%0 Thesis
%A Vollbrecht, Moritz Cornelius
%T Software and hardware development for the liquid scintillator detector OSIRIS of the JUNO experiment
%I RWTH
%V Dissertation
%M GSI-2025-01092
%P 355 p.
%D 2024
%Z Dissertation, RWTH, 2024
%X The Jiagmen Underground Neutrino Observatory (JUNO) is a next-generation liquid scintillator (LS) neutrino experiment currently under construction near Kaiping, Southern China. The construction of JUNO will end in 2024. JUNO will accomplish its main goal with six years of data-taking: the determination of the neutrino mass ordering with a sensitivity of 3-4 σ. To achieve this goal, the LS used in JUNO must meet stringent requirements regarding its radiopurity. For validation of these requirements during the months-long filling of JUNO, the Online Scintillator Internal Radioactivity System (OSIRIS) monitors the LS radiopurity via <sup>214</sup>Bi-<sup>214</sup>Po / <sup>212</sup>Bi-<sup>212</sup>Po coincident decays in the respective decay chains of <sup>238</sup>U and <sup>232</sup>Th. In OSIRIS, German workgroups have a leading role. This thesis presents contributions to the development of both hardware and software for OSIRIS. The hardware-focused part of the thesis details the author's construction of an ultrapure-water-based cleaning facility at RWTH Aachen as well as the design of a holder structure for OSIRIS's photomultiplier tubes (PMTs). The cleaning facility was used successfully to prepare many German working groups' contributions prior to their shipment to China. The holder development was aided by simulations based on finite element analysis. Both holder design requirements and simulation results were confirmed in load tests of the holder structure, first at RWTH and later again on-site at JUNO in 2022. The holder as originally planned for the intelligent PMT system (iPMT) had to be adapted at JUNO to accommodate the Chinese Large PMT system (LPMT), because the iPMTs were found to be non-functional upon arrival at JUNO. During the on-site mission in 2022, also studies of airborne and surface dust levels within the OSIRIS detector were carried out by the author. The investigations proved that a) OSIRIS's air filtration unit maintained the required ISO-7 clean room environment and b) the accumulated dust on OSIRIS's central vessel was uncritical for the experiment's future. The measurements also showed a substantial decrease in surface particle counts after a subsequent detector cleaning. The commissioning of one of OSIRIS's calibration systems, the Automated Calibration System (ACU), was supervised by the author in 2023 as another part of the hardware-related work described here. The ACU contains a radioactive multi-gamma source (<sup>137</sup>Cs/<sup>65</sup>Zn/<sup>60</sup>Co) and a 435 nm LED to perform calibrations of LPMT timing and charge as well as calibrations of detector energy and vertex reconstructions. A <sup>40</sup>K-source serves as a standard candle during normal operation and is used to monitor the optical properties of the LS. During the ACU commissioning, the ACU hardware and software, the related safety mechanisms, and the positioning of the sources were evaluated. In the positioning calibrations, a precision of  ≈ 4 mm was achieved, which is well below the sub-cm requirement of OSIRIS. Overall, the results of the commissioning successfully prepared the ACU for nominal operation in OSIRIS. The calibration of OSIRIS with the ACU was also the key aspect of the software-focused part of the thesis. The presented GEANT4-based energy calibration studies improved the analysis algorithms in terms of stability and speed, while also determining a minimal statistic of 30k events needed for each of the involved radioactive isotopes to reach required precision levels. The analyses of different source capsule designs validated their usability in OSIRIS and led to an approval for the calibration source production in late 2022. Also a first set of ACU calibration data, recorded with the LED of the ACU, was analysed by the author. The analysis focused on evaluating the overall detector performance and identified irregularities for PMTs close to the LED cable, promptly triggering investigations by the OSIRIS group. The analysis also confirmed both the principle of aligning the PMT timings and the expected detector behaviour for different illumination levels.
%K Hochschulschrift (Other)
%K liquid scintillator detectors ; neutrino physics ; hardware development ; software development ; particle physics; Flüssigszintintillatordetektoren ; Neutrinophysik ; Hardwareentwicklung ; Softwareentwicklung ; Teilchenphysik ; JUNO ; OSIRIS (Other)
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2025-02894
%U https://repository.gsi.de/record/362228