ETD

• characterization, simulation,pixel sensors

The Belle II experiment at KEK, Japan, investigates electron–positron collisions to perform precision measurements of flavor physics and search for phenomena beyond the Standard Model. To meet the requirements of the future high-luminosity phase, the current vertex detector will be replaced by the VerTeX Detector (VTX), based on radiation-hard CMOS pixel sensors. This thesis presents the characterization and Technology CAD simulation of TJ-Monopix2, the direct predecessor of the OBELIX sensor developed for the Belle II VTX. The work combines laboratory measurements, test beam studies, and Synopsys Sentaurus simulations of irradiated sensors exposed to fluences up to 5 × 10¹⁴ 1-MeV-neq/cm².
Laboratory measurements showed that irradiation-induced leakage current strongly affects threshold and noise performance, particularly at high temperatures. Test beam measurements performed at DESY in 2025 demonstrated that the DCC front-end architecture significantly outperforms the HVC design after irradiation, maintaining hit efficiencies above 99% under the most demanding operating conditions. These results led to the selection of the DCC architecture for the OBELIX sensor and established a target operating temperature of 30 °C for the VTX.
A charge calibration procedure was developed to reconstruct collected charge from Time-over-Threshold measurements, revealing the spatial dependence of charge collection within the pixel cell. TCAD simulations reproduced the observed trends in leakage current, charge collection, and charge sharing, providing further insight into the sensor behavior after irradiation. A simplified front-end model was also developed to qualitatively assess the effect of leakage current and signal shaping on charge measurement.The results of this work provide key inputs for the design and operation of the Belle II VTX and support the ongoing development of the OBELIX sensor.