Ion Beam Analysis, IBA PIXE, PIGE, BS, IL, DPAA
Ion Beam Analysis (IBA) techniques allow us to study the composition of materials thanks to the particles and/or radiations emitted by the interaction of an accelerated proton beam (in the MeV range) with the atoms and the nuclei of the target. Thanks to IBA, we can acquire both qualitative and quantitative information, even though the latter with few limitations. Measurements are neither invasive nor destructive. The object, i.e. the artwork, to be analysed can be studied without the necessity of collecting any sample and just in atmosphere (actually, the proton beam is extracted into the atmosphere). The only real limitation is thus represented by the fact that the artwork has to be transported in the laboratory.
Different IBA techniques can be implemented according to the particles/radiation to be collected.
PIXE (Particle Induced X-ray Emission): X-rays, whose energies are typical of the atomic species, are detected. Exploiting an external beam set-up, by PIXE, we can determine elements with Z ≥ 11, with a very good sensitivity even when elements are present as traces.
PIGE (Particle Induced Gamma-ray Emission): γ rays, whose energies are typical of the isotopic species, are detected. Thanks to PIGE, some limitations of PIXE can be overcome: for instance, it can be exploited to study light elements in thick targets.
BS (Backscattering spettroscopy): backscattered protons, whose energies depend on the mass of the nuclei involved in the interaction, are detected. Light elements (Z > 1 ) can be identified and information on the possible stratigraphy of the analysed objects can be acquired.
IL (Ionoluminescenza): radiation in the UV/VIS/IR range is detected. The photons emitted arise from de-excitation processes of the shallower energy levels of hit atoms, so that the IL response may be strongly dependent on the neighbours of the emitting atoms. This means that the IL technique can be extremely sensitive to the structure of the hit material and on the impurities present in the bulk, providing information about both crystal structure and chemical state of the atoms present in the sample.
DPAA (Deep Proton Activation Analysis): particles produced in the nuclear interactions of the proton beam with the nuclei of the target are detected. It allows for the analysis of the inner area of the samples (400-600 microns).
Performed both on canvas and panel paintings, – digital – X-ray radiography allows us to study the conservation state of the object, the characteristics of its support, the production process and the possible layers underlying the uppermost pictorial film.
X-ray tomography can be successfully applied to all those artworks for which the third dimension is also important, i.e. statues, vases, ornaments or other artefacts. It allows us to investigate the inner parts of the object in a non-invasive way, obtaining fundamental information on conservation state and construction techniques. The developed instruments are characterised by different spatial resolutions and are able to analyse objects of different sizes and typology.
K-edge radiography is an imaging technique which consists in performing two digital radiographs with X-ray energies, respectively, slightly greater and slightly less than the K-edge of the element that has to be identified. The attenuation coefficient of this element undergoes a substantial change in the interval between the two energies employed; from the digital subtraction of the two radiographs it is thus possible to obtain the spatial distribution of the considered element. The produced elemental map has the typical advantages of an imaging technique with respect to a sampling technique: it allows avoiding the arbitrariness in the sample choice, and recognising restored parts. K-edge radiography is very useful for the non-invasive study of the pictorial materials of a painting, as a particular chemical element is often characteristic of a particular pigment (e.g. zinc in zinc white, mercury in cinnabar, etc.).
X-ray diffraction (XRD) is used to determine the mineral phases characterising the materials. This technique is particularly useful in the study of cultural heritage and archaeological materials, especially for the determination of pigments in paintings, frescoes, mural paintings, parchments and illuminated manuscripts. Other applications are related to the characterisation of corrosion or degradation patinas in ancient metals and architectural materials in order to determine the nature of the degradation processes and to identify appropriate conservation protocols. XRD has also been used in forensics for attribution and authentication of historical and artistic materials.
Raman spectroscopy is based on the inelastic scattering of light by the molecules and it allows obtaining information on the molecular composition, the bonds, the chemical environment, the phase and the crystal structure of the sample. The analysed materials can be gas, liquids and amorphous or crystalline solids.
Many applications in archaeometry can be performed, such as the physical-chemical characterization of the pigments, of the minerals and the gemstones in the artworks.
Thanks to the coupling with a microscopy, Raman microspectroscopy can analyse small details as small as few microns.
Fourier Transform Infrared Spectroscopy (FTIR) is a non-destructive and non-invasive (or micro-invasive) diagnostic technique that gives information about the molecular composition of non-metallic materials. In particular, it allows analysing the organic component, not detectable by other diagnostic techniques. This technique can give qualitative and, in some cases, quantitative information. FTIR spectroscopy is increasingly being used for surface analysis of materials and artefacts that have to be restored, helping to get useful information for choosing the best operational methodology.
Laser Induced Fluorescence (LIF) can be a valid complement to traditional compositional techniques in the cultural heritage field. It is based on the characteristic emission spectra of pigments/binders, etc. that made them recognisable on the paint surface. The analysed sample is excited by a laser source; the study of the emission spectra of the compounds allows then for the material identification. Advantages of this technique are: high sensitivity, non-invasiveness and immediate answer.
On the other hand, one problem is represented by the difficulty to separate the contribution of different compounds when analysing a mixture (for example pigment + binder). This problem can be overcome by the time-resolved analysis of the spectra (TR-LIF), which allows for the compound identification through the study of the decay times observed in the emissions.
Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) is particularly indicated for identification and localization of organic compounds such as resins, binders, dyes, lacquers, etc. It is able to provide information on the chemical composition of materials, also obtaining elemental maps.
When applied in dynamic mode, this technique provides the chemical stratigraphy of the material (depth profiling). SIMS is considered the more sensitive technique for surface analysis (ppm/ppb), able to get a sub-nanometer resolution along z and a lateral resolution of 60 nm.
ICP-MS allows us to determine the elemental composition of a sample. It is an extremely sensitive technique capable of operating in the field of ultra-traces, allowing the detection of concentrations equal to or lower than 10-12 g of analyte per g of sample. It can thus give information on possible contaminations in the analysed sample or on traces that can support provenance studies. Mass spectrometry can be considered a milli-destructive technique, given that the amount of sample required for the measurement is pretty small, generally in the order of a few tens of mg. TIMS, especially in the case of a set-up equipped with a multiple collector, is particularly suitable for measuring the isotope ratios of the elements. It allows us to obtain very high precision measurements that can be used to carry out studies and investigations on the origin of a find or, in the case of artifacts, of the material used for their realization. In some cases this technique is also used to demonstrate the authenticity of works of art. Generally, the elements most exploited for these applications are strontium, in case of biological samples, and lead for inorganic materials.
Stable isotope mass spectrometry allows the measurement of isotope ratios of elements such as carbon, nitrogen, oxygen, hydrogen and sulfur, in solid, liquid or gaseous materials. The potential of this technique lies in the capability to determine the origin of a certain substance in the environment. For example, the isotopic analysis of C and N present in bones allows us to reconstruct the diet of the populations of the past, while C from residues organic in archaeological ceramic finds allows to trace the type of food used, distinguishing between vegetable and animal fats. From the isotopic ratios of strontium on tooth enamel, on the other hand, it is possible to reconstruct the possible migrations that have affected individuals of a specific population.
The profilometer is an instrument used to measure a surface’s profile, in order to quantify its roughness. Vertical resolution is usually in the nanometer scale, while lateral resolution is usually lower.