Radioisotope X-Ray Fluorescence Spectrometry

TECHNICAL REPORTS SERIES No. 115

INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA,1970

RADIOISOTOPE X-RAY FLUORESCENCE SPECTROMETRY The following States jre Members of the International Atomic Energy Agency:

AFGHANISTAN GREECE NORWAY ALBANIA GUATEMALA PAKISTAN ALGERIA HAITI PANAMA ARGENTINA HOLY SEE PARAGUAY AUSTRALIA HUNGARY PERU AUSTRIA ICELAND PHILIPPINES BELGIUM INDIA POLAND BOLIVIA INDONESIA PORTUGAL BRAZIL IRAN ROMANIA BULGARIA IRAQ SAUDI ARABIA BURMA IRELAND SENEGAL BYELORUSSIAN SOVIET ISRAEL SIERRA LEONE SOCIALIST REPUBLIC ITALY SINGAPORE CAMBODIA IVORY COAST SOUTH AFRICA CAMEROON JAMAICA SPAIN CANADA JAPAN SUDAN CEYLON JORDAN SWEDEN CHILE KENYA SWITZERLAND CHINA KOREA, REPUBLIC OF SYRIAN ARAB REPUBLIC COLOMBIA KUWAIT THAILAND CONGO, DEMOCRATIC LEBANON TUNISIA REPUBLIC OF LIBERIA TURKEY COSTA RICA LIBYAN ARAB REPUBUC UGANDA CUBA LIECHTENSTEIN UKRAINIAN SOVIET SOCIALIST CYPRUS LUXEMBOURG REPUBLIC CZECHOSLOVAK SOCIALIST MADAGASCAR UNION OF SOVIET SOCIALIST REPUBLIC MALAYSIA REPUBLICS DENMARK MALI UNITED ARAB REPUBLIC DOMINICAN REPUBLIC MEXICO UNITED KINGDOM OF GREAT ECUADOR MONACO BRITAIN AND NORTHERN EL SALVADOR MOROCCO IRELAND ETHIOPIA NETHERLANDS UNITED STATES OF AMERICA FINLAND NEW ZEALAND URUGUAY FRANCE NICARAGUA VENEZUELA GABON NIGER VIET-NAM GERMANY, FEDERAL REPUBLIC OF NIGERIA YUGOSLAVIA GHANA ZAMBIA

The Agency's Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is "to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world".

Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, Kamtner Ring 11, P.O. Box 590, A-1011 Vienna, Austria.

Printed by the IAEA in Austria June 1970 TECHNICAL REPORTS SERIES No. 115

RADIOISOTOPE X-RAY FLUORESCENCE SPECTROMETRY

REPORT OF A PANEL HELD IN VIENNA, 13 - 17 MAY 1968

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1970 RADIOISOTOPE X-RAY FLUORESCENCE-SPECTROMETRY IAEA, VIENNA, 1970 STI/DOC/10/115 FOREWORD

Radioisotope X-ray fluorescence is one of the more recent techniques developed as a result of the general availability of sealed radioisotope sources. It is mainly used for non-destructive elemental analysis and for the measurement of coating thickness. In both these applications it has become complementary to, and competitive with, several older-established methods. A great variety of portable equipment for the analysis of alloys, both in industrial plant laboratories and in the field, now uses radioisotope X-ray fluorescence techniques as a basic element in its design. This equipment is in wide use in geophysical prospecting and in mine development and control. Also important in mining is the development of borehole probes for use in percussion-drilled holes; this is an attractive alternative to the analysis of drill cores. On-line equipment incorporating these techniques has been developed for use in mineral processing plants, especially for analysis of lead, zinc, tin, copper, iron, calcium and silicon. Although industry was slow to introduce this on-line application, several commercial installations are now reported to be in operation. For on-line measurement in particular, the combination of radioisotope X-ray fluorescence with neutron activation analysis (using radioisotope neutron sources) is now being developed. This will be especially useful where several elements have to be analysed simultaneously. Commercial equipment, designed for use in computer-control systems, is now available for regulating the thickness of coatings in the tinning and galvanizing processes. An important development is the work on low-Z analysis by alpha excitation. This method of analysis can now be seriously considered for industrial application, especially since an intrinsically safe alpha source, 242Cm, has recently become available commercially. Also important in extending the range of applications is the development of solid-state detectors, particularly the Si(Li) type, with which inter-element effects can be reduced and limits of detection can be lowered; a main reason for the improvement here is that balanced X-ray filters are not required. The two problems limiting higher accuracy are those of inter-element effects — already largely solved — and of particle-size effects. Many experts are confident that present work aimed at overcoming particle-size effects will be successful. Although development work is proceeding in many directions, there is a wide range of possible uses that have as yet been barely explored. The Panel whose findings are reported here met in Vienna on 13—17 May 1968. Its task was to review current techniques and applications, to comment on the factors that limit performance, and to consider promising lines of development and extensions of present usage. The report is based on Panel discussions and on written contributions from each Panel Member, as well as on a paper by B. Dziunikowski, Institute of Radioisotope Techniques, Academy of Mining and Metallurgy, Cracow, Poland. The International Atomic Energy Agency is greatly indebted to C. G. Clayton, Chairman of the Panel, for compiling and editing the final text of the report and ensuring that the information is up to date. CONTENTS

A. GENERAL CONSIDERATIONS . 1

A.l. Comparison between radioisotope X-ray fluorescence and other analytical techniques 1 A.1.1. General comparisons 1 A. 2. Comparison between radioisotope and conventional X-ray fluorescence analysis 4 A. 2.1. Relative performance 4 A.3. Comparison between primary and secondary alpha, beta, gamma and X-ray excitation 6 A.3.1. Principal characteristics of the different methods of excitation 6 A.3.2. Choice of source 9 A. 3.3. Comparison of the available excitation sources 15 A.3.4. A comment on proton excitation 16 A.4. Present status of the theory of radioisotope X-ray fluorescence 16

B. INSTRUMENTATION AND TECHNIQUES 19

B.l. Components of radioisotope X-ray fluorescent analysers 19 B.l.l. Radioactive sources 19 B.l.2. Detectors , 25 B.1.3. Electronic instrumentation 28 B.l.4. Windows 28 B.1.5. X-ray filters for energy selection 29 B.l.6. Cooling systems 30 B.l.7. Multi-element computers 31 B.2. Analytical Instruments and systems 32 B.2.1. Commercially available systems 32 B.2.2. Other systems in use or under development 40 B.3. Techniques for minimizing interference 41 B.3.1. Matrix effects 41 В. 3.2. Heterogeneity effects 42 B.4. Calibration 44 С. APPLICATIONS 45

C.l. Metalliferous mineral exploration and development 45 C.l.l. Unprepared rock surfaces 45 C.l.2. Drill cores 46 C.1.3. Boreholes 47 C.l.4. Particulate samples 47 C.2. On-stream process control 50 C.2.1. Introduction 50 C.2.2. Analysis of slurries 51 C.2.3. Analysis of crushed materials 63 C.2.4. Analysis of solutions 70 C.3. Alloy analysis 70 C.4. Coating thickness measurement 71 C.4.1. Introduction 71 C.4.2. X-ray fluorescence methods of measuring coating thickness 71 C.4.3. Applications 71 C.4.4. Tin coating gauges 71 C.4.5. Zinc coating gauges 77 C.5. Miscellaneous applications 78 C.5.1. Medical 78 C.5.2. Other applications 79 C.6. Health and safety considerations 79 C.6.1. External hazards 79 C.6.2. Internal hazards 80

D. PROSPECTS AND REQUIREMENTS 81

E. RECOMMENDATIONS TO THE AGENCY 83

REFERENCES 85

APPENDIX I — Manufacturers' specification forms 89

APPENDIX II — Classification of preferred terms and definitions relating to radioisotope X-ray fluorescence analysis 99

LIST OF PARTICIPANTS AND SECRETARIAT 101 A. GENERAL CONSIDERATIONS

A. 1. COMPARISON BETWEEN RADIOISOTOPE X-RAY FLUORESCENCE AND OTHER ANALYTICAL TECHNIQUES

Radioisotope X-ray fluorescence analysis is one of a number of techniques now available to determine the concentration of elements in different materials. Its suitability for any application depends on a variety of factors, such as the environment in which the analysis is to be carried out and the limits of detection required. In the present comparison, radioisotope X-ray fluorescence is first examined in a general manner and then compared specifically with conventional X-ray fluorescence analysis.

A. 1.1. General comparisons

(a) Analyses in the laboratory ,

A general comparison is difficult to make because of the many factors which have to be taken into account, but an attempt has been made to summarize the present situation in Table I. The different analytical techniques have been assigned a figure of merit, i.e. 1, 2, 3, 4 or 5 (1 indicating highest performance and 5 lowest performance) for the particular characteristics defined below.

(i) Selectivity

This is the ability of the technique to ensure that the signal being measured originates from the element being determined.

(ii) Limit of detection

This applies to the sample being examined but does not .take into account concentration or dilution factors.

(iii) Accuracy

This refers to the degree of correctness with which a method of measurement yields the true value of the quantity being measured.

(iv) Equipment cost

. This is the cost of all the equipment required to complete an analysis.

1 TABLE I. COMPARISON BETWEEN THE LABORATORY APPLICATION OF RADIOISOTOPE X-RAY FLUORESCENCE AND THAT OF OTHER ANALYTICAL TECHNIQUES

Limit of Equipment Laboratory Technique Selectivity Accuracy detection cost cost

Atomic absorption 1 1 3 2 2

Atomic fluorescence 1 1 3 2 2

Polarography 4 2 3 2 2

Colorimetric 4 2 3 1 1

Fast neutron 3 4 2 4 4

Radioisotope X.R.F. 3 4 2 2 1

Tube X.R.F. 2 3 1 4 3

Direct reader 2 2 1 3 3

Analytical time Operator time and grade Technique Massive Massive Powders Liquids solids solids Powders Liquids

Atomic absorption 4 3 1 3 3 1

Atomic fluorescence 4 3 1 3 3 1

Polarography 5 4 3 4 4 3

Colorimetric 5 4 3 4 4 3

Fast neutron 2 1 2 2 1 2

Radioisotope X.R.F. 3 2 2 1 1 1

Tube X.R.F. 2 1 1 1 1 1

Direct reader 1 2 1 1 2 1

(v) Laboratory cost

This is the cost of installing the equipment and is a frequently neglected factor. Housing a • neutron generator will be much more expensive than, for example, a simple radioisotope X-ray analyser, which can, if required, be completely portable so that laboratory costs are virtually zero.

(vi) Analytical time

This is the time between taking a sample and receiving the analysis. Transfer of the sample to the laboratory may be a major factor, and in this case a simple robust instrument operating at the sampling point may be quicker than a more sophisticated instrument located some distance away. Generalizations are difficult to make in this connection, and the quoted figures of merit should be treated with caution.

2 (vii) Operation time and grade of operating staff

When the analytical time is not the determining factor, the time spent by the operator and the grade of the staff required to carry out the work may be the determining factor governing the choice of technique. The grade of staff required depends not only on the technique but also on the instrument being used: for example lower grade staff would be required for a completely automatic colorimetric analyser than for a manual X-ray spectrometer.

Conclusions. Radioisotope X-ray fluorescence analysis is at present of limited use in a well-equipped analytical laboratory. However, in such a laboratory its main function would be to reduce the load on conventional X-ray equipment or to examine samples which, because of their dimensions or for other reasons, cannot be analysed in an X-ray tube instrument. When conventional X-ray equipment is not available, a simple non-dispersive X-ray spectrometer is a. possible alternative which is competitive in both cost and speed of analysis and is adequate provided that the required .analyses are within its somewhat limited capabilities. The most important advantages of radioisotope X-ray fluorescence analysis appear to be the low -cost of the equipment, the small size and the portability which enable analyses to be removed from the laboratory and carried out on the plant, on the factory floor or in the field. This manner of operation requires simple, rugged mains- or battery-powered instruments with unequivocal readout, and equipment fulfilling these requirements is now available commercially.

(b) Analyses on-stream

The main techniques competing with radioisotope X-ray fluorescence analysis for on-stream applications are conventional, dispersive X-ray analysis and neutron activation analysis, with either a radioisotope source or a neutron generator. When a number of elements have to be analysed, especially if low concentrations are involved, conventional X-ray spectrometry may be preferred. However, the need to bring process streams to the equipment increases its already high cost and complicates plant design. If the lighter elements such as aluminium, silicon, fluorine or phosphorus have to be determined, neutron activation, usually in a recirculating form, is the preferred method, since very thin windows are avoided and heterogeneity effects are of very little importance — both grave limitations in the on-stream determination of these elements by X-ray fluorescence analysis. Where the concentration of a heavy element, such- as uranium or lead, has to be measured on stream, a strong competitor for radioisotope X-ray fluorescence is preferential X- or y-ray absorption. This technique has the advantage of very limited heterogeneity effects and avoids the use of thin- window sample presenters.

3 (с) Analyses in the field

For portability of instrumentation, radioisotope X-ray fluorescence has little or no competition. - However, where truck-borne instrumentation is considered, many of the techniques considered for laboratory use are definitely competitors — for instance atomic absorption and calorimetric methods. The X-ray fluorescence technique has the advantage that sample preparation is normally confined to preparing the sample in a powdered form, whereas other techniques require preparation of a solution. The latter may be an advantage where the required determination is the acid soluble portion of the element — which is often the case in the analysis of minerals.

A.2. COMPARISON BETWEEN RADIOISOTOPE AND CONVENTIONAL X-RAY FLUORESCENCE ANALYSIS

Conventional X-ray spectrometers at present range from the simple, manually-operated, sequential instrument to fully automatic multi-channel equipment, capable of examining a hundred or more samples or several slurry streams without human intervention. The latter is often equipped with on- line computers which are programmed to make corrections for interelement effects and to print the required concentrations directly. They can be used to determine concentrations of all elements from fluorine upwards in the periodic table, and through most of this range the limit of detection is of the order of a few parts per million. Direct electron-excitation instruments are also available and give a greatly increased fluorescent yield for the lighter elements; they are also available in sequential or simultaneous form. Radioisotope instruments are at present mainly of two types: the portable analyser designed for field, base-camp or factory-floor use, and the on- stream analyser for use in process control. Both types of instrument are available commercially. Instruments with lithium-drifted silicon and ger- manium detectors have recently become available. These have a very high energy resolution, and are giving promise of extending the field of application of radioisotope X-ray fluorescence analysis into that now dominated by conventional X-ray spectrometers.

A.2.1. Relative performance

Until the advent of lithium-drifted detectors, the conventional X-ray spectrometer was superior to the radioisotope instrument in almost every respect; it could be used to analyse a wider range of elements with lower limits of detection and greater precision. The advantages of radioisotope instruments were associated with characteristics other than analytical performance. With the introduction of the solid-state detector, there is now a region of the X-ray spectrum — 20 to 100 keV approximately — where a non- dispersive instrument could be superior in performance to a Bragg spectro-

4 X-RAY ENERGY (keV)

FIG, 1. Comparison of resolutions of various X-ray detectors.

meter (see Fig. 1). This region contains the К X-rays of the medium- and high-atomic-number elements and these X-rays are preferred to the corresponding L X-rays for powder and slurry analysis, since heterogeneity effects are markedly reduced with the higher energy radiation. Even where the resolving power is inferior to that of a Bragg spectrometer, limits of detection comparable to those obtained with the conventional instrument can be achieved. This is because a non-dispersive system can use a source-target assembly to provide efficient excitation of any particular radiation. However, it should be pointed out that a low-power X-ray tube with a suitable series of targets may be preferred to a radioisotope source for use with solid-state detectors. At present, solid-state detectors of high resolving power have a small aperture, and the additional intensity available from an X-ray tube would be a distinct advantage. The main disadvantages of solid-state ' detectors are their poor performance with the lighter elements and their relatively high cost — a complete system can be almost as expensive as a manual conventional X-ray spectrometer. The production of a low-cost, sealed solid-state detector with

5 Joule-Thomson cooling, capable of being used both in the field and in on- stream systems, would markedly increase the range of application of these devices and is not beyond existing technology. The main advantages of radioisotope X-ray sources are their low cost, compactness, reliability and stability. This makes them particularly useful in portable equipment for field work and in process control analysis, where they permit the construction of low-cost sensors for remote operation on the plant. The latter enables on-stream analysis to be introduced where the cost of conventional equipment would preclude its introduction, due to the un- certainty of the benefits derived from it. Another important advantage of the radioisotope X-ray source compared with the conventional X-ray spectrometer is that the former, in its most effective form, emits essentially monochromatic X-rays. This allows more adequate compensation for effects caused by changes in X-ray absorption coefficients of the analysed material, by the use of transmitted or Compton-scattered primary radiation.

A.3. COMPARISON BETWEEN PRIMARY AND SECONDARY ALPHA, BETA, GAMMA AND X-RAY EXCITATION

A.3.1. Principal characteristics of the different methods of excitation

(a) Alpha excitation

For targets of atomic number below 29, practically the only phenome- non involved is direct excitation of the К shell by interaction between the a-particle and the orbital electrons. Excitation of the nucleus and interaction of the a-particle with the Coulomb field of the nucleus give rise to a negligible intensity of X-radiation — about 1СГ4 less than the intensity of characteristic X-rays. L and M excitation takes place with a probability which is about one order of magnitude less than for К X-ray excitation [1]. Conse- quently, the use of a-particles gives very pure line spectra. From theoretical considerations, the yield of characteristic X-rays is approximately proportional to the fourth or fifth power of the energy of the bombarding a-particle. However, measurements obtained by Sellers and Zeigler [1] give a slope somewhat different from that expected theoretically, as shown in Fig. 2. Results obtained with a 210Po source having a 0.2-mil stainless-steel window are given in Tables II and III. Comparison is made with theoretical figures derived from a proton excitation model.

(b) Beta excitation

When a target is irradiated by /З-particles from a radioisotope source, the following phenomena takeplace:

(i) direct excitation of the energy levels of the atoms and subsequent К and L X-ray production;

6 (ii) backscatter of/3-particles;

(iii) production of bremsstrahlung in the target which gives rise to a continuous spectrum of X-rays, ranging from 0 to the maximum energy of the /3-particles;

ALPHA-PARTICLE ENERGY (MeV)

FIG. 2. Aluminium X-ray cross-sections for alpha particles.

TABLE П. ТШСК TARGET К X-RAY YIELDS

Yield (photons/alpha sr) X-ray energy Element Z Target (keV) , Experiment Estimated

0. ,277 Carbon • 6 С 1, ,22 XlO"' 1.50 xl0"!

0, ,392 Nitrogen 7 BN 2, ,94 xlO"4

0, ,525 Oxygen 8 2, ,85 x10~4 "s80»

0, ,677 Fluorine 9 CFj 5, .82 xlO"4

1,,04 1 Sodium 11 NaBH4 1. ,74 xlO-4

1,,25 4 Magnesium 12 Mg 5, ,29 xlO"4 2.86 xl0"s

1, .487 Aluminium 13 Al 2. ,67 xlO"4 1.81 xl0"s

For estimate Y„(Ea) « 4 Yp(E0/4)

where Ya = yield for 1 orparticle and Yp = yield for 1 proton.

7 ¡я- a = S. я

* 2 2

IN IN N M

.2 e с g = 3 Ü -о л I E S ¿ (¡v) excitation of К and L X-rays of the target by the bremsstrahlung

produced in the target and by bremsstrahlung radiated by the source;

(v) Compton scattering of the bremsstrahlung from the source;

(vi) Compton and Rayleigh scattering of the characteristic X-rays of the source window.

Theoretical derivations of the relative intensities of all the components of the radiations emitted by the target have been made by Filosofo [2] and Leontiadis [3], but they are too involved to be discussed in detail here. The intensity of the raditions mentioned in (ii), (iii), (v) and (vi) is by no means negligible. The bremsstrahlung produced in the sample is responsi- ble for most of the background in devices used in /З-excitation. This bremsstrahlung increases with the atomic number of the target and is mainly emitted in the direction of the incident /^-particles. However, it is worth noting that the absorption of ^-particles in the target is dependent to a very small extent on the target's atomic number.

As already mentioned, Compton, photoelectric and Rayleigh effects are the main phenomena that occur when a photon interacts with matter. For photon energies higher than the К absorption edges, the cross-sections for photoelectric effect vary as (E)~2-8 and increase with the atomic number of the target. On the other hand, the cross-section for Compton scattering decreases relatively slowly as the energy of the photon increases and depends only on the number of electrons per unit mass of the target. Values of the cross- section are derived from the well known formulae of Klein and Nishina. Rayleigh scattering, or elastic scattering of photons, is important only for relatively low-energy photons; the photons scattered by this process are mainly in the direction of the incident radiation. In general, therefore, Compton scattering makes the greatest contribution to the radiation intensity from the sample when excitation is by high-energy y-rays. For exciting energies below 60 keV, and for elements having excitation thresholds just below the exciting energies, the photoelectric effect is by far the most important process.

