Neutron Activation Analysis and Provenance Research in Archaeology

Measurement Science and Technology

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This content was downloaded from IP address 128.194.172.218 on 10/07/2018 at 17:46 INSTITUTE OF PHYSICS PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY Meas.Sci.Technol. 14 (2003) 1516–1526 PII: S0957-0233(03)56714-5 Neutron activation analysis and provenance research in archaeology

Michael D Glascock1,3 and Hector Neff2

1 Research Reactor Center, University of Missouri, Columbia, MO 65211, USA 2 Department of Anthropology, California State University, Long Beach, CA 90840, USA

E-mail: [email protected] and [email protected]

Received 27 November 2002, accepted for publication 7 April 2003 Published 29 July 2003 Online at stacks.iop.org/MST/14/1516 Abstract Neutron activation analysis is a powerful quantitative analytical technique with application in a broad range of disciplines such as agriculture, archaeology, geochemistry, health and human nutrition, environmental monitoring and semiconductor technology. Due to its excellent sensitivity, great accuracy and precision, and versatility, the technique is a suitable method for analysing many different types of samples. Archaeologists, in particular, have made extensive use of neutron activation analysis for the purpose of characterizing archaeological materials and determining their provenance. This paper presents a brief history of the technique and its application to archaeology, describes the physics behind the analytical method, and explains how the method is generally employed to determine the sources of archaeological materials.

Keywords: neutron activation analysis, nuclear reactions, cross-sections, thermal neutrons, gamma-ray spectra, provenance, ceramics, clays, obsidian, chert, multivariate statistics

1. Introduction mass spectrometry (ICP-MS) on liquid samples, NAA offers sensitivities that are superior to those possible by all other The application of chemical analytical methods to archaeo- analytical methods. Moreover, the accuracy and precision logical materials in support of provenance research has grown of the technique are such that NAA is still one of the rapidly over the past few decades. Provenance research en- primary methods employed by the National Institute of tails the use of compositional profiles of artefacts and source Standards and Technology to certify the concentrations of materials to trace individual artefacts from their find spot to elements in standard reference materials. Applications of their place of origin. The information obtained is used by NAA are by no means limited to archaeology, but include a archaeologists to investigate questions regarding the location broad range of disciplines such as agriculture, geochemistry, of prehistoric production areas, the identification of routes of health and human nutrition, environmental monitoring and trade and exchange of raw materials and artefacts, and the mo- semiconductors. bility patterns of prehistoric peoples. Although a number of The NAA technique involves the irradiation of a sample techniques have been employed to characterize archaeologi- by neutrons to make the sample radioactive. After irradiation, cal materials, the analytical method with one of the longest the gamma rays emitted from the radioactive sample are and most successful histories of application for provenance measured to determine the amounts of different elements research has been neutron activation analysis (NAA). present in the sample. As a result, NAA has a number Neutron activation analysis isasensitive technique useful of advantages over most other analytical methods when for qualitative and quantitative multi-element analysis of investigating archaeological specimens. First, it is nearly free major, minor, and trace elements present in many sample of any matrix interference effects because the vast majority matrices. With the exception of inductively coupled plasma- of archaeological samples are transparent to the probe, the neutron, and the emitted analytical signal, the gamma ray. 3 http://www.missouri.edu/∼glascock/archlab.htm Second, because NAA can be applied instrumentally (without

