Note: Descriptions are shown in the official language in which they were submitted.
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"DETECTION OF DIAMONDS"
BACKGROUND TO THE INVENTION
This invention relates to the detection of diamonds.
The invention is applicable to the detection of diamonds as individual, free
particles, as embedded in host bodies typically of kimberlite or as particles
included in a mass of other particles.
Referring to the application of the invention in the detection of diamonds in
host bodies of kimberlite it is recognised that it would be highly desirable
in
diamond recovery operations to have the facility to detect, at an early stage,
kimberlite particles which are host to diamond inclusions. It would then be
possible to reject the barren kimberlite particles and continue with
processing
of only those particles which are indicated as containing diamond inclusions.
With barren particles rejected at an early stage, the downstream processing
equipment could have a reduced capacity requirement.
It would in addition be advantageous to have the facility to detect not only
the
presence of a diamond inclusion but also the size and relative position of
that
inclusion in the host kimberlite body, since this information could be used to
regulate subsequent crushing operations used to liberate the diamond
inclusion to ensure that it is not physically damaged.
CONFIRMATION COPY
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In known proposals for detecting a diamond inclusion in a host kimberlite
body, the body is irradiated with X-radiation or with neutrons. In the former
case, the differential absorption of X-rays by diamond and kimberlite provides
an indication of the presence of a diamond inclusion. However this technique
suffers from the disadvantage that there is a small difference only between
the X-ray attenuation coefficient for diamond and the host kimberlite, so the
contrast which is obtained is limited. Furthermore there is a severe
limitation
on the size of the particles which can be analysed in this way because of the
substantial X-ray attenuation which takes place in kimberlite. The known
neutron irradiation technique relies on neutron resonance absorption, but also
has limitations insofar as detectable contrast between the diamond and the
surrounding kimberlite rock, and complexity of the method in practice, are
concerned.
SUMMARY OF THE INVENTION
One aspect of the present invention provides a method of detecting the
presence of diamond in a particle, wherein the particle is irradiated with
photons of selected energy at which the GDR (giant dipole resonance) is
excited for the nuclear reaction of the photons with carbon, and identifying
the
particle as potentially a diamond or diamond-containing particle according to
its interaction within the incident photons. The particle may be irradiated
with
bremsstrahlung encompassing a range of energy levels including a
characteristic GDR energy level, typically 22MeV for carbon.
In the preferred embodiment, the particle is identified as potentially a
diamond
or diamond-containing particle according to whether the isotope "C, with a
characteristic half-life of approximately twenty minutes, is produced by the
photon/carbon nuclear reaction and/or according to whether coincident and
collinear gamma ray photons at a distinctive energy level are emitted by the
particle.
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According to another aspect of the invention there is provided an on-line
particle sorting method comprising the steps of irradiating particles with
photons of gamma radiation at an energy level at which GDR (giant dipole
resonance) is excited for the nuclear reaction of the photons with carbon,
identifying particles as potentially diamond or diamond-containing particles
according to whether the isotope "C, with a characteristic half-life of
approximately twenty minutes, is produced by the photon/carbon nuclear
reaction and whether coincident and collinear gamma ray photons at a
distinctive energy level are emitted by the particles, and separating from
other
particles those particles which are identified as potentially diamond or
diamond-containing particles.
Still further the invention provides an apparatus for detecting the presence
of
diamond in a particle, the apparatus comprising means for irradiating the
particle with photons of gamma radiation at a selected energy at which the
GDR (giant dipole resonance) is excited for the nuclear reaction of the
photons with carbon, and means for identifying the particle as potentially a
diamond or diamond-containing particle according to the interaction of the
particle with the incident photons.
The invention also provides an on-line particle sorting apparatus comprising
irradiation means for irradiating particles which are to be sorted with
photons
of gamma radiation at an energy level at which GDR (giant dipole resonance)
is excited for the nuclear reaction of the photons with carbon and
identification
means for identifying particles as potentially diamond or diamond-containing
particles according to whether the isotope "C, with a characteristic half-life
of
approximately twenty minutes, is produced by the photon/carbon nuclear
reaction and whether coincident and collinear gamma ray photons at a
distinctive energy level are emitted by the particles, and means for
separating
from other particles those particles which are identified as potentially
diamond
or diamond-containing particles.
