## 2 X-ray Detectors
### X-ray Detectors--XCT
The CCD-based detector systems, quite widespread some years ago, consist of a CCD camera optically coupled, through a lens and a 45\({}^{\circ}\) mirror, with a scintillator screen (columnar CsI(Tl), or Gadolinium O oxysulfide Gd\({}_{2}\)O\({}_{5}\):Tb), positioned in a light-tight box [127]. In this configuration, X-rays interact with the scintillator, producing visible-light photons, deflected by the mirror and directed on the CCD. This design avoids the direct irradiation of the CCD--ideal configuration for high-energy X-ray imaging [128]--and for this reason similar systems are still in use in Neutron tomography [129]. Nowadays, most of the systems for XCT adopt an in-line configuration, with CCD and flat-panel detectors [130]. The CCDs exploited are generally coupled with a scintillator plate for an indirect conversion and can be small-size detectors, with an area of some square centimeters and a pixel size of few microns, for high-resolution imaging such as micro-XCT [131], or linear detectors. The latter are bigger detectors, with a length of some tens of centimeters  
Figure 5: X-ray tomography facilities devoted to in situ CH investigations, developed by the X-ray tomography laboratory of Bologna. (**a**) picture of the setup designed for medium-size objects (up to 50 by 50 cm) during the analysis of ancient Japanese theatre masks in Rome; (**b**) photo of the facility for large-size objects, up to 150 by 150 cm, during the investigation of a Celestial Globe in Venice. and a pixel size from tens to hundreds of microns and are commonly used for in-line inspections. However, thanks to their large dimension and their fast acquisition, they have been successfully applied in a XCT facility for investigations of large CH objects [132; 133]. Indubitably, flat-panel detectors are the most common detectors for works of art investigations. Characterized by a large detection area, up to 45 \(\times\) 45 cm\({}^{2}\), and good spatial resolution, with a pixel size from tens to a few hundred microns, these detectors generally exploit a CMOS or an Amorphous Silicon technology and take advantage of scintillator plates (CsI(Tl) or GOS (Gd2O2S:Tb) for indirect conversion [134]). Direct conversion FDPs are also available, which exploit an amorphous Se layer to directly convert X-rays to charge. The use of this kind of detector can overcome the intermediate step of X-ray conversion into visible light and generally enables achieving better spatial resolution than indirect FDPs for the same pixel size. The emerging technique of the dual-energy XCT is opening new perspectives to X-ray imaging: the use of two carefully chosen irradiating spectra of the primary X-rays--or acquired energy ranges--enables the enhancement of the small differences in absorption of two or more materials, such as human tissue and a contrast agent in medical imaging. This can be achieved using several configurations, from the use of two orthogonal source-detector systems to acquire simultaneously two XCT datasets with different spectra, to the acquisition of two successive CTs by switching the voltage of one X-ray tube [135]. For its applications in the CH field, where the investigated materials are disparate and the acquisition of two spectra can be reductive, the most promising setup involves the use of large-area hybrid photon-counting detectors (HPCDs). Similarly to detectors used for X-ray spectroscopy, these solid-state detectors allow the simultaneous readout of X-ray fluxes and spectral energies, enabling the acquisition of an energy resolved tomography, the so-called spectroscopic or \"color\" tomography. In recent years, some large-area HPCDs have been developed, with a high spatial resolution and fully spectroscopic capabilities [136] or with spectral discrimination achieved using two/four energy thresholds to be opportunely set [137; 138].