Elemental analysis by IBA and NAA — A critical comparison

370

ELEMENTAL

Nuclear

ANALYSIS

Instruments

and Methods

BY IBA AND NAA - A CRITICAL

in Physics Research B35 (1988) 370-377 North-Holland, Amsterdam

COMPARISON

J.I.W. WA’ITERSON Wits-CSIR Schonland Research Centre for Nuclear Sciences, University of the Witwatersrand, Republic of South Africa

Johannesburg,

2050,

In this review neutron activation analysis (NAA) and ion beam analysis (IBA) have been compared in the context of the entire field of analytical science using the discipline of scientometrics, as developed by Braun and Lyon. This perspective on the relative achievements of the two methods is modified by considering and comparing their particular attributes and characteristics, particularly in relation to their differing degree of maturity. This assessment shows that NAA, as the more mature method, is the most widely applied nuclear technique, but the special capabilities of IBA give it the ability to provide information about surface composition and elemental distribution that is unique, while it is still relatively immature and it is not yet possible to define its ultimate role with any confidence.

1. Introduction In undertaking this invited comparison between the techniques of neutron activation analysis and ion beam analysis, a major problem is to establish appropriate terms of reference. Neutron activation analysis (NAA) and ion beam analysis (IBA) are both nuclear techniques, but with very different backgrounds, histories and uses. NAA is the most established and widely used nuclear technique while the widespread development and acceptance of IBA is a more recent phenomenon. It is not possible to provide detailed reviews of the research in these two large fields within the confines of a short article but the scientometric results of Braun [1,2] and Lyon [3,4] provide a useful approach. These results can be used as a measure of the degree of acceptance and use of these techniques within the broad field of analytical science. In this article, after a short historical review, these results will therefore be used to provide a perspective on the relative degree of acceptance of these two fields and then their unique attributes will be contrasted against this background. After the original experiment of Hevesy and Levi [5] in 1936, NAA grew rapidly as a result of its practical use in the first reactors during the war, so that by 1943 it had become “an every day procedure at the Oak Ridge National Laboratory” [6]. Immediately after the war, as a result of the work on radiochemical separations by Smales at Harwell[7], it made major contributions in the fields of geochemistry and cosmochemistry. It is a method that has been developed largely by chemists, perhaps as a result of this early reliance on chemical separation techniques. In the 1960s it was 0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

transformed by the development of the lithium-drifted germanium detector which led to the rapid development of instrumental methods of analysis. Today it is a well established method, competing directly with the many other methods of instrumental analysis on the basis of cost, convenience, sensitivity and accuracy. The history of IBA goes back almost as far as that of NAA. Shortly after Lawrence’s invention of the cyclotron, Seaborg and Livingood [8] used 6 MeV deuterons to determine 6 ppm of Ga in Fe, the first charged particle activation analysis (CPAA). There followed a period without widespread activity in which the work of Albert [9] in France on the use of CPAA for the measurement of trace elements in metals should be mentioned. It seems that there were two factors in the revival of IBA and its rather phenomenal growth in the last fifteen years, since the first IBA conference. The first factor was undoubtedly the slowing down of nuclear physics programmes. As accelerator physicists cast around for alternative support and began to look at so-called relevant research, the analytical capabilities of backscattering came to prominence and new methods, such as the production of X-rays by particle emission, were discovered [lo]. At more or less the same time the interest in materials science mushroomed: surfaces became of the greatest importance and distributions of elements became of interest. This happy coming together of demand and availability has produced a tremendous amount of research and has converted a scene in which IBA was used perhaps as an excuse to keep accelerators going to one in which accelerators are built and marketed specifically for IBA.

