## 2 X-ray Detectors
### X-ray Detectors--XCT  
Until recently, computed X-ray tomography (XCT) was confined to the medical and industrial field, but nowadays its application is expanding in several other fields, including CH. The high penetration power of X-rays and their safety for non-living items make this imaging technique the optimal approach to analyze the entire volume of works of art and archaeological findings in a non-invasive and non-destructive way. XCT imaging reveals crucial information about artifacts, such as inner morphological features, conservation status, manufacturing technique and constituent materials [118; 119; 120; 121]. Radiographs are commonly acquired using indirect-conversion detectors, exploiting suitable scintillator screens to convert photons from X-rays to Visible light and then collect them by optical systems and CCD cameras. X-ray tomography is achieved by acquiring multiple radiographic projections of the item over 360 (or 180) degrees. The elaboration and reconstruction of the projections with ad hoc algorithms result in a set of cross-sectional images, the so-called slices, from which a rendering software re-creates a complete 3D imaging of the object, from the outside morphology to the inner details. Similarly to the other techniques borrowed from more traditional research fields, the setups, the instrumentations, and especially the detectors, have been optimized for the new scope. In medicine, the XCT facilities are generally designed to acquire the projections rotating the source-detector system over 360 degrees around the patient. In this configuration, the source-detector distance is fixed, resulting in an optimized but limited space available for the patient, and different acquisition procedures can be used, from circular acquisition to a spiral path with the use of single or multi-slice detectors [122; 123]. Being the patient safety of primary importance, to avoid any unnecessary dose, the X-ray beams are generally highly collimated. Additionally, the energy ranges are optimized for the human body and the spatial resolution commonly achievable with this kind of apparatus is quite limited--generally 0.5 mm. For specific anatomical investigations, such as dental XCT or head XCT, systems with a higher resolution are used, typically 0.2-0.3 mm [124]. These characteristics are the result of decades of design to optimize the XCT for the human body and, as can be easily understood, the use of these instruments for CH investigations poses severe limitations. The large variety of sizes and compositions that is typical of CH objects--from tiny jewels to 2 m high wooden statues, from canvas to ceramics and metals--needs peculiar designs. In these investigations, similarly to industrial XCT, the dose requirements are less stringent than in the medical field, but, more crucially, the availability of an in situ diagnostic technique is requested, which can be applied directly inside museums or restoration laboratories, in order to avoid the transfer of precious and fragile artifacts to an external laboratory. For these reasons, the facilities devoted to CH are generally designed with flexible mechanics. Ideally un-bounded, these structures are more easily transportable on-site and with the possibility to vary the source-detector and the object-detector distances to fit a large variety of objects and to accord the voxel size to the object peculiarities. Figure 5 shows two examples of the transportable facilities for CH investigation developed by the INFN-CHNet X-ray tomography laboratory of Bologna, especially designed for medium-size (Figure 5a) [125], and large-size (Figure 5b) [126] works of art. In this case, the projections are acquired rotating the investigated objects and several are the possible configurations and detectors.