CA1160364A - Device for determining the proportions by volume of a multiple-component mixture by irradiation with several gamma lines - Google Patents

Abstract

IN THE UNITED STATES PATENT OFFICE APPLICATION OF WALFRIED MICHAELIS and HANS-ULRICH FANGER FOR "DEVICE FOR DETERMINING THE PROPORTIONS BY VOLUME OF A MULTIPLE-COMPONENT MIXTURE BY IRRADIATION WITH SEVERAL GAMMA LINES" A device for determining the proportions by volume of an n-component mixture whose components have differing mean atomic numbers includes a source for irradiating the mixture on a common axis with n-1 gamma lines of different energies, that is to say gamma radiation with n-1 energy peaks at different energies, at which the absorption coefficients are different. A number of radiation detectors equal to the number of gamma lines used are provided each of which measures the intensity of only a respective one of the gamma lines and calculating unit uses the measured intensities to calculate the said proportions by volume, 1.

Description

~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 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
. ,.

Claims (7)

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

1. A device for determining the proportions by volume of an n-component mixture, the components of said mixture having differing mean atomic numbers, said device including radiation means for irradiating said mixture on a common axis with n-1 gamma lines of different energies, the absorption coefficients of said components being different at said different energies, n-1 radiation detec-tors, each of said detectors being responsive to substantially only a respective one of said gamma lines and being arranged to measure the intensity of its respec-tive gamma line after said respective gamma line has passed through said mixture, and calculation means connected to said radiation detectors arranged to calculate said proportions by volume from said measured intensities of said gamma lines.

2. A device as claimed in Claim 1 wherein said gamma lines pass, in use, in a direction of irradiation and said detectors are arranged one behind the other of said common axis in said direction of irradiation

3. A device as claimed in Claim 2 including absorber means between each two of said detectors, said absorber means being adapted to permit the passage only of those gamma lines to which the subsequent detectors in said direction of irradiation are responsive.

4. A device as claimed in Claim 2 wherein said detectors are responsive to gamma lines of increasing energy in said direction of irradiation.

5. A device as claimed in Claim 4 in which that detector which is closest to said radiation means is a scintillation counter and that detector which is furthest from said radiation means is a Cerenkov counter.

6. A device as claimed in Claim 3 wherein said detectors are responsive to gamma lines of increasing energy in said direction of irradiation.

7. A device as claimed in Claim 6 in which that detector which is closestto said radiation means is a scintillation counter and that detector which is furthest from said radiation means is a Cerenkov counter.