GB2379732A

GB2379732A - Diamond examination

Info

Publication number: GB2379732A
Application number: GB0122053A
Authority: GB
Inventors: Philip Maurice Martineau; Simon Craig Lawson; Paul Martyn Spear
Original assignee: Gersan Ets
Current assignee: Gersan Ets
Priority date: 2001-09-12
Filing date: 2001-09-12
Publication date: 2003-03-19
Also published as: GB0122053D0

Abstract

In order to determine whether a diamond 1 is a natural/synthetic doublet, it is irradiated with uv light of 325 nm wavelength, and luminescence from 330 to 450 nm is detected using a confocal microscope 3 and a spectrometer 8. The focal plane 9 is scanned vertically though the diamond 1. An abrupt change in luminescence with increasing depth indicates that the diamond is a natural/synthetic doublet. The invention can used on diamonds down to ten points, and on diamonds set in jewellery.

Description

Diamond Examination Background of the Invention The present invention relates to a method of examining a diamond, primarily for detecting whether the diamond is a natural/synthetic doublet.

Natural/synthetic doublets can be made by depositing synthetic diamond on a natural diamond, normally in its polished or part-polished state, to form part of the crown or pavilion of the doublet.

There are techniques for detecting whether the diamond is a doublet-see for instance WO 94/20837, WO 95/20152, WO 96/07895, WO 96/07896, WO 97/04302 and WO 97/04303.

It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior techniques, or to provide a useful alternative.

It is generally desirable to be able to examine automatically, and to provide a technique which can be used for loose diamonds or diamonds set in jewellery.

The Invention In its broadest aspect, the present invention provides a method as set forth in Claim 1 or 15 and apparatus as set forth in Claim 14 or 16.

In general terms, any change in the material of which the diamond is composed may be detected. However, the method is primarily used for detecting whether the diamond is a natural/synthetic doublet. If the diamond is a doublet, there is a change in

the luminescence when the detection reaches the depth where the change between natural and synthetic, or vice versa, occurs.

In the method of the invention, any characteristic of luminescence can be compared, but preferably the intensity of a spectral feature of the luminescence is compared.

The whole procedure can be automated. The invention can be used for doublet detection in diamonds down to about ten points (0.1 carats) in weight, and possibly less.

If stimulating radiation capable of penetrating the whole depth of the diamond is focused within the depth of the diamond, the luminescence from different depths can be detected, e. g. by substantially preventing detection of luminescence which is not substantially in the focal plane. A suitable technique is a confocal technique, using a confocal spectrometer. A confocal aperture placed at the back-focal plane of a microscope ensures that only luminescence from the focal point of the objective reaches the spectrometer detector. Luminescence from other parts of the sample fails to pass through the confocal aperture and so is not detected. The area of the selected region depends upon the diameter of the confocal aperture and the magnification of the microscope objective. The luminescence is collected from a volume effectively comprised of the selected area, determined by the confocal aperture diameter and objective magnification, and the depth of focus of the objective, determined by its numerical aperture. A processor 12 is indicated.

Claims 2 to 13 set forth preferred or optional features of the invention.

Preferably the stimulating irradiation is radiation of about 300 to about 400, for instance about 325, nm wavelength and luminescence from about 330 to about 450 nm is detected. However a change in the rate of decrease of the Raman signal with depth, due to differential absorption of the stimulating irradiation, could alternatively be used to indicate a change in material of which the diamond is composed.

Although the method is normally carried out at room temperature, a lower temperature may be used by employing a cryostat such as the Microstat N from Oxford Instruments.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic vertical cross-section through apparatus in accordance with the invention, showing a diamond being examined in accordance with the method of the invention; Figure 2 is a photoluminescence/Raman spectrum of a typical type la natural diamond; Figure 3a is the depth profile of N3 luminescence intensity for a first doublet, the distance being the distance moved by the doublet; Figure 3b corresponds to Figure 3a, but the depth is the distance moved by the focal plane within the diamond; Figure 4a is the depth profile of normalised N3 luminescence intensity for a second doublet, the distance moved being the distance moved by the diamond ; and Figure 4b corresponds to Figure 4a, but the depth moved is the distance moved by the focal plane within the diamond.

Figure 1 Figure 1 shows a polished diamond I on a mount 2 below a confocal microscope 3. Though not illustrated, the mount 2 is carried on a table which can be moved up and down by a stepping motor. The microscope 3 has an objective lens 4 and a confocal aperture 5. Above the microscope 3, there is a beam splitter 6, a laser 7 for irradiating

the diamond 1, and a spectrometer 8. All the parts are illustrated extremely schematically.

The confocal aperture 5 prevents light out of the focal plane from entering the spectrometer 8. The instantaneous focal plane is indicated at 9 and the arrangement is such that the focal plane 9 can be scanned right through the diamond from the topmost point (here the table 10) to the bottommost point (here the culet 11). Scanning is most conveniently done by moving the mount 2 vertically, in predetermined intervals, say of 10 ! lm or 100 um. The laser beam, however, is refracted as it enters the diamond and therefore the distance travelled by the focal point of the laser (within the stone) at a wavelength of e. g. 325 nm is approximately 2.51 times greater than the distance travelled by the stone itself (2.51 is the refractive index of diamond at 325 nm).

