Nuclear Instruments and Methods in Physics Research B56/57 (1991) 917-920 North-Holland

917

Industrial on-line bulk analysis using nuclear techniques

G. Vourvopoulos ~e~~rtme~t of Physics and Astronomy, Western Kentucky Uniuersity, Bowling Green, KY 42101, USA

The industrial need for on-line determination of various parameters such as ash content, concentrations of slagging elements (Al, Ca, Si, Fe), concentration of environmentally hazardous S etc., has led into the development of several on-line nuclear techniques. Characteristic examples of these techniques along with their successes and limitations are presented. The need to determine on-line the con~n~ation of a number of elements excitable through different neutron techniques (inelastic scattering, thermal capture, neutron activation) brings into the forefront the use of pulsed neutron generators. The coal industry, cement industry and organizations with concern for hidden explosives could benefit from the development of such devices.

1. Introduction

Chemical and atomic methods have been and con- tinue to be the traditional means for the measurement of various bulk parameters of industrial interest. Param- eters such as the sulfur content of coal, its ash content, the chemical composition of materials in cement, etc., have been measured in an analytical laboratory environ- ment with accuracy and precision. A number of these methods have been standardized by the American Society for Testing Materials (ASTM) and fairly elaborate sampling procedures have been established in order to minimize parameter variation due to erroneous sampling. Most of these methods require the analysis of a representative sample which has been removed from the bulk of the material, i.e. they are methods for off-line analysis.

These off-line procedures, however, do not constitute an accurate record if the elemental composition of the material tested continuously changes. Coal for example, is heterogeneous and its composition depends on its depositional environment and the geochemical and geo- logical history of the strata into which the coal was laid down. Variations exist not only from one coal vein to another but even within the same vein. The mineral composition of coal varies appreciably and parameters such as ash content and moisture must be monitored for the most efficient burning of coal. Parameters such as sulfur on the other hand must he monitored for en- vironmental considerations. In cement production, qual- ity control during the fabrication process must be con- tinuously maintained for product uniformity.

Demands such as the above cannot be met by devel- oping a more accurate laboratory method. What is needed is the development of on-line methodology that would allow the continuous monitoring of the parame- ters deemed important to the industry. Any method

developed for on-line analysis should be able to pro- duce results which are (a) precise, (b) accurate, (c) continuous, and (d) rapid. Furthermore, the method should be able to operate in an instrumentally hostile environment (e.g. a coal conveyor belt, or a cement hopper).

Nuclear methods have been shown to fulfill most of the requirements for on-line analysis. In the particular case of the coal industry, on-line analyzers for ash content, density, and sulfur content are commercially available, and a number of them have been installed in power plants. Reviews of the nuclear techniques as pertaining to the coal industry can be found in refs. [1,2]. These techniques utilize X-rays, gamma-rays and neutrons. In al1 cases, the radiation measured is either result of natural radioactivity or is produced from a radioactive source. The neutrons are also produced from radioactive sources.

In the following section, examples of on-line applica- tions using radioactive sources will be discussed. A brief review will be presented for a method used in the coal industry that has been shown to be successful. Problems associated with the on-line analysis of sulfur will be discussed. Finally, the utilization of neutrons produced from a pulsed neutron generator for on-line analysis in the cement and coal industries and possible other appli- cations will be discussed.

2. Applications using radioactive sources

2.1. Ash content of coal

The ash content of coal is measured by using either X-rays or gamma-rays [3,4]. In the case of X-rays, the principle of the method (ref. [5] and references therein) is based on the variation of the backscattered X-rays

0168-583X/91/$03.50 0 1991 - Elsetier Science Publishers B.V. (North-Holland) XI. ACTIVATION

918 G. Vouroopoulos / Indwtrial on-line bulk analysis

with the Z of the scattering atoms. In a two-parameter model, one assumes that ash is primarily composed of combustible elements with Z = 6 and the mineral frac- tion with Z = 12. The existence of Fe (Z = 26) as a major element in coal with large variations (0.2 to 5 wt.%) must be taken into consideration. This can be done either by the use of Al filters of the appropriate thickness so that the Fe characteristic K X-rays will not reach the detector, or by using a detector of sufficient resolution to differentiate the Fe K X-rays from the backscattered ones.

In the use of gamma-rays, two methods are utilized for measuring the ash content: a two gamma-ray source method and a pair production method. In the two gamma-ray source method, the ash content is calculated from the attenuation of two gamma-rays of different energies [6,7]. The absorption of the lower energy gamma-ray ( < 0.3 MeV) is mostly through the photo- electric effect and has a Z5/A dependence. It thus greatly depends on the chemical composition of the absorbing material. The absorption of the higher energy gamma-ray (0.3 MeV <E, < 3 MeV) is through the Compton effect which has a Z/A dependence. This dependence is the electron density dependence which is related to the density of the medium. The transmitted intensities are combined to give the ash content inde- pendent of weight per unit area [6]. A schematic di- agram of an ash gauge is shown in fig. 1.

