Recent Devetopments in Activation Analysis T. B. PIERCE Analytical Sciences Diuisiow A .E.R,E. HavwdL Berks. Contents General considerations Introduction Techniques of activation analysis Chemical separation techniques Radiation measurement Reactor activation analysis-Delayed techniques -Prompt techniques Accelerator neutron sources-Delayed techniques -Prompt radiation Radioisotope neutron sources Delayed techniques Prompt techniques-Elastic scattering Methods of isolation of activity Neutron-activation analysis Charged-particle activation analysis -Prompt gamma radiation -Particle group measurement -Nuclear positive ion microprobe Gamma photon activation analysis 13 134 PIERCE General Consideration Introduction Activation analysis has been recognised as an analytical technique for over 30 years and for the last 20 years of that time has been intensively investigated at specialist establishments that have had available the experimental facilities necessary for supporting the work.The cost of these facilities particularly of suitable radiation sources has often been high limiting both the number of centres at which the full range of activation methods can be investigated and the avail-ability of shared irradiation facilities. As a result activation analysis has not been able to enjoy the support of many of the organisations dedicated to the develop-ment of analytical techniques for specific applications particularly for industrial use which have played so important a part in bringing other analytical techniques from the research phase through the development stages needed to mould the method into an approved tool that can be entrusted with routine widespread application.In spite of the fact that only a limited number of centres have been sufficiently well equipped to pursue the development of a range of activation analy-sis techniques potential advantages of the method have been sufficient to justify the investment of substantial effort a t some large establishments with more specialised effort at smaller laboratories with restricted access to irradiation and other facilities so that the technique has been under continuous development. This interest has been reflected in the number of papers published; a recent bibliography contains more than 4,000 references and shows that the growth of publications on the subject of activation analysis is almost exponential with a doubling time of little over 3 years.l Initially the technique of activation analysis was fairly clearly defined.A sample would be irradiated in a field of particles or electromagnetic radiation usually neutrons and most frequently those from a nuclear reactor. After the completion of irradiation the elemental concentration of one or more components of the sample was determined by measuring the induced radioactivity usually after isolation by radiochemical separation of the activity to be counted from all others produced in the sample. However the last 5 years have seen increasing interest on the part of the activation analyst in the application of techniques that are no longer based solely on the measurement of induced radio-activity but which still involve an irradiation step as an integral part of the analytical procedure and are consequently often grouped together under the general heading of activation analysis.Thus activation analysis has become a very much more diffuse subject than it was some years ago as analytical scientists have harnessed more of the properties of the nucleus to provide relevant information, and instrumentation and techniques have been developed that have resulted in the less convenient methods being put on a more competitive analytical basis. To the non-specialist activation analysis can now appear to be a somewhat arbitrary accumulation of separate techniques a view that is not helped by the fact that there is no comprehensive up-to-date text-book available which treats the subject in a systematic manner.While a detailed theoretical discussion and comparison of th RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 135 nuclear interactions exploited by activation analysis as a means of obtaining analytical information is out of place in a review article such as this a short sub-section gn techniques of activation analysis has been included in the first part of this review in an attempt to give the reasons for the structure chosen for the article and also in the hope that it will give an idea to the non-specialist albeit in a very superficial way of the interconnection between the techniques discussed. The majority of the references used for this review have appeared in print within 3 years of June 1970 but reference is made to earlier papers when these are of special interest and illustrate some particular facet of activation analysis not well covered elsewhere.It is pertinent to conclude this short introduction with a few comments on the general trends that have become apparent in activation analysis over recent years. Many developments in techniques have followed directly from improvements in instrumentation and it is encouraging to see that instrument manufacturers and machine operators are becoming more prepared to provide equipment tailored to meet the exacting demands of the analyst rather than the activation analyst having to make do with equipment primarily designed for an entirely different purpose.Improvements to methods of y-ray spectroscopy common to so many activation techniques have extended the scope of intact methods of analysis. These have consequently taken over some of those determinations that would previously have required chemical manipulation of the sample ; y-ray spectroscopy of partially separated mixtures of radionuclides is now also a more attractive proposition. However the strength and permanence of chemical separations in activation analysis are clearly illustrated by work in laboratories that are well versed in sophisticated instrumental techniques as well as radiochemical methods, and which have still found the need to develop new methods of chemical separation in spite of being well equipped with advanced instrumentation for y-ray spectro-scopy.Clearly activation analysis has as yet made little impact on industrial applications where analyses of large numbers of samples are required and for which in some ways activation analysis is well suited. Partly because of the cost (although some activation systems can be produced that are competitive in price with larger conventional analytical techniques such as X-ray fluorescence or mass spectroscopy) but also because of lack of effort at existing activation laboratories, the possibility of using activation analysis for specific determinations of interest to industry has not always been investigated in sufficient detail to warrant the expenditure by a potential user necessary to equip a laboratory with suitable equipment. It is therefore of considerable interest that a very large British industrial organisation with previous experience of reactor neutron-activation analysis has seen fit to invest in a nuclear reactor of its own to be used in part for analytical work.Techniques of activation analysis As activation analysis now embraces a number of different techniques, apparently chosen in a somewhat haphazard way (elastic scattering based o 136 PIERCE Coulomb repulsion is generally included in works on activation analysis whereas Mossbauer spectroscopy is normally considered to be an entirely separate subject) to avoid confusion a review such as this one must identify the different fields of interest by sub-division. To explain the method by which sub-divisions used in this review have been chosen and to acquaint the non-specialist with the inter-relationship of the various methods a very short discussion of the techniques of activation analysis and the relevant processes occurring during a nuclear reaction are outlined below.In the author’s opinion the usual procedure normally followed in works on activation analysis of considering solely the formation and decay of radioactive species is no longer adequate for describing activation procedures as an increasing number of publications are appearing that are concerned with the detec-tion of radiations other than those derived from the decay of radioactive nuclei. The irradiation stage of an activation procedure is an intrinsic part of the analytical method and distinguishes activation analysis from isotope techniques such as radioactive isotope dilution radio release and isotopic exchange in which production of the radioactive species is carried out quite independently of the analysis.One way of illustrating the different techniques of activation analysis is by considering a nuclear reaction occurring according to Bohr’s theory of compound nucleus fomation. The nuclear reaction occurs by two independent steps the first being the absorption of the incident particle to form a compound nucleus and the second the subsequent decay of the compound nucleus. The first step assumes that the incident particle is absorbed by the target nucleus to form a compound nucleus in a highly excited state with an energy level E in the nucleus +’ *$ as shown in Fig. 1. A+a A t a - I C z + z z t z - I Fig.1. Schematic representation of a nuclear reaction A + a - l B z + RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 137 The excitation energy derived from the binding energy of the incident nucleus and the kinetic energy of the incident particle can be rapidly re-distributed amongst the nucleons in the compound nucleus and if at any stage sufficient energy is concentrated on a single nucleon or a group of nucleons in the nucleus these may be ejected. The second stage of the nuclear reaction therefore is the decay of the compound nucleus feeding levels in the product nucleus shown in Fig. 1 as emission of the proton groups Po PI and P, although of course other particles, for example neutrons or a-particles may be emitted during nuclear decay.If a particle is not ejected the excited levels of the compound nucleus may lose their excitation energy in the form of y-radiation (yl y2 ys and y4 in Fig. l) which is characteristic of the nuclear transitions occurring. All this takes place very rapidly, nearly always in less than 1 ps and thus radiations resulting from compound nucleus decay cannot usually be measured after physical transfer of the sample from the place of irradiation to the radiation counter as is usual in traditional methods of activation analysis They are ‘prompt’ radiations which must be measured while the irradiation is in progress. Particle emission may leave the product nucleus in either the ground state or in excited states and if an excited state is produced the excitation energy may again be lost in the form of y-radiation as represented by transitions y6 and y7.The product nucleus ;$ ZiC having been formed artificially may be unstable in its ground state decaying by the emission of radiation familiar in the decay of radioisotopes. In Fig. 1 the product nucleus is assumed to decay by /3-emission (Po and /?,) to form the final stable nucleus 2 $ -’B although emission of /?+ electron capture etc. might be alternative modes of decay. If /3-emission feeds excited states of this residual nucleus then again the excitation energy may be lost by emission of y-radiation occurring very rapidly after emission of the /3-particle (yJ. As the half-lives associated with /3-decay are generally relatively long emission of the /3-particles and those radia-tions following /3-decay (y8) can often be detected after removal of the sample from the place of irradiation to a counter.Prompt radiations are therefore usually nuclear reaction products ; delayed radiations are most frequently derived from the decay of radionuclides. The decay scheme shown in Fig. 1 is by no means universal as not all reactions occur through compound nucleus formation and decay may take place by more complex modes than those shown in Fig. 1. Nevertheless the scheme does illustrate many of the types of decay that are now used as a basis for activation analysis. The general types of over-all reaction are summarised in Table I below. TABLE I TYPES OF ACTIVATION PROCEDURE ;A + [;$:B] -$A + :a . . . . (1) __+ $A+ + fta * . * (2) -+ZA$rtB + y .. . (3) -++$:ItD+td .. .. (4 138 PIERCE In equations (1) and (2) the same particle is emitted as is absorbed and the reactions are called scattering reactions. In equation (1) kinetic energy is conserved and therefore the reaction appears to the observer to be a typical ‘billiard-ball’ interaction but in equation (2) kinetic energy is lost and is converted to excitation energy of the product nucleus. Equations (3) and (4) are transmutation reactions in that the product nucleus is different from the target nucleus but in equation (3) the product nucleus is formed by particle-capture with the emission of y-radiation, whereas in equation (4) another particle is emitted resulting in a different product nucleus. Neutrons charged particles and y-photons are all regularly used to irradiate samples and to initiate nuclear reactions for analytical purposes so that three general categories of activation analysis based on the type of irradiating radiation, can be distinguished.The character of the different radiations and the methods of their production profoundly influence the nature of the analytical information produced and the experimental techniques that are acceptable. The analytical procedure chosen will of course be dependent on the type of interaction upon which measurement is based. However the options open must clearly differ for prompt and delayed radiation as for the measurement of prompt radiation counting conditions will be influenced by the proximity of the radiation sources and there is no chance of using chemistry to isolate the radiation to be measured from all other emitted from the sample between irradiation and counting.The following chart summarises the different categories of radiation available for sample irradiation, but neutrons have been so extensively used for activation analysis that neutron techniques have been sub-divided into three categories based on the available Particles available for irradiation I I y-Phbtons Neutrons Chaiged particles I I I I I Reactor neutrons Neutrons from accelerators Radioisotope sources sources of neutrons reactor accelerator and radioisotope. The second chart shows the options available for activation procedures that are based on methods available for isolation of the radiation to be determined. This review therefore is divided into three major sections one for each of the three types of particles used to irradiate samples in activation procedures as shown in the charts but the section dealing with neutron-activation analysis is sub-divided into three further section RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS Radiation measurement procedures I 139 Methods based on measurement of delayed radiation I I Methods based on measurement of prompt radiation I I Instrumental Chemical methods separation of activity covering analytical methods based upon reactor accelerator and radioisotope neutron sources.Each section consists of two parts wherever appropriate the first being concerned with methods based upon the measurement of delayed radiation which make up the greater part of activation procedures as they have been more extensively investigated and applied while the second considers those methods that are dependent upon the measurement of prompt radiation for their success.Where prompt and delayed techniques axe combined as in certain neutron generator applications these are included in the section covering prompt methods. In addition a separate section is included that is concerned with methods of isolating the radiation to be measured from other emission from the sample as these are common to most activation procedures. Methods of Isolation of Activity During irradiation it is extremely fortuitous if the only activity induced in the sample is that upon which the analytical measurement is to be based; conse-quently some means must be found for isolating the required activity from all other radiations emitted from the sample.Initially chemical separation was used almost exclusively to perform this function but following extensive development, instrumental methods of radiation measurement have assumed an increasingly important role in activation analysis. Over the last few years significant advances have been made in the manufacture of new y-ray detectors in instruments to handle detector output and in mathematical and computer methods used for data processing. It must remain very reassuring however to the trained chemist that, in spite of sophisticated instrumentation now available to the activation analyst, chemical separation is often the only satisfactory method of achieving the high decontamination factors necessary for isolating very small amounts of active material and its activity from one delayed source from very large amounts of activity from others.Intact methods of analysis are most frequently based on