Study of monolithic CMOS pixel sensors in the Belle II experiment upgrade

Sumitted to PubDB: 2024-05-04

Category: Master Thesis, Visibility: Public

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Authors Mara Calo', Francesco Forti
Date 2024-01-01
Belle II Number BELLE2-MTHESIS-2024-016
Abstract Belle II is a particle physics experiment at the KEK laboratory in Tsukuba, Japan. Its detector is a general-purpose spectrometer designed to study electron-positron collisions produced by the SuperKEKB accelerator, the world’s highest-luminosity collider (Lpeak = 4.7 × 10³⁴ cm⁻²s⁻¹). SuperKEKB, the upgrade of KEKB (1998–2010), is a 3 km asymmetric collider operating near the Υ(4S) resonance (√s = 10.58 GeV) and began data-taking in March 2019. Over the next decade, it aims to reach a luminosity of 6 × 10³⁵ cm⁻²s⁻¹ and collect 50 ab⁻¹, enabling precise studies of charge-parity violation in B mesons and searches for physics beyond the Standard Model. Higher luminosities bring both larger datasets and harsher radiation and background conditions, especially affecting subdetectors near the beam pipe. The vertex detector (VXD), consisting of an inner pixel detector (PXD) and an outer silicon strip detector (SVD), plays a critical role in reconstructing charged particle tracks and decay vertices. Current studies suggest safe operation up to Linst = 2 × 10³⁵ cm⁻²s⁻¹, but margins are limited. Thus, an upgrade is planned, replacing the VXD with the proposed VerTeX Detector (VTX), built from five layers of fully pixelated sensors using CMOS Depleted Monolithic Active Pixel Sensor (DMAPS) technology. Proven in the ALICE experiment at CERN, this technology offers radiation tolerance and high performance, with ongoing developments to achieve faster readout and low occupancy under Belle II conditions. Within this framework, a new sensor named OBELIX is being developed using the 180 nm TowerJazz process. Designed to be faster, lighter, and more granular, it reduces material budget and improves tracking and vertexing. OBELIX builds on prototypes such as TJ-Monopix2, whose characterization is central to this thesis. Extensive laboratory and beam tests, including those at DESY (June 2022), evaluated efficiency before and after irradiation, power consumption, and electrical behavior. Key studies include pixel matrix response, threshold dispersion and noise, calibration of Time-Over-Threshold curves via internal injections and radioactive sources (e.g., ⁵⁵Fe), and optimization of register settings to sustain low thresholds crucial for post-irradiation efficiency. During testing, a cross-talk issue was identified and investigated, providing insights for mitigation. The results obtained have directly informed the OBELIX design, which is expected to be submitted for fabrication in the coming months.
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