Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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APPARATUS FOR BULK MATERIAL CONSTITUENT CONTENT DETERMINATION
USING PULSED NEVTRON RADIATION ~ND MET~IOD EMPLOYED
The present inven-tion relates to novel measurement
apparatus employing pulsed neutron radiation for determining
the conten-t of the various constituents of a bulk material.
It also rela-tes to a method and an installation for determin-
ing this content and employing the measurement apparatus.
The problem of-ten occurs in the mining and mineral
indus-tries, for example in mines and cement works, of de-
termining the content of a material as regards its various
elements in order to know the composition of the raw material
during continuous extraction thereof or while, for example, it
is being introduced into a cement making plant. Among those
lS constituents or elemen-ts whlch are the most important in
cemen-t-making techniques the following can be ~entioned: Si,
~1, Fe, Ca, Mg, K, C and H.
Various procedures are known for carrying out~continuous
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analysis of a material in order to determine the content of
its various constituents. As examples, we can clte, among
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others and limi-ting ourselves to those of most recent design,
methods employing neutron irradiation or emission and which
enable a defined zone, ~`orming a so-called sphere of influence
produced by the emission source, to be investiga-ted using
hig~ly accurate nuclear analysis techni~ues.
The simplicity of -the principles employed is another
advantage of the use of neutron analysis. The source is
selected so tha-t the nuclear reaction during interaction with
-the element -to be analyzed -takes place in the thermal neutron
energy domain with an energy that is as low as possible, the
neutrons producing various types of gamma radiation notably as
a result of the so-called "neutronic activation" phenomenon
and by the "capture" phenomenon an example of which is the
reaction:
Ca48(n, gamma)Ca49
which makes it possible to determine calcium. Generally
spealcing, when neutrons collide with the target nucleus (for
example Ca as mentioned above) there is an initial brief
emission of prompt gamma radiation after which the radioactive
isotope formed returns -to its stable state by emitting gamma
~adiation known as decay radiation. This behavior, which is
typical for calcium (Ca) should no-t be considered however as
characteristic for all neutron interac-tions with the various
ma-terials used as a target. Detection of the -types of gamma
radiation emitted b~ the isotope produced enables, firstly,
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identifica-tion of the body -that is -to be determined (by
Measuring th~ energy of tlle various radia-tions emitted and the
half~ e o~ the element produced) and, ~econdly, i-t enables
-the bod~ to be determined by measuring -the intensity of the
radiation emit-ted (number of pulses received at the detector).
In order to put the principles mentioned above into
practice, numerous systems have been proposed which, at best,
enable results that are uniform to be obtained by continuous
operation of a neutron bombardmen-t ana]ysis installation.
~R-A-2 618 225 describes a method, appara-tus and instal-
lation employing a neutron source which is activated intermit-
-tently at fixed periods. A gamma radiation detector counts
the pho-tons only a~ter neutron emission has stopped duriny the
separate periods of time corresponding respectivel~ to the
capture and the neutronic activation phenomena. The di~ferent
signals are numerically processed using two separate channels
and the results from them are automatically combinsd. No
actual taking of samples is necessary. Each application
requires that the relevan-t energy bands be correctl~ defined
'0 in accordance with -the elements tllat are being investigated.