A.3.2. Choice of source

A number of radioisotope sources are available and the choice is dictated by considerations of the physical and technological aspects of the analysis as well as of the environment in which the measurement is to be carried out.

(a) High-energy photon sources (E >150 keV)

The most important sources are 192Ir, 137Cs and 60Co. They are easily available, inexpensive, sturdy and of high specific activity.

9 It has been shown [4] that, for X-ray fluorescence analysis, these sources can be used with advantage for measuring the concentration of high- atomic-number materials (Z >60) in light matrices. In this case it is generally better to excite the К X-rays of the heavy element, as this reduces attenuation and grain size and matrix effects. The Compton backscattered y-rays can be used to overcome density and matrix effects. In the presence of medium- atomic-number impurities they offer fewer advantages. In general, these sources are not used in portable equipment, because of the large weight of shielding required. Such sources can be used in proximity detectors. If a heavy metal can be selected as the target, problems of environment such as the presence of dust and humidity and variations in distance between source and target are less acute than when low-energy exciting sources are used. However, no practical applications of this kind have as yet been reported. In general, these are applications where a poor geometry is often unavoidable, and where high activities are needed.

(b) Medium-energy photon sources (60 keV

The most important sources are 07Co and 153Gd. These sources are suitable for analysis of high-Z elements by means of their К X-rays [5] and analyses can be carried out in the presence of medium- or low-Z elements. It is possible to construct portable detectors with sources in this energy range. These sources can also be used for analysis of medium-Z elements when it is necessary to eliminate density effects; for instance by separate measurement of the К X-rays and of the backscattered y-rays [6].

These sources have been used extensively in X-ray fluorescence analysis, absorption analysis, thickness and coating-thickness measurement and in package monitors. Generally they are the best sources from the point of view of cost, low radiation hazard, compact geometry of the measuring system and the ability to use elemental filters for energy selection. Most radioisotope X-ray fluorescence equipment now in use incorporates sources in this energy range. Monochromatic or quasi-monochromatic sources of high intensity are available, the most important being 55Fe (20 mCi), 238PU (30 mCi), 109Cd (20 mCi), 125I (10 mCi), 210Pb (100 mCi), 241Am (100 mCi). Each of these sources has an activity of the order of 107—10s photons/ s. Each monoenergetic source has its own energy range in which X-rays of high intensity can be produced accompanied by low-intensity background radiation. Figure 3 shows such energy ranges for various sources. The hatched area represents the energy region where coherent and Compton scatterings take place, and the shadowed area represents the Compton continuum.

10 SECONDARY X-RAY SOURCES

Fe 55

Pu 238

• Cd 109

I В 1125

Pb 210

IlAm241

. 5 10 50 100

ENERGY (keV) FIG. 3. Most suitable energy regions foi various monoenergetic y- and X-ray sources. In the hatched area coherent and Compton scatterings take place. The shadowed area represents the Compton continuum.

SAMPLE ANNULAR X-RAY FILTER SODIUM . IODIDE CRYSTAL TUN6STEN ALLOY SHIELD TARGET

-PERSPEX- PRIMARY LIGHT GUIDE SOURCE SHIELD

PRIMARY SOURCE PHOTOMULTIPLIER

2 INCHES

FIG. 4. Source-target combination mounted in a scintillation counter.

Tominaga and Enomoto [7] have obtained the B/Nz ratios (background over К X-ray peak) for К X-rays of several elements excited by an 241Am source and these ratios are between 10"3 and 10"4 for X-rays in the energy range 15—40 keV.

11 The choice of source is made, when possible, by taking exciting energies just above the К absorption edge of the element to be measured. In some cases a slightly higher excitation energy is preferred, since it diminishes the contribution of the Compton-scattered X-rays to the intensity of the measured peak.

(d) Secondary, low-energy photon sources

The most popular and efficient secondary sources are of the type shown in Fig. 4, which was developed mainly by Watt [8] and Rhodes [9]. The main primary sources are loaCd, 147Pm/Al and И1Ат. By suitable choice of target it is possible to select the most efficient excitation energy for the element to be measured. The spectrum emitted by these sources is essentially composed of the К and L X-rays of the target. In practice this type of source assembly is used to produce nearly monochromatic X-rays in the energy range from 5 keV to 100 keV. Secondary X-ray sources have considerably enriched the number of low- energy sources available and have greatly widened the already varied range of possible applications of radioisotope X-ray fluorescence analyses. However, these sources cannot be used at present for applications requiring 'point' X-ray sources, since the smallest diameters available are about 2 cm. In addition, the intensity of the characteristic X-rays from the secondary target is low in the case of elements of atomic number below about Z = 30. This results in count rates of the order of hundreds rather than thousands per second, and this is too low for many applications. Beta-excited X-ray sources are also available [10—15]. An important contribution has been made by Preuss [16], who produced a catalogue of /3-excited X-ray spectra, and a comparison has been made of spectra obtained by means of (i:)Ni, 45Ca, 147Pm, M,T1, 32P and !,"(Sr + Y) under various conditions. Beta-excited X-ray sources emit characteristic X-rays of the target material accompanied by a continuous bremsstrahlung spectrum. Sources of H7Pm mixed with Al, Ag or Sb have been used for absorption measurements, but now their use has generally been abandoned in favour of low-energy photon emitters such as 241 Am.

(e) Very low-energy photon sources (E<4 keV)

There is a restricted number of radioisotope sources in this energy range and the 3H/ Zr source, giving 2-keV Zr L X-rays is the only photon source of any importance. It has been used to excite the characteristic X-rays of Si, Al and Mg in synthetic samples [17 ]. The possibility of using a-excited X-ray sources has been considered by Sellers and Ziegler [1] and the main considerations in their further development are choice of a-emitter and secondary target, self-absorption effects,

12 mechanical integrity of the target and generation of unwanted (3- and y-radiation. The application of a-excited X-ray sources in analysis is for elements of atomic number below Z = 20, and in this range they compete with direct a-excitation of characteristic X-rays. However, a-excited X-ray sources are preferred when preferential excitation of an element is required in the presence of another element of neighbouring and higher Z, such as for Na in the presence of Al. The technology for constructing a-excited X-ray sources has not yet been sufficiently developed for a final comment to be made on their usefulness.

(f) Alpha sources

Alpha sources have been used for the analysis of elements of low atomic number (below Z = 15) [18—21] and are the most important sources for X-ray fluorescence analysis of elements in this atomic number range. Alpha sources have been used for measurement of C, Na, Mg, Al and Si in solids, and for chlorine and sulphur in solution or in organic materials. If a sealed radiation detector is used, then, because of the low energy of the excited radiation, the measuring system must be contained in a vacuum or in an atmosphere of light gas such as hydrogen or helium. Asan alternative to the use of a sealed detector, a demountable detector may be used, in which case the measuring system is mounted within the detector [19]- This method permits immediate detection of contamination from the source and limits this contamination to the detector itself. Some of the problems that arise in the use of a-sources are listed below.

(i) Integrity of the source: this is the main problem, since all currently available a-emitters have a high toxicity. However, sources of 21"Po with nickel windows 6 ¡J.m thick are now available for use with activities over 10 m Ci.

(ii) Manufacture of the source window and sealing the source.

(iii) Production of unwanted /3- or -y-radiation: for this reason, but also due to their specific activity, 22(5Ra and M1Am have so far not been used. Their usefulness depends on the method of excitation and detection and also on the methods used to eliminate unwanted radiations.

(iv) Scarcity of pure a-emitters: 242Cm and 2J1Cm sources are expected to be generally available in the future. These sources are safer than 2lnPo sources, since the chemical form in which the isotope is used is not volatile, as is the case with 21(lPo.

13 (g) Beta sources

These have been used for about 15 years for direct excitation of characteristic X-rays [10—16, 20, 22], but they have found little practical application. This is mainly because low-energy photon sources are a better means of excitation and avoid the production of bremsstrahlung radiation in the sample. In direct excitation it is necessary to eliminate /З-particles backscattered towards the counter by using elemental filters and magnetic fields, but because of these requirements, the efficiency of the detection system is low. However, there are several important characteristics of this method and these are listed below.

(i) There is little matrix effect on the exciting radiation.

(ii) The non-compact geometry permits better filtering of the characteristic X-rays and allows several detectors to be used.

(iii) The samples can be examined through relatively thick windows, such as 0.3-mm polyethylene, more easily than with low-energy photon sources. Such windows contribute only a little bremsstrahlung whereas photon sources give rise to intense backscattering.

(iv) Analyses can be carried out on areas of down to 5-mm diam. and less.

(v) A wide range of elements can be analysed.

The most important sources are 3H/Zr, 147Pm, 2U4T1, S5Kr and 90(Sr + Y). In a vacuum, or in an atmosphere of He, /З-particles emitted from a 3H/Zr source can be used to bombard the sample and to excite low-energy X-rays [23]. However, a-emitters are superior to /З-emitters in the energy region below 1.5 — 2.0 keV because of the higher X-ray yield and lower background. A /З-source of 147Pm can be mounted in an apparatus with a geometry of high efficiency because unwanted, scattered /З-particles from this source can be absorbed by a thin Be foil in front of the counter window. This source can' be used to excite characteristic X-rays of elements of Z = 20 to 35 effectively [24]. The efficiency of X-ray generation by this source varies more, slowly with the energy of characteristic X-rays to be excited than when mono- energic X-ray or a-sources are used. Photon counts above 105 counts/ s can be obtained with a 1-Ci source for the elements from Ti to Zn. The main difficulty is the increase in counting error with decrease in concentration of the. wanted element, which is due to a high background radiation intensity. The low dependence of the mass attenuation coefficient on Z has another advantage: the l1IPm /3-source can be used conveniently for thickness gauging of coating materials of low Z.

14 Tanemura [25] reports that the sensitivity attained with this source hi measuring the thickness of a low-Z coating on steel is 1.5 times as large as that attained with a 3H/ Zr source. 90(Sr -I- Y) is available as an industrial source with stainless-steel windows. It has been used to excite characteristic X-rays from Z 92 to Z = 19. Comparison of this source with a 3H/Zr source to measure the copper concentration of aqueous solutions indicated that the /3-excitation method was preferred. Although this method of excitation has a number of useful characteristics, in general the advantages are not decisive for most analytical applications and better results can be obtained by using the most appropriate photon source.

A. 3.3. Comparison of the available excitation sources

Primary and secondary X- or y-ray excitation is by far the most widely used method in radioisotope X-ray spectrometry. At all energies, except in the range below about 2 keV, X-ray excitation is best, and useful sources and source-target assemblies are available for the whole energy range. Excitation by monochromatic X-rays is preferred to the use of bremsstrahlung sources because it is then generally possible to isolate the required characteristic X-rays from the unwanted scattered radiation by pulse-height analysis. Thus 3H/Zr and 147Pm/Al bremsstrahlung sources are being superseded by sources such as lmlCd, 23SPu and MlAm (which emit line spectra), now that the latter have become commercially available. Gamma- or X-ray-excited X-ray sources have found some use in the energy range 15 to 50 keV where they yield high output and spectral purity. There are two main energy regions which are still not adequately covered by available X- or y-ray sources. In the first, efficient excitation of Cr, iMn and 1-е К X-rays is not possible, as an exciting energy of 7 to 10 keV is required. The second region lies below 2 keV, where X-ray excitation becomes very inefficient due to low values of fluorescent yield and radioisotope X-ray sources are not available. Beta-particle excitation has a number of disadvantages associated with low excitation efficiency and high background count rates due to both bremsstrahlung and backscattered уЗ-particles. The advantage that the absorption of the incident /З-particles is independent of the matrix atomic number has been exploited very little. The main applications of direct уЗ-excitation have been carried out in France and Japan, but monochromatic photon sources of high activity have now become available and have superseded both /3-sources and /З-excited X-ray sources. Heavy-particle excitation of soft X-rays has the important advantages of high cross-section, low intensity of backscattered radiation and negligible bremsstrahlung production. The main drawback of cc-sources is the extreme fragility of any window having a reasonably high transmission for a-particles.

15 Nevertheless, they are the only sources available for efficient excitation of elements of very low atomic number.

A.3.4. A comment on proton excitation

Recently, proton excitation of soft X-rays has been investigated by using commercially available, low-voltage Cockcroft-Walton accelerators to produce proton beams of a few /¿A with an energy between 50 and 300 keV. High excitation efficiencies and extremely high signal-to-background ratios have been obtained experimentally [26]. At present, the main applications have been restricted to the determination of X-ray yields and ionization cross- sections and to the measurement of the thickness of very thin coatings. In this X-ray energy region (0.05 to 1 keV), quantitative elemental analysis is probably less important than studies of surface physics and surface chemistry and investigations of changes in energy and line shape of characteristic X-rays with different chemical binding energies. Excitation of soft X-rays with protons is highly efficient and is accompanied by such a low background that the number of its applications is bound to increase compared with the more conventional use of direct electron excitation or of high-current soft X-ray tubes.

A.4. PRESENT STATUS OF THE THEORY OF RADIOISOTOPE X-RAY FLUORESCENCE

The basic theory of X-ray excitation is adequately described in standard texts on conventional X-ray spectrometry. In radioisotope X-ray spectrometry, workers have been mainly concerned with simplified equations aimed at facilitating feasibility studies and optimizing design parameters of measuring systems. Three main assumptions are made in describing fluorescent X-ray excitation: monochromatic exciting X- or y-radiation, collimated primary and secondary beams, and short X-ray penetration into the sample compared with the dimensions of the apparatus. For polychromatic beams it is easy in principle to integrate over the spectra. However, experiments indicate that the narrow-beam theory fits broad-beam conditions fairly well. The third assumption is critical, enabling much simpler equations to be used because integration over the sample volume is avoided. All known' equations make this assumption. The assumption breaks down for broad-beam geometry, such as is used in radioisotope X-ray fluorescence at high X- or y-ray penetration, as in the К X-ray excitation of heavy elements in light matrices. Otherwise there is surprisingly good agreement between experiment and simplified theory. Simple practical formulae using a self-consistent set of symbols have been reported by Rhodes [27] for fluorescent, backscattered and transmitted X-ray yields as functions of sample thickness, composition and attenuation and scattering coefficients. Included are equations for calculating optimum sample thicknesses, optimum thicknesses for balanced filters, average mass

16 attenuation coefficients of compounds and mixtures, source emission efficiencies and detector absorption efficiencies. References to relevant data, such as excitation and scatter cross-sections, fluorescent yields and X-ray energies, are also given. Equations for X-ray intensities excited by penetrating y-rays, and for the yield of backscattered y-rays, have been derived and verified by Martinelli and Blanquet [28] for defined geometries. Watt [8] has derived and verified an equation for the yield of fluorescent and scattered X-rays from source-target assemblies. The equations referred to above [27] explain matrix absorption effects in homogeneous samples quantitatively. However, matrix enhancement effects have not been satisfactorily quantified because the degree of enhancement depends on the spatial origin of the enhancing fluorescent radiation in the sample. Until recently the theory of heterogeneity effects was mainly confined to reports by workers in conventional X-ray spectrometry, Claisse's equation describing the fluorescent X-ray yield as a function of particle size being the best known [29]. Recent research has shown that Claisse's equation is sometimes inadequate and Lubecki [30] has developed an alternative theory that includes the effect of packing density. Heterogeneity effects in radioisotope X-ray fluorescence analysis are currently under active investigation by a number of workers. Beta excitation of both characteristic X-rays and bremsstrahlung has been investigated theoretically and experimentally by Cameron and Rhodes [31] and by Filosofo et al. [32]. Reasonable agreement was obtained between experiment and theory and useful semi-empirical equations were derived. Heavy particles (e.g. a-particles and protons) can excite characteristic X-rays with negligible interference from bremsstrahlung and backscatter radiation. Excitation cross-sections increase rapidly with decreasing atomic number, so that a- and proton excitation is confined to light-element analyses. Sellers [ЗЗ] has just completed a survey of a-excitation theory and has measured some light-element cross-sections. Proton excitation (with acceler- ator proton sources) also has potential and basic and empirical theories have been reviewed by Khan and Potter [34] and by Marks et al. [35].

В. INSTRUMENTATION AND TECHNIQUES

B.l. COMPONENTS OF RADIOISOTOPE X-RAY FLUORESCENT ANALYSERS

В. 1.1. Radioactive sources

The sources used in radioisotope X-ray fluorescence instruments emit either a- or /З-particles or X- or -y-radiation. Once the choice of method of excitation, has been 'made, the choice of the best source within each type is made' on the basis of such factors as half-life, purity, available specific activity, radiation energy and cost and from general safety considerations. "

(a) Alpha sources < •

Until recently it was generally considered that high activity a-sources were not sufficiently safe for use outside the laboratory. However, in view of the important work reported by Sellers and Ziegler and the general interest now shown in this technique, further consideration should be given to such sources. A list of suitable sources is given in Table IV, but so far most of the work reported has been done with polonium-210. However, commercially available sources of this isotope have a high failure rate. Curium-242 is best suited for the manufacture of a safe'a-source in that it is chemically more stable than polonium-210 and its half-life is reasonably short, so that long-term damage to the window will be restricted. The higher energy of the alpha particles permits a thicker window, to be used than with polonium-210. • Various windows have been used for a-sources, including aluminium, stainless steel, iron, nickel and platinum.

(b) Beta sources •

An excellent compilation of/3-excited X-ray spectra has recently been produced by Preuss [16] based on studies with 147Pm', 45Ca, 32P, 63Ni, 204T1 and 30Sr. It is suggested that promethium is the most suitable ¡3-source for most applications but that there might be a problem with the production of high activity sources. Ptomethium - 147 sources with specific activities greater than lCi/cm2 are now commercially available.

A photon source will be required for most applications in radioisotope X-ray fluorescence analysis. Three groups of photon source can be dis-.

19 tinguished: 'high-energy* sources, low-energy' sources and source-target assemblies.

(i) High-energy photon sources

These sources, including 192Ir,137Cs and 60Co, are easily available, inexpensive and sturdy; high activities can be obtained without difficulty. However, their use in radioisotope X-ray fluorescence is limited.

(ii) Low-energy sources

The development and production in the United Kingdom of low- energy photon sources (including bremsstrahlung sources) which are suitable for use in radioisotope X-ray fluorescence applications have recently been reviewed by Ansell and Stevenson [36] and by Stevenson and Myerscough [37]. The properties of these sources are given in Tables V - VIII. The following points are worth noting:

— Nuclides which decay by a-emission have the advantage of negligible bremsstrahlung, but the disadvantage of high toxicity.

— Nuclides which decay by /З-emission are less toxic than a-emitters but are accompanied by strong bremsstrahlung radiation.

— Nuclides which decay by electron capture have low toxicity and those now in use have litde supplementary (unwanted) radiation.

— Tritium bremsstrahlung and 56Fe sources are the only sources not currently available as sealed sources, but sealed beryllium capsules will be available in the near future.

TABLE IV. AVAILABLE ALPHA EMITTERS RELATIVELY FREE FROM HIGH-ENERGY GAMMA RADIATION

Alpha energy High-activity sources NucUde Half-life (MeV) available

ZlOp 138 d 5.3 Yes

»Cm 160 d 6.1 On loan in United States of America Commercially 1958/59 in United Kingdom

шСш 17.6 yr . 5.8 No

238p„ 86 yr 5.4 Yes

"'Am 458 yr 5.4 Yes

239Pu 24 000 yr 5.1 No

20 TABLE V. PROPERTIES OF NUCLIDES USED AS PRIMARY X-RAY SOURCES

Photon emission Half-life Mode of Nuclide (yr) decay keV °!o

55 Fe 2.7 ECa 6 Mn К X-rays 28.5

238pu 86.4 a 12-17 U L X-rays 13

i°'Cd 1.27 EC 88 4 22 Ag К X-rays 107

125[ 0.16 EC 35 7 27 Те К X-rays 138

"°Pb 22 в 47 4 11-13 Bi L X-rays 24,- plus bremsstrahlung up to 1.17 MeV

"'Am 458 a 60 36 14-21 Np L X-rays 37 662 10"3

153 Gd 0. 65 EC 103 20 97 30 70 2.6 41 Eu К X-rays 110

"Co 0. 74 EC 700 0.2 136 8.8 122 88.9 14 8.2 ' 6.4 Fe К X-rays

a Electron capture.

