Note: Descriptions are shown in the official language in which they were submitted.
~b d
~6~336~
BACKGROUND OF THE INVENTION
The invention relates to a device and method for
determining the proportions by volume of a multiple
coMponent mixture in which the mixture is irradiated with
5. two or more gamma lines and the intensities of the radia-tion
which pass through the mixture are measured and used to
calculate the said proportions. A gamma line is gamma
radiation with a substantial energy peak~ that is to
say that the substantial majority of the radiation has an
10. energy equal to or very close to a single known value. This
fac-t means that the radiation is essentially monochromatic,
tha-t is -to say it is of substan-tially only one frequency,
but in the field of gamma radiation it is conventional to
refer to energy rather than frequency.
15. In indus-trial technology there is an increasing need
for methods for a contac-t-free, rapid and continuous
determination of the concentration by volume of one or
more of the components of a multiple-component mixture.
This need is due to, amonst other things, the increasing
20. importance of hydraulic transport of solid materials.
Generally the objects to be measured are opaque bodies,
that is to say the bodies thermselves or, for instance
the surrounding conveyor pipe is opaque, so that for
contact-free measurement only the use of penetrating gamma
25. radiation and the analysis of the interaction of the
gamma quanta with the object to be studied is practical.
A method of this general type is described in
German Auslegeschrift No. 2622175 and in the periodical
"meerestechnik" (Marine Technology) 10, 1979, No. 6,
30. pages 190-195, which method is essentially based upon the
~,, , ~.
3 . 3 ~ 66~3~ ;
fact that for two substances (~ and q)` with sufficiently
different mean atomic number Z -the ratio of the gamma
absorption coefficients ~u in -the region of low gamma
energy up to approximately 1.5 MeV is highly energy
dependent. Using this fact it is possible for the two
unknown component proportions or volume proportions of
these components vp and vq in a three component mixture to
be unambiguously determined from two equations by the .
measurement of the radiation in-tensities J wi-th and
, lOo without the presence of the absorbing bodies at -two
differen-t gamma radia-tion energies (El, E2). Since
the dimensional parameters during measuring are normally
fixed and thus the length of the transmission path ~ in
the irradiated medium is constant, the third cornponent is
15. given as a by-product from the limiting condition that the
sum of the three proportions by volume must be 100~. When
one is concerned with hydraulic conveying sys-tems the
third component is generally water (w) 9 which as a rule
takes up the space in the conveyor pipe which is ieft
200 free by the solid components ~ and qO In this case it is
convenient to select not the absorber-free (vacuum)
intensity, -that is to say the radiation intensity when no
: : material wha-tever is present, but the intensity Jw f
the gamma radiation for solid-free water as the reference
25. :parameter. Thus the two transmission equations for the
energies El and E2 have the form
tl = ~ = e L [vp~pl -~ v~uql - (vp + v ),u
~wl
300
4. ~G03~gL
t2 = - ~ e L [V~Up2 ~ Vq~Uq2 (~p vq)Ju
Jw2
with vp ~ vq ~ VW ~ 1.
Solution for v and v gives
v = (LN) 1 ~-lnt1(,uq2 ~ ~w2) ~ lnt2(~q1 ~wl~
or
10. vq = (LN) ~nt1(~p2 ~ ~w2) lnt2(~p1 ~wl)]
where
N (,up1 Pw~ q2 ~w2) (~p2 ~2)(~ql ~wl~
The two gamma lines advantageously pass through the
volume to be measured on a common radiation axis and
1 thus can be used to determine these volume proportions
exactly. Differences in physical structure which would
lead to errors of inhomogeneity if the transmission of the
two lines occurred at different places in the volume to be
measured do not therefore cause any problem.
Naturally this process can be applied to mixtures
with more than three components. A further gamma line
is then necessary for each additional component. In the
calculation a further transmission equation is produced
for each additional component.
The errors in de-termining the volume concentrations
o
depend upon the precision of the measurement of the gamma
intensities. The influence of the relative errors in
the measured gamma intensities is independent of the volume
concentration and inversely proportional to the length of
the transmisslon path L.
~6~3~
Since the proportions of solid material are often
only in the region of a few percent, i-t is desirable tha-t
for sufficiently accurate measurements the rela-tive
errors should lie in the region of, or only slightly
5~ abo~eg O.a%~
In the method disclosed in German Auslegeschrift
No. 2622175 the intensities of the two gan~a lines are
determined separately by pulse heigh-t analysis of the
pulses emit-ted in a conventional scintilla-tion counter
10. which responds -to the two lines together. The counter
must show an adequate energy resolution so -tha-t reciprocal
interference remains low. The most suitable substance
for use a a scintillator material is sodium iodide (NaI)
doped with thallium.