y-ray spectroscopy particularly for measurement of the induced radiation emitted by radioisotopes as contributions from several sources can often be distinguished 140 PIERCE from the same sample and the convenience of the technique is a great attraction where the alternative chemical manipulation is complex or when large numbers of samples must be processed consequently making heavy demands upon laboratory effort. Therefore when induced radioactivity is measured and y-ray spectroscopy cannot adequately distinguish between the induced radioactivities of a complex mixture there has been a noticeable trend to combine y-ray spectroscopy with chemical separation by carrying out group separations in which y-emitters are not isolated individually but are separated into a few convenient groups and the contribution of individual emitters to the total activity of the groups is then distinguished by y-ray spectroscopy.By this means a compromise is achieved whereby chemical separation is used to remove interferences that restrict the application of y-ray spectroscopy but the heavy investment in laboratory effort that would be needed to carry out complete chemical separation of the components to bring them to a high state of radiochemical purity is reduced by using y-ray spectroscopy to distinguish the several y-ray emitters in the simplified mixtures.Chemical separation techniques Chemical separation techniques are primarily applied to the separation of a radionuclide from an irradiated sample between irradiation and counting and are particularly valuable when small amounts of active species are to be isolated from a much more active matrix. Inactive carriers are usually added to permit calcula-tion of chemical yield and many procedures are still designed to provide the product in a state of high radiochemical purity so that counting can be carried out with relatively simple equipment and the heavy investment in instrumentation required for more complex techniques is avoided. Separations must clearly be completed within the time-scale permitted by the half-life of the active species so that the preferred chemical and experimental methods may have to be modified to cope with the rate of decay of the radionuclide measured.Pre-separation of specific components of a sample is occasionally carried out when separation between irradiation and counting is not feasible but the method is susceptible to errors caused by reagent blanks which are avoided by post-irradiation separations to provide one of the major advantages of activation analysis for trace-element determinations. Much information is available in the literature about chemical separation procedures and the series of monographs issued by the U.S. National Academy of Sciences2 remains a useful source of information on the behaviour of individual elements that can be used as a basis for devising separation procedures.Also included are a number of works on specific separation techniques. Precipitation for so long used as the principal separation tool of the radio-chemist continues to appear with regularity in published radiochemical separation procedures although the general lack of selectivity of the method frequently limits its use to primary fractionation of the components of a mixture for example by scavenging or for the preparation of sources for chemical yield determination. Salutsky’s review3 provides a useful introduction to the precipitation method and Bock Werthmann4 has summarised the use of some organic reagents for precipita RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 141 tion thus employing a technique for so long applied to non-active analytical work.5 Solvent extraction figures prominently in separation procedures by virtue of its versatility convenience and the simple equipment needed.Good selectivity can often be achieved by careful choice of extractant and conditions. The book by Morrison and Freiser,G although first published some years ago provides a valuable introduction to the use of solvent extraction for analytical separations and contains much useful data. A more recent book by Stary' considers the use of chelates in solvent extraction techniques. In addition an immense amount of data is available in open literature describing extraction mechanisms and experimental behaviour of solvent extraction systems. Separation on organic ion exchangers continues to be the most widely used technique based on differential migration methods and will be as familiar to those analysts concerned with the separation of inactive materials as to those requiring the separation of radioactive species.The monograph by Samuelson8 continues to be a useful source of information for the practicing analyst and a newer book by Inczedy9 is also biased towards the analytical use of ion exchangers. Reversed-phase partition chromatography which aims to combine the choice of extractants available for solvent extraction with the multi-plate effect experienced in differen-tial migration by immobilising an extractant on a suitable carrier has been used for radiochemical separations for more than 10 years and is being increasingly applied to activation analysis.Comprehensive reviews on the subject are difficult to find but the article by CerrailO demonstrates the capability of the technique. Thin-layer chromatography so widely applied to organic chemistry can be adapted to provide inorganic separationsll and its use extended for radiochemical separations by reversed-phase techniques,12 but like paper chr~matographyl~ it suffers from the limitation of having only a low capacity if spot distortion is to be avoided. Gas chromatography has been extensively developed for organic applica-tion and is an attractive method for fast separations provided that the active species can be converted to a suitably volatile form. The method is applicable to many element combinations as a very large number of all the elements in the periodic table have been separated by gas chromatography.14 Radiochemical separation on inorganic precipitates is a field of separation technology that has been receiving increasing attention over recent years as reaction with ions in solution can occur by mechanisms such as re-crystallisation precipitation isotopic exchange and redox reactions to alter the elemental concentrations and the results of about 2,000 absorption experiments designed to assess the retention of a large number of ions on several precipitates have been summarised by Girardi et ~ 1 .~ 5 Distillation, which has long been used for the separation of certain radioactive species in activa-tion procedures for example for the separation of fluorine-18 produced by charged-particle activation analysis can offer quick and rapid separations of suitable elements,la and other less widely used methods such as isotopic exchange,17 electrochemical methods1* and separation on micro-crystalline cellulose columns1g have all been used to augment the repertoire of the activation analyst 142 PIERCE Of the two further aspects of radiochemical processing sample solution and chemical yield measurement rapid and complete solution of the analytical specimen often proves to be the most troublesome stage of sample treatment.The method chosen must ensure complete solution of major and minor constituents and Girardi2* has summarised a number of techniques in general use for biological materials. Chemical yield determination by weighing a precipitate is still widely used but other established methods of quantitative analysis axe increasingly used to determine the amount of element associated with the radioactivity measured.Re-act ivat ion and radioactive isotope dilution provide alternative radio analytical methods of mass measurement but considerable interest has been shown in recent years in so-called substoicheiometric finishes to activation procedures. These methods require the measurement of activity associated with a predetermined amount of active species sampled by adding to the system a known amount of reactant which is insufficient to combine with all the active species present in the system,21 and has the advantage that small masses of material can be examined by this means. Procedures have been developed for very many elements and these have been summarised in a book by Ruzicka and Stary.22 From the data available on separation techniques chemical procedures can be devised that permit one or a number of elements to be separated from complex matrices and determined at very low levels.For example ten trace elements have been determined in sixty-seven different meteorites by using radiochemical separation following neutron-activation analysis in some cases at levels of as low as 0*0005 ~ . p . m . ~ ~ Wile complex radiochemical separations are still necessary for many trace element determinations there has been a noticeable increase over the last few years in the use of group separations to avoid overlap of interfering y-peaks and to prevent the dominating activities induced in the sample from masking the radiations of low intensity.Examples of group separation are to be found in many fields of application of activation analysis. For example six ele-ments arsenic chromium cobalt copper molybdenum and tungsten have been determined in steels by using first a-benzoinoxime in chloroform followed by an ether extraction of chloride complexes and then counting the induced activity with either a germanium counter or a sodium iodide scintillator when high resolu-tion or high efficiency were required.24 Rock standards have been fractionated into six groups by volatilisation extraction with hydrated antimony pentoxide, ion exchange and solvent extraction with tributyl phosphate and then forty-five elements determined by y-ray spectroscopy on the separated groups.25 Group separation has also been used for the automatic processing of irradiated biological samples the separation being obtained by absorption on columns of specific materials.Column materials included Kiesehlguhr impregnated with di- (2-ethylhexyl) orthophosphate anion exchangers chelating resins and zirconium ortho-phosphate providing fully automatic separation in 40 minutes.26 In view of the substantial effort that is being devoted in non-active laboratories to automated chemical procedures in an attempt to reduce the investment in labora-tory effort required for each analysis automation of radiochemical separation RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 143 is an expected development where large numbers of samples are to be handled.Relatively simple systems have been devised to operate in a fixed mode and are clearly adequate for many routine operations where similar chemical operations can be guaranteed for a substantial period of time. More complex systems based on computer programmes27 have also proved to be of value when greater flexibility is required; digital computers have been used for treatment of experimental data.% It would seem logical to assume that with the rapid decrease over recent years in the cost of very small digital computers these machines will find their way into radiochemical laboratories for controlling larger chemical separation units and for carrying out some reduction in the handling of experimental data where this is feasible. Radiation measurement The primary purpose of radiation measurement is to isolate or determine the radiation emission derived from the sample upon which analytical measurement can be based.Consequently the counting system must be chosen not only to be sensitive to the type of radiation emitted from the sample (y-photons charged particles or neutrons) but also to provide data that can be identified with the component of the total radiation spectrum which is to be measured. A simple counting system is likely to be adequate for counting a radiochemically pure source produced by chemical separation but systems capable of discriminating between radiation from several sources present in the same sample are clearly liable to demand more complex instrumentation. Many radioactive isotopes and excited species decay by y-ray emission and the high potential information content of y-ray spectra has resulted in the widespread use of y-ray spectroscopy for the examination of multi-component mixtures while the convenience of the technique has ensured its use for more simple counting applications.Consequently y-ray detection is now the major measurement tool in the radiochemical laboratory and all other counting techniques must to some extent be regarded as special-purpose applications. Undoubtedly the most important single development that has occurred in y-ray spectroscopy over recent years has been the appearance of the lithium-drifted germanium diode as a viable y-ray detector to supplement the long established thallium-activated sodium iodide scintillator and Heath29 has reviewed the various aspects of germanium counter usage in detail.The high resolution of the germanium counter is the characteristic that is of major interest to the analyst, and full width at half maximum peak height of about a few kilovolts or better can now be achieved which is substantially better than that attainable with sodium iodide. This enables individual y-sources emitting lines of closely similar energies, to be distinguished in mixtures in a way that was not previously possible with non-dispersive y-ray detectors. The disadvantages that accrue from the use of germa-nium counters quite apart from the need to keep the detectors permanently cooled include low efficiency and high cost a 40-cm3 germanium counter together with cryostat and pre-amplifier costing about the same as a sophisticated atomic absorption spectrophotometer.The basic advantage of the germanium counter i 144 PIERCE derived from the low average energy of 2.98 eV required for the production of an electron-hole pair in germanium. However to exploit the inherent energy resolu-tion limited only by statistical considerations not only must the detector itself be as free from defects as possible but the other components of the counting system must have suitable properties. A major problem is the deterioration in performance of the over-all counting system at high count rates and new signal processing systems have been devisedSo that enable high counting rates to be accepted with relatively little deterioration in system resolution.A consequence of the improved resolution is the need for multi-channel pulse-height analysers with larger numbers of channels and a compromise must usually be made between the number of channels theoretically desirable and the financial investment called for. However, 4000 and 8000 channel systems are encountered relatively frequently in larger installations. Fixed-wire multi-channel pulse-height analysers are generally used for accumulation of data but small digital computers provide an alternative method of direct data acquisition and can offer very flexible systems as operation is under programme control and can often be varied relatively easily. The fall in price of small computers over recent years now makes them an altogether more attractive proposition for pulse-height analysis although the ‘word-length’ of the smallest machines may be inadequate to permit a single ‘word’ being allocated to each channel.81 Increase in computer size permits a more flexible system to be developed and the additional functions required for activation analysis such as sample transfer can be more easily put under computer control.32 The small computer has the additional attraction of being capable of being applied to other problems such as data processing and presentation when not in use as a multi-channel pulse-height analyser.= Very small computers have also been used to permit rapid transfer of information from a number of hard-wired analysers on to a magnetic tape store,w and direct connection between multi-channel analysers and a large computer capable of carrying out complex calculations has also been estab-lished.% To exploit the full potential of the high-resolution germanium counter for isotope identification precise y-line energies of nuclear transitions are required and recently amongst the tabulations that have been produced is one containing 2000 y-line energies from about 250 isotopes produced by neutron activatiomS6 The relative merits of using sodium iodide scintillators and germanium detec-tors for radiation counting in activation-analysis procedures will of course be entirely dependent on the composition of the radiation emitted but the two detectors have been compared for the analysis of a number of elements in lanthanum - tungsten bronzes and the examination of mixtures of rare earths.37 Comparison of the use of the two detectors with each other and with group separation procedures for the analysis of glass suggested that automated group separation probably provides the best cornpromi~e.