0957-0233/03/091516+11$30.00 © 2003 IOP Publishing Ltd Printed in the UK 1516 Neutron activation analysis and provenance research in archaeology sample digestion or dissolution), there is little opportunity for Perlman and Asaro (1969) provided a description of the reagent or laboratory contamination. Third, the preparation standard-comparator method of NAA as applied to provenance of samples from most matrices (especially geological sample determination at the Lawrence Berkeley Laboratory (LBL). types) for analysis by NAA is extremely easy—in most Archaeologists turned more frequently to NAA during the instances a portion of the sample need only be weighed and 1970s and 1980s to determine the sources of pottery, obsidian, place in an appropriate container. In contrast, the difficulty chertand other materials (Hughes et al 1991, Kuleff and of achieving complete digestion of geological samples for Djingova 1990, Neff and Glascock 1995). The NAA analysis by ICP-MS can be challenging and the labour costs are laboratories at BNL and LBL were joined by laboratories at much greater. Finally, although nuclear reactors are becoming the University of Michigan, University of Toronto, Hebrew less available while ICP-MS instruments are becoming University, National Institute of Standards and Technology, more widely available, many reactors offer competitive low- University of Missouri, and a number of smaller university cost analyses on projects involving collaborative academic research reactors. By the early 1990s, NAA was regarded as research. the technique of choice for provenance research (Bishop et al This paper presents a brief history of NAA and its 1990, Gilmore 1991). application to archaeology, describes the physics behind the Although the late 1980s and 1990s, saw reactor NAA technique, and explains how the method is generally decommissioning and retirements of key personnel which employed to determine the provenance of archaeological concluded a number of the major programmes (i.e. BNL, materials. The finalsection presents a few examples of NAA LBL, the University of Toronto and Hebrew University), the applied to recent archaeological studies. demand for NAA has not decreased. Fortunately, although there are fewer places to perform NAA today, the remaining 2. A brief history programmes have been able to increase their capacity such that access to NAA is still readily available. The notion that nuclear reactions might be used for quantitative analysis first occurred to Georg Hevesy and Hilde Levy in 3. Theory 1936 when they exposed rare-earth salts to a naturally emitting Ra(Be) neutron source (Ehmann and Vance 1991). They found 3.1. Nuclearreactions that many of the rare-earth elements became highly radioactive Nuclear reactions occupy a central role in all methods of upon bombardment with neutrons,and the radiation emitted activation analysis. In a nuclear reaction, an incident particle from the different elements decreased according to different (e.g. neutron, proton or alpha particle) interacts with a target timeconstants. From this discovery, they recognized the nucleus either by scattering or by absorption. The diagram potential for identifying elements present in mixtures of shown in figure 1 illustrates a typical nuclear reaction involving samples through measurement of different radiations and half- absorption of the incident particle followed by the emission lives of the radioactive elements. of both particles and prompt gamma rays and by production The 1950s and 1960s saw the construction of nuclear of a radioactive nucleus. If the incident particle is a thermal reactors with neutron fluxes sufficient to allow sensitivities for neutron, a prompt particle is rarely emitted. If the reaction NAA at levels of interest to solving real analytical problems. involves an incident fast neutron or a charged particle, a prompt In addition, there were improvements in the sophistication particle is always emitted. In either case, prompt gamma rays and sensitivity of instrumentation used to make nuclear will always occur. The reaction can also be described by the measurements, and hundreds of experiments were performed expression to measure the basic nuclear parameters (i.e. reaction cross- a + A → [X] → b + B + Q (1) sections, half-lives, gamma-ray abundances and branching ratios) associated with nuclear reactions. or in shorthand notation A(a, b)B.Wherethesymbol a The potential of NAA as an archaeological tool was first represents the incident particle, A is the target nuclide, [X]is recognized by Robert Oppenheimer in the autumn of 1954, thecompound nucleus in a state of excitation, B is theproduct when he suggested its use to Dodson and Sayre of Brookhaven nuclide (often radioactive), b is the exiting particle or radiation, National Laboratory (BNL) as a possible way to establish the and Q accounts for the amount of energy released or absorbed provenance of archaeological ceramics (Harbottle 1976). The during the reaction. If Q is positive, the reaction is called experimental work was undertaken by Sayre and reported to exoergic. If Q is negative, the reaction is called endoergic. archaeologists and chemists at Princeton in 1956 (Sayre and Nuclear reactions caused by neutrons are typically Dodson 1957). At around the same time, a group at Oxford exoergic, and the required threshold energy is zero. Thus, began experimenting with the use of NAA on pottery and the reaction can take place when the incident neutron has a coins (Emeleus 1958, Emeleus and Simpson 1960). The initial kinetic energy of nearly zero. This is the situation for thermal applications were hampered by the poor resolution of available neutron capture or (n, γ )reactions. When the incident neutron detection systems (e.g. Bennyhoff and Heizer 1965), but the energy exceeds the threshold energy for a reaction involving advent of lithium-drifted germanium Ge[Li] detectors in the particle emission, reactions such as (n, p), (n, α), (n, n),and early 1960s offered significant improvements in resolution and (n, 2n) are possible. In most applications involving neutron prompted a flurry of archaeological applications (Harbottle activationanalysis, the product nuclide B is radioactive and is 1976). It is interesting to note that a paper by Sayre (1965) followed by emission of one or more delayed gamma rays. on the analysis of ancient glass was the first to report the use Techniques based on measurement of the prompt and and advantages oftheGe[Li] detector in NAA. delayed radiation are referredtoaspromptgamma neutron

1517 MDGlascock and H Neff

Figure 1. Diagram illustrating of the nuclear processes occurring during interaction of an incident neutron, proton or other nuclear projectile with a target nucleus. activation analysis (PGNAA) and delayed gamma neutron activation analysis (DGNAA), respectively. The latter Figure 2. Atypical neutron energy spectrum from a nuclear ﬁssion technique is frequently referred to as instrumental neutron reactor. activation analysis (INAA) when the procedures employed do not involve extra pre- or post-irradiation steps such as chemical separations.

3.2. Reactor neutron spectrum and reaction cross-sections Reactors based on the fission of 235Uofferthemost intense neutron sources currently available for NAA. The spectrum of neutrons in a reactor consists of three parts as illustrated in figure 2. In the high-energy region, neutrons still have most of their original energy from fission (e.g. roughly 2–6 MeV) and are frequently calledtheprimary fission neutrons or ‘fast neutrons’. Fast neutrons lose energy rapidly through elastic scattering with moderating materials in the reactor, such as water and graphite. Neutrons in the energy range from 0.5 to 1MeVare commonly referred to as ‘epithermal neutrons’ and have been partially slowed down. The distribution of neutrons in the epithermal region approximates a 1/E slope. Below Figure 3. Cross-section versus energy for a common neutron ( ,γ) energies of 0.5 eV, the neutrons are usually referred to as capture n reaction involving thermal and epithermal neutrons. ‘thermal neutrons’. Thermal neutrons have approximately the same velocity (v) distribution as the molecules and atoms of of the compound nucleus. To facilitate calculation of reaction theirsurroundings (i.e. a Maxwell–Boltzmann distribution) of rates in the 1/E-flux region an epithermal cross-section I the form (also called the effective resonance integral) is defined by the dn 4n −(v/v )2 = √ v2e 0 . (2) expression dv v3 π ∞ σ(E) 0 I = dE. (3) In a typical light moderated reactor, the fluxes of epithermal 0.5eV E andfastneutrons are on the order of 2–3% and 7–10% Because both thermal and epithermal neutrons can induce respectively of the thermal neutron flux. Neutron fluxes are (n, γ )reactions, a non-rigorous but commonly used expression − − generally expressed in units of neutrons cm 2 s 1. for the total reaction rate for a particular nuclide assuming n Tabulations of cross-sections for thermal neutron reactions target atoms in the sample is given by assume a velocity of 2200 m s−1 which is the most probable ◦ neutron velocity for neutrons at 20 Cand corresponds to a R = n[φthσth + φepi I](4) most probable energy of 0.0253 eV. Reaction cross-sections −24 −2 are usually expressedinbarns(b)where1b= 10 cm . where φth and φepi are the thermal and epithermal neutron For most nuclides at low neutron energies the cross-section fluxes respectively. In most cases, the thermal and resonance for the (n, γ )reaction obeys a 1/v law as shown in figure 3. cross-sections are similar in magnitude, but the number of Although there are small deviations from the 1/v shape in the thermal neutrons is much greater. Therefore, thermal neutron cross-section curves for some nuclides, the average thermal activationgenerally accounts for a majority of the induced neutron cross-section σth is assumed to be approximately the activity in most elements. Examples of nuclides illustrating same as the cross-section at exactly 2200 m s−1. the range of variability of thermal neutron (n, γ )reaction As neutron energies increase above 0.5 eV, the cross- cross-sections are 27Al (0.226 b), 45Sc (26.3b), 58Fe (1.31 b), section curves for most target nuclides are characterized by a 138Ba (0.405 b) and 164Dy (2725 b). number of resonance peaks added on top of the 1/v curve. The At higher neutron energies, the cross-sections for (n, γ ) energies of the resonance peaks coincide with the excited states reactions are very small, and nuclear reactions that result in the