(iii) Bremsstrahlung sources

Bremsstrahlung produced by the passage oí /í-particles through matter has a continuous energy spectrum up to the maximum energy of the уЗ-particles. The average energy of the bremsstrahlung is only about 10°/. to 20°/. of this energy. Although a number of pure /3-emitters exists, the production of bremsstrahlung sources hàs been restricted mainly to aH and H7Pm, with a lesser demand for S5Kr and Э05г/90У. The characteristics of these sources are given in Table VI.

(d) Source design

The basic problem of source design is the combination of effective containment of the radioactive materials with efficient emission of the required radiation. The radiation output depends on 'internal' absorption within the source material and also on 'external' absorption in the source window. The higher

21 the specific activity the lower the internal absorption and, for bremsstrahlung sources, the lower the effective energy of the emitted radiation.

(i) Disc sources

This shape of source is that most commonly used and the construction of sources of this type for nuclides in the energy ranges 10—50 keV and above 50 keV is shown in Figs 5 and 6.

(ii) Point, line and cylindrical sources . ..

For particular applications sources of other shapes are sometimes preferable to disc sources.

TABLE VI. PROPERTIES OF NUCLIDES USED IN BREMSSTRAHLUNG SOURCES

Efficiency of Useful Max. Half-life bremsstrahlung energy Nuclide В-energy Target (yr) production • range (keV) (photons/e) (keV)

3H 12.3 18 Zr 4 x 10"5 2 - 12

»'Pm 2.6 220 Al 2.5 x 10"3 10 - 100

85Kr 10.7 670 С 1 x 10"2 25 - 100

90Sr/90Y 28 2270 Al 6 x Ю"г 50 - 200

TABLE VII. PURITY OF LOW-ENERGY X-RAY SOURCES

Other radioactive nuclides and subsidiary Nuclide radiation which may be present

' ! -ssFe 59Fe and MMn may be present but activity is < 0. l^o

• 238pu < 0.17» of other y-emitting impurities, decay products of other Pu isotopes

• Cyclotron production: no other y-emitting isotopes: though'occasionally 65Zn is present 1091 Cd Reactor production: a cheaper method of production but has lower specific activity and moretimpurities

1251 No impurities

21»pb Bremsstrahlung up to 1 MeV. Up to 0.1 % of 226Ra and decay products

"'Am 10-37o, 0.662 MeV y-rays ' •

i53Gd Virtually pure

0.2%, 700 keV y-radiation 58Co and56 Co also present to < lío

22 Point sources are useful for inserting into narrow apertures and for measurements on small surface areas. Glass beads of 2-mm diam. containing up to 14 mCi 2_llAm and 1.25 Ci 1J7Pm are now available for this type of application. Line sources, mainly for applications on cylindrical and elongated surfaces, have been prepared by filling a groove in a stainless steel or platinum block with a suitable enamel containing the radionuclide. The nuclide is then covered with a thin protective window. Cylindrical sources are useful for applications inside tubes. The increased application of radioisotope X-ray fluorescence to borehole logging is likely to see a greater demand for sources of this shape.

(iii) Source-target assemblies

An arrangement to give characteristic X-rays virtually free from scattered radiation is shown'in Fig. 4 [8, 9, 38]. This arrangement is well-tried and. has been used mainly in 'on-stream' applications where the relatively low photon output can be tolerated. A different design of source-target combination which has recently been developed by Ansell [39] is shown in Fig. 7. This design enables much greater activities to be used and the present size is almost small enough to be incorporated into 2-in. diam. scintillation counter assemblies, or to be used with near standard proportional counters. So far, Pm/Al sources with an activity of 12 Ci (=1 mCi 109Cd) have been incorporated into this construction.

TABLE VIII. COMPARISON OF THEORETICAL AND AVAILABLE SPECIFIC ACTIVITIES OF LOW-ENERGY X-RAY SOURCES

Commercially Theoretical available specific Method of Nuclide specific activity production activity ' (Ci/g) (Ci/g)

55 Fe 2.2 xlO3 3-10 Reactor. Could use cyclotron and approach theoretical specific activity

Z38Pu 16.8 13 Reactor

>°'Cd 2.55 x 103 ~ Theoretical Cyclotron

I2SJ 1.07 XlO4 ~ Theoretical Reactor

zupb . 88 5-50 Natural radioisotope

"'Am 3.3 Theoretical Reactor

ls3Gd 3. 5 x 103 16 Reactor

"Co 8. 5 x 103 6 x юз Cyclotron

23 STAINLESS STEEL 60LD FRICTION WELD ALUMINIUM

FIG. 6. Source capsule for 10 to 50 keV.

FIG. 7. Small, high-intensity source-target combination.

(e) Cost

The cost of a source depends upon the cost of the original nuclide and the Complexity of the source design. Some nuclides, particularly those produced by cyclotron techniques, are expensive. To obtain an approximate cost comparison, the cost of the quantity of each nuclide required to give a total emission of 10' photons/ s is shown in Table IX. .. . .

24 TABLE IX. COMPARISON OF COSTS OF RADIOISOTOPE SOURCES BASED ON A TOTAL EMISSION OF 107 PHOTONS/SECOND

Energy Activity Nuclide £. (sterling) (keV) (mCi)

241Am 60 0.7 0.35

238pu 14 - 17 2 10

109Cd 22 0.25 25

USj 27 + 35 1.9 5

55 Fe 6 1 10

210pb 10 - 13 1.1 1.7

!S3Gd 97 + 103 0.5 25

"Co 122 0.3 3

w'Pm/Al 10 - 40 130 Less than 1

3H/Zr 3 - 12 5000 7.5

Prices are based on figures obtained from R. С. C. Amersham. The price of 238Pu may fall in the near future. The price of ls3Gd is much lower in Australia.

B.1.2. Detectors

For most practical purposes only four types of detector need be considered: scintillation detectors, sealed gas-filled proportional counters, ion chambers and solid-state detectors. Each type has its own special characteristics which make it suitable for a given application. In général, scintillation detectors are used for X-ray energies above about 5 keV. However, recent improvements in photomultipliers have led to the application of Nal(Tl) X-ray detectors for the measurement of sulphur К X-rays (2.3 keV). This is the lower practical limit of detection in many cases, since, below this energy, air paths at atmospheric pressure between sample and detector result in a high attenuation of the fluorescent X-rays. Proportional counters offer the advantage of higher resolution coupled with an ability to discriminate against backscattered primary radiation by choosing the correct filling gas. Argon, xenon, neon and krypton, when mixed with a suitable quenching gas, may each be used as a filling gas in a proportional counter. As a result of improvements in thin, vacuum-tight beryllium windows, flow proportional counters need not be used above 1.48 keV (Al K), although at this energy a vacuum, or hydrogen or helium path for the К X-rays is required. When their superior energy resolution is an important requirement, proportional counters can be used as efficient X-ray detectors up to about 20 keV, an energy which includes К X-rays from light and medium atomic weights and L X-rays of heavy elements. Certain undesirable characteristics in the operation of proportional counters at these energies have been

25 reported. For example, space charge effects are known to reduce the pulse height as the detected count raté increases, and wall effects reduce the useful energy resolution by producing a non-Gaussian photopeak. These effects can be particularly important when measuring elements at low concentrations. The space charge effect can result in a significant dependence of gain upon count rate and this is illustrated in Fig. 8. However, careful attention to the design of the detector and the use of a thin anode wire can reduce this effect. On the other hand, the use of a wire which is too thin can lead to trouble from microphony.

TOTAL INPUT COUNT RATE (counts Is)

FIG. 8. Variation of gain with count rate in a proportional counter due to the effect of space charge.

For low-energy work with, sealed counters, it is . essential to use the thinnest possible window, which must, however, be adequately impermeable. Grid-supported beryllium windows only 25 /¿m thick have been used satisfactorily. The thinriest unsupported window reported to date is 50 /jm, It may be noted that sealed proportional counters have performed well over periods of many months in unfavourable industrial environments. In contrast, serious temperature effects have been reported in scintillation detectors, giving rise to significant changes in gain and count rate. Sealed proportional counters are relatively free from-this defect. The use of ion chambers is generally restricted to industrial applications such as coating thickness measurements, where ruggedness and stability are the primary requirements. The advent of lithium-drifted silicon solid-state detectors with a resolution of better than 400 eV promises to extend and improve non-dispersive X-ray fluorescence techniques. The main advantage .of this detector is its ability to collect a complete and well-resolved spectrum simultaneously from a wide range of elements in the sample. This is ideal for qualitative analysis of unknown samples, and • equipment available at present is well suited to this application.

26 The need to cryogenically cool the detector and first amplifier stage is not considered to be a, significant practical drawback. The stability of the whole spectrometer should be superior to that of spectrometers with X-ray tubes, photomultiplier tubes or proportional counters, firstly, because solid- state circuitry is used throughout and the critical components are maintained at a constant temperature, and secondly, because the total system gain.is in the electronic circuitry and therefore capable of being stabilized by feedback.

X-RAY ENERGY (keV)

FIG. 9- Efficiency of .Si(Li) detector of 3-m'm thickness for detection of X-rays in the energy range 3-100 keV.

• A figure of merit for overall performance compared with other systems is not easy to quantify/but energy resolution, lower energy limit, size of detector and count-rate-gain stability are probably the most important factors, in, that order. Figure 1 compares the energy resolution as a function of energy of Si(Li), scintillation, proportional and dispersive systems in the range 1 to 100 keV. Solid-state detectors are clearly superior for detection of medium- and heavy-element К X-rays. Howeveir, they are not suitable for detection of X-rays .with energies less than about 3 keV. The present low- energy limit is a function of system noise and resolution and is likely to be lowered as resolution improves. The main obstacle to improvement in resolution is noise rather than intrinsic lack of resolution in the detector. The Fano factor for Si (and Ge) has been shown to be as. low as 0.15, yielding an. intrinsic value for the resolution (full width at half maximum) of 0.12 keV at 5 keV (0.11 keV for Ge). High analytical sensitivity is achieved not only through good energy resolution, but also by obtaining large detected signals. At present this is the weakest feature of solid-state X-ray spectrometers. Firstly, energy resolution falls off rapidly, with increasing detector area, practical resolutions being achievable only with detectors of area 80 mm2 or less. The poor geometrical efficiency obtained means that curie-strength sources (or even X-ray tubes) are required to realize the potential of parts per million sensitivity in short counting times. In the past year, the tolerance of Si(Li) detectors to high

27 count rates has improved, and count rates of 2 X 104 counts/s now have little effect on the resolution of commercially available detectors. The efficiency of Si(Li) and Ge(Li) detectors for 2- to 100-keV photons can be calculated from tabulated values of their mass attenuation coefficients. For the lower energy photons, a 0.12- to 0.25-mm (5-10 thou) Be window is provided in the front face of the cryostat. Figure 9 shows the variation in efficiency with photon energy for a typical 3-mm-thick Si(Li) detector behind a 0.13-mm-thick (0.005 in.) Be window. Over this same energy region, a Ge(Li) detector is almost 100% efficient, except in the low-energy region where it follows the Si(Li) efficiency curve.

B.1.3. Electronic instrumentation

Commercially available electronic equipment is capable of meeting all normal requirements and no further comment is necessary. However, in industrial environments care may be needed to protect the equipment, especially against excessively high temperatures and against dust.

B.1.4. Windows

В. 1.4.1. Analysis cell windows

Continuous analysis of aqueous dispersions is normally conducted in an analysis cell through which the sample flows, sometimes under a pressure of a few pounds per square inch. Cell windows must therefore be capable of use at the prevailing pressure without creep, be capable of resisting puncture, abrasion and chemical attack, and exhibit low absorption characteristics. Cell windows should also be of a form which facilitates rapid replacement and accurate tensioning and positioning, since window distension can give rise to substantial errors. 'Melinex'^is a preferred material from which strong, stable, preformed windows can be made in thicknesses ranging from 10 to 125 /u.m. To provide additional abrasion resistance, such windows can be coated on the inside face with boron or some other suitable material. At- tempts have been made to produce 'Melinex' windows 5 fim thick and rein- forced with a nylon grid, but so far they do not appear to have been successful. Polypropylene has recently been used to make thin cell windows and this material offers the advantage of rigidity combined with an unusually low absorption coefficient for soft X-rays.

В. 1.4.2. Windows for powder presenters

It is generally advantageous to examine flowing powder samples through a window in a sample presenter rather than from above the material onto a free powder surface. Requirements here are similar to those indicated

1 ) Proprietary brand of polyethylene terephthalate film.

28 above but are generally less severe as regards mechanical strength and abrasion resistance since pressure is negligible. Plastic materials such as 'Melinex' or polypropylene have been used satisfactorily and in one application, where operation at an elevated temperature was required, beryllium was employed [40]. It is generally agreed that plastic windows are suitable for these applications and that wear is not a serious problem if attention is paid to calibration. Deposition of solids on the window, by either chemical or electrostatic means, may be a problem but can be reduced by increasing abrasion, by the use of a suitable sequestering agent and by anti-static sprays.

В. 1.5. X-ray filters for energy selection

In radioisotope X-ray fluorescence, X-ray filters are now in general use for energy selection. They are simple, inexpensive and stable and they are capable of giving a high energy resolution sufficient to isolate the К X-rays of most elements. They are usually in the form of thin sheets covering the detector window.

В.1.5.1. Construction of X-ray filters

(a) Elemental foils

Filters of the elements Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Zr, Mo, Rh, Pd, Ag, W, Re and Ir and the alloy Mn/Ni (85 %/15 %) can be made in the form of metal foils of a thickness varying between 0.0005 in. and 0.005 in. The thickness of most of these foils can be reduced by etching in either dilute HNO3 or aqua regia (2 parts HC1 to 1 part HNO3); however, Ti is etched in dilute HF solution.

( b ) Polythene filters

If the filter cannot be made as a foil it can be made as a plastic disc by mixing a fine powder (200 B.S. mesh) of a suitable compound of the element (usually the oxide) with polythene heated to its melting point and by hot pressing, with annular spacers used to achieve the required thickness. The maximum ratio of powder to polythene which can be tolerated is about 1/3. Filters of the elements Ga, Ge, Sr and Y have been made in this way and there seems to be no reason why the same technique could not be used for other elements. If other fabricating compounds are chosen (e.g. metallurgical mounting plastics), care must be taken to ensure that the element does not react with the chosen material. This problem arises when introducing Os into some plastics.

29 (с) Ероху resin filters

For high-Z elements in particular, it is more convenient to manufacture the required range of filters from a mixture of the element, or one of its compounds, with an epoxy resin. A high ratio of powder to resin of about 5/2 can be achieved and this enables an acceptably thin filter to be used and the desired source-detector separation to be preserved. Great care must be taken to ensure that the element is thoroughly mixed in the resin and that no air bubbles, for instance, are introduced during the mixing process. Filters made- of elements from Al to Ca in the periodic table, require special techniques because of their very low mass per unit area (1 to 3 mg/cm2). For example, silicon filters have been made by depositing 1 to 2 mg/cm2 Si on terylene film 0.00025 in. thick. Two new methods are worth mentioning, both suitable for making filters for light-element analysis. Huth [41] reports the use of vacuum evapo- ration to deposit a thin layer of the required element on'the window of his avalanche detector. Dunne [42] reports a method which involves suspending the powdered element (or suitable salt) as a slurry in a solution of polystyrene in toluene. This is then spread out in an even film of Controlled thickness with a standard film-casting knife. After drying, the film is peeled off and the filters cut out. Area densities of film are adjusted by varying the toluene to polystyrene ratio of the slurries. Sulphur and phosphorus balanced filters have been made successfully by this method. The choice between metal foil and plastic filters when both are available appears to be a matter of personal preference. No systematic comparison has been carried out. However, the flexibility of polythene filters is consistent with the development of a rapid filter-changing mechanism similar to that of photographic film in a camera.

B.1.6. Cooling systems

Cryostats are needed for use with solid-state detectors and a number are now available commercially. Details of one assembly2,^ the design of which is typical of most others, is given below.

В.1.6.1. Cryostat assembly

In one cryostat assembly, the detector is thermally attached to a copper rod by means of a piece of beryllium which also serves to insulate it electri- cally from earth potential. A vacuum of 10"8 Torr is maintained around the detector by means of an ion pump to prevent any surface contamination. Since the device is to operate with a leakage current of less than 1040A, clean operating conditions are essential. X-rays enter the cryostat through a 0.005-in. beryllium window situated within a few mm of the detector. The first stage of the preamplifier is also cooled. Since the FET incorporated in

2) Supplied by Nuclear Enterprises Ltd.

30 this stage has optimum noise at 140 °K, it is mounted on a thermal insulator and maintained at this temperature by its own electrical' heat input. . : A constant supply of liquid nitrogen is provided by a> 10- or 25-litre reservoir which guarantees operation for a minimum of 10 days .and usually longer, depending on ambient conditions. > At present the use of solid-state detectors is restricted to . the laboratory, but should they be required for field use further development of cryostat systems will be needed. . '

B.l.7. Multi-element computers

In conventional X-ráy spectrometry, the use of either on-line or off-line computers has been established practice for several years. They enable standardization, interelement corrections and conversion to the required output to be made automatically. The use of computers with radioisotope X-ray fluorescence analysis appears at first sight to be ruled out because of the cost. However, a closer examination shows this is not so, especially for on-stream analysis. In a typical system the input from three or four sensing points would consist of between ten and twenty digital signals. A conventional measuring system would employ an equal number of scalers controlled by a timer. Processing of the output data would be complicated. With a low-cost computer, after a simple divide by 'n' stage (the latter being selected to fit the system's count rates), the computer can either cycle through the input points from the divide by n stages, store any n'th pulse, and carry out the calculations at the end of a. selected time, or it can carry out the calculations continuously, interrupting these to store any incoming n'th pulse as it arrives. The former method is more suited to laboratory instruments, where a definite measuring break between sample loading and unloading exists; the latter method is particularly attractive for on-stream analysis. A further major advantage of using a computer is that it allows the use of a fully automatic standardization cycle, which is essential for on-stream analysis and is difficult to achieve by other methods. In many on-stream applications, the conditions of measurement can be adjusted so that thé calculations are simple arithmetic functions. Wherè this is not so, either the storage of nomograms or suitable mathematical models are required. The building of the latter should be facilitated with radioisotope X-ray fluorescence analysis, since in its most efficient form it employs essentially monochromatic excitation. This allows a precise treatment of primary mass absorption coefficients, which is difficult to carry out with the polychromatic excitation obtained from an X-ray tube. For the simpler cases, the cost of such a digital data processor would probably not be acceptable. A new method of analog computation offers promise of . a technically satisfactory and economical means of overcoming this difficulty. A relatively simple and potentially cheap device is currently being developed to solve automatically nomograms involving two independent var- ables [43].

31 Matrix correction and similar procedures by means of nomograms are already well established [44,45], and it is anticipated that this new computing aid will permit the application of these methods in the field of continuous analysis. Adler and Tromka [21] have demonstrated that a comparatively small digital computer is capable of resolving overlapping peaks by a matching procedure so as to produce effective resolutions of the same order as those obtained in conventional dispersive analysis. This procedure is compatible with transmission of primary measurements from a number of remote sensing heads to a central computer for processing.

B.2. ANALYTICAL INSTRUMENTS AND SYSTEMS

B.2.1. Commercially available systems

В.2.1.1. Portable and laboratory instrumentation

At least ten manufacturers now offer portable and/or laboratory radioisotope X-ray fluorescence systems for sale. As far as is known these firms are:

United Kingdom

Ekco Electronics Ltd., Southend-on-Sea, Essex. Hilger and Watts Ltd., 98 St. Paneras Way, Camden Road, London N.W. 1. Nuclear Enterprises Ltd., Beenham Grange, Aldermaston Wharf, Nr. Reading, Berks.

Telsec Instruments Ltd., Sandy Lane West, Littlemore, Oxford.

United States of America Nucleonic Data Systems, Inc., 7461 Lorge Circle, Huntington Beach, Calif. Panametrics, Inc., 221 Crescent Street, Waltham, Mass. 02154. Texas Nuclear Corporation, 9Ю1 Highway 183, P.O. Box 9267, Austin, Texas 78756.

Austria

EIC Elektronische Instrumente und Kontrolleinrichtungen Ges.m.b.H., Opernring l/E/628, Vienna.

France

Nucléomètre, 17 Boulevard de la Liberation, 93 Saint Denis (Seine).

Japan

Rigaku, 9-8, 2 Chôme, Sotokanda, Chiyoda-Ku, Tokyo.

