Radon-Induced Lead Isotopes in XENONnT: Studies on Branching Ratios and Surface Background

The XENONnT experiment, a dual-phase liquid Xenon time projection chamber (TPC), hosted in the Laboratori Nazionali del Gran Sasso underground facilities, is designed to achieve unprecedented sensitivity in the direct detection of WIMP dark matter. As the sensitivity of liquid Xenon detectors improves, background contributions from intrinsic radioactive contaminants become increasingly significant. Among these, Radon-induced Lead isotopes Pb-212, Pb-214 and Pb-210 pose a major challenge, affecting both low-energy electron recoil searches and WIMP detection. These isotopes originate from Radon outgassing and Radon progeny plate-out on detector surfaces, constituting bulk and surface backgrounds through beta decays and subsequent radiation emissions. A precise characterization of their decay properties, deposition mechanisms and impact on the experimental signal region is essential for optimizing background rejection techniques and improving sensitivity to rare-events and Beyond Standard Model physics searches. Dedicated measurements of the beta decay branching ratios of Pb-212 and Pb-214 have been performed using XENONnT data, with the aim of significantly reducing systematic uncertainties for future low-energy rare-events electron-recoil searches. The improved branching ratios measurements enhance the ability to constrain Pb-212 and Pb-214 induced backgrounds in the electron recoil low-energy region, particularly relevant for searches targeting Solar-pp neutrinos and other Beyond Standard Model studies. Furthermore, the refined measurements of Pb-212 and Pb-214 ground state branching ratios contribute to the broader field of low-background physics, informing Monte Carlo simulations and improving the accuracy of background models used in rare-event searches. In addition to bulk Lead contamination, the Pb-210 isotope represents a critical background component due to its long half-life (of about 22 y) and its accumulation on detector Teflon surfaces. A novel six-dimensional physics-driven model has been developed to describe the Pb-210 surface background in XENONnT, incorporating charge loss effects, refined spatial events distributions and energy deposition simulation. This model, validated against XENONnT science runs SR0 and SR1 data, provides a refined estimate of the Pb-210 wall background activity, estimated to be 19.0(10) Bq/m2 . The improved understanding of the surface background enables the definition of more precise fiducial volumes in future science runs and potentially improve WIMP searches, by gaining exposure. The impact of these studies extends beyond XENONnT physics goals, providing critical insights for future noble liquid TPCs, such as XLZD, both for refinement of sensitivity studies and for improvements in the materials handling and cleaning procedures. Indeed, on the one hand, the methodology developed for Pb-212 and Pb-214 Lead isotope characterization offers a framework for assessing and mitigating similar backgrounds, while the direct measurements of their ground state branching ratios provides improved constraints for future rare-events analysis. On the other hand, the advancements obtained in the TPC surface background analysis once more validate the Radon progeny plate-out origins of this background and reveal the importance of the human-factor in the enhancement of its rate. Indeed, as evident by the study results, given the non-uniform spatial distribution observed in the data, the explanation for the modulation of the surface background rate can be related to cleaning and handling procedures of Teflon materials. By advancing the precision of Radon-induced background characterization and surface event modeling, this Ph.D Thesis represents a major step forward in the optimization of liquid Xenon experiments for fundamental physics searches. The results presented here contribute directly to the improvements of electron recoil backgrounds constraints and optimization of data selection, pushing the discovery potential of both rare-events and direct WIMP dark matter detection, closer to the ultimate sensitivity limit imposed by the neutrino floor.