Patent Number: 056152450
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described in detail with reference to accompanying drawings. The present invention, however, is not to be limited to the details given herein. FIG. 4 shows a monochromator for radiant X-rays according to an embodiment of the present invention. The monochromator is composed of a first crystal 10 which has a first surface of incidence 12 having a concave letter V-shaped groove 11 and a second crystal 20 which has a second surface of incidence 22 having a letter V-shaped convex 21. The first crystal 10, as shown in FIG. 5, internally includes a transport pipe 30 (cooling means) for flowing a cooling material 31 behind the first surface of incidence 12 along the concave letter V-shaped groove 11. The transport pipe 30 extends toward the bottom portion (intensely heated zone) 13 of the concave letter V-shaped groove 11 from the bottom portion of the first crystal 10, branches off in opposed directions just under the intensely heated zone 13 so as to run substantially along the inclined surfaces 12, which are the first surface of incidence of the letter V-shaped groove, and reaches side walls of the first crystal 10, from which the cooling material 31 is discharged outside. Stainless steel or Teflon which is heat-resistant and pressure resistant is used as material for the transport pipe 30, and water or liquid gallium having a good cooling efficiency is used as the cooling material 31, and also the cooling material 31 is applied to the intensely heated zone 13 in the form of a water jet. FIG. 6 is a schematic showing a lattice distortion of the first crystal 10 caused by heat when a pencil of radiant X-rays enters the monochromator for radiant X-rays of the present invention. In FIG. 6, horizontal lines 40 denote lattice planes. Initial lattice planes before entry of a pencil of X-rays are represented with dotted lines, and thermally deformed lattice planes are represented with solid lines. As seen from FIG. 6, a portion (denoted by 42) of the inclined surface 12 near the intensely heated zone 13 expands in an inward direction of the concave letter V-shaped groove 11, but a deformation of lattice planes themselves is slight. That is, lattice planes deform in such a manner that ends thereof slightly rise with resect to a virtual line extended from the bottom portion 13 of the concave letter V-shaped groove 11. However, thus deformed lattice planes do not deviate too much from initial lattice planes. Also, in the intensely heated zone 13, since the inclined surfaces of the concave letter V shaped groove 11 converge, thermally generated stresses cancel out each other and are attenuated, whereby thermal distortion is suppressed. In a zone 44 subject to a deformation which is directed toward the inside of the first crystal 11 underneath the intensely heated zone 13, the deformation terminates within the crystal without reaching the bottom portion and side surfaces of the crystal. Accordingly, the influence of the thermal deformation concentrates on the periphery of the intensely heated zone 13, and the influence on the entire crystal can be minimized. Thus, the influence of thermal deformation on lattice planes is very small which is observed with the monochromator for radiant X-rays of the present invention, and hence any large warp of lattice planes does not take place which is observed with conventional plate type monochromators. FIG. 7 shows a path of a parallel pencil of X-rays which enters the first crystal and then exits the second crystal. A parallel pencil of radiant X-rays enters the first crystal 10 at the first surface of incidence 12 having the concave letter V-shaped groove 11 (angle between the letter V-shaped inclined surfaces: 2.alpha.). The first surface of incidence 12 is inclined at an angle of .theta. from a pencil of incident X-rays A.sub.0 B.sub.0 C.sub.0 (minor axis 2S.sub.v, major axis 2S.sub.h), and thus the shade A.sub.1 B.sub.1 C.sub.1 of X-rays on the surface of incidence elongates to a half-ellipse (minor axis 2s.sub.v /sin.theta., major axis S.sub.h /{tan.alpha. sin.theta.}. Then, the X-rays which have expanded on the first surface of incidence 12 reflect therefrom. Thus reflected X-rays enter the second crystal 20 at the second surface of incidence 22 having the letter V-shaped convex 21 (angle between the letter V-shaped inclined surfaces: 2.alpha.). The concave letter V-shaped groove 11 and the letter V-shaped convex 21 fit into each other and are positioned so as to align with each other. As a result, the first surface of incidence 12 and the second surface of incidence 22 are arranged in parallel with each other and spaced by D. Accordingly, a pencil of X-rays reflecting from the first surface of incidence 12 impinges in parallel on the second surface of incidence 22, thereby forming a half-elliptical shadow A.sub.2 B.sub.2 C.sub.2 of X-rays having the same size on the second surface of incidence 22. As a result of arranging the first surface of incidence 12 and the second surface of incidence 22 in parallel with each other, a pencil of