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,~ 25 l active volume) Xenon-based TPC, read out in chargemode with an 0.8 cm × 0.8 cm-segmented 700 cm2 plane(1152 ch) of the latest microbulk-Micromegas technology. It hasbeen recently commissioned at University of Zaragoza as part of theR&D of the NEXT 0νββ experiment, although theexperiment's first stage is currently being built based on aSiPM/PMT-readout concept relying on electroluminescence. Around 2million events were collected during the last months, stemming fromthe low energy γ-rays emitted by a 241Am source wheninteracting with the Xenon gas (Eγ = 26, 30,59.5 keV). The localized nature of such events around atmosphericpressure, the long drift times, as well as the possibility todetermine their production time from the associated αparticle in coincidence, allow the extraction of primordialproperties of the TPC filling gas, namely the drift velocity,diffusion and attachment coefficients. In this work we focus on thelittle explored combination of Xe and trimethylamine (TMA) forwhich, in particular, such properties are largely unknown. This gasmixture offers potential advantages over pure Xenon when aimed atRare Event Searches, mainly due to its Penning characteristics,wave-length shifting properties and reduced diffusion, and it isbeing actively investigated by our collaboration. The chamber iscurrently operated at 2.7 bar, as an intermediate step towards theenvisaged 10 bar. We report here its performance as well as a firstimplementation of the calibration procedures that have allowed theextension of the previously reported energy resolution to the wholereadout plane (10.6% FWHM@30 keV).

Note:

22 pages, 13 figures

xenon: gas
time projection chamber
diffusion
drift velocity
calibration
performance
Micromegas
NEXT
gas: admixture
organic compounds

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