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

Abstract

NEXT-MM is a general-purpose high pressure (10 bar, ∼ 25 l active volume) Xenonbased TPC, read out in charge mode with an 0.8 cm×0.8 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 0νβ β experiment, although the experiment's 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 γ-rays emitted by a 241 Am source when interacting with the Xenon gas (E γ = 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 α 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@30 keV).

Radiopurity assessment of the tracking readout for the NEXT double beta decay experiment

“…was screened. Material is reported as Plexiglas GS/XT from Evonik Industries AG 10 . Results are shown in row #14 of table 3, setting upper limits to the analyzed radioisotopes.…”

“…As illustrated in figure 1, there will be separate energy and tracking readout planes, located at opposite sides of the pressure vessel using different sensors: photomultiplier tubes (PMTs) for calorimetry (and for fixing the start of the event) and silicon photomultipliers (SiPMs) for tracking. While work on prototypes is still ongoing [5,6,7,8,9,10], the installation of shielding and ancillary system started at LSC in 2013. Underground commissioning of the NEW detector using ∼10 kg of xenon has begun at the end of 2014 and first data are expected along 2015.…”

Readout technologies for directional WIMP Dark Matter detection

“…Microbulk detectors are currently being developed in the context of double beta decay searches, for which good energy resolution and operation at high pressure in Xe mixtures is required. Energy resolutions of 7.3% (9.6%) FWHM in 1(10) bar at 22 keV in Xe and trimethilamine has been demonstrated [106,107,108].…”

Electron drift properties in high pressure gaseous xenon

“…The coefficients have been extrapolated to normal temperature, 20 • C, and transformed into a most common representation with the following expression [30]:…”