Example In one suitable apparatus, the laser 7 is a He-Cd laser having a 10-100 mW output at 325 nm. The laser 7 can be supplied together with the confocal microscope 3 and the spectrometer 8 as a LabRam Infinity confocal spectrometer system, manufactured by J Y Horiba. Luminescence from about 330 to about 450 nm is detected. In diamond, this system enables depths of 0 to 500 u. m to be probed using a xlOO objective and a 50 ! lm confocal aperture 5. Depths of 0 to 10 mm may be probed using a x20 objective and a 200 u. m confbcal aperture 5.

A processor 12 with suitable software can indicate automatically whether the diamond is a doublet. The software normalises the integrated intensity of the N3 zero phonon line relative to the integrated intensity of the diamond Raman line. This normalisation procedure allows results to be corrected for changes in collection efficiency or size of stone. If the Raman signal falls to less than 10 per cent of its initial value, it can be assumed that the focal point of the probe is no longer within the diamond. By choosing the appropriate grating, CCD detector and central wavelength position of the spectrometer grating (in the spectrometer 8), both the N3 and Raman signals may be captured within the same spectrum. Software, such as that provided with

the LabRam Infinity confocal spectrometer, was configured to provide a real-time display of the depth profile.

Figure 2 Figure 2 is a typical photoluminescence/Raman spectrum for type la natural diamond, collected confocally at room temperature with 325 nm He-Cd laser excitation.

It contains the N3 zero phonon line at 415 nm with associated vibronic structure at longer wavelengths. More than 95% of all natural diamonds have the N3 zero phonon line; those that do not are selected out beforehand. The spectrum also contains the Raman line at approximately 339 nm.

A similar spectrum for CVD synthetic diamond would not contain the N3 zero phonon line at 415 nm or its associated vibronic structure.

Figures 3a and 3b Figures 3a and 3b show the measured confocal depth profiles of normalised N3 luminescence for a first doublet, which was produced for experimental purposes only.

The first doublet was a round brilliant, partly composed of natural type la diamond and partly of CVD synthetic diamond. It has a CVD synthetic diamond crown and the interface between this component and the natural diamond component is 0.86 mm below the table, the total depth of the stone being 3.19 mm.

The centre of the surface of the table of the doublet 1 was first positioned at the focal point of the laser beam and spectra were recorded at 100 um intervals as the doublet 1 was moved upwards towards the objective lens 4 that focused the laser. This process was equivalent to collecting spectra as the focal point of the laser was scanned into the doublet 1 via the table.

As explained above, the distance travelled by the focal point of the laser within the stone is approximately 2.51 times greater than the distance travelled by the stone

itself. In Figure 3a, the x-axis is the distance travelled by the stone from the position in which the table is at the focal point of the laser. In Figure 3b, the x-axis is this distance multiplied by 2. 51. This corresponds approximately to the depth of the focal point of the laser beam below the table.

The change in the graph of Figures 3a and 3b is not abrupt because of the relatively poor resolution at the depths that are being probed, and the intervals between measurements. However the precise depth of the interface is not usually of concern, only whether or not there is an interface.

Figures 4a and 4b Figures 4a and 4b correspond closely to Figures 3a and 3b, but show the spectra for a second doublet, which was also produced for experimental purposes only. The second doublet was a round brilliant, partly composed of natural type la diamond and partly of CVD diamond. It has a natural type la diamond crown and the interface between this component and the CVD synthetic diamond component is 0.75 mm below the table, the total depth of the stone being 1.64 mm.

The second doublet was positioned as for the first doublet of Figures 3a and 3b. x x x Unless the context clearly requires otherwise, throughout the description and the claims, the words'comprise', 'comprising', and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense ; that is to say, in the sense of "including, but not limited to".

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. The invention also consists in any individual features described or implicit herein or shown or implicit in

the drawings or any combination of any such features or any generalisation of any such features or combination.

Claims

CLAIMS:

1. A method of examining a diamond, comprising irradiating the diamond to stimulate the emission of luminescence, detecting the luminescence at different depths within the diamond, and comparing the luminescence so detected so as to detect whether there is a change of the material of which the diamond is composed.

2. The method of Claim 1, wherein the intensity of a spectral feature of the luminescence is detected and compared.

3. The method of Claim 1 or 2, wherein the stimulating irradiation is capable of penetrating the whole depth of the diamond but is focused within the depth of the diamond, and the luminescence is sensed by collecting luminescence from said different depths.

4. The method of Claim 3, carried out using a technique which substantially prevents detection of luminescence which is not in the focal plane at said depth.

5. The method of Claim 3, and carried out using a confocal technique.

6. The method of Claim 3, and carried out using a confocal spectrometer.

7. The method of any of the preceding Claims, wherein the luminescence detected is normalised by ratioing it with a luminescence emission characteristic of all diamonds.

8. The method of Claim 7, wherein said characteristic luminescence emission is Raman.

9. The method of any of the preceding Claims, and used to detect whether the diamond is a natural/synthetic doublet.

10. The method of Claim 9, wherein the depth at which luminescence is detected moves automatically by fixed increments, and an automatic indication is given as to whether the diamond is a natural/synthetic doublet.

11. The method of any of the preceding Claims, wherein the stimulating irradiation is irradiation of about 300 to about 400 nm wavelength.

12. The method of Claim 11, wherein the stimulating irradiation is irradiation of about 325 nm wavelength.

13. The method of Claim 11 or 12, wherein luminescence from about 330 to about 450 nm is detected.

14. Apparatus for carrying out the method of any of the preceding Claims, and including software for detecting whether there is a change of the material of which the diamond is composed.

15. A method of examining a diamond, substantially as herein described with reference to the accompanying drawings.

16. Apparatus for examining a diamond, substantially as herein described with reference to Figure I of the accompanying drawings.