The Jiangmen Underground Neutrino Observatory (JUNO) is a next generation 20 kt liquid scintillator (LS) neutrino detector in China, which has completed construction and has been taking data since August 2025. Located 650 m underground and 52. 5 km from both the Taishan and Yangjiang nuclear power plants, its primary goal is to determine the neutrino mass ordering, one of the most forefront open questions in neutrino physics. This is achieved by measuring the coincidence signals of inverse beta decay (IBD) events from reactor antineutrinos. With its large target mass and excellent energy resolution (2. 95\% at 1 MeV), JUNO is expected to reach a 3 sensitivity to the NMO within 6. 5 years of data taking. Besides, JUNO is also able to explore other important topics in neutrino and astroparticle physics, including the precise measurement of oscillation parameters, and the detection of solar, geo-, atmospheric, and supernova neutrinos. The radiopurity of the LS is critical for JUNO's success, with requirements of 10^-16 g/g for ^238U and ^232Th in IBD-related measurements. To ensure these levels, the Online Scintillator Internal Radioactivity Investigation System (OSIRIS) was designed and built as a pre-detector to monitor LS radiopurity. It can tag the Bi-Po coincidences in the corresponding U/Th decay chains. OSIRIS consists of an inner detector (ID) equipped with 64 PMTs observing 18 tons of LS contained in an acrylic vessel, and an outer detector (OD) with 12 PMTs serving as a water Cherenkov veto detector. The OSIRIS Event Builder (EB) software, as a core part of the DAQ system, was developed to read, sort, trigger, and package the raw data into built events according to user-defined trigger conditions in different trigger modes. The EB was optimized for better performance. Trigger studies in the iPMT scenario were carried out using a workflow that combined OSIRIS Geant4 simulation, DAQ simulation, and the EB. Simulated data of ^14C decay electrons and cosmogenic muons were used to determine the optimal trigger conditions for the ID and OD, respectively, in coincidence mode, where a certain hit multiplicity must be reached within a certain time window. The optimized trigger conditions are (70 ns, 5) for the ID and (150 ns, 5) for the OD. The pre-time and post-time window lengths were also determined to capture prompt signals from the 85Kr minor decay branch and afterpulses of the PMTs. Several aspects of on-site work are also presented, including the setup of a testing chain for OSIRIS LPMT electronics, environmental control of the electronics cabin, and preparation of the Automatic Calibration Unit (ACU) hardware. The fitting model for the ACU multi-gamma source (^137Cs, ^65Zn, and ^60Co) was validated with large samples of simulated calibration data. The first ACU LED calibration commissioning air-run data was analyzed in depth. A method was developed to filter out noisy and flat waveforms, as well as to exclude multi-photoelectron waveforms. Charge reconstruction and timing alignment were performed for the selected waveforms of all the inner PMTs with different LED positions.
Runxuan Liu (Wed,) studied this question.