Electron Spin for CULtural heritage And PEople's safeguard – ESCULAPE

EPR allows the detection of unpaired electrons that are present in a large number of metallic species (Ti3+, V4+, Cr3+, Cr5+, Fe3+, Mn2+, for example), certain organic or inorganic molecules (radicals) as well as numerous atomic defects (vacancies, interstitials, impurities etc.) in solid materials. The EPR spectra of these paramagnetic species are very sensitive to the nature and structure of their chemical environment. Thus, these species are used as probes to «spy from the inside« on solids at the nanoscale. ESCULAPE has two distinct purposes. The first consists of developing methodological approaches in EPR using «classical« EPR spectrometers to carry out analyses on millimetre-scale samples (samples, reconstituted samples), in order to provide new information that cannot be provided by other techniques. The second, instrumental, purpose is the development of a transportable device that will allow the analysis and mapping of the paramagnetic species of whole, large and non-displaceable objects (very precious or very fragile objects, etc.). This totally original instrument is based on the use of «microstrip« type resonators (used in telecommunications) to carry out magnetic resonance over a very small area (approximately 300 ?m).

An important result of the project is the realisation of a portable EPR spectrometer (about 20 kg) operating at 5 GHz, allowing the detection of EPR signals at any point on the surface of an object, with a spatial resolution of less than 500 ?m. A displacement system for mapping the paramagnetic species of the object has yet to be developed. Several ongoing research projects using classical EPR are already showing the great interest of this technique in providing new information on heritage-related issues. Examples include the mechanism of blackening of green pigments in Renaissance paintings, or the balms of Egyptian mummies, which has provided us with first-class information on a human mummy of unknown origin.

The first results and ongoing studies have shown the full potential of EPR to answer questions concerning the origin, structure and evolution of the materials composing heritage objects, although commercially available equipment can only be used to analyse small samples (i.e. samples or fragments of objects). The portable RPE equipment developed in this project makes it possible to remove the limitation on the size of the objects studied, thanks to the technology of microstrip resonators. The technology has been demonstrated and validated. Thanks to its small footprint, the equipment can eventually be moved to the site (museum, building ...). In terms of TRL (Technical Readiness Level), we were at TRL1 when the project was submitted, and are currently at TRL5-6 (end of 2021). This instrumental aspect of the project is currently being published. For the future, the transition to TRL8-9, corresponding to the development of paramagnetic imaging on heritage objects, requires the design of a positioning-displacement system in x, y, z of high precision. One way to take this last step could be to take advantage of the experience acquired by the Louvre particle accelerator team (AGLAE), which has made a positioner with characteristics quite similar to those envisaged for the ESCULAPE spectrometer.

So far this work has been published in 3 peer-reviewed journals (Anal. Chem. 2020, J. Phys.Chem.C 2021 and Inorg. Chem. 2019). One publication is also submitted and is available as a preprint (DOI: 10.21203/rs.3.rs-960462/v1). Another publication is in preparation on the determination of the distribution grain orientations of “Egyptian blue” pigment in paint layers. An article is also in preparation on the instrumental part of the project.

Non-invasive chemical analysis and imaging is a scientific challenge for in situ study of cultural heritage objects and works. A first challenge of the heritage sciences is to understand their history, the composition and origin of the constituent materials, but also to understand their alteration processes in order to stop them. A second challenge in this area is to authenticate objects and works by fine chemical analysis, because more and more false archaeological pieces and false works circulate in the art markets, some of which are probably still older acquisitions present in museums as recent examples show. The heritage sciences are developing and perfecting heavy analytical techniques, such as those based on synchrotron radiation and ion beams. However, size, scarcity, fragility or the need for in situ analysis require the development of non-invasive and transportable spectroscopic techniques at sites (museums, archaeological sites) allowing the identification and in situ mapping of chemical species (portable X-ray fluorescence X, portable Raman etc ...). The challenge of the ESCULAPE project is the development of a Portable Electron Paramagnetic Resonance (EPR) spectrometer with a "microstrip" surface resonator for the analysis and imaging in situ of paramagnetic species: transition metal ions (TiIII, VIV, CrIII, CrV, MnII, FeIII, CoII, CuII), organic and inorganic radicals, atomic and molecular defects, in large archaeological pieces and works of art (paintings for example).The second application envisaged for this mobile and non-invasive device is in the current context of the possibility of radiological terrorist acts (radioactive substances, for example in cinemas or public transport). The non-destructive determination of defects created by irradiation in mobile phone screens should be carried out with a rapid population sorting objective in order to determine the need for care of persons and Decision of the treatment to be applied according to the dose received.
Standard laboratory RPE equipments are very massive (about 1 ton), and only small samples can be analyzed because they use closed resonators (resonant cavities). The three challenges identified in the ESCULAPE project are 1) the obility (very low mass), 2) the possibility of analyzing large objects, and 3) the possibility of selective mapping of paramagnetic species on these objects. The device will operate at a frequency between 1 and 5 GHz, which will reduce the magnetic field and therefore the mass (5 to 10 kg maximum). The project's flagship idea is to use "microstrip" type surface resonators from telecommunication technologies, thus making it possible to analyze totally non-invasively, with great sensitivity and on very small surfaces (about 0.2x0 .2 mm2) paramagnetic chemical species in objects of large size or too precious to be displaced. By moving the resonator and the magnet to the surface of the object using a stepping motor, it will also be possible to construct a mapping of the detected chemical species. The analysis and deconvolution of the spectra, as well as the reconstruction of the chemical images, will be carried out numerically using the "Chebfun" paradigm. The project involves four partners: (i) Chimie-ParisTech, which is an institutionally associated with the Center for Research and Restoration of French Museums; II) the Laboratoire de Spectrochimie Infrarouge et Raman of Lille; III) the Institute of Materials, Microelectronics and Nanosciences of Provence of the University Ax-Marseille) and IV) the Institute of Radioprotection and Nuclear Safety of Fontenay-aux-Roses.