GB2100421A - Neutron method for elemental analysis independent of bulk density - Google Patents

Abstract

Apparatus for quantitatively measuring the concentration of a first element or a material containing the first element in a sample (3) containing the first element and other element or elements comprises source (1) yielding neutrons having energy sufficient to produce first gamma -rays by neutron inelastic scattering or thermal neutron capture by atoms of the first element and other gamma -rays by neutron inelastic scattering or thermal neutron capture by atoms of the other element or elements, detector means (7) adapted to detect the first gamma -rays and the other gamma -rays, shield means (6, 8) associated with the detector means to reduce the intensity of direct source gamma - rays and neutrons, energy analysis means (10) associated with the output of the detector means adapted to distinguish between the first gamma -rays and the other gamma -rays and calculating means (11) associated with the output of the energy analysis means to calculate the concentration. Directions in the measured carbon to hydrogen ratio from that of dry coal (i.e. from the same seam) enable the moisture content of coal to be determined. Sulphur in lead ore may be measured using both sulphur and lead measurements (the concentration of lead remaining essentially constant). A density determination of the sample is not required. <IMAGE>

Description

SPECIFICATION Neutron method for elemental analysis independent of bulk density The present invention provides an improved apparatus and method of elemental analysis based on the simultaneous measurement of the intensities of two or more neutron-induced 1,-rays from different elements in a sample. These intensities are combined to yield an elemental abundance which is essentially independent of bulk density.

The method is particularly applicable to situations where either (a) one of the elements is of fixed or known concentration or (b) there is known relationship between two of the elements in the sample. Two favourable potential applications of the technique are for the analysis of sulfur in lead sinter feed product and the analysis of moisture in coal or other organic material.

When a sample is irradiated with fast neutrons, these neutrons interact with the sample to produce prompt y-rays mainly by inelastic scatter and thermal neutron capture. As well, thermal neutron capture can lead to the formation of an unstable nucleus which emits delayed y-rays. Measurement of the intensity of these prompt or delayed 1,-rays can be used for elemental analysis of static bulk samples or for on-line analysis (e.g. of ores on conveyor belts).

Work in this field has recently been reviewed by Nargolwalla, S. S. et al., in "Nuclear Methods in Mineral Exploration and Production", ed. J. G.

Morse (Elsevier, Amsterdam, 1977), p.113. and Berry. P. F. and Martin, T. C., Adv. Activation Analysis2 (1972) 89.

Accurate elemental analysis often requires a method of compensating the 1,may yields for changes in bulk density. Methods of compensation include separate measurement of y-ray transmission, reported by Gozani, T. et al AN L-7 9-62, 266, and by Holmes, R. J.

Messenger, A. J., and Miles, J. G., Prow; Australas.

Inst. Min. Metall. 274 (1980), 17., separate measurement of "matched" y-ray back-scatter, defined in United States Patent No.4,314,155 and Sowerby, B. D., Nuclear Instruments and Methods, 160(1979)173 and normalisation to another peak in the y-ray spectrum, reported by Stewart R. F., Hall, A. W., Martin, J. W.i Farrior, W. L. and Poston, A. M., U.S. Bureau of Mined Technical Progress Report 74, Jan. 1974 and Hall, A. W., Martin J. W., Stewart R. F. and Poston, A. M.

U.S. Bureau of Mines Report BM-Rl-8038 (1975).

The work reported by Stewart et al., is the closest in concept to the present invention. This work concerns the development of neutron capture techniques for the on-line bulk analysis of sulfur in coal. The coal was contained within a bin of diameter 1 m and height 4 m and sulfur was measured using the 5.42 MeV thermal neutron capture y-ray from sulfur. One method used for the compensation of variations in moisture and bulk density was to normalise the measured sulfur y-ray yield to the area under the 2.22 MeV H capture peak. However this procedure alone will not determine elemental abundance independent of bulk density except over small ranges of variation in moisture and ash. Variations in these parameters will affect the hydrogen content. For example, consider a coal containing 20 wt. % ash, 0% moisture and 4 wt.% H.If moisture increases to 5 wt. %. the hydrogen content of the sample will increase to approximately 4.5 wt %. Also, if ash increases to 25 Wt %, the hydrogen content will drop to about 3.75 wt. %. In fact, Stewart et al. found that hydrogen normalisation did not improve the accuracy of results.

