Patent Number: 056152450
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spectroscope which is installed in an X-ray beam line using an undulator as a light source in high luminance radiant beam facilities for synchrotron, and particularly to a monochromator for radiant X-rays which is used as part of apparatus for X-ray analysis of structure and material evaluation. 2. Prior Art According to a conventional monochromator, as shown in FIG. 14, one surface of a plate-like monochromator or a plate crystal serves as a reflecting surface, and another surface serves as a cooling surface. The reflecting surface is heated, and the cooling surface is cooled by a cooling material or a coolant such as water or liquid metal. In a certain type of monochromator, the cooling surface is finned or processed likewise so as to increase a cooling efficiency. However, in such plate type monochromators, heat applied to the reflecting surface causes stress deformation, and thus the surface of a crystal thermally deforms with a resultant problem of the scattering of emissive X-rays. FIG. 1 shows typical thermal deformations of a monochromator crystal. As shown in FIG. 1(a), when the entire surface of the plate crystal is irradiated with incident X-rays and thus heated uniformly and also when the other surface is cooled, only a grating constant expands uniformly, while planes of atoms remain uninclined. Thus, a uniform expansion (lattice expansion) occurs in which an expansion reduces as it goes downward. In this case, a warp (bowing) (FIG. 1(c)) occurs with the heated surface being convexed. If a local zone is irradiated, only the irradiated portion swells like a knot or a bump (FIG. 1(b)). These thermal deformations of a plate crystal cause a pencil of emissive X-rays to diverge as represented with E' and E" when the surface of a crystal is irradiated with a parallel pencil of incident X-rays. That is, X-rays are reflected in nonparallel, causing a marked deterioration in spectroscopic performance (energy resolution and intensity) of X-rays. Particularly, in designing a beam line in high-luminance radiant beam facilities most of which are occupied by an insertion light source, the aforesaid thermal deformation of a crystal is a most serious problem. Various devices have been adopted, but most of them are intended to physically improve a cooling capability. A monochromator with practical specifications has not been obtained yet. Too much emphasis is rather placed on an improvement of cooling capability, leading to a problem of instability at a lower accuracy, difficulty in use, higher costs, complicated maintenance and the like. FIG. 2 shows an inclined crystal monochromator which has been developed for use with APS, U.S.A. (Advanced Photon Source, U.S.A.; energy 7.0 GeV; characteristic photon energy 19.0 KeV; 34 beam lines +.alpha.; circumferential length 1104 m). An angle between incident X-rays and lattice planes can never be changed, but the surface of a crystal can have an arbitrary angle. As shown in FIG. 2, (111) lattice planes (crystallographic planes related to spectroscopy) of a single crystal of silicon are laid horizontally, and the single crystal is cut at an angle of near 90 degrees from crystallographic planes to obtain an inclined surface. When X-rays impinge on the inclined surface, their shadow is cast long thereon, and thus a heat flux per unit area becomes smaller, whereby generated heat can be reduced. This means that the shadow should be cast long so as to reduce generated heat, denoting a need for a large crystal. FIG. 3 shows measurements of a ratio of an irradiated area on the surface of a crystal to the orthogonal cross-sectional area of a beam as an angle of inclination of a crystal from incident X-rays is varied. As seen from FIG. 3, as the angle of inclination increases, the ratio increases. That is, as the angle of inclination of a crystal from a pencil of incident X-rays increases, an area of shadow of incident X-rays expands on the surface of a crystal, whereby a heat flux per unit area reduces. Also, in this geometry, a direction of thermal deformation mostly falls on crystallographic planes of a crystal, thereby producing an advantage that lattice planes related to diffraction are free from large distortion. However, in spite of these advantages, the inclined crystal monochromator is said to have the following disadvantages: (1) a large crystal needs to be used so as to reduce generated heat; (2) adjustment is difficult to make (an adjustment error is greatly amplified); and (3) a fluctuation in the position of an incident beam causes instability of the position of an emissive beam. Thus, problems with respect to practical use remain to be solved. SUMMARY OF THE INVENTION An object of the present invention is to provide a monochromator for radiant X-rays which minimizes a thermal deformation of a crystal, thereby overcoming the aforesaid difficulties involved in conventional apparatus; it also solves the problems with respect to practical use. Specifically, an object of the present invention is to provide a monochromator for radiant X-rays which uses a small crystal and minimizes thermal deformation of the