32 It is claimed for all portable systems that they are useful for laboratory type measurements, i.e. that they are capable of operating from external power supplies and of having an optional sample holder and shield. How- ever, only four firms provide a strictly non-portable laboratory model which cannot be operated from batteries. Six of the firms incorporate a measuring head containing a proportional counter as the primary X-ray detector, but some of them offer a scintillation detector as an alternative for some applications. Four firms use only scintillation counters in their measuring heads. One firm Uses a dual proportional counter as a means of facilitating the use of balanced filter techniques. The measuring head is arranged so that each counter observes only one-half of the sample, which can be rotated, allowing duplicate measurements to be made and thereby removing effects of sample inhomogeneity. At least two firms offer the option of filter holders having up to six or more filters to enable multi-element analyses to be made. All of the systems available provide some form of electronic pulse-height discrimination, a timing function (some having a single fixed time) and a readout in either digital or analog form. However, only two of the units have an analog readout which reflects the fact, indicated in field experiments, that user readout preference is: digital, integrating ratemeter and conventional ratemeter, in that order. Detailed specifications of these instruments are given in Appendix I. Some of the instruments are shown in Figs 10 — 15.

B.2.1.2. On-line systems

At least four firms offer on-line radioisotope X-ray fluorescence analysis equipment: Cartner Group Limited (Mintek Division) and Nuclear Enterprises Ltd. in the United Kingdom and Nucléomètre and CGEI Lepaute in France. A brief description of equipment made by these firms is given below.

(a) Cartner Group Limited (Mintek Division)

A range of units is available for the continuous analysis of minerals, ores and allied materials either as dry powders or aqueous dispersions. The equipment is modular in concept so that a complete system can be built up from standard units to suit a given application. Maximum use is made of micrologic circuits and measurements are made digitally with an automatic standardizing procedure. Parallel analog outputs can be provided by means of an accurate digital/analog converter and hence primary count data can be processed either in the digital or analog mode. The basic components include a powder presentation unit, a slurry sampling unit and the radioisotope X-ray fluorescence unit. The powder presentation unit supplies samples of dry powders (nominally finer than 350 ¡xm) at controlled mass per unit area and bulk density to a measuring head for either transmission or backscatter measurements. The slurry sampling unit permits selection of a slurry sample from up to six streams and provides facilities for mixing or dilution and the removal of coarse debris before the sample is pumped through the rest of the

33 FIG. 10. Portable isotope fluorescence analyser (Hilger and Watts Ltd.).

circuit. The radioisotope X-ray fluorescence unit provides facilities for both X-ray fluorescence and transmission (absorption) correction measurements with automatic source and filter change mechanisms which permit selection from a maximum of six sources and six filters. Either sealed proportional or scintillation counters may be used. Measurements are made under a constant pressure of about 3 lb/ in2 and a window puncture alarm can be fitted.

(b) Nuclear Enterprises Ltd.

The 'Atomat Tin Coating Gauge' which continuously monitors tin thickness is marketed by this firm. Two separate source-detector systems are mounted on each of two heads, one inspecting the top of the strip and the other the bottom. Fifteen 3H/Zr sources with a total activity of 40 Ci are mounted in a row along the centre of each detector window. To standardize the gauge, the heads are withdrawn to the side of the strip where they are positioned in 'garages' in front of steel reference plates.

34 FIG. 11. Mineral analyser (Ekco Electronics Ltd.).

The ionization current corresponding to zero tin-coating thickness is approximately 1Û"11 A. Measurements to 1 % accuracy in thickness involve measurement of ion currents to an accuracy of 0.2% of the above value, i.e. 2 X 10"14A at the bottom end of the thickness range and approximately 4 X 10"14A at the top end of the range. Solid-state amplifiers are used and the amplifier load resistor and ionization chamber are thermostated. The 0.75-in. air gap is also air purged to prevent build up of extraneous matter on sources or detectors. The required stability is then easily achieved. The potential health hazard due to the presence of the tritium source is minimal. Small amounts of tritium gas are emitted from the 3H/Zr sources (estimated at 0.5 /uCi/'Ci day) but measurements by the United Kingdom Radiological Protection Service indicated that "both the total tritium and the tritiated water vapour concentrations in the air at the exhaust point are some two orders of magnitude less than the relevant maximum permissible concentrations for unclassified workers". Tin coatings are usually between 0.2 and 1.5 lb/base box (0.35 — 2.32 fim) and the gauge is accurate to ±2% and reproducible to ± 1 % within this range when using a response time (i.e. 5 time constants) of 7.5 s. The optimum pass-line for the tin-coated sheet is 19 mm (0.75 in.) away from the face of the measuring head, and at this distance a pass-line variation of ± 2 mm (±0.08 in.) corresponds to a variation in coating thickness of less than 1 %.

35 FIG. 12. Portable mineral analyser (Telsec Instruments Ltd.).

FIG. 1 3. Portable unit, X-ray spectrochemical analyser (Panametrics Inc.). FIG. 14. Portable X-ray fluorescence analyser (Texas Nuclear Corporation).

A second version of this gauge is made for monitoring zinc thickness. This is the 'Atomat Zinc Gauge'. It contains a balanced ionization chamber, a 1-Ci americium-241 source, a fail-safe source shutter, a solid-state preamplifier, thermostating controls and two sheet-edge sensors. A precision of + 2 % of the coating weight is obtained with a 2-s time constant. The optimum pass-line for the tin-coated sheet is 20 mm from the face of the measuring head and at this distance a pass-line variation of + 2 mm (+0.08 in.) corresponds to a variation of less than 1 % in coating thickness.

This firm also manufactures on-line gauges to measure the thickness of zinc coatings in galvanized steel strip and the thickness of tin coatings in electrolytically-tinned sheet. The principles of these gauges are the same as described above. The manufacturers state that in the zinc-coating gauge measurement normally takes place on both sides of the strip simultaneously and is presented as deviation from a target value set in on a dial of digital pattern calibrated in actual weight over the range 0.25 to 3.0 oz/ft2. By duplicating

37 Electronic equipment

Measuring head (with sample holder)

FIG.15. Fluoscope (Nucléomètre).

38 the target setting controls and the indicators, it is possible to measure differential coatings, and the measurements of top- and bottom-side coatings are then summed and displayed as total coating (sum of both sides) and deviation from desired total coating. A unique arrangement of width determination enables the measuring heads to scan from edge to edge of the sheet, automatically following any change in width or sideways wander of the sheet. Further refinements include the ability to scan repetitively any preset sector of the sheet. A variation of this equipment is available for measurement and control of electrolytically-tinned sheet.

(d) Nucléomètre

On-line gauges for measuring zinc coatings are marketed by this firm. A zinc gauge, for on-line measurement of zinc coatings on either side of steel sheets, utilizes a source of americium-241 and a proportional counter to detect the zinc К X-rays. Iron К X-rays coming from the galvanized sheet are filtered by an aluminium foil which also acts as a light-tight window for the measuring head. The count rate is thus essentially due to the zinc К X-ray intensity.

(e) CGEI Lepaute

This firm manufactures an X-ray fluorescence gauge for the measurement of the calcium content of raw materials. The measuring head contains a proportional counter with two tritium bremsstrahlung sources placed one on each side of the window of the counter. The measuring head is temperature controlled for greater stability.

(f) EIC Elektronische Instrumente und Kontrolleinrichtungen Ges.m.b.H.

The EIC Model 200 on-line coating weight gauge uses digital counting techniques and thereby avoids all known sources of error common to analog systems. The EIC gauge covers, all coatings Currently applied to steel, like aluminum, zinc and tin. . By the use of a special high resolution proportional counter it is possible to achieve a sensitivity of a factor of ten in count rate between the bottom and the top end" of the measuring range. That and the better stability of the digital system result in a gauge with excellent long-term stability features. An ACC (Automatic Calibration Control) circuit corrects for any drift occurring in the amplifier, the high-voltage supply or the proportional counter tube. In on-line tests, the gauge has given accurate readouts over a period of over two months without any recalibration. The display gives the coating weight directly in the applicable units in a digital form. Typical accuracies are 1 — 2% of coating weight. Solid-state electronics is used throughout the instrument, with integrated circuits wherever feasible.

39 The detector head configuration has been made so that an air gap of 2 in. between the strip surface and the window can be achieved, with minimum sensitivity to passline variations. Pass-line variations of ±0.150 in. result in a coating weight error of ±0.01 oz/ft2. in the case of zinc or aluminium. The scanning system is controlled by solid-state logic, ensuring fail- safe operation. The modes of operation are standby, normal scan, manual positioning and automatic positioning in three preselected spots. The pre- selection switch is calibrated in inches from the edge of the strip. In this way an automatic three-spot test can be performed and the result can be printed. The instrument is manufactured in the United States of America by Nucleonic Data Systems, Inc.

B.2.2. Other systems in use or under development

B.2.2.1. Portable and laboratory instrumentation

Atomic Energy of Canada Limited is reported to be developing a portable radioisotope X-ray fluorescence analyser which may be lighter and smaller than existing systems (2.5 kg and 3500 cm3). From the point of view of the user in the field (especially the geologist), further reductions in size and weight below those of existing equipment appear to be necessary. Under development for the US National Aeronautics and Space Administration (NASA) by Panametrics Inc. is a prototype device for ultimate use on manned lunar missions to measure the composition of the lunar surface. This device uses an a-excited X-ray technique. As reported in section B.3, this technique is especially suitable for the analysis of very low-atomic-number elements. It provides a simple means of extending radioisotope X-ray spectrochemical analysis to low-Z material such as Na, Mg, Si, etc., This program has involved the development of a thin window (1-mil Be) sealed proportional counter capable of being operated in a vacuum. In addition to counter design, the materials used in the measuring head were also investigated to reduce interference from unwanted radiation. It was found th&t curium-242, which emits both a-particles and low energy X-rays, was best for this application because the a-particles excite the low-Z elements while the X-rays excite the higher Z elements. With this instrumentation a successful series of measurements on selected geological samples has been carried out. Since the lunar mission application is one which only allows a short on-site measuring time but a long period of data processing on earth, the use of computer techniques for data processing was also investigated. Such data analysis methods were applied to measurements obtained on geological samples and it was found that excellent results were obtained (in some cases equivalent to those obtainable when using a conventional dif- fracting type of X-ray spectrochemical analyser). Further testing of this proto-

40 type system is being carried out in the United States of America at the Goddard Space Flight Laboratory. In France, work on the development of the a-X-ray technique for industrial applications is being done by CEA, Saclay.

B.2.2.2. On-line systems

It has - been reported that several on-line systems are being developed or are Under evaluation in France. These include an on-line zinc coating gauge, with americium-241 as a source and a proportional counter as the X-ray detector, and a letter sorting device based on the detection of zinc К X-rays from stamps made from paper containing a high percentage of ZnO. In Australia, a gauge is under development for measuring the lead- content in slurries by using 1 m Ci 60Co and 20 m Ci 151Gd sources in a two- source attenuation gauge configuration. In England an X-ray fluorescent system is being used in pilot plant operation at Warren Spring Laboratory. The system is modular and the necessary components can be incorporated into the analysis line as the application requires. Equipment to monitor the fineness of grinding mill outputs is being developed by the Cartner Group (Mintek Division). .

B.3. TECHNIQUES FOR MINIMIZING INTERFERENCE

The accuracy of radioisotope X-ray fluorescence analysis may be affected by the presence of other elements and by the particulate nature of the sample. These phenomena are generally referred to as matrix and particle-size effects, respectively.

B.3.1. Matrix effects

There are two main matrix effects: absorption of the fluorescent radiation of the wanted element by heavy elements in the matrix and enhancement of the fluorescent radiation of this element by higher energy fluorescent radiation from other elements present in the matrix. In the last few years much work has been done to overcome matrix effects and most of these have been reviewed by Rhodes [46]. Some new and more generalized methods to eliminate these effects have since been published [47, 48]. However, a clear, self-consistent theory for fluorescence, scatter and transmission of X-rays does not.exist. The theories that do exist are to some extent mutually contradictory and limited to special cases. This makes the complete treatment of matrix effects difficult. Some techniques to overcome matrix effects include the use of computing methods based on nomograms derived from two or more separate measurements on the same sample. • Generally, two fluorescent radiations, fluorescence and absorption, fluorescence and scattered radiation, are used. Other techniques are based on special methods of preparing the sample and

41 include dilution, fusion with boric acid, grinding, or the addition of internal standards. Methods based on sample preparation cannot easily be used in the field with portable instruments.

B.3.2. Heterogeneity effects

These are due to the varying size of particles present in the sample, the varying composition of fluorescent particles and the spatial distribution of particles. Heterogeneity effects take place only in the analysis of powders, slurries and of natural samples of minerals and rocks. The general opinion now is that the early work of Claisse and Samson [29] does not take into consideration all factors which should be included in a rigorous treatment of heterogeneity effects. However, a more comprehensive theoretical treatment has recently been given by Carr-Brion [49], Lubecki et al. [50] and Berry et al. [51]. The simple equations used to predict X-ray fluorescence intensities apply only when the system is homogeneous. As soon as any degree of heterogeneity is present, deviations from the predicted intensities are found which may cause appreciable • errors in analytical results. For convenience these may be classified into particle composition and particle size effects.

В.3.2.1. Particle composition

In a heterogeneous system, the fluorescent intensity from a given concentration of an element may depend on the composition of the particles containing that element, even though the concentration of the particles is so low ás to have a negligible .effect on the overall X-ray absorption coefficients of the system. For example 0.1 % iron as pyrites present in sand will give a different intensity from 0.1% iron present as magnetite. The iron X-rays have to pass through the particle in which they originate: .they only have a statistical probability, depending on the relative concentration, of passing through, any other. This explains the importance of the composition of the fluorescing particles on the X-ray intensity. Elimination of this effect is difficult. It can -be removed by fusion or reduced by fine grinding and a limited decrease can sometimes be obtained by increasing the energy of the exciting 'radiation. Correction is complex, since it requires, either directly or indirectly, the measurement of the mass absorption coefficients of the fluorescent particles. If these are present in major amounts, the concentration of the element causing the effect could be determined — for example sulphur in a mixed sulphide-carbonate copper ore. When the particles are present as a minor component, the only apparent solution involves measuring the fluorescent intensities from the same sample under different conditions. If, for example, the fluorescent intensities are measured successively by. using two different exciting energies, the ratio of the. two intensities should depend on the composition of the fluorescing particles. It may also depend, on the size of the particles, the composition of the rest of the matrix and to a lesser extent on, the particle size of the

42 other components. If compensation has to be made for all these effects, it gives rise to a complex measuring system.

B.3.2.2. Particle size

Heterogeneity effects due to particle size are more readily compensated for. A range of particle sizes exists, above and below which there is little change of fluorescence intensity with particle size. The region over which there is a rapid change in fluorescence intensity with particle size is known as the transition zone. If a sample is made up of particles of sizes above the transition zone, it is best left alone, since grinding may well transfer the particle size into the transition zone. The approximate position of this zone for any system can be calculated by using Claisse's equations. This is a useful guide, in application work, especially with radioisotope X-ray fluorescence analysis where the primary mass absorption coefficients can be calculated. When the particle size of the material analysed is in the transition zone, several possibilities exist. Fusion can eliminate the effect and in some cases fine grinding can take the particle size out of the transition zone. However, even for a medium-atomic-number element such as tin, the particle size must be reduced to less than 5 yu-m to ensure accurate analysis with Sn К radiation. Many analytical methods for which it is claimed that grinding is used to eliminate particle-size effects merely rely on the reproducibility of the grinding systems. In true on-stream. analysis, continuous grinding of a large sample stream is difficult and indeed may be impossible as a result of other process requirements. Here a means of correcting for variations in particle size is desirable. Experimental work carried out at Warren Spring Laboratory sug- gests that the fluorescent intensity is dependent on the weight mean of the reciprocal particle size. Therefore, either a measuring system is employed which gives the particle size distribution and the weight mean reciprocal size calculated from this — a major operation — or sensors are used which enable the weight mean reciprocal particle size to be measured directly. Two such sensors are known to exist, one using X-ray transmission and the other

Xrray fluorescence to achieve this end. The first measures the transmitted intensity of two X-ray beams, the energies of which are chosen so that one is particle-size dependent and the other particle-size independent in transmission. The ratio of the two intensities depends on the weight mean of the reciprocal particle size. The second sensor measures the fluorescent intensities from the sample stream when they are successively excited by primary X-ray beams of two different energies. The ratio of the fluorescent intensities depends on the weight mean reciprocal particle size of the fluorescing particles. The output of both sensors can depend on sample composition. While the choice of suitable X-ray energies can reduce this effect, compensation may have to be made. Thus, a typical on-stream system might consist of an X-ray fluorescence analyser with means of compensating for changes in slurry composition, a particlé-size

43 sensor and y-density gauge, all coupled to a low-cost computer to produce the required process parameters.

В.3.2.3. Boundary effects

The boundary between a heterogeneous sample and the source-detector unit may be imprecise because of surface irregularities. These may be comparable in dimension to the critical depth, and produce shadowing effects which will also be dependent on particle size. The relatively broad beam geometry employed in radioisotope X-ray fluorescence analysis helps to reduce this effect.

B.4. CALIBRATION

In common with most physical methods of analysis, radioisotope X-ray techniques are not an absolute means of measurement. Hence they- must be calibrated against some accepted method. To do this it is necessary to obtain samples for which the range of variables likely to be met in the practical analytical problem is covered. These' variables include concentrations both of the elements to be determined and other elements in the sample and particle size. The collection of such a representative and accurately analysed series of samples may in itself constitute a serious problem and when this cannot be achieved it may be satisfactory to use artificially prepared samples of materials as similar as possible to the material being tested. In- adequate mixing, in the case of powdered samples, and segregation of alloys can be a serious additional limitation to the use of artificial standards. Portable radioisotope analysers are often used by persons having little knowledge of the problems or limitations of this method of analysis. Errors can occur, especially in the use of balanced filter techniques, unless allowance is made for changes in the matrix ór in particle-size distribution. Hence, it is necessary to make frequent calibration checks to ensure that the results are within acceptable limits. To ensure that the probable sources of error of the method and limitations of the equipment are appreciated, it is important that detailed instructions regarding use and calibration be included with each instrument. The calibration of on-stream isotope analysers is complicated by the necessity to ensure that the sample seen by the analyser is the same as that collected for analysis by the accepted method. Further, the normal variations occurring in a plant process stream may not change sufficiently for calibration purposes in a reasonable time. In many cases both of these limitations may be overcome by the use of a closed calibration loop. Sample composition can then be varied at will and an adequate sample can be taken with little trouble. The calibration of gauges to measure the coating thickness of materials such as tin píate and zinc-coated steel depends to a limited extent on the thickness of the alloy layer. However, in practice it has been found that this alloy layer is sufficiently constant to cause no appreciable error in the measurement.

44 С. APPLICATIONS

C.l. METALLIFEROUS MINERAL EXPLORATION AND DEVELOPMENT

Metalliferous mineral exploration can be conveniently sub-divided into four successive stages: (1) regional reconnaissance, (2) localized, more detailed investigations, (3) evaluation of individual occurrences, and (4) development prior to exploitation. At the present time, radioisotope X-ray fluorescence instrumentation is most relevant to stages (2) to (4). The problems to which the technique can be applied include the detection and semi-quantitative analysis of specific elements in unprepared rock surfaces, drill cores and boreholes, and the detection and (under favourable circumstances) quantitative analysis of specific elements in particulate natural samples, such as soils and heavy mineral concentrates, and in artificially prepared samples such as powdered rocks.

С. 1.1. Unprepared rock surfaces

The examination of unprepared rock surfaces, which include natural exposures and rocks artificially exposed in mine and quarry workings, presents the most unfavourable circumstances for the application of radioisotope X-ray fluorescence. This is due to uncontrolled probe-sample geometry, uncontrolled grain size and heterogeneity within the sample. Never- theless, there occur many situations in mineral exploration when the ability to indicate rapidly the presence or absence of a specific element in a rock, or to indicate that it is present in large, moderate, or small amounts, is very desirable and economically important. Until the first portable radioisotope X-ray fluorescence analyser became available (the first commercial instruments were marketed in 1965) there was no way in which this could be done if the ore-minerals were not readily visible to the eye. Mineral exploration is an activity dating back into prehistory, and consequently it is not surprising that the full advantages of 'instant recognition' have yet to be fully appreciated and applied. The following are examples of situations where 'instant recognition' is of value:

(a) Testing rock surfaces with no obvious visible indications of valuable metal content, e.g. fine-grained argillite for copper mineralization, impure dolomite for zinc mineralization, ultrabasic rock for nickel mineralization.

(b) Identifying from a variety of vein structures intersecting a rock those which carry the elements of economic interest.

(c) Identifying limits of economic mineralization on a rock face in a mine working in order to delineate areas suitable for extraction.

