Resistive Plate Chambers for brain PET imaging and particle tracking and timing (TOF-tracker)

Alberto Blanco; Antero Abrunhosa; Cust\'odio F. M. Loureiro; Filomena M. C. Clem\^encio; Jan Michel; Jo\~ao Saraiva; Lu\'is Lopes; Michael Traxler; Miguel Castelo-Branco; Miguel Couceiro

REVIEW 4 major objections 5 minor 7 references

Resistive Plate Chambers for brain PET imaging and particle tracking and timing (TOF-tracker)

T0 review · 4 major / 5 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read Multigap gas detectors over 625 cm² of active area can track charged particles to about 130 μm per plane, time them to 61 ps per plane, and localize a beta-plus source to 0.49 mm FWHM from its annihilation radiation.

desk verdict Large-area RPC TOF-tracker with real but incremental progress; the 61 ps timing claim needs the electronics floor explained before it supports the headline. read the letter →

arxiv 2501.07768 v1 pith:FF3MB2SF submitted 2025-01-14 physics.ins-det

Paulo Fonte , Luís Lopes , Filomena M. C. Clemêncio , Miguel Couceiro , Susete Fetal , Custódio F. M. Loureiro , Jan Michel , João Saraiva

show 4 more authors

Michael Traxler Antero Abrunhosa Alberto Blanco Miguel Castelo-Branco

This is my paper · ORCID

classification physics.ins-det

keywords ResistivePlateChambersTOF-trackertime-of-flightPETpositionresolutiontimingchargeinterpolationannihilationphotonlocalizationmultigapRPC

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

The paper attempts to establish that multigap Resistive Plate Chambers over a 625 cm² active area can act simultaneously as precise charged-particle trackers and as gamma-ray detectors for positron-emission tomography. In a four-layer cosmic-ray telescope it measures per-layer position resolutions of 130 $\mu$m $\sigma$ (X) and 128 $\mu$m $\sigma$ (Y) and, with a simplified time readout, a per-layer timing resolution of 222 ps $\sigma$. A two-plane version with the standard two-ended strip readout reaches 61 ps $\sigma$ per layer. In a PET-like arrangement with a 0.2 mm $^{22}$Na source, the intrinsic localization precision of the annihilation radiation is reported as 0.49 mm FWHM, extrapolated to zero distance between the detecting planes. If these figures hold together, large-area gas detectors would combine sub-100 ps timing, sub-150 $\mu$m tracking, and sub-millimetre PET localization without scintillator crystals.

What carries the argument

The central object is the multigap Resistive Plate Chamber plane: two 5-gap RPCs sandwich a central X readout electrode, while the two exterior faces form Y readout electrodes. The X electrode is organised as ten structures electrically equivalent to readout strips, and each strip can be timed at one or both ends; avalanche positions come from charge interpolation over 48 strips per coordinate. The argument is carried by comparing the simplified one-ended wired-OR time readout (222 ps $\sigma$ per layer) with the standard two-ended readout (61 ps $\sigma$ per layer), and by using the plane of least confusion of the annihilation line-of-response distribution to separate source-localization accuracy from photon non-colinearity. Timing differences are corrected empirically for correlations with avalanche charge and position.

What would settle it

Repeat the PET-like measurement with the same 0.2 mm $^{22}$Na source and electronics at head separations from about 50 mm down to the closest mechanical limit, and fit the width of the plane-of-least-confusion distribution as a function of separation; the 0.49 mm FWHM claim stands only if the fitted intercept is independent of the fitted distance range and of the acollinearity distribution (for example, after changing the source housing material).

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Extended reading notes

Core claim

On the paper's own terms, the discovery is that the TOF-tracker concept—multigap RPC planes with position readout by charge interpolation and timing readout on strips—holds over 625 cm² of active area: per-plane position resolution of about 130 $\mu$m $\sigma$, and, when each timing strip is read at both ends, per-plane timing resolution of 61 ps $\sigma$. It also reports that annihilation photons from a 0.2 mm $^{22}$Na source can be localized with an intrinsic precision of 0.49 mm FWHM, obtained as the zero-distance intercept of the measured width of the line-of-response intersection distribution. The paper interprets these results as demonstrating that one gas detector technology can provide the timing and position accuracy needed for time-of-flight particle tracking and for PET imaging.

