Silicon Photomultipliers for Orbital Ultra High Energy Cosmic Ray Observation

A. Haungs; A. Menshikov; JEM-EUSO Collaboration; M. Karus; M. Oehler; M. Renschler; T. Huber; W. Painter

REVIEW 4 major objections 5 minor 16 references

Silicon Photomultipliers for Orbital Ultra High Energy Cosmic Ray Observation

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The paper claims that the 256-channel SiECA camera demonstrates silicon photomultipliers can replace photomultiplier tubes in orbital cosmic-ray fluorescence detectors.

desk verdict Useful engineering progress report on a 256-channel SiPM camera with credible lab single-photon evidence, but the abstract's 'proves viable' claim outruns the data. read the letter →

arxiv 1908.09136 v1 pith:HTZNNVD6 submitted 2019-08-24 astro-ph.IM astro-ph.HEphysics.ins-det

W. Painter , A. Haungs , T. Huber , M. Karus , A. Menshikov , M. Oehler , M. Renschler , JEM-EUSO Collaboration This is my paper · ORCID

classification astro-ph.IMastro-ph.HEphysics.ins-det

keywords siliconphotomultiplierSiPMMPPCECAcameraultra-high-energycosmicraysfluorescencedetectionflat-fieldingsingle-photoncounting

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 the development and laboratory calibration of SiECA, a 256-channel camera built from silicon photomultiplier arrays for use in orbital telescopes that watch for the ultraviolet fluorescence of cosmic-ray air showers. The authors claim that the camera can count single photons, that channel-by-channel bias tuning makes the sensor array substantially more uniform than a photomultiplier-tube camera could be, and that this makes silicon photomultipliers a viable replacement for multi-anode photomultiplier tubes in future space-based detectors. The motivation is practical: silicon sensors are more robust, compact, and lower-voltage, and they tolerate high-luminosity exposure without the damage that photomultiplier tubes suffer. A first balloon flight carried the camera but produced only a few hours of data with no cosmic-ray signatures, so the viability case rests mainly on laboratory characterization.

What carries the argument

The load-bearing object is the SiECA camera itself: four 64-channel multi-pixel photon counter arrays, giving 256 channels total, on a front board read out by eight Citiroc ASICs and controlled by an FPGA. The Citiroc is a mixed-signal readout chip that amplifies, shapes, and discriminates each channel's pulses, and here it runs in a peak-counting mode in which the FPGA records, once per 2.5 microsecond gate time, how many pulses on each channel pass threshold. The mechanism that produces the uniformity claim is per-channel biasing: each group of channels has its own bias voltage generator, and the ASIC's digital-to-analog converters can adjust gain and threshold channel by channel, so the camera can be flat-fielded on either signal amplitude or detection efficiency rather than relying on one global operating point as with photomultiplier tubes.

What would settle it

A decisive check would be a high-altitude or orbital run in which SiECA is triggered by a known ultraviolet source or an actual air-shower event: if the single-photon counting, flat-field uniformity, or gain stability achieved in the laboratory does not survive the temperature and pressure profile, the viability claim is falsified. In the lab, recovering the expected roughly 7.5 photons per 2.5 microsecond gate from the 3-photon, 1 MHz illumination after flat-fielding would test the camera's absolute calibration, since the paper reports a substantial signal loss before correction.

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Extended reading notes

Core claim

The central claim is that a 256-channel camera built from silicon photomultiplier arrays and read out by Citiroc ASICs is a workable photon sensor for space-based ultra-high-energy cosmic-ray fluorescence detection. The camera, called SiECA, is designed to attach to existing fluorescence telescopes and share their optics, so it can be evaluated in parallel with the usual multi-anode photomultiplier tubes under identical measurement conditions. Laboratory scans over bias voltage, preamplifier gain, and discriminator threshold resolve single-photon peaks, and illuminated flat-field tests show that per-channel tuning can make the array uniform either in gain or in photon detection efficiency. The authors conclude from this that silicon photomultipliers are viable replacements for photomultiplier tubes in future non-terrestrial instruments, particularly where high-luminosity exposure could damage tube-based cameras.