45 Measurements obtained in rock surfaces have been reported by Darnley and Leamy [45], Gallagher [52] and Fox and Bird [5 3] who report encourag- ing results on one particular zinc ore body. Only in favourable circumstances, when the variables involved are minimal, can satisfactory quantitative results on unprepared rock surfaces be obtained without the necessity for taking a prohibitive number of measurements. However, conventional (i.e. channel) sampling techniques leave much to be desired in practice and there is scope for careful comparisons to be made over a period of time in favourable situations between conventional and the new instrumental techniques. Favourable situations are most likely to be found in fine-grained, evenly disseminated, homogeneous mineral deposits in either stratiform sediments or in igneous rocks. Generally speaking, in-situ measurements, even in the most favourable circumstances, will have lower sensitivities and higher limits of detection than measurement on the same samples in powdered form. The possibility of in-situ radioisotope X-ray fluorescence analysis ever taking the place of conventional sampling for ore reserve and valuation purposes thus seems remote.

С. 1.2. Drill cores

The problems that apply to measurements on unprepared rock surfaces are somewhat less severe for core measurements because the geometry of sample presentation is more regular. The surface is cylindrical and relatively smooth (assuming the core has not been broke into fragments by the drilling process, which may often happen), although the diameter will vary according to the size of drill used. Problems caused by uncontrolled grain size and heterogeneity within the core remain. Under most circumstances the nature of the mineralization likely to be intersected in a drill core will be known in advance, but in certain circumstances, as when a drill hole has been located solely on the basis of geophysical indications, this may not be the case. There may be a large number of possible mineralization host structures in a core without visible indications of mineralization and the ability to determine rapidly which, if any, contain elements of interest greatly expedites the planning of subsequent drilling. Conventional chemical analysis is normally undertaken in a laboratory remote from an exploration drill site and analyses are not usually available for several days. As drilling-rigs normally operate on a round-the-clock basis, immediate information is of obvious value. Measure- ments on cores have been reported by Darnley and Leamy [45] for tin and copper and by Gallagher [54] for tin, lead, zinc and iron. The spread of values obtained by comparison with corresponding chemical analyses ranges from +100% to ±20% on different groups of core samples, depending primarily on the heterogeneity of the mineralization. Conventional laboratory analyses may be expensive and, in the case of drilling operations at remote sites, removal of samples for analysis may have to take place by air, which is also expensive. There is thus some in- centive to remove only the minimum number of samples. A field instrument, even in a semi-quantitative role, can be used to select and reject material for analysis, thereby ensuring maximum information at minimum cost.

46 С.1.3. Boreholes

Opinions in the mining industry seem to vary greatly concerning the extent to which there is a demand for borehole probes. As far as exploration holes are concerned, it seems that for the foreseeable future there will always be a requirement for core-drilling, because physical examination of the rock provides more positive information than could conceivably be obtained from any combination of probes. However, in many situations, notwithstanding the skill employed in the drilling technique, there are portions of a hole, from which no core can be recovered, and sometimes these are within, or adjacent to, known mineralization. As a means of investigating these sites a probe could be of obvious use. Obviously there are severe limitations on the employment of an X-ray probe in a water-filled, mud-coated hole, and only for the heaviest elements (Z >40) could any useful information be- expected under these conditions. Probably the main scope of X-ray borehole probes is in development drilling on the margins of known ore bodies. Such drilling may be undertaken from underground workings or in the walls of an open pit, and in both situations most holes drilled will not contain standing water. X-ray fluorescence probes can then be applied for the full range of elements heavier than titanium. In areas of development drilling the necessity for core is much reduced, and economy of operation could be expected by using non-coring bits substituting a probing operation for core collection. In addition to problems presented by water or solid contamination of the hole, the basic problems encountered by analysing solid rock remain. Development and testing of radioisotope X-ray fluorescence borehole probes is going ahead in thé United Kingdom, the United States of America and Australia at the present time (Clayton, Rhodes and Rawlings: oral com- munications) but only in the United Kingdom has a probe been subjected to trials under field conditions. X-ray fluorescêncè is the most specific of the available possible borehole probe techniques for elemental analysis (the others being gamma-gamma and neutron activation logging) but it seems probable that these, and particularly XRF and y —y, will be complementary , to one another.

С. 1.4. Particulate samples

C.l.4.1. Natural samples

The level of detection and sensitivity available from current equipment (other than solid-state detector devices) is insufficient by an order of magnitude to permit results to be obtained from any normal soils such as are sampled as part of a geochemical exploration program. Atomic absorption has now become the standard analytical technique, having taken over in recent years from colorimetry and emission spectrography. The use of solid-state detectors in field camps may be commonly adopted in the near future for preliminary screening of samples and to avoid the necessity for chemical dissolution required for atomic absorption. However, problems of quantifying

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49 results from multi-element samples via solid-state detectors have still to be solved. Certain sediments in streams, lakes, rivers and oceans may contain natural concentrations of dense minerals of economic significance: cassiterite for example (БпОг) may be found in such a situation. Systematic examination of such sediments in geologically promising regions would be within the capacity of existing instrumentation employing the balanced filter technique.

C.l.4.2. Artificially prepared samples

To reduce problems presented by irregular and variable grain size and heterogeneity of rocks, powdering the samples is desirable whenever circumstances permit. Obviously the problems may be further reduced (as in conventional X-ray fluorescence analysis with X-ray tube and crystal diffracto- meter) by fusing and diluting the samples with borax or another additive, but this step is not normally envisaged in what is primarily a rapid and inexpensive technique intended for use in a field or plant laboratory with minimum facilities. A special category of particulate samples is occupied by the fragments produced by drilling operations. These are flushed from the hole by air or water and useful results have been obtained for zinc sulphite distribution within an ore body by on-the-spot measurement of the fragments. The present state of the art with respect to the limits of detection achieved is given in Table X. The source, source strength and filters used are listed. It must be emphasized that due to the multiplicity of variables in geological materials these are only an indication of what is possible for a certain element in a certain environment. In other environments substantial deviations in effectiveness of the technique may be encountered, and it should always be assumed that, if there are any physical or chemical differences between two ore bodies of the same element, then, two calibration curves will be required. Calibration samples should always be selected from the same locality and type of mineralization as the material to be analysed.

C.2. ON-STREAM PROCESS CONTROL

C.2.1. Introduction

The introduction of automatic control of industrial processes has many potential benefits, including the manpower saving and improved product quality and economy in plant operation. In many industries this automation can only be realized if rapid analyses can be made of materials in various process streams about the plant. There is .a growing awareness of the potential of radioisotope X-ray techniques for these analyses and the first installations of a new generation of radioisotope equipment in industrial plants have now been made. They

50 consist of apparatus for the continuous or frequent sampling of a process stream and of radioisotope X-ray equipment to analyse the sample. Systems that are being developed and installed for the rapid plant analysis of slurries, crushed materials and solutions are listed in Tables XI — XIII. Some of the more developed systems are discussed in the following sections.

C.2.2. Analysis of slurries

The rapid analysis of slurries is important in mineral dressing and in other industries. Large variations in the composition of slurries occur because of changes in the plant feed material, and close plant control is possible only if a number of plant slurry streams are continuously analysed. The prime requirement in the mineral dressing industry is the on- stream determination of one or two elements to a precision between 5 and 10% of the element concentration in the. slurry solids. This concentration may range from 75 wt% in concentrates to less than 0.1 wt% in residues. On-stream analysers incorporating an X-ray tube and spectrometer were introduced into mineral dressing plants in 1956. The mineral industry has been slow to accept these techniques because the equipment is bulky and expensive. Normally, slurry streams have to be brought through long runs of narrow diameter pipe to the instrument, making the whole installation complex. Radioisotope ori-stream analysers have great potential application in mineral dressing plants. The low-cost detection head units may be mounted close to sampling points throughout the plant and the electrical output signals fed to a centrally located computer for processing. The use of a relatively inexpensive and commercially-available computer removes the need for most nucleonic equipment other than the detector heads. A small fraction of the process stream is usually continuously sampled from a constant head tank. Provided that the size of the tank is comparable in dimensions to the inflow pipe, experimental work has shown that a representative sample can be obtained in spite of wide variations in the inflow rate and particle size [5 5]. The sampled fraction of the process stream flows continuously past the radioisotope X-ray analyser. Thin windows between the stream and analyser are necessary when X-ray fluorescence techniques are used. Melinex windows have been found to be suitable, and the life of 25-;u,m windows should exceed 1000 hours [56]. No radiation damage to the windows occurs because of the low X-ray intensities from radioisotope sources. Both radioisotope X-ray preferential absorption and fluorescence techniques are being developed for the on-stream analysis of slurries. The systems are outlined in Table XI and some of the more highly developed ones are discussed in the following text. In addition to the preferential absorption or fluorescence measurements, a determination of the slurry solids content is required. The solids, content is related to the slurry density [56], which is usually determined by measurement of the transmission of 137 С s y-rays by the slurry.

51 i n plan t i n pilo t effect s fee d pipelin e i n

w Statu s an d remark H y-ray s fo r matri x correction flotatio n Installe d on-strea m Plan t trials . Collimate d bea m Routin e us o n th mai plan t Plan t trials . Solid s mus b e particle-siz e I n us e on-strea m S geometry . Ha s backscattere d groun d t o < 5 0 fu n avoi Plan t trial s plan t ¡=> fr> w cT 0> CO I

ce 59 ] 60 ] m 1 î* [56 ] ! [57,58 , [47,59 , [28,69 ]

H i t—1 J Xi л o. a. CL S # o" w S ' Ю (1 < t f « « Precisio n Detectio n Detectio n limi t limi t И о О о о U O.Olf t S n 0.01-0.1 % B a i n slurr y fe P О ¡5 и« XI С XI CL. и a. ^ XI с CL 00 ( wt % i n dr y solid s w ( wt % i n dr y solid s о £ f (N со unles s otherwis e stated ) fc unles s otherwis e stated ) CO f t О t 4 ю Concentratio n i n slurr y t—( Ю «H «N с- о 0-10 % B a H с о

Он Он I • <с • Filte r p.h . a e o r p.h.a.

(Л Pd-A g (balanced ) И I—I tí tí и D о J y •o v Na l Na l m 4) Z Na l chamber s Х е prop ' 1 counte r fe a Tw o io n О w и (J (н В J U б «¡ о e Е c- w 5 Й activity* •и «¡ Sourc e an d p jjи- 24 1 137 24 A m (2. 5 mCi) C s (1 5 Ci ) wit h 'A m (1 4 mCi ) 13 7 C s (30 0 mCi) wit h B a targe t wit h S m an d 225-ke V back - scattere d y-rays , B a target s 1 ! <3 и tí u. u, 1

H XR A

XR A XR F XR F Й w g ! 3 Technique

X H J n < Measuremen t Lea d i n flotatio Lea d i n or e Calciu m i n cemen t slurrie s Ti n i or e slurrie s Bariu m i n or e slurrie s fee d slurrie s slurrie s H

52 I 1 1 1 ' i Б • 1 • I Ne i 0 ' . 1 1 1 1 1 j ю ОгНЮ . О«-HP- fe и -о ГНю< 1со # N с tIN^ O*r-í ^О e

Zinc in slurries of ГН СО XXX с

tails Cu-Ni balanced с Closed-loop experiments Xe prop' 1 i

heads and p.h. a. N [93] under laboratory conditions

counter # concentrates with plant mineral samples. No matrix corrections necessary because of low solids content of slurries о & о о Copper in Cu 238 Pu (30 mCi) Nal Cu 17-26% Cu о [47] X CO fe concentrate X with Ga target, Cu and Zn 0.9-1.9% Cu [47] All cases in Ref. [4] were flotation feed with Ga and laboratory feasibility studies Ge targets using dry samples taken from i plant streams. The copper fe was mined as a Cu-Pb-Zn Zinc in Zn 238 Pu (30 mCi) Nal ООО л ore, and lead and zinc as a = о 0о # concentrate with Ge target Zn 52-55% Zn NM [47]

o Pb-Zn ore. On-stream trials с с

flotation feed with Ge target Zn 5-23% Zn CNI [47] are to be carried out for residue with Ge target Zn and Al 0.1-1.5 Zn [47] copper, zinc, lead, and tin and 241 Am (5 mCi) with Cs target S w < 5 ю <лО a и во. E и r-^ ïS «о sgo с

Tin in mixed -g1 Nal Note: For the Zn in К CC Li. Ll.ti X

ores and X Ag and Pd 0-23% Sn 0.21% Sn flotation feed and residue, concentrates, for Cs and Sn the Pb concentration is used slime sulphides, targets, 0.5-3% Sn 0.034% Sn as a matrix correction. Pb slime rougher Oí •respectively 0.08-0.2% Sn 0.006% Sn is determined separately for tailings plant control • C S ъ

Lead in Zn j Nal 1ЛI tï j. r-i тН1 < Xi CL S f-t соj Xi со } о о о CL Xi 1 1 о' оj

in <£> CO CO и ю UD Ю Oi. . et

3 о 1-© H Precisio n limit s detectio n 0.01 % i 0.01 %

3 и

(wt % i n dr y solid s t

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Concentratio n о о ! i , e tfl Filte r o r p.h . a p.h.a . j Y-Z r balance d Sr- Y Ô,

a -a ti с лe» -«X >M. с y Argo n

counte r £L s « Argo n prop ' 1 prop ' 1 2 S S counte r an d Na l Oi «= дz Detecto r U +3 ? fe 4) OJ 4 £ x rt « 5 2 •и Jn3i "-О2 с «J £ So .5 Э ~ E м " o j: • B Ю .2 К ° DO о и S = 'S С S Я tí S а

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< e ё U, tfa ей « § X И z XR F s g XR F 3 Technique

я plant s

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54 55

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eo e? 2 t- ю Réf . £ !

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о « J= H • • • c Filte r o p . h a у 3 ь о. s с 0 E <ч o r Na l 1 Io n chambe r Io n chambe r Argo n prop l I counte r

H D J б U О U с? E w гн о E С о о н M Е m о E Ь о О < ® ^н activit y "Ё

S S> XR A ! XR A XR A X

S a Techniqu e PS ¡H и i 'I-a S о к si

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J Measuremen t petroleu m 8 solutio n Sulphu r i n U i n aci d solutio m s C u i n electrolyt e

57 FIG. 16. Continuous determination of lead in flotation feed slurries.The y-ray transmission gauges shown are mounted directly on the plant pipeline.

C.2.2.1. Lead in flotation feed slurries

Lead in flotation feed slurries is determined continuously on-stream at the plant of North Broken Hill Ltd. at Broken Hill, Australia. The technique depends on separate measurements of transmission of 660- and 225-keV y-rays by the slurry [57,58]. These measurements are combined electronically to give a reading proportional to the lead concentration. The ion-chamber detectors are mounted directly on the plant pipelines (Fig. 16). The plot of combined gauge signal as a function of sampled lead concentration, shown in Fig. 17, is the result of a plant feasibility test carried out before routine use of the gauges on stream. The lead was determined to 4% of the mean lead concentration both in the slurry and in the solids for slurry density variations in the range 1.36 to 1.68 g per cm3. . A different technique is used to determine lead in flotation feed slurries at the plant of the Zinc Corporation Ltd., Broken Hill. Lead is determined

58 u POINTS

5i _ 1 1 ^ 5 IS 25 Э9

LEAD CONCENTRATION (% BY WEIGHT OF SOLIDS)

FIG. 17. Determination of lead in flotation feed slurries: results of a plant feasibility experiment. by combining separate measurements of transmission of 153Gd and 60Co y-rays [59,60]. The main advantage of this technique compared with that described in the previous paragraph is that 153Gd y-rays (about 100 keV) are more preferentially absorbed by lead than the 225-keV y-rays. However, continuous sampling of the main slurry stream is necessary. In a plant feasibility study, lead in the range 14 to 20 wt% in the slurry solids was determined to 0.4%. Slurry densities varied between 1.47 and 1.61 g per cm3. The variation of lead in the slurry solids, as determined by the radioisotope gauges in two days of plant operation, is shown in Fig. 18.

C.2.2.2. Calcium content of cement kiln feed

In the cement industry it is important to monitor the calcium content of the kiln feed. The feed consists of about 80% СаСОз, and the precision required is about 1 wt% (95% confidence limit). The plant may operate on wet or dry processes. Determination of calcium has been found to be feasible in both cases [61]. The sample preparation and presentation scheme for slurries is shown in Fig. 19. Calcium К X-rays are excited by a 55Fe source (2-3 mCi); Fe К X-rays are not excited by this source. An argon-filled proportional counter of 5-cm diameter and with a 250-jtim-thick beryllium

59 TIME FIG. 18. Variation in lead content of flotation feed slurry as measured by radioisotope gauges during two days' operation at the Zinc Corporation plant at Broken Hill, Australia. window is used to count both fluorescent and backscattered X-rays. Particle- size effects have been found to be very severe and current practice is to pre- mill the sample to less than 50 fim. Work is in hand to obviate the necessity for this pretreatment by the use of a particle-size correction measurement. A typical calibration line for the slurry is shown in Fig. 20. The analysis is expressed as СаСОз per cent by weight in the solids with the X-ray reading normalized to a slurry solids content of 38% by means of a y-ray solids measurement. The accuracy achieved for both wet and dry processes is of the same order as that of the normal process control test (acid/alkali titration) and corresponds to a standard deviation of about 0.40% СаСОз on individual readings. Figure 21 shows instrument readings obtained over several hours' operation in a wet process cement works. The sinusoidal variations revealed by the instrument have been found to be typical of the process although they were not previously picked up by the spot sampling technique used.

C.2.2.3. Barium in barytes slurries

On-stream barium determinations are made in a fully instrumented barytes flotation pilot plant at Warren Spring Laboratory, United Kingdom [56]. The intensities of barium К X-rays, and backscattered X-rays from the

60 slurry, are determined with two source-target assemblies. Combination of the outputs of the two detectors gives barium concentration essentially independent of chemical composition of the solids. The solids content of the slurry is measured by y-ray absorption techniques. Four analysers were connected directly into the slurry streams, analysing the feed, the concentrate, the tailing and a recycle line. The outputs from these analysers were automatically sequentially sampled and fed into a single pair of ratemeters, interconnected to give the net barium К intensity as •output.

C.2.2.4. Tin concentration of tailings streams

The tin content of tailings streams required monitoring to assist in the rapid optimization of prototype concentration equipment [56]. The tin concentration in the solid- ranged from 0.1 to 1.0% and the solid weight fraction from O.'l to 0.3%, giving a minimum tin content in the slurry of 0.01%. This proved to be the practical limit of detection of the analyser. The analyser used two source-target assemblies and a y-ray density gauge. The accuracy required was low, ranging from the ability to distinguish clearly between 0.1 and 0.2% tin at the lowest concentration to + 0.1% at the highest. This was achieved despite widely varying iron content in the gauge. The system has been used in a prototype tin concentrator in Cornwall. In a second application the analyser was connected for a month directly to the plant slurry line.

61 FIG. 20. Calibration line for cement raw material slurry.

C.2.2.5. Copper, zinc and tin

Laboratory feasibility studies for the determination of copper, zinc and tin in various ores and mineral products have been made with X-ray fluorescence techniques by the Australian Atomic Energy Commission [47]. Source-target assemblies and scintillation detectors without pulse-height selection were used in each case. - Detailed knowledge of variations in chemical composition and particle size distribution of mineral products is rarely possessed by the mineral processing industry. Feasibility studies are therefore best carried out on samples taken directly from the plant slurry streams. To approximate normal long-term variations in these streams about 25 samples were taken from each stream over periods of at least six weeks. The results of these tests on concentrates, feed, tailings and residues are summarized in part of Table XI. The precision' obtained was in the range 1 to 10% of the mean element concentration, except for copper in tailings.

62 FIG. 21. Variation of slurry composition with time as shown by radioisotope analyser.

The results were obtained with dry solids, but the same order of precision is likely in slurries. On-stream plant tests are to be undertaken in Australia (see'Status and Remarks'in Table XI).

C.2.3. Analysis of crushed materials

A rapiid means of análysis of crushed materials such as powders, aggre- gates and ore is necessary for process control in some industrial plants. Presentation of a sample in a suitable form for radioisotope X-ray analysis is a greater problem with solids than with slurries because of the larger particle size and surface' irregularities of the solids. After continuous or batch sampling of the crushed material from a conveyor belt or hopper, it is usually necessary to-grind the sample before analysis. Various plant installations and feasibility studies for determination of constituents of crushed materials are listed in Table XII. Determination of the ash content of' coal, calcium and other elements in cement raw mix, basicity ratio of blast furnace feed materials, and zinc in slag are discussed in more detail. . •

C.2.3.1. Ash content of coal

Coal ash is the oxidized residue left after burning coal and is an important factor in determining the calorific value of the coal. There is a considerable demand for a simple continuous method of monitoring coal ash, not only in the coal industry to control cleaning and blending operations, but also in the electricity generating, steel and chemical industries for automatic control of combustion efficiency.