J.I. W. Watterson 2. Nuclear

/ Elemental analysis by IBA and NAA

techniques in perspective

371

phy and, interestingly enough, beating by far flow injection analysis, identified as one of the “hot spots” in analytical development, which ranks 14th in this list with 0.8% of all cited uses. Thus, looking broadly at analytical science, the science of the characterisation of materials, we see that atomic absorption spectrophotometry is the dominant method, then comes ordinary spectrophotometry followed by emission spectrometry. When NAA and X-ray fluorescence are added, 72.4% of all analytical applications have been accounted for. It must be borne in mind that this analysis is in terms of the research applications of the analytical methods only; if purely routine applications w&e considered, the results would look rather different and it is clear that atomic absorption, together with spectrophotometry, emission spectrometry and Xray fluorescence would dominate the other methods overwhelmingly. In spite of obvious shortcomings, this analysis does give an overall perspective, showing NAA as an accepted and highly ranking technique, at the time of this survey, while IBA was relatively less important in terms of frequency of use, but it provides only a crude way to look at analytical science. There are many different trends and forces, needs and demands that all together determine where the research emphasis goes in analytical science. New industrial technologies make new demands while new analytical methods open up opportunities for new processes. The demands of the different fields of technology, the different ways in which materials need to be analysed, the distribution of elements, the need for speciation, or knowledge of how the elements occur, the levels of concentration that are important, the chemical nature of the materials to be analysed: all of these factors lead to the need for many physical attributes to be brought into play in this field. It is also true that even if one method could account for the great majority of all analytical needs, there might be one or two critical applications in which another method could only be used. In that case that second method could be of equal importance. This is appropriate to the role of IBA. In terms of this comparison between IBA and NAA this scientometric analysis shows that, from the point of

It is a formidable task to put nuclear techniques into perspective in relation to the field of analytical science, the science of the characterisation of materials, but we can start to see a picture thanks to the work of Braun [1,2] and Lyon [3,4], amongst others. Braun has taken 15 elements, shown in table 1, and counted the number of times that various types of techniques have been used to determine these elements, using Analytical Abstracts as a database, and covering the period 1981-1984. Classifying the various analytical methods according to the physical principles involved in the measurement process, then, according to Analytical Abstracts, there are 143 different techniques. Of these 37 are nuclear. Many of these methods overlap considerably or are really different expressions of the same thing. For example gamma activation and photon activation are given as different techniques, as are charged particle activation, proton activation and alpha activation. Also a method such as nuclear reaction analysis (NRA) is missing from this list. Nevertheless this serves the purpose of providing a perspective. Braun found that 44 of these 143 methods had more than 7 cited uses in this period. Of these 44 different techniques 4 are nuclear, with proton-induced X-rays and particle-induced X-rays cited as separate techniques. Correcting this error then three are nuclear. Table 2 shows the eleven most frequently used techniques. The most interesting if not entirely surprising feature in this table is the overwhelming domination of the top few techniques. Atomic absorption accounts for one quarter of all reported uses. Adding spectrophotometry and emission spectrometry brings the total to almost 60%. It is surprising that neutron activation ranks as high as number 4, by far the most frequently used nuclear method, accounting for 8.5% of all methods applied to these fifteen elements. It ranks above X-ray fluorescence (6.6%) and thin layer chromatography and shows up as one of the work horses of the analytical world. Taking proton-induced and particle-induced X-ray emission together, they account for 1.3% of all uses and rank 10th between polarography and ion chromatogra-

Table 1 Elements chosen for the comparison of techniques [2]; the numbers indicate frequency of use of the techniques Method

ODtiCd

Nklear Electrical Chromatog. Misc.

Element

[!%]

Ag

As

Au

Bi

Co

Cr

Cu

Fe

Ga

Hg

Mn

MO

Sb

U

V

Total

59.1 14.1 15.7 3.5 7.7

70.4 15.1 7.8 4.2 2.5

59.1 25.9 10.0 2.7 2.3

67.9 3.3 15.9 9.3 3.7

62.9 13.6 9.5 10.0 4.1

70.2 14.0 7.4 5.8 2.6

66.3 8.2 14.6 6.8 4.1

70.8 13.5 5.6 5.2 4.9

71.6 11.9 12.8 1.8 1.8

65.9 12.9 9.8 9.1 2.3

73.2 16.2 3.9 4.1 2.6

70.1 12.6 9.6 4.8 3.0

59.5 21.6 11.4 3.4 4.2

47.2 29.1 13.9 6.8 2.9

74.7 11.1 7.2 3.6 3.4

66.9 13.8 9.8 5.8 3.7

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TECHNIQUES

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J.I. W. Watterson / Elemental analysis by IBA and NAA

Table 2 Uses of instrumental techniques in research papers, 1981-1984 Technique

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

Atomic absorption Spectrophotometry Emission spectrometry Neutron activation analysis X-ray fluorescence Thin-layer chromatography High performance liquid chromatography Atomic-emission spectrometry Polarography Particle-induced X-ray emission Ion chromatography

[2] Use

Cumulative

no.

