Characterization and Operation of Final DEPFET Pixel Detector Modules for the Belle II Experiment

Abstract

The Belle II experiment at the SuperKEKB super B-factory collects data from asymmetric-energy $e^+e^-$ collisions at the PUpsilonFourS resonance since 2019. Its goal is to collect a PB{}APB data set 50 times larger than that of the predecessor experiments Belle and Babar enabling new levels of accuracy in measurement of standard model processes and search for new physics at the luminosity/precision frontier. <br /> A vertex resolution of O(10)~unit{micrometre} is required for the precise discrimination of decay vertices and lifetime measurements of short-lived particles. This is made possible by the Belle II VerteX Detector (VXD). It consists of the ultra low-mass PiXel Detector (PXD) based on the DEpleted P-channel Field Effect Transistor (DEPFET) sensor technology surrounded by a double sided silicon strip detector (SVD). <br /> The PXD features self supported all-silicon modules with qty{75}{micrometre} thin sensor areas bringing down the material budget to an average of $sim0.21%~X_0$ per layer inside the physics acceptance. This constitutes the lowest mass tracking detector in a running high energy physics experiment to date. It features ladders of up to qty{17}{centimetre} length with pixel sizes of times55$ to times85$~unit{micrometresquared}. <br /> Results from the characterization and operation of final DEPFET PXD modules are presented in this thesis. The characterization results focus on the currents passing through the silicon bulk as well as the FET currents. Both parameters are important for successful operation of the detector. <br /> The bulk currents are investigated after X-ray irradiation of a module as functions of electrode voltages as well as voltage attached to the underside of the module. The results point to an effect of the charged-up buried oxide on the bulk currents, most likely by avalanche electron-amplification. <br /> The effect of the on-module electronics, production processes and sensor layout on the FET currents is investigated in detail, separating its effects and size of contributions. It is found that the lithography process of the sensor production and the channel-to-channel variation of the readout electronics cause relatively large variations in the FET currents. <br /> Finally, the operation of the first modules in the final Belle II environment is described. The modules successfully recorded data over a period of several months and demonstrated good performance under experiment conditions. From detector side this constituted the last step of a full-system verification before commissioning of the full vertex detector in 2018.