1518 Neutron activation analysis and provenance research in archaeology ejection of one or more particles—(n, p), (n, α), (n, n),and irradiation and in the interval between the end of irradiation and (n, 2n) reactions—dominate. These transmutation reactions thebeginning of measurement. Thus, the rate of production, occur only when the neutron energy is above the minimum half-life, length of irradiation (Ti ),length of decay (Td ),and ( ) threshold energy ET required for the particular reaction to length of measurement Tc are important factors for NAA. occur. Because the neutron energy distribution is complex Mathematically, the rate of change in the number of at high energies, the average cross-section for a fast neutron radioactive atoms during irradiation is the difference between reaction σ¯ f is defined by the expression the rate of production and the rate of decay, i.e. ∞ σ(E)φ(E) dE dN = − λ . σ¯ = ET . R N (10) f ∞ (5) dt φ(E) dE ET Assuming there were no radioactive atoms present at t = 0, the Using this average cross-section, the total reaction rate for a number of radioactive atoms present at the end of irradiation fast neutron reaction is given by (EOI) is R −λTi NEOI = (1 − e ). (11) R = nφ f σ¯ f (6) λ In general, measurement of radiation emitted from a sample where φ f is defined the average fast neutron flux. There are only a few instances in which fast neutrons are important in made radioactive does not begin immediately after irradiation, butafter a period of decay. Under normal conditions, short NAA. Among these are the measurement of nickel using the decay times are used for radionuclides with short half-lives and 58Ni(n,γ)58Co reaction and corrections to (n,γ)reactions for long decay times are used when the half-life is long. The long several light-mass elements (e.g. Na, Mg, Al) from interference decay time allows possible short-lived nuclides with initially due to fast neutrons. high activities time to become insignificant. The equations describing the number of radioactive atoms present at the 3.3. Radioactive decay start of the counting period (SOC) andendof counting period Radioactive decay is the spontaneous transformation of atoms (EOC), respectively, are by emission of particles or gamma rays from the nucleus, −λ R −λ −λ N = N Td = ( − Ti ) Td or of x-rays after capture of shell electrons by the nucleus. SOC EOI e λ 1 e e (12) Radioactive decay is a statistical process. For a sample containing a large number of radioactive atoms, there is no and waytopredict which atom will be next to decay, but the −λ −λ R −λ N = N e Td (1 − e Tc ) = (1 − e Ti ) decay characteristics of the entire sample can be described. EOC EOI λ −λ −λ The number of atoms that decay per unit of time is defined × e Td (1 − e Tc ). (13) as the activity A andisdefined by the fundamental law of radioactivity 3.4. Decay schemes and gamma-ray spectroscopy dN A =− = λN (7) dt In NAA, the radioactive nuclei produced usually decay into where N is the number of radioactive atoms and λ is the decay daughter nuclei by emitting beta particles. The daughter nuclei constant. created areoften in excited states, and undergo emission of The value of λ is different for each species of radionuclide. one or more gamma rays before arriving at a ground state. An equation describing the timedependence of the number of Measurement of these gamma rays yields the information atoms of the radionuclide is necessary to determine the abundances of elements in the irradiated sample. −λt N(t) = N0e (8) The decay schemes of nuclei range from simple to complex. A simple decay scheme is exemplified by the decay = 28 where N0 is the number of radioactive atoms at time t 0. of Al (t1/2 = 2.24 min),whichisproduced by the irradiation Therefore, the process of radioactive decay is an exponential of 27Al (e.g. Parry (1991), figure 4.2). The 28Al nucleus decays law, andthe activity of the radionuclide is controlled by a via β− emission into the 1779 keV excited state of the daughter characteristic property known at the half-life (i.e. the period nucleus 28Si. In this case, the transition from excited state to of time during which half of the original atoms of that nuclide ground state always leads to emission of a gamma ray with an have decayed). The half-life is related to the decay constant energy of 1779 keV and branching ratio of 100%. Other decay according to schemes are more complex. . ln 2 0 693 Gamma rays interact with matter in several ways, one t1/2 = = . (9) λ λ of which is an absorption process in which the energy of The half-lives of different radionuclides range from thegamma ray is transferred to photoelectrons inside a milliseconds to several times the age of the universe. semiconductor detector. In general, the energies of gamma For NAA using delayed gamma rays, the number of atoms rays from a neutron-irradiated sample will range up to decaying during the post-irradiation measurement period is about 3200 keV, creating a spectrum of gamma rays whose measured. The number of radioactive atoms present at any time characteristics are representative of the sample. The spectrum depends on the total number of radioactive atoms produced of gamma rays measured from a sample of pottery is shown in during irradiation less the number that decayed both during the figure 4.

