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Abstract

Many X-ray diffraction experiments at fourth generation light sources, such as Coherent Diffraction Imaging or Serial Femto-second X-ray Crystallography, rely on a wide-angle scattering geometry with a pixelated semiconductor detector positioned perpendicular to the beam axis and close to the sample position. Diffracted photons arrive on the sensor surface under a considerably oblique angle of incidence. At normal incidence the X-ray point spread function of the sensor approximately follows a Gaussian distribution and depends on detector operating conditions and photon energy. The known shape of the distribution enables reconstruction of a photon's point of impact on the sensor surface with a precision below the pixel pitch from the given detector signal in low noise systems. Experimental studies in the soft X-ray, single photon regime show that sub-pixel position resolution below 5% of the pixel pitch in point of impact reconstruction are attainable when considering charge sharing between neighboring pixels. For photon energies above 8 keV and large angles of incidence however, the point spread function can be significantly distorted from a symmetric shape due to a drop in quantum efficiency, referred to as Parallax Effect. This effect is so far only corrected for in a few application specific scenarios and detailed measurements of the point spread function at high angle of incidence and under controlled conditions are sparse. The present work quantifies the systematics induced by the Parallax Effect. Using a highly collimated X-ray beam at the Petra III P65 beamline, the signal distributions resulting from X-rays at normal as well as oblique incidence at energies between 10-20 keV and angles of incidence up to 50° are characterized.

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