63 Mined coal has a wide range of particle sizes (up to tens of centi- metres) and a mineral content of heterogeneous distribution and variable composition. Mechanical mining has tended to reduce the average particle size but to increase the amount of shale present in the coal. Washing and screening remove much of the shale and separate the coal into different size and ash ranges. Blending of cleaned, graded coal is intended to yield a product of uniform quality. . Coal-ash monitors are required in the coal industry to control washeries and blending systems and to measure the amount of ash in run-of-mine coal. The blended and pulverized coals which coal users require to be analysed are generally more homogeneous than coals analysed by the coal industry. The accuracy required depends on whether the result is used to group coal into broad ash ranges or to control a washery or the operating parameters of a furnace. The total analysis time is dictated by the process being controlled and may vary from less than 30 seconds when monitoring furnace feeds to several minutes when checking the ash content of coal wagons at a coal preparation plant. This time must include sampling, sample preparation, measurement of ash content and data processing. Radioisotope X-ray gauges for the continuous determination of the ash content of coal are installed in industrial plants and coal mines. The two methods used are based on measuring the intensity of X-rays backscattered or transmitted by the coal. The detected intensity in each case depends on the absorption of X-rays by the coal. The absorption increases with increasing atomic number, Z, and thus is higher for ash (average Z about 11) than for combustible material (average Z about 6). Errors in ash determination result mainly from variations in chemical composition of the ash, particle-size effects, and moisture variations in the coal. Variations in iron concentration (up to 40% of the coal ash) are the main cause of error due to changes in chemical composition of the ash. The absorption of X-rays by iron is large compared with that by materials of mean Z about 11. Correction for this relatively great absorption by iron may be made with the backscattered X-ray method by deliberately exciting iron К X-rays and detecting them together with the backscattered X-rays. The intensity of iron К X-rays increases with increasing iron content and this compensates for the decrease in backscattered intensity due to iron. However, this technique can only be used on relatively fine coal particles because of the low penetration of iron К X-rays. Larger particles can be assayed by higher X-ray energy absorption techniques, but no compensation is made for variations in iron content. Corrections for the effect of moisture variations on the ash determination may be made by means of a separate measurement of neutron backscatter. One instrument designed by the United Kingdom Atomic Energy Authority [62,63] has a high accuracy and is used for coal that has been crushed, dried to less than 2% free moisture and ground to a maximum particle size of less than 0.5 mm. The total measurement time, including coal sampling and preparation, is about 15 minutes. The coal is gravity fed from a hopper (which acts as a reservoir) onto a belt where its top surface

64 ft Scintillation Counter

Smoothed Bed of Coal

FIG. 22. Continuous measurement of. the ash content of coal by X-ray backscatter techniques with tritium bremsstrahlung. The scintillation counter and smoothed bed of coal sample is shown in the diagram. is smoothed by means of a vertical scraper. Tritium bremsstrahlung (3H/Zr) is used, and the backscattered X-rays and iron К X-rays (for compensation) are detected by a scintillation detector (Fig. 22). A wide range of United Kingdom coals have been analysed with accuracies (95% confidence limits) varying from + 0.2% ash at 5% ash to ± 1% ash at 50% ash. This type of instrument is used by the United Kingdom National Coal Board at several coal blending plants. Another instrument was designed by the UKAEA to overcome the cost and time involved in sample preparation and to operate with as large a 'top size' of coal as possible [64]. A presentation system similar to that

65 FIG. 23. Continuous determination of the ash content of coal by y-ray absorption techniques with 241Am and 137Cs. The gauge head units and coal presentation system are shown in the photograph.

described in the previous paragraph is used but the top surface is smoothed by a grooved roller and compacted by a tamping device. Any irregularities in the surface introduced by tamping are removed by a second, smooth roller. The coal bed is 15 cm wide and 6 cm deep and the belt speed is 12 cm/s. Cadmium-109 sources (22 keV) are used, the X-ray energy being high enough to penetrate 1.5-cm particles and yet low enough to permit compensation for variations in iron concentration. The accuracy of ash measurement depends, on . 'top-size' and decreases with increasing 'top-size'. Precisions obtained with coal. containing high and variable iron concentrations ( 7 to 20% Бег-Оз) in the ash were 0.4, 0.7, 1.2 and 1.5% ash for 'top-sizes' of 1/8, 1/4, 1/2 and 3/4 in., respectively. These results are uncorrected for moisture content, which varied in the range 6 to 9%. At higher moisture levels the effect of change of 1 wt% moisture is approximately equivalent to a change in ash.content of 1%.

66 The transmission or scattering of higher energy X-rays is used in coal mines and industrial plants in Japan [65], the United Kingdom [63,66] and the Federal Republic of Germany [63]. Figure 23 shows one type of equipment used to determine coal ash in Japan. Coal travels down a steel pipe from the bottom of the hopper. Separate measurements of the transmission of 241Am and 137Cs X-rays are made. The 241Am measurement depends on the mean density and ash content and the 137Cs measurement on mean density. The detector signals are combined electronically to give an output of coal ash content. The unit has been used for two years at the Yoshikuma coal mine in Kyushu, Japan. The precision of ash measurement has been found to be ± 0.60% in on-line use.. Grain size up to 25 mm can be tolerated. Moisture content of more than 9% affects the accuracy but can easily be corrected by the simultaneous use of a neutron moisture gauge.

C.2.3.2 Cement raw mix and product clinker

A radioisotope X-ray fluorescence apparatus was installed in 1964 at the Kanda cement plant of Ube Industries Ltd. at Ube, Japan. This apparatus uses two excitation sources: a 210Po a-ray source for the determination of Al and Si, and a 3H/Ti bremsstrahlung source for the determination of Ca and Fe [67]. Two units are now in use for the analyses of the raw material mixture and the product clinker, and for the determination of fly ash contents in Pozzolan cement. The radioisotope X-ray analyser in conjunction with an electronic computer automatically controls the raw material mixture process. The computer also makes the corrections for the matrix effects in the X-ray analysis. The total time necessary for the analysis of one sample is twenty minutes: ten minutes for the preparation of the sample by fusing the raw material with an equal amount of lithium borate, and ten minutes for the X-ray intensity measurement. The raw material is analysed every thirty minutes, and the clinker every three days. The accuracies and precisions of the analyses are shown in Table XII. The following economic and technical advantages were reported to have been achieved by the application of the radioisotope X-ray emission technique:

( 1 ) economy of manpower in the raw material production process; (2) lower dispersion in the hydraulic, the silicate and the iron moduli of the raw material .which is introduced into the kiln; (3) in consequence, the improvement of the quality of the clinker product and the stable operation of the kiln.

Another cement corporation in Japan has applied radioisotope X-ray techniques to the on-line determination of CaO in cement raw mix [68]. Powdered cement raw mix is introduced from the main line to a belt conveyor by-path. A 55Fe source of 2 mCi excites Ca К X-rays from the continuously moving sample. An argon-methane gas-flow proportional counter detects the X-rays. A ratemeter with a time constant of 100 seconds is used for the intensity measurement. The counting rate exceeds 104 counts per

67 SAMPLER

DISINTEGRATOR 99e/o <80 MICRON

TO ANALYSER-*

FIG. 24.' Sampling and preparation unit used to present sample of raw sinter mix to radioisotope X-ray analyser.

68 second for СаСО'з contents of about 70%. A precision of 0.3 wt% СаСОз is obtained. Similar installations have been made in the United Kingdom [61] and in France [69, 70].

C.2.3.3. Basicity ratio of blast furnace feed materials [61]

The primary requirement in the iron making industry is for equipment to monitor the basicity ration (CaO/SiCb) of blast furnace feed material. The samples can be taken either before or after sintering, although the former is preferred because of the practical difficulties involved in sampling and preparing the hot fused sinter cake. Figure 24 is a diagram of a sampling installation designed to provide raw sinter mix samples for an experimental analytical unit about to be commissioned in United Kingdom steel works [61]. The equipment is designed to yield material 99% finer than 80 цт. Analysis is carried out semi-continuously on pellets which are automatically pressed and presented to two measuring heads simultaneously by an ingenious rotary device. A third pelletizing position and measuring head are also available for 1-е determination if required. The pellets are presented 'naked' and with dust-free surfaces. The accuracies (67% confidence levels) aimed for are: CaO — 0.2% at a nominal level of 12%; SiOî — 0.2% at a level of 8%. Ca is determined with an air path by means of a 7-mCi 55Fe source and a Xe-fillèd sealed proportional counter with a 250-/u.m beryllium window; Si is determined with a helium path by means of a 4.5-Ci 3H/Zr source and a Ne-filled proportional counter with a 50-^tm beryllium window. The counter gas is chosen to discriminate against Ca and Fe. Both channels have conventional counting electronics with pulse-height analysers housed in an air-conditioned cubicle. It has been reported [71] that satisfactory repeatability on a given sample has been achieved in preliminary trials, but adequate overall accuracy has yet to be proved on run-of-the mill material.

C.2.3.4. Zinc in slag

A portable mineral analyser is in routine use for the determination of zinc in slag at the plant of Broken Hill Associated Smelters Pty. Ltd., Pt. Pirie, Australia [72]. To operate the slag fuming process on an economical basis, it is necessary to stop the treatment of each slag charge when the zinc concentration is reduced to a particular level. This is done by plotting the zinc level against time and extrapolating ahead to determine the cut-off point. This method can only be used if a rapid method of analysis is available, and the radioisotope method with a portable isotope fluorescence analyser meets this requirement very well. The precision required is about 10% of the mean zinc concentration, particularly for samples near the cut-off point of about 3 wt% zinc. No detailed comparison of radioisotope and conventional chemical methods has been made with samples from the plant working under normal operating conditions. Tests on pilot plant samples indicated that a precision approaching the requirements could be obtained.

69 С.2.4. Analysis of solutions

Continuous on-stream analysis of solutions is a relatively simple problem with radioisotope X-ray techniques. With a homogeneous material representative samples may easily be obtained, and X-ray preferential absorption techniques may be used even for determination of low-atomic-number elements. Some applications and feasibility studies are listed in Table XIII.

С.2.4.1. Sulphur-in petroleum products

The use of radioisotope preferential absorption instruments to measure the sulphur content of hydrocarbons is now a widely accepted technique. Major advantages of this method are that it is rapid, simple and accurate, and gives an analysis independent of C/H ratio. In most instruments 147Pm/Al bremsstrahlung is the source used [65]. By proper choice of sample thickness and detector a mean energy of about 20 keV is detected; at this energy the mass absorption coefficients of carbon and hydrogen are equal. On-line instruments have ionization chamber detectors and are approved for use in areas where fire is a hazard. Compensation for density changes is achieved by means of a density sensor which regulates the weight of sample in the measuring cell. Both laboratory and on-line instruments can measure all types of oil from light fuel oils to heavy diesel oils and cover the range 0.05 to 5% sulphur with a precision of + 0.03% (95% confidence level). Separate calibration curves are necessary for hydrocarbons containing different amounts of dissolved chemicals (e.g. Br and V), since the absorption technique is not specific for sulphur.

C.2.4.2. Plutonium in solution

Preferential absorption techniques with 60-keV 241Am y-rays have been used to measure and control the in-stream concentration of a plutonium product evaporator [7 3]. A high background results from X- and y-rays emitted by the heavy element solution, and is particularly high in the case of purified plutonium extracted from highly irradiated fuel. However, by • careful design of the collimation system and by restricting the path length of the cell, the background signal can be made sufficiently small to make the transmission measurement feasible. It is estimated that a precision of about 0.3% (3 о ) at 300 g of plutonium per litre should be possible under laboratory conditions. •

C.3. ALLOY ANALYSIS

The use of portable analysers for alloy analysis has been proposed a number of times. The main applications appear to be for alloy identification of components such as sheet, strip, tubes, bars and forgings, and for scrap

70 sorting prior to melting-down [74]. A number of feasibility studies have been carried out aimed at laboratory [75] and plant application and a few instruments are in routine use. Table XIV summarizes the feasibility studies known to have been performed.

C.4. COATING THICKNESS MEASUREMENT

С.4.1. Introduction

There is an increasing need for a rapid, non-destructive versatile method of measuring coating thickness both in the laboratory and on-line. Of several possible methods, X-ray fluorescence techniques with sealed radioisotope exciting sources have important advantages. Firstly, the methods are rapid and non-destructive and can be applied to a wide range of coating and substrate materials. In addition, they are unaffected by such properties as hardness and degree of magnetization of the substrate and practically unaffected by thin surface coatings of mineral oils which are sometimes used to preserve surface finish. On very thin coatings, high sensitivity and accuracy are obtainable. Compared with electrical X-ray generators, radioisotope sources have the advantages of compactness, freedom from maintenance and the need for high voltage supplies, constant energy output and low price. The sources are completely sealed and subject to strict tests before sale to ensure that they will not leak during service. A combination of compact source-sample- detector geometry, high-efficiency detectors and fail-safe radiation shields makes it easy to keep dose rates in the vicinity of the measuring head to well below the maximum levels permitted by the authorities so as to ensure complete safety for operators and maintenance staff.

C.4.2. X-ray fluorescence methods of measuring coating thickness [76-78]

The thickness of a coating is derived either by measuring the increase in intensity of an X-ray excited in the coating as the coating thickness is increased or by exciting an X-ray in the backing and observing the decrease in intensity due to attenuation in the coating as coating thickness is increased.

C.4.3. Applications

At the present time, the most important applications are to tin and zinc coatings on steel. The coating thicknesses for which these techniques can be used are summarized in Table XV.

C.4.4. Tin coating gauges

Tin coatings were originally measured with /З-backscatter gauges but, because of the very thin coatings used, the precision of the method was poor. This, coupled with errors caused by variations in the temperature of the air

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Pb in Sn/Pb XRF Pb К p.h. a. alloy

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p.h. a. 0.08% Ni Proportional counter Ni in steel XRF Ni К in the source-sample-detector gap and in the hardness and composition of the base steel, has resulted in the X-ray fluorescence technique now being preferred for this measurement. The majority of/З-backscatter tin gauges have now been withdrawn.

C.4.4.1. Choice of method

To measure coatings of tin on steel one can excite Sn L (3.4 keV), Sn К (25.2 keV) or Fe К X-rays (6.4 keV). An exact and concise comparison of these three alternatives is virtually impossible on account of the permutations involved in choice of source, source activity, detector, or whether, for example, a laboratory or on-line measurement is required. In general, the sensitivities with the different sources do not differ enough to be a major consideration and can be neglected. The choice of method is made more on the practical grounds outlined below.

(a) Excitation of Sn L X-rays

3H/ Zr bremsstrahlung, which is a convenient source to excite Sn L X- rays, also excites Fe К X-rays and makes it necessary to use some method of energy selection to isolate the Sn L X-rays. A secondary emission source can be used for selective excitation but the low intensity available restricts the detector to some type of counter. The most important consideration, however, is the relatively low energy which entails thin windows and possible troubles from changes in air-gap density. Because of these difficulties, this method is not used in practice.

(b) Excitation of Sn К X-rays

Sn К X-rays are most conveniently excited by using 241 Am either directly [79,80] or in a secondary-emission source with a Cs target [59,81,82]. In the equipment developed by Watt [59,81,82] an Ag absorption edge filter is used to reduce the intensity of the backscattered primary rays. A linear relationship is obtained between detector output and coating thickness which is practically unaffected by non-uniformities in the tin layer (e.g. matt finishes) or of the surface roughness of the iron base. Minor difficulties are experi- enced due to dependence of the detector output on the thickness of the iron base and excitation of the tin layer on the remote side of the sheet. Compen- sation can be made for both these effects: the former can be neglected if measurements are made as the sheet passes over an iron roller. Two on-line gauges are in use based on excitation of Sn К X-rays, one using direct 241Am excitation and a proportional counter [79] and the other an 241Am/Cs source and a. scintillation counter [59]. The precision, claimed for the former is + 5% at 0.2 ¡xm and ±1% at 4/u.m, while that for the latter is ±3% at 0.2 mg/cm2 and ± 1.5% at 1 mg/cm2.

76 (с) Excitation of Fe К X-rays '

3H/Zr bremsstrahlung is a convenient source [83-86]. Sn L X-rays are also excited- but their intensity relative to the iron is low and when a filter is used- they can be neglected. Saturation thickness of iron is approximately 0.05 mm (2 thou) so that variations iñ base thickness can be neglected. Scintillation and proportional counters [84] and ionization chambers are used for laboratory instruments, but in all the on-line instruments manufactured to date, ionization chambers are preferred. A precision of about ± 0.015 lb/b.b.3) is obtained with a time constant of 1.5 s.

C.4.4.2. Other sources of error in tin coating gauges

Some researchers have shown that measurement with Fe X-rays is affected by the surface finish of the base steel and non-uniformities of the tin coating layer [81] (matt finishes) and both these effects would be expected in principle. However, four such gauges have 'been in satisfactory routine use on-line for three to four years giving excellent continued corre- lation with chemical stripping methods, so it must be concluded that these two effects are negligible in practice due to the fact that the finish and type of coating are sufficiently constant. Variations in the depth' of the Sn/Fe alloy layer also have a small effect on the accuracy of a gauge, as would be expected, but again it has been found that, in practice, the thickness of the alloy layer is constant enough not to cause appreciable errors. Variations in absorption caused by changes in air density can generally be neglected with the geometry used. For example, when using Fe К X-rays, if the detector is 5 mm from the sample, a 10% change in air density will change the count rate by about 0.2%, which corresponds to an error in thickness of about 0.5%. Commercially available tin coating gauges are mentioned in section 15.2.1.

C.4.5. Zinc coating gauges

Zinc coatings are obtained by electrolysis1 (électrozinc) and by hot dipping (galvanized steel), and on-line measurement is required to control the latter, particularly where the thickness approaches 2 oz/ft2 and 2.5 oz/ft2, usually on each side. Zinc coatings were also first measured by /З-backscatter techniques. However, the total change in signal from steel to an infinite layer of zinc is only about 8% (c.f. about 300% with X-ray fluorescence)-so an extremely high precision gauge is required.. Although the measurement problem is eased by the use of higher energy 90Sr/90Y /З-particles, the precision is still

3) Base box: unit of area for tin plate and temeplate equal to 31 360 in2 or 217.78 ft2 or 20.232 square miles = area covered by 112 plates of 14 X 20 in. each.

77 limited by errors similar to those given above for tin-coating gauges, and the intrinsic accuracy of a /З-backscatter gauge is not so good as that of an X-ray fluorescence gauge. The present position is that some уЗ-backscatter gauges are still in operation but others have been abandoned because of unsatisfactory performance. All except one of the gauge manufacturers now offer radioisotope X-ray fluorescence gauges for zinc-coating measurement.

C.4.5.1. Choice of method

To measure coatings of zinc on steel, one can either excite the Zn (8.6 keV) or Fe К X-rays. Absorption in the zinc layer limits the iron fluorescence method to thin electroplated zinc; the mass attenuation coefficient of Zn for Fe К X-rays is 100 cm2/g so that attenuation of a 60 .mg/cm2 layer is exp (-6) = 0.002 which is an unworkable attenuation. A3H/Zr source also excites Zn К X-rays, so a 0.013-mm-thick Ni absorption edge filter is used to reduce their intensity. A gauge based on a 3H/ Zr source and a Ni filter will cover the range up to 200 fxin. Hot-dipped Zn coatings go up to 0.0035 in. and to measure this thickness it is necessary to use Zn К X-rays. However, even then one is nearing saturation and it is imperative to use a high-energy exciting source; 241 Am is ideal. This source also excites Fe К X-rays and the problem is to eliminate interference from these X-rays. Two approaches are used. In the instruments designed by Cavailles and Martinelli [80] and Margolinas [79], an aluminium filter which absorbs 85% of the Fe К X-rays but only 50% of the Zn X-rays and a sealed proportional counter are used. Rhodes [87] suggested the use of differential Ni, Cu filters and a balanced ionization chamber. Either two chambers or one chamber divided in two are used, each with an end window, one covered with a Ni filter and the other with a Cu filter. Thus, the net detector output is a measure of the intensity of the Zn X-rays, and scattered radiation and Fe К X-rays are automatically subtracted. Commercially available zinc-coating gauges are mentioned in section B.2.1.

