## 2 X-ray Detectors
### X-ray Detectors--Synchrotron-Based Techniques--XRF and XAS  
It is worth mentioning here that third- and fourth-generation synchrotron light sources have revolutionized scientific and technological research in many fields. Projects have beenpresented to develop new energy resolving imaging systems based on SDD, able to cope with the high photon count rates provided by the new light sources [105]. Synchrotron radiation sources offer brilliance which is orders of magnitude higher than X-ray tubes. XRF end stations are typically equipped with monochromators which grant tunable monochromatic primary beams, important to perform selective excitation of the elements to be studied, maximizing the sensitivity of detection, in particular for light elements which are more difficult to detect. The energy tunability achievable with synchrotron radiation gives the possibility to also gain information on the local atomic coordination, thanks to the use of X-ray Absorption Spectroscopy (XAS) (see [106] for a summary of the XAS technique and the XAS experimental setup at the DA\(\Phi\)NE-Light experimental facility). XAS studies the valence states but can also monitor the chemical transformation to which original materials, constituting CH artifacts, were subjected. XAS spectra, that are element-specific, can be studied finely varying the X-ray beam energy in detail around the X-ray absorption edges of the atomic elements to be studied. Depending on the sample nature, measurements can be performed in transmission the mode, detecting the incoming and the transmitted X-ray beams; otherwise, e.g., in the presence of diluted or supported samples in the fluorescence mode, monitoring the incoming X-ray beam and the fluorescence signal, using a detector set at 90 degrees with respect to the incoming beam to reduce contributions due to elastic scattering. Within INFN-CHNet, at the DA\(\Phi\)NE-Light synchrotron radiation facility of the Frascati National Laboratory (INFN-LNF), there is a soft X-ray beamline, DXR1, used to study all elements with X-ray absorption edges in the energy range between 900 eV and 3000 eV. Some XAS spectra of samples containing Si, S, Cl, that have a relevant interest in the CH field, have been measured. As an example, in Figure 2 we report the XAS spectrum at the S K-edge of a reference lazurite sample [107], a mineral used for ages to produce the most famous blue pigment, the ultramarine blue. This spectrum was acquired during an ongoing study on ultramarine pigments. In order to decrease the XAS spectra scan times and collect measurements with high signal to noise ratios, a four-channel silicon drift detector, ARDESIA, financed by INFN and developed by INFN, the Politecnico di Milano and Fondazione Bruno Kessler (FBK) [108] has been installed at the DXR1 beamline in 2019. ARDESIA is very useful when dealing with low and high count rates (>1 Mcps per channel) due to all the elements present in the sample, contributing to the total XRF signal and when good energy resolution (e.g., below  
Figure 1: Schematic representation of the three-SDD MA-XRF scanner of INFN-CHNet **(left)**. Picture of the measuring head taken during the acquisition of elemental maps on the [PERSON]’s Fornarina, 80 \(\times\) 60 cm\({}^{2}\) panel at Palazzo Barberini, Galleria Nazionale Arte Antica, Rome (**right**). Two of the three detectors are visible, as well as the two laser beams (which allow for the correct aiming of the point of interest). 150 eV of full width at half maximum at peaking times faster than 200 ns) is needed. The ARDESIA detector is composed of a monolithic array of SDDs, with a squared geometry of 5 mm \(\times\) 5 mm each and each one collimated to an area of 20.3 mm\({}^{2}\). The standard thickness of the SDDs is approximately 450 \(\upmu\)m. ARDESIA has been engineered in a finger-like structure that gives the possibility to place it very near to the sample using a specific vacuum-tight translating system.