DIAMOND DETECTION

DETECTION DE DIAMANTS

English 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
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

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THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1.
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.

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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.
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.
5.
A method according to 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 claim 2 wherein the particle beam is a deuteron beam
and the target is a deuterium gas target.

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7.
A method according to claim 6 wherein the fast neutron beam is modulated
between energy levels of 8 MeV and 7 MeV respectively.
8.
A method according to claim 7 when used to detect a diamond in a body of
kimberlite.

Description

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DIAMOND DETECTION
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.

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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
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
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 non-
resonant energy levels for diamond, obtaining corresponding first and second

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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
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.

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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
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.

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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
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
be 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

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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.

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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.