Industrial applications of activation analysis with 14 MeV neutrons

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A N D M E T H O D S 92

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INDUSTRIAL A P P L I C A T I O N S OF ACTIVATION ANALYSIS W I T H 14 MeV NEUTRONS D. E. W O O D

Kaman Sciences Corporation, Colorado Springs, Colorado 80907, U.S.A.

1. Introduction Activation analysis is a promising technique for both laboratory and industrial process control problems. Laboratory systems have been in use for many years to analyze small samples of material with the results occasionally applied to industrial problems. Recent developments in equipment and techniques make it possible to consider the placement of sample analysis systems near the production floor for intermittent sampling and control over production processes as well as for extension to on-line and "in-situ" analysis problems. The potential for application of individual sample analysis systems arises because they can do certain special analyses fast enough to be applied directly to production control problems in the metals and feed industries, among others. In the metals industries the primary elements of interest are oxygen and silicon. In the feed industry, nitrogen is analyzed as a measure of protein content. The intermittent sampling results available from these systems are quite adequate for tightening production controls, since the results are obtained much faster than from conventional chemical methods. Continuous on-line analysis for solids on belts or liquids in pipes is similar in principle to the sample analysis system except that the transport method consists Of the belt or pipe itself. This technique is somewhat more difficult, but actual tests have been made for copper ore sorting, analysis of coal, silicon in taconite, fluorine in water, sodium to phosphorus ratio in detergents, palladium and uranium in water, nitrogen in slurries, sulfur in oil, and trace metals in oil. Recent developments of miniature sealed tube ion accelerators have permitted the possibility of "insitu" analysis since the accelerator can be moved with the detector, if necessary, instead of the sample. This technique has been tested for analysis of oxygen in titanium welds and the analysis of calcium and phosphorus in human beings. Field systems, using a similar technique, have found applications in the areas of oil and mineral surveys. Development work is proceeding for systems for remote analysis of the lunar surface and ocean beds.

excellent results using differentially pumped accelerators, solid state detectors as well as sodium iodide, multichannel analyzers, and computers to handle the data that comes out. For typical industrial or field applications, these subsystems should be simplified in order to improve reliability and reduce maintenance. Actually, even the complex subsystems mentioned have been used successfully in field applications on trucks. However, the level of reliability achieved was not adequate to provide the type of system that industry would really like to have. The accelerator can be simplified by using the newer sealed tube type of miniaturized ion accelerators, which eliminate the tritium handling problems and are usually more highly automated to provide simpler operation. The detector for industrial systems is usually a NaI(T1) crystal because of the difficulty of maintaining a supply of liquid nitrogen for a Ge(Li) detector in the field or factory. Single channel analyzers are preferred to the multichannel in order to provide simple data output that is easy for an operator to interpret. Gain stabilizers can be employed to avoid the need for frequent recalibration or resetting of windows during long term operation in the field. At first glance it appears undesirable to use something as complex as a computer to handle data output for industrial or field applications. However, remote terminals have been developed which are suitable for industrial applications, and small rugged computers have been developed which are useful for field operation in a truck to provide complete data analysis on site. Thus computers are becoming accepted as part of a complete system used for activation analysis.

2. Industrial equipment Laboratory activation analysis systems have achieved

3. Sample analysis systems The simplest analysis by 14 MeV neutrons is the analysis of oxygen, typically in metals1). The analysis is performed through the 160(n,p)16N reaction, where the product 16N emits gamma rays at 7.1 and 6.1 MeV with a half-life of 7.35 sec. The high gamma ray energy permits the analysis to be made with essentially no interference except from fluorine. The short half-life allows a very fast analysis to be made with typical analysis times ranging from 30 sec to 1½ min. Since the standard vacuum fusion technique is difficult to perform and uncertain in results on refractory metals,

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activation analysis has become a very attractive alternative to the earlier technique. In fact, activation is rapidly becoming accepted as the primary standard for oxygen analysis in metals. The National Bureau of Standards has recertified its oxygen in steel standards by neutron activation analysis. The activation system includes a neutron source for activation of the samples, a pneumatic transfer system to move the sample back and forth between neutron source and detector, NaI(TI) detectors of various sizes up to 5"× 5", and a single channel counting system to select the high energy gammas from 16N. The pneumatic system is usually a dual tube system so that a sample and a reference can be irradiated together, and then counted either simultaneously or sequentially for the analysis. The reference serves as a neutron flux monitor and is needed since the output of a neutron generator is constant only within a few percent, which is sometimes not good enough for the precision required in the analysis. Rotation of the sample and reference around two axes is customarily performed to provide the highest precision. At least ten systems of this type have been constructed for use in the metals industry with the primary application the analysis of oxygen. Silicon is another element which is easily done by fast neutron activation analysis through the 28Si(n,p)ZSAl reaction. The product 28A1 emits a gamma ray of energy 1.78 MeV with a 2.3 min half-life. Silicon is of interest in both the metals industry and geology. Several geological laboratories are involved in making major constituent analyses of rock samples, including lunar samples, by analyzing silicon, aluminum, and oxygen to give an almost complete analysis of rock composition2'3). The aluminum is analyzed through the 27Al(n,p)27Mg reaction. The product 27Mg emits gamma rays of 0.84 and 1.01 MeV with a half-life of 9.5 min. In the food industry, the analysis for protein is customarily done by analyzing total nitrogen and then applying a factor to convert the nitrogen to protein. The 14N(n,2n)13N reaction can be done by fast neutrons 4) in order to provide a fast analysis for protein in food samples. ~3N is a positron emitter with a 10 rain half-life. The annihilation gammas at 0.51 MeV are counted, either singly or in coincidence, as a measure of the nitrogen content of the product. Kaman has developed a system which can perform this analysis at the rate of 30 samples per hour. Since the classical Kjeidahl technique takes several hours to perform, the activation technique offers the possibility for process control, with answers being

