A Geant4 simulation of not irradiated and irradiated pixel detectors

This note presents a numerical simulation of the response of irradiated and not irradiated silicon pixel detectors to ionizing particles. The simulation was developed as a stand-alone Geant4 application with the goal of understanding the response of a detector using a detailed description of the charge drift and signal induction processes. The radiation damage is simulated through the modifications it induces on the thickness of the active region, the electric field strength and distribution inside the sensor, and the charge trapping probability. This approach is different from those used in the simulations developed in the more general framework of the ATLAS detector simulation, which is subjected to severe CPU and memory usage constraints and can not afford the detailed description of charge drift and signal induction processes described here. The simulation strategy is described in detail. The simulated detector response before and after radiation damage is presented and it is compared with data taken at the test beam.