1519 MDGlascock and H Neff

Figure 4. Gamma-ray spectrum for a pottery sample made radioactive by NAA and counted with a high-purity germanium detector.

The actual number of gamma rays collected by the high- However, for overlapping multiplets, peak fitting routines purity germanium (HPGe) semiconductor detector used to relying on the Gaussian shapes of peaks are commonly used to measure them is smaller than the total number of decays due determine the areas of individual peaks. The measured activity to several factors: is then calculatedbydividingthe peak area by count time. The uncertainty in peak area is reported by calculating (1) The intensities (gamma-ray branching ratios) of emitted the relative standard deviation in per cent. One of the gamma rays are in many cases less than 100%. more common measures of standard deviation is given by the (2) Some of the emitted gamma rays do not reach the detector. expression √ Due to the isotropic nature of gamma-ray emission, only %s.d. = (100 S +2B)/S. (16) those headed in the direction of the detector can be measured. The limit of detection for a peak depends on how well the (3) A portion of the gamma rays will pass through the detector background is known. The usual definition for the detection without interacting, especially high-energy gammas. In limit is based on the two-sigma (∼95%) probability that a peak other cases, some of the gamma rays lose part of their should be observed above the background. energy by Compton scattering and pair production effects, thus contributing to the gamma continuum beneath the 3.5. Calculating concentrations photopeaks of primary interest. In order to convert measured activities into concentrations The second and third factors comprise a characteristic of the for the sample one can use an absolute method by which detector called the geometric efficiency ε.Atenergies above knowledge of all nuclear and experimental parameters is 200 keV, the log of efficiency approximates a linear function necessary such that the following equation expressing the of the log of energy activity present at any time can be used log ε = b log Eγ +loga (14) m A = N θ (φ σ + φ I)Pγ εSDC (17) M A th th epi where constants a and b are dependent on the sample-to- detector distance and dimensions of the detector crystal. where m = mass of sample (g), M = atomic −1 Various techniques are used to extract qualitative and weight (gmol ), NA = Avogadro’s number (6.02 × 23 −1 quantitative information from gamma-ray spectra. Given a 10 molecules mol ), θ = isotopic abundance, Pγ = gamma-ray spectrum, the first task is to identify the nuclides intensity of the measured gamma ray, ε = efficiencyofthe responsible for the various peaks and the second task is detector at the energy of the measured gamma ray, S = −λ −λ to determine the peak areas. Peak identification can be irradiation factor (1 − e Ti ), D = decay factor (e Td ) and −λ accomplished by consulting a number of compilations of decay C = count factor (1 − e Tc ). schemes and tables listing the associated gamma-ray energies However, the absolute activation analysis procedure is andbranching ratios (Erdtmann and Soyka 1979, Firestone rarely used in archaeology where comparator methods are et al 1996, Glascock 1998). For most peaks, the peak area normally preferred. The equationusedtocalculate the mass (sometimes called the signal) can be determined by summing of an element present in the unknown relative to a comparator thetotal number of counts under the peak and subtracting an standard of known concentration is estimated background as follows −λ A m (e Td ) sam = sam sam (18) −λT S = T − B. (15) Astd mstd(e d )std

