Low-energy calibration, reconstruction software and light-collection efficiency parametrization of the NEXT-White detector

Resum

Neutrinos are, arguably, the less understood particles in the Standard Model of particle physics. In the past decades, it has been discovered through neutrino oscillation experiments that these are massive particles, contradicting the assumptions of the theory. The possible Majorana nature of neutrinos could explain not only their seemly arbitrary low mass, but also the matter-antimatter asymmetry of the universe. Thus, the research in neutrino physics has been recently focused on the search for the neutrinoless double beta decay (ββ0ν), a hypothetical nuclear process that would prove the Majorana nature of neutrinos. The standard mode of the double beta decay (ββ2ν), with two-neutrino emission and allowed within the standard model, has already been observed in a number of isotopes. The neutrinoless mode, on the other hand, can only occur if neutrinos are Majorana particles and there has not been any convicing evidence of its occurrence so far. There are multiple projects implementing different techniques to search for the decay, amonst them is the NEXT experiment. NEXT will search for ββ0ν in 136Xe with a High-Pressure Xenon TPC. The NEXT-100 detector consists of an electroluminiscent TPC with O(100 kg) of xenon enriched to 91 % in 136Xe at 15 bar. The detector is instrumented with PMTs to measure the event energy and with an array of SiPMs for tracking purposes. The technique is focused on achieving an excellent energy resolution (< 1 % FWHM at Qββ) and significant background rejection using topological information. The operation of the NEXT-White (NEW) detector constitutes the first phase of the project and is currently ongoing. Built in 2015-2016, NEW is a ∼1:2 scale model of the NEXT-100 detector and uses the same materials and technology. Its primary purposes are the validation of the technological solutions for NEXT-100 in a large-scale detector, the assesment of the background model and the measurement of the ββ2ν half life. This work focuses on three different, yet related, aspects of NEXT-White: the simulation of the optical response, the implementation of the reconstruction software and the low-energy calibration of the detector with 83Krm data. The optical simulation of the detector contributes to the simulation software of the experiment. The photon detection probability model of the detector is parametrized to achieve an accurate description of the detector response, whilst maintaining the memory capability requirements within attainable values. The method can also be tranferred to larger detectors, hence providing a tool to simulate large-statistics Monte Carlo datasets for the NEXT-White and NEXT-100 detectors, which would be otherwise impossible. The implementation of the reconstruction software constitutes the second contribution to NEXT. This thesis contains a through description of each stage of the reconstruction chain and the software structures created to hold the information. Furthermore, we discuss the software framework (IC), the software philosophy behind it, and its main feature: the dataflow scheme. The low-energy calibration methodology using 83Krm data taken with the NEXT-White detector contributes to the data analysis of the experiment. These data are used to correct for the finite electron lifetime and for the dependence of the measured energy with the event position. After producing calibration maps to correct for both effects, we measure an excellent energy resolution for 41.5 keV point-like deposits of (4.553 ± 0.010 (stat.) ± 0.324 (sys.)) % FWHM in the full volume and (3.804 ± 0.013 (stat.) ± 0.112 (sys.)) % FWHM in a restricted fiducial volume for a 7.2 bar Run. A naive E−1/2 extrapolation yields energy resolutions at Qββ of (0.5916 ± 0.0014 (stat.) ± 0.0421 (sys.)) % FWHM in the full volume and (0.4943 ± 0.0017 (stat.) ± 0.0146 (sys.)) % FWHM in the fiducial volume. Similar results were obtained for a 9.1 bar Run.