First demonstration of a bubble-assisted Liquid Hole Multiplier operation in liquid argon

Amos Breskin; Andrea Tesi; David Vartsky; Eran Erdal; Shikma Bressler

REVIEW 3 major objections 7 minor 34 references

First demonstration of a bubble-assisted Liquid Hole Multiplier operation in liquid argon

T0 review · 3 major / 7 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read A bubble trapped under a perforated electrode lets liquid argon detect alpha particles and image them with a silicon-photomultiplier array.

desk verdict First LAr bubble-assisted LHM demonstration is credible and worth refereeing, but the bubble itself is inferred, not seen. read the letter →

arxiv 1908.04974 v2 pith:QMMTKOSQ submitted 2019-08-14 physics.ins-det

Eran Erdal , Andrea Tesi , Amos Breskin , David Vartsky , Shikma Bressler This is my paper · ORCID

classification physics.ins-det

keywords liquidargondetectorsholemultiplierTHGEMelectroluminescencenoble-liquidTPCchargemultiplicationSiPMimagingalphaparticles

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper demonstrates, for the first time, that a bubble-assisted Liquid Hole Multiplier can operate in liquid argon at roughly 90 K. The detector couples a THGEM electrode to a gas bubble held underneath by a wire grid; alpha-particle ionization electrons are focused into the electrode holes, cross the liquid-gas interface, and produce electroluminescence inside the bubble. The pulse area scales linearly with the voltage across the THGEM at low transfer field, and raising the transfer field adds a second electroluminescence component and eventually about tenfold charge multiplication. A four-element SiPM readout reconstructs the annular shape of the alpha source, so the device can image ionization patterns. The authors present these as preliminary results and note that the bubble is inferred from pressure-response tests rather than directly observed.

What carries the argument

The load-bearing object is the bubble-assisted Liquid Hole Multiplier: a 0.4 mm thick THGEM electrode with hexagonal 0.3 mm diameter holes spaced 0.7 mm apart, immersed in liquid argon. A grid of 55 μm heating wires, placed about 1.6 mm below the electrode, is used once to generate a stable vapor bubble under the THGEM and then defines the transfer field $E_t$ between the electrode bottom face and the wires. Ionization electrons collected into the holes cross the liquid-gas interface and excite electroluminescence in the bubble; the transfer field controls where that EL is produced and, at high values, drives charge multiplication near the wires. EL photons are read out by a TPB-coated PMT for pulse shapes or by a windowless quad-SiPM for two-dimensional position reconstruction.

What would settle it

Fit the cryostat with a side window or fast camera and cycle the pressure while imaging the space below the THGEM. If the EL signals change without the bubble vanishing, or if the same pressure-dependent EL appears when no bubble can exist, the bubble mechanism would not be established.

Watch

Extended reading notes

Core claim

The paper's central claim is that the Liquid Hole Multiplier concept—a perforated electrode immersed in noble liquid with a stable gas bubble underneath—works in liquid argon, not only in liquid xenon. Using a THGEM with a wire-generated bubble and a $^{241}$Am source, the authors observe S1 scintillation and S2 electroluminescence pulses whose area grows linearly with $\Delta V_{\mathrm{THGEM}}$ at zero transfer field. Increasing the transfer field to several kV/cm first adds a second EL component near the heating wires and, above roughly 4 kV/cm, produces about a tenfold charge multiplication. With a quad-SiPM and center-of-gravity reconstruction, the annular $\alpha$-source pattern is recovered, demonstrating 2D imaging; at high transfer field the 59.5 keV gamma line also becomes visible. The paper notes that the observed 3–4 μs decay constant in LAr is not explained by known argon scintillation or electron-emission timescales, and that the energy and imaging resolutions are preliminary and poorer than the corresponding LXe results.

Load-bearing premise

The load-bearing assumption is that the pressure-dependent electroluminescence really comes from a stable gas bubble trapped under the THGEM; the cryostat has no side window, so the bubble is inferred from EL disappearing on pressure rise and reappearing on pressure drop rather than from direct observation.