1520 Neutron activation analysis and provenance research in archaeology where A = activity of the sample (sam) and standard (std), m = mass of the element and Td = decay time. When performing short irradiations, the irradiation, decay and counting times are normally identical for all samples and standards (i.e. the steps are performed on individual samples and standards sequentially and all counting is performed in areproducible geometry) such that all time-dependent and geometric factors will cancel. Thus, the previous equation simplifies to Wstd Asam csam = cstd (19) Wsam Astd where c = concentration of the element in sample and standard and W = weight of sample and standard. The majority of laboratories involved in NAA utilize Figure 5. Two approaches to provenance determination. one or more multi-element calibration standards. The basic procedure is to collect gamma-ray spectra for both sourcing, individual raw material samples are compared to the unknowns and standards under conditions that are as similar range of variation between ceramic reference groups. as possible, then to use one of the equations above to calculate concentrations of the various elements whose peaks appear 4.2. Suitability of different archaeological materials in the gamma spectra. Quality control or check standards are generally included in each irradiation in order to provide As mentioned previously, obsidian artefacts are relatively easy an independent check on the quality of the data and in to source by chemical analysis. In addition to the fact that most order to identify mistakes, such as weighing errors in the obsidian sources are extremely homogeneous, the volcanic standards. Certified reference standards can be purchased from sources are limited geographically to certain regions. Obsidian the National Institute for Standards and Technology. Non- is high in silica, but the trace and minor element constituents certified standards can also be obtained from the United States sometimes differ by orders of magnitude between sources. Geological Survey and other sources. If all possible sources have been sufficiently characterized, the reliability of matching an obsidian artefact to its proper 4. Methodology source is excellent and the number of elements required to identify the source may be very small. However, the reliability 4.1. The provenance postulate of obsidian sourcing is sometimes challenged by processes such as weathering and erosion which may distribute obsidian The basic proposition underlying chemistry-based provenance cobbles far from their source vent or by obsidian sources with determination was understood by the early 1970s (e.g. Bieber multiple flows, between which the differences may be subtle. et al 1976, Harbottle 1976, Perlman and Asaro 1969, Sayre For the latter, the determination of larger numbers of elements et al 1971). But, it was Weigand et al (1977) who first by ahigh-precision multi-element technique such as NAA can stated explicitly that the effort to link artefacts to sources be essential. through compositional analysis depends on the postulate Chert and flint are sedimentary rocks high in quartz ‘that there exist differences in chemical composition between that were commonly used in tool making and for which different natural sources that exceed, in some recognizable source determination by chemical characterization is often way, the differences observed within a given source’. Today, challenging (Luedtke 1992). Hoard et al (1992, 1993) this statement is known as the‘provenance postulate’ by successfully differentiated several archaeologically important archaeologists. It is interpreted to mean that the raw material Oligocene-age chert outcrops in the Great Plains, and Malyk- source for responsible for an artefact can be successfully Selivanova et al (1998) were successful in separating chert determined through chemical analysis as long as between- bearing formations from Alaska. On the other hand, efforts to source chemical differences exceed within-source differences. differentiate between different chert outcrops in Belize have Source determination efforts based ontheprovenance failedtoproduce reliable source distinctions, the whole area postulate can follow one of two separate paths as explained being essentially a single chert source (Cackler et al 1999). by Neff (2000) and illustrated in figure 5. If the sources Clearly the geographic extent and geological context of chert are localized and relatively easy to identify, as in the case of sources are crucial to determining whether chert provenance volcanic obsidian flows, raw materials from the known sources analysis will yield answers to archaeological questions. areusually characterized andthen artefacts of unknown Clays are so ubiquitous and their geological histories provenance can be compared to the range of variation of the are so varied that the reliability of distinguishing between known source groups. On the other hand, if sources are natural sources varies widely. In general, if the parent rock widespread, as is especially true in the case of ceramic raw contributions and weathering histories of two argillaceous soils materials, the prospects of sampling and characterizing most or or sediments are sufficiently distinct, then the provenance all of the possible sources are impractical. As a result, ceramic postulate will apply. Unfortunately, geological processes provenance research generally involves an alternative approach of clay formation often do not create discrete, chemically by which reference groups are created from the unknown homogeneous sources but instead produce extensive deposits ceramic samples. In this more common approach to ceramic that vary in composition both vertically and horizontally.

1521 MDGlascock and H Neff More generally, in sedimentary clay deposits, the proportions specimen database can be made to account for any observed of material from different source rocks may vary gradually ‘drift’ in the analytical data. across horizontal space, while textural variation and chemical weathering may create vertical differentiation within a 4.4. Evaluation of data single stratigraphic column. Such conditions highlight the importance of explicitly identifying the geographic scale at The volume of compositional data generated in most NAA whichthe source is conceived (Neff 2000). After manufacture, studies of archaeological materials is substantial (ranging the use of an artefact and post-depositional processes may up to 35 elements). As a result, multivariate statistical modify its chemistry. Lithic materials (i.e. rocks) are rarely methods are often required to quantify the similarities and affected by this problem, but for ceramic compositional studies differences between specimens and groups of specimens. diagenetic alteration is always a potential concern. Compositional groups can be viewed as ‘centres of mass’ in the Steatite, pipestone, turquoise, limestone, marble, basalt, compositional hyperspace described by the data. An individual ancientglass, native copper, coins and other archaeological group is characterized by the location of its centroid and the materials have been analysed by NAA with various degrees unique correlations of element concentrations to one another. of success for archaeological interpretation (Harbottle 1976, Pattern recognition methods such as cluster analysis, plots of Mead 1999, Truncer et al 1998). the original data in two or three dimensions, and principal components analysis (PCA) are customary approaches to data handling. These methods are described extensively elsewhere 4.3. Analytical procedures (Baxter 1994, Davis 1986, Glascock 1992, Neff 2002) and will In our laboratory, preparation of archaeological specimens be described only briefly here. for instrumental NAA begins by removal of soil and other Cluster analysis is a general term that applies to a variety foreign materials adhering to the surface. For lithic samples, of specific techniques but the essential components are a we usually obtain the analytical sample from clean interior measure of the similarity–dissimilarity between specimens fragments after breaking or crushing the specimen. For (i.e. distance) and an algorithm that groups specimens on the ceramics, the procedure involves burring away the surface with basis of the defined measure. The results of cluster analysis are atungsten carbide tool to remove glazes or slip material that generally presented in the form of dendrograms that show the may be contaminated by weathering or other post-depositional orderand level of specimen clustering. Because interpretation processes. Lithic samples need only be fragmented into small of dendrograms is highly subjective, it is normally only used chips 25–50 mg in size, while the interior paste from a ceramic to identify possible groups after which other techniques are sherdmust be ground to a fine powder and homogenized. employed for group refinement and classification. Individual samples are prepared for short and long irradiations Bivariate and trivariate plots are used to examine the correlations between variables, identify obvious groups and by weighing into clean polyethylene and high-purity quartz detect outlier specimens. Confidence ellipses (e.g. probability vials (i.e. low blanks) respectively. All sample weights are intervals) are usually drawn around groups to emphasize recorded to the nearest 0.01 mg. Reference standards are thedifferences between groups or to show the associations similarly prepared. between individual specimens and known groups. Aseriesoftwo irradiations and three gamma counts are PCA involves a transformation of the dataset on the performed. The short irradiation is carried out on the samples basis of eigenvector methods to determine the magnitude and and standards in polyethylene vials using the pneumatic tube direction of maximum variance in the dataset distribution in irradiation system at the Missouri University Research Reactor hyperspace. The PCA transformation provides a new basis (MURR) (flux of 8 × 1013 neutrons cm−2 s−1) in which for viewing the entire data distribution to reveal structure not the samples are sequentially irradiated for 5 s, decayed for readily observed when plotting the original variables. 25 min and counted for 12 min. The nine short-lived elements Cluster analysis and many other multivariate approaches measured are Al, Ba, Ca, Dy, K, Mn, Na, Ti and V.The samples to data handling are dependent on the use of Euclidean and standards in quartz vials used for long irradiation are distances to measure the dissimilarity between specimens. bundled together in batches of 35–50 samples, four unknowns However, Euclidean distances are not always the best approach and three quality control standards and irradiated in the reactor when working with geochemical data because they do not × 13 −2 −1 pool (flux of 5 10 neutrons cm s )using24 h for pottery account for the correlations between variables. Instead, a or 70hfor lithic artefacts. After a 7-day decay, the samples distance measure known as the Mahalanobis distance (MD) arecounted for 2000 s (the ‘middle’ count) to measure seven is very useful. The MD is defined as the squared Euclidean medium-lived elements, including As, La, Lu, Nd, Sm, U and distance between the specimen and group centroid, divided by Yb. Following an additional 3- or 4-week decay, the samples the group variance in the direction of the specimen. The MD arecounted again for 10 000 s. This last measurement yields from a specimen k to the centroid of a group of specimens A thefollowing 17 long-lived elements, including: Ce, Co, Cr, is written as follows: Cs, Eu, Fe,Hf,Ni,Rb,Sb,Sc,Sr,Ta,Tb,Th,ZnandZr. n n Standards and quality controls made from SRM-1633a D2 = [C − A ]I [C − A ](20) Coal Flyash, SRM-278 Obsidian Rock, SRM-688 Basalt Rock, kA ik i ij jk j i=1 j=1 and Ohio Red Clay have been irradiated and counted with each batch of samples. These reference materials have been used where Ai and A j are the mean concentrations of elements i continually since the laboratory began in 1988. If necessary, and j in the group and Iij is the ijth element of the inverse of corrections or adjustments to batches of samples in the 50 000 the variance–covariance matrix. The MD statistic incorporates

