3D Monte Carlo simulations of pixelated CdZnTe detectors under high photon fluxes

In this paper a direct approach for 3D simulation of pulse waveforms induced on CdZnTe detector with 11×11 pixelated anodes is presented. In high flux applications the signal induction from the drift of charge carriers and the variation of space charge can no longer be decoupled. Calculation of induced pulse waveforms on electrodes can be achieved by determining the charge induction from the Shockley-Ramo theorem and electron and hole transport from the continuity equations coupled with Poisson´s equation. A complete 3D simulation code has been developed coupling Monte Carlo modeling of photon interactions with matter, electric field and weighting potential using the finite element method (FEM), electric field perturbations due to space charge carriers using the method of images (or mirror images), and electron and hole transport taking into account diffusion and trapping of both charge carriers. The entire calculation is carried out for a number of interaction positions in the detector in each time step, thus providing the overall field due to both space charge accumulation and external applied bias. A large number of finite cubes (136,803,300) within the detector volume of 15×15×10mm³ have been considered in these simulations under varying photon fluxes. Experimental studies using a Cs-137 irradiator and a digital waveform acquisition system in continuous mode will provide verification of simulation results.