## 2 X-ray Detectors
### X-ray Detectors--Conventional XRF  
Similar to PIXE, as concerns the detected radiation, X-ray fluorescence (XRF) is an analytical technique based on the detection of characteristic X-rays induced in the target material by primary X-ray beams, with typical energy spectrum ranges of hundreds of eVs to tens of keVs. XRF is an analytical technique that enables detecting/quantifying elemental composition of the sample under investigation. XRF [83; 84; 85; 86; 87] can be carried out with handheld instruments, laboratory spectrometers or large facilities. In XRF applied to CH, many applications now exploit macro-XRF (MA-XRF) scanners to provide both elemental imaging and point analyses and we will limit our discussion to them, being the most advanced and best-performing instruments, preferred because they allow for a better understanding of sample composition and structure [88]. Many custom and commercial MA-XRF scanners have been presented and are currently used, most of them based on a single SDD detector; see, for example, [89; 90; 91; 92; 93; 94]. A custom ultra-portable MA-XRF scanner was developed also within INFN-CHNet, exploiting a single SDD detector and battery powered [95; 96; 97]. Actually, differently from the particle X-ray production cross sections, which have a decreasing behavior with increasing atomic number (Z) of target atom, when ionization is induced by a given X-ray beam, both the ionization cross sections and the fluorescence yields increase with increasing Z of target atoms, so increasing the number of detectors does not help to balance the detection count rates of low and high energy. However, multi-detector instruments are very appealing also in XRF applied to CH, where analyses are normally carried out in the imaging mode. In this way, it is possible to reduce the measurement time, which can easily be of several hours or even more when big samples are studied, such as paintings of great dimensions. Moreover, increasing the number of detectors is of great interest when high-intensity radiation sources are available, because while a single large-area detector would be affected by noticeable pile-up and saturation effects, adopting multi-element detectors of the same area, these effects can be avoided, reducing the collection solid angle, and so the data throughput of each single element of the detector. Two-SDD detector MA-XRF scanners have been recently developed; see, for example, [98; 99; 100]. A three-SDD detector MA-XRF scanner has been developed within the INFN-CHNet in the framework of the Mu.S.A. (Multi-Channel Scanner for Artworks) project, which involved INFN Roma Tre in collaboration with Ars Mensurae, an enterprise working in the field of diagnostics. The energy resolution is approximately 127 eV at the Mn\({}_{\text{K}\alpha}\) energy, and the image resolution is approximately 1 mm\({}^{2}\) per pixel. The use of three SDD detectors enables faster measurements, maintaining a good S/N ratio (2 mm/s, using a 100 \(\mu\)A X-ray tube). In 2020, the three-detector MA-XRF scanner was used for the first time on the famous masterpiece \"La Fornarina\" painted by [PERSON] (manuscript in preparation); see Figure 1.  
The previous two-detector version of the scanner was tested on a variety of different materials and samples [101; 102; 103]. It is also worth mentioning an approach based on a conventional CCD detector, the Full Field X-ray Pinhole Camera, not widely used yet, but promising [104]. It consists of a CCD detector coupled to a pinhole collimator of small diameter, with the X-ray fluorescence induced on the samples by an external X-ray tube. This approach enables good energy resolution (133 eV @ 5.9 keV Mn\({}_{\text{K}\alpha}\) line) and makes possible the acquisition of two-dimensional elemental maps of the irradiated samples.