Alignment procedure of silicon pixel detectors for ion-beam therapy applications

In ion-beam therapy, the elastic and inelastic interactions of the primary beam interacting with different mediums are of strong interest. Cross section and scattering measurements are important in order to provide accurate basic data for the treatment planning system. Using several planes of a high spatial resolution vertex detector based on monolithic CMOS pixel sensor technology is a common concept for measuring precise trajectories of charged particles before and after interactions. To reach high spatial resolution, the alignment of the sensors is mandatory. In this work, an alignment procedure based on a global ${\chi }^2$ cost function, that simultaneously optimizes the alignment parameters for all events is presented and shows accurate results which allows the detectors to reach high spatial resolution. The procedure was benchmarked with simulated and experimental data for the Mimosa28 pixel detector and compared to a state of the art alignment algorithm. The results of the alignment were in agreement with the requirements needed for precise measurements in particle therapy. The spatial resolution reached after the alignment was better than $10\mu m$.