PI

PI

1648 1271 1005 585 452 149 117 116 94 85 81

24.1 18.5 14.1 8.5 6.6 2.2 1.7 1.7 1.4 1.3 1.2

view of frequency of use, NAA finds many more uses, but quantity and quality must not be confused. In making a more detailed comparison of IBA and NAA it is important to emphasise that neither is a single technique. To make a valid comparison it is important to consider not only the general characteristics of the fields, but also to look at the subfields.

3. Neutron activation analysis The particular attributes of NAA derive from the physical nature of the neutron and the photon. They interact comparatively weakly with matter and hence are penetrating. To them matter is mostly holes and they can sample large volumes of it before being absorbed. It is this unique attribute, together with the properties of neutron capture reactions and the interaction of gamma rays with matter, that has built up the usefulness of NAA, in spite of the relative expense of the method and the comparative rarity of nuclear reactors. Another notable attribute of neutron activation analysis is the simplicity of the physical model that is applicable, at least when absorption is negligible. Over a wide range of commonly encountered conditions the total signal is a linear superposition of the individual single element components in neutron activation. The signal is directly proportional to the mass of the analyte and the behaviour with the irradiation, decay and counting times is governed by a simple exponential law. In this method the limiting sensitivity and the reproducibility are given by the Poisson law of counting statistics and this must be, for small samples, one of the best understood methods that there is. With some variations all of neutron activation depends on these characteristics of neutron reactions and gamma ray detection.

24.1 42.6 51.3 65.8 12.4 74.6 16.3 78.0 19.3 80.6 81.8

3.1. Reactor activation analysis Here the individual properties of this subfield are determined by the large thermal neutron fluxes associated with fission reactors, giving this method its characteristic sensitivity, ranging down to the ppb level in favourable cases. It is this method which has provided the great bulk of the analyses that have given NAA its rank in the analytical hierarchy. This method makes use mainly of the neutron capture reaction with its large cross sections and these reactions have another characteristic that bears directly on the success of the instrumental method in nondestructive analysis. Because of the role that neutron capture followed by beta decay (as the s-process) played in the synthesis of the elements, the natural abundances of the elements bear an inverse relationship to their capture cross sections. This provides one of the reasons for the ability of instrumental neutron activation analysis to determine trace elements in rocks [ll], without the signal being completely swamped by the major elements. Groups of elements such as the rare earths [12] and the platinum group elements [13], that are difficult to determine by other methods, are measured routinely and make up a large portion of the uses of this method. The disadvantages of this method are related to the characteristics of reactors as large, expensive, centralised facilities and the difficulties associated with the introduction of samples into them and the automation of batch irradiations. 3.2. Isotope source activation analysis This method is so different in application from the previous one that it seems almost anomalous to call them both NAA. Here the absolute sensitivity is in general low, while the neutron source is readily accessi-

313

J.I. W. Watterson / Elemental analysis by IBA and NAA ble and large samples can be analysed nondestructively. In principle the method is adaptable to many on-line or in situ analyses [14]. The (a, n) neutron sources that are available have limited the possible applications because of their low intensities. The introduction of the “‘Cf fission source appeared at one stage to be able to overcome these limitations but it is clear that this source has not yet achieved its initial promise [15]. One area where this method may prove itself is in the analysis of large samples of materials such as gold ore, where sampling is a major problem. Another area is in the online determination of the calorific, ash and sulphur contents of coal, where the use of prompt gamma ray spectroscopy shows promise [16]. 3.3. Fast neutron activation analysis This name has become primarily associated with the use of 14 MeV neutrons from small 150-300 keV deuteron accelerators using tritium targets. In this method and the following one, where ion beams are used for the production of neutrons, there is an overlap between IBA and NAA. These sources have been successful in the determination of oxygen and silicon, but they have also been limited by their comparatively large sensitivity to these common elements as well as to aluminium. Although this method is used fairly commonly, it has not established itself in any widespread way. This is also because attempts to reach the “magic” figure of 10” n/s with a cheap small accelerator and a reasonable target life have proved elusive. Available neutron fluxes are often an order of magnitude or so too low to solve many real analytical problems. On the other hand, the number of analytical instruments that can operate down boreholes is very limited and here this type of method has found a real niche with its use in borehole logging in the petroleum industry. Efforts to extend this use by applying it to in situ logging for precious elements have not been fruitful so far, although there is a tantalising feeling that this could be useful, provided that a sufficiently powerful D-D machine, based on the 2H(d, n)3He reaction could be developed. Such a machine would revitalise this field in many applications.

