Effects of Oblique Incidence in Pixel Detectors on Diffraction Experiments

The European X-ray Free Electron Laser (XFEL.EU) is a high-intensity X-ray light source currently being commissioned in the area of Hamburg/Schenefeld, Germany, that will provide spatially coherent X-rays in the energy range between 0.25keV and 20keV. The machine will deliver a unique time structure, consisting of up to 2700 pulses, with a 4.5MHz repetition rate, 10 times per second at very high photon fluxes up to 10 ¹³, photons per pulse. The LPD, DSSC and AGIPD detectors are being developed to provide mega-pixel imaging capabilities at the aforementioned repetition rates for a dynamic range spanning from single photon sensitivity to 10 ⁴, - 10 ⁵, photons per pixel. Data analysis of Bragg diffraction experiments relies on localizing and indexing of diffracted X-rays in the detector image. Pixelated, two dimensional X-ray detectors add uncertainty to the localization. The finite pixel size, scattering of diffracted photons in the sensitive sensor layer and the entrance window, and charge diffusion, contribute to a non-trivial signal distribution Using X-ray Detector Simulation Pipelines (XDSPs) the uncertainty in correct attribution of the point of impact (POI) of diffracted photons on the detector's pixel matrix is evaluated ovetthe XFEL.EU's energy range and relevant pixel sizes. We quantify how steeper angles of incidence (AOIs) cause the signal distribution's center of mass to shift away from the X-ray's POI. Shifts increase with the photon energy, as far as 255μm for E ₚₕ =25keV and θ ₘₐₓ =50° AOI in our model detector.