Characterisation of the charge sharing in pixellated Si detectors with single-photon processing readout

Pixellated silicon detectors with a thickness of 300 and 700 μm bonded to the MEDIPIX2 readout chip have been characterised using a monoenergetic microbeam at the ESRF. The spectral response when a 10×10 μm2 wide 40 keV beam is centred on a single pixel is achieved. When the beam is scanned over the pixel, the charge sharing will increase when the beam approaches the border of the pixel.The experimental results have been verified by charge transport simulations and X-ray scattering simulations. Agreement between measurements and simulations can be achieved if a wider beam is assumed in the simulations. Widening of the absorption profile can to a large extent be explained by backscattering of lower-energy photons by the tin/led bump bounds below the detector. Widening of the detected beam is also an effect of angular alignment problems, especially on the 700 μm detector. Since the angel between the depth and a half-pixel is only 2.2°, alignment of thick pixellated silicon detectors will be a problem to consider when designing X-ray imaging setups.