Impact of Radiation Damage on Performance of Monolithic CMOS Pixel Sensor for Vertex Detector Upgrade at Belle II

Sumitted to PubDB: 2025-08-28

Category: Master Thesis

Tags: PXD SVD Upgrade

Principal Authors Shijie Wang
Date 2025-08-05
Belle II Number BELLE2-MTHESIS-2025-018
Abstract Belle II, a particle physics experiment operating at the B factory, SuperKEKB accelerator, explores physics beyond the Standard Model using large amount of $\Upsilon(4S)\to B\bar{B}$ events generated from $e^+e^-$ collisions. Due to the re-design of Interaction Region for further performance improvement of SuperKEKB, an upgrade project of the innermost vertex detector named VTX, is being studied. It is also expected to provide better background tolerance and physics performance. VTX makes use of Depleted Monolithic Pixel Sensor (DMAPS) technology in its sensor unit. The same matrix design with a DMAPS prototype, TJ-Monopix2, is used in the sensor development. Since the innermost vertex detector is exposed to a large amount of radiation during the operation of the Belle II experiment, it is necessary to evaluate the impact of radiation to the characteristics and performance of the TJ-Monopix2 prototype, in order to understand the potential performance degradation and characteristic change of VTX after long-term operation. In this work, a TJ-Monopix2 sensor is irradiated up to accumulated NIEL flux of $5.57\times 10^{14}~n_{\mathrm{eq}}\mathrm{cm^{-2}}$ corresponding to irradiation expectation at Belle II experiment using the $90~\mathrm{MeV}$ electron beam at RARiS (Research Center for Accelerator and Radioisotope Science), Tohoku University. Laboratory characterization and test beam campaigns at DESY II Test Beam facility and KEK PF-AR Test Beam Line were performed before and after irradiation, to check the performance degradation and whether the performance still meets the requirement at the Belle II experiment. The performance comparison between DC and AC front-end designs of TJ-Monopix2 also provides a reference for the future design of sensors which is being developed based on the TJ-Monopix2 design.

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