The method and devices accordiny to FR-A-2 61~ 225,
while nevertheless constituting progress over ~nown methods
and devices, lead to difficulties which it has not yet been
possible to overcome in an optimal fashion. Effectivel~, the
sphere of ~nfluence prod~ced starting trom the point source
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and within which the nucl~ar reac-tion takes p-ace, is tangen-
tial -to -tl~e wall of -the conical hopper used for pouring the
ma-terial. Because of -this, systematic sources of error may be
present which are a resul-t of the more or less permanent
presence of residual heaps of material which ge-t created
during emp-tying of the hopper and which, because the particles
are lumped together and tend -to stick, can give rise -to
sys-tema-tic errors in measuremen-t. The fact should also be
mentioned that two radioac-tive isotopes can decay while
emitting radiation of close or equal energy which leads to
interference in -the energy area. This is the case for the
reactions:
Fe56(n p)MnS6
Al27(n p)Mg27
Moreover, correct energy defini-tion of the source needs
to be guaranteed: effec-tively, the "effective ac-tiva-tion
cross-section'l which is directly proportional to the collision
probability and hence to nuclear reaction, varies with the
energy of the projec-tile neutron. If this latter has a too
broad spectral distribution, the same radioactive isotope can
be obtained starting from -two or several elements. Thus, to
take an illustrative case:
A127(n, gamma)
Si (n, p) ____> ~l ff
p31 (n, ~)
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Such a phenomenon, if physically present, can make it
impossible to de-termine an element. Only carefully performed
standardization and -the use of scanning using discrete and
meaningfully distinc-t values can enable these sources of error
to be eliminated.
Methods of this types are also described for example in
EP-A-O 095 900, EP-A-O 171 256, AT-B-295 ~93, GB-A-2 101 304
and FR-A-] 514 030 which give rise -to systema-tic disadvantages
both as regards difficul~ies in carrying out measurement as
~0 well as regarding their accuracy.
The present invention enables the disadvantages of the
methods and apparatuses described in the prior art, apart from
F~-~-2 618 225, to be overcome. Moreover, i-t provides an
improvement to -the method and apparatus described in
FR-~-2 618 225.
The present inven-tion firstly provides an appara-tus for
measuring the content of the various consti-tuents of a bulk
material using pulsed neutron radiation comprising an enclos-
ure in which an endless belt enabling continuous passage of a
~O bulk ma-terial to be provided is adapted to travel, said enclos-
ure being cons-tituted of a ma-terial the molecule of which
includes a high proportion of hydrogen and containlng, on
respective sides of the endless belt, a pulsed neutron rad~ation
source and measurement means for the various gamma radiations
produced by the bu k material const:tutiny the target for the
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neutrons emit-ted by said s~uroe.
In an embodimen-t of the measuriny appara-tus, the source
is covered by means for stopping or slowing down the pulsed
neutrons emit-ted, consis-ting of a shield formed of a heavy
metal. One can, for e~ample, use lead for this.
The invention also provides a method for determinlng the
content of the various cons-ti-tuen-ts of a bulk material in
which said bulk material is continuously moving and is con-
veyed through an irradiation and measurement region where the
various types of gamma radiation produced by saicl irradiation
are measured, said method comprising supplying the spec-tral
data resulting from measurement to a computing area where
standard spectral data are stored resulting from measur~ments
carried ou-t under iden-tical conditions for each one of said
constituents, the presence of which is to be determined, in
the pure state and performing numerical processing in said
computing area employing a metllod consisting of dividing the
measured spectrum up into energy bands and comparing the
height of each segment wi-tll-the heights of the segments for
eac11 one o~ the corresponding standard spectrum energy bands
in order to determine, on at least part of said energy bands,
values ~or each one of said contents and to deduce therefrom a
resulting~average value~
~ccording to one aspect of~-the method, -the material is
first made homogeneous~over the whole volume~-thereof~to be
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investiga-ted before it en-ters -the irradia-tion and gamma
radia-tion measurement zone, the particle size distribution of
the ma-terial being brought to a prof:ile that i.s very close to
the particle size dis-tribution of the pure cons-tituents tha-t
5 were used to obtain the s-tandard values. If necessary, this
is done by successive screening operations enabling a sequence
of particle layers of prede-termined mean diameter to be
obtained.
According -to another aspec-t, con-tinuous determination of
the content of -the various cons-tituents of a bulk material
using pulsed neutron radiation is carried out by selective
measurement of gamma radiation emitted that is specific to
said elements, neutron emission being typically provided by a
neutron generating -tube, controlled whereby neutron emission
is periodically in-terrupted, the measuremen-t phases occurring
during or after termina-tion of neutron emission in order to :'
supply an indication of the spectra of the various types of
resulting gamma radia-ti.on.
