Development of a time projection chamber prototype with micromegas technology for the search of the double beta decay of 136xe

Resumen

The main goal of this work is the study of the viability of the Micromegas technology in the NEXT project. This project proposes a TPC equiped filled with 100 kg of xenon enriched in 136Xe for an experiment of double beta decay. Three parameters have been studied at the expected energy of this process (Qbb = 2.48 MeV): the energy resolution, the background level and the detector efficiency. For the energy resolution, two different setups have been used: one previously used by the HELLAZ experiment and a completely new one, where the experience learnt in the former setup has been applied. An energy resolution of 2.5% FWHM at 5.5 MeV in pure xenon at 4 bar has been measured using an 241Am alpha source. At the same pressure, a value of 1.8% FWHM has been estimated using a selection of risetimes. This value implies an energy resolution of 2.8% FWHM at Qbb. Two simulation codes have been developped for the physics and for the specific features of a TPC in order to estimate the background level and the detector efficiency. In addition, the topology of the neutrinoless signal (bb0n) has been compared to that of background events. In this way, three discrimination methods have been developped based on three features: the distance to the walls, the existence of only one connection and the charge at the two ends of the event. The background of two conceptual geometries, based in two possible materials of the vessel (stainless steel and copper) have been calculated after the applications of the discrimination methods. The values obtained are respectively (1.27 +- 0.04) x 10^{-3} and (3.3 +- 0.2) x 10^{-4} counts/keV/kg/yr, which are an order of magnitude better than the measured values by other collaborations like IGEX and Gothard. The detector efficiency is (28.4 +- 0.1)% and the rejection factor is more than 1600 for 208Tl contaminations in the range of interest of the bb0n signal. The results obtained for the energy resolution, the efficiency and the background level have been combined to estimate the sensitivity of NEXT. Effective neutrino masses below 150 meV can be explored with a detector of 100 kg and 5 years of data-taking in an optimistic scenario (1% FWHM at Qbb and a copper vessel). For a mass of 1 tonne and a further background decrease (214Bi and 208Tl around 10 uBq/kg), a value of 33 meV can be reached in the same period of data. Better results seem feasible with a further development of discrimination techniques.