~~ Multi-detector systems have been developed to suppress Compton event~393~0 and for y,y-coincidence spectro-metry,41 but although these techniques have proved invaluable for certain applica-tions the competitive position of the activation technique with respect to other analytical methods becomes weaker as greater capital investment is required fo RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 145 satisfactory instrumentation.Correct processing of accumulated data be it a simple y-ray spectrum or a more complex mixture of many sources is an essential part of any activation procedure and Yule42 has traced the development of methods used for the processing of y-ray spectra and given examples of the methods of mathematical manipulation of data in general use.Manipulation of data accumulated in y-ray spectrometers with the aid of large computers is now in widespread use and programmes are used for peak location, the measurement of peak energies the determination of y-line intensities and the calculation of elemental content of the sample. Many variations on the method of least squares fitting have been developed and are now extensively used where libraries of suitable standard spectra are available for mathematically constructing the necessary equations.43 These techniques undoubtedly increase the power of activation analysis and lighten the burden of onerous manual calculations but perhaps comments on y-ray spectroscopy should finish with a note of encourage-ment to those prospective activation analysts who do not have at their disposal the exceedingly expensive instrumentation required to pursue highly sophisticated techniques of y-ray spectroscopy.Not only can complex calculations be success-fully carried out on laboratory c a l c ~ l a t o r s ~ ~ but many valuable activation deter-minations can still be achieved with more mundane but in very many cases highly effective peak-area calculation techniques such as that of C~veIl*~ or the familiar base-line method. Although y-ray counting dominates the techniques of radiation measurement used in activation analysis certain other types of counter have found limited application for specialised purposes.Demands for charged-particle spectrometry have increased with the greater interest in the measurement of the prompt products resulting from nuclear inter-actions. Magnetic techniques** offer high resolution which permits detailed examination of complex spectra but semiconductor detectors are more convenient:' are non-dispersive and provide a resolution that is adequate for many applications. Neutron time-of-flight spectroscopy a technique widely applied in nuclear physics laboratories for the determination of neutron energies has been used to identify neutron groups emitted after sample irradiation with pulsed charged-particle beams.48 The measurement of tracks in polymeric materials such as cellulose acetate can provide a valuable positional indication of the presence of impurities when highly ionising radiations are and the work involved in track counting can sometimes be reduced by the use of instrumentation primarily intended for metallurigcal investigation,50 which enables tracks in a given field to be automatically totalled.Neutronlactivation Analysis Sample irradiation with neutrons has been extensively used in activation procedures and consequently is considered first in this review. Neutrons are most frequently produced in a nuclear reactor but accelerator and radioisotope neutron sources as shown on pp. 138 and 139 provide alternatives that although providing 146 PIERCE substantially lower neutron output can be used for certain applications and indeed may have some advantages where a high flux is not required.Neutrons themselves are a penetrating radiation and therefore interaction can be obtained with rela-tively large volumes of sample. However reactor irradiations are more frequently used to examine small samples perhaps of the order of a few hundred milligrams as the sensitivity of the technique is often adequate and the production of large amounts of unwanted activity is thereby avoided. Reactor activation analysis Delayed techniques. The use of a nuclear reactor as a neutron source is attractive to the analyst carrying out measurements at trace levels as not only are high sensitivities attainable as shown in Fig. 2 (taken from Ref. 51) but the high -8 z V Q) v) > > .-CI .- .- CI UJ .-% s $2 r r .-L.UJ +-’ 0) - G Q, y. 0 13 t z 5 I0 I 10-1 10-2 I O - ~ I O - ~ Sensitivity pg per 10Xn cmm2 s-’ Fig. 2. Sensitivities available for thermal neutron-activation procedures sensitivity of the method can often be exploited in realistic systems by using chemical separation as reagent blanks can be avoided and corrections can be made for chemical yield. It is advisable for the analyst to regard the reactor as something more than a ‘black box,’ particularly taking the trouble to find out the relevant characteristics of the irradiation station in so far as they influence his measure-ments. Guinn52 has summarised many of the characteristics of water reactors of the TRIGA type which are likely to be of interest to the activation analyst.Although most analytical applications are based on interaction of thermal neutrons high energy neutrons are available in a reactor and in some cases these may prove to be preferable for a specific determination. Assessment of the value of a particular reaction can often be made from a knowledge of cross sections if these are available (a) ( b ) Fig. 3. prepared for microcircuit production Photomicrograph (a) and the corresponding autoradiograph ( b ) of a silicon slice [To face Page 14 RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 147 both for the reaction to be used as the basis for measurement and also for those other reactions occurring with constituents in the sample that could produce activities likely to interfere with the determination.Tables of cross sections have been compiled which facilitate this type of calculation.63 Reactor fast neutrons, besides interfering with thermal neutron procedures offer the opportunity of determining a number of elements for which normal capture reactions provide inadequate sensitivity or which do not yield activation products suitable for measurement. A detailed study of the behaviour of thirty-five elements has shown that six, oxygen silicon phosphorus iron yttrium and lead have detection limits for reactor fast neutron-activation analysis that are lower than for thermal neutron irradiation while a further fifteen elements have similar sensitivities for fast and thermal activation.= However the limitation to the use of reactor fast neutrons is often not the inherent sensitivity of the method but the accompanying flux of low energy neutrons which may cause interfering nuclear reactions.Resonance neutrons also have uses for sample irradiation and their applica-tions have been reviewed.55 The possibility of nuclear interference must always be considered in activation procedures and a compilation has appeared of information on second-order reaction^.^^ Reactor pulsing was proposed some years agos7 so as to improve the sensitivity of methods employing short-lived nuclides and a number of recent applications have been described that emphasise the value of the technique.58 Considerable interest has also been devoted to devising methods for measuring short-lived nuclides to extend the scope of activation procedures and sensitivities and cadmium ratios for the production of a number of nuclides with half-lives varying from 2.5 s to 21 min have been determined practically and the results reported.59 Activation procedures based on the measurement of short-lived nuclides introduce special problems of counting timing and sample handling but equipment used for the determination of lead by the measurement of 800 ms lead-207 m has been described,BO and add - subtract sequence being used to distinguish between long-lived and short-lived components of the y-ray spectrum.The implication of counting short-lived nuclides has previously been considered in some detaiP and a more recent report has been published that is concerned with problems of decay curve analysis.62 Positional location of impurities by counting tracks formed in polymeric materials such as cellulose acetate,s3 or the long established technique of auto-radiography has found application when the radiation from the component to be investigated can be distinguished from other sources of radiation induced in the sample.When the over-all composition of the sample is suitable useful informa-tion can be obtained from such position sensitive techniques as shown in Fig. 3, (Pierce T. B. and Peck P. F. unpublished work) in which the photomicrograph of a silicon slice prepared for microcircuit production [Fig. 3(a)] is compared with an autoradiograph of the induced activity [Fig. 3(b)] taken after neutron irradiation, when the silicon activity has decayed away.The major component of the induce 148 PIERCE activity was caused by gold and the distribution of the element can be seen to be correlated with structural features of the slice. The distribution of copper and other elements in silicon has been examined by autoradiography to assist in the production of pure materials and the half-lives of components found by photo-metric examination of autoradiographs taken over an extended period of time.s4 Autoradiographic techniques have also been used to examine the inhomogeneous distribution of tantalum and tungsten in zone-refined niobiumJ65 while auto-radiography of oil paintings following neutron irradiation has revealed structural detail enabling pigments to be identified.66 In the latter case an absorbed dose of 50 rads was needed to provide adequate activity for autoradiography but even after a dose two orders of magnitude greater there was no sign of any changes in colour hardness flexibility or solubility in the paintings which were examined up to 3 years after irradiation.Secondary particles produced as a result of the interaction of neutrons with some component in the sample can provide a worthwhile alternative to neutron activation in certain cases. The best known application that of the determination of oxygen by tritons produceds7 as a result of the reaction 6Li (n,cc)T has found further application for the determination of oxygen in microcrystalline carbon68 and organic compo~nds.~~ Following the earlier work of Born a secondary reaction technique has also been used for the measurement of lithium,'O and deuterium has been determined by counting fluorine-17 produced by the reaction 160(d,n)17F which was induced by recoiling deuterium nuclei from the (n - d) collision.In this case the reaction 160(n,p)16N was used for internal standard-isation. Application-s. Reactor neutron-activation analysis has found very wide application which makes a critical survey impossible in the space available in a review such as this. However as the general theory and experimental techniques involved in many programmes of work utilising activation analysis are well understood interest now often lies in the results and their interpretation rather than in the method whereby the results are obtained always assuming that this has been reliably carried out.Applications must therefore figure in any discussion of activation analysis and the purpose of this section is to outline some of the sectors of application that have been found for neutron-activation analysis so that the reader can subsequently pursue the subject in more detail if he so desires. The high sensitivity attainable by neutron-activation analysis together with the lack of any reagent blanks and the ability to calculate chemical yields has made neutron-activation analysis a favoured method for the determination of many elements at low levels in a variety of complex matrices. One of the major fields of application of reactor neutron-activation analysis has therefore been the examination of a very wide range of materials of geological cosmological and bio-logical interest and examples of these applications are given below.In addition, the convenience of reactor activation analysis particularly where examination of the intact sample is possible or when only limited chemical separation is necessary, has established its use as the preferred technique for other purposes RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 149 Rocks meteorites minerals etc. Neutron-activation analysis has always been attractive for the analysis of geological and allied materials and there has been no dearth of examples of this particular type of usage in recent years. Undoubtedly one of the most interesting applications of neutron-activation analysis that has been reported recently has been the analysis of the lunar samples brought back by the Apollo 11 mission and reported in a copy of Scieme devoted to the proceedings of the Apollo 11 Lunar Science Conference held at Houston from the 5th to 8th January 1970.A number of analytical papers on the composition found for lunar material are included and neutron-activation analysis figures largely in the analytical methods chosen. Some laboratories have applied several analytical methods of which neutron activation is only ~ n e ~ ~ ~ ~ ~ but many data have been accumulated on the composition of rocks soil and core ~amples.~~-~7 Spallogenic manganese-53 was determined after reactor irradiation by counting 300-day manganese-54. In this case chemical yield was measured with carrier-tree manganese-52 as tracer.7s The extensive information obtained by activation analysis for a very large number of meteorites has been summarised demonstrating the scope of the technique for the analysis of both stony and iron meteorite^.^^ Neutron-activation analysis has been used to obtain information about the elemental abundances of various elements in the Allenda meteorite that fell on February 8th 1969 and was of the relatively rare carbonaceous chondrite type.791so An interesting develop-ment following an application of neutron-activation analysis established by Smales in 1958 was the determination of nickel cobalt manganese and iridium in cosmic spherules between 100 and 350 pm in diameter which were separated from 1.5 tons of Pacific Ocean sediment.For the measurement of short-lived nuclides the spherules were mounted on mica sheets for irradiation and y-ray spectrometry was used to isolate contributions from the individual elements.sl Among the wealth of papers that have been devoted to the analysis of rocks and ~ltramafic:~ metamorphics4 and igneous rockss5 have been amongst those examined.The value of germanium counters in the examination of geological materials has been clearly demonstrated by Gordon,86 who has deter-mined twenty-three elements in igneous rocks by high resolution y-ray spectro-metry following neutron irradiation and has considered the possibility of measur-ing several others. Identification of the source of y-lines in the complex spectra may prove difficult and principal y-rays are listed together with likely interferences to aid identification.Chemical separations have been used after instrumental examination of the sample to extend the range of elements determined,s7 while y - y coincidence spectrometry has also been found to provide extra selectivity in certain cases.s* The measurement of uranium in geological material has again been made by measurement of a fission product of uranium-235,89 in this instance tellurium-132, to complement the more usual technique of delayed neutron measurement and the ratio of protactinium-231 to uranium-235 in rocks has been measured by neutron activation and a-particle spectr~metry.~~ Activation analysis has been used t 150 PIERCE demonstrate that gold is not homogeneously distributed through standard rocks G-1 and VV-1 by division of a large sample and subsequent measurement of the smaller fractions .91 Soils have also been examined in some detail by neutron-activation techniques, and an attempt has been made to determine whether the y-spectra of soils can be used to trace their geographical Those nuclides that could be detected after sodium-24 activity had decayed were examined by y-ray spectrometry but soils of dissimilar origins sometimes gave rise to similar y-spectra thus throwing doubts upon the possibility of identifying the source of soil specimens from the induced y-ray spectra.Neutron-activation analysis and atomic-absorption analysis have been recommended as a particularly suitable combination of analytical techniques for soil analysis because of the range of elements that can be deter-mined with good precision and accuracy.93 Epithermal neutrons can sometimes prove more satisfactory than thermal neutrons for sample irradiation and rubi-dium caesium strontium antimony tantalum and uranium are elements that are favourable for this type of a c t i ~ a t i o n .~ ~ Where they can be applied the convenience of rapid methods of intact analysis permits the high throughput of samples and consequently enables the areal distribution of elements to be examined.g5 Similar techniques have been used to examine certain elements present in volcanic ash from the Taal volcano.98 Analysis of certain components in sediments from the South Pacific have enabled the behaviour of trace elements during sedimentation processes to be under~tood.