At each side of the drift cathode array, p+ implants (guard strips) grade the potential from the highest negative voltages to grounded outer n+ implant ring. Since this region should be as small as possible the electric field needs a careful evaluation. Furthermore, as the voltage difference between two consecutive strips is 16V (see detector description), the punch-through phenomenon should be carefully evaluated (this is presented in the "Surface effects" paragraph).
Building a structure, able to simulate the electric parameters as they are in the real working conditions, is not easy. Using nine cathodes, as for the drift region, we obtain a cell length of only 0.4mm that is too small compared with the device depth. In this case, the solution in the region of interest would be affected by the calculation in the boundary region. On the other hand we can not increase the number of cathodes because of the limited amount of memory. To overcome this problem we realised a cell having lateral edges approximatively shaped as the potential lines foreseen for the real device (see figure below). In this way the almost correct solution at the edges of the structure will lead to a correct result in the region of interest. The metal covering the right edge is the anodic contact, and its potential is 30V higher than the potential of the first couple of cathodes (this is the expected depth of the potential gutter between two opposite cathodes).