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REVIEW 3 major objections 6 minor 21 references

Signal shape studies and rate dependence of HFO-based gas mixtures in RPC detectors

T0 review · 3 major / 6 minor · reviewed 2026-08-09 · deepseek-v4-flash

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Pith's one-line read HFO/CO2 gas blends can replace the standard RPC gas, at a cost in working voltage and charge per hit.

desk verdict Useful beam-test data on HFO/CO2 RPC mixtures, but the headline 1.6/1.7 charge-per-hit ratio rests on a rate-independent assumption the paper does not justify. read the letter →

arxiv 2502.02374 v1 pith:XVFYMZOE submitted 2025-02-04 physics.ins-det nucl-ex

L. Quaglia , M. Abbrescia , G. Aielli , R. Aly , M. C. Arena , M. Barroso , L. Benussi , S. Bianco
show 51 more authors
F. Bordon D. Boscherini A. Bruni S. Buontempo M. Busato P. Camarri R. Cardarelli L. Congedo D. De Jesus Damiao F. Debernardis M. De Serio A. Di Ciaccio L. Di Stante P. Dupieux J. Eysermans A. Ferretti M. Gagliardi G. Galati S. Garetti R. Guida G. Iaselli B. Joly S.A. Juks K.S. Lee B. Liberti D. Lucero Ramirez B. Mandelli S.P. Manen L. Massa A. Pastore E. Pastori D. Piccolo L. Pizzimento A. Polini G. Proto G. Pugliese D. Ramos G. Rigoletti A. Rocchi M. Romano P. Salvini A. Samalan R. Santonico G. Saviano M. Sessa S. Simone L. Terlizzi M. Tytgat E. Vercellin M. Verzeroli N. Zaganidis
This is my paper · ORCID
classification physics.ins-detnucl-ex
keywords ResistivePlateChamberseco-friendlygasmixturesHFO/CO2tetrafluoropropenebeamtestratedependencegammairradiationagingstudies
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 reports beam-test studies of resistive plate chambers (RPCs) operated with eco-friendly gas mixtures in which the standard high-global-warming refrigerant C2H2F4 is replaced by tetrafluoropropene (HFO) diluted with CO2. It tries to establish that increasing the HFO fraction improves the detector response, at the price of raising the working point by roughly 1 kV for every 10% HFO added, and that the average charge released per gamma hit at the working point is about 1.6/1.7 times higher than with the standard mixture. This matters because EU regulations are phasing down the fluorinated greenhouse gases currently used in LHC muon detectors, and a drop-in alternative must be shown to maintain efficiency, rate capability, and long-term stability. The paper's rate-dependence analysis uses the linear relation between current density and measured gamma cluster rate to extract the average charge per hit, which is then used to flag a potentially faster aging that a dedicated long-term campaign is checking.

What carries the argument

The load-bearing object is Eq. (1), $I/A = \langle Q\rangle\cdot N_{\gamma-\mathrm{detected}}/(A\cdot\Delta t) + \mathrm{DCD}$, which states that the current density drawn by an RPC is a linear function of the measured gamma cluster rate, with slope equal to the average charge released per detected gamma hit and intercept equal to the dark current density. The analysis fits this line to current-versus-rate data taken at the working point and uses the slope as a mixture-by-mixture estimator of charge per hit, connecting detector chemistry to aging risk. Two auxiliary tools carry the comparison: the working point defined as the knee of a logistic fit to the efficiency curve plus 150 V, and a correction for the voltage drop across the resistive bakelite electrodes, which is estimated as the product of circulating current and electrode resistance.

What would settle it

Compare the slope of current density versus gamma cluster rate with the average charge per hit measured directly from digitized waveform integrals, at several irradiation levels and voltages. If the slope changes with rate or voltage, or disagrees with the direct integral, the single-charge linear assumption fails.

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Extended reading notes

Core claim

The paper claims that replacing C2H2F4 with HFO/CO2 is viable: all tested HFO-based mixtures reached full muon efficiency in avalanche mode, and the efficiency curves show a slight improvement in maximum efficiency as HFO concentration rises. The cost is an increase of the detector working point of about 1 kV per 10% HFO, and prompt-charge distributions shifted to higher values, meaning larger avalanche signals and more large-signal contamination. At the working point, the average total charge released per detected gamma hit, extracted as the slope of current density versus gamma cluster rate, is about 1.6/1.7 times higher for the eco-friendly mixtures than for the standard gas; this higher charge per hit is the paper's main concern for long-term aging. The paper also reports that one year of irradiation (about 115 mC/cm2 on one detector) produced only a small working-point shift and no significant efficiency degradation, so the aging question remains open.

