Simulations of charge summing and threshold dispersion effects in Medipix3

A novel feature of the Medipix3 photon-counting pixel readout chip is inter-pixel communication. By summing together the signals from neighbouring pixels at a series of ``summing nodes{''}, and assigning each hit to the node with the highest signal, the chip can compensate for charge-sharing effects. However, previous experimental tests have demonstrated that the node-to-node variation in the detector's response is very large. Using computer simulations, it is shown that this variation is due to threshold dispersion, which results in many hits being assigned to whichever summing node in the vicinity has the lowest threshold level. A reduction in threshold variation would attenuate but not solve this issue. A new charge summing and hit assignment process is proposed, where the signals in individual pixels are used to determine the hit location, and then signals from neighbouring pixels are summed to determine whether the total photon energy is above threshold. In simulation, this new mode accurately assigns each hit to the pixel with the highest pulse height without any losses or double counting. (C) 2011 Elsevier B.V. All rights reserved.