1522 Neutron activation analysis and provenance research in archaeology the lowlands of northern Guatemala and southern Mexico, including the Yucatan Peninsula. Chichen Itza, in northern Yucatan, was occupied during the Classic and subsequent ∆ Early Postclassic period, when central Mexicoan people are thought to have intruded into the Maya area. Since Chichen Itza is located approximately 700 km from the nearest obsidian ∆ sources in Guatemala and more a 1000 km from the sources in central Mexico (ﬁgure 6), changes in relative frequencies of obsidian provide evidence about changing interaction patterns of the site’s inhabitants. Acollection of 421 artefacts from Chichen Itza and nearby sites was submitted to the Archaeometry Lab for NAA by a collaborator (Geoffrey Braswell). The samples Figure 6. Map of the Mesoamerican region showing the locations of major obsidian sources. The sources in Guatemala are: (1) San were analysed using a short-irradiation NAA procedure earlier Lorenzo, (2) San Martin Jilotepeque, (3) San Bartolome Milpas proven successful for this region (Glascock et al 1994) and Altas, (4) Laguna de Ayarza, (5) El Chayal, (6) Sansare, (7) Jalapa, were compared to a database of previously analysed obsidian and (8) Ixtepeque. The sources in Mexico are: (9) Pico de Orizaba, sources. Figure 7 shows a bivariate plot of Mn versus Na for the (1) Guadalupe Victoria, (11) Zaragoza, (12) Paredon, (13) Santa Chichen Itza artefacts compared to 95% probability conﬁdence Elena, (14) Tulancingo, (15) Tepalzingo, (16) Otumba, (17) Malpais, (18) Pachuca, (19) Zacualtipan, (20) El Pariaso, ellipses for nine sources located in Guatemala and Mexico. (21) Fuentezuelas, (22) Ucareo, (23) Zinapecuaro, (24) Tequila, The comparison was highly successful with sources for nearly (25) Magdalena, and (26) Teuchitlan. all of the artefacts securely established. Nineteen (i.e. 4.5%) of the artefacts with the lowest probabilities of membership on information about the correlations between pairs of elements the Mn versus Na plot were submitted to the long-irradiation as derived by the off-diagonal terms of a variance–covariance procedure. Examination of the additional data found that 17 matrix, which simple Euclidean distance does not. Thus, it of the artefacts agreed with the most likely sources suggested permits calculation of the probability that a particular specimen by the short-irradiation procedure. The two remaining samples belongs to a group based not only on its proximity to the group were found to be tektite (i.e. a type of glass thought to be caused centroid but also on the rate at which the density of data points by the impact of meteorites) instead of obsidian. decreases away from the centroid in that direction. The Chichen Itza example illustrates that obsidian provenance studies can be very valuable to archaeologists interested in studying long-distance interactions between 5. Examples prehistoric humans in the form of trade and exchange. Many interesting questions about the inhabitants of Chichen Itza and 5.1. Sourcing obsidian artefacts from Chichen Itza their contacts with peoples living near the obsidian sources can The Maya civilization was famous for its knowledge of be examined with the data from obsidian provenance studies. astronomy and for developing a writing system. The Maya In this case, the importance of interactions with central Mexico of the Classic period (AD 300–900) built cities throughout was established conclusively. Other objectives of obsidian

Figure 7. Bivariate plot of NaversusMnshowing 421 obsidian artefacts from the site of Chichen Itza projected against the 95% conﬁdence ellipses for sources in Mexico and Guatemala.