3.4. Other accelerator sources of neutrons Neutrons are produced by accelerators using several other reactions, traditionally most often the reaction 9Be(d, n)9B. In this case medium energy accelerators are used but the small beam currents normally available with these machines together with the high neutron energies of the strongly exoergic (d, n) reactions have limited the usefulness of this method. In some recent work, including a paper presented at this conference, it has been suggested that a high-current accelerator with an energy of 4-5 MeV could provide a powerful method for the application of NAA to the rapid determination of certain of the precious elements with possible future applications in on-line analysis and in situ analysis. Such a machine would produce a high intensity source of lower energy neutrons through the use of endoergic (p, n) reactions. There is an increasing interest in high-intensity accelerator sources because of developments in neutron therapy, in fusion studies and in neutron imaging and a high-current accelerator with an energy of several MeV could greatly increase the sphere of application of neutron activation analysis.

4. Ion beam analysis In contrast to NAA, IBA methods, in general, make direct use of charged particles that interact strongly with the sample matrix. As a result, they produce methods that are sensitive to surface effects, to lattice position and to element distribution, giving analytical techniques that are perhaps not as widely applicable as those of NAA, but that have unique analytical capabilities. The physical model of the processes in IBA is more complicated than in NAA and the interpretation of the results is often an art in itself. For example, in Rutherford backscattering (RBS) the results must often be compared with a computer simulation, and there can be a considerable degree of ambiguity in the interpretation.

Table 3 Analytical applications of small accelerators (after ref. [17]) Application

Beam

Typical energy

Refs.

NW Particle-induced X-ray emission Rutherford backscattering Channeling Microprobes Nuclear reaction analysis

P? u (1 (I

Mass spectrometry

heavy ions

Hydrogen profiling

P? a P. a

heavy ions

1 -3 0.4-4 OS-4 1 -3 0.5-5 4 -10 2 -20

WI 1191 WI PI WI v31

(241 IV. COMPLEMENTARY

TECHNIQUES

374

J.I. W. Whrterson / E~ement~Ianalysis by IBA end NAA

90

Backscattering

80

60

0 1945

1950

1955

1960

1965

1970

1975

1980

Year Fig. 1. Relative numbers of papers in seven fields of prompt nuclear analysis, showing the activity in the different fields (after Lyon

One of the characteristics of IBA is that it has spawned a multitude of methods, several of which can often be used together to increase the power of the technique. Table 3 [17-241 summarizes a number of these methods and the development of these methods in relation to each other is shown in fig. 1 taken from Lyon [4]. This figure shows the relative number of papers in seven fields of prompt nuclear analysis between 1949 and 1976 and it reflects the introduction of new fields in the 195Os, channeling passing through a phase of high fashion and RBS asserting its dominance but making way towards the end of this period to an increased interest in NRA (identified as (ch. p, ch. p) and (ch. p, y)}. This figure shows only the trends within the field and the curves are smoothed. 4.1. Rutherford backscattering This method goes back to the dawn of nuclear science. It makes use of the large-angle scattering of particles by the nucleus, together with the ease with which charged particles can be detected, to achieve a high sensitivity to surface composition and layer thickness. The emphasis here is on depth information and the method is particularly useful for the determination of a heavy layer on a light substrate. With the explosion in the development of integrated circuits and particularly with the role of ion implantation in the modification of materials this method has

come into its own. It has been used in such a wide variety of applications that they cannot all be described here, but typically it can be used to measure the thickness and composition of the metal layer and the oxide layer on the silicon substrate. Its special attributes enable it to be used in the depth-profiling of elements, particularly heavy elements in light matrices. This is a rather specialised attribute and it might easily have not found any profound use but for the semiconductor industry and its great need to understand the behaviour and dist~bution of dopants in silicon. 4.2. Particle-induced

X-ray emission (PIXE)

PIXE has established itself as a powerful method [25] for the analysis of very small samples. It appears that the good signal-to-noise ratios combined with the sensitivity of solid state detectors in so-called energydispersive spectroscopy give this method a distinct advantage over either conventional X-ray fluorescence, or electron-induced X-ray analysis in the analysis of small samples. The good signal-to-noise ratio follows from the fact that the bremsstrahlung associated with the proton beam is very much less intense than that associated with an electron beam in electron-induced X-ray emission. As a result of this, the PIXE method is much better able to analyse small samples and it has found its niche because of the analytical need to examine airborne particulates.