Load-bearing premise

The analysis assumes that the current density is a linear function of the gamma cluster rate with a single rate-independent average charge per hit; if the average charge drifts with rate or voltage, the mixture comparisons and the aging-risk estimate lose their footing.

Editorial extensions

If this is right

  • Adding HFO to the gas raises the working point by roughly 1 kV per 10% HFO, so selecting a mixture is a trade-off between eco-friendliness and available high voltage, not a free upgrade.
  • The average charge released per gamma hit is about 1.6/1.7 times higher for HFO/CO2 blends than for the standard mixture, which the paper treats as a concrete aging risk that the ongoing long-term campaign is designed to test.
  • Increasing the HFO fraction improves the detector response, but the large-signal contamination grows more steeply with voltage than for the standard gas, narrowing the useful operating window.
  • Under gamma irradiation, efficiency curves shift to higher voltages and maximum efficiency drops, an effect the paper attributes to the voltage drop on the resistive electrodes as absorbed current rises.
  • After one year of irradiation corresponding to about 115 mC/cm2 on one detector, only a slight working-point increase and about 2% efficiency loss were observed, with no significant performance degradation.

Reading between the lines

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

  • If the single-charge linear model is exact, the ratio of slopes between mixtures predicts the relative aging rate; a detector run on a 35% HFO blend would be expected to accumulate charge about 1.6/1.7 times faster than on the standard gas at equal rate, so the aging campaign should compare integrated charge rather than calendar time.
  • The 1 kV-per-10% HFO rule implies that a mixture with high HFO fraction may exceed the comfortable voltage range of existing chambers; a practical drop-in would therefore need either a lower-HFO blend with acceptable response or a reduction of the electrode gap or resistivity.
  • The same linear-fit method could be applied to the large-signal (streamer) contribution separately, separating the avalanche charge and streamer charge per gamma hit; the paper does not separate them.
  • A testable prediction: if the bakelite voltage drop is the cause of the rate-dependent efficiency loss, the extracted slope $\langle Q\rangle$ should appear constant at low rates and deviate only once the voltage drop becomes appreciable; checking that boundary would validate the method.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 6 minor

Summary. The paper reports beam-test studies of Resistive Plate Chambers operated with HFO/CO2-based gas mixtures as an eco-friendly alternative to the standard C2H2F4-based mixture. The authors present source-off efficiency curves and prompt-charge distributions for several mixtures plus a standard reference, rate-dependent behavior when the GIF++ 137Cs source is turned on, and a linear extraction of the average charge per gamma hit from current density versus measured gamma cluster rate. They also compare source-off and source-on performance before and after one year of irradiation, corresponding to an integrated charge of about 115 mC/cm2 for the EP-DT detector. The central claims are that increasing HFO concentration improves the RPC response, raises the working point by about 1 kV per 10% HFO, and that HFO-based mixtures release about 1.6/1.7 times more charge per hit than the standard gas, motivating an ongoing aging campaign.

Significance. If the quantitative charge-per-hit result holds, the paper provides important input for the LHC RPC community's transition away from fluorinated greenhouse gases. The study is significant for its systematic comparison across many mixtures, using two independent detector systems (ALICE and EP-DT) and a well-characterized irradiation facility. The qualitative trends—working-point increase with HFO fraction, higher prompt-charge distributions, and rate-dependent efficiency loss—are direct measurements and are likely robust. The main quantitative inference, the average-charge ratio from Eq. (1), is a simple linear-fit interpretation, and the aging comparison provides useful long-term data. The paper's central claims are not circular; they rest on direct measurements rather than an assumed model. The main weaknesses are the lack of uncertainties on the presented data and the insufficiently supported assumption of a rate-independent average charge per hit.

