Characterization of irradiated monolithic CMOS active pixel sensors for the upgrade of the Belle II Vertex Detector

Sumitted to PubDB: 2025-11-03

Category: Master Thesis

Tags: Upgrade

Principal Authors Guglielmo Francesco Benfratello, Francesco Forti, Giuliana Rizzo
Date 2025-10-22
Belle II Number BELLE2-MTHESIS-2025-023
Abstract Belle II is a particle physics experiment, operated at the SuperKEKB B-Factory. The experiment studies $e^+e^-$ collisions at the $\Upsilon(4S)$ resonance. The ambitious physics and luminosity goals of Belle II and SuperKEKB require an upgrade of the interaction region and of the vertex detector. The proposal chosen for the upgrade of this detector is the VerTeX Detector (VTX), which is a new vertex detector with six layers of a new pixelated sensor, called OBELIX, based on the CMOS Depleted Monolithic Active Pixel Sensor (DMAPS) technology to replace the VXD. This new detector is being designed to have higher granularity in both space and time, which gives greater robustness against beam-induced backgrounds and also improves tracking efficiency and resolution, especially at low momentum. This thesis presents the characterization of the precursor of OBELIX, the TJ-Monopix2 sensor, focusing on its performance after irradiation. Laboratory and beam tests to study the efficiency as a function of threshold and temperature have been conducted on four devices irradiated with fluences from $1\times 10^{14}$ $1-MeV-n_{eq}/cm^2$ to $5\times10^{14}$ $1-MeV-n_{eq}/cm^2$, which describe the full range of the radiation damage expected during the 10-year operation of VTX. The characterization campaign has been performed both with laboratory and beam tests, in the INFN-Pisa laboratory and the DESY Test Beam facility. The sensors have been characterized by studying the dependence of their threshold, noise and detection efficiency on the operating temperature, which has been helpful for the design of the VTX detector. The comparison between the DC- and AC- coupled front-ends of TJ-Monopix2 has also been done by studying their detection efficiency at increasing threshold, prompting the choice of the former as the final front-end for the OBELIX sensor. Finally, a cross-talk effect of the clock counter on the pixel threshold has been extensively studied, prompting its mitigation in OBELIX.
Institute Pisa

Files