Saturation effects of CdTe photon counting detectors under high photon fluxes

Abstract

Pixelated photon counting semiconductor detectors enable to resolve the spectral composition of the incident photons in X-ray imaging, provided that a detector is operated in the absence of pulse pile-up and sensor polarisation. The purpose of this study was to examine the imaging properties of Medipix2 MXR arrays under high photon fluxes, which do not meet this requirement. The detectors studied feature a 1 mm thick cadmium telluride sensor and pixel pitches of 110 μm and 165 μm. We show that the critical photon flux, at which the detector's linear response breaks down, can be shifted towards higher values by increasing the I Krum current in the charge sensitive preamplifier, which corresponds to reducing the pulse shaping time. The negative impact of this procedure on the energy response function, which was measured using an 241Am source, seems only moderate. Furthermore, we describe various kinds of image artefacts due to sensor defects. While bulks of non-counting pixels always appear at the same position and shrink with increased I Krum values, non-counting single pixels randomly show up at different positions during image acquisition and their number rises with an increased photon flux. Additionally, we demonstrate that the critical flux varies among the pixels, leading to large-scale image inhomogeneities at high photon fluxes. © 2012 IOP Publishing Ltd and Sissa Medialab srl.