In the pair production method, the gamma-ray at- tenuation through pair production is utilized [8]. Such attenuation has a Z2/A dependence, i.e. it is very sensitive to the presence of heavy elements. The 0.511

EHT Signal to amplifier

Sodium iodide Detector

/-collimator

belt

241Am and ‘W3a sources

Fig. 1. Schematic of the SIROASH gauge showing transmis-

sion of narrow beams of 24’Am and ‘33Ba gamma-rays through coal on a conveyor (from Fookes et al. [6]).

*szCf NEUTRON SOURCE PARAFFIN

BORATED EPOXY + Pb SHIELD

Pb SHIELD

(&) VOAL CONTAINER

bl HPGE DETECTOR

Fig. 2. Schematic diagram of the PGNA setup for the de- termination of the sulfur content of coal.

MeV annihilation radiation and the Compton back- scattered radiation are simultaneously measured. It is thus possible to determine the ash content of bulk coal samples with approximately 10% variation from chem- ical laboratory ash measurements.

2.2. Sulfur content of coal

Prompt gamma neutron activation (PGNA) is the nuclear method used in the determination of the S content of coal [9]. Neutrons produced from a source such as 252Cf are partially thermalized and impinge on a coal sample (see fig. 2). Further thermalization occurs within the coal bulk. Upon activation, gamma-rays are emitted from the various nuclides present in the coal sample. Some of the nuclides, such as 32S, have a large thermal neutron capture cross section, so that a suffi- cient number of gamma-rays can be produced within a reasonable time. The gamma-rays are subsequently de- tected, typically with a NaI(T1) detector. The predomi- nant prompt gamma-ray from 32S is at 5420 keV along with its first and double escape peaks. Because of the large number of elements present in coal having large neutron capture cross sections, many other gamma-rays are emitted in the vicinity of the 33S 5420 keV gamma- ray. The resolution of the NaI(T1) detector is not suffi- cient to detect these individual gamma-rays, which are best seen with a high resolution detector such as a high purity germanium (HPGe) detector [lo]. Fig. 3 shows a comparison between a NaI(T1) spectrum and a germanium detector spectrum over the same energy region. The inability of the NaI(Tl) detector to actually measure the S content of coal is apparent. One can therefore only infer the amount of S present after making some assumptions and (or) measurements on the concentrations of the interfering elements. A major difficulty in the development of an algorithm for a NaI(Tl) detection system is the uncorrelated variation of the concentration of these elements. Utilizing a Ge detector, one can measure directly the S content. The

G. Vourvopoulos / Industrial on-line bulk analysis 919

disadvantages however are the low efficiency of the detector, and its resolution degradation due to radiation

damage. The severity of the latter can be lessened by using n-type detectors.

From the above two examples one can see that, in spite of the difficulties and the problems that still persist, nuclear methods can be successful as on-line methods. Certain compromises must be reached among the requirements of speed of measurement, precision, cost, and radiation safety.

3. Applications using neutron generators

Not all elements are amenable to analysis via ther- mal neutrons. One element that is not is oxygen. It is an important element in the cement industry where the accurate determination of lime (CaO), silica (SiO,),

alumina (Al,O,) and iron oxide (Fe,O,) is required for quality control. In the on-line detection of hidden ex- plosives, oxygen is one of the three elements (the other two being C and N) the determination of which can narrow the number of false alarms. Oxygen, however, has a low thermal neutron capture cross section and can only be measured with fast neutrons. The (n,n’y) reac- tion has a threshold energy of 6.4 MeV and an in- tegrated cross section of 96 mb for En = 14.1 MeV [ll]. The 160(n, p)16N reaction has a threshold energy of 10.3 MeV, and a cross section of approximately 38 mb. Because the i6N nucleus decays with a half-life of 7.13 s, although the (n,p) cross section is smaller, the 6.13 MeV gamma-ray of I60 would be easier to observe via activation analysis than through the (n, n’v) reaction. In order to be able to detect the largest number of ele- ments in the cases where on-line elemental composition is of interest, it would be desirable to have available all

Fig. 3. Portions of gamma-ray spectra between 4.2 MeV and 5.8 MeV resulting from the bombardment of a coal sample with thermal neutrons. The lower spectrum was taken with a HPGe detector and characteristic gamma-rays of various elements in the vicinity of

the 5420 keV sulfur full energy peak and its first escape and double escape peaks are seen. They are marked with their energy in keV,

the element producing them and by the characterization as a single escape (S) or a double escape (D) peak. The upper spectrum is

taken with a 5 X 5 NaI(Tl) detector with an 8% resolution. The vertical scale for the two spectra is different.

