When working at a field of 600V/cm, the drift speed of the charge cloud will be about 8 um/ns with a maximal drift time of about 4.3us. The drift cathodes have a pitch of 120um and are 70um wide, while the cathode metalization exceeds the p+ implant by 5um at both edges, to form a symmetrical field plate. The guard cathodes have a pitch of 32 um. There is one guard every two drift cathodes, so at a given bias the voltage difference between adjacent guards is twice that between adjacent drift cathodes. There are separate integrated high-voltage dividers to bias independently the drift and guard cathodes. Thereby. possible distortions of the potential distribution on the guard cathodes, due to breakdown or punch-through, will not be directly transferred to the drift region.
The drifting charge is collected by 256 anodes with a pitch of 294um. The figure below shows a close up on the collection zone. It consists of eight cathodes biased independently from the high voltage divider. The first, around the anodes, has been called "grid" cathode, and has the function of separating the anodes. Meanwhile three "pull-up" cathodes opposite to the anodes force the drifting charge towards the anode array. Four outermost cathodes (on both faces of the detector) serve to ensure electrical separation of anodes from the n+ ring at the edge of the detector. The design of this complex region has been carried out following the criteria reported in the paragraph "simulations of the collection zone".
