GB2056056A

GB2056056A - Detection or particles containing predominantly low atomic number nuclei

Info

Publication number: GB2056056A
Application number: GB8020773A
Authority: GB
Original assignee: De Beers Industrial Diamond Division Pty Ltd
Current assignee: De Beers Industrial Diamond Division Pty Ltd
Priority date: 1979-06-26
Filing date: 1980-06-25
Publication date: 1981-03-11
Also published as: GB2056056B; AU530206B2; AU5946780A

Abstract

A method of detecting desired particles, e.g. diamonds, containing predominantly atomic nuclei having low atomic numbers in a particulate material including other particles containing atomic nuclei having higher atomic numbers, includes the steps of irradiating particles, advantageously individually, of the particulate material with incident X-rays, conveniently in the form of a monoenergetic beam, to produce scattered X-ray spectra for said particles, determining the peak strengths due to Compton scattering and/or Rayleigh scattering of the incident X-rays by the particles, at a selected scattering angle (preferably an angle other than one at which a Bragg peak [coherent diffraction peak] occurs for the desired particles in polycrystalline form) relative to the incident X-rays, and utilizing the peak strength information to identify said desired particles. Preferably, for diamond, the ratio of the Compton peak to the Rayleigh peak is used, as being substantially greater for diamond than for non-diamond specimens.

Description

SPECIFICATION Detection of particles containing predominantly low atomic number nuclei This invention relates to a method of detecting particles containing predominantly low atomic number nuclei and particularly to the detection of diamonds.

South African Patent No. 78/0569 describes a method of detecting particles with nuclei of low atomic number in which each particle is irradiated with incident X-rays to produce a secondary X-ray spectrum for the particle and the particles with nuclei of low atomic number are identified as being those particles which have a secondary X-ray spectrum intensity, at a selected energy region, above a predetermined value.

It is an object of the present invention to provide an alternative method of detecting particles with nuclei of low atomic number.

According to one aspect of the present invention we therefore provide a method of detecting desired particles, in particular diamonds, containing predominantly atomic nuclei having -iow atomic numbers in a particulate material including other particles containing atomic nuclei having atomic numbers which are higher, the method including the steps of irradiating particles, advantageously individually, of the particulate material with incident X-rays, conveniently in the form of a monoenergetic beam, to produce scattered X-ray spectra for said particles, determining in the scattered X-ray spectra the peak strengths due to Compton scattering and or Rayleigh scattering of the incident X-rays by the particles, at a selected scattering angle (preferably selected to be an angle other than an angle at which a Bragg peak [coherent diffraction peak] occurs for the desired particles in polycrystalline form) relative to the X-rays incident on the particles, and utilizing the peak strength information to identify said desired particles. The peak strength may be determined from the peak intensity or height or from the integrated intensity.

The particles with nuclei of low atomic number may be distinguished from those particles with nuclei of higher atomic number on the basis of peak strengths due to Compton or Rayleigh scattering. Alternatively, the ratio of peak strengths due to Compton and Rayleigh scattering may be determined and then the desired particles identified as those particles having ratios above a predetermined value. When simply using the peak strengths due to the Compton or Rayleigh scattering for the identification, the area of the incident X-rays should be less than the specimen area to avoid size effects. When the area of the incident X-rays is greater than the specimen area a blank reading due to the specimen chamber, support, etc. should be taken and deducted.When the ratio information is used for the identification then the area of the incident X-rays is not critical.

The peak strengths may be determined by examining the scattered spectrum with either a wavelength or an energy dispersive mode.

The scattered X-ray spectrum of the particle may be directed into at least two detecting means for separately obtaining a measure of the peaks due to Compton and Rayleigh scattering. Such would be used particularly when ratios of peak strengths are to be used in the identification.

When using the method in the energy dispersive mode, only one detector is necessary. In the wavelength dispersive mode, the method can be used either in the scanning crystal mode or in the fixed crystal mode. In the scanning mode, the scattered radiation is allowed to impinge upon an analysing crystal which is made to scan over a range of angles. The radiation is diffracted from the crystal into a detector which scans at twice the rate of the crystal.

In the fixed mode two analysing crystals are employed. One diffracts the Compton peak into a suitably placed detector and the other diffracts the Rayleigh peak into another suitably placed detector. The fixed crystal mode is considerably faster than the scanning mode. Hence the fixed crystal mode would probably be better suited to production conditions.

A preferred embodiment of the method of the present invention is further described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic illustration of an apparatus for use in the method according to the invention and Figures 2 and 3 show graphs of scattered X-ray spectra as functions of wavelength and energy respectively.

Figure 1 illustrates an X-ray tube 10, two crystals 12 and 14 respectively, two X-ray detectors 1 6 and 1 8 respectively and a particle 20 mounted on a holder 22 which is in the form of a metal cylinder 24 with a thin film Mylar window 26 which is transparent to X-rays. The particle 20 rests on this film. A third crystal (not shown) to obtain a blank reading may be provided.

