Gaseous time projection chambers for rare event detection: results from the T-REX project. I. Double beta decay

Abstract

Abstract. As part of the T-REX project, a number of R&D and prototyping activities have been carried out during the last years to explore the applicability of gaseous Time Projection Chambers (TPCs) with Micromesh Gas Structures (Micromegas) in rare event searches like double beta decay, axion research and low-mass WIMP searches. In both this and its companion paper, we compile the main results of the project and give an outlook of application prospects for this detection technique. While in the companion paper we focus on axions and WIMPs, in this paper we focus on the results regarding the measurement of the double beta decay (DBD) of 136 Xe in a high pressure Xe (HPXe) TPC. Micromegas of the microbulk type have been extensively studied in high pressure Xe and Xe mixtures. Particularly relevant are the results obtained in Xe + trimethylamine (TMA) mixtures, showing very promising results in terms of gain, stability of operation, and energy resolution at high pressures up to 10 bar. The addition of TMA at levels of ∼1% reduces electron diffusion by up to a factor of 10 with respect to pure Xe, improving the quality of the topological pattern, with a positive impact on the discrimination capability. Operation with a medium size prototype of 30 cm diameter and 38 cm of drift (holding about 1 kg of Xe at 10 bar in the fiducial volume, enough to contain high energy electron tracks in the detector volume) has allowed to test the detection concept in realistic experimental conditions. Microbulk Micromegas are able to image the DBD ionization signature with high quality while, at the same time, measuring its energy deposition with a resolution of at least a ∼3% FWHM @ Q ββ . This value was experimentally demonstrated for high-energy extended tracks at 10 bar, and is probably improvable down to the ∼1% FWHM levels as extrapolated from low energy events. In addition, first results on the topological signature information (one straggling track ending in two blobs) show promising background discrimination capabilities out of reach of other experimental implementations. Moreover, microbulk Micromegas have very low levels of intrinsic radioactivity, and offer cost-effective scaling-up options. All these results demonstrate that Micromegas-read HPXe TPC remains a very competitive technique for the next generation DBD experiments.

Background assessment for the TREX dark matter experiment

“…New activity measurements for the microbulk Micromegas and Cu-kapton-Cu foil samples (previously measured with Ge spectroscopy) were carried out profiting the great capabilities of the BiPo-3 detector operating at LSC (#28,31 of table 1). For both cases, only limits to the contamination in 208 Tl and 214 Bi can be deduced, which improve the Ge spectrometry limits by more than 2 orders of magnitude, pointing to contaminations at the level of, or below, ∼0.1 µBq/cm 2 [14]. A more sensitive measurement for microbulk Micromegas produced at CERN was made in 2016, using two capsules of the BiPo-3 detector (30×30 cm 2 each); results shown in #33 of table 1 point to a very significant reduction of the upper limits of both 208 Tl and 214 Bi.…”

“…An exhaustive material radioassay campaign for TREX-DM was undertaken a few years ago [11,14,31], as made in other experiments in the context of rare event searches (see for instance recent results in [32,33,34,35,36,37,38]). It has allowed, on one side, to design and construct the detector and shielding according to the radiopurity specifications and, on the other, to provide inputs to build the experiment background model.…”

“…It is also worth noting that no indication of a possible 210 Pb contamination has been found in any of the measurements for this sample, as there is no excess over background at the 46.5 keV peak. A number of other samples involved in various Micromegas fabrication processes have also been measured [14]. A sample of pyralux sheets, used in the construction of bulk Micromegas, showed good radiopurity first in germanium screening (#37 of table 1) and afterwards using the BiPo-3 detector (#38 of table 1).…”

“…Different technologies have been built: bulk Micromegas have the readout plane and the mesh all in one and microbulk Micromegas are in addition more homogeneous and radiopure [13]. They offer important advantages for rare event detection [12,14]: the possibility of scaling-up, topological information to discriminate backgrounds from the expected signal (just a few microns track for dark matter particles, giving a point-like event) and low intrinsic radioactivity as they 1 Electron equivalent energy. arXiv:1812.04519v2 [astro-ph.IM] 22 Sep 2019 are made out of kapton and copper, potentially very clean.…”

TREX-DM: a low-background Micromegas-based TPC for low-mass WIMP detection

“…Being gaseous detectors, the scaling-up prospects of gas TPCs are typically considered modest. However, advances in electronics and novel micro-pattern gas readout planes (especially Micromegas) are changing this view (see [24, 25] and references therein). The objective of the T-REX project [26, 27] has been to study the applicability of Micromegas readouts TPCs to rare event searches (not just to WIMP searches, but also axions [28] and double beta decay [29]).…”