1523 MDGlascock and H Neff cultural development were located along the Ica and Grande drainages (figure 8). Recently, a MURR collaborator (Kevin Vaughn) conducted excavations at the Early Nasca (AD 1–450) domestic site of Marcaya. Ceramic data and radiocarbon dates from the site indicate a relatively short span of occupation (Vaughn 2000). Marcaya’s ceramic assemblage consists of a high percentage of fineware pottery, especially when compared to other sites in the central Andes. Visual examination of the finewarepastes under low-power magnification identified three different paste types designated paste type A, B and C. To date, no evidence of pottery production has been found at Marcaya. Asampleof100 excavated pottery sherds from Marcaya wassubmitted to MURR for NAA to determine if the pottery assemblage exhibited compositional variation. A total of 32 of the 33 elements normally sought by NAA were measured. The sole exception was the element nickel, which we found below our limit of detection by NAA in all 100 specimens. The data were transformed to base-10 logarithms Figure 8. Map of the IcaandRioGrande river drainages in and submitted to PCA. The transformation to logarithms southern Peru where the Nasca civilization prospered. before PCA serves to make the data more normally distributed and compensates for the different weighting research are to say something about the people who used effects caused by using high concentration (e.g., Al and Fe) obsidian and why their exploitation or trade patterns changed in and low concentration (e.g. rare-earth elements) elements antiquity. A comprehensive database of obsidian sources such simultaneously. Examination of biplots of the samples as that we have established at MURR is essential to answering and element vectors against the first and second principal questions such as these. components (see figure 9) and the first and third principal components showed that the majority of specimens fell 5.2. A pilot study on pottery from southern Peru into asingle homogeneous compositional group, designated group 1, with a relatively small number of specimens The Nasca culture developed along the coast of southern clustering separately. The main group was evaluated and Peru during the Early Intermediate Period (about AD 1–750). refined using methods described in greater detail elsewhere The Nasca ceramic tradition is distinguished by a fineware (see Glascock 1992, Neff 2000). Specimens with probabilities polychrome pottery, found throughout the southern coastal of membership in group 1 of less than 1% were excluded. region and renowned for its elaborate iconography, artistic Some of the specimens not assigned to group 1 were clearly style and technical quality. The primary centres of Nasca associated with two other distinct clusters of specimens

Figure 9. Biplot showing both the pottery samples and the element vectors on principal components 1 and 2 for the three Nasca pottery subgroups identified by INAA. The magnitude and direction of the vectors represent the contribution of individual elements to the principal components. Projections of the vectors on to the axes are equal to the coefficients calculated by PCA. Confidence ellipses shown on the plot have been drawn at the 90% probability level for each group.

1524 Neutron activation analysis and provenance research in archaeology

Figure 10. Bivariate plot of samarium versus lanthanum for the three Nasca pottery subgroups with confidence ellipses drawn at the 90% probability level. designated group 2 and group 3. Small sample sizes prevented The current grant number is BCS-0102325. We are very testingthe latter groups as rigorously as group 1. Figure 10 appreciative of this support. shows a bivariate plot of the data for the elements lanthanum andsamariumwith thedifferent pottery groups surrounded References by 90% confidence ellipses. Fourteen of the 100 specimens were not assigned to any of the groups. Unassigned sherds Baxter M J 1994 Exploratory Multivariate Analysis in Archaeology from paste types A and B (not shown) were found to have less (Edinburgh: Edinburgh University Press) than 0.000 0005% probabilities of membership in group 1. Bennyhoff J A and Heizer R F 1965 Neutron activation analysis of The lack of analyses of clay sources precludes making some Cuiculco and Teotihuacan pottery: archaeological astrong conclusion that the pottery was not locally made. interpretation of results Am. Antiquity 30 348 Bieber A M Jr, Brooks D W, Harbottle G and Sayre E V 1976 However, the data from this investigation serve as an Application of multivariate techniques to analytical data on initial database against which future pottery analyses can be Aegean ceramics Archaeometry 18 59 compared. The Nasca study demonstrates the strong need for Bishop R L, Canouts V, Crown P L and De Atley S P 1990 surveys of raw material sources followed by compositional Sensitivity, precision, and accuracy: their roles in ceramic analysis. compositional data bases Am. Antiquity 55 537 Cackler P R, Glascock M D, Neff H, Iceland H, Pyburn K A, Hudler D and Hester T R 1999 Chipped stone artefacts, source 6. Summary areas, and provenance studies of the northern Belize chert-bearing zone J. Archaeol. Sci. 26 389 After more than three decades of successful application in the Davis J C 1986 Statistics and Data Analysis in Geology (New York: field of archaeology, the reliability of NAA-based provenance Wiley) Ehmann W D and Vance D E 1991 Radiochemistry and Nuclear determination has been firmly established. In addition to its Methods of Analysis (New York: Wiley) sensitivity, accuracy and precision, the success of NAA is due Emeleus V M 1958 The technique of neutron activation analysis to the versatility of the method and the ease of preparing applied to trace element determination in pottery and coins archaeological materials for analysis. Archaeologists have Archaeometry 1 6 relied on NAA studies of artefacts to investigate human Emeleus V M and Simpson G 1960 Neutron activation analysis of ancient Roman potsherds Nature 185 196 activities such as trade and exchange, population mobility and Erdtmann G and Soyka W 1979 The Gamma Rays of the settlement patterns. Although the number of NAA labs has Radionuclides: Tables for Applied Gamma Ray Spectroscopy decreased in recent years, the capacities of the remaining NAA (New York: Verlag Chemie) labs have been increased to keep pace with the still growing Firestone R B, Shirley V S, Baglin C M, Chu S Y and Ziplin J 1996 demand. Table of the Isotopes Vo l I a n d II, 8th edn (New York: Wiley) Gilmore G R 1991 Sources of uncertainty in the neutron activation analysis of pottery Neutron Activation and Plasma Emission Acknowledgments Spectroscopic Analysis in Archaeology: Techniques and Applications (British Museum Occasional Papers No 82) ed The authors acknowledge their collaborators Geoffrey MJHughes, M R Cowell and D R Hook (London: British Braswell andKevin Vaughn who supplied the samples of Museum) Glascock M D 1992 Characterization of archaeological ceramics at obsidian and pottery used as examples in this paper. The MURR by neutron activation analysis and multivariate National Science Foundation has supported the Archaeometry statistics Chemical Characterization of Ceramic Pastes in Laboratory at MURR on a continuous basis since 1988. Archaeology ed H Neff (Madison: Prehistory Press)