crystal by increasing the cooling capability, allows easy adjustment, suppresses a fluctuation in the position of a pencil of incident X-rays, provides a stable pencil of emissive X-rays, i.e., useful light at high accuracy, is easy to use and economical, and allows easy maintenance. In accordance with the present invention, X-rays which are emitted from an X-ray source and enter a first crystal are called incident X-rays (or incident beam), X-rays which reflect from the first crystal and enter a second crystal are called reflected X-rays (or reflected beam), and X-rays (monochromatic X-rays) which exit the second crystal are called emissive X-rays (or emissive beam). A monochromator for radiant X-rays of the present invention is characterized by comprising a first crystal which has a surface of incidence having a shape of a concave letter V-shaped groove and cooling means for flowing a cooling material behind the surface of incidence along the letter V-shaped groove, and a second crystal which has a letter V-shaped convex having a convex letter V-shape to fit the concave letter V-shaped groove. The bottom portion of the concave letter V-shaped groove of the first crystal, i.e., a most acute-angled portion is largest in terms of the volume of a pencil of incident X-rays. As a result, the center of heat generation (i.e., the portion which receives the most intense heat, or an intensely heated zone) is formed at the bottom portion. In the first crystal, inclined surfaces of the letter V-shaped groove surround the periphery of the center of heat generation, and thus a lattice distortion due to a thermal deformation can be suppressed by the peripheral volume of the crystal. A pencil of radiant X-rays which has impinged on the concave letter V-shaped groove of the first crystal reflects therefrom, and thus reflected X-rays impinge on the letter V-shaped convex of the second crystal. The reflected X-rays are rearranged on the second crystal to the same size as that of a pencil of incident X-rays. Thus rearranged X-rays exit the second crystal as a pencil of emissive X-rays. Accordingly, to accurately obtain a pencil of emissive X-rays, it is preferable that the first and second crystals be set within a slight range of error. Specifically, it is most preferable that the bottom portion of the concave letter V-shaped groove of the first crystal and the letter V-shaped convex of the second crystal align with each other along a centerline. However, a deviation of 0.01 mm or less is acceptable between them. The aforesaid cooling means is provided behind the surface of incidence along the letter V-shaped groove so as to promote thermal diffusion within the first crystal. Preferably, the cooling means is provided behind the surface of incidence along the letter V-shaped groove with a starting point thereof located just under the center of heat generation. More preferably, a transport pipe is directed toward the intensely heated zone from the bottom portion of the fist crystal, branches off in opposite directions with a starting point thereof located just under the center of heat generation, and allows the cooling material to flow behind the surface of incidence along the letter V-shaped groove. Stainless steel or Teflon (registered trademark of Dupont) is preferably used as material for the transport pipe in view of heat resistance and pressure resistance and so as to use as the cooling material water and/or liquid metal such as liquid gallium or the like having high cooling efficiency. The operations of the above-mentioned structure will be described below. (1) As X-rays impinge on an inclined surface of a letter V-shaped groove, the shade of X-rays can elongate on a surface of incidence, whereby a heat flux can be made smaller. (2) Unlike conventional inclined crystal monochromators having one inclined surface, two inclined surfaces which form a letter V-shape are provided, whereby a size in a longitudinal direction can be reduced (half the size of conventional inclined crystal monochromators). (3) A heat flux is intensest at the bottom portion of a letter V-shaped groove to form the center of heat generation. However, heat generated at the center of heat generation diffuses radially along the peripheral inclined surfaces of the letter V-shaped groove and toward the inside of a crystal underneath the groove, whereby the diffusion of heat is promoted within the crystal. (4) Since a crystal surrounds the center of heat generation, generated thermal stresses cancel each other, whereby a distortion of the crystal due to heat (thermal deformation) can be suppressed. (5) Cooling means is provided behind a surface of incidence along the letter V-shaped groove, whereby heat whose generation is caused by incident X-rays can be cooled efficiently. (6) When cooling means is in the form of a transport pipe formed of stainless steel or Teflon, as cooling material water and/or liquid metal such as liquid gallium or the like can be used intact. (7) By adopting the letter V-shaped groove which allows generated stresses to cancel each other out, a water jet which otherwise is likely to cause a local deformation of a crystal can be used as cooling means.