X-rays produced by the X-ray tube 10 are directed onto the particle 20 and part of the scattered X-ray spectrum produced by the particle is directed onto the crystals 12 and 14. These crystals are adjacent to one another and thus the spectra of X-rays scattered from the particle 20 which impinge on them are at substantially the same scattering angle. The crystals are positioned so as to diffract a separate stream of X-rays into each of the detectors 1 6 and 18.

Figure 2 is obtained by using the wavelength dispersive method in the scanning mode and is a plot of the intensity I of the scattered X-ray spectrum produced by a particle as a function of wavelength, wI, over a given wavelength region.

The curve illustrates two intensity peaks Pr and Pc at wavelengths .Ir and wIc due to Rayleigh scattering and Compton scattering respectively of the incident X-rays. It is found that the ratios of peak intensities Pc to Pr produced from particles with nuclei of low atomic number are greater than those produced from particles with nuclei of high atomic numbers. This phenomenon permits one to distinguish particles with nuclei of low atomic number from particles with nuclei with atomic numbers which are higher. In particular it permits the identification of diamond in a particulate mass where the gangue particles contain nuclei of higher atomic numbers.

Figure 3 illustrates a curve of intensity I against photon energy E over a given region of scattered X-ray spectrum produced in an energy dispersive system. In this case the scattered peak produced by Compton or inelastic scattering is denoted Pc and is centred on an energy Ec. The peak produced by Rayleigh or elastic scattering is denoted Pr and is centred on an energy denoted Er. As with the wavelength dispersive method, the ratios of Pc to Pr for particles with nuclei of low atomic number are greater than similar ratios produced with particles with nuclei of higher atomic number. This phenomenon permits one to distinguish the desired particles from particles with nuclei with atomic numbers which are higher.

The following table reflects the Compton and Rayleigh ratios for a variety of diamond and nondiamond specimens: SN 0.04 0.30 0.46 0.25 0.48 0.44 0.66 SD 0.92 1.52 1.42 6.80 7.16 5.36 1.90 CN CD SN = and SD RN RD where CN = Compton peak height for non-diamond CD = Compton peale height for diamond RN = Rayleigh peak height for non-diamond RD = Rayleigh peak height for diamond It should be noted that the ratios 5N and SD may change with a change of specimen orientation. Hence specimen spinning during measurement may be helpful in obtaining a reliable mean value. The ratios have been corrected for background radiation.

Claims

1. A method of detecting desired particles containing predominantly atomic nuclei having low atomic numbers in a particulate material including other particles containing atomic nuclei having atomic numbers which are higher, the method including the steps of irradiating particles of the particulate material with incident X-rays to produce scattered X-ray spectra for said particles, determining in the scattered X-ray spectra the peak strengths due to Compton scattering and/or Rayleigh scattering of incident X-rays by the particles, at a selected scattering angle relative to the X-rays incident on the particles, and utilizing the peak strength information to identify said desired particles.

2. A method as claimed in claim 2 wherein the desired particles are diamonds.

3. A method as claimed in either claim 1 or 2 wherein successive particles are irradiated individually.

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4. A method as claimed in any one of claims 1 to 3 wherein the selected scattering angle is other than an angle at which a Bragg peak (coherent diffraction peak) occurs for the desired particles in polycrystalline form.

5. A method as claimed in any one of the preceding claims wherein the area of the incident X-rays is less than that of the particles and the desired particles are distinguished from those particles containing atomic nuclei having higher atomic numbers on the basis of peak strengths due to Compton or Rayleigh scattering.

6. A method as claimed in any one of claims 1 to 4 wherein the ratio of peak strengths due to Compton and Rayleigh scattering is determined and the desired particles identified as those particles having ratios above a predetermined value.

7. A method as claimed in any one of the preceding claims wherein the peak strength is determined from peak intensity or integrated intensity.

8. A method as claimed in any one of the preceding claims wherein the peak strengths are determined by examining the scattered X-ray spectra by either a wavelength or an energy dispersive mode.

9. A method as claimed in claim 8 wherein the wavelength dispersive mode is used, the scattered radiation being allowed to impinge upon an analysing crystal which is made scan over a range of angles and the radiation is diffracted from the crystal into a detector which scans at twice the rate of the crystal.

10. A method as claimed in claim 8 wherein to measure a scattered X-ray spectrum the wavelength dispersive system employing two analysing crystals is used, one crystal diffracting the Compton peak into a suitably placed detector and the other diffracting the Rayleigh peak into another suitably placed detector.

11. A method of detecting desired particles containing predominantly atomic nuclei having low atomic numbers in a particulate material including other particles containing atomic nuclei having atomic numbers which are higher, substantially as herein described with particular reference to any one of Figures 1 to 3 of the accompanying drawings.

12. Diamonds or other desired particles whenever detected by a method as claimed in any one of the preceding claims.