major comments (3)
  1. [Section 3.1, Eq. (1), Fig. 6] The average charge per gamma hit <Q> is extracted from the slope of a linear fit of I/A versus measured gamma cluster rate, but the data points are taken at working points estimated for each irradiation condition. Because Figure 5a shows that the efficiency curves shift with rate and the text attributes part of this shift to the voltage drop on the bakelite electrodes, the actual voltage across the gas gap may differ from point to point. If <Q> is rate-dependent, for instance because of gain changes from the bakelite I*R drop, the fitted slope becomes a rate-weighted average rather than a mixture property, and the comparison across mixtures is biased. This is load-bearing because the conclusion that HFO mixtures release about 1.6/1.7 times more charge per hit and the associated aging-risk estimate rest directly on this slope. The paper should state whether the effective high voltage was corrected for the bakelite voltage drop before the working point was chosen, and should report fit residuals, fit-parameter uncertainties, or a rate-restricted repeat of the fit to demonstrate that a constant slope is tenable.
  2. [Figures 3, 5, and 6] The central quantitative claims—the approximately 1 kV per 10% HFO working-point increase, the charge-per-hit ratio of 1.6/1.7, and the rate-dependent efficiency loss—are presented without statistical or systematic error bars. This makes it impossible to assess whether differences between mixtures are significant or whether the reported trends are consistent within uncertainties. Please add at least statistical uncertainties on the efficiency points, current densities, gamma cluster rates, and the linear-fit parameters, and discuss systematic uncertainties such as the absolute normalization of the gamma rate, the choice of working point, and the uncertainty on the effective high voltage.
  3. [Section 3.2, Fig. 7] The approximately 2% decrease in maximum efficiency between July 2023 and July 2024 is attributed to "a geometrical misalignment of the detector in 2024, not present in 2023," but no alignment measurement, survey, or data supporting this statement is presented. Since this attribution is used to argue that no significant performance degradation occurred apart from the working-point increase, it is load-bearing for the aging conclusion. Please either provide quantitative evidence for the misalignment or present the aging comparison without that attribution.
minor comments (6)
  1. [Abstract vs. Section 3.1] The abstract and conclusions quote the charge-per-hit ratio as about 1.6/1.7 times higher than the standard gas, while Section 3.1 states "this quantity is ≈1.5/2 times higher"; please reconcile the quoted ratio.
  2. [Section 2.1] In the description of the analysis output, "the trend of the aforementioned vales" should read "the trend of the aforementioned values."
  3. [Section 3.2] The phrase "a a geometrical misalignment" contains a duplicated article; please correct it.
  4. [Figure 5a caption and text] The caption of Figure 5a refers to "Efficiency/large signal probability vs HV eff" while the text in Section 3.1 refers to "efficiency and large signal contamination"; please use consistent terminology.
  5. [Section 2.1 and Eq. (1)] The gamma cluster rate is measured by the RPC itself with a detection efficiency of the order of one per mille; the paper should state explicitly whether the rate entering Eq. (1) and Figure 6 is the raw detected rate or a corrected rate, since the equation formally uses the measured cluster rate.
  6. [Table 2] MIX0 contains no HFO but 95% CO2, so it is not an HFO/CO2 blend in the same sense as the other mixtures; a sentence explaining why this mixture is included in the HFO/CO2 scan would improve clarity.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the central claims are direct measurements; the only model input, Eq. (1), is a linear parameterization used to define <Q> rather than a prediction derived from itself.

full rationale

The paper's central results are direct experimental measurements: source-off efficiency curves, prompt-charge distributions, and current-versus-rate data. The quantitative claim that HFO/CO2 mixtures release about 1.6/1.7 times more charge per hit than the standard mixture comes from the slope of Eq. (1), I/A = <Q> * (gamma cluster rate) + DCD, fitted in Figure 6. This is a measurement procedure: <Q> is defined as the slope of current density versus measured gamma cluster rate, and the cross-mixture ratio is a ratio of fitted slopes. It is not a prediction of data that were used to fit the parameter, and it does not reduce to an input by construction. The possible rate dependence of <Q> due to the bakelite-resistance voltage drop, raised in the skeptic's analysis, is a correctness or robustness concern about the linear model, not circularity, because Eq. (1) is not claimed to be derived from the very data it is used to interpret. The paper self-cites the collaboration's prior work, notably [12] for the HVeff definition and the bakelite voltage-drop correction and [19] for the aging-test methodology, but these citations are contextual and methodological, not the load-bearing justification for the main trend. The qualitative response improvement with HFO is also supported by the directly measured efficiency curves and charge spectra in Figures 3 and 4, independent of the fitted <Q> values. No step in the paper equates its conclusion with its assumptions by definition, and no fitted parameter is renamed as a prediction.

Assumptions & free parameters 3 free parameters · 4 assumptions · 0 invented entities

The paper's central quantities come from fits and standard detector-model assumptions. The <Q> values in Figure 6b are slopes fit to Eq. (1); the efficiency knee voltages come from logistic fits; the rate dependence analysis assumes the bakelite-resistance voltage-drop model. No wholly new entities are introduced.

