Toward Tonne-Scale NEXT Detectors: SiPM Energy-Tracking Planes and Metalenses for Light Collection

Abstract

The best method of proving the nature of neutrinos is discovering neutrinoless double beta decay (0νββ), as this decay is only possible if the neutrino is Majorana in nature. Measuring the half-life of 0νββ would also provide constraints on the absolute neutrino mass and the neutrino mass ordering. The NEXT collaboration searches for this decay in 136Xe using a high-pressure gaseous-xenon Time Projection Chamber. After demonstrating this technology, NEXT is proposing tonne-scale detectors to achieve the required sensitivity of the inverted neutrino mass ordering. This thesis presents research on new technology needed to reach this goal, in particular, the development of a highly dense SiPM plane proposed to perform the energy measurements previously done by PMTs. First, to demonstrate the viability of such a plane, a novel method of measuring energy with SiPMs is developed and tested with NEXT-White SiPM data. With only ∼1% coverage, these SiPMs achieve an energy resolution of (10.38 ± 0.26)% (FWHM) at 41.5 keV. Extrapolating these results suggests an optimal SiPM coverage of ∼30% needed to achieve excellent energy resolution. Second, simulations of dense SiPM planes are created to optimize energy resolution, confirming the data-driven extrapolation of an optimal ∼30% coverage, and informing future prototype and tonne-scale detector designs. Finally, research and development into metalenses, a new technology to improve light collection in noble element detectors like NEXT, is presented. A novel method of characterizing the metalenses for light collection is developed, and two lenses are studied, including the very first metalens designed to focus VUV light.