DE19631367C1 - X-ray identification method for cut diamonds - Google Patents

Abstract

The invention concerns a method used for radiographically identifying diamonds, in particular for determining whether a completely cut diamond is identical to or different from a known completely cut diamond. To that end, the position of the table facet of the diamond relative to the diamond lattice is determined by means of two crystallographic polar angles and then at least one radiographical transmission recording of the internal defects of the diamond is realized by monochromatic X-radiation at the Bragg angle. The identity of the diamond can be checked by comparing the polar angles determined and the X-ray topograms with polar angles and X-ray topograms of known decorative diamonds obtained by the same procedure.

Description

The invention relates to a method for X-ray identification of cut diamonds, especially to determine whether a fully ground Diamond identical to or different from a known one is completely cut diamond.

Diamonds cut for jewelry purposes (brilliants, Navette, Drops, triangles, trapezes, baguettes, emerald cuts, etc.) usually represent a high monetary value. Their value determines weight, color, purity and cut quality. This four value features of a cut diamond are identified by a professional, according to internationally established rules diamond grading to be carried out. With ground Jewelry diamonds of higher value, their weight usually is above 0.5 carat, the graduation result will be in Form of a certificate in writing and traceable that is attached to the graduated diamond.

Everyone receives from the information contained in the certificate cut jewelry diamond an individuality that in the case loss through carelessness, theft, etc. Recovery usually allows its recognition. This recognition is made by the slightest changes on Cut and thus also questioned the weight of the stone,  made impossible if there are no other typical features, such as the nature of a characteristic inclusion are.

There has been no lack of efforts in the past, to develop a characterization process that cut jewelry diamonds makes it unmistakable. Nature remains unaffected by external manipulations on the stone his characterized by individual construction errors Crystal lattice. These internal construction defects can be caused by diffraction of X-rays on the crystal lattice of the diamond X-ray sensitive films or with others Recording techniques are made visible and recorded. Such a mapping of crystal defects (X-ray topogram) is the fingerprint of the examined jewelry diamonds and as such distinctive, since there - like the fingerprint in humans - practically none there are two diamonds that match x-ray topograms deliver. An X-ray topogram would be preserved on microfilm attached to the certificate and a copy of it in an archive to deposit. If the stone is lost or is it stolen, so can be used again after recovery recorded X-ray topogram whose identity beyond any doubt determine.

The unique and therefore unmistakable identification of raw, partially or completely cut jewelry diamonds using x-ray topography (fingerprinting) has been around for many Years (see e.g. A. R. Lang et al., Fingerprinting Diamonds by X-ray Topography, Industrial Diamond Review, March 1976, pp. 96-103).

In the practice of trading in high quality ground Jewelry diamonds have so far not prevailed because the Procedure used for the prior art Creation of x-ray topograms too cumbersome and too is complicated. Because of the high crystallographic symmetry of the diamond lattice currently need for basic characterization  of a diamond many x-ray topograms (mother topograms) be recorded with a single control topogram (Daughter topogram) the proof of identity succeeds beyond doubt.

In U.S. Patent No. 4,125,770 "Diamond Identification" from November 14, 1978 and German patent DE 25 59 245 C2 "Diamond Identification Process" of May 26 1988, the state of the art of X-ray topography to identify cut diamonds in their "Descent" of rough diamonds is used when used of monochromatic X-rays in the "simplest Routine procedure "(simplest routine) of a number of twelve projection topograms to be recorded, with the X-ray diffraction at network planes of type {400} is made because cube areas have the least multiplicity exhibit. Using the symmetry after adjustment Such a network level, four X-ray topograms in succession produced by diffraction on {400} network planes perpendicular thereto, the crystal between the shots by 90 ° around the Network plane normal is rotated. The exact direction of projection remains open in the two patents.

The direction of projection is given by the path of the X-ray crystal lattice diffracted in diamond. she will determined by the network plane array reflecting the beam (hkl) in connection with the azimuthal orientation of the Diamond crystal around the associated network plane normal [hkl]. This means that the position of the irradiated crystal in the "Dispersion level" (= level of incident and diffracted X-ray beam) only by specifying the network plane set used (hkl) and one in the (hkl) level Crystal direction [uvw], which z. B. perpendicular to Dispersion level is defined.

