Patent Number: 063174839
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION In accordance with this invention, an x-ray reflective device shown in FIG. 1 comprises a curved crystal 10 and support base 12. Crystal 10 has a set of curved atomic reflection planes 14. On every point of the crystal surface, atomic plane set 14 is near parallel to the crystal surface 16 in the embodiment shown. The spacing between the atomic planes, d, varies continuously from d.sub.1 at one end of the crystal to d.sub.2 at the other end of the crystal. Values of d.sub.1 and d.sub.2 are determined from the Bragg equation where the Bragg angles are the incident angles .theta..sub.1 and .theta..sub.2, respectively. According to the Bragg equation, the Bragg angles of reflective planes 14 for x-ray photons of wavelength .lambda. vary with the d spacing profiles. The configuration of the crystal surface 16 can be spherical, ellipsoidal, paraboloidal, toroidal, or other type of doubly curved surface. The profile of the d spacing for the crystal planes is designed to allow the incident angles of monochromatic x-rays from a divergent laboratory source to match the Bragg angle on each point of the crystal surface. The optical device according to the present invention can be fabricated by bending a flat thin crystal slab 10 as shown in FIG. 2 with a desired d spacing profile to a preselected geometry. One bending method is the fabrication process described in co-pending, commonly assigned U.S. patent application Ser. No. 09/342,606, entitled "Curved Optical Device and Method of Fabrication," the entirety of which is hereby incorporated herein by reference. The variation of the d spacing of the crystal planes can be produced by growing a crystal made of two or more elements and changing the relative percentages of the two elements as the crystal is grown. For instance, the lattice parameter of a Si--Ge crystal varies with a change in concentration of Ge. Therefore, a crystal material with a graded lattice parameter can be obtained by growing a Si--Ge crystal and controlling the concentration of Ge during growth. Such crystal planes are commercial available and can be purchased, for example, from Virginia Semiconductor, Inc. of Fredericksburg, Va. One embodiment of the present invention providing point to point x-ray imaging is illustrated in FIG. 3A. Crystal planes 14 are curved to an ellipsoidal shape and the d spacing of the planes varies along the direction parallel to the optical axis 2--2. For the symmetrical configuration shown in FIG. 3A, the d spacing of reflection planes has a maximum value d.sub.0 at the center point O and decreases as edge E is approached. With the decrease of the d value from the center O of the crystal to the edge E, the Bragg angle for x-rays of wavelength .lambda. increases, which matches the increase of the incident angles from O to E for x-rays diverging from the left focus of the ellipse. A cross-section of the crystal taken along line 4--4 is shown in FIG. 3B. In this plane, the crystal planes are circular and the d spacing does not vary. FIG. 3C shows an asymmetrical arrangement of a point to point focusing geometry, which provides demagnification of source S. The ellipsoidal crystal element in FIG. 3A can be made by bending a flat crystal 10 (see FIG. 2) to an ellipsoid, where the d spacing of the flat crystal 10 varies along the X direction but is constant along the Y direction (see FIG. 2). The optical element shown in FIG. 3A may be fabricated in two pieces such that two identical flat crystal slabs with graded d spacing from d.sub.0 to d.sub.E can be used as shown in the exploded view of FIG. 4. In this embodiment, the two crystals are joined at O and the surface is ellipsoidal. This approach allows the grading to increase in one dimension. Conversely, the element in FIG. 3A requires a grading profile that increases and then decreases. A curved crystal device with paraboloid geometry is shown in FIG. 5A. This device produces a monochromatic collimating x-ray beam from a point source S. The d spacing of the reflection plane of the crystal 10 is graded from a value of d.sub.1 to d.sub.2. To satisfy the Bragg condition, the d spacing profile is linear for the first order approximation and increases from point A to B. Alternatively, a collimated beam can be directed to a focal spot as shown in FIG. 5B. The focusing and collimating of x-rays can also be obtained with a spherical geometry at near normal incidence using crystalline planes with graded d spacing. Spherical mirrors are well known as imaging devices for normal incident visible light optics. A conventional spherically bent crystal can demagnify (or magnify) and collimate x-rays from a divergent x-ray source for some particular wavelength at near normal incidence. However, the numerical aperture of this type device is too small to be useful. The numerical aperture can be improved substantially with the use of graded d spacing, doubly curved crystals in accordance with the present invention. FIG. 6A shows a set of spherical curved crystal planes according to another embodiment of the present invention, which provides a demagnified image of the x-ray source. The d spacing of the crystal planes has a symmetrical profile about the optical axis and varies along the transverse direction. It increases across the surface from points A to B. The normal projecting view along the optical axis is illustrated in FIG. 6B. The d spacing profile of this device may be difficult to obtain. In practice, it can be approximated by using multiple pieces of crystal slabs with a simple graded d profile as shown in FIG. 6C. Each piece of crystal is curved to a spherical shape with the reflection planes parallel to the surface. The d spacing profile of the reflection planes is one-dimensionally graded along the radial direction passing the center of each crystal. If an x-ray source is placed at the focus of the concave spherical device similar to the orientation shown in FIG. 6A, a collimating x-ray beam is obtained. Strong demagnification can be obtained if two spherical crystal devices are combined. One preferred geometry is the Schwarzschild configuration which is used to image soft x-rays in conjunction with a multilayer coating. Graded crystals with the Schwarzschild geometry provide imaging for hard x-rays as shown in FIG. 7. The reflection planes of primary crystal 18 has a d spacing profile of d.sub.1 (r) to reflect x-rays from a source emitting x-rays at a near normal incident angle. The reflection planes of the secondary crystal 20 have the desired profile d.sub.2 (r) to match the incident angles of the x-rays reflected off the primary crystal 18. This system produces a final image of the source at I. While the invention has been described in detail herein in accordance with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is intended by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.