US 3310675 A
Description (OCR text may contain errors)
March 2l, 1967 R, PRlCKETT ET AL 3,310,675
ADJUSTABLE OFFSET COLLIMATOR Foa USE 0N x-RAY POWDER CAMERAS Filed July 7, 1964 2 Sheets-Sheet 1 llllllll ....Illl A ""IIII March 2l, 1967 R. l... PRICKETT ET AL ADJUSTABLE OFFSET COLLIMATOR FOR USE ON X-RAY POWDER CAMERAS Filed July 7, 1964 PRIOR ART capa L mty" 2 Sheets-Sheet 2 ATTORNEYS 3,310,675 ADEUSTAELE UFFSET CLLEMATGR FR USE N X-RAY PWDER CAMERAS Robert L. iriekett, 2855 nrnhangh Road, Dayton, @his 45432, and fhndahaklrsh S. Mazdiyasni, Dayton, @fno (2218 Upper-Eeiihroolr Road, Xenia, @frio 453%) Filed .fnly 7, 1964, Ser. No. '33@,959 3 Claims. (Cl. 250m-MES) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.
This invention relates to crystallography, and more specifically to rcamera apparatus used in this field of science. Camera apparatus is used to record, on photographic film, a series of line-like patterns produced by sample materials placed in an X-ray beam. The film so produced is used to identify the sample material and in the study of Various characteristics thereof.
Electromagnetic radiation impinging upon a surface is normally reflected, the angle of incidence being equal to the angle of reiiection. When electromagnetic waves become very short, i.e. X-rays, reflection does not occur at the surface because the rays penetrate into the solid, being reflected in part from successive layers of atoms. This reflection, which in X-ray work is usually referred to as ditfraction, is governed by Braggs law which states: nk=2d sin 0; where n=order of the diffraction, Xzthe wave length of the X-ray used, d=the interplanar distance of the atomic planes causing the diffraction, and =thc angle of incidence and diffraction of the Xrays with respect to the planes.
A single crystal placed in an X-ray beam will produce a pattern of spots, while moving the crystal slightly Will produce line movement of the spots. If a mass of finely powdered crystals with theoretically random orientation is placed in an X-ray beam, a series of line-like patterns will be produced. These line-like patterns, which represent the summation of all the theoretical spot positions derivable from a single crystal, are commonly called a powder pattern.
This invention is concerned with improved apparatus and methods for deriving powder patterns. A powder pattern is most satisfactory when it Vis derived from a small sample in a finely collimated X-ray beam. The material to be sampled is usually ground and passed through a screen of 200 to 400 mesh. Even this finely divided material will produce a nonuniform line powder pattern, i.e. a pattern containing spottiness. The elimination of spottiness is best accomplished by rotating the small sampie in the X-ray beam, the rotation changing the orientati-on of the individual small crystallites with respect to the beam and thus causing the spots to move `and become blended into lines.
Since electromagnetic radiation is a wave-like phenomenon, and since X-rays are reiiected from successive layers of atoms, the presence or absence of a detectable reflection is dependent upon whether the phases of the reflected waves are added or subtracted over the successive layers of atoms, i.e. whether the didraction pattern has constructive or destructive interference. The symmetry .of a solid directly controls whether destructive or constructive interference will result. A very symmetrical material, for example: in crystallographic terminology,
i a member of the cubic system such as table salt, will have more destructive interference than constructive interfer- United States Patent C) der diffraction pattern; each line being characteristic of a particular type of pla-ne, and with the number of types of planes being theoretically infinite.
A material of relatively simple atomic composition might be expected to have a relatively high symmetry because of its simplicity of atomic packing. In contrast, an extremely complex molecule cannot be packed very efficiently and will have an extremely low type symmetry structure. For this reason sodium chloride, for example, has a very simple diffraction pattern while tobacco mosaic virum would have one of the most complex diffraction patterns. Y
When studying low symmetry triclinic materials by powder pattern diffraction techniques,'it is mandatory for best crystal structure studies, to see each of the lines individually. in many powder patterns of triclinic materials, the large number of lines Vresults in frequent overlap. Separation of some of the lines can be accomplished only with great difficulty.
