## 2 X-ray Detectors  
X-ray detectors can be divided into two main categories, photon counters and spectroscopic (energy dispersive, ED) detectors. Photon counters are typically used in scattering experiments (e.g., XRD) or X-ray absorption measurements (number of photons impinging on the sample and transmitted photons), whereas energy dispersive detectors are most often used for X-ray emission spectrometry techniques (e.g., XRF and PIXE). Photon counters are typically gas ionization or scintillation detectors, where the photons generated in scintillating material are converted in an electrical signal by a photomultiplier (either in more conventional PM tube or in more innovative Si PM), or solid-state semiconductor detectors such as strip-detectors or hybrid pixel detectors, where the number of pixel can extend up to Mpixel and the lateral resolutions can go down to 50 \(\upmu\)m.  
Spectroscopic ED semiconductor detectors are generally based on a P-I-N diode structure. When the diode is reverse biased, a large, depleted region is formed where charges, created upon interaction with the photons, are collected by the electrodes. Designs differ mainly for the geometry of the electrodes which define the charge collection scheme. Thick P-I-N structures can be obtained by lithium diffusion into p-type silicon (SiLi detector) or germanium (GeLi detector). High-purity germanium (HPGe detector) can also be used to have thick intrinsic regions. High-resistivity silicon wafers may be used to produce P-I-N detectors up to a thickness of 1 mm. In typical designs of P-I-N-based detectors, charge travels mostly perpendicularly to the electrodes. A breakthrough in ED detectors has been the development of silicon drift detectors (SDDs). Silicon drift detectors adopt a design scheme where electrons travel laterally to reach a very small anode, compared to the sensitive area, with a great advantage in terms of capacitance and the associated noise term. The P-I-N structure is also the basis of semiconductor strip and pixel detectors, where one or both sides are segmented to obtain isolated contacts allowing the collection of charge only from a limited portion of the sensor and achieve laterally resolved detection. Every channel has then to be read independently from the others and hence independently connected to the read-out ASIC. In pixelated detectors with a high density of pixels, wire bonding becomes impossible and flip-chip (bump bonding) technologies are adopted; such detectors, called hybrid pixel or hybrid photon-counting detectors, are ideal for imaging and scattering applications. Well-known detectors in this category are those combined in ASICs, developed by the Medipix collaboration, and the detectors based on chips, developed at the Paul Scherrer Institut (PSI), which will be presented in the following Section 2.4. For a review on readout ASICs for hybrid pixel detectors, see [1]. Two parameters of crucial importance for detectors are the intrinsic detection efficiency (DE) and energy resolution (ER). The intrinsic DE is defined as DE = (number of pulses recorded)/(number of radiation quanta incident on detector). For X-rays, ER is normally defined as the full width at half maximum (FWHM) of the MnK\({}_{\alpha}\) (5.9 keV) line of the \({}^{55}\)Fe X source. The Fano limit for energy resolution at MnK\({}_{\alpha}\) (5.9 keV) for Ge is approximately 100 eV and it is approximately 115 eV for Si. Better energy resolutions can be obtained in superconducting detectors (transition-edge-sensor bolometer (TES), kinetic inductance detector (KID), and superconducting tunnel junction detector (STJ)). The energy of the pairing in Cooper Pairs is in the meV range and energy resolutions down to a few eVs at MnK\({}_{\alpha}\) can be obtained.