Editorial extensions

If this is right

The LHM principle now spans two noble liquids, so liquid-argon detectors could adopt the same bubble-assisted electrode readout for ionization and, with a future CsI coating, for scintillation photons.
At low transfer field the linear EL response provides an energy-measuring mode for alpha particles in LAr, with a measured RMS resolution of 13.5% in this uncalibrated setup.
At high transfer field the roughly tenfold charge multiplication near the wires offers a gain stage inside the liquid, which could amplify small ionization signals before readout.
The quad-SiPM reconstruction of the annular source shows that 2D imaging of ionization-induced EL is feasible in LAr, a basic requirement for a position-sensitive TPC readout.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

The unexplained 3–4 μs decay constant suggests electron transfer across the LAr bubble interface is slower than in LXe, which would need to be understood before timing-based S1/S2 separation is designed in argon.
If the pressure-response signature is confirmed visually, the same wire-grid scheme could be used to study bubble stability and EL yield as a function of pressure and temperature, turning the current proof-of-principle into a quantitative characterization.
Because 59.5 keV gammas become visible at high $E_t$, a CsI-coated LAr LHM might detect S1, S1', and S2 in a single-phase volume, but that combination has not yet been demonstrated in argon.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

First, the thing to know: this is a genuine first demonstration of a bubble-assisted LHM in liquid argon, not a routine transfer from the group's LXe work. They had to form and sustain a bubble at 90 K, reoptimize fields, and show EL, modest gain, and imaging with a quad-SiPM. The paper is clearly written and honestly flags its own limitations.

What it does well: the pressure-response test is a reasonable indirect check for the bubble—EL disappears on pressure rise, reappears on drop—and the linear pulse-area vs ΔV_THGEM trend is what you expect for EL without gain. The ~10-fold multiplication at high transfer fields is a real observation, and the annulus imaging, though qualitative, reproduces the source shape. The authors also cite their earlier LXe work appropriately; the novelty is the LAr operation itself.

Soft spots, in order: (1) the bubble is never directly observed. The cryostat has no side window, so the claim rests on the pressure-response signature. That is a legitimate inference, and the prior LXe direct observation makes it plausible, but it is not unique—boiling at the wires or a change in electron extraction could in principle mimic the on/off behavior. A direct optical check or an independent bubble sensor would settle it. (2) The 3–4 µs decay constant is unexplained; the authors say the known LAr timescales don't account for it. That is a loose end, not a fatal one. (3) The light yield is uncalibrated, the energy resolution has no error bars, and the SiPM cryogenic operation is not fully characterized. These are reasonable for a proof-of-principle, but they limit how far the quantitative claims can be pushed.

The stress-test note about the bubble inference is fair and I think it lands. But it does not sink the paper. The central claim—first LAr LHM operation—is credible, and the authors are appropriately cautious. The citation pattern looks fine; they are not overclaiming novelty relative to their own LXe work.

Who is this for? Anyone working on noble-liquid TPCs, especially LAr dual-phase alternatives or local readout concepts. It deserves a serious referee; the right referee will push for a direct bubble check and more quantitative characterization, but this is exactly the kind of incremental but real result that should go through peer review rather than be desk rejected.

Referee Report

3 major / 7 minor

Summary. The paper reports the first operation of a bubble-assisted Liquid Hole Multiplier (LHM) in liquid argon at ~90 K. A THGEM electrode is immersed in LAr, a bubble is formed underneath by a heating-wire grid, and alpha particles from an 241Am source produce electroluminescence signals read out by a TPB-coated PMT and a quad-SiPM array. The authors report a linear dependence of the EL pulse area on the THGEM voltage, a 13.5% RMS energy resolution, the appearance of a second EL component and ~10-fold charge multiplication at high transfer fields, and a qualitative 2D reconstruction of the annular alpha source. The existence of the bubble is inferred from the disappearance/reappearance of EL signals under sudden pressure changes, because the cryostat has no side window.

Significance. If the central claim holds, this is a useful first step toward extending the LHM concept, previously validated in liquid xenon, to liquid argon, with potential applications in dark-matter, neutrino, and neutron detection. The paper benefits from a clear experimental methodology, a falsifiable pressure-response test, reproduction of the known LXe phenomenology, and a direct comparison with prior LXe-LHM results. The work is explicitly preliminary and uncalibrated, and the authors are transparent about the missing direct observation of the bubble and about the unexplained slow decay constant. The significance is moderate: it is a proof-of-principle demonstration, not a full characterization, and the main claim depends on an inference whose exclusion of alternative explanations is not yet complete.

major comments (3)