J.I. W. Watterson / Elemental analysis by IBA and NAA

PIXE has stood out as the method of choice in this area of research with its profound implications for public health. PIXE is one of the methods of IBA which has found that it can satisfy the criteria of being able to fill a real need at an acceptable cost, and there are several facilities that are able to offer a commercial service. 4.3. The proton microprobe The low bremsstrahlung associated with positive ion beams in comparison with electron beams, together with the much lower scattering angles, has the consequence that the positive ion microprobe has the ability to attain greater sensitivities and, with improved lens development, to do this at smaller spot sixes than are obtainable with electron machines such as the electron microprobe and the scanning electron microscope. After the pioneering work at Harwell [26] achieved elemental mapping to be carried out at the level of 1 ppm [28]. With new lens designs and higher brightness ion sources, a resolution of 0.1 urn with a useful beam current is within reach. This will lead to important breakthroughs in elemental microanalysis. 4.4. Accelerator mass spectrometv One of the unique ways in which ion beams can be utilised for analysis is by using the accelerator as a mass spectrometer. The high energies given to the beam in an accelerator mean that individual particles, ablated from the sample in the ion source, can be detected as heavy ions, thus greatly increasing the sensitivity over conventional methods. This field started effectively in 1977 when 14C was first detected at natural abundances [29,30]. The analytical advantage of AMS is its ability to handle samples of l/1000 of the sample size for radiometric methods, and because of its sensitivity, to measure ages greater than the conventional limits of 30-40000 years. Its greatest analytical problem is to achieve precisions of less than 1%. Other nuclides such as “Be, z6Al, 36Cl, 41Ca and lz91 can also be used for dating. It is interesting that the IsoTrace laboratory is one example of a commercial service offered by an IBA laboratory. They perform some 300 high precision carbon “dates” per year. AMS can also be used for the determination of ultratrace levels of elements. At present this powerful method has not yet found a niche, perhaps because of the lack of demand for this information, or because of sampling problems [31] and as a result of the complexity of ion production processes.

375

the use of this method it is possible to locate the lattice position of impurities and to decide, for example, whether they are interstitial or substitutional [32] from angular scans of backscattered ions near the main crystal axes. Recently a transmission channeling method [33] has been used to give important information about the position of adsorbed atoms on crystal surfaces. 4.6. Nuclear reaction analysis Charged-particle activation analysis is capable of great sensitivities. For example, in the investigation of trace elements in GaAs a sensitivity for boron of 3 X 1Ol4 at./cm3 has been quoted [34]. This method uses a deuteron beam and the reactions “B(d, n)“C with a 3 PA beam of 8 MeV deuterons and a 20 min irradiation. A limit of detection of 8 X 1013 at./cm3 (0.3 ppb by mass) has been achieved for carbon using the reaction ‘*C(d, n)t3N. These sensitivities are equal to or better than those that can be achieved by spark source mass spectrometry or secondary ion mass spectrometry but they can only be achieved by the use of a radiochemical separation step in both cases. More often it is used as a prompt method for the determination of elements at higher levels and for the profiling of depth distributions, and also in conjunction with other IBA methods. For example, hydrogen profiling using the reaction ‘H(“N, cuy )“C has become an established routine method. 4.7. Secondary ion mass spectrometry (SIMS) Heavy ions produce sputtering in a target even at a very low energy (down to a few keV). The secondary ions that are produced can be analysed by a mass spectrometer to give comprehensive information about the composition of surfaces and the distribution of elements with depth into the material [35]. SIMS does not require an accelerator and, for this reason together with its good sensitivity and depth resolution, it is very widely used, particularly for the depth profiling of surface layers and in the development of techniques for the production of VLSI devices. The technique is highly developed with several commercial machines available with scanning techniques that allow for the imaging of element distributions. Some problems exist because the complexity of the sputtering process makes the quantification of the results difficult, although a large measure of success has been obtained with the use of implanted standards matched to the sample. Another problem in this method is the dynamic alteration of the element profile during the sputtering process [36]. 4.8 Combinations of IBA methoak