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920 G. Vourvopoulos / Industrial on-line bulk analysis

three reactions, namely, (n,n’y), (n, y), and neutron activation. Such a task can be accomplished with the use of a pulsed neutron generator. These generators employ the D-D and D-T reactions for neutron pro- duction, and their design and performance characteris- tics vary depending on the principle of their operation and the manufacturer [12-141. These devices typically produce series of pulses of a few us with a repetition rate either of 10 Hz or variable to 10 kHz and a fast neutron yield of 106-lo8 neutrons/pulse. An on-line system that we are developing for sulfur analysis utilizes a 10 us neutron pulse followed by a 20 ms pause. The gamma-rays from the thermal neutron capture are acquired in the first 200 us of the pause followed by the accumulation of gamma-rays from neutron activation. A system developed for quality control in the cement industry [15] utilizes a 100 ps pulse followed by a 200 us pause, with a 20 ms pause every 100 pulses. In this latter arrangement, the elements of primary importance Si, Ca, and Fe are measured from neutron capture and 0 from neutron activation.

One further advantage in the utilization of neutron generators is that of radiation safety. The use of either

an 241Am-Be or 252Cf source, because of their high activity, requires stringent radiation protection and safety rules, Massive shielding is needed whether or not the source is used. In the cases of the on-line analysis performed in a public place (e.g. an airport luggage conveyor belt) added concern would be the deliberate detonation of an explosive device hidden in a piece of luggage to be analyzed. In a pulsed neutron generator, the neutron emission occurs through the deutetium acceleration via electronic means. The only radioactive substance present would be the small amount of tritium within the generator itself. The accidental release of tritium would not constitute a major radioactivity con- tamination.

4. Conclusions

Nuclear methods have been successfully utilized for on-line analysis in a number of industrial concerns. In

many cases these methods have been commercialized and are routinely available. The majority of the meth- ods developed utilize radioactive sources, possibly be- cause of the industrial demand for continuous and reliable performance. Neutron generators, however, especially of the pulsed type have advantages that war- rant an effort towards their further development. We could expect to see an increase of information on their uses and performance as more research is performed in that direction.

References

[l] Nuclear Assay of Coal, Electric Power Research Institute Report EPRI CS-989 (EPRI, Palo Alto, 1984).

[2] Gamma, X-Ray and Neutron Techniques for the Coal Industry, Advisory Group Proc. (IAEA, Vienna, 1986).

[3] A. Trost, Radioisotope Instruments in Industry and Gec- physics (IAEA, Vienna, 1966) p. 435.

[4] I.S. Boyce, C.G. Clayton and D. Page, Nuclear Tech- niques and Mineral Resources (IAEA, Vienna, 1977) p. 135.

[5] C.G. Clayton and M.R. WormaId, Int. J. Appl. Radiat. Isot. 34 (1983) 3.

[6] R.A. Fookes et al., Int. J. Appl. Radiat. Isot. 34 (1983) 63. [7] T. Gozani, J. Stenftenagel and P. Shea, Nuclear Assay of

Coal, Report EPRI CS-989 (Palo Alto, 1984) p. 2-1. [8] B.D. Sowerby and V.N. Ngo, Nucl. Instr. and Meth. 188

(1981) 429. [9] T. Gozani, in: Capture Gamma-Ray Spectroscopy and

Related Topics 1984, ed. S. Raman (American Institute of Physics, New York 1985) p. 828.

[lo] G. Vourvopoulos and P.C. Womble, Nucl. Instr. and Meth. B36 (1989) 200.

[ll] W.J. McDonald, J.M. Robson and R. Malcolm, Nucl. Phys. 75 (1966) 353.

[12] D.H. Jensen, IEEE Trans. Nucl. Sci. 28 (1981) 1685. [13] P. Bach, in: Applications of Nuclear Techniques, eds. G.

Vourvopoulos and T. ParadeBis (World Press, Singapore, 1991) p. 321.

[14] R.W. Hamm, these Proceedings (11th Int. Conf. on the Application of Accelerators in Research and Industry, Denton, TX, 1990) Nucl. Instr. and Meth. B56/57 (1991) 1039.

[15] J.P. Barron and L. Debray, in: Applications of Nuclear Techniques, eds. G. VourvopouIos and T. Paradelhs (World Press, Singapore, 1991) p. 268.