The above discussion relates mainly to on-lihe and bulk analysis of samples. Another application area in which neutron techniques are used for elemental analysis is borehole logging, such as reported by Caldwell, R. L. et al., in "Nuclear Techniques and Mineral Resources", (Proc. Symp.

Vienna, 1977), IAEA, Vienna (1977), p.3. In this area, techniques have been recently developed for the determination of elemental ratios (carbon/oxygen and silicon/calcium) to distinguish oil/water ratios in porous rocks. The technique depends primarily on neutron inelastic scatter.

However, the technique is not used to obtain elemental concentrations.

The measured flux Ffi of a promp 1,may produced by neutron interaction in a sample is given by an equatiaon of the form:

where 0(r)=neutron flux r=radial distance from source and detector nl=number of 1,-rays produced per neutron interaction in element p=bulk density NO=Avogadro's number Cl=concentration of element A,=atomic weight of element neutron cross section f(ss4r)=function of y and r (,u-ray absorption coefficient) v=volume considered The form of this equation is valid for both neutron inelastic scatter and thermal neutron capture.

To a first approximation the ratio of measured y-ray yields Fl and Fj is given by

This equation is accurate for two 1,-rays of similar energy produced by the same interaction process (i.e. inelastic scatter or neutron capture) but can be subject to errors when the 1,-rays are produced by different interaction processes. As the values of n, a and A are known for elements i and j, the ratio F*/FI can be used to deduce CZ if C is known.

To a-good approximation, the result will be independent of bulk density.

In general, it is difficult to determine exact yray yields independent of background and a correlation equation of the following form can be used C1=a F,+b FJ+C where a, b, c are constants The method therefore can be used for the measurement of elemental concentration independent of bulk density where the concentration of one element is known or where the relationship between elemental concentrations are known.

The method and apparatus of the invention find special use in the determination of moisture in coal. The most common nuclear method for the measurement of moisture is that based on the slowing down of fast neutrons by hydrogen in the sample, reported in international Atomic Energy Agency, "Neutron Moisture Gauges", Tech.

Report Series No. 112, IDEA, Vienna, (1970). This method is relatively straight forward for many applications provided the results are compensated for bulk density and matrix effects.

However a neutron moisture gauge based on this principle is incapable of distinguishing between hydrogen in coal and hydrogen in water. As coal matter contains about 5 wt. % H and as 1 wt. % water contains only 0.11 wt. % H, variations in the hydrogen content of coal must be very small for accurate moisture measurements.

One method for the measurement of moisture in bulk coal samples involves the accurate determination of carbon and hydrogen using a neutron gauge and a matched y-ray backscatter gauge, described in U.S. Patent No.4,314,155, and reported by Sowerby, B. D., Nuclear Instruments and Methods, 160(1979)173.

However, if carbon and hydrogen assays are not required, moisture in coal can be determined with a y-ray backscatter gauge using the present method.

This results in a considerable simplification for the accurate determination of moisture in coal.

In the present preferred method, moisture is determined essentially from deviations in the measured carbon/hydrogen ratio from that of dry coal. From the published assays of 112 Australian black coals, Joint Coal Board/Queensland Coal Board, Australian Black Coals Report, Sept. 1976, the dry coal ratio is 16.45+1.78. Deviations in this ratio are much reduced for coal from a particular seam. A 1 wt % change in moisture will cause the C/H ratio to change by about 2.8% relative.

The present invention therefore provides a measuring apparatus for quantitatively measuring the concentration of a first element in a sample containing said first element and other element or elements, said apparatus comprising a source yielding neutrons having energy sufficient to produce first 1,-rays by neutron inelastic scattering or thermal neutron capture by atoms of said first element and other 1,-rays by neutron inelastic scattering or thermal neutron capture by atoms of said other element or elements, detector means adapted to detect said first 1,-rays and said other y-rays, shield means associated with said detector means to reduce the intensity of direct source 1,-rays and neutrons, energy analysis means associated with the output of said detector means adapted to distinguish between said first 1,-rays and said other 1,-rays and calculating means associated with the output energy analysis means to calculate said concentration.

The invention also provides a method for quantitatively measuring the concentration of a first element in a sample containing said first element and other element or elements, said method comprising combining the results of (i) a measurement of first 1,-rays generated by neutron inelastic scattering or thermal neutron capture by atoms of said first, element, and (ii) at least one measurement of other 1,-rays generated by neutron inelastic scattering of thermal neutron capture by atoms of said other element or elements.