4.5. Channeling The increased penetration of ion beams along particular crystallographic axes is termed channeling. By

A number of very interesting results have been obtained by a combination of IBA methods. For example, in an interesting article a group at the University of IV. COMPLEMENTARY TECHNIQUES

J.I. W. Watterson

376

/ Elemental analysis by IBA and NAA

Surrey [37] has combined RBS backscattering, channeling and a microbeam to investigate 10 pm semiconductor structures and observe features at the interface between a nickel silicide layer and the silicon substrate as regards the stoichiometry, crystallinity and roughness of the interface. These are surely attributes that could be measured by no other method. Many subtle and remarkable results have been achieved by combining RBS with NRA. In another case it has been combined with Coulomb excitation in a coincidence mode to determine the components of layers where the constituents have similar masses [38].

haps later, to reactors, it might be expected that the particular physical attributes of NAA and PIXE will move them up in the ranking of routine analytical methods, in spite of the increases competition of plasmaand laser-based analytical methods, and that the unique properties of the other IBA methods will broaden their contribution to the fundamental problems of the understanding of surface behaviour and the performance of devices, problems that would not be solved without their help.

References 5. Discussion

VI T. Braun, E. Bujdosb and A. Schubert, Literature of

A number of interesting points occur from a comparison of these different nuclear analytical methods and on the use of scientometrics to obtain a perspective on nuclear methods as a whole in the field of analytical science. Clearly, even in a research context, the nuclear method that has been able to compete best, as far as the sheer number of analyses is concerned, with conventional techniques such as atomic absorption is NAA, with 8.5% of all uses in table 2. PIXE is also doing remarkably well in this rating with a ranking of tenth, and has in all probability risen since this table was compiled. This is one perspective interesting in itself but concealing a great deal of the specific value of IBA methods. A consideration of the individual methods as they have been outlined above, shows that the special contribution of IBA methods lies in the type of information that they provide, information on the composition of surfaces and the distribution of elements. This distributional information is outside the scope of NAA (except for the specialised field of fission and alpha track etching) and the only competition is provided by electron methods, such as the microprobe and the scanning electron microscope. Here the ion beam methods offer greater sensitivity but at a greater cost. They also provide the unique capability to determine depth distributions. The IBA method that competes most directly with NAA is PIXE, but neither of these methods is a replacement for the other. In the field of the analysis of airborne particulates PIXE is clearly the method of choice, while NAA is the best method for the determination example,

of trace rare

earths

elements

in many

in geological

cases

materials.

as,

for

In other

cases, such as the determination of the platinum group, a method has been developed that uses both these techniques in a complementary way [39&l]. It is difficult to forecast the future, but as new technologies are applied to accelerators and also, per-

Analytical Chemistry: A Scientometric Evaluation (CRC Press, Florida, 1987). PI T. Braun, Statistical Evaluation of Recorded Knowledge in Nuclear and Other Analytical Techniques, in: Comparison of Nuclear Analytical Methods With Competitive Methods, Proc. IAEA Advisory Group Meeting, Oak Ridge, October 1986 (IAEA-TECDOC-435, Vienna, 1987) pp. 9-36. [31 E. Bujdoso, W.S. Lyon and I. Noszlopi, J. Radioanal. Chem. 74 (1982) 197. [41 W.S. Lyon, Use of Scientometrics to Assess Nuclear and Other Analytical Methods, in: Proc. IAEA Advisory Group Meeting, Oak Ridge, October 1986 (IAEATECDOC-435, Vienna, 1987) pp. 37-49. [51 G. Hevesy and I-I. Levi, K. Dan. Vidensk. Selsk. Mat. Fys. Medd. 14 (1936) no. 5. Fl G.E. Boyd, Anal. Chem. 21 (1949) 335. [71 A.A. Smales, Anal. Chem. 24 (1952) 717. PI G.T. Seaborg and J.J. Livingood, J. Am. Chem. Sot. 60 (1938) 1784. 191 P. Albert, in: Proc. 2nd Conf. on Practical Aspects of Activation Analysis with Charged Particles, ed. H.G. Ebert, Euratom Report EUR 3896 d-f-e (1968) 3. WI T.B. Johansson, R. Akselsson and S.A.E. Johansson, Nucl. Instr. and Meth. 84 (1970) 141. PII J.I.W. Watterson, J.P.F. Sellschop, C.S. Erasmus and R.J. Hart, Int. J. Appl. Radiat. Isot. 34 (1983) 407. 1121 I.H. Campbell, CM. Lesher, P. Coad, J.M. Franklin, M.P. Gorton and P.C. Tburston, Chem. Geol. 45 (1984) 181. 1131 E.L. Hoffmann, R.G.V. Hancock, A.J. Naldrett, J.C. van Loon and A. Manson, Anal. Chim. Acta 102 (1978) 4.55. P41 T.C. Martin, J.R. Rhodes and J.B. Waters, On-line Nuclear Analysis Measurements in Process Control Applications, USAEC Report ORO-2980-16 (1967). 1151 e.g. On-stream Monitoring of Si and Fe in Taconite Slurries, Californium-252 Prog. 19 (1975) 30. WI H.R. Wilde and W. Herzog, J. Radioanal. Chem. 71 (1982) 253. [‘171J.B. Schroeder, C.W. Howell and G.A. Norton, Nucl. Instr. and Meth. B24/25 (1987) 763. WI J.L. Campbell and J.A. Cookson, Nucl. Ins&. and Meth. B3 (1984) 185. 1191 J.F. Zielger (ed.), New Uses of Ion Accelerators (Plenum, New York, 1975) p. 96. PO1 Ibid, p. 229.