However, the method only works reliably, i. H.  Only then will the mother and daughter diagram be practically identical be, if this projection direction is also fixed, e.g. B. by a <110 <or <100 <direction perpendicular to Dispersion level is made. Definitely will unique identification with only one daughter topogram a total of at least 12 mother topograms are mandatory, because on 3 network levels of type {400} are in 4 projects each direction to produce topograms.

Our own experiments have shown that even this number for clear identification by 1: 1 comparison of mother and Daughter topogram is not sufficient. Although remain after the Friedel's law in a centrosymmetric crystal lattice the diffraction conditions for direction and opposite direction are the same, but the projection conditions change because of the volume of the diamond differ in direction and opposite direction is irradiated. By different geometrical Superimposition of the construction error contrasts as a result of Absorption with larger crystals can therefore be such Distinguish x-ray topograms from each other. So you would have to Receive a complete fingerprint of the stone, four each Images around each of the three cube axes in the "positive" direction and four shots around each of the three cube axes in Make "negative" direction. This means the recording of a total of 24 x-ray topograms. Only then is ensures that a single daughter admission is enough to complete agreement with one of the 24 mother admissions bring about.

The entire procedure requires qualified personnel, one high work and time expenditure and prohibitively high Costs. This is the reason why fingerprinting is different from valuable, cut jewelry diamonds, although the highest desirable to protect property, as commercial So far, not enforcing sub-technology of diamond grading could.

The object of the present invention is to provide a method create with the comparatively simple and fast it is possible to identify cut diamonds.

To solve the problem it is proposed that
using a goniometer with five circles (2Θ, Ω, X, Σ, Δ) with monochromatic X-rays in an X-ray diffraction adjustment routine, the position of the table facet of the diamond relative to the diamond grating is determined by two crystallographic pole angles (ρ, ϕ),
an orientation of the diamond is determined from this,
subsequently, with this orientation, at least one X-ray topographical transmission image of the internal defects of the diamond is made by means of monochromatic X-ray radiation at Bragg's angle,
and the identity is checked by comparing the determined pole angles and X-ray topograms with pole angles and X-ray topograms of known cut diamonds obtained according to the same procedure.

This is a much easier procedure for the X-ray gene diffractometric / X-ray topographic characterization of cut jewelry diamonds. It will here X-ray diffractometrically two angles ("pole angle") on DUT determined with high accuracy and then in the Usually a single mother topogram is created. Together with the In the two determined angles, this mother topogram becomes the Diamond grading certificate on microfilm or other Disks enclosed to be used with a single after identical Procedural regulation included daughter topogram identity or To prove non-identity. More than a mother topogram will compulsory only in very few, precisely specifiable cases may be necessary, although the production of a maximum of four Mother topograms e.g. B. for very defect-free diamonds Security is advised.  

A goniometer with five circles is used as an adjustment device (2Θ, Ω, X, Σ, Δ), an X-ray source and a sample holder as well as a Detector used. With the help of a preferably motorized Goniometers that involve some or all circles a special control program a fully automatic Orientation of the examinee relative to the recipient X-ray makes the angle measurement on the circles Σ and Δ carried out exactly and the preparation of the topogram / of the topograms for shadow-free defect mapping in Transmission made up to an asymmetry angle of 40 ° will.

The procedure can be carried out routinely by technical personnel be performed. Extensive automation as well as significant reduced time and personnel costs of the new process open, in contrast to the prior art, the economic application of the X-ray topographical identifier (Fingerprinting) in the practice of high quality certification cut diamond. It is also suitable due to specific growth characteristics for the identification of synthetic Diamonds, which in future will be increasingly available in interesting sizes, Colors and purities will appear in the diamond trade. Embodiments of the invention are in the subclaims listed.

Below is the invention with its essentials Details described in more detail.