15. The critical disadvantage of conventional spectroscopy~
with the NaI referred to is the relatively long
fluorescent time constant of this scintillator of
0.25 ~s. Of necessity this results in a pulse leng-th in
-the region of microseconds which at high counting rates
20. leads -to pulse pile-up and displacement of the
zero line and thus finally to inaccuracies in determining
-the intensities. A practical upper limit for the
counting rate is approximately 50,000 pulses/s. if the
; errors given above are not to be markedly exceeded by
25. sys-tematic errors. For statistical reasons this counting
rate in turn implies minimum measuring times purely
arithmetically of approximately 40 s and in practice
generally in the region of 50 s. Therefore the process
can only be described as quasi-continuous. Faster
30. scintillation detec-tors do exis-t but they do not permit
36~
adequate energy discrimination.
SUMMARY OF THE IN~ENTION
An object of the present invention, therefore, is to
provide a device of the type referred to which avoids the
known problems and in particular makes shorter measuring
times possible,
According to the present invention there is provided
a device for determining the proportions by volume of an
n-compOnen-t mixture, the componen-ts having differing
10. mean atomic numbers, including means for irradiating the
mixture on a common axis with n-~ gamma lines of different
energies at which the absorption coefficients of the
components are different 9 a number of radiation de-tectors
equal to the number of gamma lines used, each detector
15. being responsive to substantially only a respective one
of the gamma energies and arranged to measure the intensity
of its respective gamma line after it has passed through
the mixture and calculation means connected to the
radiation detectors arranged to calculate the said
20. proportions by volume from the measured radiation
intensities.
According to the invention a separate detector is used
for each gamma line and essentially measures the in-tensity
of only this line. The difficulties which occur when
25. several gamma lines have to be discriminated in one
deteCtor and which lead to considerable reduction in the
achievable counting rate are thus avoided. The pulse
height analysis which is electronically complicated and
lowers -the counting rate can be dispensed with, The
30. individually determined counting rates can be directly
O
7.
used in calculations in a simple manner. With -this
arrangement it is easily possible to use detectors
which each have substantial sensitivity only for the gamma
line which it is to detect. The residual sensitivity
of a detector for the other lines can be compensated for
either arithmetically or by suitable arrangement or
selection if it signi*icantly alters the measured
result. Possible arrangements of the different detectors
are arrangement adjacent to one another or arrangement
10. behind one another in the path of -the radiation. In
addition it is also possible -to split -the ray by means
of radiation splitters, for example in a frequency
selec-tive manner by means of crystal lattices or the like.
The essential advantage of the prior art described above
15c is retained, namely the common irradiation of the volume
to be measured with all the gamma lines used on a single
axis.
The detectors are prefer-ably arranged one behind
the other on the common axis. With the detectors
20. arranged adjacent to one another in the radiation, each
detector sees only a part of the cross-sec-tion of the
radia~tion. Thus slight variations in in-tensity caused
by inhomogeneities in the volume to be measured may cause
inaccuracies. This does not occur when the detectors are
25o arranged one behind the other, since all the detectors
are subjected to the same or the entire cross section of
the radiation. The different gamma lines can be easily
discriminated in -the respectively associated detectors
in two ways. On the one hand selective detectors can be
30. used which are sensitive only -to the energy of one
3~
respective gamma lineO Howeverg even with identical
detectors arranged one behind the other different
gamma energies can still be selectively detected.
The exemplary table below shows the result using two
5. lines (low energy and high energy) and two detectors
(D1 and D2)
Dl D2
10. low 95% 5%
high 20% 16%
D1 and D2 are two identical detectors, of which D2
is arranged behind D1. Both are subjected to the same
15. cross-section of the radiation. The detectors have
differing sensitivity to the two lines and absorb in
each case 95% of the energy incident on it of the low
energy line and 20% of the high energy line. Thus in
the detector D1 95% of the first line is absorbed
(and thus indicated). The second detector can only
receive the remaining 5% and indicates 95% of this,
i.e. approximately 5%. In the first de-tector 20% of
the higher energy line is absorbed. Thus 80% reaches
the second detector and 20% of this 80% is absorbed there~
25. i.e. approximately 16%. Thus it will be seen that the
two lines of differing energy are sufficiently
discriminated in both detectors, namely with difference
factors 95 : 20 or 5 : 16. These ratios can be further
improved by differing thicknesses of the detec-tors. Thus
30. in the present example the thickness of the first
~6~3~
detector D1 can be chosen so -that it absorbs subs-tantially
100% of the line of lower energy~ Then -the second
detector no longer sees this line. This would give
the following result:
Dl D2
low 100% o%
high 21% 16%
10 .
