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

The TREX-DM collaboration successfully implemented a CNA-produced $^{37}$Ar calibration source and a novel GEM-Micromegas readout system to detect low-energy argon decay emissions, achieving an unprecedented energy threshold approaching the single-electron ionization limit for low-mass dark matter searches.

The Gist

Imagine the universe is filled with invisible, ghostly particles called Dark Matter. Scientists think these particles might occasionally bump into normal atoms, but because they are so light and shy, these bumps are incredibly tiny—like a feather landing on a trampoline. To catch them, we need detectors that are sensitive enough to feel even the faintest whisper of energy.

This paper is about a team of scientists who built a super-sensitive "feather-catching" machine and proved it works by teaching it to hear a very quiet sound.

The Machine: A Giant, Pressurized Balloon

The scientists used a device called TREX-DM. Think of it as a large, heavy copper tank filled with pressurized Argon gas (the same gas used in lightbulbs). Inside this tank, they created an electric field. If a dark matter particle hits an Argon atom, it knocks an electron loose. That electron zips through the gas, creating a tiny spark that the machine can see.

The challenge? The "bump" from a dark matter particle is so small that it might only knock loose a single electron. Most machines are too "noisy" or "heavy" to hear such a faint signal.

The Teacher: A Special Radioactive Gas

To test if their machine was sensitive enough, they needed a "teacher" that could make a sound at the exact volume of a dark matter bump. They created a special gas called Argon-37.

• How they made it: They took a bag of calcium powder (like chalk dust) and shot it with a high-speed beam of neutrons at a facility in Spain called CNA HiSPANoS. This is like using a particle cannon to turn the calcium into the special Argon-37 gas.
• Why it's a good teacher: When Argon-37 decays, it doesn't just make a loud bang; it makes two very specific, quiet "pings." One is a standard ping (2,820 electron-volts), and the other is an ultra-quiet whisper (270 electron-volts). The quiet one is the real test.

The Trick: The Double-Stage Amplifier

The machine has a special reading system made of two layers: a GEM and a Micromegas.

• Think of the GEM as a pre-amplifier (like a microphone that boosts a voice before it hits the main speaker).
• Think of the Micromegas as the main speaker.

By stacking them, the scientists created a "double-boost." When a tiny electron signal comes in, the GEM boosts it 50 to 60 times before the Micromegas even sees it. This is crucial because it turns a whisper into something the machine can actually hear without getting confused by background noise.

The Results: Hearing the Whisper

When they pumped the Argon-37 gas into the machine, here is what happened:

1. They heard the loud ping: They easily detected the 2,820 eV signal.
2. They heard the whisper: Remarkably, they also detected the 270 eV signal. This is a huge deal because 270 eV is incredibly close to the energy of just one electron.
3. The "Threshold": The machine proved it could detect signals as low as 20 to 30 eV. To put that in perspective, the energy needed to knock a single electron loose in Argon is about 26 eV. The machine is now operating right at the physical limit of what is possible for this type of gas detector.

The Map: Even Distribution

The scientists also checked if the gas spread evenly inside the tank. Imagine spraying perfume in a room; you want to know if it smells the same everywhere or if it's only strong in the corners. They found that the gas was perfectly uniform. The machine "heard" the gas equally well in every corner, meaning it won't miss dark matter just because it's hiding in a blind spot.

The Bottom Line

The paper concludes that the TREX-DM detector, using this new double-boost system and the special Argon-37 gas, is now sensitive enough to hear the faintest possible signals. It has successfully demonstrated that it can reach the "single-electron" level. This proves the machine is ready to start hunting for light dark matter particles that were previously too quiet to be heard.

Technical Summary

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

Problem and Motivation
Direct detection of low-mass Weakly Interacting Massive Particles (WIMPs) requires detectors capable of achieving sub-keV energy thresholds, as WIMP-induced nuclear recoils in this mass regime can deposit energies of only a few keV or less. While the TREX-DM experiment utilizes a high-pressure time projection chamber (TPC) with noble gases (argon or neon) to enhance sensitivity to these low-energy events, characterizing detector response at the single-electron level remains a critical challenge. Standard external calibration sources often suffer from self-shielding, limited penetration depth, and a lack of discrete emissions in the sub-keV range. To address these limitations, the authors sought to implement a gaseous $^{37}$Ar calibration source, which offers homogeneous distribution within the active volume and discrete monoenergetic emissions at 2.82 keV (K-shell) and 270 eV (L-shell).

