Modeling Charge Collection in Silicon Pixel Detectors for Proton Therapy Applications.

Monolithic active pixel sensors are used for charged particle tracking in many applications, from medical physics to astrophysics. The Bergen pCT collaboration designed a sampling calorimeter for proton computed tomography, based entirely on the ALICE PIxel DEtector (ALPIDE). The same telescope can be used for in-situ range verification in particle therapy. An accurate charge diffusion model is required to convert the deposited energy from Monte Carlo simulations to a cluster of pixels, and to estimate the deposited energy, given an experimentally observed cluster.We optimize the parameters of different charge diffusion models to experimental data for both proton computed tomography and proton range verification, collected at the Danish Centre for Particle Therapy. We then evaluate the performance of downstream tasks to investigate the impact of charge diffusion modeling.We find that it is beneficial to optimize application-specific models, with a power law working best for proton computed tomography, and a model based on a 2D Cauchy-Lorentz distribution giving better agreement for range verification. We further highlight the importance of evaluating the downstream tasks with multiple approaches to obtain a range of expected performance metrics for the application.This work demonstrates the influence of the charge diffusion model on downstream tasks, and recommends a new model for proton range verification with an ALPIDE-based pixel telescope.