The XENONnT Neutron Veto: Performance in the Gd-doped phase

Many observations point to postulate that Dark Matter (DM) constitutes about 85% of the universe’s total mass. DM still remains undetected, with Weakly Interacting Massive Particles (WIMPs) being one of the primary DM candidates. The XENONnT experiment, located at the underground Gran Sasso laboratory, aims to detect WIMPs using a dual-phase TPC with a 5.9 tonnes active liquid Xenon target. XENONnT is equipped also with a Muon and a Neutron Veto (NV) water Cherenkov detectors to reduce backgrounds. Since 2021, XENONnT has performed three Scientific Runs. In the first two runs, the NV was operated with demineralized water, while in the current run, starting in October 2023, water was doped with 500 ppm of Gd sulphate (GdSO), to improve the neutron capture efficiency. The NV performance, in particular in the Gd-doped phase, is presented in this thesis. XENONnT uses a comprehensive Montecarlo (MC) simulation framework for detailed simulations of each subdetector. A Hitlet Simulator is specifically developed for the NV and allows to extend the simulation chain up to the photomultipliers' response, by generating simulated signals with charge and timing that closely mirror real data. The NV response to neutron captures is calibrated using an Americium Beryllium (AmBe) neutron source. Experimental data from AmBe calibration demonstrate a significant reduction in the average neutron capture time from 163 µs (pure water) to 75 µs (Gd-doped water). The neutron tagging efficiency of the NV increased from 53% with water to 77% with Gd water. MC simulations of AmBe calibration agree well with real data, thus confirming these findings. Overall, the introduction of Gd significantly enhanced the NV system’s ability to suppress neutron-induced backgrounds by a further factor 2 with respect to the previous Science Runs with demineralized water. Based on MC simulations, a neutron tagging efficiency of 85% will be achieved with the final 5000 ppm GdSO concentration.