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Introduction to Synchrotron Radiation: Application to the Study of Cultural Heritage Materials and Biominerals

Catherine DEJOIE1, Pauline MARTINETTO2 and Nobumichi TAMURA3

1 European Synchrotron Radiation Facility, Grenoble, France

2 Institut Néel CNRS/UGA, Grenoble, France

3 Advanced Light Source, Lawrence Berkeley National Lab, USA

1.1. Introduction

Cultural Heritage materials are often complex and heterogeneous with a hierarchical architecture spanning from the nanometer range to the macroscopic. An ancient ceramic is usually made of a clay body, on which a surface decoration is apposed (clay patterning, slip, glaze, etc.). The chemical and structural composition of the raw materials and the firing technique (temperature and atmosphere control) affect the end quality and intrinsic properties of the product for its everyday use. An ancient iron axe preserves traces of the manufacturing process in its inner structure, hinting at the knowhow of the craftsman who manufactured it. The passing of time, storage conditions and conservation intervention are assessed by the presence of corrosion products and/or passivation layers at the surface of the axe. Cultural Heritage does not only refer to manufactured objects, but also encompasses the tools and techniques developed by ancient societies. In such a context, Roman concrete has shown exceptional resilience over time, as evidenced by Roman monuments still standing today after two millennia. Finally, preservation and conservation of our Cultural Heritage is of societal concern, a legacy from the past to be transmitted to future generations.

Cultural Heritage is at the junction of several disciplines, such as history, art, archaeology, materials science, chemistry, physics, geology and biology. The reasons for studying Cultural Heritage materials are diverse: knowledge of ancient societies, evolution of practices, development of trading exchanges, material properties, artifact dating, artwork preservation and restoration, etc. Today, the study of Cultural Heritage materials often requires scientific collaborations across multiple disciplines, necessitating combined approaches and promoting interactions between different scientific communities. The rarity, fragility and complexity of Cultural Heritage materials makes them challenging to study, justifying the use of the most advanced scientific tools such as synchrotron radiation facilities to decipher the secrets hidden in their structure.

Synchrotron radiation is the electromagnetic radiation emitted when charge particles travelling at relativistic velocities are radially accelerated. One of the most spectacular manifestations of synchrotron radiation is the Crab Nebula (and associated pulsar, a 30.2 Hz spinning neutron star), remnant of a supernova observed in 1054 by Chinese and Japanese astronomers (Figure 1.1a) (Burbidge 1957; Caroff and Scargle 1969; Bychkov 1973). The strong magnetic field produced by the pulsar bends the electron path, thus generating synchrotron radiation.

Figure 1.1. a) Crab nebula mosaic image, taken by NASA Hubble Space Telescope (credit: NASA, ESA). b) View of the European Synchrotron Radiation Facility (ESRF), Grenoble, France (credit: ESRF/D. Morel).

Part of the theory around synchrotron radiation was formulated at the end of the 19th century (Liénard 1898). The first particle accelerators emerged in the first half of the 20th century, and synchrotron radiation was observed for the first time at General Electric in 1947 (Goward and Barnes 1946; Elder et al. 1947). In the beginning, such radiation, causing the particles to lose energy, was mainly seen as a nuisance in high-energy electron accelerators (Blewett 1998). Nevertheless, the possibility to produce X-rays of unprecedented brilliance was soon recognized. Synchrotron beams were first used in a parasitic way by scientists at particle accelerators. Today, more than 50 fully dedicated synchrotron sources exist, distributed all over the world (Figure 1.1b)1.