C.5. MISCELLANEOUS APPLICATIONS

С. 5.1. Medical

Only a few applications of radioisotope X-ray fluorescence techniques in medicine have been reported to-date. The most promising appear to be for in-vivo studies of the mineralization of bone on the basis of the measurement of calcium content. A new method to determine the size of the thyroid gland has been developed [88] in which the (inactive) iodine normally present in the gland is excited by using a 159Dy source. This method is said to reduce the radiation dose delivered to the patient by a factor of between 5 and 10 compared

78 with that in normal diagnostic studies in which radioactive iodine is used. However, this method does not give any direct information on the 'turnover rate' of iodine in the thyroid and therefore must be regarded as complementary to the use of radioactive tracers. Rapid measurement of potassium in the blood during surgical operations is said to be important and a method is now being developed. A method of measuring the concentrations of copper, sodium and phosphorus in plasma is also being worked out. Radioisotope X-ray fluorescence techniques are currently being developed to measure the concentration of sulphur in protein fractions in vitro and the iodine concentration in blood by circula- ting blood samples through a specially designed flow cell.

C.5.2. Other applications

An instrument has been developed for the United States Customs to detect gold, and an instrument to identify bullet-holes has been developed for use in crime detection [89]. A technique to determine the amount of silver in photographic emulsions has been developed and an instrument based on this technique is in routine use during production of photographic material [90]. Ostrowski et al. [91] have developed a method to measure the diameter- of tungsten, molybdenum and copper wires.

C.6. HEALTH AND SAFETY CONSIDERATIONS

C.6.1. External hazards

It is generally agreed that adequate safety precautions have been designed into all currently available portable instruments and that, if used in accordance with manufacturers' instructions, they are completely safe for routine use by unskilled personnel. Field experience indicates that some of the early portable models may even have been overdesigned with respect to safety, to the detriment of practical field utilization. On the basis of the considerable experience accumulated throughout the world, it' is generally agreed that radiation safety is not the controlling problem in the routine use of portable instruments. There can, however, be a K.odest radiation hazard to the skin and eyes with some instruments if they are misused. This could occur, for example, if manufacturers' instructions were deliberately disregarded and the measuring head were dismantled by unskilled personnel. The dose rates associated with even the largest radioisotope X-ray sources used in portable or on-line equipment are trivial compared with those from X-ray tubes used in similar applications. However, doses of the order of roentgens per hour occur within a few inches of some sources, so they must be treated with respect. The dose from low-energy sources falls off very sharply with distance, due to air attenuation, and there is generally negligible radiation hazard at distances beyond a few inches from such a source.

79 The problem of accurately measuring the dose rates from low-energy sources (<100 keV) is not simple and very few laboratories have the special- ized equipment to make such measurements. Nevertheless, considering all the factors, the Panel was of the opinion that an indication of the dose rate at one foot from an exposed source should be given by instrument manufacturers in addition to the usual operational information which is generally given.

C.6.2. Internal hazards

There is a potential ingestion hazard from most of the sources employed in X-ray fluorescence analysis, although the magnitude of this hazard varies widely with the type of source employed. As is well known, a-emitters present the most formidable problems in this respect. However, proper source and instrument design have been, well combined in currently available commercial equipment so that these problems do not present a major re- striction on the use of X-ray fluorescent instruments at the present time. The most difficult problems are connected with the use of a-emitting sources but these problems are surmountable by means of proper design, e.g. provision can be made to lock the source-sample chamber automatically in the event of alpha contamination. Polonium-210, in particular, poses a special problem because it is impossible to bind it chemically in a manner which prevents leakage and contamination in the event of fracture of the encapsulating foil. The anticipated availability of 242Cm and 244Cm should alleviate this problem. With proper source and instrument design, adequate leak-testing procedure and careful use, there appears to be no reason why a-sources cannot be introduced into commercial instruments for routine application.

80 D. PROSPECTS AND REQUIREMENTS

As a result of the considerable development work on solid-state detectors reported by members of the Panel, a significandy wider application was foreseen. It was thought that the introduction of these detectors would lead to radioisotope X-ray fluorescence equipment acquiring a more competitive position compared with other analytical techniques, quite apart from the recognized advantages of low cost, reliability and the non-destructive nature of the analysis. An increase in the availability of instruments based on particle excitation was also forecast and this, it was considered, would lead to a range of analytical instruments for the analysis of elements of low Z. The introduction of small Joule-Thompson cooling systems was expected and this should give rise to a greater mobility and wider range of application of these detectors. However, even the continued use of liquid- nitrogen cryostats was not thought to be an important deterrent where sufficiently great user benefits could be established. A greater degree of control and understanding of interelement and particle-size effects would increase the precision and accuracy of analytical applications, especially in mineral and in on-line slurry analysis. An increase in analytical systems based on a combination of techniques was expected. This would be especially true in on-line analysis, and neutron techniques, especially those possible with isotopic neutron sources, are expected to be introduced on an increasing scale. The most important areas of application are for elements of medium and low Z where (for some elements) neutron techniques have lower limits of detection and are not sensitive to changes in particle size. An increase in the number and range of applications is expected. This is especially true in mining where there is already an awareness of the value of radioisotope X-ray fluorescence techniques. In particular, it seems that the preliminary work reported on borehole logging is sufficiently promising to gain general acceptance of this technique. The first industries to benefit from the application of radioisotope X-ray fluorescence techniques were the metal processing and metal component manufacturing industries, closely followed by the mining industry. An in- creasingly wider range of industries is likely to be using these techniques in the future. The low limits of detection (a few ppm) now being reported for some elements will help to accelerate the rate at which new applications are introduced. The possible combination of static field analytical systems with telemetering systems may enhance field applications in remote areas. The Panel recognized the importance of an improved theory to predict particle-size effects in the analysis of powders and slurries and the need for supporting experimental work. The need for new photon sources of increased intensity (compared with existing isotopic X-ray sources) and emitting monochromatic radiation to

81 achieve lower limits of detection than those obtainable with current systems is becoming more apparent. For uranium exploration, portable instruments are now appearing, including 3- or 4-channel y-ray spectrometers. The experience already gained with such equipment should be of use in the X-ray fluorescence field. In on-stream analysis it is generally difficult to acquire sufficient data to understand the factors controlling the production process. Continuous data collection with computer control is urgendy required in plant of this type. Radioisotope instruments are important in this connection because, with their small size and rugged construction, they can easily be adapted for plant installation and hence avoid the need for long pipes to carry slurry such as when dispersive systems are used. In addition, only the measuring head need be subjected to the plant environment. For computer applications in particular, there is a strong need for more accurate data, especially of attenuation and excitation co-efficients for low-energy X-rays, to improve the accuracy of performance calculations in feasibility studies. This is also important in micro-probe applications.

82 E. RECOMMENDATIONS TO THE AGENCY

Reluctance on the part of labour and management to accept a radioactive source as an integral part of a commonplace instrument was seen to be an important barrier to the rapid introduction of these instruments in industry in some countries. It is strongly recommended that the Agency make every effort to introduce a Code of Practice to indicate the operational requirements for these instruments. The accuracy and limits of detection of analytical instruments based on radioisotope X-ray fluorescence analysis are strongly governed by interelement effects in alloys and by interelemerit, particle-size and heterogeneity effects in minerals. Because of the lack of suitable materials of known composition which exhibit these effects in a known manner, a complete figure of merit for the performance of analytical instruments of this type cannot be obtained at present. The lack of suitable standard materials also inhibits an intercomparison of instruments on an international basis and is thought to deter a more rapid acceptance of these instruments, especially in the mining field. The Panel strongly recommends that the Agency take whatever steps are necessary to overcome this deficiency, by taking responsibility for the development and production of a series of samples composed of appropriate combinations of elements of neighbouring and widely differing atomic number arranged in a particulate and non-particulate form, homogeneously and heter- ogeneously spatially distributed so as to form a series of satisfactory test specimens with regard to interelement, particle-size and heterogeneity effects. When an internationally acceptable series of such test specimens is prepared, the Agency is requested to arrange that it be made available throughout the world. It is recommended that the Agency should consider the advisability and practicability of fitting a visible notice to radioisotope instruments to indicate the dose rate at some fixed point or the longest safe exposure when the shutter is open. The object of such a notice would be to reassure the user that certain operations could be carried out on the instrument with negligible radiation risk. It was generally agreed by the Panel that more accurate fundamental data, especially attenuation and excitation co-efficients for low-energy X-rays, are required to improve the accuracy of performance calculations. The Agency is therefore asked to encourage, by financial support, work of this kind whenever possible.

REFERENCES

[1] SELLERS, В., ZIEGLER, С. A., "Generation and practical use of monoenergetic X-rays from a-emitting isotopes", Symp. on Low-Energy X- and Gamma Sources and Applications, Chicago, Rep. ORNL-IIC-5 (1964) 353. [2] FILOSOFO, I., Isotopic sources of secondary radiation, Rep. ARF 1122 (1961). [3] LEONTIADIS, I., Réalisation d'une jauge à fluorescence /3-Х pour l'analyse chimique, Rapport CEA-R 2970 (1966). [4] ENOMOTO, S., Heavy element concentration determination by the X-ray fluorescence analysis using radioisotope y-ray sources, Report CEA No. 33-69, Japan Atomic Energy Research Institute (1968). [5] FORBERG, S., ROMANO DE RUVO, A., "Analysis of uranium solutions by scintillation detection of X-ray fluorescence excited by 57Co 122 keV y-rays", Radiochemical Methods of Analysis (Proc. Symp. Salzburg, 1964) П, IAEA, Vienna (1964) 485. [6] LUBECKI, A., WASILEWSKA, M., GORSKI, L., On the application of Compton scattering to the elimination of matrix effects in non-dispersive X-ray fluorescence analysis, Spectrochim. Acta 23A ( 1967) 831. ' [7] TOMINAGA, H., ENOMOTO, S., Fifth Annual Meeting on Radioisotopes in Science and Industry, Tokyo (1968). [8] WATT, J. S., Gamma-ray excited X-ray sources, Int. J. appl. Radiat. Isotopes 1_5 (1964) 617 [9] RHODES, J.R., AHIER, T.G., BOYCE, I. S., Determination of tin in tin ores by radioisotope X-ray fluorescence, Radiochemical Methods of Analysis (Proc. Symp. Salzburg, 1964) Д, IAEA, Vienna (1965) 431. [10] KEREIAKES, J.G., KREBS, A. T., Optimum X-ray yields in/3-excited X-ray sources, 2nd Int. Conf. peaceful Uses of atom. Energy (Proc. Conf. Geneva, 1958) 20, UN New York (1958) 234. [11] REIFFEL, L., Beta excited low energy X-ray sources, Nucleonics (March 1955). [12] STARFELT, N.. CEDERLUND, J., LIDEN, K., The yield of characteristic X-rays excited by/3-rays, Int. J. appl. Radiat. Isotopes 2 (1957) 265. [13] MULLER, R.H., Particle interactions with matter; backscattering and excitation of characteristic X-rays. Progress in Nuclear Energie, Series IX, Analytical Chemistry 2(1961). [14] EZOP, J.J., STINCHCOMB, T.G., "Study, design and applications of/3-excited X-ray sources", Symp. on Low-energy X- and Gamma Sources and Applications, Chicago, Rep. ORNL-IIC-5 (1964) 4. [15] CAMERON, J. F., RHODES, J. R., "Beta excited sources of electromagnetic radiation", Radioisotopes in the Physical Sciences and Industry (Proc. Conf. Copenhagen, i960) П, IAEA, Vienna (1960) 23. [16] PREUSS, L.E., A compilation of/З-excited X-ray spectra, Reps TID-22361 (1966) and TID-22361 (Part 2) (1967). [17] LETRAON, J.Y., SEIBEL, G., Mesure d'intensité de rayonnements X d'énergie voisine en spectrométrie non dispersive, Int. J. appl. Radiat. Isotopes 1_4 (1963) 365. [18] MELLISH, C.E., X-ray fluorescence spectroscopy, Research 12 (June 1959) 212. [19] ROBERT, A., MARTINELLI, P., "Méthode radioactive d'analyse par fluorescence X des éléments légers", Radiochemical Methods of Analysis (Proc. Symp. Salzburg, 1964) П, IAEA, Vienna (1965) 401. [20] UCHIDA, K., TOMINAGA, H., IMAMURA, H., "Light elements simultaneous analyser by the X-ray emission method, using a- and X-ray sources for cement raw mix control», Radioisotope Instruments in Industry and Geophysics" (Proc. Symp. Warsaw, 1965) 1, IAEA, Vienna (1965) 113. 121 ] ADLER, I., TROMBKA, J., "Rock analysis by a-excitation of X-rays. A possible lunar probe", 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 2 (1967) 929.

85 [22] MARTINELLI, P., SEIBEL, G., Nouveaux développements de l'analyse et de la mesure des épaisseurs par excitation des raies de fluorescence au moyen de par- ticules/3, Rapport CEA n" 1786 (i960). [23] TANEMURA, T., Jap. J. appl. phys. 3 (1964) 208. [24] TANEMURA, T., Jap. J. appl. phys. 1 ( 1966) 51. [25] TANEMURA, T., Private communication. [26] RHODES, J.R., FLORKOWSKI, T., CAMERON, J.F., Analysis of sulphur and cobalt in hydrocarbons using a 147Pm/Al bremsstrahlung source", UKAEA Research Group Report AERE —R 3925 (1962). [27] RHODES, J.R., "Radioisotope techniques for coating thickness measurement and analysis", Non-Destructive Testing, Oxford Univ. Press (Egerton, H.B. (Ed.)) (1969) 132. [28] MARTINELLI, P., BLANQUET, P., "Heavy elements content measurement by means of gamma excited X-ray fluorescence", Proc. Symp. on Low-Energy X- and Gamma Sources and Applications, Chicago, Rep. ORNL-IIC-5 (1964) 107. [29] CLAISSE, F., SAMSON, C., "Heterogeneity effects in X-ray analysis", Advances in X-ray Analysis 5 (1962) 335. [30] LUBECKI, A., HOLYNSKA, В., WASILEWSKA, M., Grain size effect in non-dispersive X-ráy fluorescence analysis, Spectrochim, Acta 23B 7 (1968) 465. [31] CAMERON, J.F., RHODES, J.R., "Beta-excited sources of electromagnetic radiation", Radioisotopes in the Physical Sciences and Industry (Proc. Conf. Copenhagen, i960) H, IAEA, Vienna (1962) 23. [3 2] FILOSOFO, I., REIFFEL, L„ STONE, C.A., VOYVODIC, L., "Design and characteristics of beta-excited X-ray sources", Radioisotopes in the Physical Sciences and Industry (Proc. Conf. Copenhagen, 1960) II, IAEA, Vienna ( 1962) 3. [33] SELLERS, В., PAPADOPOULOUS, J., WILSON, H.; Generation and practical use of monoenergetic X-rays from alpha-emitting isotopes, Rep. NYO-3491-2 (1967). [34] KHAN, J.M., POTTER, D.L., Phys. Rev. 133 ЗА (Í964) A890. [35] MARKS, C.L., SAYLOR, W.P.,. STARK, A.A., "X-ray analysis by proton bom- bardment", 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 2 (1967) 587. [36] ANSELL, К. H., STEVENSON, J., "The development of low-energy photon sources in the United Kingdom", 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 (1967) 764. [37] STEVENSON, J., MYERSCOUGH, L.C., "Sources of low energy gamma radiation for industrial instrumentation", Paper presented at the Annual Meeting of the Japanese Radioisotope Association, Session on Radioisotopes in Physical Sciences and Industry, Tokyo ( 1966). [38] MACKAY, K.J.H., Non-dispersive X-ray fluorescence analysis of uranium-containing solutions, J. inorg.' nucl. Chem. 19 ( 1961 ) 171. [39] ANSELL„K.H., Private communication (1968). [40] C. G. E. I. Lepaute, Paris, Technical Report Ed. 5.67. [41] HUTH, G.C. et al., I.E.E.E. Trans, on Nuclear Science (1965) 275. [42] DUNNE, J. A., NICKLE, N.L., Balanced filters for the analysis of Al, Si, K, Ca, Fe and Ni, Proc. 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, • Austin,-ORNL-IIC-10 (1967),336. [43] HALL,E., Private communication, Littlemore Scientific Engineering Company, Oxford. [44] NIEWODNICZANSKI, J., "Analysis of copper ores in mine conditions by X-ray fluorescence induced by isotope sources", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965)1, IAEA, Vienna (1966) 173.. [45] DARNLEY, A.G., LEAMY, С. C., "The analysis of tin and copper ores using a portable radioisotope X-ray fluorescence analyser", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965)1, IAEA, Vienna (1966) 191. [46] RHODES, J.R., Radioisotope X-ray spectrometry, The Analyst 91 (1966) 683. [47] ELLIS,'W.K., FOOKES, R.A., GRAVITIS, V.L., WATT, J.S.; Radioisotope X-ray techniques for on-stream analysis of slurries. Feasibility Studies using solid samples of mineral products, J. appl. Radiat. Isotopes 20 10 (Oct. 1969) 691.

86 [48] LUBECKI, A. J., The sensitivity of complex radioanalytical methods, J. Rad. Analyt. Chem. 1(1968)413. [49] CARR-BRION, K. G„ MITCHELL, P.J., An on-stream X-ray particle size sensor, J. scient. Instrum. 44 (1967) 611. [50] LUBECKI, A., HOLYNSKA, В., WESILEWSKA, M., Grain size effect in non-dispersive X-ray fluorescence analysis, Spectrochim. Acta 23B (1968) 465. [51] BERRY, P.F., FURUTA, T., RHODES, J.R., Particle size effects in radioisotope X-ray spectrometry, Adv. in X-ray Analysis 1_2 (1969). [52] GALLAGHER, M., Determination of molybdenum, iron and titanium in ores and rocks by portable radioisotope fluorescence analyser, Trans. Inst. Min. Metall. 76 (1967)B155. [53] FOX, J. G.M., BIRD, G.R., Private communication. [54] GALLAGHER, M., "Portable X-ray spectrometers for rapid ore analysis", 9th Commonwealth Mining and Metallurgy Congress (1969) Paper 22. [55] CARR-BRION, K. G., Private communication. [56] CARR-BRION, K.G., Performance of an on-stream radioisotope portable analyser, Trans. Inst. Min. Metall. 76 ( 1967) C94. [57] ELLIS, W.K., FOOKES, R. A., WATT, J.S., HARDY, E.L., STEWART, C.C., Deter- mination in ore pulps by a technique using two gamma-ray absorption gauges, Int. J. appl. Radiat. Isotopes 18 (1967) 473. [58] STEWART, C.C., On-stream analysis of lead in flotation feed pulps, Paper presented at 75th Conf. Australasian Inst. Min. Metall., Broken Hill, Australia (1968). [59] WATT, J.S., Recent developments in low energy X- and gamma ray sources and applications in Australia, ORNL-IIC-10 2 ( 1967) 663. [60] HINCKFUSS, D.A., RAWLING, B.S., "The development and application of an on-stream analysis system for lead at the Zinc Corporation Limited", Paper presented at the 75th Conf. Australasian Inst. Min. Metall., Broken Hill, Australia (1968). [61] STARNE S, P. E., Oral communication ' [62] RHODES, J.R., DAGLISH, J.G., CLAYTON, C.G., "A coal-ash monitor with low dependence on ash composition", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965) I, IAEA, Vienna (1966) 447. [63] CAMERON, J. F., "Measurement of ash content and calorific value of coal with radioisotope instruments", Proc. 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 2 (1967) 903. [64] CLAYTON, C.G., Oral communication. [65] KATO, M., "Present status of research and application of low-energy X- and gamma- ray sources in Japan", Proc. 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 2 ( 1967) 723. [66] CAMERON, J. F., Oral communication. [67] UCHIDA, K„ TOMINAGA, H„ IMAMURA, H., "Light-elements simultaneous analyser by the X-ray emission method, using alpha- and X-ray sources for cement raw-mix control", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965)1, IAEA, Vienna (1966) 113. [68] TANEMURA, T., Oral communication. [69] MARTINELLI, P., ROBERT, A., CAVAILLES, J., "Progress in the study and applications of low-energy-photon radioactive sources", Proc. 2nd Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 2 (1967) 696. [70] MARTINELLI, P., Oral communication. [71] WHITE, G„ Proc. 19th BISRA Chemists Conf., London (April 1966). [72] BLANKS, R.F., Private communication, Broken Hill Associated Smelters Pty. Ltd., Pt. Pirie, Australia. [73] WHITTAKER, A., GREEN, G., GARNETT, J.E., "Advances in the gamma-ab- sorptiometric determination of uranium and plutonium in solution", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965) i, IAEA, Vienna (1966) 271. [74] RHODES, J.R., PACKER, T.W., BOYCE, I. S., "Rapid X-ray analysis of copper alloys and steels using a radioisotope portable analyser", Radioisotope Instruments

