first reflected by the multilayer mirror surface of the outer portion  38 , and then reflected by the total external reflection mirror surface of the inner mirror portion  40 , so as to reach the focus  18 . The total external reflection mirror surface of the inner portion reduces the divergence of the x-rays by reflecting them to a longer focus distance. The inner portion  38  may also be moved along the common longitudinal axis relative to the outer portion so as to vary the focus distance. As with each of the embodiments herein, the relative position of the multilayer mirror surface and the total external reflection surface may be reversed. 
     While the invention has been shown and described with reference to a preferred embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.