US3032656A - X-ray refracting optical element - Google Patents

Description

May 1, 1962 R. HOSEMANN ETAL X--RAY REFRACTING OPTICAL ELEMENT Filed Aug. 15, 1957 II x N 10 W a,

II I I R2 4 22 *!Eii 2 um um L/s 'F" RIEIQS Inventors ROLF HOSEMANN ANTON WEBER Q I By /W i {W Attorneys United States Patent @fiice 3,032,656 X-RAY REFRACTING OPTICAL ELEMENT Rolf Hosemann and Anton Weber, Berlin, Germany, assignors to Licentia Patent-VerwaltungsG.m.b.I-I., Hamburg, Germany Filed Aug. 15, 1957, Ser. No. 673,762 1 Claim. (Cl. 25051.5)

refracting elements at all angles including angles larger than the so-called critical .angle' which special type of refraction is conventionally called reflection.

It is an object of our invention to provide an X-ray refracting element of satisfactory size adapted for use as an X-ray monochromator or X-ray analyzer which is capable of producing X-ray images of higher light intensity and freer from distortion than the known crystals.

It is a further object of the invention to provide an X-ray refracting element having the aforesaid properties which is particularly easy to manufacture on an industrial scale.

It is yet another object of the invention to provide X'ray refracting elements whose optical properties such as the intensity of the reflected or refracted X-rays, their freedom from distortion, a sharp selectivity with regard to the refracted frequency band, and the like, can be determined selectively, at will, Within certain limits in order to adapt them to a given purpose.

It is well known to use suitable crystals as monochromators or analyzers in X-ray diifractometry and X-ray spectroscopy by interposing these crystals in the path of the X-ray beam.

If these crystals are to be used as monochromators in X-ray dilfractometry, they are interposed in the path of the X-ray tube and the'preparation so as to filter the beam and obtain a monochromatic X-ray light'capable of producing a clear diffraction image or pattern. If these crystals are to be used as analyzers in X-ray spectroscopy, they are interposed between the preparation and the X-ray detector and are rotated or displaced in a suitable manner together with the latter so as to disperse spectrally the characteristic radiation emitted by the preparation under examination.

It is desirable to obtain X-ray images, i.e. diffraction patterns and X-ray spectra which are of high light intensity and free from distortions. This can only be achieved if the aforesaid crystal element has the shape of a plate or disk of largest possible size and consists of a single well-grown crystal. In practice, quartz plates have been used for this purpose which were cut from specimens of outstanding quality selected from among quartz crystals found in nature. Such crystals are relatively rare. Therefore, crystals of various materials such as sodium chloride, calcium fluorite, lithium fluorite, sugar, mica or pentaerythrite, have been grown artificially. However, neither the crystals found in nature nor those artificially grown do usually possess, except in very rare cases, the necessary high quality of their internal texture, if they are of suflicient size; or, if they satisfy the necessary high demands of quality, they can only be found in small-sized specimens.

Furthermore, it was hitherto believed in the art that the aforesaid crystals were particularly well suited for use as X-ray refracting elements, it they possess, particularly in the layers adjacent their refracting surface, a mosaic or tesselated structure. Such structure is brought about in all of the aforesaid crystals by the work process applied in giving the crystal a desired shape. The changes effected especially in those lattice planes of the crystals which are adjacent the refracting surface to give the same a mosaic structure, are also furthered by the heating eifects occurring during the working steps of shaping the crystal. These Working steps comprise conventionally, for instance, the application of a concave reflecting surface on one side of the crystal, and often the application of a convex side on the crystal opposite the concave one.

In contrast to the hitherto held belief that crystals having at least in their surface a mosaic structure are best sutied for use as X-ray monochromators and analyzers, we have discovered that crystals are much better suited for use as such X-ray refracting elements, and have considerably improved optical properties which comprise a higher intensity of the refracted or reflected X-ray light, a greater freedom from distortion, and a sharper selectivity of the refracted or reflected frequency bands, as these crystals approach, at least in the layers adjacent the retracting surface, the state of an ideal crystal.

An ideal crystal is a theoretical crystal which is free from any distortions in its crystal lattice. v

On the basis of our discovery, the aforesaid objects can be obtained by using as an X-ray retracting element such as a monochromator or analyzer for X-rays having wave lengths from about 4 to 0.01 Angstroms, an artificially grown monocrystal of an elementary semiconductormaterial selected from the group consisting of germanium and silicon and being shaped or cut as a plate of satisfactory size, which is, at the same time, capable of producing X-ray diffraction patterns or spectra of satisfactory intensity and free from distortion.

We have found that these elementary semi-conductor monocrystals show outstanding mechanical and optical properties which make them superior to all hitherto known crystals. Thus, quartz and all other above mentioned crystals are inclined to adopt a mosaic structure. during the mechanical treatment giving the crystal its desired shape. Monocrystals of the semi-conductor elements of germanium and silicon possess a much better thermal conductivity and can therefore be worked to give them the desired shape in accordance with the present invention, while preserving their almost ideal structure, and, as a consequence thereof, their superior optical properties.

