AU707986B2

AU707986B2 - Diamond detection

Info

Publication number: AU707986B2
Application number: AU15067/97A
Authority: AU
Inventors: Robert Hamm; Ulf Anders Staffan Tapper
Original assignee: De Beers Consolidated Mines Ltd
Current assignee: De Beers Consolidated Mines Ltd
Priority date: 1996-03-04
Filing date: 1997-03-03
Publication date: 1999-07-22
Anticipated expiration: 2017-03-03
Also published as: CA2198990A1; GB2310927A; AU1506797A; CA2198990C; GB9704354D0; GB2310927B; RU2151409C1

Description

-2- BACKGROUND TO THE INVENTION THIS invention relates to diamond detection.

The invention has particular application to the detection of diamonds within host kimberlite particles. In practice in diamond recovery operations, it would be highly desirable to detect kimberlite particles that are host to internal diamond inclusions since it would then be possible to reject those kimberlite particles which are barren and to continue with processing of only those particles known to include diamonds. With barren particles rejected at an early stage, the load on and capacity required of the downstream processing equipment could be vastly reduced.

The specification of South African patent 94/10192 describes a fast neutron imaging technique for use, inter alia, in detecting the presence of diamond inclusions in particles of kimberlite. In one version of the technique, individual kimberlite particles are irradiated with a fast neutron beam (i.e.

the neutrons have a kinetic energy of the order of mega-electron volts) at two distinct energy levels, one of which is a resonant energy level for diamond and the other of which is a non-resonant level for diamond. The aforementioned patent describes the use of beam energy levels of about 7,8 MeV and 7 MeV respectively.

-3- Irradiation at the higher energy level gives a large contrast between the diamond and the host rock in the absorption image which is obtained, because the rock is very much more transparent to the incident radiation than the diamond at this energy level. However in the absorption image which is obtained at the lower energy level, there is little or no contrast between the diamond and the host rock because at this energy level both the diamond and the rock are similarly transparent to the incident radiation. The absorption images obtained at the two energy levels are subtracted from one another to provide a third image from which the image attributable to the rock is essentially removed, and the third image is analysed for the presence of diamond. The analysis is carried out by a computer which, in response to the detection of a diamond in a particular particle, triggers the operation of a i~ particle separation system which separates the diamond-containing particle from other, barren particles.

The fast neutron beam is typically generated by a nuclear reaction between a deuteron beam and a deuterium gas target. The neutron energy in the beam i which is produced is linearly related to the kinetic energy of the incident deuteron beam.

SUMMARY OF THE INVENTION According to the invention there is provided a method of detecting the presence of diamond in a body, the method comprising the steps of irradiating the body with a fast neutron beam modulated between first and second, distinct energy levels which are respectively resonant and nonresonant energy levels for diamond, obtaining corresponding first and second 4 absorption images for the body, deriving from the first and second absorption images a third absorption image in which absorption effects attributable to the presence of non-diamond material in the body are eliminated or at least reduced, and analysing the third absorption image for the presence of diamond in the body, wherein the energy modulated fast neutron beam is produced by nuclear reaction between an energy modulated particle beam and a target, the energy modulated particle beam being produced by: passing a particle beam sequentially through a first particle accelerator which accelerates the particle beam and a second particle accelerator to which the accelerated particle beam is delivered by the first particle accelerator, operating the second particle accelerator alternately in first and second modes to produce, from the particle beam delivered by the first particle accelerator, a particle beam which is modulated between relatively high and relatively low energy levels.

In the preferred embodiment, the second particle accelerator is operated in its first mode to boost the energy of the particle beam delivered to it by the first particle accelerator and in its second mode to apply no boost to the 0 energy of the particle beam delivered to it by the first particle accelerator.

Typically, the first and second particle accelerators are RFQ accelerators and the second particle accelerator is switched from its first mode to its second mode by detuning it so that its RF and accelerating fields are out of phase.

SPECIFIC DESCRIPTION In one application the invention is used to produce a modulated energy deuteron beam. The modulated energy deuteron beam is used to produce a modulated energy fast neutron beam. This is achieved by directing the modulated energy deuteron beam at a deuterium gas target. The fast neutron beam is then employed in a diamond detection and sorting technique as described above.

The fast neutron beam is produced as a result of the nuclear reaction between the deuteron beam and the deuterium gas target. As mentioned previously, the energy level of the fast neutron beam is related linearly to the kinetic energy level of the incident deuteron beam. As also mentioned rirr previously, the diamond detection technique in which the invention finds application requires a fast neutron beam at an energy level modulated between resonant and non-resonant values. In the present example, it is required to produce a fast neutron beam modulated between a resonant ~energy level of 8 MeV, corresponding to the previously mentioned 7,8 MeV level, and a non-resonant energy level of 7 MeV.

It is known that to produce a fast neutron beam at an energy level of 8 MeV in the nuclear reaction, the deuteron beam should be accelerated to an energy level of 5 MeV, due to a 3 MeV positive Q-value in this particular reaction.

To produce a fast neutron beam at the non-resonant energy level of 7 MeV, the deuteron beam should be accelerated to an energy level of 4 MeV, once again because of the 3 MeV positive Q-value. It is accordingly necessary to modulate the deuteron beam between 4 MeV and 5 MeV energy levels.

6 In this embodiment, the modulation of the deuteron beam is effected with the use of two RFQ (radio frequency quadrupole) particle accelerators arranged in series, and preferably connected to one another to form a unitary apparatus, with the first RFQ particle accelerator being arranged to deliver its output directly to the second RFQ particle accelerator.