~~ Application to life sciences. Life sciences provide a field where activation techniques have found profitable application and over the last few years a large number of reviews have a~peared.~8-lO~ These cover the use of reactors both for conventional activation procedures and for more general applications and include the use of secondary reactions for the determination of some elements that are not conveniently activated by reactor neutrons directly.Fields of application include metabolic and kinetic studies the determination of stable added tracers elemental determination for research and diagnostic purposes and even activation analysis in vivo. The possibility of obtaining an early diagnosis of cystic fibrosis by the analysis of clippings from toe and fingernails for sodium potassium and chlorine has been inve~tigated,l03~~04 and the ability to determine the sodium-to-potassium ratio by neutron-activation analysis improves the prospect of performing a success-ful sweat test.Of the many measurements carried out on the elemental content of various types of tissue a study of differences in trace element contents between normal and atherosclerotic aorta~,~05 investigations into the relationship between the occurrence of amyotrophic lateral sclerosis and manganese metabolism,lo6 and analysis of lung tissue from workers from certain mineral mining and processing industries107 have been reported. Neutron-activation analysis has also been used to monitor the levels of sodium magnesium manganese and sulphur in commercially available DNAse I (bovine pancreatic)lo8 and for dose control of suppositories of atropine methobromide contained in sealed polythene tubes ; measurement was based on the determination of the induced bromine-82 activity.log The value o RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 151 reactor neutrons for in vivo activation analysis is limited by the relatively low energy of the neutrons available but iodine has been locally activated in the thyroid by using an internal standard to allow for neutron absorption in tissue,l1° while the mass ratio of calcium to chlorine in human tibia has been measured by a neutron capture y-ray method.lll Substantial use has also been made of activation analysis in dental investiga-tions,l12 and the antimony arsenic copper gold iron mercury silver sodium and tin contents of dental pulp from diseased teeth have been examined so as to assess the biological importance of these elements.l13 Marine life and waters.Neutron activation has been used to support pro-grammes of investigation into fish and marine organisms. Elemental concentra-tions in tissue of species of Pacific salmon have been investigated114 and variations found between species and between sexes of the same species. All salmon were found to concentrate caesium relative to potassium. Levels of various elements in midline muscle dorsal muscle brain spinal cord liver heart spleen kidney and blood of rainbow trout raised under controlled conditions have been found and compared with that of the food fed to the fish.ll6 Variations in the trace element concentrations of corals have shown that it may be possible to distinguish reef from near-reef specimens by the higher uranium content of the latter.l16 The calcium content of two groups of zooplankton from four lakes Lago Maggiore Lago di Varese Lago di Comabbio and Lago di Monate have been compared,l17 and the effect of salinity on the deposition of calcium magnesium manganese strontium and sodium has been investigated.lls In spite of the difficulties associated with the relatively high induced radio-activity from sodium and chlorine the sensitivity and convenience of activation analysis has proved attractive for the examination of trace elements in a variety of natural waters.For example vanadium has been determined at concentrations ranging from 0.2 to 49.2 pg 1-1,119 and several elements have been measured directly in sea water after a 4 to 5-week cooling period has allowed the sodium-24 activity to decay.120 Activation analyses have supported studies with sea water,121 spring water,122 river rain water124 and mine water.125 Lanthanide distribu-tions in water from the Gulf of Mexico have also been determined by using neutron activation and a seaward decrease in elemental concentration was observed which has been correlated with river run-off.(A maximum in the depth profile was observed similar to that obtained with other elements in the vicinity of 1000 m.12s) High purity materials. The high sensitivity of neutron-activation analysis has been well exploited for the examination of the trace element contents of a variety of high purity materials including semi-conductors,l27 to aid impurity studies and to assist in improving manufacturing processes.The influence of surface impurities on the reproducibility and accuracy of the determination of the copper gold and silver contents of high purity germanium and silicon have been studied and correct surface treatment before activation has been found to be essential so as to obtain reliable results.128 Frequently concentration profiles or elemental contents of thin layers are required and an anodisation-peeling technique has been used in conjunc 162 PIERCE tion with neutron-activation analysis to obtain the concentration profile of energetic ions implanted in silicon crystals.12@ Neutron-activation techniques have also been used to follow the presence of impurities during the growing of doped laser crystals and to assess the extent of crystal contamination.130 Among the many other pure materials analysed are s e l e n i ~ m ~ ~ ~ ? ~ ~ ~ aluminiumls~l~ metallic uranium1% and plates of crystalline quartz.l= High induced activities of gallium-72 and arsenic-76 formed from the matrix when gallium or gallium arsenide is irradiated present special problems of handling as the sample activity may be 1 to 10 Ci and the separation of induced activity particularly of short-lived radio-nuclides that must be measured before the matrix activity has had time to die away can present special problems.These have been considered in some detail, and sensitivity under optimum irradiation conditions has been investigated for various impurities present at very low 1e~els.l~' Separation and determination of contaminants in the processing agents used in the telecommunication industry, such as silicochloroform distilled water nitric acid and carbon tetrachloride have been made by using a paper-chromatographic separation followed by y-spectro-scopy after neutron irradiation.l58 Oil pesticides and tobacco.The use of activation analysis in the mineral oil industry can be used to provide information concerning the composition of crude oil finished products wear elements engine deposits and residues. A knowledge of the source of an oil causing pollution of waterways is clearly important in sub-sequently deciding legal responsibility and in an attempt to provide a means of characterising oil sources sixteen different marine fuel oils have been analysed by neutron-activation analysis for vanadium manganese sodium cobalt antimony, arsenic copper and zinc.As part of this project prolonged exposure of the oils to sea water has been examined to assess the effect it may have on their elemental composition.139 The use of neutron-activation analysis to assess the content of pesticides present in feeding stuff has continued and the determination of bromine in wheat and bread after thermal neutron-activation analysis has been done as a means of determining traces of bromine-containing germicides.140 An extensive programme of work into the arsenic content of fifty-two samples of thirty-three brands of American cigarettes has been facilitated by the fact that no chemical separation has been needed in the neutron-activation determinati~n,~~~ and the content of a number of elements in a standard cigarette have been deter-mined by neutron activation to support work on tobacco and health research.142 Freedom from complication in sample preparation has proved attractive for the examination of the distribution of arsenic residues in tomato plants.Discrimina-tion within the plant indicated that the soil concentration of arsenic would have to be at a level that caused reduction in the size or yield of the fruit before the con-centration in the fruit exceeded tolerance limits.143 Further examples of the application of neutron-activation analysis to pesticide residue measurementla demonstrate the relatively rapid measurement of bromine and arsenic in a wide variety of plant material such as fruit tobacco or wheat flour.An extensive com-parison between neutron-activation analysis and a variety of other analytica RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 163 techniques for the determination of a large number of elements in a standard biological material has been made by a number of collaborating laboratories.14s Trace elements in the atmosphere. An increasing amount of work has been concerned with the determination of trace elements present in the atmosphere so as to obtain information about sources of pollution. The seasonal variations of manganese and aluminium have been followed from deposits collected in the Chiba area of Japan.146 Instrumental neutron-activation analysis involving germanium counters has permitted measurement of the concentration of more than twenty elements in atmospheric aerosols of urban areas and the technique was shown to be more sensitive and accurate than the other analytical techniques in current use.147 Sodium iodide scintillators have proved satisfactory for the determination of a number of elements present in airborne particulate matter collected on cellulose-base filter-papers.l@ The distributions of a number of elements at altitudes varying from lo00 to 50,000 f t have been examined by instrumental activation analysis in an attempt to establish the origin of aerosols in the atmosphere and the concentra-tion of iron was found to decrease by a factor of eight between the altitudes 10,000 and 50,000 ft.149 Anomalously high concentrations of both bromine and chlorine have been detected above the surface of Lake Michigan150 near urban localities and the distributions of trace elements in marine aerosols have also been examined.151 Forensic ap$&cations.The application of neutron-activation techniques to the analysis of samples of forensic interest has long been discussed in neutron-activa-tion circles and has been used to provide legally acceptable evidence in a number of cases. As more detailed information has been accumulated there has been an opportunity to assess the value of the technique in greater detail. Trace element content of hair for long suggested as a possibility of identifying an individual has been considered in depth with samples from a large number of volunteers and the analytical results have been treated statistically to assess their ~alue.l5~ Incorpora-tion of thallium into hair and nails has been correlated with the time a patient was known to have taken the element,l= and the results from neutron activation have been compared with those obtained by analysis using spark source mass spectro-scopy for the determination of a number of elements in human hair.154 The elemental content of samples of glass from a suspect's shoe has been compared with that from the broken window in a house that was burgled and results showed that glass from the shoe and from the window probably came from the same batch of manufactured glass155 ; the origin of glass fragments found duringan investigation of an accident between two cars was also used to ascribe responsibility for the acci-dent.lW Other investigations have included the detection of gun-shot residues by the presence of barium and antimony the determination of antimony arsenic, aluminium and silver in bullet lead for bullet characterisation discrimination between paint pigments and the determination of mercury in oats alleged to have poisoned animals.15' Art and archaeology.Use has been made of neutron-activation analysis to examine the elemental contents of small samples of materials removed from paintings to ascertain if trace element contents of pigments could be sufficientl 164 PIERCE characterised for dating to be possible and thence to reveal forgeries. The trace element content of white lead has been examined in some detail showing that impurity concentrations can give some indication of age.15*v15Q Contents of certain elements in pottery sherds from Hajar bin Humeid and related areas have been examined to study correlations in composition,160 and those from Knossos and Mycenae have also been investigated; in the latter case some samples from each group examined appeared not to belong to the supposed compositional category.lsl The geologic origins of prehistoric obsidian artifacts have been found by matching the sodium manganese lanthanum samarium scandium iron and rubidium contents of the artifacts with those of obsidian from geologic sources.162 An attempt has also been made to establish the origin of amber samples found during archaeological investigation with the assistance of neutron-activation analysis,16s and the silver content of coins dating from the Roman period has been traced by the analysis of 700 ~amp1es.l~~ Non-active tracers.The use of inactive materials as tracers that can sub-sequently be determined by activation analysis is attractive for many applications as the hazards associated with the handling of radioactive tracers is avoided and although the technique is more liable to contamination from the same element present in the medium to be investigated careful choice of a tracer can limit this occurrence. Selection of the element is also governed by the desired nuclear properties as detection at levels of adequate sensitivity ought to be relatively easy, and chemical behaviour may dictate the response of the tracer to some of the processes examined.Stable tracers in conjunction with neutron-activation analy-sis have been used for large-scale environmental studies and for the examination of the behaviour of very much smaller systems The movement of sand on the Mondello Beach Palermo Sicily has been studied with silver and cobalt stable tracers mixed with sand and the distribution of tracer was subsequently found by sampling and subsequent measuring by neutron-activation analysis.16s Indium, injected at a cloud base as finely divided particles from pyrotechnic flares was subsequently determined in rain collected in an array of fourteen samplers spaced over 11 km. The sensitivity of the method was limited by the natural background of indium which was found to be 6 & 3 ng of indium per litre.166 Stable tracers have been applied to the tracing of estuary waters,l67 and the technique has also been considered for examining the surface distribution of agricultural insecticides sprayed by helicopter.16* The addition of 0.1 per cent.w/w of a rare earth oxide to gunpowder permits detection of the firing of a weapon with a high degree of probability and a cost analysis suggests that the labelling of gunpowder with europium will increase the cost of ammunition by less than 2 per cent.169 Refrac-tory materials containing tracer elements have been used to identify non-metallic inclusions in steels which originate from such refractory components as ladle-linings or the hearths of electric furnaces.170 Isotopic analysis. As a radionuclide is produced by the interaction of a neutron with a specific isotope neutron-activation analysis is essentially a method of isotopic analysis and offers the possibility of determining isotope ratios in specifi RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 156 samples.The ratio of uranium-235 to uranium-238 has been measured from the change in intensity of y-peaks of neptunium-239 and fission produ~ts,l~l-l~~ while manganese43 has been determined in meteoritic iron by a neutron-activation t e ~ h n i q u e . ~ ~ ~ Miscellarceoiws applications. Neutron-activation analysis has been used to support other programmes of work that are not conveniently covered by the head-ings given previously and a number of these studies have been chosen to demon-strate the wide field of application of activation analysis.The rate of evaporation of some solids has been measured by condensing the evaporating substance on a water-cooled lead plate and then irradiating the plate and condensate with neutrons. Subsequent measurement of the induced radioactivities permitted the measurement of evaporation rates in the range 10-7 to 10-9gcm-2 ~ e c - l . l ~ ~ Neutron-activation analysis has also provided a method for re-determining the vapour pressure of liquid cerium176 and has been used to measure individual corrosion rates of iron and chromium in sodium systems.177 The behaviour of selenium mixed with lithium fluoride and added to highly purified graphite powder as a spectrographic buffer has been followed under arc discharge by neutron techniques and rates of vaporisation of the materials have been determined under a number of different conditions.l78 Prompt techniques.An alternative to the measurement of induced radioactivity produced as a result of sample irradiation with reactor neutrons is the measurement of prompt events from nuclear interaction which occur very rapidly after the initial interaction of the reacting nucleus with the incident neutron. The capture of thermal neutrons results in the emission of y-radiation that is characteristic of the nuclear transitions occurring and can consequently provide the basis of analytical determination. The initial excited states resulting from neutron capture may have energies of 7 MeV or more so that spectra are frequently complex and the y-ray emission from multi-component samples is often difficult to analyse.However ‘capture spectra’ for a very large number of elements have been compiled for sodium iodide scintillator~l7~ and germanium countersls0 so that suitable reference data are available if use of the technique is being considered. Although the inherent sensitivity of prompt techniques is high,lsl and for many elements may exceed that available for conventional activation analysis only relatively low neutron fluxes are normally available for capture work as neutron beams are usually extracted from the side of a reactor through the biological shield, and the decrease in neutron flux over that available in the reactor may be 6 or 7 orders of magnitude hence good sensitivity can only be obtained for those elements that possess high neutron capture cross-sections.Boron which cannot be con-veniently determined by more usual neutron-activation techniques undergoes the reaction l0B (n,a)7Li with thermal neutrons producing lithium-7 in its first excited state; this isotope then decays to ground-state by the emission of the 478 keV y-line. The y-ray spectrum resulting from the interaction of boron is therefore un-complicated and the high cross-section for the production of lithium-7 provides 156 FIERCE the basis for a sensitive determination. Moreover the importance of the presence of boron in certain materials makes simple determinations of the element a matter of real interest. This method has been successfully applied to the determination of boron in steels and sensitivities of the order of a few tens of parts per million have been a c h i e ~ e d .