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Abstract: It has already been shown in literature that X-ray sensitive CCDs can be used to measure the degree of linear polarization of X-rays using the effect that photoelectrons are emitted with a non-isotropic angular distribution in respect to the orientation of the electric field vector of impinging photons. Up to now hybrid semiconductor pixel detectors like the Timepix-detector have never been used for X-ray polarimetry. The main reason for this is that the pixel pitch is large compared to CCDs which results in a much smaller analyzing power. On the other hand, the active thickness of the sensor layer can be larger than in CCDs leading to an increased efficiency. Therefore hybrid photon counting pixel detectors may be used for imaging and polarimetry at higher photon energies. For irradiation with polarized X-ray photons we were able to measure an asymmetry between vertical and horizontal double hit events in neighboring pixels of the hybrid photon counting Timepix-detector at room temperature. For the specific spectrum used in our experiment an average polarization asymmetry of was measured. Additionally, the Timepix-detector with its spectroscopic time-over-threshold-mode was used to measure the dependence of the polarization asymmetry on energy deposition in the detector. Polarization asymmetries between 0.2% at 29keV and 3.4% at 78keV energy deposition were determined. The results can be reproduced with our EGS4-based Monte-Carlo simulation. [Copyright &y& Elsevier]

Abstract: In this paper we discuss the physical processes from the first interaction of an X-ray photon to the spatial distribution of charge carriers at the electrodes of a pixel detector. The implications of these processes on the achievable position and energy resolution have been investigated with simulation calculations and with measurements employing the Medipix detectors. As the next generation detectors will be capable of energy resolution we present two methods of energy exploitation in X-ray imaging, i.e. energy weighting and material reconstruction. [Copyright &y& Elsevier]

Abstract: In the last decades, phase-contrast imaging using a Talbot–Lau grating interferometer is possible even with a low-brilliance X-ray source. With the potential of increasing the soft-tissue contrast, this method is on its way into medical imaging. For this purpose, the knowledge of the underlying physics of this technique is necessary. With this paper, we would like to contribute to the understanding of grating-based phase-contrast imaging by presenting results on measurements and simulations regarding the noise behaviour of the differential phases. These measurements were done using a microfocus X-ray tube with a hybrid, photon-counting, semiconductor Medipix2 detector. The additional simulations were performed by our in-house developed phase-contrast simulation tool “SPHINX”, combining both wave and particle contributions of the simulated photons. The results obtained by both of these methods show the same behaviour. Increasing the number of photons leads to a linear decrease of the standard deviation of the phase. The number of used phase steps has no influence on the standard deviation, if the total number of photons is held constant. Furthermore, the probability density function (pdf) of the reconstructed differential phases was analysed. It turned out that the so-called von Mises distribution is the physically correct pdf, which was also confirmed by measurements. This information advances the understanding of grating-based phase-contrast imaging and can be used to improve image quality. [Copyright &y& Elsevier]

Abstract: In this paper we present results from simulations on grating-based phase contrast imaging. Herein, the performance of a grating-based interferometer concerning the X-ray''s energy is shown. Additionally, geometric parameters of the interferometer were investigated. In the experimental part of this work we present the dependency of the Talbot-Lau interferometer setup on the lower energy threshold of the Medipix-detector. These are the first energy sensitive measurements acquired within the applied method. Furthermore, we demonstrate the difference between a differential and an integral measurement by comparing the phase and absorption image. The conclusion of this work is the detailed characterisation of grating-based phase contrast imaging regarding its energy dependent behaviour and geometric parameters of the gratings. [Copyright &y& Elsevier]

Abstract: We outline in this paper that the noise of a photon counting pixel detector depends on the detection efficiency and the average multiplicity of counts per interacting photon. We give a simple expression for the signal-to-noise ratio (SNR) and zero-frequency detective quantum efficiency (DQE). We describe a method to determine the DQE from measured data and to optimize the DQE as a function of energy threshold. [Copyright &y& Elsevier]