Laboratories working on the analysis of ancient materials are heavy users of infrared spec- troscopy, X-ray diffraction, X-ray fluorescence, and radiography, which are all among the top ten methods used by these institutions (see Fig. 2). These methods find direct counter- parts at synchrotron sources with improved analytical sensitivity, ultra-short acquisition times, and a micrometre-range resolution. Indeed, even hard X-ray beams can now be focussed down to micrometric or sub-micrometric spots by using synchrotron sources.
In this chapter, imaging is understood in its broad meaning, including both raster scanning and full-field methods, leading to the collection of 2D and 3D maps with a micrometre-level resolution. In total, this represents about one-fifth of the total synchro- tron publications for the heritage field known to us.1The main techniques dealt with are listed in Table 1, and the reader is invited to refer the chapter on Synchrotron Radiation Synchrotron Imaging for Archaeology, Art History, Conservation, and Palaeontology 99
Fig. 2. Most widely used methods in European laboratories working on ancient materials.
Methods that find a direct counterpart at synchrotron facilities are shown in black and grey.
Adapted from the LabsTech 2003 data (LabsTech European Coordination Network (1999–2002)).
Optical microscopy Photography Electron microscopy Infrared spectroscopy (60%) X-ray diffraction (58%) Chromatography Colorimetry Radiography (44%) Environmental weathering X-ray fluorescence (42%)
0 20 40 60 80
1Synchrotron SOLEIL Heritage and Archaeology Liaison Office – reference database website available at http://www.synchrotron-soleil.fr/heritage/
and its Use in Art, Archaeometry, and Cultural Heritage Studies (D. Creagh, Chapter 1) for an introduction on the corresponding methods.
A survey of the synchrotron heritage publications clearly shows that the use of synchro- tron techniques in microfocussed modeexceeds that of large beam. Microfocussed beam usage ranges from 100% for FTIR to 49% for XRD (see Fig. 3). Microbeams are either used for single-spot acquisitions on minute samples or to acquire 2D (3D) raster scans. Such scans (a) Scanning experiments
Wide-angle X-ray diffraction and scattering Crystalline structure (mXRD and mWAXS)
Small-angle X-ray scattering (mSAXS) Organisation at the nanometre scale
X-ray fluorescence (mXRF) Elemental content
X-ray absorption (mXAS, mXANES, and Oxidation state and local
mEXAFS) environment
Fourier-transform infrared spectromicroscopy Chemical bond identification (mFTIR)
(b) Full-field experiments
Micro-computed X-ray tomography (mCT) 3D radiography
X-ray microscopy (XRM) Radiography
Fig. 3. Distribution of the synchrotron publications on heritage according to the technique used. Microbeam experiments are shown in dark grey. For experimental method abbreviations, please refer to Table 1 (data time span 1986–2005). A list of synchrotron publications on heritage is maintained at the SOLEIL synchrotron website (http://www.
synchrotron-soleil.fr/heritage/).
0 20 40 60 80 100
XRF XRD XAS FTIR OTHER
can lead to very precise mapping of composition, structure, and chemical information at a micrometre-length-scale level, crucial for the understanding of the materials, their ageing, and the treatments applied to them. Using multitechnique synchrotron beamlines, elemental, chemical, and structural information can be collected at the same acquisition point.
Reducing the beam footprint can additionally decrease the complexity due to the number of chemical species contributing to each spectrum collected, thus simplifying further data processing. This is a very important factor for chemical speciation of heterogeneous samples through X-ray absorption spectroscopy, and is used extensively for other fields such as envi- ronmental sciences (Isaure et al., 2002; Manceau et al., 2002). Comparison and clustering of individual spectra can then contribute to the understanding of the underlying compositional and structural correlations. However, reducing the spot size diminishes the representativeness of the experiment. It can also be at the origin of specific crystal orientation issues in 2D mono- chromatic mXRD mapping when the spot size reaches that of the crystallite. Very few, if any, crystal plans are in a good orientation to diffract, and almost no spot is observed in the diffrac- tion image. In such a situation, polychromatic (“white”) beam techniques are preferred.
Alternatively, full-fieldimaging methods rely on the interaction between a wide beam and the sample: X-ray microscopy, X-ray micro-computed tomography, etc. The resolution is then primarily limited by that of the detector and can be less than a micrometre at synchrotron tomography beamlines. Some of the major specificities of the synchrotron radiation (high monochromaticity, intensity, nearly parallel geometry, and coherence) are particularly suited to the observation of samples that are difficult to image using conventional radiography and microtomography equipment (Salvo et al., 2003). In particular, the contrast of materials that have a rather homogeneous density distribution can be strongly enhanced using phase contrast imaging, and the beam hardeningeffect, due to the polychromaticity of laboratory X-ray sources, is suppressed. These two points are essential for palaeontological research (Tafforeau, 2004; Tafforeau et al., 2006). Recent additional solutions to increase the contrast, developed primarily for biomedical synchrotron analysis, include diffraction-enhanced imaging (DEI).
The main advantages sought by current synchrotron users from the heritage community are therefore:
1. the use of microfocussed setups for methods that they also generally have access to using laboratory equipment; and
2. the access to methods that are specific to synchrotron radiation, as X-ray absorption that requires the tuneable and highly monochromatic beam delivered at synchrotron facilities.
Most heritage users rely on a conjunction of synchrotron and non-synchrotron imaging techniques. The latter can be elementally or chemically selective, such as Raman microscopy, scanning electron microscopy coupled to energy dispersive spectrometry (SEM-EDX), transmission electron microscopy (TEM), laboratory X-ray techniques (XRF, XRD), FTIR, ion beam analyses (IBA: PIXE, etc.), etc. In particular, Raman microscopy is currently leading to unprecedented developments for the understanding of art and archaeological samples at the micrometer-scale level (Neff et al., 2004) and can now be coupled to synchrotron characterisations (Davies et al., 2005). To date, cross-technique correlation remains difficult, and more research has to be done to attain a satisfactory complementary use of synchrotron and non-synchrotron techniques.
Synchrotron Imaging for Archaeology, Art History, Conservation, and Palaeontology 101