Suitable sources include various (a,n) sources with beyllium targets (e.g. plutonium-238/Be, radium-226/Be, americium-241/Be, plutonium239/Be, polonium-210/Be, actinium-227/Be), spontaneous fission sources (e.g. californium252) and neutron generators utilising the (d,T) and (d,d) reactions.

Suitable y-ray detectors include scintillation detectors such as Nal(TI), Csl(Na) and bismuth germanate and solid state detectors such as Ge(Li) and intrinsic Ge.

Suitable neutron shields contain a thermal neutron absorber such as boron or lithium. They can also contain hydrogen to slow the neutrons to thermal energies. The shield between source and detector is primarily to reduce the flux of source 1,-rays reaching the detector. Suitable materials are the high density metals such as tungsten, lead and bismuth.

It is preferred that the signal or signals from the detector are amplified preferably in a pre-amplifier or gain stabilised amplifier before being fed into the energy analyses means (energy analyser). The energy analyser can comprise a hardwired multichannel analyser (e.g. Hewlett-Packard 5401 B, Canberra models 30, 35, 40 or 80, Nuclear Data Models 60, 600, 660). Alternatively, it may comprise an analog-to-digital converter (either Wilkinson-type or successive approximation) interfaced to a computer, or it may comprise a series of single channel analysers.

The calculating means is preferably a computer or calculator to calculate the elemental concentration from the measured count rates. A suitable computer would be the PDP 1 1/03: The preferred embodiments are described with reference to the drawings in which: Fig. 1 is a cross sectional view of an assembly used in a determination according to the invention; Fig. 2 is a pulse-height spectrum obtained using the assembly shown in Fig. 1; Fig. 3(a) shows the net count rate of 2.22 MeV H capture 1,-rays plotted against chemical laboratory moisture; Fig. 3(b) shows the chemical laboratory moisture plotted against the measured intensity of both 2.22 MeV H capture 1,-rays and 4.43 MeV C inelastic scatter y-rays.

Fig. 4 shows chemical laboratory sulfur plotted against the measure intensity of both 2.23 MeV S inelastic scatter 1,-rays and 2.62 MeV Pb inelastic scatter 1,-rays.

The method was tested by measuring the moisture content of coal samples using the experimental assembly shown in Fig. 1. In this assembly a neutron source 1 (e.g.238Pu-Be) emits 4.43 MeV 1,-rays and neutrons, which neutrons follow a path such as 2 into a sample 3 (e.g. a coal sample) where they strike atoms of the element being determined at 4 thereby producing 1,-rays which follow a path such as 5 through a neutron shield 6 to a detector 7 which is shielded from the neutron source 1 by a further shield 8. A suitable shield 6 is boron trioxide which shields the detector 7 from scattered neutrons whereas tungsten, lead or bismuth would be suitable as shield 8 to shield the detector 7 from the source 1. A suitable detector 7 would be a 1 50 mm diameterx 100 mm thick Nal(TI) crystal.

Pulses from a Nal(Tl) detector in the neutron assembly were amplified using a gain stabilised amplifier 9 and fed into a Hewlett-Packard 5401 B multichannel analyser 10. Count rates and backgrounds were determined by setting windows in the pulse height spectra. A typical pulse-height spectrum is shown in Fig. 3.

Elemental concentration was calculated from the measured count rates by PDP 11/03 computer 11.

The source and detector may also be located on opposite sides of the sample when the sample under analysis is of substantially uniform thickness.

In figure 2 curves B and C represent 10 and 50 times magnification, compared with curve A.

Peak 1 9 represents 1.78 MeV Si, 20 represents 2.22 MeV H, 21 represents 3.92 MeV 12C escape and 22 represents 4.43 MeV 12C. The 3.92 MeV peak results from the escape of one 0.511 MeV annihilation y-ray from the interaction of a 4.43 MeV '2C y-ray detector.

Fig. 3(a) shows the net count rate of 2.22 MeV H capture 1,-rays plotted against chemical laboratory moisture. These results were obtained using one coal sample with added water and in which the bulk density was deliberately varied by compaction. The 2.22 MeV count rate shows a very poor correlation with moisture. However if moisture is correlated with the measured intensities of both the 2.22 MeV H capture y-ray and the 4.43 MeV C inelastic scatter y-ray using equation (3), the moisture can be determined to within 0.36 wt. % as shown in Fig. 3(b). No improvement in accuracy is obtained using the combination neutron and y-ray backscatter method, U.S. Patent No.4,314,155 and Sowerby, B. D., Nuclear Instruments and Methods, 160, (1979) 173.

