Abstract:
An x-ray analysis system including a focusing optic for focusing an x-ray beam to a focal point, a first slit optically coupled to the focusing optic, a second slit optically coupled to the first slit, and an x-ray detector, where the focal point is located in front of the detector.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an x-ray analysis application. More specifically, the present invention relates to an apparatus and method for generating, forming, and directing an x-ray beam used in x-ray analysis. 
     A common method used to study moderately ordered structures, i.e. those structures which have short range ordering but lack long range ordering, is small angle x-ray scattering. The method is based on illuminating a sample structure with a beam of x-rays. A portion of the x-ray beam is not able to travel directly through the sample structure, rather some rays are deflected or scattered and emerge from the sample at varying angles. The incident x-rays make their way along the spaces between the atoms of the structure or are deflected by the atoms. Since the structure is ordered throughout with short range ordering, the scattering from the structure will create a diffused x-ray pattern at a very close range to the x-rays traveling directly through the structure. This diffused pattern corresponds to the atomic structural arrangement of the sample. 
     Small angle x-ray scattering can be done in one or two dimensions. One dimensional small angle x-ray scattering utilizes a line source to maximize x-ray flux. The resultant diffusion pattern formed by the line source reveals information in only one dimension. Two dimensional x-ray scattering utilizes an x-ray point source which makes it possible to reveal two dimensional information. Although a rotating anode is preferred as a laboratory x-ray point source, other x-ray generators, including sealed tubes, may be used. A synchrotron has also been used in two-dimensional applications due to its well-collimated and high intensity beam. 
     Traditionally, an x-ray beam used in two dimensional small angle scattering is formed by a series of slits or pinholes to collimate the divergent beam and limit scattering effects from the slits. For samples with strong scattering power or a large scattering angle, such as crystals, parasitic scattering from pinholes and mirrors can be ignored. A two pinhole system may be used in such an application. For samples with weak scattering power or a small scattering angle, such as those contemplated by the present invention, a three pinhole system is preferably used. The current techniques for small angle scattering involve the use of pinhole systems, filters, and total reflection mirrors. A Ni filter, graphite or other crystals are used in a pinhole system or a pinhole+total reflection mirror system to reduce the Kβ radiation or other continuous spectrum radiation. Total reflection mirrors such as Kirkpatrick-Baez or cross-coupled mirrors are frequently used with the pinhole systems (both with two-pinhole systems and three pinhole systems). Presently, the focal point of a total reflection mirror used with a pinhole system is always set at the detector position, creating a loss of flux. Parabolic multilayer optics (Kirkpatrick-Baez, or cross-coupled) are also used in small angle scattering systems but fail to enhance the beam at the sample position effectively. 
     Small angle x-ray scattering systems presently used in the art suffer from noise problems caused by pinhole scattering and limited x-ray flux used for generating x-ray scattering patterns. Thus, there is a need in the art for a small angle x-ray scattering system which eliminates diffraction noise and increases the flux on a sample. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for generating an x-ray beam used in small angle x-ray scattering applications. The present invention uses optics to focus and increase the flux of an x-ray beam generated by an x-ray point source and a system of slits or pinholes to shape the x-ray beam. The optical system can be configured in either a two pinhole system for maximum flux or a three pinhole system for low background noise and a small minimum accessible angle. 
     An object of the present invention is to reduce the beam divergence of an x-ray beam used in small angle x-ray scattering applications. 
     A further object of the present invention is to increase the flux of an x-ray beam on a sample in small angle x-ray scattering applications. 
     A still further object of the present invention is to have a small “minimum accessible angle.” 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to those skilled in the art after reading the following specification and by reference to the drawings, in which: 
     FIG. 1 is a diagrammatic view of the optical scheme of the present invention according to the preferred embodiment; 
     FIG. 2 is a diagrammatic view of the alignment mechanism of the present invention according to the preferred embodiment; and 
     FIG. 3 is a diagrammatic view of an alternate embodiment of the alignment mechanism of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a diagrammatic view of the optical system  10  of the present invention. An x-ray beam  12  is generated by an x-ray source  14  that is directed towards an optic  16 , such as an elliptical mirror, that focuses the x-ray beam  12 . The optic  16  has a reflective surface which may be comprised of bent graphite, bent perfect crystal, a total reflection mirror, a mulitlayer Bragg reflector which may be depth or laterally graded, or any other x-ray reflective surface known in the art. The optic  16  directs the x-ray beam through a first slit (or pinhole)  18  and a second slit (or pinhole)  20  to form and define a coherent x-ray beam  21 . Scattering and interference patterns or noise created by the first slit  18  are blocked by the second slit  20 . The focal point  22  of the x-ray beam  21  is located between the second slit  20  and an x-ray detector  30 . A sample chamber  24 , containing a sample structure  26  to be analyzed, includes a third slit  28  to eliminate scattering and interference patterns created by the second slit  20 . 
     The x-ray beam  21  flux at the sample chamber  24  and the x-ray beam  21  size or incident area on the x-ray detector  30  depend on where the focal point  22  of the optic  16  is located. Flux passing through the second slit  20  and reaching the sample chamber  24  is the greatest when the focal point  22  of the optic  16  is positioned on the second slit  20 , and the x-ray beam  21  size on the x-ray detector  30  is also the greatest in this situation. The x-ray beam  21  size on the x-ray detector  30  is the smallest if the focal point  22  of the optic  16  is positioned on or at the x-ray detector  30 , therefore the resolution of a system using this focal point  22  position would be the greatest. However, the flux in this case would also be the smallest. Therefore, the position of the focal point  22  in the system is determined by the trade-off between intensity and resolution of x-rays incident on the x-ray detector  30 . 