1525 MDGlascock and H Neff Glascock M D 1998 Activation analysis Instrumental Multielement Neff H 2000 Neutron activation analysis for provenance Chemical Analysis ed Z B Alfassi (Dordrecht: determination in archaeology Modern Analytical Methods in Kluwer–Academic) Art and Archaeology (Chemical Analysis Series) vol 135, ed Glascock M D, Neff H, Stryker K S and Johnson T N 1994 Sourcing ECiliberto and G Spoto (New York: Wiley) archaeological obsidian by an abbreviated NAA procedure Neff H 2002 Quantitative techniques for analyzing ceramic J. Radioanal. Nucl. Chem. 180 29 compositional data CeramicProduction and Circulation in the Harbottle G 1976 Activation analysis in archaeology Radiochemistry Greater Southwest: Source Determination by INAA and vol3,edGWANewton (London: The Chemical Society) Complementary Mineralogical Investigations ed Hoard R J, Bozell J R, Holen S R, Glascock M D, Neff H and DMGlowacki and H Neff (Los Angeles, CA: Costen Institute Elam J M 1993 Source determination of White River group of Archaeology, University of California) Monograph 44 Neff Hand Glascock M D 1995 The state of nuclear archaeology in silicates from two archaeological sites in the Great Plains Am. North America J. Radioanal. Nucl. Chem. 196 2 Antiquity 58 698 Parry S J 1991 Activation Spectrometry in Chemical Analysis Hoard R J, Holen S R, Glascock M D, Neff H and Elam J M 1992 (Chemical Analysis Series vol 119) (New York: Wiley) Neutron activation analysis of stone from the Chadron Perlman I and Asaro F 1969 Pottery analysis by neutron activation formation and a Clovis site on the Great Plains J. Archaeol. Sci. Archaeometry 11 21 19 655 Sayre E V 1965 Reﬁnement in methods of neutron activation Hughes M J, Cowell M R and Hook D R (ed) 1991 Neutron analysis of ancient glass objects through the use of lithium activation analysis procedures at the British Museum Research drifted germanium diode counters Proc. VII Int. Congr. on Laboratory Neutron Activation and Plasma Emission Glass (Brussels, June 1965) Spectroscopic Analysis in Archaeology: Techniques and Sayre E V, Chan L-H and Sabloff J A 1971 High-resolution gamma Applications (British Museum Occasional Papers No 82) ray spectroscopic analyses of Mayan Fine Orange pottery (London: British Museum) Science and Archaeology ed R H Brill (Cambridge, MA: MIT Kuleff I and Djingova R 1990 Activation analysis in archaeology Press) Activation Analysis vol 2, ed Z B Alfassi (Boca Raton, FL: Sayre E V and Dodson R W 1957 Neutron activation study of Chemical Rubber Company) Mediterranean potsherds Am. J. Archaeol. 61 35 Truncer J, Glascock M D and Neff H 1998 Steatite source Luedtke B E 1992 Archaeologist’s Guide to Chert and Flint (Los characterization in eastern North America: new results using Angeles, CA: Costen Institute of Archaeology, University of instrumental neutron activation analysis Archaeometry 40 23 California) Vaughn K J 2000 Archaeological investigations at marcaya: a Malyk-Selivanova N, Ashley G M, Gal R, Glascock M D and village approach to nasca sociopolitical and economic Neff H 1998 Geological–geochemical approach to sourcing of organization PhD Dissertation University of California, Santa prehistoric chert artifacts, northwestern Alaska Barbara Geoarchaeology 13 673 Weigand P C, Harbottle G and Sayre E V 1977 Turquoise sources Mead A H 1999 Design analysis and chemical characterization of and source analysis: Mesoamerica and the Southwestern USA non-tubular stone pipes of the Great Plains and Eastern US Exchange Systems in Prehistory ed T K Earle and PhD Dissertation University of Missouri, Columbia JEEriscon(NewYork: Academic)

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