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arxiv: 2510.05877 · v2 · submitted 2025-10-07 · ⚛️ physics.ins-det · hep-ex

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

J. Castel , S. Cebri\'an , T. Dafni , D. D\'iez-Ib\'a\~nez , A. Ezquerro , B. Fern\'andez , J. Gal\'an , J.A. Garc\'ia
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C. Guerrero I.G. Irastorza G. Luz\'on C. Margalejo H. Mirallas L. Obis A. Ortiz de Sol\'orzano O. P\'erez J. Porr\'on M. J. Puyuelo A. Quintana
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Pith reviewed 2026-05-18 09:04 UTC · model grok-4.3

classification ⚛️ physics.ins-det hep-ex
keywords dark matter detectionlow energy threshold37Ar calibrationTREX-DMGEM-Micromegas readouttime projection chamberargon ionizationneutron activation source
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The pith

A novel GEM-Micromegas readout in TREX-DM detects ³⁷Ar decays at 2.82 keV and 270 eV, reaching near single-electron ionization thresholds in argon.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that a ³⁷Ar source produced by neutron activation of CaO can be used to calibrate the TREX-DM high-pressure time projection chamber at very low energies. With a combined GEM-Micromegas readout, the team recorded the expected X-ray and electron emissions from the source decays and demonstrated that the detector can resolve signals close to the energy needed to produce a single ionization electron in argon. A sympathetic reader would care because low-mass dark matter particles are expected to deposit even smaller amounts of energy, so pushing the threshold this low directly expands the range of masses and cross-sections that can be probed in direct detection experiments.

Core claim

The TREX-DM detector, equipped with a novel combined GEM-Micromegas readout system, successfully detected both the 2.82 keV and 270 eV characteristic emissions from ³⁷Ar decays in a high-pressure argon-filled time projection chamber. The source was produced via fast neutron activation of CaO powder at the HiSPANoS facility, yielding O(1) kBq activity. This performance establishes an energy threshold in TREX-DM that approaches the single-electron ionization energy of argon.

What carries the argument

The combined GEM-Micromegas readout system in the high-pressure time projection chamber, which amplifies and records ionization signals from low-energy deposits in argon.

If this is right

  • The detector can now be calibrated at energies directly relevant to low-mass dark matter interactions.
  • Background rejection and signal reconstruction have been validated down to the single-electron regime.
  • The activation-based source production method provides a repeatable, low-activity calibration tool for similar detectors.
  • This threshold performance supports extended searches for dark matter particles below current experimental limits.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the low threshold holds during longer background runs, the experiment could set new limits on dark matter-electron scattering in previously inaccessible mass ranges.
  • The readout approach may transfer to other noble-gas TPCs seeking single-electron sensitivity for neutrino or rare-event physics.
  • A direct next step would be to apply the same calibration to quantify the actual dark matter exclusion reach with this configuration.

Load-bearing premise

The observed signals at 270 eV and 2.82 keV come from ³⁷Ar source decays rather than background, noise, or instrumental artifacts, and the source activity matches the expected value from neutron activation.

What would settle it

Absence of peaks at the predicted energies or a measured rate inconsistent with the O(1) kBq source activity in a background-subtracted spectrum would falsify the detection claim.

read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports the production of an ^{37}Ar calibration source via fast neutron activation of CaO powder at the HiSPANoS facility (yielding O(1) kBq activity), its deployment in the TREX-DM high-pressure TPC, and the detection of the characteristic 2.82 keV and 270 eV decay lines using a novel combined GEM-Micromegas readout. The central claim is that this setup achieves an energy threshold in TREX-DM approaching the single-electron ionization energy of argon.

Significance. If the signals are robustly attributed to ^{37}Ar, the work would provide a practical low-energy calibration method for gaseous TPCs targeting low-mass dark matter, where sub-keV thresholds are essential. The combined readout approach could represent a technical advance for threshold performance in high-pressure argon detectors.

major comments (2)
  1. [Results section (spectra and rate analysis)] The results section does not present a background-subtracted spectrum or a quantitative comparison of the observed count rate in the 270 eV and 2.82 keV windows to the expected rate from the stated O(1) kBq source activity (accounting for branching ratios, solid angle, and detection efficiency). Without this, the attribution of the peaks to ^{37}Ar decays rather than background or instrumental effects remains unverified and is load-bearing for the central claim.
  2. [§5 (threshold performance discussion)] The threshold performance claim (approaching single-electron ionization) lacks an explicit derivation or measurement of the effective ionization energy and resolution at the 270 eV scale; the manuscript should show how the threshold is extracted from the data (e.g., via efficiency curves or noise characterization) rather than stating it qualitatively.
minor comments (2)
  1. [Figure captions] Figure captions should explicitly state whether the displayed spectra are raw, background-subtracted, or efficiency-corrected.
  2. [Abstract and §2] The abstract and introduction would benefit from a brief statement of the measured activity (with uncertainty) and a reference to the activation cross-section or monitoring method used at HiSPANoS.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive comments, which help clarify the presentation of our results. We address each major comment below and will revise the manuscript accordingly to strengthen the evidence for the ^{37}Ar attribution and the threshold extraction.

read point-by-point responses

  1. Referee: [Results section (spectra and rate analysis)] The results section does not present a background-subtracted spectrum or a quantitative comparison of the observed count rate in the 270 eV and 2.82 keV windows to the expected rate from the stated O(1) kBq source activity (accounting for branching ratios, solid angle, and detection efficiency). Without this, the attribution of the peaks to ^{37}Ar decays rather than background or instrumental effects remains unverified and is load-bearing for the central claim.

    Authors: We agree that a background-subtracted spectrum and quantitative rate comparison would strengthen the attribution. In the revised manuscript we will include a background-subtracted spectrum and a direct comparison of the observed event rates in both the 270 eV and 2.82 keV windows to the expected rates calculated from the measured O(1) kBq source activity, incorporating the known branching ratios, solid-angle acceptance, and detection efficiency. This addition will make the ^{37}Ar origin of the peaks explicit and quantitative. revision: yes

  2. Referee: [§5 (threshold performance discussion)] The threshold performance claim (approaching single-electron ionization) lacks an explicit derivation or measurement of the effective ionization energy and resolution at the 270 eV scale; the manuscript should show how the threshold is extracted from the data (e.g., via efficiency curves or noise characterization) rather than stating it qualitatively.

    Authors: We acknowledge that the threshold extraction requires a more explicit description. In the revised manuscript we will add a dedicated subsection in §5 that derives the effective ionization energy and resolution at the 270 eV scale from the data. This will include the noise characterization, the construction of efficiency curves as a function of energy, and the quantitative criterion used to define the threshold, thereby demonstrating that the performance approaches the single-electron ionization energy of argon. revision: yes

Circularity Check

0 steps flagged

No circularity: pure experimental observation of known decay lines

full rationale

The paper describes production of a 37Ar source via neutron activation and its use to calibrate a high-pressure TPC with GEM-Micromegas readout. The central claims are empirical: detection of the 2.82 keV and 270 eV lines and achievement of low energy threshold. No equations, derivations, fitted parameters, or self-citation chains are present that could reduce any result to its own inputs by construction. Signal attribution rests on standard nuclear data and background subtraction, which are externally verifiable and not self-referential.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This experimental calibration report relies on standard nuclear decay properties and detector physics rather than new postulates or fitted parameters.

axioms (1)
  • standard math The decay scheme and emission energies of 37Ar are known and correctly applied to interpret the observed signals.
    Detection of characteristic 2.82 keV and 270 eV lines assumes established nuclear data for 37Ar electron capture decay.

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Reference graph

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