Upgrade of the Belle II Vertex Detector with Depleted Monolithic Active Pixel Sensors

The Belle II experiment, operating at the SuperKEKBe+e- collider, is developing a new pixelated vertex detector (VTX) to handle an higher luminosity, up to 6 × 1035cm-2s-1. The VTX will feature 5–6 layers, with a material budget under 3% ofX 0, and will utilize depleted monolithic active pixel sensors. The chip developed for the upgrade of the vertex detector, OBELIX, is based on the TJ-Monopix2 sensor which uses Tower Semiconductor 180 nm CMOS process. OBELIX sensor offers a 33 μm pitch, 50 ns time-stamping, and a digital trigger logic that matches Belle II's 30 kHz trigger rate. The sensor is designed to withstand radiation levels up to 5 × 1014neq/cm2and 1 MGy while maintaining low power dissipation (200 mW/cm2). This paper outlines the motivation for the upgrade and the VTX concept, and presents characterisation results and beam test campaigns on the TJ-Monopix2 sensor, including both non-irradiated and irradiated samples, which are crucial for validating key performance characteristics for the OBELIX design.