Abstract: We propose a new detector concept, GraXe (to be pronounced as grace), to search for neutrinoless double beta decay in Xe-136. GraXe combines a popular detection medium in rare-event searches, liquid xenon, with a new, background-free material, grapheme. In our baseline design of GraXe, a sphere made of graphene-coated titanium mesh and filled with liquid xenon (LXe) enriched in the Xe-136 isotope is immersed in a large volume of natural LXe instrumented with photodetectors. Liquid xenon is an excellent scintillator, reasonably transparent to its own light. Graphene is transparent over a large frequency range, and impermeable to the xenon. Event position could be deduced from the light pattern detected in the photosensors. External backgrounds would be shielded by the buffer of natural LXe, leaving the ultra-radiopure internal volume virtually free of background. Industrial graphene can be manufactured at a competitive cost to produce the sphere. Enriching xenon in the isotope Xe-136 is easy and relatively cheap, and there is already near one ton of enriched xenon available in the world (currently being used by the EXO, KamLAND-Zen and NEXT experiments). All the cryogenic know-how is readily available from the numerous experiments using liquid xenon. An experiment using the GraXe concept appears realistic and affordable in a short time scale, and its physics potential is enormous.

Abstract: NEXT-DEMO is a prototype of NEXT (Neutrino Experiment with Xenon TPC), an experiment to search for neutrino-less double beta decay using a 100 kg radio-pure, 90 % enriched (136Xe isotope) high-pressure gaseous xenon TPC with electroluminescence readout. The detector is based on a PMT plane for energy measurements and a SiPM tracking plane for topological event filtering. The experiment will be located in the Canfranc Underground Laboratory in Spain. Front-end electronics, trigger and data-acquisition systems (DAQ) have been built. The DAQ is an implementation of the Scalable Readout System (RD51 collaboration) based on FPGA. Our approach for trigger is to have a distributed and reconfigurable system in the DAQ itself. Moreover, the trigger allows on-line triggering based on the detection of primary or secondary scintillation light, or a combination of both, that arrives to the PMT plane.

Abstract: High-pressure xenon gas is an attractive detection medium for a variety of applications in fundamental and applied physics. In this paper we study the ionization and scintillation detection properties of xenon gas at 10 bar pressure. For this purpose, we use a source of alpha particles in the NEXT-DEMO time projection chamber, the large scale prototype of the NEXT-100 neutrinoless double beta decay experiment, in three different drift electric field configurations. We measure the ionization electron drift velocity and longitudinal diffusion, and compare our results to expectations based on available electron scattering cross sections on pure xenon. In addition, two types of measurements addressing the connection between the ionization and scintillation yields are performed. On the one hand we observe, for the first time in xenon gas, large event-by-event correlated fluctuations between the ionization and scintillation signals, similar to that already observed in liquid xenon. On the other hand, we study the field dependence of the average scintillation and ionization yields. Both types of measurements may shed light on the mechanism of electron-ion recombination in xenon gas for highly-ionizing particles. Finally, by comparing the response of alpha particles and electrons in NEXT-DEMO, we find no evidence for quenching of the primary scintillation light produced by alpha particles in the xenon gas.