Fiber-coupled Digital Photo Sensors for Large Time Projection Chambers

Andrew Young; David Jackson; Elizabeth Triller; James Robert Sinclair; Pieter Alexander Breur; Xavier Defay

REVIEW 4 major objections 6 minor 16 references

Fiber-coupled Digital Photo Sensors for Large Time Projection Chambers

T0 review · 4 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read Fiber-coupled digital photo sensors inside the TPC volume can deliver prompt scintillation light everywhere, enabling nanosecond-level event timing and particle identification in large liquid-argon detectors.

desk verdict A plausible in-volume light-readout concept for large LArTPCs, but the field-integrity and heat-load premises are asserted rather than shown; deserves a generous but demanding referee. read the letter →

arxiv 2502.09729 v2 pith:R4WGYF6W submitted 2025-02-13 physics.ins-det hep-ex

Pieter Alexander Breur , Xavier Defay , David Jackson , James Robert Sinclair , Elizabeth Triller , Andrew Young This is my paper · ORCID

classification physics.ins-dethep-ex

keywords timeprojectionchambersliquidargondetectorsdigitalsiliconphotomultiplierspromptscintillationlightpoweroverfibersignalparticleidentificationneutrino

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 proposes that a large liquid-argon time projection chamber (LArTPC) can be read out from the inside: arrays of digital photo sensor units, each combining a digital silicon photomultiplier (dSiPM) with readout electronics, an optical power converter, and a silicon-photonic data transmitter, are deployed in the drift volume and dielectrically coupled to the outside by optical fibers. The point is to keep every optical path shorter than the Rayleigh scattering length so the fast (nanosecond) component of argon scintillation is not smeared away before it arrives at a sensor. If that works, prompt-light timing becomes available at every vertex, which the authors argue improves neutrino vertex finding, neutron energy measurement by time of flight, and particle identification through decay lifetimes such as the charged kaon's. For a full-size vertical-drift geometry, they estimate that about 2500 sensor units, spaced at 1.5 times the scattering length, would cover the entire volume.

What carries the argument

The key object is the Digital Photo Sensor Unit (DPSU), a self-contained module that integrates dSiPM pixels, threshold timing electronics, an optical power converter, and a silicon-photonic transmitter, with power and signal carried by optical fibers rather than conductive cables. The mechanism it exploits is argon's prompt singlet scintillation, whose $O(1\ \mathrm{ns})$ lifetime would otherwise be lost to Rayleigh scattering once optical paths reach a meter or more; spacing sensor strings at 1.5 times the scattering length preserves the fast signal. The digital output matters because it turns light detection into precise timestamps without waveform analysis, and the dielectric fiber link is what makes it safe to embed the units in a high-voltage region.

What would settle it

In the planned 50 cm drift demonstration, position a pulsed UV laser at a known point and compare the DPSU timestamp with the drift-time prediction from a calibrated electric field; the central claim fails if the observed drift time shifts by more than the target timing resolution when the sensors are powered, or if the light timestamp jitter is above a few nanoseconds.

Watch

Extended reading notes

Core claim

The central claim is that the usual trade-off between fast timing and detector coverage in noble-liquid TPCs can be broken by instrumenting the bulk volume rather than the periphery. By powering and reading each sensor through dielectric optical fibers, the design places digital silicon photomultipliers directly between cathode and anode without, the authors assert, introducing electrical noise or disturbing the drift field. Because the dSiPM outputs a digital threshold-crossing timestamp, no waveform digitization is needed, which keeps channel density high and data transmission simple. The consequence would be that prompt scintillation light, with its $O(1\ \mathrm{ns})$ singlet lifetime, is available across the whole detector, enabling localized triggers, time-of-flight neutron spectroscopy, and lifetime-based particle tagging for channels like $p\to K^+\nu$.

Load-bearing premise

The design assumes that Digital Photo Sensor Units can sit inside the high-voltage liquid-argon volume, powered and read out through optical fibers, without unacceptably distorting the drift electric field, overheating the argon, or degrading signal integrity; this is asserted in the design section and slated for a 50 cm demonstration, with no measured results presented here.

Editorial extensions

If this is right

Prompt-light readout becomes a volumetric property, so detectors can be triggered locally on activity anywhere in the TPC rather than only near the instrumented walls.
Neutron energies lost to the calorimetric measurement can be recovered from time of flight, using the stated 12 cm per nanosecond velocity of a 100 MeV neutron, removing a known bias in hadronic energy reconstruction.
Particle identification by decay lifetime becomes possible: a charged kaon with a 12.4 ns lifetime would be tagged by scintillation from its decay products, following the same logic already used in water Cherenkov detectors.
Because readout no longer depends on the slow scintillation component, the sensor response is partially resilient to nitrogen contamination, which suppresses slow light.
A full-size vertical-drift TPC would need roughly 2500 digitally read-out sensor units spaced at 1.5 times the scattering length, a sparse instrumentation load compared to the approximately 10,000 photomultipliers in a 12 m diameter water sphere.