Load-bearing premise

The load-bearing premise is that the 0.49 mm FWHM localization figure, obtained by extrapolating the measured line-of-response width to zero distance between the detecting planes, is not contaminated by the same unexplained distance-dependent broadening that the paper cannot account for; if that broadening has a component that survives at zero distance, the quoted intrinsic precision is too optimistic.

Editorial extensions

If this is right

A large-area RPC tracker with two-ended strip readout can timestamp minimum-ionizing particles at about 61 ps $\sigma$ per plane, making it suitable for time-of-flight trigger and tracking systems.
The same 25 cm × 25 cm planes position charged particles to about 130 $\mu$m $\sigma$ per plane, so RPCs can act as precision trackers rather than only coarse trigger detectors.
The simplified single-ended time readout yields 222 ps $\sigma$ per plane, so builders can trade timing resolution for fewer timing channels when system cost is the limiting factor.
The zero-distance localization intercept of 0.49 mm FWHM implies sub-millimetre PET localization is in reach with a gas detector, which matters for brain imaging where spatial detail is scarce.
Because the measured position resolution is not dominated by electronic noise, further gains should come from reducing systematic effects rather than from lower-noise electronics.

Reading between the lines

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

Going beyond the paper: if the 61 ps per-plane timing is maintained when the two-ended readout is installed in the full four-plane telescope, a two-plane coincidence-time difference would have about 86 ps $\sigma$, placing a TOF-PET system in the regime where timing information contributes directly to image sharpening.
Going beyond the paper: the unexplained distance-dependent growth of the LOR width suggests a second localization mechanism beyond photon acollinearity, such as avalanche-size fluctuations or strip-readout granularity; identifying that mechanism could replace the zero-distance extrapolation with a predictive model and possibly lower the intercept.
Going beyond the paper: a direct next test is to operate the four-plane telescope with the two-ended readout while keeping the position readout active, because the simultaneous claim of sub-100 ps timing and sub-150 $\mu$m tracking currently rests on two separate experiments.

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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

Short version: the paper reports 61 ps per-layer timing and ~130 μm position resolution over 625 cm², plus a 0.49 mm FWHM intrinsic precision for localizing a β+ source via annihilation photons. The position numbers look solid. The 61 ps timing claim is not yet nailed down, and the PET extrapolation has a known unexplained term.

What's genuinely new: they scaled the TOF-tracker from small-area demonstrations to a 25×25 cm² active area, and they measured the localization precision as a function of plane distance. That distance study is useful and goes beyond the earlier RPC-PET demonstrator. The paper is also honest: it explicitly says the distance dependence can't be fully explained by non-colinearity and that further work is needed.

The soft spots:

First, the 61 ps. They report a 93 ps σ per TDC channel from a pulser test. In Experiment 3, each layer's time comes from a standard two-ended readout. If only the two ends of a single strip contribute, the electronics floor per layer is 93/√2 ≈ 66 ps before adding detector jitter. The reported 61 ps sits below that floor. The paper doesn't state how many TDC channels actually enter the layer time, doesn't give a TDC bin resolution for Experiment 3, and doesn't give an uncertainty on the 61 ps. Maybe common-mode jitter cancels in the layer-to-layer difference, or the pulser test overestimates the real noise, or more than one strip contributes. But the text doesn't explain which. The sub-100 ps part of the headline is therefore not established to my satisfaction.

Second, the 0.49 mm FWHM intrinsic precision. The measured width grows almost linearly with distance, faster than non-colinearity predicts. The zero-distance intercept is taken as the intrinsic precision, but if the unexplained broadening has a component that doesn't vanish at zero distance, the intercept is biased. The paper flags this itself, which is good, but it means the headline number is an extrapolation under an unverified assumption.

