shuman results2 figs

Diode Signal Distributions, Preliminary Results from Diffusion Simulation

Howard Wieman, Derek Shuman

This exercise explores diode signal distributions in our standard APS geometry as a function of X-ray conversion location. One can be convinced that solving the steady state Laplace's equation for diffusion with appropriate boundary conditions will give the final average charge on the diodes. Two questions can be addressed with this effort. 1. Does the depth of the conversion significantly affect signal distribution, i.e. can we use the distribution to measure depth and identify signals coming from the substrate? 2. How much improvement in signal to noise can we expect for signals collected on large area photo-gates versus standard small diodes?

This first pass calculation was done by Derek Shuman using the thermal part of the ANSYS finite element code. The geometry used is shown in the following figure. There were 9 diodes and the calculations were repeated using 16 separate electron release points as indicted in the figure by the red balls. The green squares represent the depletion regions around the diodes.

The diode signal distributions over the 9 diodes are shown both graphically and numerically in the following. The accuracy of this first pass is limited by the small number of diodes used, but the results seem more or less reasonable. We can already conclude that once the source location is slightly removed from the diode, the distribution is not significantly affected by the depth of the electron release point. The next calculation will be done with a matrix of either 25 or 49 diodes. Following that we will repeat the calculations using the photo-gate geometry.