Enhancing the direct search for dark matter in the XENON nT experiment with a water Cerenkov neutron detector

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Three major neutron veto milestones were reached in FY2022:1.) The gadolinium water purification system (GdWPS) was first commissioned with pure water in closed loop circulation (CLC), separate from the 700 tonne water shield that contains also the neutron veto. We thus verified the basic functions of all its components.2.) Maintaining water quality in the full 700 tonne XENONnT water shield - which comprises the detector's muon and neutron veto - our GdWPS proved that it improved the quality of the shields pure water.3.) After returning to CLC operation we finally added gadolinium sulfate (GdS) to the ~3 tons of water in the closed loop, bringing it up to the ultimate target concentration of 0.5 % of gadolinium sulfate in January. We optimize operating parameters for coming neutron veto operation with GdS dissolved in the whole water shield.Together these three milestones mean that now the neutron veto systems are all ready to be loaded with GdS and thus reach our design neutron tagging efficiency. Unfortunately the rest of the experiment is not yet in a state that we can proceed at this point in time, but we are ready and all our systems have been shown to work as expected. We can dissolve GdS at any time and reach full efficiency as soon as the GdS is evenly distributed in the 700 tonne water shield.