Development and trends in synchrotron studies of ancient and historical materials
Introduction
The study of the composition, structure, morphology and physico-chemical properties of materials from archaeology, cultural heritage and palaeontology is an essential component of the research in these fields [1], [2], [3]. It complements, when available, historical evidence and information retrieved from the geochronological context. Over the past years, progresses in the instrumentation available on site, at the laboratory and at large scale facilities led to considerable improvement in our understanding of these materials. Among them, synchrotron-based techniques are often unique in the brightness attained and the versatility of the source, allowing a very wide range of photon-based spectroscopy and imaging techniques.
Questions such as the understanding of the discolouration of smalt pigments in paint layers using synchrotron infrared and X-ray absorption spectroscopy techniques [4], the long-term alteration of bone materials in archaeological contexts through synchrotron small-angle X-ray scattering techniques [5], [6], improvements in the determination of the age at death of hominins through phase-contrast visualisation of incremental features in fossil teeth [7] demonstrate the diversity of approaches and questions in the field that benefited from synchrotron investigation in the recent years. The range of synchrotron techniques is very broad and includes hard X-ray, soft X-ray, EUV–VUV, UV/visible, near-, mid- and far-infrared techniques. Although the potentials of the source in the field of ancient and historical materials was identified as soon as the mid 80’s [8], [9], [10], [11], real developments started mainly from the years 2000 onwards [12], [13], [14] with major contributions from a restricted set of expert users of the technique. The field in some respect still lacks the strong collaborative effort that was put in place in some other fields of synchrotron studies such as for structural biology and nano-sciences.
The use of some of the core synchrotron radiation techniques for the characterisation of ancient materials has already been reviewed by several authors, including synchrotron microtomography for palaeontological specimens, synchrotron-based FT-IR and X-ray absorption spectroscopy for cultural heritage [14], [15], [16], [17], [18], [19]. However, the present article focuses on the main methodological developments and trends that are central to this field of research. We therefore recall here the physical principles of the most important synchrotron techniques, review the use of synchrotron techniques with examples of application from the open literature and the contribution of the authors in the field, and present adaptations in each of the core techniques to the specific constraints of ancient and historical materials.
Finally, we discuss and question in a full section the prominent trends that may lead to future novel developments in the synchrotron-based study of ancient and historical materials.
Section snippets
Generation of synchrotron radiation
When a charged particle is accelerated it emits electromagnetic radiation. The term synchrotron radiation (SR) is usually only used when the acceleration changes the direction and not the absolute speed (as in a magnetic field) while the opposed case of a decelerated charged particle is called bremsstrahlung. Synchrotron radiation is emitted by cosmic sources, like the electromagnetic fields around black holes, however, in this context we will only look at the radiation generated in synchrotron
Synchrotron X-ray techniques
Main processes of interaction between photon and matter and their analytical interest in the study of ancient and historical materials are summarised in Fig. 8 and in Table 2. For tabulated X-ray data, the reader is invited to turn towards more specialised reading [31]. The Center for X-ray optics at LBNL also produces a very convenient X-ray data booklet with most of its data available online at the CXRO website [32].
Synchrotron ultraviolet/visible and infrared techniques
The Born–Oppenheimer approximation leads to consider, that since the mass of nucleus is much higher than the mass of electrons, it is possible to treat separately the electronic and the nuclear wave functions, as well as electronic and nuclear energies. In first approximation, the energy can be expressed as the sum of an electronic term , linked to electrons energy, a term , linked to nucleus vibration, and a term , linked to nucleus rotation. where is
Discussion
We discuss here some of the main issues at stake and recent developments observed regarding the synchrotron-based characterisation of ancient and historical materials. In particular, we focus on those areas where improvements are foreseen in terms of methodology (spectro-imaging, nano-imaging, organic analysis, combination of techniques, non-invasive characterisation, time-resolved measurements) taking into account on-going instrumental development. Mitigation strategies to prevent radiation
Acknowledgements
The authors acknowledge critical reading by S. Hustache (synchrotron SOLEIL) and É. Anheim (UVSQ), and rereading of the presentation of their work by St. Leroy (IRAMAT, Saclay), S. Bernard (Muséum National d’Histoire Naturelle, Paris), Ph. Sciau (CEMES, Toulouse) and P. Tafforeau (ESRF, Grenoble). The Bamiyan painting study was funded by grants from ESRF (Project EC-101). The authors are grateful to Y. Taniguchi (NRICP) for active collaboration, to the Ministry of Information and Culture of
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