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

¹ Institute of High Energy Physics, Austrian Academy of Sciences, Nikolsdorfer Gasse 18, 1050 Vienna, Austria
² Department of Engineering and Applied Sciences, University of Bergamo, Viale Marconi 5, I-24044 Dalmine (BG), Italy
³ Department of Electrical, Computer and Biomedical Engineering, University of Pavia, Via Ferrata 5, I-27100 Pavia, Italy
⁴ Dipartimento di Fisica “E. Fermi”, Universita‘ di Pisa, L.go B. Pontecorvo 3, I-56127 Pisa, Italy
⁵ INFN Sezione di Pisa, L.go B. Pontecorvo 3, I-56127 Pisa, Italy
⁶ Aix Marseille Univ, CNRS/IN2P3, CPPM, Marseille, France
⁷ Laboratoire de Physique des 2 infinis Irène Joliot-Curie – IJCLab, Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France
⁸ Université de Strasbourg, CNRS, IPHC UMR 7178, F-67000 Strasbourg, France
⁹ Physikalisches Institut, Rheinische Friedrich-Wilhelms-Universität Universität Bonn, Nussallee 12, 53115 Bonn, Germany
¹⁰ University of Applied Sciences and Arts Dortmund, Sonnenstraße 96-100, 44139 Dortmund, Germany
¹¹ II. Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
¹² High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan
¹³ The Graduate University for Advanced Studies (SOKENDAI), Hayama 240-0193, Japan
¹⁴ Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa-no-ha 5-1-5, Kashiwa 277-8583, Japan
¹⁵ Department of Physics, University of Tokyo, Hongo 7-3-1, Tokyo 113-0033, Japan
¹⁶ Instituto de Fisica Corpuscular (IFIC), CSIC-UV, Catedratico Jose Beltran, 2. E-46980 Paterna, Spain
¹⁷ School of Physical and Chemical Sciences, Department of Physics and Astronomy, Queen Mary University of London, 327 Mile End Road, London, E1 4NS, United Kingdom
¹⁸ College of Physics, Jilin University, 2699 Qianjin Street, Changchun, Jilin, China
¹⁹ Universidade de Santiago de Compostela, 2010 Instituto Galego de Física de Altas Enerxías (IGFAE), Colexio de San Xerome, PZ Obradoiro, S/N. E-15782 Santiago de Compostela, Spain

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Published online: 6 November 2025

Abstract

The Belle II experiment at the SuperKEKB e⁺e⁻ collider is preparing for an upgrade of its vertex detector to cope with an increased luminosity of up to 6 × 10³⁵ cm⁻²s⁻¹. The new vertex detector (VTX) will consist of six layers of depleted monolithic active pixel sensors (DMAPS), with a total material budget of about 3% of X₀. The OBELIX chip, developed for this upgrade, is derived from the TJ-Monopix2 sensor and manufactured using the Tower Semiconductor 180 nm CMOS technology. It features a 33 µm pixel pitch, time-stamping capability with 50 ns resolution, and a dedicated digital periphery compatible with the Belle II trigger system, supporting rates up to 30 kHz. The sensor is designed to operate under the expected background hit rate at the target luminosity, with high radiation tolerance, up to 5 × 10¹⁴ n_eq/cm² and 1 MGy, while maintaining a power density in the range of 200-300 mW/cm², corresponding to hit rates from a few MHz/cm² up to 120 MHz/cm². This paper presents results from laboratory measurements and beam tests performed on TJ-Monopix2 chips, including both not-irradiated and irradiated devices. Particular focus is given to the performance of irradiated sensors as a function of temperature, a key aspect for defining the maximum allowable operating temperature for OBELIX. These studies provide essential input for the thermal design of the VTX cooling system, especially for the innermost layers where power density and hit rates are highest.