The Medipix3 Prototype, a Pixel Readout Chip Working in Single Photon Counting Mode with Improved Spectrometric Performance

A prototype pixel detector readout chip has been developed with a new front-end architecture aimed at eliminating the spectral distortion produced by charge diffusion in highly segmented semiconductor detectors. In the new architecture neighbouring pixels communicate with one another. Charges can be summed event-by-event and the incoming quantum can be assigned as a single hit to the pixel with the biggest charge deposit. In the case where incoming X-ray photons produce fluorescence - a particular issue in high-Z materials - the charge deposited by those fluorescent photons will be included in the charge sum provided that the deposition takes place within the volume of the pixels neighbouring the initial impact point. The chip is configurable such that either the dimensions of each detector pixel match those of one readout pixel or detector pixels are 4 times greater in area than the readout pixels. In the latter case event-by-event summing is still possible between the larger pixels. As well as this innovative analog front-end circuit, each pixel contains comparators, logic circuits and two 15-bit counters. When the larger detector pixels are used these counters can be configured to permit multiple thresholds in a pixel providing colour information. The prototype chip has been designed and manufactured in an 8-metal 0.13mum CMOS technology. First measurements show an electronic pixel noise of ~72 e- rms (Single Pixel Mode) and ~140 e- rms (Charge Summing Mode).