provided to adjust the industrial process in less than 20 rain. This analysis requires two channels since interference is present from 28A1 formed from silicon as well as the 31p(n,~)28A1 reaction. The 28A1 is counted in a second channel and a predetermined fraction of this activity is subtracted from the 0.51 MeV photopeak activity in order to subtract off the Compton background from 28A1. A blank resulting from 13N formed by proton recoils on 13C and 160 must be subtracted. This feature limits the nitrogen analysis to contents of 0.5% nitrogen or above. A system of this kind has been installed at the Ralston Purina Company in St. Louis, Missouri, for analysis of all types of ingredients of animal feeds as well as finished products. The analysis has been shown to correlate well with the classical Kjeldahl analysis over the range 5 to 95% protein. The above examples are the most popular analyses in the industrial area. However, more than 30 elements can be analyzed by fast neutron activation, depending upon the appropriate combination of concentrations and matrix interference. Many of these applications are being developed for later use in special industrial problems.

4. On-line systems The use of activation analysis to determine components in a flowing system in a pipeline or on a belt is very attractive. The possibility exists to analyze components without disturbing the material, changing its shape or form, or contacting it in any way. This type of analysis has been tested in several laboratories, but industrial applications are only now beginning. The system shown in fig. 1 was constructed at Kaman to test the application of liquid loop systems for several different analysesS). The delay loops are included in order to allow optimization of decay time of the material for analysis of selected elements of

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A P P L I C A T I O N S OF A C T I V A T I O N ANALYSIS WITH 14 MeV N E U T R O N S

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The belt type of analysis system is shown in fig. 2 which illustrates the system constructed by the Texas Nuclear Company. In belt systems, higher neutron output is usually required since efficient geometries are more difficult to obtain with solid materials. The belt speed is varied to obtain the appropriate decay time between irradiation and counting. However, the available variation in belt speed or distance between neutron source and counter is quite limited, so interference from other elements is typically present. When a fast neutron generator is employed, substantial interference usually arises from the 160(n,p)lON reaction. Isotopic sources can be used to obtain a neutron energy low enough to avoid activating the oxygen, but neutron output is too low to provide efficient analysis for belt systems. The system shown here has been tested for the analysis of oxygen and silicon in coal, silicon and aluminum in bauxite, cement raw mix, and tar sands, silicon and iron in taconite, and silicon and copper in copper ore. For the copper analysis, the interference from silicon was quite severe and required a decay time of about 10 rain to allow analysis of the lower levels of copper. The belt speed required to obtain this long a decay time is quite slow (about 2 ft per minute), so it would be necessary to use a side stream rather than the main conveyer belt to handle this analysis.

differing half-lives. The speed of the pump may also be varied to change delay times as well as providing different flow rates. The storage barrels are used to allow decay of activated material before retesting as well as to provide differing raw materials for analysis of mixtures. This system was used to test the measurement of salt in water, sulfur in oil, sodium to phosphorus ratio in detergents, and trace metals in hydraulic fluid and water. The Academy of Mining and Metallurgy in Krakow, Poland, has used a liquid loop system to test the feasibility of measuring water, salt, and sulfur in crude oi16). The Dow Chemical Company in Midland, Michigan, has tested the application of activation analysis in liquid loops for the determination of fluorine, silver, and selenium in waterY). The University of Toronto, Ontario, Canada, has shown the application of such a system for the measurement of fluorine, vanadium, and uranium in water solutionsS). The Texas Nuclear Company in Austin, Texas, has studied the analysis of iron and silicon in taconite slurries, and silicon and aluminum in both cement raw mix and bauxiteg). In some of these applications, the neutron output required was not very high since tubing containing the material could be wrapped around the neutron source and detector to give very efficient geometries.