In this way the effort required for performing the
calculation is reduced.
Thus the selective sensitivity to par-ticular
energies can be achieved in identical detectors~solely
by means of their arrangement Such an arrangement
preferably includes an absorber between each adjacent
pair of detectors which permits the passage only of
those gamma energies to which the subsequent detectors
are responsive. If behind one de-tector the line which is
200 to be measured by that detector is blocked by a suitable
absorber which allows the other lines to pass through
substantially unattenuated, -then this absorbed line does
not influence the subsequent detectors, so that
discrimination and subsequent calculation of the values
25. is significantly-simplified. ~ -
The detectors are preferably arranged so as -to be
responsive to increasing energies of radiation in the
direction in which, in use, the radiation passes. In
this way the physical`features of -the detectors and
30. the optional.y interposed absorbers are taken
10~ 3~
into account. Higher energies are more pene-trating and
can still be detected in the las-t detector without
significant attenuation~ whereas lower energies are
b~tter detected at the beginning of the detector chain
because they are more strongly attenuated in the
detec-tors.
The detector or detectors responsive to the lowest
or lower energy lines are preferably scintillation
counters whilst -the detector responsive to t~e highest
10. or higher energy lines are preEerably Cerenkov counters~
These -types of detec-tors are selectively responsive to
the respective energy ranges and are characterised by
high counting rates.
Further features and details of the invention will
15. be apparent from the following description of one
specific construction which is a device which can be
used in marine technology for examining a mixture of
manganese nodules~ sediment and sea water which is gi~en
by way of example only with reference to the following
20~ drawings in which:-
BRIEF DESCRIPTION OF HE DRAWINGS
Figure 1 shows the curves of the energy dependence of
the absorption of gamma energy in three different
substances~
250 Figure 2 shows the spectrum of -two different gamma
lines which are preferably used, and
Figure 3 is a block diagram of a device in accordance
with the invention for irradiation of a test volume
with two gamma lines.
3~
DESCRIPTION OF THE PREFERRED EMBODIMENT
_
Figure 1 shows the different absorption coefficients
of manganese nodules, sediment and sea water as a function
of the energy of the incident gamma radiation used~ It
can be seen that with difiering gamma energies
differences in absorption characteristics are clearly
present and measurable. Figure 2 shows the energy
spectrum of two gamma radiation emissions, namely that
from Americium 241 and Caesium 137. The two lines are
10. each shown at three different heights I, II and ~II
af~ter passing through different media, the associated
reference numerals indicating the medium as follows:
I water
II 7.5% by volume ~uartz sand in water
150 III 8.0% by volume manganese nodules in water.
The X axis of the graph shows the energy of the
gamma radiationg whilst the Y axis shows the number of
impulses produced by a scintillation counter on which
the radiation is incident and thus indicates the intensity
20. of the radiation at different energies, which is to say
at different frequencies. As can be seen, the difierent
intensities resulting from the diiiering absorption
coefficients can be easily evaluated and plotted, each
substance emitting only one pronounced gamma line.
25. One embodiment according to the invention wiIl now
be described with reierence to Figure 3.
A gamrna radiation source 1 ernits two gamma lines a-t
energies El and E2c El is chosen to have a relatively
low energy (see Figures 1 and 2) and E2 has a
30. significantly higher energy. The source 1 may be
12. ~ 3~
Americium241 and Caesium 137 either in discreet lumps
or amalgamated in a form of alloy, or any other convenient
source of two gamma lines. After collimation in a
device 2 the gamma radiation passes through a body 3 to
5~ be examined and is absorbed by detectors ~ and 5,
optionally after further collimation in a device 60 These
detectors should on the one hand be charac-terised by very
small time cons-tants and on the other hand must also be
advantageously selected and dimensioned so that the
10. first detector 4 almost completely absorbs the low energy
radiation, but allows -the high energy radia-tion to pass
through substantially unattenuated. The second de-tector
5 then in practice responds only to -the high energy
components of the gamma radia-tion. An absorber 7 of
150 suitable thickness and atomic number can optionally be
provided between the two detectors. The detectors are
generally (i.e. in light-emitting systems~ connected to
pho-tomultipliers 8a and 8_, in the case of -the first
detector conveniently via a suitably formed optical
20~ guide 9. The pulses from the two counting systems are
counted in respective electronic counters 10 and the
figures thus obtained are passed to a calculating unit 11
which performs calculations with the aid of the
transmission equations given above and reference
2S. parameters 9 which may be obtained as described above 9
and calculates the proportions by volume of the
consti-tuents in the body 3.