On the other hand, it is possible, by a selective mechanical treatment, to bring about a determined degree of recrystallization from an almost ideal structure of at least the lattice portion adjacent the retracting surface of the monocrystal to a partial or complete mosaic structure. By choosing, through a suitable mechanical treatment of the crystal in giving the same its desired shape, a determined stage intermediate the two extremes of complete ideal structure and complete mosaic structure, it is possible, in accordance with our invention, to provide monochromator or analyzer crystals of determined optical behavior.

Both germanium and silicon monocrystals possess reflecting lattice planes (111) which reflect X-rays including Gamma rays with excellent intensity.

If particularly hard X-rays are to be reflected, i.e. X-rays having a wave length from 0.01 to about 1 Angstrom, higher indexed lattice planes of germanium or silicon r'nonocrystals may also be used. In the case of germanium, for instance, the relative intensities of X-ray beams reflected by the higher indexed planes compared with the intensity of an X-ray beam reflected by the (111) Fatented May 1, 1962 aesaeee a) plane set at 1.0, are 0.9 for the (220) plane, 0.8 for the (131) plane, and 0.7 for the (242) plane.

The absolute intensities or powers of the reflected X-ray beams depend on a considerable number of factors, and their determination is well known in the art. (See, for instance, Advances in Biological and Medical Physics by John H. Lawrence and C. A. Tobias, vol. III, page 256 and-ff., published by Academic Press, Inc, 1953, New York, N.Y.)

Satisfactory intensity of the reflected X-radiation and freedom from distortion of the resulting Xray images depend to a large degree on the external shape of the crystals used as X-ray refracting elements according to the invention. Crystal plates of the proper shape are obtained by careful cutting, grinding, milling,v corroding oif, and polishing the artificially grown germanium or silicon crystal.

According to an important feature of our invention, an X-ray refracting element of particularly satisfactory properties is obtained by providing a combined monocrystal plate and convex-concave top frame system in which the refractory surface of the monocrystal is first cut with a determined curvature to be of concave shape and is assembled under pressure to the convex side of. the aforesaid frame which convex side is of a narrower curvature than the concave crystal surface, so that the concave monocrystal surface is under permanent stress.

The lattice planes in the monocrystal can be disposed parallel to the refracting or reflecting surface of the monocrystal, or'they can be inclined at an acute angle thereto. This angle should not exceed the Bragg reflection angle which is half the diffraction reflection angle. For germanium, this angle is 12 50 at the (111) lattice plane, and 22 50'- at the (220) lattice plane.

Whenever the lattice planes which reflect X-rays are inclined at an angle to the refracting surface of the monocrystal plate, the normal on the group of lattice planes must at least be perpendicular to the shorter of the edges delimiting the refracting surface of the plate.

The shorter edge or edges of that plate surface must be. of a length which is above the half value band of the depth of penetration of the X-radiation in the crystal. Otherwise too large a portion of X-ray passes through the crystal without being diffracted. The half value band is in the order of 0.1 micron for the ideal crystal and in the order of 1 micron for a crystal having complete mosaic structure in its near-surface layers.

According to another feature of the invention, that edge of a crystal having a determined different height, length, and width, which corresponds to a length intermediate that of the shortest and the longest edge, is at right angle to the normal on the lattice planes of the monocrystal lattice. If the crystal is so dimensioned, it permits to utilize fully the space angle of the radiation intensity emitted by the X-rayv tube, in particular, if line focus tubes are used.

Higher indexed lattice'planes of germaniumand silicon monocrystals can be used for X-rays having wave lengths from 0.01 Angstrom to h =2d, wherein d is the distance between two higher indexed lattice planes. The use of these higher indexed lattice planes leads to a much higher dispersion, i.e. a small change in the wave length causes a much larger change in the scattering angle than with the (111) plane.

Our invention will be still better understood from the description thereof following hereinafter in connection with the accompanying drawings.

FIGURE 1 is a perspective view-ofa monocrystal plate according to the invention;

FIGURE Z-illustrates in perspective another embodiment of a monocrystal plate according to the invention;

FIGURE 3 shows in perspective yeta further X-ray refracting element according to the invention, one of the surfaces of which is' aportion of the wall of the cylinder;

FIGURE 4 is a further embodiment of a monocrystfl X-ray refracting element in perspective view, two opposed surfaces of which are portions of the Walls of cylinders having different generating radii;

FIGURE 5 illustrates in perspective an X-ray refracting system comprising a monocrystal of the kind illustrated in FIGURE 4 filled onto a curved frame;

FIGURE 6 is a lateral view of the parts of the system shown in FIGURE 5 prior to its assembly; and,

FIGURE 7 is a lateral view in cross section of the assembled system.