X-rays impinging on the second surface of incidence 22 exits at the same angle .theta. as the Bragg angle .theta. to the first surface of incidence 12. Thus, a pencil of emissive X-rays A.sub.3 B.sub.3 C.sub.3 (minor axis 2S.sub.v, major axis 2S.sub.h) is obtained which has the same size and parallelism as the initial parallel pencil of X-rays. In this case, a cross section of the pencil of incident X-rays is seen from FIG. 8, and a cross section of the pencil of emissive X-rays is seen from FIG. 9. FIG. 8 is a distribution diagram showing a power distribution of the first beam from an undulator, representing a pencil of incident X-rays which enters the first crystal. FIG. 9 is a distribution diagram showing a power distribution of the emissive beam which exits the monochromator, representing a pencil of emissive X-rays which exits the second crystal. As seen from the figures, both cross sections agree well with each other, indicating that the pencil of emissive X-rays accurately reproduces the pencil of incident X-rays. Since the pencil of emissive X-rays is useful as a light for X-ray structural analysis, material evaluation and the like, it is found that high-accuracy X-ray spectroscopic performance can be obtained. To obtain useful light which accurately has the same size and parallelism as a pencil of radiant X-rays, it is preferable that the first crystal 10 and the second crystal 20 be arranged in such a manner that the centerline of the bottom portion 13 of the concave letter V-shaped groove aligns with the centerline of a tip portion 23 of the letter V-shaped convex 21. Suppose that the first crystal 10 and the second crystal 20 deviate from each other by spacing b. In this case, as shown in FIG. 10, X-rays which reflect from the first crystal 10 do not impinge in parallel on the second surface of incidence 22 of the second crystal 20. Unlike an initial parallel pencil of X-rays, resultant emissive X-rays become an unparallel pencil of X-rays, whose cross section is a stepped cross section having a shear of 2b cos.theta./tan.alpha., as shown in FIG. 11. For example, in a test under the conditions in which the angle, .theta., (Bragg angle) between a pencil of incident X-rays and a surface of incidence is 15 degrees, an angle of a V groove is 2.alpha., and the deviation, b, between the first and second crystals is 0.1 mm, the result is that an intensity distribution of a pencil of emissive X-rays, i.e., an intensity distribution of monochromatic light is found to have a stepped cross section as shown in FIG. 12. FIG. 13 is a graph in which a shear of the cross section of an emissive beam is plotted as the Bragg angle, .theta., and the angle, 2.alpha., of the letter V-shaped groove are varied at 0.1 mm in the deviation between the first and second crystals. As seen from FIG. 13, as .theta. and 2.alpha. approach 90 degrees, a shear of the cross section of the emissive beam becomes smaller. In other words, in order to obtain a pencil of emissive X-rays which is substantially equal to a pencil of incident X-rays at high accuracy, the deviation between the first and second crystals can also be compensated by bringing an angle of the letter V-shaped groove closer to 90 degrees. According to a monochromator for radiant X-rays of the present invention, a heat flux per unit area of incident X-rays can be reduced on the surface of a crystal, and cooling characteristics equivalent to those of a commonly used water cooled or liquid gallium cooled monochromator can be obtained. Also, the structure is such that thermal stresses due to incident X-rays cancel each other out, whereby a thermal deformation can be suppressed. Furthermore, two crystal surfaces of incidence are provided which are in the shape of concave letter V and convex letter V combined, and thus a longitudinal size can be halved as compared with conventional inclined crystal monochromators, thereby attaining a compact structure. Also, this structure is less likely to be subjected to mechanical deformation. Even when any deformation takes place, the deformation can be directed so as not to affect optical performance. In addition, cooling means extends from the bottom portion of a crystal and runs behind the surface of incidence, whereby cooling efficiency can be increased. Thus, the monochromator for radiant X-rays of the present invention is compact, suppresses a fluctuation in the position of a pencil of incident X-rays, allows easy adjustment, provides a pencil of emissive X-rays stably and highly accurately, is easy to use and economical, and allows easy maintenance. As has been stated above, having good cooling and excellent distortion characteristics, being compact and easy to use, providing a stable pencil of emissive X-rays at high accuracy, and allowing easy installation, adjustment, and maintenance, the monochromator for radiant X-rays of the present invention fully exhibits capabilities thereof when used in third-generation radiant beam facilities. From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skilled of the art are intended to be covered by the appended claims.