PoS - Proceedings of Science
Volume 448 - The 32nd International Workshop on Vertex Detectors (VERTEX2023) - Operational experience on current detectors
The Belle II Pixel Vertex Detector
A. Baur*, P. Ahlburg, L. Andricek, R. Ayad, V. Babu, F. Becherer, F. Bernlochner, L. Bierwirth, J. Bilk, T. Bilka, A. Bolz, A. Bozek, C. Camien, L. Cao, R. Dhayal, J. Dingfelder, Z. Doležal, R. Farkas, A. Frey, K. Gadow, G. Giakoustidis, M. Graf-Schreiber, D. Greenwald, Z. Gruberova, Y. Han, M. Hoek, S. Huber, P. Kapusta, R. Karl, J. Kehl, M. Khan, C. Kiesling, B. Kisielewski, P. Kodyš, C. Koffmane, I. Konorov, M. Krein, W. Kühn, H. Krüger, P. Kvasnicka, J.S. Lange, P. Leitl, D. Levit, Q. Liu, Z. Liu, T. Lück, C. Mariñas, D. Meleshko, H.G. Moser, C. Niebuhr, J. Ninkovic, B. Paschen, S. Paul, I. Peric, D. Pitzl, A. Rabusov, S.P. Reiter, R. Richter, M. Ritzert, J.G. Sanchez, J. Schmitz, B. Schwenker, M. Schwickardi, C. Sfienti, F. Simon, J. Skorupa, Y. Soloviev, B. Spruck, S. Stefkova, R. Stever, M. Takahashi, I. Vila, A.L. Virto, B.S. Wang, C. Wang, N. Wermes, J. Zhao and R. Žlebčíket al. (click to show)
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The Belle II experiment at the SuperKEKB $e^{+}e^{-}$ collider took data from 2019 to 2022 and is currently undergoing its planned first long shutdown (LS1). During its operational period, SuperKEKB achieved a record-breaking instantaneous luminosity of 4.7 x 10$^{34}$ cm$^{-2}$s$^{-1}$ and Belle II recorded a dataset corresponding to 428 fb$^{-1}$.
The Belle II Pixel Vertex Detector (PXD), which is the innermost sub-detector, is based on the depleted P-channel field effect transistor (DEPFET) technology. Along with the outer Silicon Vertex Detector (SVD), utilizing double-sided silicon strips, this forms the Belle II Vertex Detector system (VXD), enabling precise reconstruction of primary and decay vertices.
The DEPFET technology allows a $75\;\mu$m-thinned sensitive detector area with a minimum pixel pitch of 50 $\mu$m x 55 $\mu$m, while maintaining a high pixel hit efficiency of about 99 %.
Including module interconnects and ASICs integrated on the sensor with silicon frames for mechanical support, this results in a low material budget of 0.21 % of the radiation length (X$_{0}$) per layer, which reduces the effect of multiple scattering.
The previous single layer version of the PXD, installed until LS1, comprised 19 functional modules.
In this article, we will illuminate the performance and operational challenges observed during its 4-year operation in a harsh environment characterized by a high beam background level.
A fully populated detector can compensate acceptance losses by redundancy and reduce the probability of wrong hit assignment introduced by higher background levels resulting from increased instantaneous luminosity.
One of the main projects during LS1 was replacing the installed PXD1 by the fully populated PXD2, which consists of 40 modules arranged in 2 layers around the beam axis. Its commissioning and testing phase during LS1 will be described. A hit efficiency of $>$ 98 % in most of the regions was measured using cosmic particles. The current installation schedule foresees a resuming of the beam operation in winter 23/24.
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