The XENONnT Neutron Veto: commissioning of the Gadolinium-water purification system

Many observational evidence for the existence of Dark Matter have been discovered over the last century. Several techniques are employed to directly search for a Dark Matter signal; among them, the XENONnT experiment features a Xenon double-phase TPC with a target mass of 5.9 tons and an active Neutron Veto (NV) system. The NV is designed as a Gadolinium-doped water Cherenkov detector, used to capture and tag neutrons and reduce the background. The presence of Gadolinium results in a lower veto dead time and a higher neutron tagging efficiency. In this thesis, developments on two important aspects of the NV are presented. On one hand, the Reflectivity Monitor is a tool to measure the photon absorption time in the NV and monitor its optical properties over time. Data analysis has been implemented and supported by Monte Carlo simulations; the photon absorption time has been measured to be (57±1) ns, constant over time. On the other hand, a dedicated purification plant, the Gadolinium-water purification system (GdWPS), is necessary to insert Gd in the Neutron Veto and at the same time purify water from other impurities; its working principle consists in separating Gd-rich from Gd-depleted solutions and purifying this latter minimizing Gadolinium losses. Important steps in its commissioning phase have been taken, operating with pure water. During this phase, the Reflectivity Monitor showed a slight decrease in photon absorption time, not directly ascribable to GdWPS operation. After that, 15 kg of Gd salt have been inserted in the GdWPS as a test: the insertion system worked properly. An efficient method to estimate Gd concentration has been found and validated by mass-spectrometry measurements and the separation efficiency has been measured as 99.6%-99.85%. A tolerable Gd loss of 200 kg/year has been found. Transparency measurements showed that the GdWPS is able to keep photon absorption lengths of the order of tens of meters both on pure water and Gd-water samples.