The type of information which may be derived mathematically by studying powder diffraction patterns extendsv from the simple identification of materials (each material having its own characteristic diffraction pattern because of its unique atomic packing) to total crystal structure determination (precise atomic positions and bond lengths and directions for each atom in the crystal).
There are not many materials found in nature which are pure materials having a tricli-nic structure. Most materials have a more symmetrical packing of their atoms and thereby have a higher degree of symmetry such as monoclinic, orthorhombic, hexagonal, tetragonal or cubic.
Most crystallography studies are on the relatively high symmetry crystals, and apparatus for their study is well known in the art. However, these standard commercial powder diffraction cameras and diffractometers will not completely separate the lines in triclinic specimens. To further complicate the situation, some triclinic specimens are far worse than other specimens. A crude and imperfect picture, which is possibly suitable for powder identification, can frequently be derived with commercial equipment, particularly if the specimen can be placed in a small enough area of the X-ray beam. Under Braggs law, a specific specimen reflects for a particular interplanar spacing at a specific angle 6 for a given radiation A. The linear arc length equivalent to this angle 0 as recorded on the film or diffractometer is directly dependent on the sample to film or detector distance. If this sample to film distance is shifted slightly, the position of the angular intercept of the diffraction pattern lines on the circular film will be moved. The ideal situation would call for an infinitely narrow rod sample to reduce the spread of the recorded line. Normal procedure is to use a practical sized capillary .3 to .7 mm. in diameter, and a convenient X-ray source collimator approximately 1 mm. in diameter.
It becomes apparent, from the above discussion, that utilizing small capillaries and small collimators will assist in resolving triclinic powder patterns. When an unstable material is involved, grinding and sieving a sample fine enough that it may be placed into a .3 mm. capillary is not very practical or satisfactory. A .7 mm. capillary is about the smallest which can be used with some unstable materiais, and this is frequently very unsatisfactory for triclinic materials in commercial equipment, since many groups of lines on the film will be unresolved.
As was previously mentioned, spottiness is diminished by rotating the sample in the X-ray beam. In commercial equipment the X-ray beam is wider than the capillary holding the sample and the rotating capillary `is bathed within the beam. The invention to be described below reduces the spread of the X-ray diffraction lines on the powder pattern by controllably moving the Xray beam to one side of the centrally located rotating specimen. This, as will be shown, causes the edge of the X-ray beam to intercept a chord through the capillary which is less than the ,great diameter, and results in photographs having marked superiority because of an improved diffraction pattern.
One object of this invention is to provide means for improving the resolution of diffraction patterns produced by low symmetry materials.
Another object of this invention is to improve the separation of diffraction lines on photographic lm.
A further object of this invention is to provide a variable collimator in which the transmitted X-ray beam may be manipulated to strike a Variable portion of a sample holding capillary.
Yet another object of this invention is to 4permit-the use of larger capillaries to produce results which on commercial machines could be obtainable only with much smaller impractical capillaries.
A still further object of this invention is to provide a powder diffraction system in which the graphic iindings .are a true measurement of the radiated sample.
Another object of this invention is to provide a new lmethod for producing improved powder diffraction film patterns having superior line separation, especially but `not limited to low symmetry materials.
Additional objects, advantages and features of the invention reside in the construction, arrangement and `combination of parts involved in the embodiment of the .invention as will appear from the following description .and accompanying drawings, wherein FIG. 1 is a plan view of a typical powder camera with the light tight cover removed and showing a strip of film against the circular wall of the camera,
FIG. 2 is a horizontal section view of the inner end lof the collimator, and showing the improvement constituting this invention,
FIG. 3 is an end view of the structure shown in :section vby FIG. 2,
FG. 4 is a horizontal sectional view similar to FiG. 12 and showing the prior art,
FIG. 5 is a schematic illustrating the principle of :the new offset collimator,
FIG. 6 is a schematic view of a portion of the film istrip in the powder camera of FlG. 1 and showing the prior art method of photography, and
FIG. 7 is a schematic view similar to FIG. 6 and :showing the photography method employing the improved offset collimator of this invention.
