Nuclear non-invasive methods for the study of materials of archaeometric interest

Alessandro Migliori, Nuclear non-invasive methods for the study of materials of archaeometric interest, Tese de Doutoramento, Università Degli Studi di Firenze, 2004

Among the various analytical techniques applied to the study of works of art, Ion Beam Analysis (IBA) investigations are assuming an increasing importance. The main aspect that makes these techniques so suitable for the analysis of this kind of objects is their non-destructive character. Among IBA, PIXE is generally the most applied in the field of Cultural Heritage, but also PIGE and RBS are used to obtain peculiar information.

This thesis work is dedicated to the implementation of experimental set-ups and of data analysis procedures that can exploit at best the potentials of IBA techniques.

In the first Chapter I introduce the features of IBA techniques that are most widely used in the study of works of art. I report in some detail the advantages and the limitations of these analytical techniques when applied in the field of Cultural Heritage. The performance of PIXE is compared to other techniques, widely used in Archaeometry, based on X-ray spectroscopy. A generic outlook of the works of art most suitable to be analysed by IBA techniques is then reported. I finally briefly describe the main characteristics of the old and the new accelerator exploited by our research group, focusing particularly on the improvements obtained with the latter.

Chapter 2 is dedicated to the development of systems for the measurement of beam current; monitoring this parameter is very important for both maintaining safe measuring conditions (avoiding damage due to proton irradiation) and obtaining quantitative results from IBA spectra. Two systems developed as indirect beam current meters are described, and results are reported concerning the correlation between beam parameters and the characteristics of the damage induced by proton irradiation on glazed ceramics.

In the third Chapter I discuss about the extraction of quantitative results from PIXE spectra. My attention is focused on data processing by means of a particular software package, GUPIX, which is considered to provide the most reliable results in the analysis of PIXE spectra. An application to the analysis of glazed ceramics is reported. Finally, I introduce the portable PIXE system, developed in another laboratory, and report about a campaign of in-situ measurements, performed with this apparatus, to which I collaborated.

In Chapter 4 I discuss a particular situation arising when analysing a wood or canvas painting. The presence of the protective varnish is a problem for the application of some analytical techniques; in particular, the effectiveness of PIXE for the analysis of pigment layers is limited by the absorption of soft X-rays in the varnish. Pigments characterized by light elements, like lapis-lazuli, can be instead identified by PIGE; the varnish composition can be determined by differential PIXE and its possible contribution to PIXE and PIGE spectra taken into account. I report about a first application to the painting “Madonna dei fusi” by Leonardo da Vinci.

Chapter 5 is dedicated to the application of PIXE to the study of metal point drawings. The particular structure of these works of art requires the use of a proton beam with very small dimensions. The micro-beam facility installed in our laboratory and its performance are described, and I present preliminary applications to samples prepared by restorers of the Opificio delle Pietre Dure, to find the best measuring conditions just in view of future campaigns of analysis on metal point drawings.

The sixth Chapter deals with analyses performed on damaged frescoes. First, I discuss some results of the campaign on paintings in the Cortile di Michelozzo in Palazzo Vecchio in Florence: we analysed by micro-PIXE the composition of some samples picked up from one of the frescoes. Finally, I report about the analysis of samples picked up from VIII-IX century frescoes of the Longobard temple of Cividale del Friuli in North-Eastern Italy.

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