Time-of-Flight Neutron Diffraction (TOF-ND) Analyses of the Composition and Minting of Ancient Judaean “Biblical” Coins

Hindawi Journal of Analytical Methods in Chemistry Volume 2019, Article ID 6164058, 18 pages https://doi.org/10.1155/2019/6164058 Research Article Time-of-Flight Neutron Diffraction (TOF-ND) Analyses of the Composition and Minting of Ancient Judaean “Biblical” Coins Stephen E. Nagler,1 Alexandru D. Stoica ,1 Grigoreta M. Stoica ,1 Ke An,1 Harley D. Skorpenske ,1 Orlando Rios,2 David B. Hendin,3 and Nathan W. Bower 4 1 Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 3 American Numismatic Society, New York, NY 10013, USA 4 Chemistry and Biochemistry, Colorado College, Colorado Springs, CO 80903, USA 2 Correspondence should be addressed to Nathan W. Bower; Received 1 November 2018; Revised 8 January 2019; Accepted 3 February 2019; Published 3 March 2019 Academic Editor: Alessandro Buccolieri Copyright © 2019 Stephen E. Nagler et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. TOF-ND elastic scattering of thermal neutrons offers some important advantages over X-ray diffraction (XRD), X-ray fluorescence (XRF), and metallography for the study of archaeological and numismatic problems. Traditional analytical methods are usually destructive and often probe only the surface. Neutrons deeply penetrate samples, simultaneously giving nondestructive bulk information about the crystal structure, composition, and texture (alignment of crystallites) from which thermomechanical manufacturing processes (e.g., cast, struck, or rolled) may be inferred. An analysis of the metal composition and minting processes used for making ancient Judaean bronze and leaded bronze coins from first century BCE and CE is used as a case study. One of the first ND analyses of the temperature used for striking bronze coins is also presented. 1. Introduction Neutron sources with sufficient flux intensity for practical neutron diffraction (ND) studies of small cultural objects have only become available in the last decade or two [1]. These fluxes can be achieved with nuclear reactors or accelerator particle beams that knock neutrons from nuclei in a target by a process called spallation. Pulsed neutron sources allow for very efficient and low background measurements with timeof-flight (TOF) methods that provide wavelength-resolved diffraction measurements across a broad band of wavelengths. This approach is used at the high flux, spallation neutron source (SNS) VULCAN instrument at the Oak Ridge National Laboratory (ORNL) [2]. It can simultaneously and nondestructively probe materials’ crystal structures, compositions, and grain orientations, making it particularly valuable to analytical chemists and materials scientists who need to examine the entire volume of cultural objects. Archaeological materials characterization via ND can be used to help determine the bulk composition hidden by corrosion [3, 4], for help with authentication, for reconstructing past technologies [5, 6], and for developing conservation plans by identifying artifact instabilities, such as internal corrosion. Studies using multiple techniques have included metallography, X-ray fluorescence (XRF), X-ray diffraction (XRD), and modern reference samples with known thermomechanical histories that help researchers interpret ND analyses [1, 7–9]. Despite progress for a number of artifact types, relatively few ND studies have been applied to numismatic questions. These include checking authenticity, identifying methods of minting, and determining changes in composition of silver coins from different eras and regions [10–13]. However, previous studies using ND for the most common ancient coinage alloys, copper-tin and leaded copper-tin (Pb-Cu-Sn) bronzes, appear to have been limited to a total of 20 late Roman coins [14–16]. To our knowledge, only one study has used ND analyses to infer whether ancient coins were struck while the metal was hot, and that was using silver coins [17]. Hot striking offers advantages in terms of the hammer force 2 needed to produce an image, but it also affects the rate of coin production. The degree to which hot striking was used is an open question in numismatics. In this study, we examine 28 bronze coins with different amounts of Pb from Judaea minted under different authorities during the first centuries BCE and CE. We use multiple techniques to interpret results from the different methods, and we use the analyses to deduce whether hot striking of bronze coins was common in this era and locale. We open our case study with an overview of the method’s basic principles. 2. Background 2.1. Neutron Diffraction. There are many kinds of neutron diffraction instruments for probing materials [18], and a number of texts [19–21] and monographs [22, 23] summarize their principles. TOF-ND with instrumentation such as the VULCAN used in this study has a number of similarities to conventional X-ray powder diffraction, but also some important differences that go beyond the obvious difference in their beam sources. Both give information about the crystal structure, including the distances between the atoms in a solid that can be used to identify the elements and molecules that are present. Relative peak intensities for both are related to the relative quantities of different molecules and to the orientation of larger domains that hold them, such as crystallite phases that are preferentially aligned in one direction. The distributions of orientations are called textures. Peak widths for both XRD and ND are affected by instrumental parameters, composition, residual microstrain, and crystal grain size, with larger crystal grains giving narrower peaks. When applied to crystalline materials both ND and XRD produce Bragg peaks. The probe can be thought of as a wave with wavelength, λ, that reflects from planes of atoms separated by a distance d. Constructive interference of waves reflected from different planes creates strong signals when the angle of reflection θ satisfies Bragg’s law, 2d sin θ � nλ, where n is a positive integer [24]. For polycrystalline samples, the resulting spectrum is usually presented as a plot of counts versus d, θ, or 2θ. Fundamentally, ND differs from conventional XRD in how neutrons and photons (or electrons) interact with matter. X-rays are sensitive to charge distributions and interact with the electron cloud around atoms [22]. Therefore, X-rays are most sensitive to elements with large atomic numbers, and XRD patterns are relatively insensitive to the distribution of atoms of elements with small differences in atomic number. Conversely, neutrons are sensitive to nuclear interactions with the atom’s nuclei, characterized by a scattering length that depends in detail on the specific isotope and spin state of the nucleus [25]. The actu (...truncated)