It shows:

Fig. 1 is a stereographic projection of the crystal directions <100 <<110 <and <111 <a diamond with einzeichneten areas "A" and "B" in the standard spherical triangle,

Fig. 2 is a goniometer with position of the goniometer axes (2Θ, Ω, X, Σ, Δ) and orientation of the diamond crystal at the beginning of the search for the reference direction,

Figure 3 shows the position of the normal to the table facet, where ρ. By the polar angle and φ in the standard triangle of the stereographic projection,

Fig. 4 is a stereographic projection of the position symmetrical equivalent reflections of the type hk0 on the zone circle of the reference direction <100 <greater for the case that the accurate pole angle ρ _{mes, 100} ρ as a resulting of the measuring accuracy of the goniometer limit angle value _lim, wherein the reference direction is in the center of the projection and the pole of the table facet is deflected with respect to the reference direction with the azimuth ϕ₁₀₀ by the pole distance ρ₁₀₀; the angular distances ε _i between the pole of the table facet and symmetrically equivalent (hk0) poles are clearly distinguishable;

Fig. 5 is a stereographic projection of the position symmetrical equivalent reflections hk0 for the case where → _{mes, 100} <ρ _lim wherein the reference direction in the center of the projection is of the type on the zone circle of the reference direction <110 <and the pole of the table facet opposite the Reference direction with the azimuth ρ₁₁₀ is deflected by the pole distance ρ₁₁₀; the angular distances ε₁ between the pole of the table facet and symmetrically equivalent (hk0) poles are clearly distinguishable;

Fig. 6 is a stereographic projection of the position symmetrical equivalent reflections of the type hk0 on the zone circle of the reference direction <100 <for the case where ρ _{mes, 100} ρ _lim, wherein the reference direction is exactly in the center of the projection and the pole of the table facet practically is indistinguishable from the same place and

Fig. 7 is a stereographic projection of the position symmetrical equivalent reflections of the type hk0 on the zone circle of the reference direction <110 <for the case where ρ _{mes, 110} ρ _lim, wherein the reference direction is exactly in the center of the projection and the pole of the table facet practically is indistinguishable from the same place.

As the practice of diamond grinding shows, the rough stone is usually supplied as a diamond octahedron, which is sawed into a large and a small rough diamond parallel to the cube surface {100}. The sawing surface becomes the later table facet for both rough stone parts. Even if the rough stone is a rhombic dodecahedron, it is usually sawn along the surface of the cube. So it happens that in about 80% of all cut jewelry diamonds, the cube axis and the normal to the table facet are approximately parallel and at most include an angle ρ₁₀₀ <10 ° with each other. In the case of irregularly shaped rough stones or those whose cut is different due to the material economy, larger angles can result between the cube axis and the table normal. In principle, the plate normal always falls in a solid angle that is limited by the directions <100 <, <111 <and <110 <. This is shown in Fig. 1 with the aid of stereographic projection ("Wulff's network"). Because of the high symmetry of the diamond, all of the following considerations can basically be traced back to this solid angle range (spherical "standard triangle").

In the vast majority of cases, the table normal is in the Cut out near a <100 <direction (cube axis). In the  Projection topography is the rounded level of the ground Diamonds distorted as little as possible on the X-ray topogram map to the projected cross section and thus the Maximize the information content of the topogram. The one <100 <direction, which is the smallest angle with the Forms table norms, therefore, becomes crystallographic Reference direction for determining the pole angle and the clear adjustment of the crystal before recording Topograms.

In such cases, if the angle between the <100 <direction and the plate normal is greater than 27 °, the plate normal no longer falls into the region "A" in FIG. 1 with the consequence that the plane of the girdle on a transmission X-ray topogram is very strong would be distorted. In this case, the one with the smallest angle with respect to the normal to the crystallographic reference direction is explained among the neighboring <110 <directions. For the subsequent topographical image (s) only those reflections are used that lie on the zone circle to the respectively selected reference direction.