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In accordance with an aspect of the invention, there is provided a method of
detecting
the presence of diamond in a particle, wherein the particle is irradiated with
photons
in an energy band window encompassing a characteristic value of about 22MeV at
which the GDR (giant dipole resonance) is excited for the nuclear reaction of
the
photons with carbon, and identifying the particle as potentially a diamond or
diamond-
containing particle according to whether the isotope "C, with a characteristic
half-life
of approximately twenty minutes, is produced by the photon/carbon nuclear
reaction.
In accordance with another aspect of the invention, there is provided an on-
line
particle sorting method including the steps of irradiating particles with
photons is an
energy band window encompassing a characteristic value of about 22MeV at which
GDR (giant dipole resonance) is excited for the nuclear reaction of the
photons with
carbon, identifying particles as potentially diamond or diamond-containing
particles
according to whether the isotope "C, with a characteristic half-life of
approximately
twenty minutes, is produced by the photon/carbon nuclear reaction and whether
coincident and collinear gamma ray photons at a distinctive energy level are
emitted
by the particles, and separating from other particles those particles which
are
identified as potentially diamond or diamond-containing particles.
In accordance with another aspect of the invention, there is provided an
apparatus for
detecting the presence of diamond in a particle, the apparatus including means
for
irradiating the particle with photons in an energy band window encompassing a
characteristic value of about 22MeV at which the GDR (giant dipole resonance)
is
excited for the nuclear reaction of the photons with carbon, and
identification means
for identifying the particle as potentially a diamond or diamond-containing
particle
according to whether the isotope 11C, with a characteristic half-life of
approximately
twenty minutes, is produced by the photon/carbon nuclear reaction.
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In accordance with another aspect of the invention, there is provided an on-
line
particle sorting apparatus including irradiation means for irradiating
particles in an
energy band window encompassing a characteristic value of about 22MeV at which
GDR (giant dipole resonance) is excited for the nuclear reaction of the
photons with
carbon, identification means for identifying particles as potentially diamond
or
diamond-containing particles according to whether the isotope 11C, with a
characteristic half-life of approximately twenty minutes, is produced by the
photon/carbon nuclear reaction and whether coincident and collinear gamma ray
photons at a distinctive energy level are emitted by the particles, and means
for
separating from other particles those particles which are identified as
potentially
diamond or diamond-containing particles.
Other features of the invention will appear from the following description and
the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the invention will now be described in more detail, by way of
example
only, with reference to the accompanying drawings in which:
Figure 1 diagrammatically illustrates an on-line sorting method according to
an aspect
of the invention;
Figure 2 diagrammatically illustrates the detection of coincident and
collinear gamma
rays in a method according to an aspect of the invention; and
Figure 3 diagrammatically illustrates components of an ore processing plant
which
can be used to implement a method according to an aspect of the invention.
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SPECIFIC DESCRIPTION WITH REFERENCE TO THE DRAWINGS
In the first aspect of the invention as summarised above, bremsstrahlung is
produced by means of a particle accelerator of sufficient energy to excite
GDR in carbon nuclei which may be present in particles undergoing analysis.
It is noted that GDR is a fundamental mode of excitation of all nuclei,
including
carbon nuclei, characterised by its considerable intensity, width and median
energy. The full, continuous bremsstrahlung can be used with an energy end
point exceeding the upper end of the energy band window which
encompasses the characteristic GDR value for carbon, i.e. about 22 MeV.
The bremsstrahlung may be monochromatised, typically by collimation of
particular anglers of emission, to have an energy bandwidth sufficient to
cover
the characteristic GDR width at a selected median value.
Particles, typically kimberlite particles, which are undergoing analysis for
the
presence of diamond, are individually irradiated with the bremsstrahlung. In a
case where a mass of diamondiferous kimberlite particles is to be analysed
with a view to separating from barren particles those particles which are
diamond-containing particles, the particles may for instance be transported in
single file or in a monolayer through an irradiation station at which they are
individually irradiated. This may, for instance, be on a conveyor such as a
conveyor belt, or during free fall of the particles from a discharge point.