Pioneering work using synchrotron radiation for the study of Cultural Heritage materials was performed in the 1990s at both the Synchrotron Radiation Source (SRS) at the Daresbury Laboratory in the UK (Pantos 2005) and at the European Synchrotron Radiation Facility (ESRF) in France (Walter et al. 1999). The use of synchrotron radiation for the study of Cultural Heritage materials is today more common. Over the last 20 years, the potential of such an approach, the main relevant synchrotron methods and their applications to Cultural Heritage material studies have been extensively reviewed (for example, Bertrand et al. 2012; Dejoie et al. 2018b; Cotte et al. 2019; Janssens and Cotte 2020). The same synchrotron tools used in Cultural Heritage materials are also relevant to other fields such as paleontology and biomineralization. Like in Cultural Heritage, seashells and other biominerals have a complex and multiscale hierarchical structure, the properties of which we have only started to understand. Millions of years of evolution have led nature to create materials that are unique in their properties and a new branch of science, biomimetics, which seeks to understand how nature does it and how we can create new materials with that knowledge.

The objective of this book is to show some recent applications of synchrotron radiation in the field of Heritage Science through a series of contributions about ancient ceramics, the corrosion of iron-based materials, the concrete used in Roman monuments and in the related field of biomineralization. In this introductory chapter, a few elements concerning synchrotron radiation will be briefly described, before introducing the contents of the book in more detail.

1.2. What is synchrotron radiation?

Synchrotron radiation is emitted when charged particles travelling at relativistic speed are accelerated in a curved trajectory. In a synchrotron facility, electrons are circulated in a storage ring near the speed of light. They are guided by magnetic fields coming from bending magnets that cause deflection of their trajectory in the horizontal plane, generating synchrotron radiation tangentially to the electron orbit. The storage ring has a polygonal shape, made of a series of cells, alternating bending magnets and straight sections, with insertion devices (wigglers or undulators) sources occupying the straight sections (Figure 1.2a). Bending magnets generate a continuous polychromatic X-ray spectrum with maximum intensity at a bending magnet critical energy. Insertion devices are made of arrays of magnets that provide a sinusoidal magnetic field, thus causing the trajectory of the electrons to oscillate, and, in so doing, to emit synchrotron radiation at each trajectory bend. In an insertion device, each emission of synchrotron radiation either adds up to produce a photon energy spectrum similar to a bending magnet but brighter (wiggler) or interfere constructively at certain energies, resulting in a series of radiation peaks (called harmonics) an order of magnitude brighter than a bending magnet or wiggler (undulators). On each turn in the storage ring, radio frequency (RF) cavities, which contain electromagnetic fields oscillating at radio frequencies, restore the energy lost by the electrons as they circulate and emit synchrotron radiation (Figure 1.2a). Beamlines where synchrotron radiation is used (mainly X-ray photons, and also IR and gamma rays) are constructed tangentially from both bending magnets and straight sections. Additional information on synchrotron radiation can be found in Margaritondo (1988), Als-Nielsen and McMorrow (2001), Kim (2001), Fitch (2019) or Hwu and Margaritondo (2021).

The principle attributes of synchrotron radiation can be defined as follows:

• high brightness, so a highly collimated and intense beam emitted from a small source size, delivering high flux of photons to the sample. This brightness parameter will be discussed further in the next paragraphs;
• a tunable range of wavelengths, extending from infrared, to soft and hard X-ray regimes, depending on the synchrotron facility. A specific wavelength can be chosen, or variable energy used, for example, for spectroscopy;
• an X-ray beam with a certain degree of coherence that can be exploited for specific experiments, for example, ptychography;
• a polarized source, as the synchrotron radiation is normally linearly polarized in the plane of the synchrotron orbit;
• a pulsed source, as the electrons do not circulate individually in the storage ring but are confined in bunches. The distribution of the bunches allows the time structure to be exploited for specific experiments.

Figure 1.2. a) Schematic representation of a synchrotron storage ring. Adapted from Fitch 2019. b) X-ray beam profile obtained at the ID22 beamline (ESRF) on September 2020. As a result of the EBS (Extremely Brilliant Source, see below) upgrade, the beam is quasi-symmetric.

Two parameters are often highlighted when discussing storage rings and beamline performance: the energy of the electrons in the storage ring and the spectral brightness (or brilliance). These two parameters will be discussed further in the next few paragraphs.

The energy Ee of the electrons circulating at a speed v is given by:

where me is the mass of the...