~ ~ ~ J ~ ~ The technique has also proved useful for the determination of a number of other elements including gold,lS4 samarium and gadolinium,lS5 and a number of elements in biological materials including hydrogen calcium phos-phorus and chlorine.lS6 Several methods for the analysis of fissile materials based upon the measurement of prompt radiation measuring neutrons or y-rays have been developed.187 Accelerator neutron sources While nuclear reactors have received the major attention as neutron sources for activation analysis neutrons may also be produced by the interaction of accelerated charged particles with target nuclei of suitable materials. Much information is available in nuclear physics literature on the nuclear parameters of neutron-producing reactions and monographs have been producedlS8 giving neutron energy as a function of reaction particle energy and angle for a number of the more widely-used nuclear reactions.While a careful choice of neutron energy can provide selectivity of value in reducing the effect of nuclear interferences in practice large accelerators are generally needed to achieve this variation in neutron energy and it is therefore not surprising that much analytical work carried out with accelerator neutron sources has been based on the use of so-called neutron generators which are usually low-voltage accelerators producing neutrons by means of the reaction A major attraction of this particular reaction is that substantial outputs of 14-MeV neutrons can be produced with accelerating voltages of the order of 120 KV.Thus the use of accelerator neutron sources can fulfil two functions firstly to provide irradiation facilities that are smaller and cheaper than nuclear reactors but which are still capable of producing a useful neutron flux and secondly to provide a neutron output with an energy distribution that is substantially different from that obtained in a nuclear reactor but which can be valuable for the determination of certain elements. Early neutron generators were almost all large drift-tube machines requiring substantial shielded space for safe operation. This sometimes made them uncompetitive when their requirements and performance were com-pared with those of other analytical techniques.However from the point of view of the analyst requiring an instrument that is relatively easy to install the newer, sealed-tube neutron generators appear to have a number of advantages.ls9 One is that they can be installed in the now well known hole-in-the-floor shielding. Although such shielding presents difficulties if frequent access to the generator target is required it makes relatively little demand on laboratory space. Delayed techniques. The primary use of neutron generators is for analytical methods based on the measurement of radioisotopes formed as a result of the *H + SH -+*He + In + 17.6 Me RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 157 mteraction of the high-energy or ‘fast’ neutrons with elemental constituents of the sample and the reactions most likely to occur are (n,a) (n,p) (n,2n) (n,n’).In addition there may also be a component in the induced activity produced by capture of low-energy neutrons particularly if substantial amounts of moderator are present near the neutron emitting target. Fast neutron-activation analysis is therefore capable of providing radioisotopes from irradiated elements which differ from those produced in largely thermal irradiation facilities and in certain cases permit the determination of those elements that cannot be conveniently determined as a result of thermal neutron irradiation. The variety of nuclear reactions that may occur under fast neutron irradiation, while giving some scope for the choice of radionuclide upon which quantitative measurement may be based also leads to the most serious limitation of the applica-tion of fast neutron-activation analysis to practical problems.The limitation is created by the production of the same radionuclide from more than one element (nuclear interference). For example chromium can be determined after irradiation with fast neutrons by measurement of the vanadium-52 produced by the reaction 52Cr(n,p)52V. Vanadium-52 is a 7-emitter decaying with a half-life of 3.76 min and with E = 1-43 MeV. However should any manganese be present in the sampIe the vanadium-52 will also be produced from the manganese by the reaction 55Mn(n,a)52V and if an appreciable flux of thermal neutrons is also present at the irradiation position vanadium-52 could be produced from vanadium-51 by the capture reaction 51V(n,y)52V.Thus the total yield of vanadium-52 will be a function of the amounts of all three elements present in the sample. If an accelera-tor is available which can substantially vary the energy of the neutrons produced, then a judicious choice of neutron energy can sometimes reduce the contribution made by an interfering reaction or decrease the yield of a major component to the y-ray spectrum.1g0 However neutron generators do not normally have such flexibility and therefore the scope for such choice of neutron energy is very limited. The ratio of thermal to fast neutrons can be varied by the use of a moderator placed near to the neutron-emitting target sometimes allowing the contribution from capture reactions to be but the penalty of using a moderator may well be one of lowering the induced radioactivity in the sample and thus limiting the scope of the analytical determination.The use of moderators alters the energy spectrum of neutrons irradiating a sample and can in some cases lead to changes in the relative induced activities formed by fast reactions,192 but in general these effects are relatively small and are not sufficiently large to improve substantially the scope for many analytical determinations where interference is serious. Another problem that critically affects the design of neutron generator systems for activation analysis stems from the rapid variation in flux with distance from the neutron-emitting target varying along the central axis as l / d near the target and l / d 2 at a greater distance where d is the distance between the target and the point at which the flux is being measured.lg3 Flux distributions for typical generator systems have been calculated194 and the results emphasise that not only will the gradient through a thick sample vary substantially even if there are no self 168 PIERCE shielding corrections to be made but also that if a sample is irradiated very near to a neutron-emitting target careful positioning is necessary to ensure that successive samples experience the same neutron flux for a given generator output.However, by careful design high precisions can be obtained by using a single-transfer system,ls5 but alternatives are to rotate the sample,lgg to use a dual rotatorl97 in which the sample and some form of standard are irradiated simultaneously or to irradiate the sample and standard together without rotation.lS8 Because of the limited target life of many high-output neutron generators and the decrease in a flux with distance from the neutron-emitting target that limits the number of samples that can be realistically irradiated at any one time neutron generator applications are normally limited to relatively short irradiations and measurement of the induced activity of short-lived radionuclides.The major interest in fast neutron-activation analysis initially stemmed from the possibility of determining oxygen by the reaction 160(n,p)16N,19s as 7.3 s nitrogen-16 is a convenient radio-nuclide for production by generator irradiations and the unusually high y-line energy of 6-1 MeV emitted as a result of the decay of the nitrogen-16 can be measured relatively easily.As a result fast neutron-activation analysis has been used for the determination of oxygen in a very wide variety of matrices. Some of the corrections involved in the application of fast neutron-activation analysis have been considered200 and must be applied when the highest accuracy is required. A significant development in the use of neutron generators has been a careful evaluation of the technique for the determination of oxygen in steel,201 and sub-sequent use of the technique in industrial operation.202 Special standards have been produced for the determination of oxygen in steels under industrial conditions.20s Another light element which because of its importance has received con-siderable attention is nitrogen particularly when present in foods and feeding stuffs as very large numbers of nitrogen determinations are at present carried out by the feeding stuffs industry to assess the protein content of the material.A detailed investigation has been made involving several hundred determinations on sixteen different types of material and good linear correlation was obtained between fast neutron-activation results and those obtained by the more traditional Kjeldahl techniques204 A similar method has been applied to the determination of nitrogen in maize seeds205 and in flour,206 and the same nuclear reaction [14N(n,2n)13N] has been used to measure the amount of nitrogen in petroleum products. In this case the effect of recoil protons was considered in some detail as their interaction with carbon present in the sample can produce nitrogen-13 by secondary reactions, including 12C(p,y)13N and 13C(p,n)13N but the yield of nitrogen-13 was found to be a function only of the weight fraction of carbon or hydrocarbons e~amined.~O' Neutrons (14MeV) have also been used for the determination of oxygen, silicon and aluminium in lunar rocks and soils and oxygen abundances were found to be lower than those in most common terestial rocks comparable to the levels present in certain types of stony meteorites.208 Among the many other applications of fast neutron activation reported is the analysis of biological materials for silicon, chlorine potassium phosphorus calcium and aluminium209 ; halogens have bee RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 159 determined in photographic emulsions2lO and copper silver and gold in ancient coins.211 Fissile material remaining after the dissolution of irradiated fuels has been determined by neutron-generator irradiation and delayed-neutron counting techniques and the sensitivity of the methodhas been assessed at 20 mg of uranium-23tL212 A mixture of thermal and fast neutrons has proved to be satisfactory for the determination of aluminium and chlorine in intact solid composite propellents.With a precision of better than &l per cent. the technique appears to be adequate for quality control purposes.213 To extend the scope of fast neutron-activation analysis to the measurement of radionuclides having a half-life of less than 1 s, fifteen nuclear reactions have been studied to obtain information about the forma-tion of a number of radioisotopes with half-lives varying from 0.5 to 870 ms.214 Unfortunately only a very limited number of radioisotopes can be produced with half-lives of this order.This somewhat restricts the application of the technique, but the measurement of short-lived nuclides may provide a useful alternative in limited cases when the counting of other radioisotopes presents some difficulty, for example the measurement of the sodium-24 nuclear isomer with a half-life of 20 ms has been proposed as an alternative to the use of 15-h sodium-24 as a basis for magnesium analysis.216 The penetration of fast neutrons has stimulated an interest in the possibility of in vivo measurements by activation analysis; an interesting example is the determination of total nitrogen and protein in mice.21e While most generators operate on the basis of the 3H(d,n)4He reaction an alternative is the reaction 2H + 2H+3He + ln + 3-28 MeV.This can be produced in small positive ion accelerators of the neutron generator type although with rather lower cross-section. A survey of sensitivities for 3-MeV neutron irradiations has been made for a large number of elements217 and an assessment of the possi-bility of using both 3-MeV and 14-MeV neutrons for precious metal exploration218 has been made. In this case a mobile 150-kV positive ion accelerator was used as the neutron source and the measurements were based upon the determination of the induced radioactivity.14MeV neutron-activation analysis has been used for compositional mapping of sea floor sediments by analysing collected samples but an in situ instrument could be designed to operate on the same principles.2fQ The relatively small size of generator neutron sources has prompted some workers to consider the possibility of using fast neutron systems in conjunction with 'y-ray spectroscopy for on-stream analysis. The high degree of penetration of both neutrons and y-rays enables relatively large volumes of material to be sampled. This is clearly desirable when the sample stream is inhomogeneous and rugged containment of both generator and detector without the need for thin windows is possible thus permitting the equipment to withstand an industrial environment.Extensive assessments of the technique have been made and some experience has been obtained with an experimental conveyor A number of cells have been designed to permit both liquids and solids to be examined including one with provision for irradiation of both sample and standard streams.