Neutron Veto of XENONnT: final construction design of the system and extensive study and calibration of the PMTs.

Numerous and convincing are the experimental evidences that led to the believe that most of the Universe is constituted by the so-called Dark Matter (DM). Its research has attracted many experimental efforts to the point of being considered the greatest mystery of physics. The most promising candidates to DM are generically named as WIMPs, an acronym for Weakly Interacting Massive Particles.The detectors that have reached the most competitive sensitivity levels have in common the characteristics of being built with ultrapure materials and located underground, shielded from cosmic rays. The XENON project,located at the Laboratori Nazionali del Gran Sasso (LNGS). The XENON1T experiment employed a dual-phase Time Projection Chamber (TPC) with 2t of xenon active mass. The next step of the detector is XENONnT, and is currently in the final assembling stage. With a bigger TPC containing 6t of xenon active mass, the main goal is to improve the detection potential for WIMPs by increasing the sensitivity of one order of magnitude in five years of data taking. That result can be achieved only with a substantial background lowering attained with a reduction of the contaminants of the xenon as well as of the detector materials and finally, with the Neutron Veto system (nVeto); the latter is a detector which tags the radiogenic neutrons coming from the materials of the experiment. The nVeto will employ the gadolinium-loaded water technology to significantly enhance the neutron capture cross-section. It will be instrumented with 120 8'' Hamamatsu R5912 photomultipliers (PMT) featuring high quantum efficiency and low radioactivity. Most of the work done to design and realize the nVeto is the argument of this thesis. In addition, an extensive study of the nVeto PMT performance has been done in a dedicated experimental setup at the LNGS. The complete description of the setup,as well as the results of the nVeto PMT tests, is the other big topic of this thesis.