The bremsstrahlung is absorbed to a far greater extent by carbon, i.e.
diamond, in the particles at the characteristic photon energy than it is by
host
rock in which the diamonds are embedded. From a derived, differential
absorption image it is then possible to detect the presence of diamond in the
particle. Imaging may be achieved by simple linear geometry detection arrays
or by more complex tomographic systems. In either event, standard image
enhancement techniques can be used to improve the contrast between
diamond and the associated rock in the image. The generally low
concentration of carbon, homogenously distributed in the associated rock
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forms a faint outline background on which the higher density, higher
concentration of carbon in diamond is superimposed.
In cases where the above analysis indicates the presence of diamond in a
particle, that particle is separated from other, barren particles, typically
with
the use of conventional sorting equipment. For instance, where the particles
are transported and analysed on a conveyor belt, and are then projected from
an end of the belt to fall freely under gravity, selected particles may be
deflected out of the falling stream by means of suitable air blast ejectors
operating under the control of a computer which performs the analysis.
It will however be understood that any suitable form of sorting apparatus can
be used to separate particles for which there is a positive identification of
diamond presence from other, barren particles for which there is no such
identification.
In the preferred, second aspect of the invention as summarised above, the
particles undergoing analysis are again irradiated with gamma ray
bremsstrahlung at a predetermined energy. The incident photons activate the
carbon content of relevant particles through the nuclear reaction:
12C(y,n)-- 11C with Q = - 18.7215 MeV
11C-> R++(3- with Q = + 1.982 MeV
,c(11C) = 20 min
The twenty minute half-life for the decay of 11C makes the reaction
distinctive
and represents a convenient period of time for the application of subsequent
interrogative procedures, as described below.
When the positron comes to rest it promptly annihilates with an electron, as
follows:
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+R =Y+
The two gamma rays are coincident and collinear and each has a distinctive
energy of 0.511 MeV, making them readily detectable. Their unique signature
(back-to-back, time coincident and energy resolved) can be used to locate
and image the source of the collinear pair of photons each of 0.511 MeV in
energy, as described further below.
The sensitivity of the method just described can be enhanced by careful
selection of the incident photon energy. With a Q value of -18.7215 MeV the
threshold energy level for the reaction to occur is +18.7215 MeV. However, as
stated above, the characteristic GDR value for carbon, i.e. diamond, is about
22 MeV. It is therefore considered that the incident photon energy should
optimally extend to a value beyond 30 MeV. This can be provided either in the
form of continuous bremsstrahlung with its end-point in this range or by an
energy window of photons with a width sufficiently broad to embrace the full
GDR spectrum and a suitable median energy. By carefully selecting the
incident photon energy level in this way, it is possible to reduce the amount
of
radiation damage suffered by the particles undergoing analysis without,
however, reducing the detectability of the response.
As in the method according to the first aspect of the invention, particles
which
are positively identified as having diamond inclusions are separated from the
other, barren particles for which there is no positive identification. The
distinctiveness of the "C half-life and two coincident and collinear gamma ray
photons each with energy 0,511 MeV, together with the imaging of the source
points of this radiation, makes the method suitable for distinguishing and
separating not only diamonds which are fully or partially embedded inclusions
in host particles, but also free diamonds as discrete particles whether in
isolation or mixed with other particles, eg in a container, in a gravel
concentrate or during conveyance on a conveyor belt or the like.
An important feature of the second method described above arises from the
high penetrative power of the incident photons at the selected energy which is
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consistent with the generation of a GDR effect in diamond and also the high
penetrative power of the emitted gamma ray photons.
This is important both from the point of view of how large each particle can
sensibly be and how uniformly a particle can be irradiated with the photon
flux. Referring to the incident photon flux, it can for instance be shown
theoretically that for a typical kimberlite having a density of 2.8gcm-3 ,
only
50% of an initial 30 MeV photon flux is attenuated by passage through 13cm
of the kimberlite sample and for sample thicknesses of 10cm and 44cm, the
corresponding attenuation values for the same initial photon flux are 22% and
90% respectively. Thus, to take 10cm kimberlite particles as an example,
adequate activation of any diamond inclusion can readily be achieved at the
photon energy of 30 MeV. It is in any event possible to apply appropriate
corrections to take account of the expected attenuation.