=l Application of the method to the determination of a number of materials of industrial interest ha l60 PIERCE been assessed; the materials included water salt and sulphur in crude oil= and copper in copper ore .223 Although from considerations of cost neutron generators are an attractive form of accelerator neutron source for analytical purposes when they are available and the experimentation can stand the higher operating costs such large accelera-tors as electrostatic generators or even cyclotrons provide a higher neutron output and greater flexibility.Interest in small cyclotrons for charged-particle activation analysis has also been extended to the use of these machines for the production of neutrons and assessments have been made of their characteristics for neutron-activation analysis.224s226 Photoneutrons produced by high energy Bremsstrahlung, generated in an electron linear accelerator by interaction of the electron beam with a suitable converter can also be used for neutron-activation analysis.226 The ability to vary neutron energy which is usually feasible with these larger machines by choosing suitable target materials and accelerating energy can be used to improve conditions for analysis. Neutrons generated by cyclotrons have been used for whole-body assays of sodium and calcium by irradiation of cadavers and of human volunteers analytical measurement being based on the activity of calcium-49 and sodium-24.227 Irradiation doses received by three volunteers were approximately 1300 mRem.Prompt radiation. Prompt events induced by accelerator neutrons are clearly dependent upon the energy of the neutrons being used to irradiate the sample. Although reactions such as (n,p) and (n,a) produce particles that can be measured directly and are likely to yield residual nuclei in excited states that may subsequently de-excite by the emission of characteristic y-rays the primary reaction that has been of interest to analysts is inelastic scattering as y-rays emitted during de-excitation can provide the basis of analytical determination.Interest in inelastic scattering has been centred around the measurement of light elements such as carbon which are difficult to determine by more conventional activation techniques or in methods operating under specialised analytical con-ditions such as those required for on-line sensors where the immediacy of the y-line yield may prove an advantage. Substantial information about inelastic scattering is available in nuclear physics literature and y-rays have been produced from a variety of materials.228 A major limitation to the analytical use of inelastic scattering methods is imposed by the high radiation background in the presence of which the necessary measurement must be made ; anticoincidence systems have been found to improve the accumulated y-spectra but these add substantially to the cost of the detector systems.Because of the relatively high level of background events detected by the counting system up to the present time inelastic scattering has been primarily concerned with the determination of major element levels and the possibility of determining carbon in and aluminium iron and magnesium in silica based materialsm0 has been examined. Moderation of fast neutrons will often occur in large samples so that capture spectra will be observed in addition to the contribution from inelasticneutro RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 161 scattering. The two types of radiation can be distinguished by the time of radiation emission following a neutron pulse and further analytical information about the analytical composition of the sample can be derived from a measurement of thermal and epithermal neutron die-away.Neutron die-away inelastic neutron scattering, neutron capture and activation have been combined in a single experiment that has been devised for the analysis of rocks and bore holes.=l Radioisotope neutron sources Conventional radioisotope neutron sources which are manufactured by mixing two components a radioactive isotope and a material that emits neutrons upon interaction with the radiation emitted by the radioisotope have the advantages of stability of output small size reliability and zero power requirements. Unfortu-nately the neutron outputs of these sources are relatively low typically varying from 106 to 10s neutrons s-1 total for those in general use although larger installa-tions have been devised.Isotopic neutron sources can be produced with a number of neutron energy di~tributions,2~~ although in some cases the neutron outputs available are very low. Analytical applications have been devised that are based on the measurement of induced radioactivity or of prompt events but the former have been more numerous. The use of fast and thermalised neutrons from a polonium -beryllium neutron source has been exploited to provide a determination of alumina and silica in bauxite samples and the method has been found to be less time consuming and more accurate than the wet chemical methods previously In some cases the lower neutron energy of radioisotope neutron sources can prove an advantage over the 14-MeV neutrons obtained from neutron generators as inter-fering reactions may be avoided.Thus fluorine can be determined by the measure-ment of nitrogen-16 produced by the reaction l9F(n,a)l6N but in this case there is no interference from nitrogen-16 produced from oxygen-16 by the (n,p) reaction as would be experienced with generator activation as the neutron energy is below the threshold value.= High output neutron sources can be obtained by irradiating americium - beryllium mixtures to produce (241Am-2Q2Cm-Be) sources and the y-spectra and sensitivities produced from elements irradiated with this type of source have been Fluorine has been determined with such a source by counting nitrogen-16 activity and a sensitivity of 0.4 mg of fluorine in a 10-g sample was found for a source emitting 4.8 x lo9 neutrons s-l total; the available flux was 1.4 x lo8 fast neutrons cm-2s-1.B6 The reliability of the radioisotope neutron sources and their minimum maintenance requirements make them attractive for on-stream applications for industrial purposes.The low sensitivity achieved with small sources that are relatively easy to incorporate into industrial systems remains a limitation and in an effort to improve the over-all sensitivity sample re-circulation has been proposed237 and the benefits of the technique demonstratedJB8 although the response of such a system must inevitably be relatively slow. Very much larger systems have been considered including one based on an antimony-124 - beryllium source capable of handling up to 7,200 Ci of antimony-124.This system can pro 162 PIERCE vide a thermal neutron flux of approximately 2 x lo8 neutrons cm-2s-1 on the inside surface of a cylindrical irradiation but clearly very special facilities are required for handling such large amounts of radioactivity. A radio-isotope system has also been devised which is based upon the use of a much smaller neutron source (5 Ci of americium - beryllium) for the off-line analysis of discrete samples for process control; silicon aluminium and chromium have been deter-mined simultaneously and the advantages of the technique were found to be the simplicity of sample preparation and the insensitivity to particle size and matrix effects.240 The fluorine reaction mentioned above has provided a means of determin-ing fluorite in ore by a probe-type technique.241 Prompt radiation measurements based on the use of radioisotope neutron sources have been carried out by the measurement of both inelastic scattering and capture y-rays.The determination of carbon in iron ore in sinter mix and in fly-ash containing 2 to 16 per cent. of carbon has been based upon the 4-43-MeV y-ray produced by the (1) -+ (0) transition in carbon-12 resulting from the inelastic scattering of isotope source neutrons; the precision found was 0.2 per cent. of carbon.242 Prompt radiation from source neutron irradiation has also been ex-amined in some detail particularly in Russia with the aim of using the method for mineral exploration and bore hole logging.A very large number of papers have appeared considering different aspects of the subject and a combination of neutron sources can help to improve the quality of the analytical data derived from the measurements. A pulsed radioisotope neutron source has been constructed for neutron die-away systems by separating several alpha sources from beryllium targets with a rotatable shutterN3 operating at 12,000 r.p.m. thus giving 500-ps neutron bursts. Field equipment based on an antimony - beryllium neutron system a germanium counter and a small computer has been installed in a mobile laboratory for ore assay rock identification and exploration.244 Undoubtedly one of the most interesting developments that has taken place recently in the development of radioisotope neutron sources has been the avail-ability to certain workers of the spontaneous fission emitter californium-252.The neutron output of this isotope is 2-34 x 10l2 neutrons s-l g-l from a volume of less than 1 cm3 and the half-life of decay is 2.6 yearsa5; the neutron energy spectrum is somewhat similar to that of the thermal-fission neutron spectrum of uranium-235. It thus provides a neutron energy distribution that differs from many of the radioisotope sources generally used for neutron-activation analysis. Investigations have been carried out to assess the value of californium-252 for activation analysis, and a 0-37-mg source with a neutron emission intensity of 8.6 x lo* neutrons per second placed at the centre of a 1-ft moderator cube was found to give thermal, epithermal and fast fluxes of 9.7 x lo6 9.4 x 104 and lo8 cm-2 s-l mg-l respec-tively with a cadmium ratio of 7*6.248 Interference-free sensitivities and detection limits have also been found by using this source.A substantial programme of work has been carried out by a number of laboratories to assess possible outlets for californium-252 and these have included bore-hole logging applications247 and in sit% analysis of the ocean floor.= In the latter case neutron capture gamm RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 163 radiation was measured to identify manganese in nodules and gold in ore in a simulated marine environment. However while the potential of this new neutron source for a number of activation applications has clearly been demonstrated its real value to activation analysis will only become apparent when supplies of the isotope become more generally available.Charged -particle Activation Analysis Charged particles lose their energy rapidly during passage through matter and so differ radically from neutrons in their behaviour. Low penetration endows charged-particle activation analysis with characteristics that may be either dis-advantageous or advantageous depending upon the type of analysis being carried out but many of the recent applications of the technique have been designed to exploit low penetration and to provide specific analytical information from a very limited thickness of material. However radiation yields can be obtained from sample thicknesses that exceed 100 ~ m 2 * ~ so that the mass of sample analysed can often compare favourably with that examined by other instrumental analytical techniques particularly if the ion beam is spread to cover a large surface area.The yield will however vary with depth because of the change in cross-section with energy. Delayed techniques The majority of the applications of charged-particle activation analysis have been based upon the measurement of induced radioactivity and have been designed to exploit the high sensitivity of the technique for the determination of light elements such as carbon oxygen and nitrogen which are of considerable importance as impurities in many fields of material science but for which conventional neutron-activation techniques often do not provide adequate sensitivity. Variations of both the type of particles (for example protons deuterons helium-3 and helium-4) and their energy are frequently possible with positive ion accelerators so that the options open to the activation analyst can be numerous.The most suitable reaction is normally chosen only after consideration of both the element to be determined and the sample matrix as the interfering nuclear reactions may well limit apphca-tion of the technique. The convenience of the method must also be considered as a suitable particle accelerator must be available. Samples must of course be suitable for irradiation under the normal conditions that apply when using particle accele-rators and they must be capable of dissipating the heat resulting from interaction of the sample with the incident ion beam.One of the problems that has accom-panied the use of charged-particle activation analysis has been standardisation in view of the different stopping powers of various matrices and the use of the concept of equivalent thickness250 or average ~ross-section~~1 has simplified the application of the technique. Computer transformation methods carried out in conjunction with accurate activation curves for specific nuclear reactions hav 164 PIERCE simplified the analysis of different matrices by establishing a curve for one matrix and converting it for use in another with the aid of differential range-energy table~.2~~ Helium-3 was proposed for sample irradiation by Markowitz and Mahony2= some time ago as the particle undergoes many exoergic reactions with a large cross-section at low kinetic energy and has now been shown to be of con-siderable interest to workers at a practical level.Not surprisingly semi-conductor materials in which light element impurities play an important role in modifying behaviour have been examined by helium-3 activation and measurements of the oxygen content of gallium phosphide silicon and germanium by this method have yielded results that are in good agreement with theoretical estimates2= Deter-mination of oxygen and carbon by helium-3 irradiation has been supplemented by proton activation analysis of nitrogen for a combined determination of the three elements at concentrations as low as 10 parts per lo9 in semiconductor sili~on,2~~ and interferences in the use of helium-3 and helium-4 activation for the determination of carbon and oxygen in high purity iron nickel and chromium have been studied with helium-3 ions of up to 30 MeV and helium-4 of up to 54 MeV.256 Comparison standards for charged-particle activation analysis particularly for helium-3 irradiations have been produced by the anodic oxidation of tantalum foil and the isotopic composition of the oxide has been varied by using isotopically enriched water in the ele~trolyte.~~' Oxide films were thus produced with thickness varying from 0.5 mg cm-2 to over 35 mg cm-2 and were capable of reproducing beam degradation conditions from negligible energy loss to complete stopping of a 10-MeV helium-3 beam.Total reaction cross-sections of the reactions 1°B(d,n)llC, 14N(d,n)150 and 160(d,n)l7F have been examined to assess their value for activation analysis and at 3-MeV deuteron energy theoretical detection limits of 3 x 10-lo g, 5 x 10-11 g and 4 x 10-ll g have been calculated for boron nitrogen and oxygen, respectively.258 Deuteron irradiation has also been used to support studies into the diffusion of oxygen in gallium ar~enide,2~~ while protons with energies higher than is usual for activation analysis (185 MeV) have been used to determine the nitrogen content of seeds by the reaction 14N(p,d)13N.260 The excitation function of the reaction 160(3H,n)18F has been examined so as to assess the possibility of using accelerated tritons for oxygen determination261 and provides an alternative to the use of secondary tritons produced by reactor neutron irradiation of lithium for oxygen measurement (considered in a previous section).Oxygen has been deter-mined in pure zirconium and aluminium at surface oxygen levels of from 100 to 10-3 pg cm-2 by this method. The low depth of penetration of helium-3 ions has been exploited to enable the depth profile of oxygen to be determined in silicon.262 Recoil ranges of fluorine-18 predicted from considerations of total momentum transfer were found to agree with experimental values. Charged-particle irradia-tion has been combined with autoradiography to examine the positional location of light elements.263 After irradiation the sample was placed in contact with a suitable photographic film to record the distribution of the induced activities and the different activities identified by calculating half-lives from a number of exposures of the same sample taken at different times after the completion of irradiation RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 165 Prompt techniques A major change in emphasis that has been apparent in the application of charged-particle techniques over the last few years has been the increasing interest in analytical methods based upon the measurement of the prompt nuclear reaction products emitted during irradiation.Prompt events occurring during charged-particle irradiation can often be detected fairly readily and background from the incident radiation is less of a problem than for example when fast neutrons are used to irradiate the sample. The growth in interest in prompt methods has stemmed not only from an improvement in instrumentation which has enabled the activation analyst to apply the techniques to samples of real interest rather than to demonstration materials but also from a growing awareness that the methods more often associated with nuclear physics investigations can sometimes be adapted to provide specific analytical information.Elastically scattered particles prompt gamma radiation and particle groups emitted as a result of nuclear interaction have all been used as a basis of analytical methods and are considered briefly below. In general particle energies used for the production of prompt events are lower than those used for conventional charged particle activation although there are exceptions. Elastic scattering. The principle of elastic charged particle scattering as applied to analysis has been described by R ~ b i n ~ ~ in some detail and examples are given that have been based on the use of a magnetic spectrograph to examine scattered ions.A more recent review264 considers not only the general principles of elastic particle scattering but also use of the technique to provide structural information in single crystals by making use of the phenomenon of channelling. The energy loss of a particle scattered elastically is a function of mass of the scattering and of the scattered nuclei and a plot of scattered particle yield against particle energy will show a peak for scattering from a thin film or a Rutherford Plateau with a high-energy cut-off characteristic of the scatterer when the targets are thick.Thus elastic scattering is best applied to the determination of a thin film of a heavy element present on a lighter substrate as the contribution from the heavy film is clear of the substrate plateau and is therefore easier to measure quantita-tively; in some cases depth distributions have been calculated from the shapes of particle peaks. Elastic scattering has been used to examine the surfaces of a variety of materials and 1-MeV helium ions have been shown to be capable of achieving a limit of detectability of atomic fraction with a depth resolution of 300 A.2s6 The technique has been used to examine surface contamination on silicon wafers occurring during washing with a contaminated solution of hydrofluoric acid in de-ionised and less than one half of a monolayer of gold and copper con-tarninant could be resolved and identified.Peak shape analysis has been used to study the diffusion of gold into copper at temperatures in the range 360" to 500" C267 and enables elemental variations with depth to be followed with greater resolution than is possible by using mechanical sectioning techniques. An interesting use of elastic particle scattering has been the development of a relatively compac 166 PIERCE instrument based on a curium-242 a-particle source and semi-conductor detectors for instrumented lunar missions,268 which subsequently operated successfully on the moon’s surf ace.269 Prompt gamma radiation. Prompt gamma radiation can be excited from most of the light elements under suitable irradiating condition~~~o and application of the traditional methods of y-ray spectroscopy can enable several elements to be determined simultaneously in a sample.Several workers have determined fluorine by the reaction 19F(p,a y)l60 counting the 6.1-MeV y-line and measurements have been carried out in gases solids and liquids down to levels of the order of 20 ~ . p . m . ~ ~ l The use of sharp resonances in this reaction has permitted depth profiles to be obtained by gradually increasing the accelerating particle energy so that the major reaction yield occurs at different distances below the sample ~urface.