The process can be divided into six steps:

• 1. The angle between the plate normal and the nearest <100 <or <110 <direction is determined by diffractometry in order to determine the crystallographic reference direction and the further procedure. For this purpose, the diamond 4 is mounted in the center of a special 5-circuit goniometer 1 (with the axes 2Θ, Ω, X, Σ, and Δ) in the zero position ( FIG. 2). In this position, the table facet of the diamond is parallel to the rotating circles X and Δ, the Ω axis and the Σ axis are parallel to each other and lie in the plane of the X and Δ rotating circles.
  Here, as in the following, the axes of the goniometer 1 are identified with large Greek letters (2Θ, Ω, X, Σ, and Δ), while the pole distance ρ and azimuth ϕ of the table facet as well as the rotary movements around the goniometer axes (2Θ, ω, χ , δ and σ) are denoted by small Greek letters.
  The reference direction is found using a search procedure with systematic rotary movements of some or all of the goniometer circles, in that a suitable reflex (e.g. 400 or 220) is sought and precisely in symmetrical theta / 2theta (Θ / 2Θ) retroreflective geometry with monochromatic x-ray radiation from an x-ray source 2 is adjusted.
  The search procedure as an adjustment and measurement routine can be carried out manually or more fully automatically using computer-controlled special software. The signal from the X-ray detector 3 is recorded, evaluated by the computer and used to optimize the angular positions.
  In the end position there is either <100 <or <110 <as the crystallographic reference direction in parallel to the x-axis of the goniometer. Now the angle σ can be read on the goniometer. It corresponds to the deflection of the normals on the table facet from the reference direction and thus indicates the pole distance Q₁₀₀ (or ρ₁₁₀) of the table facet ( Fig. 3).
• 2. Now the azimuth ϕ of the table facet normals is determined. Starting from a basic position (χ = 0 °), the goniometer and detector arm are set to symmetrical Θ / 2Θ transmission geometry for reflections of type hk0, the poles of which lie on the zone circle to the reference direction (e.g. 220). Knowledge of the 220 reflex position on the zone circle in relation to the reference direction now permits the exact positioning of the plate normals in the standard triangle of the stereographic projection. The position of the plate normals can be given in the case that <100 <was used as the reference direction, by the pole angles ρ₁₀₀ and ϕ₁₁₀ or if the reference direction <110 <was used, by ρ₁₁₀ and ϕ₁₁₀. Taken together, the two angle specifications ρ and definieren uniquely define the position of the table normals of the test specimen in relation to the crystallographic reference direction ( FIG. 3).
• 3. The two pole angles can be set reproducibly at any time within the goniometer tolerances. They are now used to determine the orientation of the jewelry diamond for the X-ray topographic image and the number of X-ray topograms required. The goniometer tolerances, which are partly due to the production technology, essentially determine the accuracy with which the goniometer angles can be set on the X-ray diffractometer and thus indirectly the accuracy of the pole angle reading. The limit angle (mainly due to production technology) is currently realistic for the pole distance ρ _lim = 0.2 °, the limit angle for the azimuth ϕ _lim is also dependent on the deflection ρ. For ρ ≈ 0.3 ° ϕ _{lim is} currently about 3 °, for ρ = 10 ° about 0.2 °; below ρ _lim , ϕ _lim no longer makes sense. ϕ _lim is determined by the control software as a function of ρ _lim . The number of minimally increasing mother topograms is determined by the deviation of the measured pole angles ρ _mes and ϕ _mes from their critical angles. If the symmetrically equivalent reflections on the zone circle of the reference direction can be distinguished by the angle ε that the relevant network planes enclose with the table facet (FIGS . 4 and 5), the rule (probability <95%) is that due to the low equipment costs accurate polar angle determination only a single x-ray topogram may be required. As a criterion for the choice of the reflex used for this one X-ray topographical image, the angle ε between its network plane normal and the table facet normal is practically used. The automatic procedure can, for. B. be set up in such a way that among the symmetrically equivalent 220 network planes on the zone circle of the reference direction, that network plane is selected for the image whose angle with the table facet deviates the least from 90 ° (compare angle ε 1 in FIGS . 4 and 5) .
  If the angles ε _i , which include symmetrically equivalent reflections on the zone circle of the reference direction with the table facet, are indistinguishable, so in rare cases the table normal coincides with the reference direction within the goniometer accuracy ( Fig. 6, 7), the inclusion of more than one mother topogram is required, namely a maximum of four for the reference direction <100 <and a maximum of two for the reference direction <110 <. Now, the method proposed here has already saved two thirds of the adjustment, measurement and evaluation time compared to the prior art and additionally provided the information that can be used for the certification that the angle ρ _mes between the table facet normal and the reference direction is smaller than ρ _lim .
• 4. Now the necessary X-ray topograms are created. An X-ray topogram can be recorded conventionally with Using the tried-and-tested long topography, d. H. under Using point focus and oscillating Translational movement of the irradiated sample and recording medium (X-ray film, nuclear trace emulsion,  Multi-channel plate, CCD camera, etc.) relative to incident x-ray. Alternatively, use steady Improvement of the spatial resolution of CCD (charged-coupled device) - image recording systems a modified Recording procedures due to much less complicated Handling (including the elimination of wet chemical Process steps), higher speed and easier Archiving option may be preferable. With this one the X-ray beam incident on the sample z. B. by Bragg reflection on meanwhile commercially available X-ray mirrors (so-called Göbel mirror or Gutman Optics) in expanded and parallelized in two dimensions and shines through the sample in full width and height at the same time, d. H. translation of sample and recording medium relative to the X-ray beam can be omitted. The CCD downstream electronic image recording system able to locate any present inhomogeneous intensity distribution in the sample incident X-ray beam by difference or on other suitable way to compensate. The following However, explanations refer to that in the current one State of the art ultra-high resolution processes: recording medium = nuclear trace emulsion. For creation of the x-ray topograms, the preselected reflex, e.g. B. from Type 220, started by computer control and via the detector signal can be optimized with regard to the angular positions. Then the adjustment panels are removed, and it anti-scatter baffles are set up. After that, at the x-ray beam placed the film cassette and the (preset) oscillating translational movement switched on. The width of the oscillation corresponds to that greatest width of the diamond parallel to the Direction of translation. The X-ray is turned on shines through the crystal and exposes the film in the light e.g. B. a 220 reflex for a defined time  (Timer).