Third, minor: no uncertainties on any of the headline values, and "below 150 μm σ" is vague when the table gives 130/128 μm.

The measurements are direct, there's no circular fitting, and the self-citation pattern is appropriate for a group extending its own demonstrated concept. This paper deserves a serious referee: the detector community will want to see whether the 61 ps survives scrutiny, and the PET distance study is a useful data point. I'd send it to review, but the referee should be asked to pin down the timing electronics floor and the zero-distance extrapolation before acceptance.

Referee Report

4 major / 5 minor

Summary. The manuscript reports three experiments on multi-gap Resistive Plate Chamber (RPC) detectors with combined timing and position readout. Experiment 1 measures the localization precision of a small 22Na source from annihilation photons as a function of the distance between detecting planes, yielding an extrapolated zero-distance FWHM of 0.494 mm. Experiment 2 uses a four-layer cosmic-ray telescope with a simplified single-ended strip readout and reports average per-layer position resolutions of 130 μm and 128 μm in X and Y, and an average per-layer timing resolution of 222 ps. Experiment 3 upgrades to a two-ended strip readout on a two-plane telescope and reports a per-layer timing resolution that improves from 82 ps to 61 ps with applied voltage. The paper concludes that these results demonstrate simultaneous high-resolution timing and bidimensional tracking over an active area of 625 cm2 and sub-mm PET localization.

Significance. If the headline numbers hold, this work would be a notable step toward large-area RPCs capable of sub-100 ps timing and sub-150 μm tracking, with potential applications in brain PET and particle tracking. The paper contains direct measurements rather than fitted derivations of the claimed quantities, and the authors are transparent about an unresolved distance dependence in the PET localization data. However, several central claims are presented without uncertainties, rest on under-specified electronics or model-dependent extrapolations, and the simultaneous timing-and-tracking capability is not actually demonstrated in a single configuration. These issues are fixable with additional information and analysis, so the manuscript merits revision rather than rejection.

major comments (4)

[Methods Experiment 3 / Results Experiment 3] The 61 ps σ per-layer timing resolution is not established as written. The only electronics-precision figure in the paper is 93 ps σ per TDC channel, measured in Experiment 2 with a different, wired-OR readout. If the same 93 ps/channel applied to Experiment 3 and only two TDC channels per layer were averaged, the floor would be about 66 ps, making 61 ps impossible. The manuscript must specify the TDC type and bin width used in Experiment 3, the number of TDC channels contributing to each layer time, and whether the 93 ps figure applies to the Experiment 3 electronics. A statistical or systematic uncertainty for the 61 ps value is also needed.
[Methods Experiment 1 / Results Experiment 1 / Figure 5] The 0.49 mm FWHM intrinsic localization precision is obtained by extrapolating the measured LOR-intersection width to zero distance between detecting planes, but the authors themselves state that the distance dependence cannot be explained solely by photon non-colinearity and that 'some further work remains.' If the unexplained broadening has a component that does not vanish at zero distance, the intercept will be biased. The paper also neglects the 0.2 mm source diameter. The authors should provide a model for the full distance dependence, quantify the systematic uncertainty of the extrapolation, or report the intrinsic precision as a range rather than a single value.
[Abstract / Conclusions] The abstract claims 'simultaneous high-resolution timing and bidimensional tracking,' but the 61 ps timing was measured in Experiment 3 without position readout, while the 130 μm position resolution was measured in Experiment 2 with a different, simplified time readout that gave 222 ps timing. The simultaneous capability is not demonstrated in a single configuration. This limitation should be stated explicitly in the abstract and conclusions, or the wording should be adjusted to indicate that the two performances were obtained in separate setups with the same detector family.
[Throughout results] None of the headline numerical claims (61 ps, 130 μm, 128 μm, 0.494 mm) is reported with a statistical or systematic uncertainty. Since these are performance figures meant to be compared with other detectors, the absence of uncertainties prevents an assessment of their significance. At minimum, the authors should provide uncertainties derived from the spread over layers and from variations in the fit range, voltage, and other analysis choices.