~~2 Sensitivities of 2 x and 1 x per cent. have been obtained for the determination of boron in silicon by measuring y-rays with energies greater than or equal to 11 and 15 MeV produced by the reaction l l B ( ~ y ) l ~ C .~ ~ ~ Particle group measurement. Measurement of particle groups emitted as a result of nuclear reactions provides yet another basis for analytical measurement. By a suitable choice of experimental conditions particle groups can often occur in a region of the spectrum where there is a very low background and hence the sensi-tivity of the method can be very high. The technique is well applied to the examination of thin films of light elements such as carbon and oxygen and particle group measurement can be used in conjunction with resonance techniques to gain information about sub-surface distribution.274 Surf ace densities of the order of 10-7 g cm-2 of sulphur can be easily identified on coins by the reaction 32S(d,p)33S?75 and depth distributions have also been obtained by the measurement of neutrons resulting from the (d,n) reaction as the energy of the emitted neutron depends on the energy of the deuteron at the position of reaction.48 Carbon nitrogen and oxygen have been determined on steel surfaces in this way and the estimated sensitivity was 0.1 pg cm-2 with a depth resolution of 4500 A.Nuclear positive ion microprobe. Use of the methods described above has not been generally concerned with the examination of small areas of sample but clearly if the size of the incident positive ion beam can be reduced a positive ion microprobe can be devised that can operate on the basis of a measurement of nuclear reactions.276 Initially relatively large beams of about 25 to 100 pm were 0btained,277,~78 but by using a quadrupole lens system in the form of the Russian quadruplet a 3-MeV proton beam of less than 4pm in diameter has been pro-d~ced.27~ The principal interest in the technique stems from the possibility of deter-mining the spatial distributions of light elements which are just those elements that are difficult to determine by more established methods of elemental distribu-tion analysis such as the electron microprobe.Applications reported include the examination of the distribution of oxygen in welds277 and particle scans of minerals and thin f i l r n ~ 2 ~ 8 while the determination of the ratios of carbon-13 to carbon-12 in biomedical samples2*0 has been proposed RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 167 Gamma Photon Activation Analysis If a y-photon with an energy that is greater than the separation energy of a particle is absorbed by a nucleus particle emission may follow and photodisinte-gration of the nucleus occurs.Interest in y-photon activation was intially stimu-lated by the sensitivity of the method for the determination of light elements such as carbon and oxygen particularly as the penetration of y-photons permits the analysis of bulk samples. Gamma photons of high energy are required to obtain reactions with cross-sections adequate for high sensitivity determinations and are normally produced as ‘Bremsstrahlung’ by allowing accelerated electrons to strike a radiator of a heavy element. Betatrons or linear accelerators are used for electron acceleration but in general the latter have found widest application as high beam currents can be produced with these machines.Like accelerator neutron sources accelerator y-photon sources produce radiation beams that show very rapid flux variations. In spite of this however special irradiation facilities including pneumatic transfer systems have been installed in a number of laboratories to satisfy the requirements of the activation analysts. Interferences in y-photon activation may arise as in the use of other high energy incident radiation by nuclear interference as well as by secondary neutrons and investigations have been carried out by a number of workers to define the products formed by irradiating different elements. These have included a comprehensive investigation of products resulting from irradiation of a large number of elements in the periodic table281 with 30-MeV photons while interaction of 35-MeV photons with eighteen elements had also been examined.282 Control of the energy of the photons enables interferences to be avoided and optimisation techniques have been developed to exploit this.283 Interest in the determination of light elements by y-photon activation analysis has been maintained and further papers have appeared describing applications.These have included the determination of carbon in vanadium at the 10 to 150 p.p.m. level284 and in high purity iron chromium nickel and molybdenum.2S6 The deter-mination of oxygen in sodium has been possible by using y-photon activation analysis at levels down to a few parts per million,286 while another method for the determination of oxygen in sodium employs a radiochemical separation involving the exchange of the active oxygen with inactive oxygen present in water and subsequent distillation of a portion of the water.%’ The technique has also been found to be satisfactory for the determination of carbon and oxygen in 0-1 cm3 of a lead - bismuth eutectic mixture; in this case the sensitivity for carbon was found to be 5 p.p.m.while the minimum level at which oxygen was determined was 3 & 2 p.p.m.288 Carbon and oxygen present as major constituents have been determined in humic acid by irradiation with Bremsstrahlung of maximum energy 24.5 MeV,S9 and the use of laboratory betatrons has also been proposed for the determination of oxygen in coals.2g* Gamma photon activation analysis has been used for the determination of other elements either where neutron-activation analysis has not been easily applied or where the existence of intense y-photon sources has made the technique convenient.Thus yttrium has been determined in rare earths which because o 168 PIERCE their high thermal neutron absorption cross-sections would provide self-shielding problems if thermal neutron activation were used.291 Fluorine chlorine bromine and iodine have been determined in a single sample by means of (y,n) reactions292 and fluorine determined in urine with a precision of 10 per cent. at the 1 to 2 p.p.m. level with a sensitivity of 0.01 pg after concentration on an anion-exchange resin.293 A number of geological applications have been devised for y-photon activation, including the measurement of zirconium and titanium complex titano - zirconium iron nickel and cobalt in iron meteorites295 and the zirconium content of zirconium ores and concentrates.296 Photo nuclear activation has been compared with fast neutron activation for the determination of copper in ores and flotation products so as to develop a method less sensitive to particle size and matrix effects than X-ray fluorescence and photo-nuclear activation has been preferred by virtue of the fewer interferences produced.223 (y,y‘) reactions can sometimes be produced with y-photons of a few MeV that is to say of very much lower energies than are normally used for y-photon activation analysis.297 While cross-sections are often very low it is possible to use a smaller and consequently a cheaper accelerator.Among the applications that have been reported have been the determination of selenium yttrium silver barium hafnium and gold based on the production of metastable states of stable isotopes with an accelerator energy of 4.2 MeV and a current of 5 p a m p ~ . ~ ~ ~ An alternative method of producing the (7,~‘) reaction is by using an intense radioactive y-source. An 80 kCi cobalt-60 source has been used for such measurements and methods have been developed for determining selenium bromine strontium silver cadmium and indium in certain mixtures and lower levels of from 1 to 70-mg amounts of individual elements could be determined.29B The low neutron binding energies of deuterium and beryllium have enabled (y,n) reactions to be used for the deter-mination of these elements and an on-line method has been developed for monitor-ing the deuterium oxide concentration of process streams at deuterium oxide levels of 0.015 to 0.13 per cent.by measuring neutrons emitted from the 2D(y,n)1H react ion .300 References 1 2 3 4 5 6 7 8 9 10 Lutz G. J. Boreni R. J. Maddock R. S. and Meinke W. W. Editors ‘Activation Analysis A Bibliography,’ National Bureau of Standards Technical Note 467 1968. Nuclear Sciences Series NAS-NS U.S. National Academy of Sciepces. Salutsky M. L. in Kolthoff I . M. and Elving P. J. Editors Treatise on Analytical Chemistry,’ Interscience Publishers Vol. 1 part 1 1959 p. 733. Bock Werthmann W. ‘Nuclear Activation Techniques in the Life Sciences,’ International Atomic Energy Agency Vienna 1967 p.173. Welcher F. J. ‘Organic Analytical Reagents,’ D. Van Nostrand and Co. 1948. Morrison G H. and Freiser H. ‘Solvent Extraction in Analytical Chemistry,’ John Stary J. ‘The Solvent Extraction of Metal Chelates,’ Pergamon Press 1964. Samuelson O. ‘Ion Exchange Separations in Analytical Chemistry,’ John Wiley and Sons, Inczedy J. ‘Analytical Applications of Ion Exchangers.’ Pergamon Press 1966. Cerrai E. Chromatog. Revs 1964 6. Wiley and Sons Inc. 1962. 1963 RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 169 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 Stahl E.‘Thin-layer Chromatography,’ George Allen and Unwin Ltd. 1969. Pierce T. B. and Flint R. F. Analytica Chim. Acta 1964 31 595. Schneer A. E. Kem. Kozlem 1969,31 23. Juvet R. S. and Cram S. P. Analyt. Chem. 1970 42 14R. Girardi F. Pietra R. and Sabbioni E. J . Radioanal Chem. 1970 5 141. De Voe J . R. ‘Application of Distillation Techniques to Radiochemical Separations,’ Qureshi J. H. and Nagi F. I. J . Inorg. Nucl. Chem. 1967 29 2879. De Wet W. J. J . S . Afr. Chern. Inst. 1969 22 168. Fritz J. S. “;“d Peters M. A. Talanta 1969 16 575. Girardi F. Nuclear Activation in the Life Sciences,’ International Atomic Energy Pierce T. B. and Peck P. F. Analyst 1963 88 603. Ruzicka J. and Stary J. ‘Substoichiometry in Radiochemical Analysis,’ Pergamon Smales A.A. Mapper D. and Fouch6 K. F. Geochemica et Cosmochinzica Acta 1967,31, Thompson B. A. and LaFleur P. D. Artalyt. Chem. 1969 41 853. Morrison G. H. Gerard J. T. Travesi A. Currie R. L. Peterson S. F. and Potter, Samsahl K. AnaZyst 1968 93 101. Goode G. C. Baker C. W. and Brooke N. M. Ibid. 1969 94 728. Girardi F. in De Voe J. R. Editor ‘Modern Trends in Activation Analysis,’ National Bureau of Standards Special Publication 312 1 577. Heath R. L. in De Voe J. R. op. cit. Special Publication 312 2 1959. Kandiah K. Sterling A. Trotman D. L. and White G. ‘Proceedings of the Inter-national Symposium on Nuclear Electronics,’ September 1968 La Socidtd Frangaise des Electroniciens et des RadioBlectriciens. Lewis A. U.K. Atomic Energy Authority Report A.E.R.E.-R.5844 1968. Thompson C. J. Nuclear Applications 1969,6 559. Pierce T. B. Webster R. K. Hallett R. and Mapper D. in De Voe J. R. op. cit., Christensen P. and Christiansen E. M. Euratom report 4289 1969 317. Cola G. D. Girardi F. and Guzzi G. Euratom report 4289 1969 313. Adams F. and Dams R. Radiochem. Radioanal. Lett. 1969 1 163. Voigt A. F. Becknell D. E. and Menapace L. in De Voe J. R. op. cit. Special Publica-tion 312 2 1035. Coleman R. F. in De Voe J. R. op. cit. Special Publication 312 2 1262. Parker J. L. Holm D. M. and Barnes B. K. in De Voe J. R. op. cit. Special Publica-tion 312 2 1075. Cooper J. A. Rancitelli L. A. Perkins R. W. Haller W. A. and Jackson A. L. in De Voe J. R. op. cit. Special Publication 312 2 1054. Michelsen 0.B. and Steinnes E. Talanta 1969 16 1436. Yule H. P. in De Voe J. R. op. cit. Special Pulication 312 2 1155. Salmon L. Nucl. Instrum. Meth. 1961 14 193. Op de Beeck J. and Hoste J. Analytica Chim. Acta 1966 35 427. Covell D. F. Analyt. Chem. 1959 31 1785. Rubin S. in Kolthoff I. M. and Elving P. J. Editors ‘Treatise on Analytical Chemistry,’ Peisach M. and Poole D. O. ‘Modern Trends in Activation Analysis,’ Proceedings of the Moller E. Nilsson L. and Starfelt N. Nucl. Instrum. Methods 1967 50 270. Fleischer R. L. and Price P. B. Science 1963 140 1221. Loveridge B. A. and McInnes C. A. J. The Microscope 1968 16 105. Smales A. A. in Meinke W. W. and Scribner B. F. Editors ‘Trace Characterization Guinn V. P. in Lenihan J. M. A, and Thomson S. J. Editors ‘Advances in Activation Aliev A.I. Drynkin V. I. Leipunskaya D. F. and Kasatkin V. A. ‘Nuclear-Physical Nuclear Sciences Series NAS-NS 3108 US. National Academy of Sciences 1962. Agency Vienna 1967 p. 117. Press 1968. 673. N. M. Ibid. 1969,41 1633. Special Publication 312 2 1116. Vol. IV Part 1 Interscience Publishers 1963 p. 2075. 1965 International Conference. Texas A and M University 1965 p. 206. Chemical and Physical,’ Monograph 100 1967 National Bureau of Standards. Analysis,’ Vol. I Academic Press (London) Ltd. 1969 p. 37. Constants for Neutron Activation Analysis Atomizdat Moscow 1969 170 PIERCE 54 Yule H P Lukens M R and Guinn V P Nucl Instrum Methods 1965 33 277 55 Proza 2 and Rakovic M Chem Lzsty 1969,63 257 56 Op de Beeck J P J Radzoanal Chem 1970,4 137 57 Lukens H R and Yule H P Nucl Instrum Methods 1965,33 273 58 Naughton W F and Jester W A zn De Voe J R op czt Special Publication 312, 1 490 59 Leushkina G V Lobanov E M Dutov A G and Matveeva N P A t Energ.