In the first RFQ particle accelerator, the deuteron beam produced by a suitable source is accelerated to a constant energy level of 4 MeV. The 4 MeV output of the first RFQ particle accelerator is delivered to the second RFQ accelerator, which is operated in an on/off mode. In the "on" mode of operation, the second RFQ particle accelerator further accelerates the deuterons so as to boost the energy level of the particle beam delivered to •it by 1 MeV to give the desired 5 MeV output. The 5 MeV output produced rirr by the second RFQ accelerator is then focused onto the deuterium gas target by suitable electromagnets, with the result that a fast neutron beam is produced at an energy of 8 MeV. This fast neutron beam is directed onto a kimberlite particle undergoing analysis, allowing a first absorption image to obtained for that particle.

In the "off' mode of operation, the second RFQ accelerator applies no further energy boost to the deuteron beam delivered to it by the first RFQ accelerator, and so passes a deuteron beam at the 4 MeV energy level. The nuclear reaction which takes place when this deuteron beam is focused onto the deuterium gas target produces a fast neutron beam at the non-resonant 7 MeV energy level. The latter neutron beam is then used to obtain a second absorption image for the kimberlite particle in question.

As explained previously, the absorption images are subtracted from one 7 another and from an analysis of the resulting, third image, a determination can be made as to whether or not the kimberlite particle has a diamond inclusion. If a diamond inclusion is detected, the kimberlite particle is separated from other particles for which the same analysis reveals no diamond presence. This may be achieved using a gas blast ejector triggered by the computer which analyses the image.

A feature of the invention is the manner in which the second RFQ accelerator is switched between the "on" and "off' modes of operation. To switch from the "on" to the "off' mode, the exciting RF (radio frequency) field in the second RFQ accelerator is detuned so as to be out of phase with the accelerating field of the accelerator, with the result that no further accelerating kinetic energy is applied to the deuteron beam delivered by the first RFQ accelerator.

It is important that the radio frequency field be designed to apply appropriate focusing effects that allow the deuteron beam to be transported through the second RFQ accelerator within a well-defined beam envelope. This applies to both modes of operation of the second RFQ accelerator. In the "off' mode of operation, even though the radio frequency field of the second RFQ accelerator does not boost the energy of the deuteron beam, the deuteron beam which is produced is nevertheless adequately focused.

The modulation of the second RFQ accelerator between its respective modes is controlled by computer, the time periods for which each mode is operative being determined by the time required for generation of the necessary radiographs, i.e. absorption images.

8 The focusing system which is used to focus the energy modulated deuteron beam onto the deuterium gas target is preferably achromatic, i.e. is insensitive to the energy level of the beam produced by the second RFQ accelerator, so that a single focusing setting can be used irrespective of the mode of operation of the accelerator.

A major advantage of producing an energy modulated fast neutron beam, using dual RFQ accelerators to modulate the energy of the exciting deuteron beam in the manner described above, is the fact that there are no moving parts.

.o 55 S S

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9 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:i.

A method of detecting the presence of diamond in a body, the method comprising the steps of irradiating the body with a fast neutron beam modulated between first and second, distinct energy levels which are respectively resonant and non-resonant energy levels for diamond, obtaining corresponding first and second absorption images for the body, deriving from the first and second absorption images a third absorption image in which absorption effects attributable to the presence of non-diamond material in the body are eliminated or at least reduced, and analysing the third absorption image for the presence of diamond in the body, wherein the energy modulated fast neutron beam is produced by nuclear reaction between an energy modulated particle beam and a target, the energy modulated particle beam being produced by: passing a particle beam sequentially through a first particle ~accelerator which accelerates the particle beam and a second particle accelerator to which the accelerated particle beam is delivered by the first particle accelerator, operating the second particle accelerator alternately in first and second modes to produce, from the particle beam delivered by the first particle accelerator, a particle beam which is modulated between relatively high and relatively low energy levels.

Claims

2. A method according to claim 1 wherein the second particle accelerator is operated in its first mode to boost the energy of the particle beam delivered to it by the first particle accelerator and in its second mode to apply no boost to the energy of the particle beam delivered to it by the first particle accelerator.
3. A method according to claim 2 wherein the first and second particle accelerators are RFQ accelerators. 4 4* 4* 4 4 *r
4. A method according to claim 3 wherein the second RFQ accelerator is switched from its first mode to its second mode by detuning it so that its RF and accelerating fields are out of phase. A method according to claim 3 or claim 4 wherein the energy modulated particle beam is focused onto the target by a focusing system which is insensitive to the energy level of the beam.
6. A method according to any one of the preceding claims wherein the particle beam is a deuteron beam and the target is a deuterium gas target. 11
7. A method according to any one of the preceding claims wherein the fast neutron beam is modulated between energy levels of 8 MeV and 7 MeV respectively.
8. A method according to any one of the preceding claims when used to detect a diamond in a body of kimberlite. DATED: 3rd March, 1997 PHILLIPS ORMONDE FITZPATRICK Attorneys for: DE BEERS CONSOLIDATED MINES LIMITED ABSTRACT The invention concerns a method of detecting the presence of diamond in a body. In method the body is irradiated with a fast neutron beam modulated between first and second, distinct energy levels which are respectively resonant and non-resonant energy levels for diamond. Corresponding first and second absorption images are obtained for the body and from these images, a third absorption image is derived in which absorption effects attributable to the presence of non-diamond material in the body are eliminated or at least reduced. The resulting image is then analysed for the presence of diamond in the body. The energy modulated fast neutron beam is produced by nuclear reaction between an energy modulated particle beam and a target. The energy modulated particle beam itself is produced by a procedure in which a particle beam is passed sequentially through first and second particle accelerators in series. Modulation is achieved by operating o o the second particle accelerator alternately in first and second modes to produce, from the particle beam delivered by the first particle accelerator, a particle beam which is modulated between relatively high and relatively low energy levels. B