Ion Beam Analysis (PIXE, PIGE, BS, IL, DPAA)
When studying a sample with IBA techniques, the object under investigation is used as target for MeV-energy ion beams; by detecting the radiation emitted in the interactions of the beam ions with the target atoms or nuclei it is possible to characterise the target both qualitatively and quantitatively. In the typical set-up used for Cultural Heritage, the sample does not need to be treated or put in vacuum and the analysis can be performed on the whole artwork, if it is possible to transport it to the laboratory.
Every IBA technique focuses its attention on a particular radiation emitted by the sample during beam bombardment.
PIXE (Particle Induced X-ray Emission): the X-radiation is revealed. It is characteristic of the atomic species that have emitted it. In external beam set-up it is possible to detect in a single measurement of a few minutes all the elements with Z≥11, even if present in traces.
PIGE (Particle Induced Gamma-ray Emission): the γ radiation is revealed. It is characteristic of the isotopic species that have emitted it. With PIGE it is possible to overcome some of the PIXE limitations: for example, it is possible to reveal the presence of light elements located in the deeper layers of the sample.
BS (Backscattering Spectrometry): BS is based on the detection of the beam ions backscattered by target nuclei, it has the advantage of allowing the detection of light elements (Z > 1 with proton beams), providing also isotopic sensitivity for the lighter elements. Due to its ability to give information on the depth structure of the sample and to detect low Z elements/isotopes, BS is a good candidate to complement PIXE.
IL (Ionoluminescence): IL is defined as the light emission in the UV/VIS/IR spectrum induced by ions of few MeV/amu energy in luminescent materials. The photons emitted arise from de-excitation processes of the shallower energy levels of hit atoms, so that the IL response may be strongly dependant 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): it is based on the production of radioactive isotopes as a result of nuclear reactions induced by a proton beam on the atomic nuclei of the sample. It allows for the analysis of the inner area of the samples (400-600 microns).
X Ray Digital Radiography (RX) and Computed Tomography (TC)
Digital radiography can be very useful especially for paintings, both on canvas and on wood. It is possible to obtain information about the conservation state, the characteristics of the painting supports, the manufacturing techniques and the underlying paint layers. The tomography can be used with success for all the works of art in which the third dimension is also important, such as figurines, vases, ornaments or other artefacts. It allows for the visualisation of the inner part of the artefacts in a non-invasive way, getting basic information about preservation state and manufacturing techniques. (See example on the left: the three-dimensional reconstruction of a head of Diana showing a very small thickness defect). With our innovative instruments it is possible to analyse objects of different sizes and types, changing in real time the acquisition conditions in order to get the best result depending on the studied materials. The fixed instrumentation is available both in different laboratories of the network and in the Centro Conservazione e Restauro “La Venaria Reale” (TO).
This term indicates 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 of zinc white, mercury of cinnabar, etc.).
X-Ray Diffraction (XRD)
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. Recently the technique has also been used in forensics for attribution and authentication of historical and artistic materials.
Fourier Transform Infrared Spectroscopy (FTIR)
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.
Secondary Ion Mass Spectrometry (SIMS)
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. Considered one of the most powerful analytical surface analysis techniques, it is able to provide information on the material chemical composition and to associate it a mapping. In depth profiling experiments (dynamic mode) this technique provides the chemical stratigraphy of the material. 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.
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.
Raman spectroscopy is maybe the most powerful among the non-invasive, non-destructive molecular analysis techniques for cultural heritage applications. It 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.
Time-Resolved Laser Induced Fluorescence (TR-LIF)
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.