Patent Number: 051270285
Section: summary

BACKGROUND OF THE INVENTION Efficient x-ray diffractors having a large collection solid angle and high spectral resolving power have surfaces and planes that are cylindrically curved to different radii as in the Johansson geometry, or are doubly curved as in the spherically bent plate or in diffractors with spherical planes and toroidal surface. However, the ideal geometries involving doubly curved diffracting planes and surfaces that are curved differently than the planes cannot always be used for diffractors fabricated from bulk single crystals for several reasons. First, natural crystal are not available with sufficiently large d spacings to diffract long wave length x rays. Second, some natural crystals with desirable d spacings and high diffraction efficiency cannot be formed to the desired configuration because of difficulty in grinding or polishing the surfaces that are not parallel to cleavage planes or because of the tendency to fracture when elastic bending is used or because of the undesirable distortion of the crystal lattice if plastic bending is used. The latter problems are enhanced when using thick pieces of single crystals to form large area diffractors because of the greater stresses produced in bending. On the other hand, layered structures obtained by multiple deposition processes have not yet been made with sufficiently small d spacings to diffract x-rays of short wave length at large Bragg angles and have not been made sufficiently thick to be configurable to the desired geometry. The purpose of the present invention is to provide a diffractor geometry with a stepped surface that will increase the possible ways for fabricating large area, high efficiency diffractors and will increase the variety and types of diffracting materials that can be used in the fabrication of these diffractors. SUMMARY OF THE INVENTION This invention is a diffractor with surface steps configured to approximate the case wherein the diffractor has doubly curved diffracting planes and a smooth doubly curved surface on the side on which the radiation impinges. Such doubly curved diffractors have been described by Wittry and Sun in the Journal of Applied Physics, Feb. 15, 1990, pages 1633-1638, and by Wittry in U.S. Pat. Nos. 4,599,741 (Jul. 8, 1986), 4,807,268 (Feb. 21, 1989), and 4,882,780 (Nov. 21, 1989). The stepped diffractor is configured so that the surface of the steps have the same shape as the diffracting planes of the continuous case. Thus, the diffracting material has diffracting planes parallel to the surface of the steps. This makes it possible to fabricate doubly curved diffractors of high radiation collection efficiency using diffracting materials that cannot normally be employed in the fabrication of high efficiency diffractors having a continuous and smooth surface. For the diffractor with doubly curved surface steps, two factors govern the width of the steps for a useful diffractor, namely, a) the effect of the width of the steps on satisfying Bragg's law within a certain range of the desired value of the diffracting angle .theta..sub.B, and b) the effect of the finite width of the steps on the focussing properties of the diffractor. As will be shown in the section on embodiments of the invention, relatively simple equations can be derived that show these relationships and indicate how the step width should vary over the surface of the diffractor in order to satisfy the constraints imposed by both the rocking curve width of the diffracting material and the desired focussing accuracy. Although stepped diffractors have been previously proposed as approximations to the Johannson geometry by Okano in U.S. Pat. No. 3,469,098 (Sep. 23, 1969) and by Wittry in U.S. Pat. No. 3,927,319 (Dec. 16, 1975), neither of these inventions took into consideration the true effects resulting from the three dimensional nature of the problem of diffracting as much radiation as possible emanating from a point source. With the results of calculations of the type made by Wittry and Sun in their 1990 Journal of Applied Physics paper it is possible to see the advantages of diffractors with doubly curved planes and doubly curved surfaces. The unique problems that occur in the fabrication of these latter types of diffractors, which were discovered only by considerable experimentation, has led to the present invention in which the steps on the diffractor have a doubly curved surface.