minor comments (5)

[Table 1] The table caption is ambiguous about which triangle (upper or lower) corresponds to 'both TDC values' and which to 'only one TDC'; please clarify directly in the caption.
[Methods Experiment 3] The text says the readout was upgraded 'to the more standard readout schematized in Figure 3,' but the standard readout is shown in Figure 4; check the figure cross-reference.
[Results Experiment 1] The line '0.494 2 / 2 0.699 mm × =' appears garbled; it should read '0.494 × √2 = 0.699 mm' to make the per-layer conversion clear.
[References / Methods Experiment 1] Reference [7] is about free-volume holes in polymers, not about the angular distribution of positron-annihilation photons; please provide a reference that actually supports the 8.4 mrad FWHM acolinearity value.
[Methods Experiment 3] The statement that crosstalk between layers was 'a few percent' after switching off the high voltage of one layer is not quantified in terms of the observable used; please describe how crosstalk was measured.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: all headline figures are direct measurements, and the explicit limitations are empirical uncertainties or under-specifications, not reductions to fitted inputs or to self-citations.

full rationale

The paper's headline results are direct experimental measurements rather than derived predictions. In Experiment 1, the 0.49 mm FWHM intrinsic localization precision is obtained by extrapolating measured widths of the LOR-intersection distribution to zero distance between detecting planes. The paper explicitly notes that the distance dependence cannot be explained solely by photon non-colinearity and that 'some further work remains to be done concerning the origin of the effect.' That is an honest modeling/extrapolation assumption and a correctness risk, but it is not circular: the measured widths are external data, and the zero-distance intercept is not imposed by the definition of the intrinsic precision. In Experiment 2, the position resolution (130 um and 128 um sigma) is obtained from weighted fit residuals to four measured points, with weights calibrated on synthetic data of known variance. This is standard deconvolution/calibration, not a fitted parameter renamed as a prediction. The timing resolution of 222 ps is derived from measured inter-layer time-difference distributions divided by sqrt(2), again a direct measurement. Experiment 3 reports 61 ps per layer from a two-layer time-difference measurement; the paper does not derive this value from the 93 ps per-TDC-channel figure quoted in Experiment 2. The apparent inconsistency with the electronics floor is a legitimate correctness concern about an under-specified measurement, but it is not a circular reduction. The cited prior works [1] and [6] are used to identify detector construction, readout architecture, and previously reported components; they do not supply the numerical claims made here. The paper's own stated limitations, such as the lack of simultaneous position resolution in Experiment 3 and the few-percent crosstalk, are disclosed uncertainties rather than circular logic. No step in the derivation chain is equivalent by construction to its own input, and no load-bearing argument reduces to a self-citation chain.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The central claims are experimental and rest mainly on the detector behaving as previously characterized and on the analysis assumptions described in the Methods.

free parameters (1)

Intrinsic localization precision (zero-distance extrapolation) = 0.494 mm FWHM
Linear extrapolation of measured LOR width vs plane distance; it is the central PET localization claim and is not accompanied by an uncertainty.

assumptions (3)

domain assumption Both layers in a pair have equal timing precision, so the per-layer resolution equals the measured time difference divided by sqrt(2).
Stated in the Methods for Experiments 2 and 3; if the layers differ, the per-layer value would change.
domain assumption The Gaussian core fitted within ±1.5 sigma represents the true resolution.
Stated in Methods; the choice excludes non-Gaussian tails and can underestimate the quoted resolution.
domain assumption The detector behavior in this experiment is consistent with the previous characterization in references [1] and [6].
The paper relies on prior papers for detector construction and readout details.