In the method described the mixture is irradiated
with the two gamma lines simultaneously and this is
300 important with mixtures of this type which are
13. ~ 3~
inhomogeneous and whose components tend to move
relative to one another since if the irradiation and
measurement of the two gamma lines were carried out
sequentially the irradiated portion of the mixture
5- migh-t have a slightly di~fering composition at the
different times which would lead to errors in -the end
result. If, however, the mixture is completely
homogeneous or entirely static, e.g. solid~ the
irradiation and measurement of the two gamma lines may
10. be carried out either simultaneously or sequentially
and it is to be understood that in -this case the gamma
radiation source may comprise two separate single line
sources which are used sequentially.
A suitable substance for the first de-tector 4 is,
150 for example, Cs~, a scintillator with a time constant
of 0~005 ~s. It only has a low light yield ~3% relative
to doped NaI) and th~s a poor energy resolution (which
is not essential here), but because of its favourable
time constant it permits very high counting rates (up to
20. the region of several MHz.). ~sF is well suited to
discrimination of both gamma energies; a 1 mm thick
detector absorbs 94~0 of a 60 keV radiation, but only
2.5% of a gamma radiation at 1250 keV. The corresponding
figures for a 5 mm thickness are 100% and 12%
25. respectivelyO Since it can be ensured that the second
counter 5 responds exclusively to the high energy
components~ the slight absorption of this radiation in
the first detector can be easily corrected for. Other
possible materials for the first detector - although they
30. have somewhat less favourable discrimination properties -
14. ~3~
are, for example~ plastic scintillators~ preferably with
Sn or Pb doping.
A Cerenkov counter 9 for example 7 is suitable for
the second detectorO If lead glass is used, a high
density and atomic number (and thus very favourable
absorption properties) can be achieved as well as a
high refractive index.
When measuring two gamma lines of different energy,
e.g. from a Co source (1.17 and 1.33 MeV), the high
10. energy component can be detected with a high degree
of efficiency if the photomul-tiplier i5 selec-ted with
regard to low photon yields. This result was confirmed
experimentally. The low energy component of the gamma
radiation is, however, not registered since the energy can
15. be so selected that the maximum speed of the electrons
produced in the counter is below the limiting speed
below which no Cerenkov radiation occurs. In this
way complete discrimination is achieved.
It is a characteristic feature of the Cerenkov effect
20. tha-t the counter responds very quickly to the gamma
quanta (in 10 1 s or less). The temporal limitation
occurs with a considerably slower multiplier (~1 ns),
Thus for the second detector which detects the gamma
quanta of the energy E2~ counting ra-tes in the region
250 of several MHz can be achieved~ ~sF can also be used
as the detector for the high energy radiation. In this
case, if necessary, residual low energy radiation can
be prevented from triggering signals in the second
de-tector by a suitable absorber 7
30. The arrangement described thus permits pulse height~
.
~6~ 4
analysis to be dispensed wi-th completely. The combination
for example of a CsF counter and a Cerenkov counter in a
~'sandwich" arrangement permits counting rates which were
not previously possible and thus measuring times in the
5~ -region of seconds or even less. In this way a truly
continuous measurement is possible9
The device illustrated uses two gamma lines for
determination of -three componentsO By analogy four
components, Eor example, can be determined with three
10. gamma lines.
The described detectors which operate by energy
selection can be arranged adjacen-t to each other in
the path of -the radiation instead of behind one another.
In addition, for example, parts of the radiation may be
150 deflected via crystal lattices and directed to
different detectors arranged at an angle to the common
axis~
It is also possible to dispense with detectors which
only respond to certain energies but completely suppress
200 others. The energy discrimination in the detectors can
be achieved solely by the arrangement of the detectors
one behind another, even when these are sensitive to
all energies, so long as the sensitivity is only energy
dependent. This method was described in greater detail
25. above.
Obviously, numerous modifications and variations of
the present invention are possible in the llght o the
above teachings, It is therefore to be unders-tood that
within the scope of the appended claims the invention
300 may be practised otherwise than as specifically described
hereinO
. ,.