: According to another feature, continuous determination
is car.ried out in a reduced sampling region, and during
determination the substan-tially unvarying par-ts of the signal .
due to the environment wi-th which the neutrons are in-teracting
are comple-tely eliminated only leaving a usable signal from
the de-tector defining an experimental spectrum, subsequent
numerical processing of which:enables the analyzed material
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content -to be determined by ma-trix computa-tion yielding,
starting from the analyzed continuous e~perimental spectrum,
areas of confidence relating -to -the resuL-t of said de-termin-
a-tion.
~rhe invention also provides an installation for de-
termining the con-tent of the various constituents of a bulk
ma-terial using pulsed neutron irradia-tion employing measure-
men-t appara-tus comprising a source of neutron irradiatioll and
a measuring region for the various types of gamma radiation,
means for providing conlinuous circula~ion of said bulk
material, means for receivi.ng the da-ta for the resulting
spectra and computing means enabling comparison to be made of
said spectral data with data resulting from measurements
carried out under identical conditions on each one of the pure
constituents the pressnce of which is being investiga-ted,
together with readout means for the averaged computed result.
Obviously, in order to obtain reliable results it is necessary
to carry out measuremen-t on pure constituents which are in a
condition where their particle sizes are practically similar~
One of the aims of the present invention is precisely
that of malcing determinations of the composition of bulk
material more reliable. Moreover, experience has shown that i:
it is indispensable for the forward movement of the materials
being examined to have a certain deyree of analogy from the
2S structural point of view with that of a system of par-ticles of
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different sizes subjected to movemellt similar -to laminar flow.
The op-timum would be for each slice of moving material, of the
same thiclcness, to possess simul-taneously the same average
density, the same average par-ticle SiZP and the same linear
velocity, said linear veloci-ty being readily able -to be held
cons-tant. A further aim of the invention is to ma~e errors
due to -the heterogeneous nature of the density distribution
and par-ticle size minimal, such errors resulting from the
corresponding anisotropy in the distribution of the radiati.on,
-the sensor only receiving a cone thereof corresponding to a
defined solid angle.
Other aims, advantages and features of the inven-tion
will become more clear from a reading of the description that
follows of an embodiment of the inven-tion provided by way of
non-limiting example and with reference to the at-tached draw-
ing in which:
- FIGURE l is a diagrammatical vlew of~an installation
according -to -the invention in which the endless belt passes
through the measuring apparatus according to the invention
- FIGURE 2 is a cross-section on a larger scale along
line Z-Z of figure l;
- FIGURE 3 is a section -throuyh the endless belt of
~igure 2, shown on an enlarged scale;
- FIGURE 4 shows the general shape of the spectra de-
tected on four pure elements these being Si, Ca, Al and Fe
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between 2.5 and 9 MeV;
- FIGU~E 5 shows a par-t of the spectrum obtained on a
raw material the composi-tion of which is being determined and,
secondly, a spectrum that has been recons-ti-tuted s-tar-ting from
the spectra of pure elements; the graphs show the spectra
obtained by counting between 0.5 and 2.5 MeV (left hand
curves) and between 2.5 and 9 MeV ~righ-t hand curves).
~ `igure 1 is a diagrammatical view of the measuring
device 1 -through which an endless belt 2 carryiny bulk ma-te-
rial 3 passes, it being desired to determine the content othe various constituents of said material during travel of
said bel-t 2. As it is required to carry out measurement on a
layer 4 of material of preferably constant thickness, levell-
ing means 5 are provided ahead of the device 1. Spectral data
receiving means 6, computing means 7 enabling said spectral
data to be compared with -those from standard spe~tra, and
readout means for the average result are provided.