Methodology
The study involved three primary phases: source production, detector integration, and spectral analysis.

1. Source Production: The $^{37}$Ar source was generated via the fast neutron activation of calcium oxide (CaO) powder ($^{40}$Ca(n,$\alpha$)$^{37}$Ar) at the HiSPANoS facility of the Centro Nacional de Aceleradores (CNA) in Sevilla. A 0.5 kg sample of high-purity CaO was irradiated for approximately 7 hours using a 5 MeV deuteron beam on a beryllium target, producing a fast neutron flux of $\sim 3 \times 10^{10}$ n/sr/s. The resulting activity was estimated to be on the order of 1 kBq, monitored indirectly via the co-produced $^{42}$K isotope. Following irradiation, the source was transported to the Laboratorio Subterráneo de Canfranc (LSC) and injected into the TREX-DM detector using a multi-cycle gas transfer process with an Ar-1% iC$_4$H$_{10}$ mixture.

2. Detector Configuration: The TREX-DM detector, a 20 L high-pressure TPC, was operated at $\sim$1 bar with a novel combined readout system consisting of a Gas Electron Multiplier (GEM) stage stacked on top of a Microbulk Micromegas detector. This configuration was designed to provide additional preamplification. Data acquisition was performed using the REST-for-Physics framework.

3. Analysis: The authors analyzed the energy spectrum to identify characteristic $^{37}$Ar peaks and assess spatial homogeneity. The energy threshold was determined by modeling the low-energy tail of the spectrum using a "cropped-peak" fit (a Gaussian multiplied by a smoothed step function) across various mesh voltage settings (280–295 V).

Key Contributions and Results

• Successful Source Implementation: The paper reports the successful production of an $\sim$1 kBq $^{37}$Ar source via CaO irradiation and its seamless integration into the TREX-DM detector, achieving a homogeneous distribution of events across the active volume.
• Detection of Ultra-Low Energy Emissions: Using the combined GEM-Micromegas readout, the experiment successfully detected both the 2.82 keV K-shell peak and, significantly, the 270 eV L-shell peak. The ratio of K-shell to L-shell events was measured at 10:1, consistent with theoretical decay probabilities.
• Gain Characterization: The GEM stage provided a preamplification factor of 50–60 compared to the Micromegas-only configuration, consistent with previous bench measurements.
• Energy Threshold Determination: The analysis demonstrated that the detector can resolve events down to equivalent energies of approximately 20 eV at optimal voltage settings (295 V). This performance approaches the single-electron ionization energy of argon ($\approx$26 eV). The threshold was shown to scale inversely with the system gain ($E_{th} \sim 1/G$), following an exponential dependence on the applied mesh voltage.
• Spatial Uniformity: A spatial homogeneity assessment confirmed that the detector response is uniform across the active volume, with no visible dead regions, validating the detector's suitability for background discrimination and signal identification.

Significance and Claims
The authors claim that this work demonstrates the capability of the TREX-DM detector, equipped with a GEM-Micromegas readout, to achieve energy thresholds at the level of single-electron ionization. The observation of the 270 eV L-shell peak is highlighted as a significant achievement, as detecting such ultra-low-energy signatures in gas-based detectors is typically challenging.

The paper asserts that these results validate the GEM-MM technology for rare event searches and provide a critical sensitivity map for low-mass WIMP searches. By achieving thresholds near the fundamental limit of argon ionization, the experiment potentially accesses unexplored regions of the dark matter parameter space. The authors note that while the current threshold analysis relies on transfer-region events (which do not fully replicate the Poisson fluctuations of drift-region few-electron events), the qualitative conclusion regarding the achievable threshold remains robust. Future work is identified as focusing on precise background characterization and the development of refined calibration techniques, such as UV laser calibration, to further probe the single-electron regime.