     In certain cases, due to the intrinsic divergence of the x-ray beam  21 , the resolution would reach its limit at certain positions of the focal point  22 . Accordingly, moving the focal point  22  closer to x-ray detector  30  would not improve the resolution and would only reduce the flux. Thus, in this case, there would be no benefit to focus the x-ray beam  21  on the x-ray detector  30 . Since the minimum accessible angle of the system is determined by the slit (pinhole) configuration, it is independent of the position of the focus. 
     The first and second slits  18  and  20  of the optical system  10  determine the size and shape of the x-ray beam  21  and the third slit  28  blocks parasitic scattering. The x-ray beam  21 , because of its focused nature, enables maximum flux to be concentrated on the sample structure  26 . The x-ray detector  30  is able to detect the diffusion pattern created by the small angle scattering from the sample structure  26  because of the increased flux on the sample structure  26  and the elimination of divergence and scattering. The x-ray detector  30  is further equipped with a beam stopper  32  to prevent direct x-ray beam damage to the x-ray detector  30  and noise. The exact location of the focal point  22  between the second slit  20  and the x-ray detector  30  depends on the desired flux and resolution characteristics of the optical system  10 . 
     The optical system  10  of the present invention is preferably enclosed in a vacuum path or pre-flight beam pipe  27  to eliminate scattering and absorption caused by atmospheric gases and particles. The pre-flight beam pipe  27  is comprised of a number of individual pipes which may be mixed and matched to optimize and change the length of the system. 
     The slits  18 ,  20 , and  28  in the preferred embodiment, are formed as pinholes that are precision machined as round holes. Rounded pinholes create significant difficulty in alignment, especially when the sizes of the pinholes are small and multiple pinholes are used. The present invention includes a pinhole plate  34  having an alignment window  36  equipped with a triangle shaped nose  38  offset and aligned with a pinhole  40 . During alignment of an x-ray beam, the x-ray beam is adjusted to enter and exit the alignment window  36 . An x-ray detector is used as feedback to ensure that the x-ray beam is passing through the alignment window  36 . The pinhole plate  34  is then moved manually or automatically in a vertical and horizontal fashion in the direction of the pinhole  40 . If the x-ray detector does not detect the x-ray beam during an indexing of the alignment window  36  relative to the x-ray beam, the pinhole plate  34  will be moved to its last position and indexed in the opposite vertical or possibly horizontal direction. In this manner, the x-ray beam position is always known and the x-ray beam may be traversed to the vertex  37  of the triangle  38 . The x-ray beam follows, in relative fashion, the cutout of the alignment window  36  until it reaches the vertex  37  of the triangle  38 . At the vertex  37  of the triangle  38 , movement will block or reduce the flux of the beam in both vertical directions and horizontal movement in the direction of the pinhole  40  will also block or reduce the beam. Accordingly, when such a condition is reached it is known that the beam is at the vertex  37  of the triangle  38 . 
     The pinhole  40  is a known fixed distance from the vertex  37  of the triangle  38 . Thus, when the x-ray beam is found to be at the vertex  37  of the triangle  38 , the pinhole plate  34  or x-ray beam may be precisely indexed this known distance to the pinhole  40 , ensuring precise alignment of the pinhole  40  and the x-ray beam. Accordingly, the position of the x-ray beam will be known. 
     In a first embodiment, the pinhole plate  34  is manually moved relative to the x-ray beam  21  using a precision x-ray table. The operator will read the x-ray detector  30  output and move the pinhole plate  34  accordingly. In alternate embodiments the operator will move the x-ray beam relative to the pinhole plate  34 . 
     In a second embodiment of the present invention, the pinhole plate  34  is moved using an automated servomotor or linear actuator system. The detector  30  feedback is transmitted to a computer which controls the x-y indexing of the x-ray beam or pinhole plate  34 . In response to feedback from the detector  30 , the computer will give the actuator system position commands to properly align the x-ray beam  21  and the pinhole plate  34 . 
     Referring to FIG. 3, an alternate embodiment of the pinhole plate  34 ′ of the present invention is shown. The pinhole plate  34 ′, as in the first embodiment  34 , includes an alignment window  36 ′ equipped with a triangle shaped nose  38 ′ having a vertex  37 ′. A rotating aperture plate  42 , having multiple apertures  44 , rotates about a point  46  in the directions of arrow  48 . The rotating aperture plate  42  allows multiple apertures  44  having various aperture diameters to be used in the present invention. Each aperture  44  may be indexed or rotated about point  46  to a position with a known offset from the vertex  37 ′ of the triangle shaped nose  38 ′. The center of each aperture  44  in the rotating aperture plate  42  is the same radial distance from the point  46 , allowing each aperture  44  to be correctly offset from the vertex  37 ′ of the triangle shaped nose  38 ′. A rotary position feedback device such as an encoder or a manual latch may be used to precisely position the apertures  44  with respect to the vertex  37 ′ of the triangle shaped nose  38 ′. 
     It is to be understood that the invention is not limited to the exact construction illustrated and described above, but that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.