FIG. 1 `shows a plan view of the circular `film box of :a typical powder camera such as is manufactured by i iemens and Halske Aktiengesellschaft of Karlsruhe, Germany, The cover fitting over the circular opening has been omitted in order to shown the interior elements. 'The circular iilm box i2 is provided, at its geometric center, with a rotatable holding means 14 for supporting .a small capillary such as previously mentioned. The capillary is so small by comparison that it is illustrated .by the dot at the center and will be referred to as capillary 16. The capillary, which is loaded with the crystals to be X-rayed, extends into the circular film box and is .rotated on the geometric axis of the box by a drive means on the lower side of the box. The rate of rotation is on the order of one revolution per minute.
Inwardly extending into the circular film box, yby passing through the vertical circular wall thereof, is a collimator comprising a collimator barrel 1S having an elongated tube, and terminating within the film box in a collimator tip a. The collimator barrel It and the collimator tip 20a are each provided with bores on their longitudinal axis. The new collimator tip 20a has the same external appearance as the prior art collimator tip 20 when joined to the collimator barrel. In the prior art shown on FIG. 4, the axis of bore 22 in the collimatorA barrel 1S', and the axis of Ibore 24 in the prior art collimator tip Ztl are coaxial. In the new collimator tip 20a, which constitutes this invention, bore 26 is made to be eccentric with 'bore 22 as best shown on FIG. 2. The film box is so constructed that the yaxis of bore 22 in the collimator barrel I8 passes through the geometric center. The exact configuration and operation of the collimator tip 2da wili be hereinafter described. Bore 22 extends through the collimator barrel and is used as the transmission path through which X-rays are introduced from external X-ray generating means. As shown on FIG. l, the path of the X-rays is through capillary 16 and then into a suitable collector device 2S where they are absorbed and dissipate-d as heat.
A film strip 3@ is placed against the interior vertical wall of the film box as shown; after which the light tight cover is positioned and held in place Iby swing bolts 32. Such swing bolts are well known in all arts. The film strip is provided with suitable holes for the passage of the collimator barrel 18 and the collector device 28.
The prior art collimator tip 20, as shown on FIG. 4, is provided with a concentric sleeve 34 which engages the bore .22 in the collimator lbarrel 1S. In the collimator tip 2da, which constitutes the invention and is best shown on FlG. 2, the bore is made smaller and the concentric sleeve 34 is replaced `by the eccentric sleeve 36; thus providing a convenient means for adjusting the lateral displacement of the bore in the tip in relation to the bore in the barrel.
The relationship between the tip and bore elements is schematically illustrated on FIG. 5. The collimator barrel IS, contains a small bore 22a in the outer end wall; bore 22a constituting the minor diameter bore limiting the cross-sectional area of the entering X-ray beam, and being concentric with the major diameter bore 22. Bore 25 in the new collimator tip is preferably made smaller than bore 22a in the collimator barrel. As the collimator tip 2da is rotated, the eccentric sleeve 36 on the tin permits the lateral displacement of bore 26 in relation to bore 22a. It is thus seen that, since the axis of bore 22a is on the geometric center of film box i2, the bore 26 in tip 24M may be adjusted to be laterally displaced from this geometric center. In actual practice this lateral displacement is on the order of .003 to .G05 inch.
The reasons for the improved diffraction patterns produced by the collimator constituting this invention are shown on FIG. 6 and FIG..7. Referring more specifically to FIG. 6, which illustrates the prior method, the capillary is shown greatly enlarged (about 8() times) in order to single out'individual crystals contained therein, and to spread the diffraction lines. The X-ray beam is somewhat broader than the diameter of the capiilary, as illustrated. P1 to P5 represent individual crystallites within the sample mass retained within the capillary. The diffraction lines .D11 D3 and D5 emanating from the excited crystals impinge onto and expose the film strip within the film box. The width of the diffraction band produced by D1, for example, represents the cross section through one of the circles on FIG. 8, D3 represents another circle, etc. Gther bands appear in the areas indicated as D2 and D4; these however have been omitted for clarity and in order to avoid confusion in the construction lines. There are, in fact, so many bands in actual practice that they overlap and interfere with each other; this interference being the primary reason for the poor quality of the film pattern. Each crystal in the capillary is not diffracting at any given instant of time. The particular crystals which are diffracting are those having a diffracting plane in proper position at any instant of time. Since the capillary is rotating, it is obvious that all crystals at some time will be in a diffracting position. Ideally, optimum results in accordance with Braggs law would be achieved by using a layer of crystals which have about 300 atoms in thickness. It is obvious that because of the minute thickness of such an atomic layer, this ideal condition cannot be achieved because of mechanical diiculties.
