Background assessment for the TREX dark matter experiment

Abstract

TREX-DM is conceived to look for low-mass Weakly Interacting Massive Particles (WIMPs) using a gas Time Projection Chamber equipped with Micromegas readout planes at the Canfranc Underground Laboratory. The detector can hold in the active volume ∼20 l of pressurized gas up to 10 bar, corresponding to 0.30 kg of Ar or 0.16 kg of Ne. The Micromegas are read with a self-triggered acquisition, being thresholds below 0.4 keV (electron equivalent) at reach. A low background level in the lowest energy region is another essential requirement. To assess the expected background, all the relevant sources have been considered, including the measured fluxes of gamma radiation, muons and neutrons at the Canfranc Laboratory, together with the activity of most of the components used in the detector and ancillary systems, obtained in a complete assay program. The background contributions have been simulated by means of a dedicated application based on Geant4 and a custom-made code for the detector response. The background model developed for the detector presently installed in Canfranc points to levels from 1 to 10 counts keV −1 kg −1 d −1 in the region of interest, making TREX-DM competitive in the search for low-mass WIMPs. A roadmap to further decrease it down to 0.1 counts keV −1 kg −1 d −1 is underway.

The Role of Small Scale Experiments in the Direct Detection of Dark Matter

“…Other types of gas detectors are being developed too. The TREX-DM experiment (Tpcs for Rare Event eXperiments-Dark Matter) uses a gas TPC equipped with Micromegas readouts [125,126,141], offering low intrinsic radioactivity and the possibility of recording topological information to discriminate backgrounds. The pressurized gas at 10 bar is held inside a vessel (with diameter and height of 0.5 m) made of 6-cm-thick copper and the largest microbulk Micromegas planes ever built (25 × 25 cm 2 ) are being used.…”

The TREX-DM experiment at the Canfranc Underground Laboratory

“…An exhaustive radioassay program has been performed over the last years to construct the detector with radiopurity specifications, mainly based on germanium gamma spectrometry at LSC and complementary measurements from GDMS, ICPMS and the BiPo-3 detector. The background model developed by Monte Carlo simulation for the present set-up points to levels at 1-10 keV −1 kg −1 d −1 in the region of interest [3]. It is based on Geant4 (for Physics processes) and a custom-made code called REST (for electron generation, diffusion effects, charge amplification and signal generation).…”

“…A roadmap to further decrease it down to 0.1 keV −1 kg −1 d −1 is underway. Prospects for exclusion plots have been evaluated for Spin-Independent interaction and standard halo model for three different experimental scenarios at a pressure of 10 bar, assuming different background levels (10, 1 and 0.1 keV −1 kg −1 d −1 ), energy thresholds (0.4, 0.1 and 0.1 keV ee ) and exposures (0.3, 0.3 and 10 kg y) [3] and are shown in Fig. 5.…”

“…Background rates expected from internal components in the region 0.2-7 keV ee using Ar (blue bars) or Ne (red bars) mixtures in TREX-DM, as estimated in[3].…”

Small scale direct dark matter search experiments

“…The single electron response (gain, drift or diffusion times) has been characterized with a laser system [51] and quenching factor measurements at Grenoble for H and at TUNL (Duke University) for Ne-CH 4 are underway. TREX-DM (Tpcs for Rare Event eXperiments-Dark Matter) is based on a gas TPC holding a pressurized gas at 10 bar inside a copper vessel, equipped with the largest Micromegas readouts ever built [52,53,54]. It operates at the Canfranc Underground Laboratory, being presently at the commissioning phase.…”