Characterization of a medium size Xe/TMA TPC instrumented with microbulk Micromegas, using low-energy $gamma$-rays

NEXT-MM is a general-purpose high pressure (10 bar, $sim25$ l active volume) Xenon-based TPC, read out in charge mode with an 8 cm $times$8 cm-segmented 700 cm$^2$ plane (1152 ch) of the latest microbulk-Micromegas technology. It has been recently commissioned at University of Zaragoza as part of the R&D of the NEXT $0 ubetabeta$ experiment, although the experiments first stage is currently being built based on a SiPM/PMT-readout concept relying on electroluminescence. Around 2 million events were collected during the last months, stemming from the low energy $gamma$-rays emitted by a $^{241}$Am source when interacting with the Xenon gas ($epsilon$ = 26, 30, 59.5 keV). The localized nature of such events above atmospheric pressure, the long drift times, as well as the possibility to determine their production time from the associated $alpha$ particle in coincidence, allow the extraction of primordial properties of the TPC filling gas, namely the drift velocity, diffusion and attachment coefficients. In this work we focus on the little explored combination of Xe and trimethylamine (TMA) for which, in particular, such properties are largely unknown. This gas mixture offers potential advantages over pure Xenon when aimed at Rare Event Searches, mainly due to its Penning characteristics, wave-length shifting properties and reduced diffusion, and it is being actively investigated by our collaboration. The chamber is currently operated at 2.7 bar, as an intermediate step towards the envisaged 10 bar. We report here its performance as well as a first implementation of the calibration procedures that have allowed the extension of the previously reported energy resolution to the whole readout plane (10.6%FWHM@30keV).

The NEXT experiment. Towards phase I

Phase I of the NEXT-100 $0 ubetabeta$ experiment (NEW) is scheduled for data taking in 2015 at Laboratorio Subterraneo de Canfranc in the Spanish Pyrenees. Thanks to the light proportional technique, NEW anticipates an outstanding energy resolution nearing the Fano factor in Xenon (0.5-1%FWHM@$Q_{betabeta,^{136}Xe}$), with a TPC-design that allows tracking and identification of the double end-blob feature of the $0 ubetabeta$ decay. When properly mastered, the combination of these two assets can suppress the irreducible $2 ubetabeta$ and (single-blob) $gamma$ backgrounds from natural radioactivity to minute levels, of the order of $5times{10^{-4}}$ ckky. Given our knowledge of the available phase-space as obtained from neutrino oscillation experiments, this feat will expectedly allow for a sensitivity to the effective electron neutrino mass of $m_{betabeta}simeq 30$ meV for exposures at the 20 ton $times$ year scale. Hence, ultimately, a full survey of the inverse hierarchy of the neutrino mass ordering appears to be within reach for a ton-scale experiment based on this technology. NEW, with 10 kg of Xenon 90%-enriched in $^{136}$Xe, sets an unprecedented scale for gaseous Xenon TPCs and will be an important milestone for its anticipated upgrades (100 kg and 1 ton). I briefly summarize the status of the NEXT experiment, from the main results obtained with $sim 1$ kg prototypes that substantiate the concept, to the ongoing works for deploying its first phase.

Present status and future perspectives of the NEXT experiment

NEXT is an experiment dedicated to neutrinoless double beta decay searches in xenon. The detector is a TPC, holding 100 kg of high-pressure xenon enriched in the $^{136}$Xe isotope. It is under construction in the Laboratorio Subterraneo de Canfranc in Spain, and it will begin operations in 2015. The NEXT detector concept provides an energy resolution better than 1% FWHM and a topological signal that can be used to reduce the background. Furthermore, the NEXT technology can be extrapolated to a 1-ton scale experiment.

Operation and first results of the NEXT-DEMO prototype using a silicon photomultiplier tracking array

NEXT-DEMO is a high-pressure xenon gas TPC which acts as a technological test-bed and demonstrator for the NEXT-100 neutrinoless double beta decay experiment. In its current configuration the apparatus fully implements the NEXT-100 design concept. This is an asymmetric TPC, with an energy plane made of photomultipliers and a tracking plane made of silicon photomultipliers (SiPM) coated with TPB. The detector in this new configuration has been used to reconstruct the characteristic signature of electrons in dense gas. Demonstrating the ability to identify the MIP and blob regions. Moreover, the SiPM tracking plane allows for the definition of a large fiducial region in which an excellent energy resolution of 1.82% FWHM at 511 keV has been measured (a value which extrapolates to 0.83% at the xenon Qbetabeta).

NEXT, a HPGXe TPC for neutrinoless double beta decay searches

We propose a novel detection concept for neutrinoless double-beta decay searches. This concept is based on a Time Projection Chamber (TPC) filled with high-pressure gaseous xenon, and with separated-function capabilities for calorimetry and tracking. Thanks to its excellent energy resolution, together with its powerful background rejection provided by the distinct double-beta decay topological signature, the design discussed in this Letter Of Intent promises to be competitive and possibly out-perform existing proposals for next-generation neutrinoless double-beta decay experiments. We discuss the detection principles, design specifications, physics potential and R&D plans to construct a detector with 100 kg fiducial mass in the double-beta decay emitting isotope Xe(136), to be installed in the Canfranc Underground Laboratory.