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5. In-situ analysis For field exploration or other situations where the sample is too large to move it, it is convenient to place the neutron generator over the sample for irradiation and then take it away to place the detector in position for counting. This "in-situ" type of analysis eliminates the pneumatic transfer system for samples, but adds the complication of a handling system for the neutron generator and detector. One example of this type of measurement is the analysis of oxygen inclusions in titanium welds in structures which are too large to be moved for analysis. A laboratory test of this technique was performed at K a m a n using equipment shown in fig. 3. The rolling dolly carries the neutron generator and the sodium iodide detector and moves back and forth across the weld position to analyze the oxygen. In the field the entire apparatus would have to be moved along the weld in steps to provide a complete scan of the welded piece. The apparatus was successful in detecting deliberate inclusions in several test welds with a spatial resolution of about one inch which was determined by the collimator opening in front of the detector. Measurement of the absolute amount of oxygen included is complicated by the attenuation in the plate itself of the g a m m a rays from activated oxygen. Thus an ab-

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solute measurement was not made, although the amounts appeared to be correct on a relative basis as compared to the measurements provided when the weld was fabricated. Several groups in the U.S. under contract to NASA are developing a miniaturized activation system for landing on lunar and planetary surfaces. The intent is to obtain an analysis of the major constituents of the surface at the point where the vehicle lands. This application involves the analysis of silicon, aluminum, oxygen, and possibly one or two other elements if the concentrations are high enough. Geological applications are a promising field for "in-situ" activation analysis. The USGS has developed a portable, truckmounted system which can detect silver on or near the surface of the ground~°). The l°9Ag(n,7) 1l°Ag reaction is used for this analysis. The product nucleus, 11°Ag, emits a 0.66 MeV gamma ray with a 24 sec half-life. Several promising tests have been made by the U S G S to indicate that silver can actually be found in relatively low grade ore. Other groups are working on development of an activation system to be placed down a bore hole for elemental analysis under the ground. The intention here is not just major constituent analysis, but also detection and identification of trace minerals of commercial value.

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A P P L I C A T I O N S OF A C T I V A T I O N A N A L Y S I S W I T H 14 MeV N E U T R O N S

Within the next year or so this technique should be tried for prospecting both at the ground surface and in bore holes. A slightly different type of"in-situ" analysis is wholebody activation analysis for calcium 11). Total body calcium is of interest in diseases involving bone deterioration as well as the phenomenon of calcium loss that occurs in the weightless state of space flight. In these studies, the 48Ca(n,7)49Ca reaction is used as a measurement of total body calcium. 4 9 C a emits a g a m m a ray of 3.1 MeV with a half-life of 8.8 min. Since thermal neutrons are used, parts of the body are covered with polyethylene moderator and the body itself is used to continue the moderation process of the neutrons. It appears possible to measure total body calcium to within 5% in this technique and to measure changes in calcium on the same individual to about 2%. Various groups from several countries are currently active in this study using both fast neutron generators and cyclotrons.

6. Summary Fast neutron activation analysis is a promising technique for laboratory and industrial analysis with emphasis on quality control applications. Laboratory studies have indicated that a number of analyses can be performed which could be useful in process control

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applications. A few of these applications have actually reached the industrial stage and further growth in this field is expected as the equipment improves in reliability and simplicity, and industrial managers begin to learn the potential benefits in their process control problems

References i) k. C. Pasztor and D. E. Wood, Talanta 13 (1966) 389. 21 W. D. Ehmann and J. W. Morgan, Science 167 (1970) 528. a) H. A. Vincent and A. Volborth, Nucl. Appl. 3 (1967) 701. 4) W. H. Doty, D. E. Wood and E. L. Schneider, J. Assoc. Off. Anal. Chem. 52 (1969) 953. 5) D. E. Wood, Development of fast neutron activation analysis for liquid loop systems, Report no. TID-24029 (Kaman Sciences Corporation, July 1967). 6) L. Gorski, J. Janczyszyn and L. Loska, Proc. 3rd Intern. Conf. Modern trends in activation analysis (National Bureau of Standards, Gaithersburg, Maryland, 1968) p. 420. 7) O. U. Anders, Nucleonics 20, no. 2 (Febr. 1962) 78. s) R. E. Jervis, H. Al-Shahristani and S. S. Nargolwalla, Proc. 3rd Intern. Conf. Modern trends in activation analysis (National Bureau of Standards, Gaithersburg, Maryland, Oct. 1968) p. 918. 9) j. R. Rhodes, P. F. Berry and R. C. Sieberg, Nuclear techniques in on-stream analysis of ores and coal, Report no. ORO-2980-18 (Texas Nuclear Corporation, Austin, Sept. 1968). 10) F. E. Sentfle and A. F. Hoyte, Nucl. Instr. and Meth. 42 (1966) 93. 11) W. B. Nelp and E. Palmer, Phys. Med, Biol. 13, no. 2 (1968) 269.

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