87 in Industry and Geophysics (Proc. Symp. Warsaw, 1965) I, IAEA, Vienna (1966) 127. ' . RHODES, J.R., FURUTA, T., "Applications of a portable radioisotope X-ray fluorescence spectrometer to analysis of minerals and alloys", Adv. in X-ray Analysis И (1968). CAMERON, J.F., RHODES, J.R., "Coating thickness measurement and chemical analysis by radioisotope X-ray spectrometry", Encyclopedia of X-rays and Gamma Rays, 150 (Clark, G.L. Ed.) Reirihold Publicity Corp., New York, London (1963) 150. . . . RHODES, J. R., Composition and coating thickness testing using radioisotope techniques, Rep. AERE — R 4457 ( 1963). CLAYTON, C.G., CAMERON, J.F., "A review of the design and application of radioisotope instruments in industry", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965) I, IAEA, Vienna ( 1966)- 15. MARGOLINAS, S., "X-ray fluorescence applied to the measurement of zinc coating . in the galvanising industry", Proc. Symp. on Low-Energy X- and Gamma Sources and Applications, Austin, ORNL-IIC-10 (1967) 805. CAVAILES, J., MARTINELLI, P., "Mesure continue des revêtements de galvani- sation par fluorescence", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965)1, IAEA, Vienna, (1966) 227. WATT, J.S., Alloy analysis and coating weight determination using gamma-ray excited X-ray sources, Rep. AAEC/TM 260 (1964). WATT, J. S., .The use of gamma-ray excited X-ray sources in X-ray fluorescence analysis, J. appl. Radiat. Isotopes 18 ( 1967) 383. CAMERON, J.F., RHODES, J.R., BERRY, P.F., Tritium bremsstrahlung and its applications, Rep. AERE —R 3086 (1959). CAMERON, J.F., RHODES, J.R., Measurement of tinplate thickness using fluorescent X-rays excited by a radioactive source, Br. J. appl. Phys. *1_1 ( i960) 49. CAMERON, J.F., RHODES, J.R., X-ráy spectrometry with radioactive sources, Nucleonics 19 6 (1961) 53. COOK, G.B., MELLISH, С. E., PAYNE, J. A., Measurement of thin metal layers — fluorescent X-ray production by radioisotope sources, Analyt. Chem. 32 (May 1960) 590. RHODES, J.R., Differential ionisation chamber with balanced filters, Br. Pat. 965, 303 (1964) HOFFER, P. В., BARCLAY JONES, W., CRAWFORD, R. В., BECK, А. В., GOTTSCHALK, A., Fluorescent thyroid scanning: A new method of imaging the thyroid, Radiology 90 2 (Feb. 1968). ZEIGLER, C.A., Oral communication. WATT, J. S., Oral communication. OSTROWSKI, K.W., GORSKI, L., NIEWODNICZANSKI, J., "Some applications - of low-energy gamma and X-ray sources in Poland", Proc. 2nd Symp. on Low-Energy X- and Gmma Sources and Applications, Austin, ORNL-IIC-10 2 (1967). STARNES, P.E., CLARK, J.W.G., "The continuous automatic analysis of dry powders and aqueous suspensions using radioisotope techniques", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965) I, IAEA, Vienna (1966) 243. RHODES, J.R., AHIER, T.G., POOLE, D.O., Analysis of zinc and copper ores by radioisotope X-ray fluorescence, Rep. AERE—R 4474, H.M.S.O., Londen (1964). TROST, A., "Détermination de la teneur en cendres de fines et mesure de l'épaisseur du verre, des matières plastiques et des métaux au moyen des rayons gamma de l'américium-241", Radioisotope Instruments in Industry and Geophysics (Proc. Symp. Warsaw, 1965) 1, IAEA, Vienna (1966) 435. RAWLING, В. S., Private communication.. WHITE, G., "The semi-continuous analysis of sinter mixtures using radioisotopes", • Proc. 19th BISRA Chemists Conf., London (1966) 17.

88 APPENDIX I

MANUFACTURERS' SPECIFICATION FORMS

Name: Hilger and Watts Ltd.

Model: . Portable Isotope Fluorescence Analyser (PIFA) (see Fig. 10)

Probe Detector type: Scintillàtion counter. Thallium-activated sodium iodide phosphor Dimensions: 210 mm long, 132 mm circumscribed diameter Weight: 1.58 kg Temperature range of operation: -20 to +50° С Detector window area: 1322.5 mm2 Mode of operation of shutter: Pistol grip lever with safety interlock Number, of filters: Two Mode of operation of filters: Sequential with swinging tray operated by bi-stable lever

Filters: In interchangeable swinging tray

Source: Interchangeable integral source and shutter assembly

Use areas: Hand-held, prepared samples, field, laboratory

Electronic unit Dimensions: 273 mm X 305 mm X 158 mm Weight: 6.5 kg Power supplies: Battery operated 2 X 6 V or9 X 1.5 V batteries Battery life: 150 h Battery type: Disposable Shockproofing: Shockproof Waterproofing: Waterproof Temperature range of operation: -20 to +50°C Form of output display: Analog Specify ranges and backing off: 102, 2.5 X 102, 103, 2.5 X 103, 104, 2.5 X 104 backing off multi-turn potentiometer Display facilities: Scaler output socket Electronic components: Ratemeter with low-level discriminator

89 II

Name: Hilger and Watts Ltd.

Model: Laboratory Isotope Fluorescence Analyser

Probe Detector type: Scintillation counter. Thallium-activated sodium iodide phosphor Dimensions: 203 mm long, 89 mm diameter Weight: 1.35 kg - Temperature range of operation: -20 to +50°С Detector window area: 1322.5 mm2 Mode of operation of shutter: Pistol grip lever with safety interlock Number of filters: Two Mode of operation of filters: Sequential with hand-Operated slide Interchangeable filter slides Interchangeable integral source and shutter assembly

Use area: Hand-held, prepared samples, field, laboratory

Electronic unit Dimensions: 317 mm X 381 mm X 216 mm Weight: 9.25 kg Power supplies: a.c. mains or 4 X 9 V batteries Battery life: 5 h Battery type: Disposable Shockproofing: Reasonably shockproof Waterproofing: Weatherproof Temperature range of operation: -20 to +50° С Form of output display: Analog Ranges and backing off: Ю2, 2.5 X'102, 103, 2:5 X 103, 104, 2.5 X 104 backing off multi-turn potentiometer Display facilities: Recorder socket, scaler socket Electronic components: Ratemeter with low-level discriminator

90 Ill

Name: Ekco Electronics Ltd.

Model: Mineral Analyser, Type 3182 (See Fig. 11)

Probe Detector type: Scintillation counter. Thallium-activated sodium iodide phosphor Dimensions: 247 mm X 159 mm X. 146 mm Weight: 1.6 kg ; Temperature range of operation: -20 to +50°C Detector window area: 1260 mm2 Mode of operation of shutter: Trigger Number of filters: Two Mode of operation of filters: Sequential with swinging tray operated by bi-stable lever Filters in interchangeable swinging tray

Source: Interchangeable integral source

Use areas: Hand-held, prepared samples, field, laboratory

Electronic unit Dimensions: 241 mm X 292 mm X 140 mm (with battery pack) Weight: 6.5 kg Power supplies: Rechargeable batteries. 4 X Deac, Type S.D.4 and 20 X Deac, Type 1000 DKZ nickel/ cadmium rechargeable cells. Alternatively mains operated with mains/ battery charger unit Battery life: 5 h continuous use Battery type: Rechargeable Shockproofing: Shockproof Waterproofing: Waterproof Temperature range of operation: -20 to +50° С Form of output display: Digital, displaying the four most significant scaler decades, two suppressed Display facilities: 'Up and down' scaler, selected by push button with difference count displayed Electronic components: Single-channel analyser, lower discriminator level preset internally. Upper discriminator level variable in ratio of 1:1 and 6.5:1 with respect to the lower discriminator. High voltage variable between 725 and 1550 V. Preset counting times "of 10 and 100 seconds

91 VIII

Name: Telsec Instruments Ltd.

Model: Portable Mineral Analyser ( see Fig. 12 )

Probe Detector type: Double-sealed proportional counters with beryllium window Dimensions: 292 mm X 292 mm X 114 mm Weight: 9 kg Temperature range of operation: 0-50°C. Cut-off operates at 50°C Detector window area: 2000 mm2 Mode of operation of shutter: Source sealed in sensing unit Number of filters: Maximum of twelve Mode of operation of filters:

Filters: Manually operated filter turret

Source: Sealed in sensing unit, can be changed after removal of cover

Use areas: Prepared samples in field or laboratory

Electronic unit Dimensions: Control box 229 mm X 127 mm X 140 mm Battery box 229 mm X 127 mm X 114 mm Weight: 6 kg Power supplies: (a) a.c. mains, (b) rechargeable batteries, (c) 20 X 1.5 V batteries Battery life: 20 h (continuous) Battery type: Rechargeable or disposable Shockproofing: Shockproof Waterproofing: Waterproof Temperature range of operation: Form of output display: Analog. Ranges 103, 104, 105 or 106 counts for f.s.d. Display facilities: Electronic components: Integrating ratemeter with timer (10,40 and 100 s integration). Display as total counts in either channel or difference in counts between channels. Alter- natively corresponding count rates can be displayed

92 II

Name: Panametrics, Inc.

Model: Laboratory unit, X-Ray Spectrochemical Analyser

Probe Detector type: Scintillation or proportional counter Dimensions: 190 mm X 102 mm Weight: 1.58 kg Temperature range of operation: 0°C to +40°С Detector window area: 1400 mm2 Mode of operation of shutter: Automatic safety interlock'actuated by lowering sample cover Number of filters: All required filters Mode of operation of filters: Sequential with manually operated switch

Filters: Interchangeable and/or integrated in measuring head

Source: Interchangeable and/or integrated in measuring head

Use areas: Hand-held, prepared samples, field, laboratory, and plant

Electronic unit Dimensions: 389 mm X 222 mm X 165 mm Weight: 6.5 kg Power supplies: a.c. mains Shockproofing: Shockproof Waterproofing: Weatherproof Temperature range of operation: 0°C to + 50° С Form of output display: Digital; four most significant decimal digits displayed; two suppressed Display facilities: + and — output for multi-channel analyser; special output for spectrum controller and display unit Electronic components: Single-channel analyser, continuously variable threshold and window variable in steps of 100 mV from 0 to 9.9 V; timer, continuously variable from 4 to 256 seconds, HV supply continuously variable from 1000 to 2800 V; charge-sensitive preamplifier and amplifier with continuously variable gain 2000: 1; reversible scaler

93 VIII

Name: Panametrics, Inc.

Model: Portable Unit, X-Ray Spectrochemical Analyser (see Fig. 13)

Probe Detector type: Scintillation or proportional counter Dimensions: 190 mm X 102 mm X 102 mm Weight: 1.58 kg Temperature range of operation: 0°C to +50° С Detector window area: 1400 mm2 Mode of operation of shutter: Automatic safety interlock actuated by positioning sample Number of filters: All required filters Mode of operation of filters:

Filters: Interchangeable and/or integrated in measuring head

Source: Interchangeable and/or integrated in measuring head

Use areas: Hand-held, prepared samples, field, laboratory and plant

Electronic unit Dimensions: 389 mm X 222 mm X 165 mm Weight: 7 kg Power supplies: 10 D-size batteries or a.c. plug-in module Battery life: Up to one week of normal operation Battery type: Available with either disposable or rechargeable Ni — Cd batteries Shockproofing: Shockproof Waterproofing: Weatherproof Temperature range of operation: 0°C to + 50° С Form of output display: Digital; four most significant digits displayed; two suppressed. Display facilities (recorder socket etci): Electronic components: Single-channel analyser, continuously variable threshold and window variable in steps of 100 mV from 0 to 9.9 V; timer, continuously variable from 4 to 256 seconds, HV supply continuously variable from 1000 to 2800 V; charge-sensitive preamplifier and amplifier with continuously variable gain 2000: 1; reversible scaler

94 VIII

Name: Texas Nuclear Corporation Ltd.

Model: Portable X-Ray Fluorescence Analyser (see Fig. 14)

Probe Detector type: Scintillation counter -, Dimensions: 8 in. long, 2.5 in. diameter; 4.5 in. flange Weight: 2.5 lb Temperature range of operation: -40°С to +50° С Detector window area: 1.5 in. long by 0.8 in. diameter Mode of operation of shutter: Automatic when sample positioned Number of filters: two- Mode of operation of filters: Bistable lever

Filters: Interchangeable, and / or integrated in measuring head

Source: Interchangeable, and / or integrated in measuring head

Use areas: Hand-held, prepared samples, field, laboratory, and plant

Electronic unit Dimensions: 10 in. X 6 in. X 7 in. Weight: 12 1b Power supplies: 10 D-size batteries or a.c. power module Battery life: 50 hours with dry cells; longer with alkaline or mercury batteries depending on temperature and mode of operation Battery type: Any type as long as D-size Shockproofing: Shockproof Waterproofing: Weatherproof Temperature range of operation: -20° С to +50° С Form of output display: Digital; four displayed, two suppressed digits Display facilities: Attachment for external multi-channel analyser Electronic components: SCA with window width variable over unlimited range; reversible scaler; timer; (automatic, 10, 20, 40 or 80 seconds, or manual)

95 VIII

Name: EIC

Model: Model 100 X-Ray Fluorescence Analyser

Probe Detector type: Proportional counter Dimensions: 283 mm X 90 mm X 90 mm Weight: 3 kg Temperature range: 0°C to +45 °C. Detector window area: Minimum: 20 mm X 50 mm Maximum: 60 mm diameter according to customer requirements

Shutter: Manually operated Filters: If required Source: Integrated in measuring head, selected for application specified Use areas: Coating weight and chemical analysis in laboratory, quality control and production

Electronic unit Dimensions: 19 in. standard rack, 3 units high, 400 mm deep Weight: 19 kg Power supply: a.c. mains, 110 V/60 cycle or 220 V/50 cycle Shockproofing: Reasonably shockproof Waterproofing: Not waterproof Temperature range: 0° С to +40° С Calibration curve linearization: 3-point adjustment to any calibration curve Form of output display: Direct digital indication in units of the quantity being measured. 3| digits Display facilities: Digital BCD output for printer, analog signal for recorder Tolerance indication: High — low — in tolerance indication Electronic components: Single-channel analyser. Window adjustment potentiometers > permanently calibrated in keV. Digital scaler/timer. Time selector switch for 2/4/8/16/32 seconds counting time

96 IX

Name: Nucléomètre

Model: Fluoscope

Probe Detector type: Proportional counter Dimensions: 390 mm X 195 mm X 160 mm Weight: 4 kg Temperature range of operation: <45° (probe is thermostated at 45°C) Detector window area: 20 cm2 Mode of operation of shutter: Manual Number of filters: Two Mode of operation of filters: Slide

Filters: Interchangeable

Source: Interchangeable

Use areas: Laboratory

Electronic unit Dimensions: Standard rack 19 in., 3 units height Weight: 10 kg Power supplies: Mains 220 V Battery type: Rechargeable, disposable Shockproofing: Not shockproof Waterproofing: Not waterproof Temperature range of operation: 0 to +50° С Form of output display: Analog digital, single-channel analyser — ratemeter — preset count scaler — time display, preset count 1 to 9 X 102 - 103 - 10" time 0.1 to 9999 seconds, recording output 0-50 mV

97 II

Name: Rigaku

Model: Radioisotope X-Ray Spectrometer

Probe Detector type: Proportional counter Dimensions: 380 mm X 116 mm X 166 mm Weight: 5 kg Temperature range of operation: -10°C to +40°C Detector window area: 25 mm X 25 mm Mode of operation of shutter: Switch manually actuated by operator Number of filters: As required Mode of operation of filters: Manually inserted sequentially by operator

Filters: Interchangeable and / or integrated in measuring head

Source: Interchangeable and/or integrated in measuring head

Use areas: Prepared samples, laboratory, and factory

Electronic Unit Dimensions: 552 mm X 546 mm X 499 mm Weight: 30 kg Power supplies: a.c. mains Shockproofing: Not shockproof Waterproofing: Not waterproof Temperature range of operation: Form of output display: Analog, 100 to 8 X 104 counts/s; digital 10e counts Display facilities: Recorder output Electronic components: Single-channel analyser, ratemeter, scaler, timer

98 APPENDIX II

CLASSIFICATION OF PREFERRED TERMS AND DEFINITIONS RELATING TO RADIOISOTOPE X-RAY FLUORESCENCE ANALYSIS

1. Methods of excitation

1.1. X-ray excitation. 1.2. уЗ-excitation. 1.3. a-excitation.

2. Methods of energy selection

2.1. Dispersive: involving the use of a crystal monochromator. 2.2. Non-dispersive: relying on the resolving power of radiation detectors, elemental filters and electronic pulse discrimination circuits. The term 'energy selection' refers to the use of radiation detectors and the term 'energy discrimination' refers to the use of elemental filters.

3. Terms relating to the sample

3.1. Wanted element. 3.2. Interfering element. 3.3. Matrix. 3.4. Matrix effects: (a) absorption, (b) enhancement, (c) heterogeneity — particle size, particle composition and particle statistics.

4. Terms relating to the source

4.1. Primary source: sealed source of a- or /З-particles or electromagnetic radiation, including bremsstrahlung. 4.2. Source-target assembly: combination of a radioisotope source aind a target to provide an essentially monochromatic source of X-rays characteristic of the target material.

5. Terms relating to the measurement

5.1. Gradient or slope of a calibration curve: the change in count rate (or detector output) caused by unit change in the concentration of the wanted element ( 91 / 9c). 5.2. Relative sensitivity (s): the ratio of the relative change in count rate to the relative change in concentration of the wanted element

s= 9I/9C I/ с

99 5.3. Precision: relates to the ability of an instrument to reproduce the same reading when the same sample is measured in exactly the same way. It is expressed quantitatively as the standard deviation of the distribution of repeated measurements. It is limited mainly by two sources of error: statistical fluctuations in the decay rate of the radioactive source and by instrumental instabilities. 5.4. Accuracy: the degree of correctness with which a method of measurement yields the true value of the meàsured quantity. 5.5. The limit of detection of a wanted element in a given matrix is the concentration of the wanted element which in a stipulated measurement time gives a detector output equal to a quoted number of standard deviations of the detector output when measurements are made of a similar sample but with zero concentration of the wanted element.

100 LIST OF PARTICIPANTS

Chairman

C. G. CLAYTON UKAEA, Wantage Research Laboratory, Wantage, Berks, United Kingdom

Panel Members

K.H.ANSELL The Radiochemical Centre, Amersham, Bucks, United Kingdom

J.F.CAMERON Nuclear Enterprises (G.B.) Ltd. Beenham Grange, Aldermaston Wharf, Nr. Reading, Berks United Kingdom

K. G. CARR-BRION Warren Spring Laboratory, Gunnels Wood Road, Stevenage, Herts, United Kingdom

Traude CLESS-BERNERT Osterreichische Studiengesellschaft für Atomenergie mbH, Lenaugasse 10, 1080 Vienna, Austria

A. G. DARNLEY Exploration Geophysics Division, Department of Energy, Mines and Resources, Geological Survey of Canada, 601 Booth Street, Ottawa 4, Ont., Canada

L. GORSK1 Institute of Radioisotope Techniques, Academy of Mining and Metallurgy, Cracow, Al. Mickiewicza 30, Poland

P.MARTINELLI Commissariat à l'énergie atomique, Centre d'études nucléaires de Saclay, B.P. n° 2, Gif-sur-Yvette (S.-et-O), France В. S. RAWLING Zinc Corporation, Broken Hill, N.S.W., Australia •

J.R.RHODES Columbia Scientific Research Institute, 3625 Bluestein Blvd., P.O. Box 6190, Austin, Texas 78702, United States of America

B.SELLERS Panametrics Incorporated, 221 Crescent Street, Waltham 54, Mass. 02154, United States of America

P.E. STARNES Cartner Group Ltd., Mintek Division, Stirling Road, Trading Estate, Slough, Bucks, United Kingdom

T. TANEMURA Rigaku Denki, 9-8, 2-chome, Sotokanda Chiyoda-Ku, Tokyo,Japan

J. S. WATT Australian Atomic Energy Commission, Research Establishment, Sutherland, N.S.W., Australia

C. A. ZIEGLER Panametrics Incorporated, 221 Crescent Street, Waltham 54, Mass. 02154, United States of America

Observers

J. CAMERON Division of Research G.B. COOK and Laboratories, T. FLORKOWSKI IAEA, Vienna, Austria

SECRETARIAT Scientific Secretaries

C. K. BESWICK Division of Research and Laboratories, J. C. DEMPSEY IAEA, Vienna, Austria

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