free parameters (3)
  • Average charge per detected gamma cluster <Q> = per mixture, about 1.5-2x the STD value (Figure 6b)
    Extracted as the slope of the linear fit of current density vs measured gamma cluster rate (Eq. 1, Figure 6). Used to compare eco-friendly mixtures to STD and to estimate aging risk.
  • Logistic-fit parameters (knee voltage, maximum efficiency) = WP = knee + 150 V; values per mixture in Figures 3-5
    Efficiency vs HVeff curves are interpolated with a logistic function to define the working point (Section 3.1); the resulting working points drive the rate-dependence comparison.
  • Dark current density DCD = intercept of the linear fit in Figure 6a (not quoted numerically)
    Nuisance parameter in Eq. (1); the paper notes an observed increase in dark current between 2023 and 2024 that partly explains higher 2024 currents.
assumptions (4)
  • domain assumption RPC signal identification: a muon hit is a signal above threshold in the muon window, with peak polarity matching the expected polarity and no cross-talk pattern (Section 2.1).
    Standard analysis convention; cross-talk events are discarded based on polarity pattern.
  • domain assumption The relation I/A = <Q> * rate + DCD (Eq. 1) is linear, with <Q> constant across the scanned voltage and rate range.
    This is the central modeling assumption for the charge-per-hit extraction; if <Q> is rate-dependent the conclusions on aging weaken.
  • domain assumption The electrode voltage drop correction, subtracting the product of absorbed current and electrode resistance from HVeff, accurately predicts detector gain at rate (Section 3.1, cf. [12]).
    Used to explain the shift of efficiency curves under irradiation; taken from prior collaboration work.
  • domain assumption Maintaining relative humidity at 40% keeps bakelite resistivity constant (Section 2).
    Required so that the resistivity, and hence the voltage-drop model, is stable across runs.

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Cite this review

Pith. "Pith review of Signal shape studies and rate dependence of HFO-based gas mixtures in RPC detectors." pith.science (2026). https://pith.science/paper/XVFYMZOE

@misc{pith2026250202374,
  author       = {Pith},
  title        = {Pith review of: Signal shape studies and rate dependence of HFO-based gas mixtures in RPC detectors},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/XVFYMZOE}},
  note         = {Machine review of arXiv:2502.02374}
}
abstract

The RPCs employed at the LHC experiments are currently operated in avalanche mode, with a mixture containing a large fraction of C$_{2}$H$_{2}$F$_{4}$ ($\approx$90\% or more) with the addition of i-C$_{4}$H$_{10}$ and SF$_{6}$ in different concentrations. However, C$_{2}$H$_{2}$F$_{4}$ and SF$_{6}$ are fluorinated greenhouse gases (F-gases) with Global Warming Potential (GWP) of $\approx$1400 and $\approx$22800, respectively. EU regulations imposed a progressive phase-down of C$_{2}$H$_{2}$F$_{4}$ production and consumption, aiming at strongly reducing its emission. This is already resulting in an increase of its price and reduction in availability. The most desirable long-term solution to this problem is to find an alternative, F-gases-free gas mixture, able to maintain similar detector performance. To address this challenge, the RPC ECOGasas@GIF++ collaboration (including RPC experts of ALICE, ATLAS, CMS, SHiP/LHCb, and the CERN EP-DT group) was created in 2019. The collaboration is currently studying a gas from the olefine family, the C$_{3}$H$_{2}$F$_{4}$ (or simply HFO, with GWP $\approx$6), to be used, in combination with CO$_{2}$, as a substitute for C$_{2}$H$_{2}$F$_{4}$. This contribution will focus on the signal shape studies that have been carried out by the collaboration during dedicated beam test periods. The methodology used in the data analysis will be presented, together with the results obtained with several HFO-based gas mixtures, and with the currently employed one. Furthermore, results on the counting-rate dependence of the RPC performance, obtained by combining the muon beam with the GIF++ $^{137}$Cs source with different attenuation factors, will also be presented.

Figures

Figures reproduced from arXiv: 2502.02374 by the authors.

Figure 1
Figure 1. Sketch of the experimental setup installed in GIF++. A detailed [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 4
Figure 4. Left panel: Source-off prompt charge distributions for the ALICE [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 3
Figure 3. Source-off efficiency vs HVeff curves for the ALICE (left panel) and EP-DT (right panel) RPCs The efficiency curves have been interpolated using a logistic function (as usually done for RPCs, as reported for example in (12)), to extract information about the detector working point (WP, i.e. the voltage at which full efficiency is reached), defined as the knee voltage (HVeff where the efficiency reaches 95% of its ma… view at source ↗
Figures from the paper (4 more)
Figure 5
Figure 5. Figure 5: Left panel: Efficiency/large signal probability vs HV [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]
Figure 7
Figure 7. Figure 7: Left panel: Source-off efficiency and large signal contamination [PITH_FULL_IMAGE:figures/full_fig_p005_7.png]
Figure 6
Figure 6. Figure 6: Left panel: Current density vs measured γ cluster rate for the ALICE RPC for all the tested gas mixtures. The straight lines shown in the panels are linear fits of the data. Right panel: Estimated average charge released in the gas per γ hit for all the tested gas mixt…
Figure 8
Figure 8. Figure 8: Left panel: Comparison of the absorbed current at working point as [PITH_FULL_IMAGE:figures/full_fig_p006_8.png]

Discussion (0). Continue with ORCID to comment.

Reference graph

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