[Section 2 and Section 3, first paragraph] The manuscript states that the cryostat has no side window and that the bubble existence was inferred only from the response of EL signals to sudden pressure changes (disappearance on pressure rise, reappearance on pressure drop). This is the sole evidence for the bubble, yet the central claims of bubble-assisted EL and charge multiplication in the bubble require a stable vapor pocket under the THGEM. A pressure-dependent process elsewhere, such as a change in electron extraction at a liquid-gas interface or local boiling at the wires, could in principle produce a similar on/off EL signature. I recommend adding direct visual observation through a viewport, an independent bubble sensor, or spatially resolved EL imaging that localizes the emission to the volume under the THGEM; without such evidence, the statement that bubble containment was demonstrated is stronger than what the data show.
[Section 3, first paragraph, and Section 4] After the initial 10 s heating pulse, no further heating was applied. At 90 K and 1365 mbar the liquid is near saturation, so the long-term stability of a vapor pocket without continuous heating is not self-evident and relies on unquantified heat leaks or metastability. The paper does not report the pressure and temperature stability over the data-taking period or the lifetime of the inferred bubble. Please provide these measurements or otherwise justify that a persistent trapped bubble, rather than intermittent boiling at the wires or a fluctuating liquid-gas interface, is responsible for the recorded S2 signals.
[Section 3.1, 'Typical signals'] The paper reports a 3-4 µs decay constant of the S2 signals and states that neither the known LAr scintillation timescales (5 ns and 860 ns) nor the electron liquid-to-gas transition timescales can explain it. Because this signal is the experimental signature of the LAr-LHM response, the unexplained slow component weakens the identification of the recorded light as electroluminescence in the bubble. I recommend adding a systematic study of the waveform shape (dependence on temperature, pressure, fields, and source position) or identifying a candidate mechanism; if the mechanism remains unknown, the claim should be correspondingly qualified.

minor comments (7)

[Abstract] The abstract contains a typo: 'We demonstrate, for the time' should read 'We demonstrate, for the first time'.
[Section 3, first paragraph] 'All measurements where conducted' should be 'All measurements were conducted'.
[Section 3.3 and 3.4] There are two subsections numbered 3.3 ('Amplification in the transfer gap' and 'Position reconstruction'); the second should be renumbered 3.4.
[Figure 9 caption] The caption refers to 'the alpha particle peak of Figure 7', but the alpha-particle peak with the Gaussian fit appears in Figure 8, not Figure 7.
[Section 3.3] 'Once can clearly see' should be 'One can clearly see'.
[Section 2 and Figure 2 caption] The text states the heating-wire grid is 1.6 mm below the THGEM, while the Figure 2 caption states 1.5 mm; please reconcile this discrepancy.
[Figure 6] The linear trend in panel (a) is presented without error bars, fit parameters, or a statement of how many events define each point; adding this information would strengthen the claim that the trend indicates EL without charge gain.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the claimed first demonstration is a direct experimental result, with prior LXe work used only as context and comparison.

full rationale

The paper reports a direct experimental demonstration of a bubble-assisted LHM in LAr, and its central claim does not reduce to any input fitted by the authors. The LAr signals (pulse-area vs. ΔV_THGEM, transfer-field amplification, and quad-SiPM imaging) are measured independently in this work; no parameter from prior LXe papers is fitted into these data to produce the claimed result. Citations to earlier LXe-LHM works [2-7] provide the concept, methodology, and comparison values, but the first-demonstration claim rests on the present LAr measurements. The only indirect step is the inference of bubble existence from EL-signal response to sudden pressure changes (Section 3), which is an experimental/diagnostic limitation rather than circularity: the pressure-response on/off behavior is an independent empirical signature, and the citation to prior work supplies context rather than the target result itself. Because no prediction is equivalent by construction to a fitted parameter or to a self-citation chain, the appropriate circularity score is 0.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

No free parameters or invented entities are used. The detector observables are measured directly; all assumptions are domain-level physics about bubble formation and electron extraction carried over from the LXe LHM program.

assumptions (3)

domain assumption A stable argon gas bubble can be created and sustained under the THGEM electrode by short resistive heating of the wire grid.
Central experimental premise. LHM operation requires a gas bubble; the paper infers it from signals responding to pressure changes because the cryostat lacks a viewing window (Section 3).
domain assumption Electrons deposited in liquid argon can traverse the liquid-gas interface into the bubble and induce electroluminescence there, as in liquid xenon.
The LHM concept is transferred from LXe (refs 1-7); the extraction and EL mechanism in LAr is assumed to follow the same physics, with LAr-specific timescales from refs 17 and 18.
domain assumption The measured pulse area is proportional to the number of photons and electrons, and its linear increase with delta V_THGEM indicates electroluminescence without charge gain.
Used to interpret Figure 6a and to classify the response as EL rather than gain. No absolute calibration of light yield is provided, so proportionality is assumed.