As is shown on FIG. 7, the capillary of this invention is a large step in the direction of the ideal condition. The X-ray beam is made much smaller and is laterally shifted so that the inner edge of the beam slices a small segment of the crystals in the capillary. A comparison of `the geometry between FIG.y 6 and FIG. 7 clearly shows the reasons for the reduction in band widths; which in turn increases the spread between bands. This in turn reduces the interference between bands; thus producing sharper 'bands with clearer lines of demarcation. FIG. 7 clearly shows that the band width is reduced because the diflracting crystals along the X-ray beam must be closer together, and because most crystals within the capillary are outside the X-ray beam, thus reducing the depthf It is obvious that -other methods than the eccentric shown may be used to provide a means for laterally shifting the X-ray beam. For example the collimator tip 20a may be joined to the capillary barrel 18 by means of a screw adjustable device for laterally shifting the bore. Another method would be -to provide the collimator support on the iilm box with a lateral adjust;` ment means. The actual position of the X-ray beam in relation to the capillary is observed on the screen at the outer end 25a of the collector device 28, as shown on FIG. l.
It is to be understood that the embodiment of the present invention as shown and described is to be regarded as illustrative only, and that theinvention is susceptible to variations, modications and changes within the scope of the appended claims.
1. An offset collimator for use on X-ray powder cameras and comprising: a collimator barrel having an elongated tube containing a major diameter bore open at one end and closed at the other end, the closed end having a bore of minor diameter therethrough and coaxial with the major diameter bore; and a collimator tip extending from and movably joined to the open end of said collimator barrel and containing an axial bore parallel to and of a diameter smaller than the bore of minor ldiameter in said collimator barrel and laterally adjustable in relation to the bore of minor diameter in said collimator-barrel to be in a displacement range of completely within to partially without the bore of minor diameter in said collimator barrel.
2. An offset collimator for use on X-ray power cameras and comprising: a collimator barrel having an elongated tube containing a major diameter bore open at one and closed at the other end, the closed end having a bore of minor diameter therethrough and coaxial with the major diameter bore; and an elongated collimator tip axially extending from the open end of said collimator barrel and having an axial bore therethrough of a diameter smaller` than the boire of minor diameter in said collimator barrel and further having an axial sleeve means for engaging the open end of said collimator barrel, the, sleeve means being eccentric with the axial lbore for rotatably adjusting the lateral displacement between the axial bore and the bore of minor diameter in said collimator Ibarrel to be in a displacement range of completely within to partially without the bore of minor diameter in said collimator barrel.
3. An oiset collimator for use on X-ray powder cameras and comprising: a collimator barrel having an elongated tube containing a major diameter bore open at one end and closed at the other end,the closed end having a bore of minor diameter therethrough and coaxial with the major diameter bore; and an elongated collimator tip axially extending from the open end of said collimator barrel and having an axial bore therethrough of a diameter less than the bore of minor diameter in said collimator barrel and further having an axial4 sleeve means for internally engaging the open end of the major diameter bore of said collimator barrel, the sleeve means being eccentric with the axial bore for rotatably adjusting the lateral displacement between the axial bore and the bore of minor diameter in said collimator barrel to be in a displacement range of completely Within to partially without the bore of minor diameter in said collimator barrel.
References Cited by the Examiner UNITED STATES PATENTS 2,533,701 12/1950 Watt et al. a 250-105 X 2,584,962 2/1952 Gross Z50- 51.5
RALPH G. NILSON, Primary Examiner.
W. F. LINDQUIST, Assistant Examiner.