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Cite this review

Pith. "Pith review of Resistive Plate Chambers for brain PET imaging and particle tracking and timing (TOF-tracker)." pith.science (2026). https://pith.science/paper/FF3MB2SF

@misc{pith2026250107768,
  author       = {Pith},
  title        = {Pith review of: Resistive Plate Chambers for brain PET imaging and particle tracking and timing (TOF-tracker)},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FF3MB2SF}},
  note         = {Machine review of arXiv:2501.07768}
}

abstract

In this work we explore readout architectures for the simultaneous high-resolution timing and bidimensional tracking of charged particles with Resistive Plate Chambers (TOF-tracker) and for the accurate detection of gamma rays for PET imaging. On 625 $cm^2$ of active area we obtained a time resolution of 61 ps $\sigma$ and bidimensional position resolution below 150 $\mu$m $\sigma$ for the tracking and timing of charged particles from cosmic rays. An intrinsic precision of 0.49 mm FWHM was determined for the localization of a small $\beta^+$ source via the detection of its annihilation radiation.

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Reference graph

Works this paper leans on

7 extracted references · 6 canonical work pages

[1]

Fonte, L

Blanco, P. Fonte, L. Lopes, P. Martins, J. Michel, M. Palka, M. Kajetanowicz, G. Korcyl, M. Traxler, R. Marques, TOFtracker: gaseous detector with bidimensional 9 tracking and time-of-flight capabilities, J. Inst. 7 (2012) P11012–P11012. https://doi.org/10.1088/1748-0221/7/11/P11012

work page doi:10.1088/1748-0221/7/11/p11012 2012
[2]

Aielli, R

G. Aielli, R. Cardarelli, L.D. Stante, B. Liberti, L. Paolozzi, E. Pastori, R. Santonico, The RPC space resolution with the charge centroid method, J. Inst. 9 (2014) C09030–C09030. https://doi.org/10.1088/1748-0221/9/09/C09030

work page doi:10.1088/1748-0221/9/09/c09030 2014
[3]

Assis, A

P. Assis, A. Bernardino, A. Blanco, F. Clemêncio, N. Carolino, O. Cunha, M. Ferreira, P. Fonte, L. Lopes, C. Loureiro, R. Luz, L. Mendes, J. Michel, A. Neiser, A. Pereira, M. Pimenta, R. Shellard, M. Traxler, A large area TOF-tracker device based on multi-gap Resistive Plate Chambers, J. Inst. 11 (2016) C10002–C10002. https://doi.org/10.1088/1748-0221/11/...

work page doi:10.1088/1748-0221/11/10/c10002 2016
[4]

X.L. Chen, Y. Wang, G. Chen, D. Han, B. Guo, F. Wang, Y. Yu, B. Wang, Q. Zhang, Y. Li, MRPC technology for muon tomography, J. Inst. 15 (2020) C12001– C12001. https://doi.org/10.1088/1748-0221/15/12/C12001

work page doi:10.1088/1748-0221/15/12/c12001 2020
[5]

R. Uda, F. Hayashi, N. Tomida, W.-C. Chang, M.-L. Chu, C.-Y. Hsieh, P.-J. Lin, K. Miyamoto, H. Noumi, A. Sakaguchi, K. Shirotori, M. Tokuda, T. Toda, Y. Yamamoto, Development of a precise time and position resolution TOF-tracker MRPC for the π 20 beam line at J-PARC, Nucl. Instrum. and Meth. in Phys. Res. A 1056 (2023) 168580. https://doi.org/10.1016/j.ni...

arXiv 2023
[6]

Fonte, L

P. Fonte, L. Lopes, R. Alves, N. Carolino, P. Crespo, M. Couceiro, O. Cunha, N. Dias, N.C. Ferreira, S. Fetal, A.L. Lopes, J. Michel, J. Moreira, A. Pereira, J. Saraiva, C. Silva, M. Silva, M. Traxler, A. Abrunhosa, A. Blanco, M. Castelo- Branco, M. Pimenta, An RPC-PET brain scanner demonstrator: First results, Nucl. Instrum. and Meth. in Phys. Res. A 105...

work page arXiv 2023
[7]

Y.C. Jean, H. Nakanishi, L.Y. Hao, T.C. Sandreczki, Anisotropy of free-volume- hole dimensions in polymers probed by positron-annihilation spectroscopy, Phys. Rev. B 42 (1990) 9705–9708. https://doi.org/10.1103/PhysRevB.42.9705

work page doi:10.1103/physrevb.42.9705 1990

Pith tools

Reviewed August 10, 2026 · model on record in the stance chip above.