If according to the criteria given above X-ray topograms of additional 220 reflexes required the film is after the first exposure temporarily taken away. Another 220 reflex will adjusted, and the width and position of the Lens hoods, if necessary, adjusted. The subsequent exposure of the film takes place after the same Procedure like the previous one. Conveniently, the still undeveloped film of the first shot again used by ensuring that the new Recording does not overlap with the previous one. For this purpose the film cassette can be placed behind an aperture be shifted by a few millimeters. after the required exposures on the x-ray film under approximately the same exposure conditions performed the film piece as a whole is developed at once and fixed. For simplicity, especially at the later identification (pattern recognition), should that Process preferably only with one reflex type (i.e. only 220 or only 400) can be operated.

The determined angle information (reference direction, pole angle, Critical angle) and X-ray topograms are included in the certificate the diamond testing center as microfilm or in some other way enclosed and a copy kept in the archive. Ideally the creation of a central archive is recommended.

If a lost polished diamond is certified with pole angles and mother topogram (s), the pole angles are measured and a daughter topogram (control recording) is taken using the same adjustment procedure. In the case of identity, the daughter topogram matches the mother topogram or one of the mother topograms. The pole angles now have two functions: on the one hand, they are parameters of the test specimen, which become worthless only by changing the angle of inclination of the table relative to the crystal lattice, and on the other hand, they serve as search parameters for quickly finding the (mother) topograms, which can be found about these two Have numerically understandable sizes conveniently located in the archive. For this purpose, a subset ρ _fil and ϕ _{fil are} screened out as search parameters of the archive, for which the following applies:

ρ _mes -ρ _lim ρ ρ _fil ρ _mes + ρ _lim
ϕ _mes - ϕ _lim ϕ _fil ϕ _mes + ϕ _lim .