J.I. W. Wafterson / Elemental analysis by IBA and NAA [21] C. Engelmann and J. Bardy, Nucl. Instr. and Meth. 218 (1983) 209. [22] J.F. Ziegler (ed.), New Uses of Ion Accelerators (Plenum, New York, 1975) p. 160. [23] J.M. Anthony, D.J. Donahue, A.J.T. Jull and T.H. Zabel, Nucl. Instr. and Meth. BlO/ll (1985) 498. [24] W.K. Chu, J.W. Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978) p. 210. [25] J.W. Winchester, Nucl. Instr. and Meth. 181 (1981) 367. 1261 J.A. Cookson, A.T.G. Ferguson and F.D. Pilling, J. Radioanal. Chem. 12 (1972) 39. [27] G.J.F. Legge, C.D. McKenzie, A.P. Mazzolini, R.M. Sealock, D.N. Jamieson, P.M. O’Brien, J.C. McCallum, G.L. Allan, R.A. Brown, R.A. Colman, B.J. Kirby, M.A. Lucas, J. Zhu and J. Cerini, Nucl. Instr. and Meth. B15 (1986) 669. [28] F. Watt, G.W. Grime, G.D. Blower and J. Takacs, IEEE Trans. Nucl. Sci. NS-28 (1981) 1413. [29] D.E. Nelson, R.G. Korteling and W.R. Stott, Science 198 (1977) 507.

377

[30] C.L. Bennett et al., Science 198 (1977) 508. [31] J.I.W. Watterson, J.P.F. Sellschop and A. Zucchiatti, Nucl. Instr. and Meth. B28 (1987) 554. (321 S.T. Picraux, in: New Uses of Ion Accelerators, ed. J.F. Ziegler (Plenum, New York, 1975) ch. 4, p. 229. [33] I. Stensgaard, Nucl. Instr. and Meth. B15 (1986) 300. [34] L.C. Wei, G. Blondiaux, A. Giovagnoli, M. Valladon and J.L. DeBrun, Nucl. Instr. and Meth. B24/25 (1987) 999. [35] P. Williams, in: Applied Atomic Collision Physics, vol. 4, ed. S. Datz (Academic Press, Orlando, 1983) p. 327. [36] W. Vandervorst, F.R. Shepherd, M.L. Swanson, H.H. Plattner, O.M. Westcott and I.V. Mitchell, Nucl. Instr. and Meth. B15 (1986) 201. [37] J.T. Hornton, R.E. Harpur and D.M. Albury, Nucl. Instr. and Meth. B29 (1987) 515. [38] T.W. Conlon, Nucl. Instr. and Meth. B24/25 (1987) 705. [39] H.J. Annegam, C.S. Erasmus and J.P.F. Sellschop, Nucl. Instr. and Meth. B3 (1984) 181. [40] C.S. Erasmus et al., in preparation.

IV. COMPLEMENTARY

TECHNIQUES