A highly innovative design of a photomultiplier: prototypes, engineering and applications in astroparticle physics.

Photon detection represents a critical issue in the current generation of high-energy astroparticle physics experiments. Indeed, many experiments in this rapidly emerging field are based on the detection of the Cherenkov or fluorescence light produced after the passage of charged particles through transparent media like air, water or ice. For these applications, the quality of the experimental results is crucially related to the performances of the adopted photodetectors, for which therefore high efficiency, photon counting capability and large sensitive surface are required. To date, this field is dominated by PhotoMultiplier Tubes (PMTs), a one-century-old technology that in the last decades has known a whirling evolution, leading to impressive improvements, especially in terms of quantum efficiency and gain, but affected by some intrinsic drawbacks that strongly limit its performances. Moreover, the next generation of experiments will set stricter requirements about photodetectors performances, in particular for what concerns photon counting capability and power consumption. For these reasons, in the last decades several alternatives to PMTs have been introduced. In this context, an interesting solution is represented by the VSiPMT (acronym of Vacuum Silicon PhotoMultiplier Tube), a novel photodetector based on the combination of PMT and Geiger-mode Avalanche PhotoDiode (G-APD) technologies. Indeed, in such device the standard dynode chain of the PMT is replaced by a G-APD, operating in electron multiplier mode, thus matching large sensitive surface and high photon counting performances. After years of preliminary work aimed at proving the feasibility of the VSiPMT, the encouraging results achieved convinced Hamamatsu Photonics, World leader company in PMT and SiPM manufacture, to realize some prototypes of the device. This thesis describes accurately all the phases of the VSiPMT project, with a particular focus on the results of the prototype characterization phase and on the possible application of the VSiPMT to the KM3NeT neutrino telescope.