Figure 2 shows a section along plane Z-Z of figure 1 on
an enlarged scale. The presen-t device 1 in which the layer 4
2n of material supported by endless belt 2 circulates, consists
o~ a high-density polyethylene housing 11 forming an enclosure
1~. Polyethylene is a material whose molecule includes numerous
hydro~en atoms, and the presence of said~high proportion of
hydrogen atoms renders the polyethylene material capable of
eliminating, through elastic difEusion, the efEect of very low
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energy ~amma rays on -the count.
The device 1 includes a neutron source 14 housed in a
bo-t-tom compartment 13 and consisting of a neutron genera-tor
tube loca-ted below -the movin~ endless belt 2. The device 1
ac-tually includes a region providing pulsed neu-tron radia-tion
resulting from -the ac-tion of the neutrons emitted by source
14, and a gamma ray measuring region using a detector 16
which, in the presen-t case, is a counter for the gamma radia-
tion emit-ted by all the elements present in the continuously
moving mass 4. Detec-tor 16 is loca-ted in enclosure 12 on the
opposite side of endless belt 2 ~o source 14. Screening
covers source 14 in order to slow down or stop emi-tted
neutrons, consistin~ of three walls 17 made of a heavy metal,
for example lead. The neutrons emitted by source 14 thus pass
through the moving bulk material 4, the -thickness of which is
determined by a straight edge means 5.
The nominal neutron flu~c can be of the order of 10 to
lO particles per second~ Detector 16 is a scintillation
counter-type element employing, for e~ample, thallium-
activated sodium iodide. The counter itself is associated
with two pro-tective shields 18 forming oblique screens made of
lead which isolate it from reflections and other spurious
influences.
Figure 3 shows a cross-sec-tional view taken in the sense
of travel of' the layer 4 of materlal. More precisely, that ~ ~;
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par-t of the layer which is meaningful for th~ measurement has
been shown in a preferred embodimen-t of -the invention, this
consists of a slab 19 about 12 cm high and about 3G cm wide.
Wi-th a throu~hput of 1 000 metric tons/hour, this type
of geome-try has enabled reproducible and reliable measurements
over an energy range of from O to lO ~eV -to be obtained.
De-tector 16 is linked to spectral data receiving means 6
which transmit their data -to compu-ting means 7 enabling
comparisons to be made with standard data. Readou-t means 8
enable the result obtained to be displayed.
Those ele~ents which are of in-teres-t and of which it is
desired to know the content in raw material are for example
eight in number. Adapta-tion of the method to a different
number of elements is readily accessible to those skilled in
the ar-t of radio-nuclear analysis. Those elemen-ts -that are
typically analyzed are, among others: Fe, Si, Ca, Al, K, C,
Mg, Tl. In one embodiment, two measuremen-t channels are
employed: the firs-t one takes account of gamma radiation due
to capture and activa-tion phenomena, the second channel only
handling activation gamma radiation. The final spectrum
(number of hits per second) is obtairled from the difference,
giving the capture gamma radia-tion spectrum. Teaching in this
matter is provided in FR-A-2 618 225.
Obviously, if it were desired to take accoun-t of other
types of gamma radiatlon, for example, those gamma rays known
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as "inelastic", one would suitably adap-t the number of mea-
surement channels.
Where two measurement channels are employed, the infor-
mation is collec-ted in the form of energy band spectra which
essentially depend on four parameters:
l. the neutron flow in the irradiated mass ("thermal"
neu-trons),
2. the density of -the irradiated mass,
3. water con-tent,
4. -the concentrations Ci of -the various elements i to be
identified and of which it is desired to determine the
content.
Figure 4 shows the spectral curves obtained for 4 pure
consti-tuents: Si, Ca, Al and Fe for an energy of from 2.5 to
9 MeV. In order not -to clutter the figure, -the spectra have
` been limited to these four pure constituents; spectra ob-tained
for other pure constituents are of the same type.