Thus it will be understood that it is possible with the method of the
invention to
analyse large particles and that the initial ore crushing steps can be
tailored
accordingly. It will also be understood that although the term "particle" is
used
throughout this specification, the invention is not limited to the analysis of
mineral fragments which are small in size.
Referring to the penetrative power and hence detectability of the
characteristic, emitted gamma ray photons, it can be shown theoretically that
for a diamond at the centre of a spherical kimberlite particle of 10cm
diameter,
the 0.511 MeV gamma ray photons which are emitted by the diamond
inclusion, and which reach the surface of the particle, are attenuated by 70%,
leaving an adequate 30% of the original flux for detection. Nevertheless,
given
that the particles undergoing analysis will be irregular in shape and that any
diamond inclusions will rarely be at the centre, it is believed that it may be
advantageous to surround the particles with detectors, thereby to improve the
likelihood of detection of the diamond.
In a practical apparatus, the particles may be irradiated with the incident
photon flux at an upstream position on a conveyor belt, with gamma ray
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detection then taking place at a downstream position on the belt selected to
take account of the characteristic 20 minute half-life. The detectors may be
used in singles mode, coincidence mode or a combination thereof.
As a further feature of the method of the second aspect of the invention it is
possible to determine not only whether a particle has a diamond inclusion, but
also the location and size of the inclusion in the particle. From the absolute
intensity of the gamma ray emission it is possible to determine the size of
the
diamond inclusion. For determination of the location of the diamond in the
particle, it would be possible to implement image reconstruction algorithms.
It
would for instance be possible to use two gamma ray detectors and to rotate
the particle between them. Alternatively it would be possible to use a PET
camera system with a large array of stationary detectors or a smaller array of
movable detectors in order to create a three-dimensional image with
adequate spatial resolution for accurate determination of the location of the
inclusion. Modern detectors with sufficient spatial resolution and
sophisticated software are available and the principle has been
experimentally verified. More is said below about a currently preferred
detector arrangement.
It is recognised that photon interaction with kimberlite could create signal
interference. 53Fe, 52Mn and "'Sr have half-lives comparable to 11C but are
found in such low concentrations in Kimberlite as to have no significant
effect
on the detection of 11C. Interference from 44K would be a concern except that
the photon energy is 0.4 MeV and suitable energy selection of the 0.511 MeV
photons would eliminate this interference.
The most common element in a kimberlite sample is oxygen, the half-life of
which, as produced by the relevant nuclear reaction, is however only 2.03
minutes. Thus the problem which the interference could cause can be
sufficiently mitigated by only performing the carbon detection steps after
several 160 half-lives, for example after ten minutes or so, after the oxygen
activation has ceased. The remaining positron decay will be dominated by the
twenty minute half-life and accordingly distinctive of carbon.
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The radioactivity of the irradiated kimberlite after it is discarded can be
shown
to be small. The majority of the elements that are activated have half-lives
from a few seconds to a few hours. After one day the radiation levels would
be significantly reduced. As for irradiated diamonds containing large
inclusions, it has been shown experimentally that by far the dominant source
of radiation is "C which decays away after a few hours.
From the above it will be understood that the identification of carbon using
the
method proposed by this aspect of the invention relies on the detection of two
coincident and collinear photons emitted from the vicinity of carbon atoms as
a result of the sequence of reactions described earlier.
Both diamond and non-diamond sources of carbon will lead to the same
signature of coincident and collinear photons. The non-diamond forms of
carbon in kimberlite are in finer particulates or are essentially
homogeneously
distributed, as compared to the diamond form of carbon in the size range of
interest. The typical concentration of non-diamond carbon is about 0,2%. The
intensity of the carbon signal alone is insufficient to recognise the
potential
occurrence of a diamond particle for host kimberlite volumes larger than about
500 times the volume of the diamond.