Reading between the lines

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

The timing argument only survives if the sensors see vacuum-ultraviolet light directly; if a wavelength shifter is required for VUV sensitivity, the time smearing it introduces could erase the nanosecond-scale benefit the whole concept depends on.
The 50 cm demonstration will be convincing only if it simultaneously measures field distortion, heat load into the liquid argon, and timing jitter; success on one axis alone would not validate the full claim.
The authors' per-event power-budget idea (charging a capacitor at a reduced duty cycle and drawing on trigger) implies that readout latency after a trigger is no longer constant, which may limit deadtime-free triggering in high-rate environments.
The same dielectric fiber coupling could be used to place other instrumentation, such as in-situ field sensors or calibration sources, inside high-voltage noble-liquid volumes if the power-over-fiber efficiency improves.

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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

The useful thing here is the concept: putting fiber-coupled digital SiPMs inside the TPC bulk instead of on the walls. That is a genuine new configuration, combining power-over-fiber, signal-over-fiber, and dSiPMs in a way I have not seen in the literature. The physics motivation is sound: prompt scintillation light is smeared by Rayleigh scattering over about a meter, so in-volume readout is a reasonable answer to vertexing and neutron time-of-flight in DUNE-scale detectors.

Credit where it is due: the paper is honest about its status. It explicitly says the 50 cm demonstration is under construction, and it flags the two big engineering risks (PoF efficiency and VUV-sensitive SPADs). The citation pattern looks fine; the 1.5 m spacing is tied to a measured Rayleigh length, not to a fitted parameter. No circularity.

The soft spot is load-bearing: Sections 4 and 6 claim deployment in the drift volume 'without introducing electrical noise or compromising detector integrity.' That is asserted, not demonstrated. Each DPSU has power conversion, readout electronics, capacitors, and conductive structures. Even with optical power and signal, the housing is a material inclusion in a high-voltage field. A floating or partially conductive unit will perturb the field and deflect drift electrons. The paper offers no electrostatic simulation and no thermal budget. With roughly 50% PoF efficiency quoted in Section 5, half the delivered optical power becomes heat in the LAr at each DPSU; 2500 of them could create local density gradients and convection that distort the primary charge readout. This is not a fatal objection—the 50 cm test might answer it—but it is a premise that currently rests on no data.

Also, the abstract and Section 6 claim improved energy resolution and robust PID as accomplishments. They are predictions from the concept. The paper does not quantify light collection, energy resolution, or timing performance. That is acceptable for a concept paper if the claims are framed as goals, but here they are sometimes stated as achievements. Needs a copy-edit.

This paper is for detector R&D people working on noble-liquid TPCs, especially DUNE and future LArTPCs. They will get a clear statement of a possible path and a list of technologies that need to mature. It deserves a serious referee—this is a JINST-style concept paper, not a PRL. The referee should push for either data or a much more careful discussion of the field-integrity and thermal-load assumptions. My recommendation: send it to peer review with the clear expectation that those two sections either get quantitative support or are explicitly reframed as open questions.

Referee Report

4 major / 6 minor

Summary. This manuscript proposes a novel light-detection architecture for large liquid-argon time projection chambers (LArTPCs), based on fiber-coupled digital silicon photomultipliers (dSiPMs) deployed inside the TPC bulk volume. The authors motivate the need for prompt scintillation light for vertex identification, neutron time-of-flight, and particle identification (notably K+ lifetime tagging for proton-decay searches). Section 4 introduces a DUNE-scale vertical-drift geometry with roughly 2500 Digital Photo Sensor Units (DPSUs) arranged on a grid with 1.5 m spacing. Section 5 describes the key technologies: power-over-fiber (PoF), signal-over-fiber (SoF), and dSiPMs, and mentions a 50 cm drift testbed under construction at SLAC. The paper concludes with claims of improved timing, energy resolution, and particle identification, but contains no simulation, no prototype data, no error analysis, and no quantitative model for light collection, field distortion, or thermal effects.

