Low-energy threshold demonstration for dark matter searches in TREX-DM with an $^{37}$Ar source produced at CNA HiSPANoS

TL;DR

The paper demonstrates a practical $^{37}$Ar calibration strategy for a high-pressure argon TPC (TREX-DM) using fast-neutron activation of CaO at HiSPANoS to produce ~$O(1)$ kBq activity. The calibration employs a combined GEM-Micromegas readout to achieve ultra-low-energy sensitivity, detecting the $^{37}$Ar 2.82 keV and 270 eV lines and reaching $ ext{O}(10)$ eV energy thresholds, approaching the argon single-electron energy of $ oughly 26$ eV. Spatial uniformity across the 20 L active volume is established, and the threshold scan shows $E_{ ext{thr}}$ decreasing with mesh gain, following $E_{ ext{thr}} \\propto 1/G$ with $G \\sim \\exp(a V_{ ext{mesh}})$, indicating a robust path toward enhanced sensitivity for light WIMPs ($m \\lesssim 10$ GeV/$c^2$). These results validate a practical calibration protocol for ultra-low-energy searches in gas TPCs and have significant implications for expanding the accessible dark-matter parameter space. $m \\lesssim 10\ \,\mathrm{GeV}/c^2$.

Abstract

We report on the successful implementation of an $^{37}$Ar calibration source in the TREX-DM detector, a high-pressure time projection chamber designed for low-mass dark matter searches. The $^{37}$Ar source was produced through fast neutron activation of CaO powder at the HiSPANoS facility of Centro Nacional de Aceleradores (CNA) in Spain, yielding $O(1)$ kBq of activity. Using a novel combined GEM-Micromegas readout system, we successfully detected both characteristic emissions from $^{37}$Ar decay (2.82 keV and 270 eV) and achieved unprecedented energy threshold performance in TREX-DM, approaching the single-electron ionization energy of argon.

Figures (7)

• Figure 1: Left: P&ID diagram with the different components of the system: stainless-steel cross used as a container for the CaO powder, particulate filters to keep the powder isolated, valves for source injection, and UHV valve through which the system is pumped before irradiation. Center: image of the actual set-up where the external support structure is visible. Right: vessel filled with 0.5 kg of powder. It can be observed that the upper part and the side filters are left unobstructed so that the $^{37}$Ar liberated from the CaO can easily flow into the gas system.
• Figure 2: Irradiation setup at CNA irradiation hall. The distance beam-vessel is 75 mm.
• Figure 3: Complete $^{37}$Ar energy spectrum obtained with TREX-DM. Total calibration time is $\sim$ 3 min due to the high rate of the source. Left panel shows the full spectrum with both characteristic peaks: 2.82 keV from K-shell electron capture and 270 eV from L-shell capture. The blue curve corresponds to the GEM-Micromegas configuration with both amplification stages active, while the red curve shows the spectrum with only the Micromegas detector powered on. Right panel shows the zoomed view of the low-energy region, showing a threshold of $O(10)$ eV in equivalent energy. The $x$ axis is also represented as the equivalent number of primary electrons, defined as the equivalent energy divided by the average energy needed to produce an electron-ion pair ($\approx 26$ eV in argon).
• Figure 4: Spatial distribution analysis of $^{37}$Ar events in TREX-DM. Left: 2D hitmap qualitatively illustrating spatial uniformity across the whole detector area. Right: 1D projection of bin counts from the inner 20$\times$20 cm$^2$ fiducial region, together with a Gaussian fit (red line). The standard deviation approximately follows the expected $\sigma=\sqrt{\mu}$.
• Figure 5: Some representative low-energy events recorded by TREX-DM using the GEM-MM readout system during $^{37}$Ar calibration under high-gain conditions. The different colors correspond to signals from individual detector strips within the same event. The signals are clearly distinguishable from baseline noise, even for events with equivalent energies of 30 eV.