GB2379733A

GB2379733A - Examining a diamond

Info

Publication number: GB2379733A
Application number: GB0122055A
Authority: GB
Inventors: Simon Craig Lawson; Paul Martyn Spear; Philip Maurice Martineau
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: GB0122055D0; ZA200402030B

Abstract

In order to determine whether a blue-to-green diamond 1 has been artificially irradiated or ion bombarded to change its colour, it is irradiated with light of 633 nm wavelength in order to stimulate the emission of luminescence, and luminescence from about 680 to about 800 nm is detected using a confocal microscope 3 and a spectrometer 8 as the focal plane 9 is scanned vertically through the diamond 1. A rapid decrease in luminescence with increase in depth indicates natural irradiation whilst a more rapid decrease indicates ion bombardment.

Description

Examining a Diamond Background of the Invention The present invention relates to a method of examining a diamond, primarily for detecting whether the diamond has been artificially irradiated or ion bombarded to change its colour.

Natural green diamonds owe their colour to irradiation by naturally occurring radio isotopes which produce alpha-particles, when the radio isotopes are adjacent the diamond in the ground. The alpha-particles penetrate only to a depth of about 30 urn below the surface of the diamond and create radiation damage to the diamond lattice, principally in the form of lattice vacancies. The vacancies give rise to a characteristic vibronic absorption system in the red end of the visible spectrum, giving rise to a blueto-green coloration.

However, artificial irradiation or ion bombardment can be used to produce a blue-to-green colour in diamonds. This treatment is usually applied to polished diamonds but the treatment can be applied to rough diamonds. Artificial irradiation is usually carried out using high-energy electrons, which have a penetration depth in diamond of a few millimetres, or using fast neutrons, which have a penetration depth in diamond of a few centimetres. High energy ions used for ion bombardment typically have a penetration depth of about I am in diamond, considerably less than that of natural alpha-particle irradiation. Hitherto, in order to be certain whether a rough or polished blue-to-green diamond had been naturally or artificially irradiated, it was necessary to destructively cross-section the diamond and observe the depth of penetration of the colour below the surface.

Because naturally irradiated diamond gemstones can command a higher price than diamonds that owe their colour to artificial irradiation or ion bombardment, for the sake of consumer confidence another method of testing is required,

It is an object of the present invention to overcome or ameliorate at least one of i 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 has been artificially irradiated to change its colour.

If the diamond has been artificially irradiated with high-energy electrons or fast neutrons to change its colour, the decrease in the luminescence detected with depth is less rapid than the decrease with depth in the case of a diamond which has been naturally irradiated. This is discussed in more detail hereafter in relation to Figures 2a, 2b and 3 of the accompanying drawings.

If high-energy ion bombardment is used, the decrease of luminescence detected with depth is more rapid than in the case of a diamond which has been naturally irradiated.

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

Although the invention is directed primarily at rough diamonds, the method of the invention can also be used to identify artificially irradiated or ion bombarded polished diamonds. When a naturally irradiated stone is polished, the shape of the stone is changed and the depth of irradiated material is no longer uniform. In the case of a polished diamond that has been artificially irradiated or ion bombarded subsequent to being polished, the change in intensity of luminescence with depth will be uniform with respect to the polished surface, clearly indicating that the irradiation is artificial.

The whole procedure can be automated. The technique can be used to detect artificial irradiation or ion bombardment in diamonds much less than about 10 points (0.1 carats) in weight, although they are preferably at least 1 mm deep.

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.

Stimulating radiation of any wavelength capable of causing luminescence from the OR1 system can be used. Preferably, the stimulating radiation is radiation of about 500 to about 740, for instance about 633, nm wavelength, and luminescence including wavelengths from about 740 to about 745 nm is detected. The vibronic absorption system which gives rise to the blue-to-green coloration in diamond, is known as Gar1. If

excited at room temperature with light in the wavelength range 500 to 740 nm, it produces luminescence with a strong line at 741 nm.

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.

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

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure I is a schematic vertical cross-section through apparatus in accordance with the invention, showing a polished diamond being examined in accordance with the method of the invention; Figure 2a shows GR1 luminescence spectra at the surface and at depth increments of 10 um below the surface of a rough naturally alpha-irradiated diamond; Figure 2b shows the normalised integrated intensity of GR1 luminescence as in Figure 2a, as a function of the depth below the surface for naturally alpha-irradiated diamond; and Figure 3 shows the normalised integrated intensity of GRI luminescence as a

function of depth below the surface for an artificially electron-irradiated diamond ; Figure I Figure 1 shows a polished diamond 1 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 from outside the focal region 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 11m. The laser beam 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. 633 nm is approximately 2.41 times greater than the distance travelled by the stone itself (2.41 is the refractive index of diamond at 633 nm). A processor 12 is indicated.

Example In one suitable apparatus, the laser 7 is a He-Ne laser having a 10-20 mW output at 633 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 680 to about 800 nm is detected. In diamond this system enables depths of 0 to 500 11m to be probed using a xlOO objective and a 50 urn confocal aperture 5. Depths of 0 to 10 mm may be probed using a x20 objective and a 200 urn confocal aperture 5.

Figures 2a. 2b and 3 Figure 2a shows a photoluminescence/Raman spectrum recorded using the confocal spectrometer with the xlOO objective and 50 urn confbcal aperture. The lines in Figure 2a are referenced with the depth below the surface, the line 0 being as recorded at the surface. The diamond Raman line is at approximately 691 run. The normalisation of Figure 2b was achieved by ratioing the integrated GR1 luminescence intensity against 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 GR1 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. A processor 12 with suitable software can indicate automatically whether the diamond has been naturally or artificially irradiated.

The centre of the surface of the table of the diamond 1 was first positioned at the focal point of the laser beam and spectra were recorded at 10 urn intervals as the diamond 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 diamond 1 via the table.

As can be seen from Figure 2b, for the naturally alpha-irradiated diamond, the GR1 luminescence was substantially confined to within 30 u. m of the surface whereas (as shown in Figure 3) for the artificially electron-irradiated diamond, the OR1 luminescence is significantly intense over 1 mm below the surface (the different scales of Figures 2b and 3 should be noted). 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 has been artificially irradiated or ion bombarded to change its colour.

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 whether the diamond has been artificially irradiated or ion bombarded to change its colour.

11. The method of any of the preceding Claims, wherein the stimulating irradiation is irradiation of about 500 to about 740 nm wavelength.

12. The method of Claim 11, wherein the stimulating irradiation is irradiation of about 633 nm wavelength.

13. The method of any of the preceding Claims, wherein luminescence from about 680 to about 800 run 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.