Vectors and submicron precision: redundancy and 3D stacking in silicon pixel detectors

Measurements are shown of GeV pions and muons in two 300 mu m thick, Si Medipix pixel detector assemblies that are stacked on top of each other, with a 25 mu m thick brass foil in between. In such a radiation imaging semiconductor matrix with a large number of pixels along the particle trail, one can determine local space vectors for the particle trajectory instead of points. This improves pattern recognition and track reconstruction, especially in a crowded environment. Stacking of sensor planes is essential for resolving directional ambiguities. Signal charge sharing can be employed for measuring positions with submicron precision. In the measurements one notices accompanying `delta' electrons that emerge outside the particle trail, far beyond the boundaries of the 55 mu m pixel cells. The frequency of such corrupted position measurements is similar to one per 2.5mm of traversed Si.