Significance. If the concept is validated, it could substantially improve the physics reach of DUNE-scale LArTPCs by providing full-volume access to prompt scintillation light with nanosecond-scale timing. The paper identifies a real physics need and leverages credible advances in photonics and digital SiPMs. Its main strengths are a concrete detector geometry, clear physics motivation, and an honest statement that VUV-sensitive SPADs and low-heat PoF must be developed. However, the central feasibility premise—embedding digitizing electronics and optical power converters inside the high-voltage TPC drift volume—is asserted rather than demonstrated, and the claimed performance improvements are not derived from any quantitative model. The paper is best read as a conceptual proposal with a promising direction, not as a completed technical study.

major comments (4)

[Section 4, Figure 2] The spacing argument is internally inconsistent: the text states that strings are placed at 1.5 times the Rayleigh scattering length in order to ensure 'the optical path is less than a scattering length.' A path of 1.5 times the scattering length is not less than one scattering length. Please clarify the intended attenuation criterion and provide a quantitative estimate of the prompt-photon survival probability and arrival-time distribution at the nearest DPSU.
[Section 4 and Section 6] The central claim that DPSUs can be embedded in the TPC bulk volume 'without introducing electrical noise or compromising detector integrity' is asserted without supporting analysis. Each DPSU contains conductive elements, capacitors, digital readout electronics, and a power converter; even with optical power and signal delivery, the unit is a material inclusion in the drift field. Please provide an electrostatic model of field distortion and ionization-electron trajectory deviations, plus a thermal budget for the proposed ~2500 DPSUs given the approximately 50% PoF efficiency quoted in Section 5. The planned 50 cm demonstration in Figure 3 is stated to be under construction with no results, so it cannot yet support the claim.
[Abstract and Section 6] The claimed improvements in energy resolution, particle identification, and event reconstruction are asserted rather than demonstrated. No light-collection model, photon-detection-efficiency input, dark-count estimate, or timing-resolution calculation is provided. Please include a quantitative estimate of the expected prompt-light detection efficiency, timing resolution, and resulting energy-resolution improvement, or explicitly scope the paper as a conceptual proposal whose performance claims require future simulation and beam tests.
[Section 5] The entire concept depends on VUV-sensitive SPADs with nanosecond timing, which the authors acknowledge are not yet available: they state that 'development of Vacuum UltraViolet sensitive single photon avalanche diodes is needed.' Without such devices, wavelength shifting would reintroduce the time smearing the design aims to avoid. This is a load-bearing technology risk that should be presented explicitly as a feasibility condition, with a discussion of achievable VUV PDE and single-photon timing jitter, rather than a peripheral development item.

minor comments (6)

[Abstract] The phrase 'and enhance event reconstruction' should read 'and enhances event reconstruction' for grammatical consistency.
[Section 4] The sentence 'The separation of and coverage at each DPSU will be further optimized' is missing a noun; it should read 'The separation of the strings and the coverage at each DPSU will be further optimized.'
[Section 2] The statement that a 100 MeV neutron travels 12 cm/ns is correct but should be accompanied by a reference or a short derivation, since it is a quantitative physics input.
[Figure 2 caption and Section 4] The geometry described as an '8 x 39 grid of strings with 8 sensors on each string' is ambiguous; please clarify whether there are 312 strings with 8 sensors each, which would give 2496 DPSUs, and reconcile this with the '2500 DPSUs' total.
[Section 3 and Section 6] The claim of 'resilience to nitrogen contamination' based on reference [10] should be justified explicitly, because nitrogen quenching can affect both fast and slow scintillation components depending on concentration.
[References] Reference [16] is cited for FBK's SuperEllen sensors, but the title '3D quantum ghost imaging' does not appear to match this content; please verify the citation.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the paper is a concept proposal with no fitted parameters, no derived equations, and no self-citation chain; its load-bearing assumptions are unvalidated but not circular.

full rationale

This is a detector-concept paper rather than a derivation or measurement paper. The central proposal is that fiber-coupled digital SiPMs (DPSUs) placed inside the TPC bulk improve prompt-light access, timing, and PID. No quantity is fitted from data and then renamed as a prediction. The 1.5 m DPSU spacing is taken directly from an external measurement of the Rayleigh scattering length in liquid argon ('To ensure the optical path is less than a scattering length, strings of sensors are displaced at 1.5 times the Rayleigh scattering length, 1.5 m'), so it is not an output of the paper's own analysis. The key enabling claims—that DPSUs can be deployed inside the drift volume without distorting the electric field or adding unacceptable heat load—are asserted in Sections 4 and 6 ('DPSUs are dielectrically coupled with optical fibers to enable deployment inside electric fields'; 'this approach allows for sensor deployment inside the TPC bulk volume without introducing electrical noise or compromising detector integrity'). These are unvalidated premises, and the paper itself flags that the demonstration is pending ('A 50 cm-drift liquid argon time projection chamber currently under construction at SLAC') and that PoF efficiency is only about 50% and 'will require optimization to reduce heat load.' Those are correctness or feasibility risks, not circularity: nothing in the argument reduces to its own inputs by construction, and no load-bearing result depends on a self-citation. The cited technologies (PoF, SoF, dSiPMs) are external component capabilities, not the authors' own prior results invoked to force a conclusion. Therefore the circularity score is 0.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

