Patent Number: 052689549
Section: summary

FIELD OF THE INVENTION This invention pertains generally to the field of monochromators, such as those used with synchrotron X-ray sources, and particularly to mounting mechanisms for monochromators and similar mechanisms. BACKGROUND OF THE INVENTION Monochromators are used to selectively filter an electromagnetic radiation beam so that only a single wavelength or a narrow band of wavelengths is passed by the monochromator. A common type of monochromator utilizes two diffractors, with the input beam diffracted by both to form an output beam which is parallel to the input beam but substantially restricted to a single wavelength. In the visible light range, where the wavelengths are relatively long, diffraction gratings may be used as diffractors. Monochromators which cover a shorter wavelength range, such as in the X-ray region, may use diffractor elements which are formed of single crystals, commonly of silicon or germanium, or multi-laminar layered mirrors that behave in the same manner as crystals. In an article by J. A. Golovchenko, et al. entitled "X-ray Monochromator System for Use with Synchrotron Radiation Sources," Review of Scientific Instruments, Vol. 52, No. 4, April, 1981, pp. 509-516, a double crystal monochromator is described in which each crystal is oriented to obtain a constant energy or direction as well as a constant beam position as the selected beam energy is varied, and to make the central ray of the beam impinge at the same point on each monochromator crystal independently of the chosen energy. This is achieved in part by sampling the output beam intensity for angular drifts out of parallelism between the two crystals, with an electronic drive then used to correct orientation of the beam. The crystals are mounted on a mounting element composed of two bars joined at a right angle in an "L" shape, with one crystal mounted parallel to one bar and the other mounted perpendicular to the other bar, so that the two crystals are held parallel to one another. The L-shaped mounting member rotates with one of the crystals about the same axis of rotation, to change the wavelength passed by the crystal, while the joint between the two legs of the L and the second crystal both translate along a straight line parallel to the input and output beams. Precision sliders are required between the mounts for the crystals and the mounting bars, as well as at the point at which the center of the L shaped mounting member is supported for translation. An improved monochromator mounting mechanism is shown in the U.S. Pat. No. 5,157,702 to Middleton and Hicks, issued Oct. 20, 1992, entitled "A Mechanically Actuated Double Crystal Monochromator." The monochromator shown in that patent is designed for operation in high vacuum so that it can be mounted to a synchrotron, and includes precision mechanical parts for allowing the crystal mounting assemblies to slide in precise relation to one another. In the foregoing systems, the parallelism of the faces of the diffracting crystals is maintained by mounting them on the L-shaped mounting member which is mounted on rotating sliders. Highly precise sliders are required in the structure since the orientation of the crystal is dependent upon the precision of the sliders, which contributes to the complexity and cost of the monochromator instrument. SUMMARY OF THE INVENTION A mounting mechanism for a double-crystal monochromator or the like in accordance with the present invention utilizes a parallelogram based mounting mechanism in which two of the vertices of the parallelogram are fixed in position along a base, and two other vertices are free to translate back and forth in a straight line parallel to the fixed base of the parallelogram. A first mounting member for one of the diffractor elements is mounted for pivoting at one of the fixed vertices of the parallelogram, and a second mounting member is mounted for pivoting at the adjacent movable vertex of the parallelogram. The surfaces of the two diffractors are maintained parallel to one another as the angle of the diffractors with respect to the input and output beams is changed to change the wavelength passed by the monochromator. The mounting members for the diffractors are operatively connected so that the change in the angle of incidence on, and reflection from the diffractors is one-half of the change in angle of a pivotable arm forming one side of the parallelogram. In a preferred structure for operatively connecting the mounting members, a large diameter wheel is connected to each mounting member to pivot around an axis which lies on the surface of the diffractor which the mounting member supports. Wheels having half of the pitch diameter of the larger wheels are mounted at the other two vertices of the parallelogram, and bands connect each pair of larger and smaller wheels so that the large and small wheels move together, with each small wheel undergoing an angular displacement twice that of the large wheel to which it is connected. The small wheel at the fixed pivot point is connected to the arm forming one side of the parallelogram so that the wheel and the arm undergo the same angular displacement about the pivot. The small wheel connected at the movable pivot point of the parallelogram is connected to the arm so that the wheel undergoes the same angular displacement as the arm. Consequently, rotation of the pivotable arm of the parallelogram will result in pivoting of the diffractors by an angle exactly equal to one half of the angular displacement of the pivotable arm. In the present invention, sliders are required only at the movable pivot point at the vertex of the parallelogram to which the small wheel is mounted and for the movable support on which the small and large wheels are mounted for translation. Thus, only two slider mechanisms are required in the present invention. Moreover, the translational support for the large and small wheels at the two moving vertices of the parallelogram can be provided by a support rod which extends outside the vacuum chamber enclosure required for applications requiring high vacuum, as with an X-ray synchrotron. Support of the sliding mechanism outside the vacuum chamber allows the use of less expensive components of the type which can be lubricated, which is not the case where the sliding supports must be mounted within the vacuum chamber. Consequently, high precision in the positioning and rotation of the mounting members, and the diffractors or mirrors supported thereon, can be obtained with relatively low cost components. The present invention may also be embodied in a plane grating monochromator in which a mirror is mounted to the movable vertex of the parallelogram structure and a diffraction grating is mounted for pivoting independently of rotation of the mirror. The parallelogram mounting apparatus of the invention orients the mirror so that the incoming beam is properly directed to the grating at all angles of incidence. Further objects, features and advantages of the present invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.