Referring now to the drawings in detail: the X-ray refracting plate 1 illustrated in FIGURE 1 has the height 4, the length 5,;and the width 6, all three being of different length. The plate is a rectangular parallelepiped, its edges 4, 5, and 6, being straight lined and at right angle to each other. The lattice planes 2 are indicated by dashed lines, and the normal 3 on these lattice planes of the monocrystal is also normal to the largest end surface 11 of plate 1.

FIGURE 2 shows a monocrystal X-ray refracting element identical with that shown in FIGURE 1 and like references numeral indicating like parts. However, the lattice planes 2% are inclined relative to the largest end surface 11 of the monocrystal plate 1, forming an acute anglea with that surface 11. Consequently, the normal 3 on the group of lattice planes 20 is inclined at that same angle to the normal 7 on plane 11, for instance, at the edge 6 which is of a length intermediate the lengths of edge 4 being the shortest, and of edge 5 being the longest of the three edges, for reasons set forth above. The angle 01-15 smaller than the Bragg reflection angle.

In FIGURE 3 the X-ray refracting element 8 has two opposed largest end surfaces 12 and 13, of which surface 12 is concavely shapedto form-a portion of the wall of a cylinder. Surface 13 opposite surface 12 is planar. Again, the monocrystal plate has-four shortest edges 4, four edges 6 of the medium length, and two straight bottomedges 14 delimiting surface 13, and two curved top edges 15 delimiting cylindrical surfaces. Again, as in FIGURE 2, the lattice planes 20 are inclined at an angle relative. to the end surfaces 12 and 13 in sucha manner that the normal 3 on the group of lattice planes 20 forms an angle 5 with the normal 7 on plane 13, for instance, at the edge 6 of medium length.

The axis formingthe'central axis of the cylinder having the generating radius R is preferably parallel to that medium long edge 6. The curved surface 12 can be ground into themonocrystal.

The embodiment of the X-ray refracting element of our invention shown in FIGURE 4, which element is designated by reference numeral 9, is similar to that shown in FIGURE 3 inasmuch as it possesses shortest cdges4, medium long edges 6, and curved edges 15 delimiting one curved end surface 12 which forms a portion of the wall of a cylinder having the central axis A and a generating radius R while the surface 16 opposite concave surface 12 is convex and forms a portion of the wall of a cylinder having the central axis A and the generating radius R Cylinder axes A and A are located in the same plane being the central plane of the X-ray refracting element 9-and parallel to the end surfaces delimited by edges 4 and 6. Radius R is longer than radius R and axes A, and A are both parallel to medium long edges 6 of element 9.

The X-ray refracting'system illustrating in FIGURE 5 is obtained from the parts thereof shown in FIGURE 6. One of these parts is a monocrystal element 9 as illustrated in FIGURE 4. The other part is a frame 19 having a central window 17 having parallel marginal portions 18 opposite each other the length of which corresponds to the length of edges 6 of element 9. Frame portions 19 interconnecting portions 18, are curved in a plane extending in the wall of a cylinder having the generating radius R;,- which is smaller than radius R pertaining to surface 12 of element 9, while cylinder axis A is parallel to cylinder axes A and A Frame 10 is placed with the convex side of curved frame portions 19 on the surface 12 of monocrystal element 9 and is pressed thereagainst, for instance, 'by clamping means 21 in such a manner that monocrystal element 9 is bent under stress so that surface 12 adopts the same curvature as frame portions 19, corresponding to radius R Radius R is preferably equal to radius R, pertaining to convex surface 16 of element 9. In the assembled system, the surfaces 12 and 16 of element 9 thus extend approximately parallel to each other at least in the central portion of element 9 facing the window 17 of frame 10'.

By adjusting the tension of clamping means 21, for instance, with the aid of screw means 22, it is possible to vary the curvature of the concave surface 12 of monocrystal 9 somewhat, and thereby to vary the angle of the lattice planes relative to an incident X-ray beam.

It will be understood that this invention is susceptible to modification in order to adapt it to different usages and conditions, and, accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claim.

We claim:

An X-ray refracting element for X-rays having wave length from 0.01 to about 4 Angstroms consisting of an artificially grown monocrystal of an elementary semi- References Cited in the file of this patent UNITED STATES PATENTS 2,474,240 Friedman June 28, 1949 2,666,147 Borst Jan. 12, 1954 2,688,093 Dumond Aug. 31, 1954 2,688,094 Dumond Aug. 31, 1954 2,816,234 Ellis Dec. 10, 1957 2,853,617 Berreman Sept. 23, 1958 FOREIGN PATENTS 727,077 Great Britain Mar. 30, 1955 OTHER REFERENCES Verma: Crystal Growth and Dislocation, 1953, pages 2-3.

Cullity: Elements of X-Ray Diflraction, 1956, page 168, lines 42 to 44, i

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