3D simulation of induced signals in the Medipix detector

The charge distribution generated by an interacting X-ray photon in the sensorlayer of an direct converting photon counting detector (as the Medipix) is transported to the pixelated electrodes and integrated during a short time. If the collected charge exceeds a threshold, a counter is increased. The generated charge will diffuse into the neighbouring pixels as the distance from the X-ray photon interaction point gets closer to a pixel edge. But even if all the charge can be projected onto a single pixel electrode, the neighbouring pixel electrodes will measure an induced signal due to the moving charge in the sensor. For being able to add these induced signals into our simulation of the whole detector device, an exact knowledge of the induced signals is necessary. Ramos formulation provides the required formula and so it is possible to calculate the induced signals for a moving charge cloud. A fully 3D simulation was performed with the finite element program COMSOL. To get a good sampling of the detector and to save computing time, the adjoint equation was solved. The calculations were carried out for better understanding of the physical processes and the imaging properties of Medipix2 and can be projected to the development for the Medipix3.