The search can also be done automatically using suitable software respectively.

With a growing number of mother topograms, one can Pattern recognition using electronic image processing within the subset with similar pole angles ρ and ϕ become meaningful. The control recording is made with the Main topogram of the (central) archive Details, d. H. the contrasts due to internal Crystal construction errors, agree. A complete one Agreement of all picture details, including the contrasts due to damage to the polish (e.g. damaged image Faceted edges) will naturally only be given if in the meantime no mechanical changes on the Gemstone were made.

Was the orientation of the table facet relative to the Crystal lattice of the diamond changed (only possible through Grinding the board) is an identification of the Crystals nevertheless by the method described  (Pattern comparison) extended to neighboring pole angles will. Stronger regrinding (e.g. after parts of the Jewelry diamonds) makes identification difficult, that up to 24 x-ray topograms were subsequently made here should be. The effort involved is likely can only be accepted in exceptional cases.

Naturally, a single X-ray topogram can only guarantee reliable identification if the jewelry diamond is "within the scope" of its defect content. X-ray topograms of very defect-free crystals offer u. Apart from growth streaks with low contrast, there may be only a few usable features. In contrast, in the case of defective crystals, contrasts that can be used per se can be rendered useless by superimposed projection. In these cases, additional X-ray topograms with a different projection direction provide greater security for recognition. X-ray topograms of this type can be generated fully automatically using the method presented above. For this purpose, after the first topogram has been recorded, a further reflex on the associated zone circle (cf. FIGS . 6 and 7) must be used for the image by rotating about the already adjusted reference direction.

Claims (9)

1. Method for the X-ray identification of cut diamonds, in particular for determining whether a fully cut diamond is identical to or different from a known fully cut diamond, in which
using a goniometer with five circles (2Θ, Ω, X, Σ, Δ) with monochromatic X-rays in an X-ray diffraction adjustment routine, the position of the table facet of the diamond relative to the diamond grating is determined by two crystallographic pole angles (ρ, ϕ),
an orientation of the diamond is determined from this,
subsequently, with this orientation, at least one X-ray topographical transmission image of the internal defects of the diamond is made by means of monochromatic X-ray radiation at Bragg's angle,
and the identity is checked by comparing the determined pole angles and X-ray topograms with pole angles and X-ray topograms of known cut diamonds obtained according to the same procedure. 2. The method according to claim 1, characterized in that several, especially up to four X-ray topographical ones Transmission recording of the diamond's internal defects be made. 3. The method according to claim 1, characterized in that by means of the adjustment routine which is normal to the Table facet of the diamond closest <100 <- or <110 <direction as the crystallographic reference direction is determined, and that the associated pole angles (ρ, ϕ) can be measured with high accuracy.   4. The method according to any one of claims 1 to 3, characterized characterized that the adjustment routine automatically computer controlled with software created for it is carried out. 5. The method according to any one of claims 1 to 3, characterized characterized that knowing the location of the normals on the table facet in relation to the crystallographi cal basis, given by the measured pole angles (ρ, ϕ), with a fixed reflection position and Projection direction preferably only one X-ray topogram (mother topogram) is recorded. 6. The method according to any one of claims 1 to 3, characterized characterized that knowing a measured Polwinkel (ρ) between the normal to the table facet and the determined crystallographic Reference direction the number of possible Reflection positions and projection directions for the X-ray topograms recorded on a maximum of four is limited. 7. The method according to any one of claims 1 to 6, characterized characterized that for an existing or new to creating diamond certificate the recorded (Mother) x-ray topograms on microfilm or on other media. 8. The method according to any one of claims 1 to 7, characterized characterized in that to identify a recovered a diffractometric diamond Measurement of the pole angle (ρ, ϕ) and then a single one X-ray topographical recording (Daughter topogram) is made. 9. The method according to any one of claims 1 to 8, characterized characterized in that for quickly finding Mother topograms to verify identity with the  Daughter topogram (control recordings) the two Polwinkel (ρ, ϕ) prepared in a computer-readable form will.