The XENONnT neutron veto: design, construction and performance

Many astrophysical and cosmological observations point to the presence of Dark Matter (DM) in the universe. Among the potential DM candidates, WIMPs emerge from theories beyond the SM. The XENON Dark Matter Project is one of the leading experiments for DM direct detection, employing a dual-phase xenon TPC. The current phase of the project, XENONnT, features a TPC with a sensitive liquid xenon mass of 5.9 t surrounded by a Gadolinium-loaded water Cherenkov Neutron Veto (NV). A critical challenge in DM detection is distinguishing WIMP-induced NR from the backgrounds. In particular, neutrons emitted by detector materials can mimic WIMP signals, posing a significant threat to the sensitivity to DM. The NV plays a crucial role in mitigating this background by tagging the escaping neutrons through their delayed capture in water. The NV surrounds the TPC, with highly reflective panels and is instrumented with 120 PMTs. In the first two science runs the NV operated with pure water. In this setup, the NV achieved a neutron tagging efficiency of 53%, contributing significantly to the first WIMP analysis in the first XENONnT science run. A blind analysis over a total exposure of 1.1 tonne-year revealed no significant excess over the expected background, establishing the lowest upper limit to the WIMP-nucleon spin-independent cross-section of 2.58×10−47 cm2 for WIMPs of 28 GeV/c2 mass at a 90% confidence level.