## 2 X-ray Detectors
### X-ray Detectors--PIXE  
Based on the detection of characteristic X-rays induced in the target material by MeV particle beams, Particle-Induced X-ray Emission (PIXE) is an analytical technique that enables detecting/quantifying elemental composition of sample [26; 27]. PIXE is possibly the most used ion beam analysis technique for CH. Easy to setup and to use, ion beam analysis can detect all the elements from Na onwards. PIXE can provide qualitative/quantitative results and is characterized by high sensitivity and accuracy. Analyses can be performed in relatively short times, in a non-destructive and non-invasive way. This last feature is mainly due to the adoption of external beam setup, where both detectors and sample are maintained out of vacuum, thus avoiding/limiting effects such as heating, selective sublimation/evaporation, charging etc. This technique has been used for CH to study many different samples, such as paintings [28; 29; 30; 31; 32; 33; 34], metal point drawings [35; 36], precious [37; 38; 39; 40] and semi-precious stones [41; 42; 43; 44; 45], ancient coins [46; 47; 48], precious metals [49; 50; 51; 52], ancient bronze statues [53; 54], terracotta, pottery, and clay [55; 56; 57; 58; 59], and ancient manuscripts [60; 61; 62; 63]. In the PIXE technique, X-rays produced by low \(Z\) elements, and thus with low energy, can escape from the sample only if they are produced in the shallower layers of the sample. On the contrary, the higher the atomic number of the emitting element, the higher the energy and thus the depth from which they can escape. To sum, low \(Z\) elements can be detected only in the shallower part of the sample and heavy elements also in deeper layers (just to give a few examples, in a SiO\({}_{2}\) matrix, 3 MeV protons have a range of 80 \(\upmu\)m. Transmission of the X-rays produced at this depth, for different elements, are: Na (\(\sim\)1 keV)\(\longrightarrow\)\(10^{-10}\), K(\(\sim\)3.3 keV)\(\longrightarrow\)\(10^{-4}\), Fe(\(\sim\)6.4 keV)\(\longrightarrow\)0.25, Pb(\(\sim\)10.6 keV)\(\longrightarrow\)0.76. PIXE has limits of detection which can be easily down to the tens of ppm or even better, in favorable conditions. This noticeable result can be obtained by optimizing the detector setup. This point is not a trivial task, due to the different production cross sections of the diverse lines of the elements. If in the sample to analyze, both light elements (low Z, thus higher X-ray production cross sections) and heavy elements (high Z, therefore lower cross section) are present, especially in the case of a light matrix with a low amount of heavy elements, as is most often the case in CH, the detection count rate will be dominated by low-energy X-rays and high \(Z\) element identification can become challenging, if not impossible [64]. To solve this problem, typically a two-detector setup is used, where one detector (SMALL, optimized for low-energy X-rays) has a small active area crystal and a small solid angle, while the other (BIG, optimized for high-energy X-rays) covers the largest possible solid angle, to compensate the low X-ray production cross section, and has a thin absorber(s) installed in front of the entrance window, to limit the count rate of low-energy X-rays. Absorbers are usually made of organic material (e.g., Mylar, Kapton) or of light-element metal foils (Be and Al). Using these absorbers, detectors are not sensitive to low-energy X-rays, which are instead counted by the other detector. For the detector optimized for low-energy X-rays, on the contrary, X-ray absorption is minimized, normally by creating a He atmosphere along the X-ray path from sample to detector, while the acceptance solid angle is small, to reduce the number of X-rays impinging on the active elements and thus reducing the count rate. With this solution, the count rate can be roughly the same for low- and high-energy X-ray detectors. Two-detector PIXE setup has become very popular; see, for example, [65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75].