(U.S S R ) 1969,26 381 60 Wiernik W and Amiel S J Radzoanal Chem 1969 3 393 61 Junod E Report CEA-R 2980 Parts A and B 1966 62 - Report CEA-R 2980(2) Part C 1970 63 Fleischer R L Price P B and Walker R M Sczence 1965 149 383 64 Martin J zn De Voe J R op czt Special Publication 312 1 517 65 Reed R E 0 R N L Report 4334 1969 p 3 66 Sayre E V Lechtman H N and Heather N Stud Conserv 1968 13 161 67 Osmond R G and Smales A A Analytzca Chzm Acta 1954 10 117 68 Mattson J S Crittenden J C and Mark H B Nucl App Technol 1969 7 383 69 Rafaeloff R Razdochem Radzoanal Lett 1969 1 199 70 Nikolov K and Todoruvsky D Isotopenpraxzs 1969 5 408 71 Smales A A Mapper D Webb M S W Webster R K and Wilson J D Sczence, 1970 167 509 72 Morrison G H Gerard J T Kashuba A T a Gangadharam E V Rothenberg A M , Potter N M and Miller G B Ibzd 1970 167 505 73 Reed G W Jovanovic S and Fuchs L H Ibzd 1970,167 501 74 Goles G G Osawa M Randle K Beyer R L JCrome D Y Lindstrom D J , 75 Waenke H Begemann F Vilcsek E Rieder R Teschke F Born W Quijano-76 Schmitt R A Wakita H and Rey P Ibzd 1970 167 512 77 Turekian K K and Kharkar D P Ibzd 1970 167 507 78 Herr W Herpers U Hess B Skerra B and Woelfle R Ibzd 1970 167 747 79 Emery J F Strain J E O’Kelley G D and Lyon W S Radzochem Radzounul Lett 1969 1 137 80 Morgan J W Rebagay T V Showalter 14 L Nadkarm R A Gillum D E , McKown D M and Ehmann W D Nature 1969,224 789 81 Millard H T zn De Voe J R op czt Special Publication 312 1 395 82 Landstrom 0 Samsami K and Wenner C -C Report A E 296 1967 83 Gangadharam E V and Reddy G R Radzochem A d a 1969 11 90 84 Green T H Brunfelt A 0 and Heier K S Eavth Planet Scz Lett 1969 7 93 85 Babaev A Babakhodzhaev S M Khaidarov A and Khudoiberganov U Dokl Akad Nauk Tadzh SSR 1969 12 24 86 Gordon G E Randle K Goles G G Corliss J B Beeson M H and Oxley S S , Geochem Cosmochzm Acta 1968 32 369 87 Fdby R H and Haller W A zn De Voe J R op czt Special Publication 312 1 339 88 Ehmann W D and McKown D M Analyt Lett 1969,2 49 89 Suttle A D O’Brien B C and Mueller D W Analyt Chem 1969 41 1265 90 Rosholt J N and Szabo B J zn De Voe J R op czt Special Publication 312,1 327 91 Fritze K and Robertson R zn De Voe J R op czt Special Publication 312 2 1279 92 Kline J R and Brar S S Isotop Radzat Technol 1970 7 319 93 Brunelle R L Hoffman C M Snow K B and Pro M J J Ass Analyt Chem 1969, 52,911 94 Brunfelt A 0 and Steinnes E Analytzca Chzm Acta 1969 48 13 95 Wainerdi R E Uken B A Santos.C G and Yule H P ‘NucIear Techniques and Mineral Resources,’ International Atomic Energy Agency Vienna 1969 p 507 96 Santos G I and Wainerdi R E zn De Voe J R op czt Special Publication 312 1 379 97 Robertson D E BNWL-1051 (Part 2) 1967 p 59 98 Lux F 2 analyt Chem 1968,243 107 99 Kasperek K Atomkernenergze 1969 14 143. Martin M R McKay S M and Steinborn T L Ibzd 1970 167 497 RICO M Voshage H I and Wlotzka F Ibzd 1970 167 523 RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 171 100 Frigerio N.A. CNM-R-2 (Vol. 2). 1967 p. 797. 101 Rakovic M. Chem. Listy 1969 63 679. 102 Comar D. in Lenihan J. M. A. and Thomson S. J. Editovs ‘Advances in Activation 103 McAndrew R. G. Smathers J. B. Wainerdi R. E. Harrison G. M. and Doggett R., 104 Woodruff G. L. Babb A. L. Wilson W. E. Yamamoto Y. and Stamrn S. J. Nucl. 105 Strain W. H. Rob C. G. Pories W. J. Childers R. C. Thompson M. F. Hennessen, 106 Yoshimasu F. Wakayama Igaku 1969,20 31. 107 Rancitelli L. A. Perkins R. W. and Renzetti A. D. BNWL-1051 (Part Z) 1969 p. 6. 108 Akaboshi M. Maeda T. Noda M. and A. Waki Zool. Mag. Tokyo 1967 76 235. 109 Holm V. Steinnes E. and Waaler T. Meddr Nwsk. Farm Selsk 1968 30 17.110 Lenihan J. M. A. Comar D. Riviere R. and Kellershohn C. Nature 1967 214 1221. 111 Comar D. Crouzel C. Chasteland M. Riviere R. and Kellershohn C. Nucl. Appl., 1969 6 344. 112 Nixon G. S. Smith H. and Livingston H. D. ‘Nuclear Activation in the Life Sciences,’ International Atomic Energy Agency Vienna 1967 p. 455. 113 Higashi T. and Kawai J. Kokobyogakkai Zusshi 1969 18 357. 114 Rancitelli L. A. BNWL-1051 (Part 2) p. 146. 115 Rancitelli L. A. BNWL-1051 (Part 2) p. 142. 116 Livingston H. D. and Thompson G. NYO-2174-96 1969. 117 Merlini M. Ravera O. and Bigliocca C. in De Voe J. R. op. cit. Special Publication 118 Gordon C. M. and Larson R. E. in De Voe J. R. op. cit. Special Publication 312,1 142. 119 Linstedt K. D. and Kruger P. Analyt. Chem. 1970 42 113.120 Piper D. Z. and Goles G. G. Analytica Chim. Acta 1969 47 560. 121 Robertson D. E. and Prospero J. M. BNWL-1051 (Part 2) 1969 p. 53. 122 Oda T. Onken Kiyo 1968,20 205. 123 Rancitelli L. A. and Tanner T. M. BNWL-1051 (Part 2)’ 1969 p. 137. 124 Bhatki K. S. and Dingle A. N. Radiochem. Rudioanal. Lett. 1970 3 71. 125 Abdullaev A. A. Zakhidov A. S. and Nishanov P. K. Im. Akud. Nauk. Uzb. SSR, 126 Hayes D. W. Ph.D. Thesis Texas A. and M. University 1969 Nucl Sci. Abs. 13779. 127 Cali J. P. ‘Trace Analysis of Semiconductor Materials,’ Pergamon Press 1964. 128 Nagy L. G. and Giber J. Period Polytech. Chem. Eng. 1968 12 37. 129 Przyborski W. Roed J. Lippert J. and Sarholt-Kristensen L. Radiat. Efl. 1969 1, 130 Ortega R. F. in De Voe J. R. op. cdt. Special Publication 312 1 536.131 Ballaux C. Dams R. and Hoste J. Analyticu Chim. Acta 1969 47 397. 132 Gilbert €2. N. and Pronin V. A. Izv. Sib. Otd. Akad. Nauk SSSR No. 4 Ser. Khim. 133 Samadi A. A. and May S. Bull. SOC. Chem. Fr. 1969 10 3776. 134 Van Reenen T. J. and De Wet W. J. J . S. Afr. Chem. Inst. 1968,21 120. 135 Alimarin I. P. Miklishanskii A. Z. and Yakovlev Y . V. J. Radioanal. Chem. 1970 4, 136 Lobanov E. M. Mamadzhanov F. I. Chubarov L. B. Dokl. Akud. Nauk. Tadzh. SSR , 137 Neeb K. H. Stoeckert H. Braun R. and Bleich H. P. 2. Analyt. Chem. 1969 245, 138 Csada I. Schneer A. and Rausch H. Kozlern 1968 16 219. 139 Guinn V. P. and Bellanca S. C. in De Voe J. R. op. cit. Special Publication 312 1 93. 140 Das H. A. Hoede D. and Zonderhuis J. report RCN. 106. 141 Lee B.K. and Murphy G. Cuncev 1969,23 1315. 142 Nadkarni R. A. and Ehmann W. D. Radiochem. Radioa?aal. Lett. 1969,2 161. 143 Geisman J. R. Carey W. E. Gould W. A, and Alban E. K. J. Fd Sci. 1969,34 295. 144 Fer A. and Fourcy A. Nucl. Appl. 1969 6 360. 145 Bowen H. J. M. Analyst 1967 92 124. 146 Yamane Y. and Miyazaki M. Eisei Kagaku 1969 15 238. Analysis,’ Academic Press 1969 p. 163. Nucl. Appl. Technol. 1970 8 290. Appl. 1969,6 352. J. A. and Graber F. M. ApPl. Spectrosc. 1969,23 121. 312 1 475. Ser. Fiz-Mat. Nauk 1968 5 60. 33. Nauk 1969 2 168. 45. 1969 12 37. 233. 172 PIERCE Zoller W. H. and Gordon G. E. Arralyt. Chem. 1970 42 267. Brar S. S. Nelson D. M. Kanabrocki E. L. Moore C. E. Burnham C. D. and Hattori, D. M. in De Voe J. R. op. cit.Special Publication 312 1 43. Rancitelli L. A. BNWL-1051 (Part Z) 1969 p. 135. Loucks R. H. Winchester J. W. Matson W. R. and Tiffany M.A. in De Voe J. R., op. cit. Special Publication 312 1 36. Dudey N. D. Ross L. E. and Noshkin V. E. in De Voe J. R. op. cit. Special Publica-tion 312 1 55. Coleman R. F. Cripps F. H. Stimson A. and Scott H. D. Atomic Weapons Research Establishment Report 0-86 1967. Henke G. Arch. Tox. 1969 25 48. Williamson T. G. and Harrison W. W. in De Voe J. R. op. cit. Special Publication 312 1 283. Coleman R. F. J . Forensic Sci. SOC. 1968 8 32. Atalla L. T. and Lima F. W. Radiochem. Radioanal. Lett. 1970 3 13. Schlesinger H. L. Lukens H. R. and Settle D. M. in De Voe J. R. op. cit. Special Publication 312 1 265. Houtman J. P. W.and Turkstra J. ‘International Atomic Energy Agency Symposium on Radiochemical Methods of Analysis,’ Salzburg 1964. Lux F. and Braunstein L. 2. Andyt. Chem. 1966,221 235. A1 Kital R. A. Chan L. and Sayre E. V. in Van Beek G. W. Editor ‘Hajer Bin Humeid Investigations a t a Pre-Islamic Site in South Arabia,’ Johns Hopkins Press, 1969. Harbottle G. Archmometry 1970 12 23. Gordus A. A. Griffin J. B. and Wright G. A. COO 912-16 1968. Das H. A. Radiochem. Radioanal. Lett. 1969 1 289. Gibbons D. and Lawson D. in De Voe J. R. op. cit. Special Publication 312 1 226. Cappadona C. in De Voe J. R. op. cit. Special Publication 312 1 72. Gatz D. F. Dingle A. N. and Winchester J. W. J . Appl. Meteorol. 1969 8 229. Channell J. K. and Kruger P. in De Voe J. R. op. cit. Special Publication 312 1 81.Bando S. Tominaga H. Kawakami Y. and Senco M. Radioisotopes Tokyo 1969 18, Pillay K. K. S. Thomas C. C. Hart D. M. Didising D. and Thomas R. C. Nucl. Rewienska-Kosciukova B. Wezranowski E. Zarzecka E. Radwan M. and Palu-Mantel M. Gilat J. and Amiel S. in De Voe J. R. op. cit. Special Publication 312, Turkstra J. and De Wet W. J. J . S. Afr. Chem. Inst. 1969 22 1. Frana J. Vorbecky M. and Kristak J. Radiochem. Radioanal. Lett. 1969 1 41. Imamura M. Matsuda H. Horie K. and Honda M. Earth Planet Sci. Lett. 1969 6, Maslov I. A. Lukhnitskii V. A. Karnaukhova N. M. and Karaganova Q. I. Nucl. Ackermann R. J. Kojima M. Rauh €2. G. and Walters R. R. J . Chem. Thermodyn., Hopenfeld J. ANL 7520 (Part l) p. 163. Osawa H. Bunseki Kiki 1969 7 619.Greenwood R. C. and Reed J. H. IITRI-1193-53 Vols 1 and 2 Illinois Institute Tech. Rasmussen N. C. Hukai Y. Inouye T. and Orphan V. J. AFCRL-69-0071 US. Air Isenhour T. L. and Morrison G. H. Analyt. Chem. 1966,38 162. Garbrah B. W. and Whitley J. E. Ibid. 1967,39 345. Juna J. Konecny K. and Vobecky M. Colln Czech. Chem. Commun. 1969 34 1605. Elkady A. and Duffey D. Trans. Amer. Nucl. SOC. 1969 12 42. Lombard S. M. and Isenhour T. L. Analyt. Chem. 1969‘41 1113. Comar D. in Lenihan J. M. A. and Thomson S. J. Editors ‘Advances in Activation Basdekas D. L. US. Patent 3,436,538 1969. 488. AppI. Technol. 1970 8 73. chowicz L. Hutnik 1967,34 676. 1 482. 165. Appl. Technol. 1969 7 389. 1969 1 527. Res. Centre 1965. Force Cambridge Research Labs 1969. Analysis,’ Vol.I Academic Press 1969 p. 163. 147 148 149 160 151 162 153 154 165 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 18 RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS 173 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 21 1 212 213 214 215 216 217 218 219 220 22 1 222 223 224 225 226 227 228 229 230 231 Hanson A. O. Taschek R. F. and Williams J. H. Revs. Mod. Phys. 1949,21 635. Hillier M. Lomer P. D. Stark D. S. and Wood J. D. L. H. in Ebert H. G. Editor, ‘Proceedings of the Third Conference on Accelerator Targets Designed for the Produc-tion of Neutrons,’ Euratom Report EUR 3895 d-f-e 1967.Steele E. L. ‘Modem Trends in Activation Analysis,’ Proceedings of 1965 International Conference 1965 p. 102. Pierce T. B. Edwards J. W. and Haines K. Talanta 1968 15 1153. Oldham G. and Darrall K. G. Internat. J . Appl. Rad. Isotop. 1969,20 29. DeSoete D. Gijbels R. and Hoste J. in De Voe J. R. op. cit. Special Publication 312 Op De Beeck J. J. Radioanal. Chem. 1968 1 313. Morgan J. W. and Ehmann W. D. Analytica Chim. Acta 1970 49 287. Anders 0. U. and Briden D. W. Analyt. Chem. 1964 36 287. Lundgren F. A. and Nargolwalla S. S. Ibid. 1968,40 672. Hoste J. De Soete D. and Speecke A. Euratom Report EUR 3565e 1967. Coleman R. F. Analyst 1962,87 590. Nargolwalla S.S. Crambes M. R. and Suddueth J. E. Analytica Chim. Acta 1970 49, Stoll N. Wagner A. and Goedert L. Euratom Report EUR 3161f 3 1966. Tyou P. Revue de Metallurgie 1969 66 621. Gijbels R. Speecke A. and Hoste J. in De Voe J. R. op. cit. Special Publication 312, Doty W. H. Wood D. E. and Schneider E. L. J . Ass. Analyt. Chem. 1969 52 953. Kosta L. ‘Panel Meeting on New Approaches to Breeding for Improved Plant Protein, Bakes J. M. and Prapuolenis A. A. Radiochem. Radioanal. Lett. 1969 1 19. Tamura M. Radioisotopes Tokyo 1969 18 252. Ehmann W. D. and Morgan J. W. Science 1970 167 528. Schramel P. J. Radioanal. Chem. 1969 3 29. Przybylowicz E. P. Smith G. W. Suddueth J. E. and Nargolwalla S. S. Analyt. Tousset J, Condamin J. and Picon M. Method. Phys.Anal. 1968 4 202. Tover. P.. CEA-CONF 1410 1969. 2 699. 425. 2 1298. Roestanga Sweden,’ International Atomic Energy Agency Vienna 1969 p. 161. Chem. 1969 41 819. kichardson A. E. and Hahison A. Analyt. Golanski A. Report CEA R-3694 1969. Givens W. W. Mills W. R. and Caldwell, Nagai T. J . Nucl. Med. 1969 10 192. Nargolwalla S. S. Niewodniczanski J. and Senftle F. E. Report TID 25169 1967. Santos G. G.. Fite. L. E. Kuvkendall. W. Publication 312 2 929. 1969 12 510. Chem. 1969 41 1396. R. L. in De Voe J. R. op. cit. Special Suddueth J. E. Trans. Amer. Nucl. Soc., E. Wainerdi R. E. Bouma A. H. and Mineral Resources International Atomic Bryant W.. R. ‘Nuclear Teihniques -and Energy Agency Vienna 1969 p. 463. Publication 312 2 918. Rhodes J.R. Isotop. Radiat. Technol. 1969 6 359. Jervis R. E. Al-Shahristani H. and Nargolwalla S. S. in De Voe J. R. op. cit. Special Gorski L. Janczyszyn J. and Loska L. Radiochem. Radioanal. Lett. 1969 1 99. Pradzynski A. ‘Nuclear Techniques and Mineral Resources,’ International Atomic Fleischer A. A. in De Voe J. R. 09. cit. Special Publication 312.2 868. Bruninx E. in De Voe J. R. op. cit. Special Publication 312 2 860. Wilkniss P. E. in De Voe J. R. op. cit. Special Publication 312 2 874. Chamberlain M. J. Proc. Roy. Soc. Med. 1969 62 370. Thompson W. E. and Engesser F. C. Report USNRDLTR-861 1965. Martin T. G. Mathur S . C. and Morgan I. L. Internat. J . Appl. Rad. Isotop. 1964,15, Pierce T. B. Peck P. F. and Cuff D. R. A. J . Radioanal. Chem. 1970 4 305. Caldwell R.L. Mills W. R. and Givens W. W. ‘Nuclear Techniques and Mineral Energy Agency Vienna 1969 p. 451. 331. Resources,’ International Atomic Energy Agency Vienna 1969 p. 397 174 PIERCE ‘Radioactive Products 196911970.’ The Radiochemical Centre Amersham Bucks p. 180. Tatar J. Banyaszat 1968 101 287. Jeffery P. G. and Bakes J. M. Analyst 1967,92 151. Wing J. and Wahlgren M. A. Report ANL 7242 1966. - and - J . Radioanal. Chem. 1969 3 37. Starnes P. E. U.S. Patent Application 62488 1966. Ashe J. B. Berry P. F. and Rhodes J. R. in De Voe J. R. op. cit. Special Publication Downs W. E. and Davis M. W. Nucl. Agpl. Technol. 1969 7 466. Kuusi J. Nucl. Apfil. Technol. 1970 8 465. Porkopchik V. I. Bushkov A. P. and Bernardskii K. G. Atomn. Energ. 1969 27 161. Stewart R.E. I.S.A. Transactions 1967 6 200. Caldwell R. L. and Givens W. W. US. Patent 3,389,257 1968. Tolmie R. W. and Thompson C. J. ‘Nuclear Techniques and Mineral Resources,’ International Atomic Energy Agency Vienna 1969 489. Crandall J. L. Isotop. Rad. Technol. 1970 7 306. Ricci E. and Handley T. H. Analyt. Chem. 1970 42 378. Perkins R. W. Rancitelli L. A. Cooper J. A. and Brown R. E. Trans. Amer. Nucl. Senftle F. E. Duffey D. and Wiggins P. F. Marine Tech. SOC. J. 1969 3 9. Rook H. L. Schweikert E A. and Wainerdi R. L. in De Voe J. R. op. cit. Special Engelmann C. C.R. Hebd Sianc. Acad. Sci. Paris 1964,258 C 4279. Ricci E. and Hahn R. L. Analyt. Chem. 1967,39 794. Rook H. L. and Schweikert E. A. Ibid. 1969 41 958. Markowitz S. S. and Mahony J. D. Ibid.1962 34 329. Kim C. K. Radiochem. Radioanal. Lett. 1969 2 53. Nozaki T. Yatsurugi Y. and Akiyama N. J . Radioanal. Chem. 1970,4 87. Debrun J. L. Barrandon J. N. and Albert P. Bull. SOC. Chim. Fr. 1969 1011. Lamb J. F. Lee D. M. and Markowitz S. S. Andyt. Chem, 1970 42 209. Wohlleben K. and Schuster E. Rudiochim. Acta 1969 12 75. Rachmann J, and Biermann R. Solid State Commun. 1969 7 1771. Johansson A. Larsson B. Tibell G. and Ehrenberg L. ‘New Approaches to Breeding for Improved Plant Protein,’ International Atomic Energy Agency Vienna 1969, p. 169. Barrandon J. N. and Albert P. in De Voe J. R. op. cit. Special Publication 312 2, 794. Lamb J. F. Lee D. M. and Markowitz S. S. Analyt. Chem. 1970 42 212. Holm D. M. Sanders W. M. Briscoe W. L. and Parker J. L.in Polishuk P. Editor, Mackintosh W. D. and Davies J. A. Analyt. Chem. 1969 41 26A. Davies J. A. Denhartog J. Eriksson L. and Mayer J. W. Can. J . Phys. 1967,45,4053. Thompson D. A. Barber H. D. and Mackintosh W. D. A@@ Phys. Lett. 1969 14, Sippel R. F. Phys. Rev. 1959 115 1441. Patterson J. H. Turkevich A. L. and Franzgrote E. J . Geophys. Res. 1965 70 1311. Turkevitch A. L. Franzgrote E. and Patterson J. H. Science 1969 165 277. Pierce T. B. and Peck P. F. in Shdlis P. W. Editor ‘Proceedings of the S.A.C. Bewers J. M. and Flack F. C. Analyst 1969,94 7. Moller E. and Starfelt N. Report AE 237 1966. Vasil’ev S. S. Mikhailov G. I. Starchik L. P. and Konanykin L. V. Zav. Lab. 1969, Amsel G. and Samuel D. J . Phys. Chem. Solids 1962 23 1707. Wolicki E. A. and Knudson A.R. Int. J . Appl. Rad. Isotopes 1967 18 429. Pierce T. B. Peck P. F. and Cuff D. R. A. Nature 1966 211 66. Price P. B. and Bird J. R. Nucl. Instrum. Methods. 1969 69 277. Pierce T. B. Peck P. F. and Cuff D. R. A. Ibid. 1969 67 1. Cookson J. A. and Pilling F. D. Atomic Energy Research Establishment Report R6300, 312 2 913. SOC. 1970 13 63. Publication 312 2 768. ‘Nucleonics in Aerospace Plenum Press 1968 p. 306. 102. Conference Nottingham 1965,’ W. Heffer and Sons Ltd. 1965 p. 159. 35 299. 1970. 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 25 1 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 27 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 RECENT DEVELOPMENTS IN ACTIVATION ANALYSIS Ricci E.and Gibbons J. H. Trans. Amer. N w ~ . SOL 1969 12 509. Baker C. A. and Wood D. A. Atomic Energy Research Establishment Report R-5818, Debrun J. L. and Albert P. Bull. SOG. Cltim. Fr. 1969 1020. Davydov M. G. and Shcherbachenko V. A. At. Energ. (USSR) 1969,27 205. Hislop J. S. and Wood D. A. Atomic Energy Research Establishment Report 33-6165, Revel G. J . Radioanal. Chem. 1969 3 421. Engelmann C. and Loevuillet M. Bull. Soc. Chim. Fr. 1969 680. Lutz G. J. Analyt. Chem. 1970 42 531. Mackintosh W. D. and Jervis R. E. in De Voe J. R. op. cit. Special Publication 312, Kasymov A. K. and Masagutov V. S. Dokl. Akad. Nauk Uzb. S S R 1969,4 18. Sulin V. V. Trudy Vses Nauchno-Issled. Inst. Yadern. Geofiz. Geokhim 1968 244. Lutz G. J. and LaFleur P. D. Talanta 1969 16 1457. Wilkniss P. E. Radiochem. A d a 1969 11 138. Ohno S. Suzuki M. Sasajima K. and Iwata S. Analyst 1970 95 260. Shornikov S. I. Trudy Vses Nauchno-Issled. Inst. Yadern. Geofiz. Geokhim. 1968 274. Meijers P. and Aten A. H. W. Radiochem. Acta 1969 11 60. Berzin A. K. Vitozhents G. C. and Sulin V. V. Trudy Vses Nauchno-Issled. Inst. Yadern. Geofiz. Geokhim. 1968 266. Engelmann C. and JBrome D. Y. in Ebert H. G. Editor ‘Proceedings of the Seconq Conference on Practical Aspects of Activation Analysis with Charged Particles, Euratom report EUR 3896 d-f-e Brussels 1968 p. 119. Kodiri S. and Starchik L. P. Dokl. Akad. Nauk. Tadzh. SSR 1969 12 17. Veres A. and Pavlicsek I. Radioanal. Chem. 1969 3 25. Stutheit J. S. and Rampey W. P. Nucl. Instrum. Methods 1969 75 43. 175 1968. 1969. 2 835