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Cite this review

Pith. "Pith review of First demonstration of a bubble-assisted Liquid Hole Multiplier operation in liquid argon." pith.science (2026). https://pith.science/paper/QMMTKOSQ

@misc{pith2026190804974,
  author       = {Pith},
  title        = {Pith review of: First demonstration of a bubble-assisted Liquid Hole Multiplier operation in liquid argon},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/QMMTKOSQ}},
  note         = {Machine review of arXiv:1908.04974}
}

read the original abstract

We demonstrate, for the time, the operation of a bubble-assisted Liquid Hole Multiplier (LHM) in liquid argon. The LHM, sensitive to both radiation-induced ionization electrons and primary scintillation photons, consists of a perforated electrode immersed in the noble liquid, with a stable gas-bubble trapped underneath. Electrons deposited in the liquid or scintillation-induced photoelectrons emitted from a photocathode on the electrode's surface, are collected into the holes; after crossing the liquid-gas interface, they induce electroluminescence within the bubble. After having validated in previous works the LHM concept in liquid xenon, we provide here first preliminary results on its operation in liquid argon. We demonstrate the bubble containment under a Thick Gas Electron Multiplier (THGEM) electrode and provide detector response to alpha particles, recorded with a SiPM - under electroluminescence and with modest gas multiplication; the imaging capability is also demonstrated.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

34 extracted references · 33 canonical work pages

[1]

Introduction ................................................................................................................................ 2

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[2]

Experimental setup & methodology ......................................................................................... 3

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[3]

5 3.1 Typical signals

Results ......................................................................................................................................... 5 3.1 Typical signals ....................................................................................................................... 5 3.2 Energy resolution ................................................

work page
[4]

Summary and discussion ......................................................................................................... 10

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Acknowledgements .................................................................................................................. 11

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References ................................................................................................................................. 11

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local dual-phase

Introduction The concept of Liquid Hole-Multipliers (LHMs) was proposed [1] for the combined detection of ionization electrons and primary scintillation photons generated along charged-particle tracks in noble liquids [2-7]. The original goal was to deploy LHM units within a single-phase noble liquid TPC, aiming at simplifying the structure and thus overc...

work page
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transfer field

Experimental setup & methodology The experiments were conducted in a dedicated LAr cryostat, WISArD (Weizmann Institute Liquid Argon Detector). It comprises a 100 mm in diameter, 150 mm tall cylindrical chamber filled with ~250 ml of LAr. The rest of the volume is equipped with various sensors, with the detector assembly suspended from the topmost flange;...

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Once formed, no further heating was applied to the wires

Results Prior to biasing of the THGEM electrode, current was driven through the heating wires in order to generate a bubble (30V across 61Ω for ~10 seconds). Once formed, no further heating was applied to the wires. As previously shown [3], the EL signals disappeared upon sudd...
[10]

We have shown that similar to LXe, a bubble can be generated in LAr and is sustained for a long period under a THGEM perforated electrode immersed in the liquid

Summary and discussion In this work, we have demonstrated, for the first time, the operation of an LHM detector in LAr. We have shown that similar to LXe, a bubble can be generated in LAr and is sustained for a long period under a THGEM perforated electrode immersed in the liq...
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Acknowledgements We would like to thank Dr. M. L. Rappaport (Weiz mann Institute of Science - WIS) and to Dr. H. Wang (UCLA) for their invaluable advices with the LAr cryostat -system design and Dr. A. Roy (WIS and Ben Gurion University) for his assistance with the LAr cryosta...
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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

[1] [1]

Introduction ................................................................................................................................ 2

work page

[2] [2]

Experimental setup & methodology ......................................................................................... 3

work page

[3] [3]

5 3.1 Typical signals

Results ......................................................................................................................................... 5 3.1 Typical signals ....................................................................................................................... 5 3.2 Energy resolution ................................................

work page

[4] [4]

Summary and discussion ......................................................................................................... 10

work page

[5] [5]

Acknowledgements .................................................................................................................. 11

work page

[6] [6]

References ................................................................................................................................. 11

work page

[7] [7]

local dual-phase

Introduction The concept of Liquid Hole-Multipliers (LHMs) was proposed [1] for the combined detection of ionization electrons and primary scintillation photons generated along charged-particle tracks in noble liquids [2-7]. The original goal was to deploy LHM units within a single-phase noble liquid TPC, aiming at simplifying the structure and thus overc...

work page

[8] [8]

transfer field

Experimental setup & methodology The experiments were conducted in a dedicated LAr cryostat, WISArD (Weizmann Institute Liquid Argon Detector). It comprises a 100 mm in diameter, 150 mm tall cylindrical chamber filled with ~250 ml of LAr. The rest of the volume is equipped with various sensors, with the detector assembly suspended from the topmost flange;...

work page