Pixel sensitivity variations in a CdTe-Medipix2 detector using poly-energetic x-rays

We have a 1-mm-thick cadmium telluride (CdTe) sensor bump-bonded to a Medipix2 readout chip. This detector has been characterized using a poly-energetic x-ray beam. Open beam images (i.e. without an attenuating specimen between the x-ray source and the detector) have been acquired at room temperature using the MARS-CT system. Profiles of various rows and columns were analyzed for one hundred, 35-ms exposures taken with a bias voltage of -300 V (operating in electron collection mode). A region of increased sensitivity is observed around the edges of the detector. A reasonably periodic, repeatable variation in pixel sensitivity is observed. Some small regions with very low sensitivity and others with zero signals are also observed. Surrounding these regions are circular rings of pixels with higher counts. At higher flux (higher tube current in the x-ray source) there is evidence of saturation of the detector assembly. In this paper we present our understanding of the origin of these features and demonstrate the improved image quality obtained after correcting for these variations.