The concept rests on several unproven domain assumptions, primarily that fiber-optic power and data links can operate inside a large liquid argon TPC without disturbing the drift field or heat budget, and that VUV-sensitive SPADs with nanosecond timing can be developed. No free parameters are fitted, and no new physical entities are introduced.

assumptions (5)

domain assumption Argon scintillation produces prompt singlet light with O(1 ns) lifetime, and Rayleigh scattering smears arrival times by O(10 ns) over O(1 m) paths.
Invoked in Section 3 via refs [5] and [6]; this motivates the need for in-volume readout.
domain assumption dSiPMs provide precise threshold-crossing times without waveform analysis, enabling simpler digital timing readout.
Stated in Section 4 as a property of the cited dSiPM technologies [13-15].
domain assumption Power over fiber and signal over fiber can operate in liquid argon with acceptable efficiency and heat load after optimization.
Section 5 cites [11] and [12] for feasibility, while noting current PoF efficiency is roughly 50% and requires optimization; the design depends on that optimization.
ad hoc to paper VUV-sensitive SPADs with nanosecond timing can be developed for argon scintillation at 128 nm without wavelength shifting.
Section 5 states this development is needed; the timing performance of the concept depends on a component that is not yet demonstrated.
ad hoc to paper Embedding DPSUs in the TPC bulk volume does not unacceptably distort the drift electric field.
Section 4 asserts dielectric fiber coupling enables deployment inside electric fields, but no simulation or measurement of field distortion is provided.

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Cite this review

Pith. "Pith review of Fiber-coupled Digital Photo Sensors for Large Time Projection Chambers." pith.science (2026). https://pith.science/paper/R4WGYF6W

@misc{pith2026250209729,
  author       = {Pith},
  title        = {Pith review of: Fiber-coupled Digital Photo Sensors for Large Time Projection Chambers},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/R4WGYF6W}},
  note         = {Machine review of arXiv:2502.09729}
}

read the original abstract

This paper presents a novel approach to addressing challenges in neutrino event reconstruction within large Time Projection Chambers (TPCs). By integrating fiber-coupled digital silicon photomultipliers, we propose a design that enhances light detection, improves energy resolution, and enhance event reconstruction. Advancements in power and signal over fiber technologies are leveraged to deploy digital sensors within the TPC bulk volume, enabling precise timing and robust particle identification.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

16 extracted references · 11 canonical work pages

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

[1] [1]

Friedland and S.W

A. Friedland and S.W. Li,Understanding the energy resolution of liquid argon neutrino detectors, Phys. Rev. D99(2019) 036009 [1811.06159]

arXiv 2019

[2] [2]

Patrignani et al.,Review of particle physics,Chinese Physics C40(2016) 100001

C. Patrignani et al.,Review of particle physics,Chinese Physics C40(2016) 100001

work page 2016

[3] [3]

MINERvAcollaboration, Measurement of𝐾+ production in charged-current𝜈𝜇 interactions, Phys. Rev. D94(2016) 012002

work page 2016

[4] [4]

Super-Kamiokandecollaboration, Search for proton decay via𝑝→𝜈𝐾+ using 260 kiloton · year data of super-kamiokande,Phys. Rev. D90 (2014) 072005

work page 2014

[5] [5]

Table-top setup for investigating the scintillation properties of liquid argon

T. Heindl, T. Dandl, A. Fedenev, M. Hofmann, R. Krücken, L. Oberauer et al.,Table-top setup for investigating the scintillation properties of liquid argon,JINST 6 (2011) P02011 [1511.07720]

work page Pith review arXiv 2011

[6] [6]

Babicz et al.,A measurement of the group velocity of scintillation light in liquid argon,JINST 15 (2020) P09009 [2002.09346]

M. Babicz et al.,A measurement of the group velocity of scintillation light in liquid argon,JINST 15 (2020) P09009 [2002.09346]

arXiv 2020

[7] [7]

IceCube collaboration, The IceCube Neutrino Observatory: Instrumentation and Online Systems, JINST 12(2017) P03012 [1612.05093]

arXiv 2017

[8] [8]

Technical Design Report, JINST 19 (2024) T08004 [2312.03130]

DUNE collaboration, The DUNE Far Detector Vertical Drift Technology. Technical Design Report, JINST 19 (2024) T08004 [2312.03130]

arXiv 2024