This problem can be addressed by the quasi-imaging of the source geometry
of the carbon signals, identifying essentially the density of the carbon in
the
source region. Equivalently the carbon signal originating from a diamond is
not seen against the whole kimberlite volume but rather against a smaller
volume, namely a minimum volume element which can be identified by the
quasi-imaging process. This technique can accordingly improve the
discrimination between the diamond and non-diamond forms of carbon.
This is based on the fact that for most types of kimberlite under
consideration,
the diamond form of carbon represents the strongest localised source of
carbon signals. The quasi-imaging technique exploits the fact that the two
photons are coincident and collinear, so that a PET type algorithm may be
used to reconstruct the source distribution of the double photon events which
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take place. A suitable PET type algorithm is one which makes use of a two
dimensional array of detectors with the source material moving relative
thereto, typically an array of PET type detectors arranged along the length of
and surrounding the conveyor system.
This is illustrated in the accompanying diagrammatic drawings. As shown in
Figure 1, the kimberlite or other source material 10 is moved at constant
velocity by a transport system, such as the illustrated conveyor belt 12,
through two imaging devices 14 and 16. The first device 14 images the rough
physical dimensions of the material particles. This could for instance be
achieved by an array of photodiodes to produce a two dimensional "shadow "
of the material which, along with the associated time component, can be
reconstructed by suitable software algorithms to create a three dimensional
representation of the transported particles.
The second device 16, which is a quasi-imaging device as referred to above,
works in a manner analogous to that of a PET device. The coincident and
collinear- photons are detected by an array of position sensitive photon
detectors 18 (Figure 2) which measure the position of the detected photon,
the time at which such measurement took place and the photon energy.
The information so obtained by the detectors 18 can then be analysed by
appropriate software which correctly assigns the detected photons into
coincident pairs at given times relative to the instantaneous position of the
source material. In effect the software algorithm freezes the motion of the
source material and employs a ray tracing technique, based on the collinear
back-to-back emission to reconstruct a density map of carbon signals from
the source material. The eventual image reconstruction is reliant on the
position sensitive nature of the photon detectors 18 as well as the identified
photon pairs and their collinear nature.
For accurate image reconstruction good time resolution of the detectors is
essential for correct identification of coincident pairs of photons. It has
been
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established that the random to real coincident ratio can be neglected if
detectors with nanosecond time resolutions are used.
Combining the reconstructed image with the physical image of the source
material particle obtained by the device 14 enables a decision to be made as
to whether the particle in question contains a localised concentration of
carbon, i.e. a localised concentration of carbon which is significantly higher
than the average carbon concentration in kimberlite particles that are barren
of diamond, and which are accordingly indicative of diamond 22, or not.
Thereafter, as explained previously, the particulate source material enters a
sorting device 20 (Figure 1) which separates the identified particles from
other
particles.
Figure 3 diagrammatically illustrates the components of a processing plant
which can be used to implement the method just described. The numeral 30
indicates a crusher set to reduce the particle size to 10cm or less. A chute
32
directs the crushed particles onto an endless conveyor belt 34 which
transports the particles through a 22MeV irradiator 36 which irradiates the
particles with gamma radiation as described above. The particles are
deposited by the belt into a hopper 38 which retains the particles for at
least a
twenty minute period before depositing them on an endless conveyor belt 40.
The latter belt transports the particles through a detection station at which
an
array 42 of detectors 44 surrounds the belt, as described above. Downstream
of the detection station, particles identified as potentially containing
diamond
are tagged by a tagging device 46, whereafter the tagged particles are
removed from the general stream of particles by a mechanical picking device
48. Barren particles 50 are deposited onto a further conveyor which transports
them to waste while selected particles 52 are if necessary crushed at crushing
stations 54 and subjected to conventional dense medium separation in dense
medium separation units 56 followed, possibly, by conventional X-ray sorting
in an X-ray sorter 58 to yield a diamond-rich product 60.
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It is envisaged that in an underground diamond mine the process described
above, at least up to the sorting stage, could be carried out underground.
The barren rocks could then be disposed of underground without the
necessity to transport them to the surface. Only the selected particles are
raised to surface for further processing.
It will be understood that the above description and the accompanying
drawing are illustrative of certain embodiments of the invention and that
many modifications are possible within the scope of the invention.