[1] [1]

Fonte, L

Blanco, P. Fonte, L. Lopes, P. Martins, J. Michel, M. Palka, M. Kajetanowicz, G. Korcyl, M. Traxler, R. Marques, TOFtracker: gaseous detector with bidimensional 9 tracking and time-of-flight capabilities, J. Inst. 7 (2012) P11012–P11012. https://doi.org/10.1088/1748-0221/7/11/P11012

work page doi:10.1088/1748-0221/7/11/p11012 2012

[2] [2]

Aielli, R

G. Aielli, R. Cardarelli, L.D. Stante, B. Liberti, L. Paolozzi, E. Pastori, R. Santonico, The RPC space resolution with the charge centroid method, J. Inst. 9 (2014) C09030–C09030. https://doi.org/10.1088/1748-0221/9/09/C09030

work page doi:10.1088/1748-0221/9/09/c09030 2014

[3] [3]

Assis, A

P. Assis, A. Bernardino, A. Blanco, F. Clemêncio, N. Carolino, O. Cunha, M. Ferreira, P. Fonte, L. Lopes, C. Loureiro, R. Luz, L. Mendes, J. Michel, A. Neiser, A. Pereira, M. Pimenta, R. Shellard, M. Traxler, A large area TOF-tracker device based on multi-gap Resistive Plate Chambers, J. Inst. 11 (2016) C10002–C10002. https://doi.org/10.1088/1748-0221/11/...

work page doi:10.1088/1748-0221/11/10/c10002 2016

[4] [4]

X.L. Chen, Y. Wang, G. Chen, D. Han, B. Guo, F. Wang, Y. Yu, B. Wang, Q. Zhang, Y. Li, MRPC technology for muon tomography, J. Inst. 15 (2020) C12001– C12001. https://doi.org/10.1088/1748-0221/15/12/C12001

work page doi:10.1088/1748-0221/15/12/c12001 2020

[5] [5]

R. Uda, F. Hayashi, N. Tomida, W.-C. Chang, M.-L. Chu, C.-Y. Hsieh, P.-J. Lin, K. Miyamoto, H. Noumi, A. Sakaguchi, K. Shirotori, M. Tokuda, T. Toda, Y. Yamamoto, Development of a precise time and position resolution TOF-tracker MRPC for the π 20 beam line at J-PARC, Nucl. Instrum. and Meth. in Phys. Res. A 1056 (2023) 168580. https://doi.org/10.1016/j.ni...

arXiv 2023

[6] [6]

Fonte, L

P. Fonte, L. Lopes, R. Alves, N. Carolino, P. Crespo, M. Couceiro, O. Cunha, N. Dias, N.C. Ferreira, S. Fetal, A.L. Lopes, J. Michel, J. Moreira, A. Pereira, J. Saraiva, C. Silva, M. Silva, M. Traxler, A. Abrunhosa, A. Blanco, M. Castelo- Branco, M. Pimenta, An RPC-PET brain scanner demonstrator: First results, Nucl. Instrum. and Meth. in Phys. Res. A 105...

work page arXiv 2023

[7] [7]

Y.C. Jean, H. Nakanishi, L.Y. Hao, T.C. Sandreczki, Anisotropy of free-volume- hole dimensions in polymers probed by positron-annihilation spectroscopy, Phys. Rev. B 42 (1990) 9705–9708. https://doi.org/10.1103/PhysRevB.42.9705

work page doi:10.1103/physrevb.42.9705 1990