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| Fig. 1: Terracotta figurines from the sanctuary of Agia Irini (Eirini) in NW Cyprus. (Source: Wikimedia Commons) |
In archaeological investigations where provenance is an aspect of important concern, it has been long recognized that systems of classification based upon form, decoration, and subtleties of workmanship should be supplemented by laboratory techniques that are able to identify distinctive chemical compositions. [1] Due to the penetrating and therefore non-destructive nature of neutrons, characterization methods based upon their usage stand out amongst the wide variety of techniques to play a fundamental role in the investigation of artifacts. [2] By allowing both surface and bulk properties to be measured, neutron methods provide researchers with the necessary means for the examination, conservation, and restoration of ancient material samples. [2] Since its first employment in the mid-1950s, Neutron Activation Analysis (NAA), in particular, has been used extensively as one of the most important neutron-based characterizing and provenancing analytical techniques; it continues to play an essential role in the field of archaeological research today. [3]
As a versatile quantitative analytical technique with remarkable sensitivity, accuracy, and precision, neutron activation analysis (NAA) is widely applied across disciplines such as archaeology, geochemistry, health, human nutrition, environmental monitoring, and semiconductor technology. [4]
Discovered by G. Hevesy and H. Levi in 1936, the method was first applied towards the investigation of archaeological research in 1956, where it was used to analyze the sherds of Mediterranean terracotta figurines such as the ones shown in Fig 1. [1,4-6] Recognizing the technique's potential for relating artifacts to source materials in a powerful yet convenient way, R. W. Dodson and E. V. Sayre of Brookhaven National Laboratory undertook this investigation at the invitation of R. Oppenheimer. [4,6] NAA grew increasingly popular amongst archaeologists during the 1970s and 1980s and, by the early 1990s, was considered to be the technique of choice for provenance research. [4]
As a highly sensitive analytical technique, NAA can be used to perform both qualitative and quantitative analysis of major, minor, or trace elements in samples of almost any type of state (i.e., solid, liquid, or gas). [3] By exposing such a sample to neutrons, nuclear reactions are induced and the result is the emission of detectable gamma rays; by measuring the characteristic energies of these gamma rays, the specific nuclear reaction(s) taking place and, therefore, the elements present in the sample, can be identified. [3]
During the most common type of nuclear reaction used for NAA, the neutron capture reaction, a compound nucleus forms in an excited state when a neutron interacts with a target nucleus via a non-elastic collision. [5] This excited compound nucleus will then immediately de-excite into a more stable configuration by emitting one or more characteristic prompt gamma rays. [5] In many cases, however, this new configuration can also yield a radioactive nucleus which also de-excites by emitting one or more characteristic delayed gamma rays, but at a rate that is much slower. [5] Depending on the time of measurement, therefore, NAA falls into two categories: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements are taken during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where measurements are taken following radioactive decay. [4,5]
In terms of reactors, the ones based on Uranium-235 fission offer the most intense neutron sources currently available for NAA. [4] Neutrons inside a reactor can be divided into 3 types based on an energy spectrum: the fast neutrons of the high-energy region (2-6 MeV), the epithermal neutrons of the 0.5-1.0 MeV region, and the thermal neutrons of the region below energies of 0.5 eV. [4]
For archaeologists, the advantages of using NAA over other analytical techniques are numerous. The preparation of samples for NAA analysis is extremely simple, requiring, in many cases, only the weighing and placing of portions into containers. [4] Additionally, because NAA can be applied instrumentally (without the need to digest or dissolve the sample), there is no concern of reagent and laboratory contamination, which also reduces labor costs. [4] And finally, NAA presents a highly precise and accurate way of simultaneously measuring multiple elements in a single sample (nowadays, up to 50-60). [2,4] Since the first application of NAA towards archaeological research in the mid-1950s, the technique has successfully established itself as a powerful, reliable, and convenient tool for provenance determination. [1,4]
© Yuchen Liu. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] I. Perlman, F. Asaro, and H. V. Michel, "Nuclear Applications in Art and Archaeology," Annu. Rev. Nucl. Sci. 22, 383 (1972).
[2] N. Kardjilov and G. Festa, eds., Neutron Methods for Archaeology and Cultural Heritage (Springer, 2017).
[3] M. D. Glascock, "Neutron Activation Analysis (NAA): Applications in Archaeology," in Encyclopedia of Global Archaeology, 2014th Ed., ed. by C. Smith (Springer, 2013).
[4] M. D. Glascock and H. Neff, "Neutron Activation Analysis and Provenance Research in Archaeology," Meas. Sci. Technol. 14, 1516, (2003).
[5] L. Hamidatou et al., "Concepts, Instrumentation and Techniques of Neutron Activation Analysis," in Imaging and Radioanalytical Techniques in Interdisciplinary Research, ed. by F. Kharfi (IntechOpen, 2013).
[6] E. V. Sayre and R. W. Dodson, "Neutron Activation Study of Mediterranean Potsherds," Am. J. Archaeol. 61, 35 (1957).
