## 2 X-ray Detectors
Beyond ER and DE, key performance indicators for X-ray detectors are sensitive area, spectral range (low energies, high energies), count rate capability, 'cleanliness' of the detector response function (peak to background or peak to tale), stability versus temperature, durability, and toughness to different environmental conditions. Depending on the application, one of the parameters may be deemed more important than others. For energy dispersive X-ray detection, only solid-state devices are now used. Si(Li) detectors were normally exploited in the past; high-purity Ge (HPGe) detectors are still sometimes used because of the higher detection efficiency for high-energy X-rays, due to the higher Z of the active element with respect to Si detectors. Both need to be LN (LiquidNitrogen) cooled. Si-PIN detectors are also used, but are generally less interesting, due to their globally worse performances. Presently, the state-of-the-art detectors for X-ray spectroscopy are SDDs, which have become the typical choice in low-cost to high-end spectrometers for X-ray based analyses. They are characterized by a very low output capacitance (order of few hundreds of fF), which does not depend on the active area of the device [2]. This feature enables SDDs to have lower electronic noise with respect to conventional silicon diodes of equivalent area and thickness. Technological advancements contributed to a substantial decrease in the detectors leakage current and to design highly customized ultra-low-noise front-end electronics [3,4]. It is thus possible to obtain good energy resolution even at high counting rates and low energies. SDDs offer excellent energy resolutions (even down to 120-130 eV at 5.9 keV) at short shaping times (1 \(\upmu\)s or less) and now even wide active areas (up to 150 mm\({}^{2}\)) are available. Maximum affordable count rates can be as high as 10\({}^{5}\) counts s\({}^{-1}\) and above [5]. SDDs are normally Peltier cooled (operational temperature down to \(-\)30 \({}^{\circ}\)C/\(-\)50 \({}^{\circ}\)C), thus no liquid-nitrogen (LN) cooling is required, a big advantage over conventional solid-state detectors in terms of ease of use, weight, and compactness. With respect to SDDs, Si(Li)s have higher cost and lower maximum count rate, lower energy resolution and need LN cooling. However, they can feature higher efficiency for high-energy X-rays than SDDs, due to the thickness, which can reach some mm instead of the \(\sim\)0.5 mm typical thickness of SDDs (1 mm thick SDDs are now also commercially available). Si-PINs are used in low-end XRF spectrometers, especially for their low cost but have low performances. HP-Ges need LN cooling. In addition, as Ge lines have higher energies than the corresponding Si lines, Ge-K escape lines (Peak Energy--9.89 keV) are much more intense than Si-K escape lines (Peak energy--1.74 keV), thus worsening the spectral response when working at excitation energies above the Ge K-edge. HpGe detectors have not been very popular, due to the higher background with respect to Si detectors, caused by to the Compton scattering in the detector active element (Compton scattering cross section is proportional to Z, the atomic number of the crossed medium, where \(Z_{\text{Si}}=14\). \(Z_{\text{Ge}}=32\)). Furthermore, they are typically bulkier, heavier, and more expensive. However, they are characterized by higher detection efficiency for higher energy radiation, due to the higher Z with respect to Si. SDD arrays allow reducing the probability of detecting multiple photons at once i.e., they are good for high-intensity applications. Two configurations are possible: multi-channel SDD arrays, where arrays of independent detectors are assembled in a single device, and single or multiple SDD arrays, where each array is built on a single silicon wafer [6]. Multi-channel SDD arrays have been presented in a commercially available solution. Each element of the array is equipped with independent preamplifiers and digital pulse processors.