Switcher and DCD ASIC irradiation studies for the Pixel Detector of the Belle II experiment

Sumitted to PubDB: 2026-05-01

Category: Master Thesis

Tags: PXD

Principal Authors Nikolas Paessler
Date 2025-08-29
Belle II Number BELLE2-MTHESIS-2026-014
Abstract Due to the significance of the SM, numerous experiments worldwide aim to test its predictions and explore its characteristics. One such characteristic is the violation of CP symmetry in B-meson decays. CP symmetry refers to the combined conservation of charge conjugation (C), which transforms particles into their antiparticles, and parity (P), which inverts spatial coordinates. Within the SM, CP violation is allowed through the presence of a complex phase in the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which governs the mixing and decay of quarks. To validate this phenomenon, the Belle experiment at the KEKB asymmetric-energy e+ e- collider was designed and successfully confirmed CP violation in a B-meson decay in 2001. This achievement was recognized with the awarding of the Nobel Prize in Physics to Cabibbo, Kobayashi, and Maskawa in 2008 for their theoretical prediction of the mechanism behind CP violation. After operating KEKB for 11 years, from 1999 to 2010, and reaching all of its technical milestones, an upgrade was undertaken to enable further studies. The new SuperKEKB collider offers a 40-fold increase in luminosity compared to its predecessor. With this increased luminosity, a much larger data sample will be available, unlocking new possibilities for measurements in heavy flavour particle physics. These studies are expected to become a crucial and unique resource for investigating new physics processes, providing complementary insights to the discoveries anticipated at future hadron colliders. To address the significantly increased background levels in this more challenging environment, the Belle detector was upgraded to Belle II. The new design largely follows the architecture of Belle with the addition of a PiXel Detector (PXD) as the innermost layer. Belle II is a multi-layer cylindrical detector system surrounding the SuperKEKB interaction point. The pixel matrix of the PXD is based on DEpleted P-channel Field Effect Transistor (DEPFET) technology and is read out and controlled by three types of Application-Specific Integrated Circuit (ASIC). As the innermost subdetector of Belle II, the PXD is exposed to elevated radiation doses. During operation, an increase in dark-current noise was observed due to the accumulation of ionizing radiation. This leads to a degradation of the signal-to-noise ratio and therefore the performance of the PXD. The hypothesis arose that this noise increase might originate from irradiation damage in the readout and control ASICs. Due to the lack of dedicated studies in this field, irradiation campaigns were conducted within the scope of this thesis to investigate the ASICs as a possible source of the observed noise increase. For this purpose, the Drain Current Digitizer (DCD) and the Switcher were individually irradiated using an X-ray tube. The DCD reads the pixel matrix and converts the drain currents into digital values, while the Switcher controls the voltages required for the readout of the DEPFET
Institute Bonn

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