Figure 5 shows the energy spectral curve for a raw
material in which the content of 8 components is required to
be known. This firstly shows the e~perimental spectrum ob-
tained on this raw material the composi-tion of which is to be
determined and, secondly, superimposed -thereon, the spectrum ~:
obtalned after calculating the composition, by oombining the
eight spectra for pure elements: lt will be noticed -that tbe :
computed spectrum is very close to the spectrum measured on
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the raw material to such an extent -that it is even difficult
-to see the differences be-tween the two spectra in figure 5
which shows them superimposed on each other. The compu-ted
spectrum in fac-t approaches -the experimentally-ob-tained spec-
trum by better than 5~.
Each ordi~ate NEi corresponding to an interval of energy [Ei,
Ei + AEi] is a con-bined value for -the contribution of neutron
impacts wi-th the cons-tituents the con-ten-t of which is to be
determined. A similar si-tuation holds in each one of the
1 024 spectral analysis channels of amplitude ~Ei.
De-termination of the conten-t of the various elements
contained in -the bulk material is based on a set of spectra of
the type of those shown in figure 4. Where, for example it is
desired to determine the content of eight elements, it will
obviously be necessary to have obtained the eight spectra, all
of which are different, corresponding to each one of the pure
elemen-ts the content of which in the bulk material it is
desired to be determined.
These spectra are recorded over a photon energy range of
0 to 10 MeV, for example on 1 024 channels (2 ). The y-axis
is the number of pulses counted per channel. The spectrum
obtained for a continuous1y moving material is spli-t up into
1 024 energy bands and the height of the segment of at least
part of the 1 024 energy bands is determined; moreover, the ~ ~-
height obtained for each standard spectrum for these same
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energy bands is de-termined. Numerical analysis on a micro-
compu-ter enables the required conten-ts -to be obtained; in this
case the eight conten-ts required. The mathematical principle
employed can be expresses by -the .+ollowing formulae expressed
in matrix form, which are the basis of the numerical calculation
enabling the conten-t of tlle various components contained in
the moving bulk material to be calculated and, in -the example
cited above, en~bling the eight contents based on 1 024 (21)
energy bands to be ob-tained. Numerical processing is based on
calculation using numerical analysis me-thods for equations
(one per channel) of the unknowns corresponding to the contents.
These are expressed analytically by:
e = Pl tl ~ P1 t2 + Pt3 t3 + P1 t4 + P1 t5 + P1 t6 1 7 1 8
~ = P2 t1 + P2 t2 ~ P2 t3 ~ P2 ~Il+ P2 t5-~ P~ ~6 P2 7 2 8
pl t + p2 t + P3 t ~ P4 tl + P5 t + p6 t + P7 t + P t
n n 1 n 2 n 3 n I ll 5 n 6 n 7 n 8
where:
e~ is the count relating to the ~th channel selected, on
the spec-trum ob-tained for the unknown material;
p~ are the various counts on each pure elem~ant spectrum
numbered i in this same channel numbered ~,
; stands for the jth channel selected from 1 024 (for -this
example), optionally this can be selected ra~ndomly,
i is the number of elemen-ts the content of which it is
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desired -to be known which, in the presen-t example, can
have a value of from l -to 8;
ti are the con-tents;
n is any value comprised between 8 and l 024.
Although, by way of example, it has been s-tated that
operation is carried out on l 024 channels or segments, it is
of course possible to employ a lower number oE channels or a
higher number of channels. It is essential in each case for n
to be sufficiently large to enable a meaningful averaged value
to be obtained. The computing means can be programmed so that
-the channels can he selected at will to have a sufficiently
large number to allow a meaningful average value -to be ob-
tained.
The method and -the continuous determination device
resulting therefrom have proved to be readily implementable
industrially and the necessary manipulations, although requir-
ing a high degree o~ accuracy it is true, can be learned by
personnel of average skill.
The present invention is obviously not limi-ted to the
2n embodiments -that have been described and shown in the drawings
but may undergo numerous variations available to those skilled
in the art without this leading to a departure from the scope
of the invention.
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