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Abstract: A pixelated X-ray semiconductor detector (=“direct converter”) is studied which contains an inhomogeneous electric field parallel to the depth axis caused by different concentrations of p- or n-doping. The X-ray energy deposition and charge movement within the detector is modeled in Monte-Carlo simulations which give access to astatistical analysis of electron drift times and current pulse widths for various degrees of static polarization. Integral charges induced on the pixel electrodes are evaluated and put to histograms of spectral detector responses and pulse height spectra (considering energy measurements before and after electronically pulse shaping, respectively). For n-doped semiconductors, the detector performance degrades due to pulse broadening. In contrast, a moderate p-doping can improve the detector performance by generating sharper electron pulses, as long as the detector is not limited by dynamical polarization.Conclusions: We performed Monte-Carlo simulations of energy deposition and charge movement within pixelated photon counting direct conversion detectors made of doped semiconductors of different acceptoror donor concentrations. Induced currents were statistically evaluated with the help of histograms of pulse width and total integral charge (represented before consideration of pulse shaper electronics in spectral responses, and after consideration of pulse shaper electronics in pulse height spectra, respectively). The electrical field close to anode pixels was identified as the main quantity defining the pulse characteristics. For n-doped semiconductors, a weaker electric field near the pixel anodes and longer total electron drift timesare seen, which results in broader pulse widths and a degradation of the spectral responses (by enhanced charge sharing) and pulse height spectra. In contrast, for p-doped semiconductors, a strengthenedelectric field near the anodes and an, on average, reduced total drift time is seen, which results in shorter pulse widths. For moderatep-doping concentrations n_A ≤ 0.9•n_max, an improvement of the spectral response (less charge sharing) and pulse height spectrumis seen. For p-doping concentrations close to the limit still allowing full depletion, n_A > 0.9•n_max, the spectral response und pulseheight spectrum degrade again due to charges which experience long drift times when created in a region with weakened electric field near the cathode. We conclude that using moderately p-doped semiconductor material, n_A ≈ 0.5•n_max, improves detector performance aslong as the detector is not limited by dynamical polarization. In the latter case, it has to be noted that a p-doped semiconductor reduces the maximum count rate at which catastrophic dynamical polarization occurs.

X-ray attenuation properties of matter (i.e. human body in medicalComputed Tomography) are energy and material dependent. This dependency is largely neglected in conventional CT techniques, which require the introduction of correction algorithms in order to prevent image artefacts. The exploitation of the inherent energy information contained in the x-ray spectrum allows distinguishing the two main physical causes of energy-dependent attenuation (photo-electric effect and Compton effect). Currently a number of methods exist that allow assessing the energy-dependent attenuation in conventional systems. These methods consist of using two distinct spectra (kVp switching ordual source) or by discriminating low and high energy photons by means of stacking two detectors. Further improvements can be achievedby transitioning to direct-conversion technologies and counting-modedetection, which inherently exhibits a better signal-to-noise ratio.Further including energy discrimination, enables new applications,which are not feasible with dual-energy techniques, e.g. the possibility to discriminate K-edge features (contrast agents, e.g. Gadolinium) from the other contributions to the x-ray attenuation of a human body. The capability of providing energy-resolved information withtwo or more independent measurements is referred as Spectral CT.A new proprietary photon counting ASIC (ChromAIX) has been developedto provide high count-rate capabilities while offering energy discrimination. The ChromAIX consists of a pixel array with an isotropicpitch of 300 µm. Each pixel contains independent discriminators which enable the possibility to discretize incoming photons into a number of energy levels. Extensive electrical characterization has been carried out to assess the performance in terms of count-rate performance and noise. Observed rates exceeding 10 Mcps/pixel (Poissonian, mean incoming rates > 27 Mcps). The energy resolution is better than4.1 keV FWHM and has been shown to be consistent with simulations. Pile-up behaviour and count-rate dependency have also been evaluated. Electrical crosstalk among pixels in terms of count-rate activity and threshold position has been assessed and show no measureable influences across the array. X-ray tests have also been performed onsamples directly flip-chip bonded to CdTe and CZT crystals. The pulse shape and spectrum obtained from a 241Am source is consistent with simulations.