The method can be used for the measurement of moisture in any organic material such as agricultural products (e.g. wool, cotton, wheat) for which the dry C/H ratio is fairly constant.

The method and apparatus of the invention also find use in the analysis of sulfur in lead smelter samples. In many lead smelter plants, lead concentrate is treated in a sinter machine prior to smelting in a blast furnace. The sinter machine converts lead sulfide to lead oxide and creates a lump size suitable for the blast furnace.

There is a lot of interest in on-line sulfur determination particularly of sinter feed to permit better control of sinter machines.

This application is ideally suited to the present technique as the concentration of lead remains constant to within about 5% relative. The energies of the prominent inelastic scatter 1,-rays from sulfur (2.23 MeV) and lead (2.62 MeV) are sufficiently similar for equation (2) to be an accurate approximation.

The method has been tested using nine samples of Mount Isa sinter feed, product and returns on a neutron assembly similar to that shown in Fig. 1. The results in Fig. 4 show that sulfur can be determined to within 0.24 wt. % using the intensities of 2.23 and 2.62 MeV yrays. For comparison the rms deviation between sulfur content and 2.23 MeV yield alone was 0.6 wt. %. The rms deviation between sulfur and 2.23 MeV intensity together with y-ray backscatter U.S. Patent No.4,314,155 was 0.5 wt. % S.

An alternative analysis method is also possible using the 5.42 MeV neutron capture y-ray from sulfur.

Claims (13)

Claims

1. Measuring apparatus for quantitiatively measuring the concentration of a first element in a sample containing said first element and other element or elements, said apparatus comprising a source yielding neutrons having energy sufficient to produce first 1,-rays by neutron inelastic scattering or thermal neutron capture by atoms of said first element and other 1,-rays by neutron inelastic scattering or thermal neutron capture by atoms of said other element or elements, detector means adapted to detect said first 1,-rays and said other y-rays, shield means associated with said detector means to reduce the intensity of direct source 1,-rays and neutrons, energy analysis means associated with the output of said detector means adapted to distinguish between said first 1,-rays and said other 1,-rays and calculating means associated with the output of said energy analysis means to calculate said concentration.

2. The measuring apparatus as defined in claim 1 , wherein said source is an (cox, n) source with a beryllium target, a spontaneous fission source or a neutron generator utilising the (d,T) and (d,d) reactions.

3. The measuring apparatus as defined in claim 2, wherein said (a,n) source is 23sPu/Be, 226Ra/Be, 241Am/Be, 239Pu/Be, 210Po/Be or 227Ac/Be.

4. The measuring apparatus as defined in claim 2, wherein said spontaneous fission source is 252Cf.

5. The measuring apparatus as defined in any one of claims 1 to 4, wherein said detector means is a scintillation detector or a solid state detector.

6. The measuring apparatus as defined in claim 5, wherein said scintillation detector is Nal(TI), Csl(TI), Csl(Na) or bismuth germanate.

7. The measuring apparatus as defined in claim 5 wherein said solid state detector is Ge(Li) or intrisic germanium.

8. The measuring apparatus as defined in any one of claims 1 to 7, wherein said energy analysis means is a multi-channel analyser, an analog to digital converter or a plurality of single channel analysers.

9. A method for quantitatively measuring the concentration of a first element in a sample containing said first element and other element or elements, said method comprising combining the results of (i) a measurement of first 1,-rays generated by neutron inelastic scattering or thermal neutron capture by atoms of said first element, and (ii) at least one measurement of other 1,-rays generated by neutron inelastic scattering or thermal neutron capture by atoms of said other element or elements.

10. The method as defined in claim 9, wherein said sample is coal and said first element is hydrogen or sulfur.

11. The method as defined in claim 10, wherein the measurement of hydrogen content is used to quantitatively determine moisture content.

12. The method as defined in claim 9 wherein said sample is lead sinter feed and said first element is sulfur.

13. Measuring apparatus for quantitatively determining the concentration of a first element in a sample containing said first element and other element or elements, said apparatus being substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.