Load-bearing premise

The claim that silicon photomultipliers are viable for orbital use assumes that laboratory measurements on the rebuilt camera represent what will happen in space; the only flight exposure lasted a few hours and produced no cosmic-ray signature with which to check that assumption.

Editorial extensions

If this is right

A 256-channel silicon photomultiplier camera can be built to match the focal-plane geometry and 2.5 microsecond gate time of an orbital fluorescence detector while running from a 5 V supply.
Channel-by-channel bias, gain, and threshold adjustment can flatten the focal surface in either gain or photon detection efficiency, a level of control uniform photomultiplier biasing does not offer.
Single-photon counting is achievable with multi-pixel photon counter arrays read out through the Citiroc ASIC, giving the sensitivity needed for faint ultraviolet air-shower fluorescence.
Because silicon photomultipliers are more robust, compact, and lower-voltage than photomultiplier tubes, future orbital detectors can use them in high-luminosity exposures that could damage tube-based systems.
The calibration procedure described here, combining parameter scans, illuminated flat fields, and neighboring-channel noise checks, can be reapplied to later silicon photomultiplier cameras.

Reading between the lines

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

An extension the paper leaves untested is that the camera's in-flight behavior, not its laboratory calibration, is what will decide whether silicon photomultipliers are truly viable in orbit.
The paper notes that temperature sensors are present but not used for live bias regulation; using them to stabilize gain across orbital thermal swings is a direct next step.
The stated 2.5 microsecond gate and slow charge-integration mode are matched to fluorescence-telescope timing, so applying the same camera concept to Cherenkov pulses would require a faster readout.
If per-channel bias flat-fielding generalizes, any array camera with pixel-to-pixel response spread could adopt the same tuning procedure instead of correcting non-uniformity in software alone.

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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: this is a useful engineering progress report with credible lab evidence for single-photon counting in a 256-channel SiPM camera, but the abstract's claim that SiECA "proves" SiPMs are viable for orbital UHECR detection is one step ahead of the data. I'd send it to review, but the authors should soften the conclusion and add missing measurements.

What's new: the full SiECA camera characterization - 256 Hamamatsu MPPC channels read out by eight Citiroc ASICs - with a parameter scan that clearly resolves 1-, 2-, and higher photo-electron peaks, plus a flat-fielding procedure that reduces response variation to less than one photon per GTU. The hardware description (mechanical layout, grounding, bias generation, FPGA timing) is detailed enough to be reproduced. The paper is also honest about the EUSO-SPB1 outcome: only a few hours of SiECA data, no cosmic-ray search possible, and one dead channel (3B3) in the lab flat field. That honesty is real credit.

The soft spots are mostly in interpretation, not engineering. First, "proves ... viable" is not supported by the body, which states the science goals have not yet been achieved and that the next step is a ground deployment. Second, the key flat-field result has no RMS, no per-channel error bars, and no comparison with a uniformly biased MAPMT baseline, even though the abstract makes a comparative claim. Third, the paper reports "a substantial loss of signal in processing": expected 7.5 photons/GTU, measured counts lower, with no full attribution. That matters because single-photon-counting capability is exactly what the viability claim rests on. Fourth, the uniformity improvements from per-channel biasing are described in the conditional future tense ("should result in ..."), so they are projections, not measured outcomes. The citation pattern is fine; earlier single-channel work [7] and SiECA R&D [5] are properly credited.

For whom: detector and instrumentation people working on EUSO/POEMMA-class cameras will get value from the practical details; nobody should cite the abstract claim as proof of space readiness. My recommendation: engage with it, but require a revision that aligns the abstract with the evidence, adds error bars and baselines to the flat-field result, and explains or quantifies the count-rate loss. That is a solid referee assignment after those changes.

Referee Report

4 major / 5 minor

Summary. The paper describes SiECA, a 256-channel silicon photomultiplier (SiPM/MPPC) camera built as an add-on for orbital ultra-high-energy cosmic ray fluorescence detectors. It presents the mechanical and electrical design, the Citiroc ASIC and FPGA readout chain, the event-timing and trigger scheme, and laboratory calibration procedures, including single-channel parameter scans and illuminated flat-field tests. The abstract claims that SiECA 'proves SiPM are viable sensors to replace Multi-Anode PhotoMultiplier Tubes' and that channel-by-channel biasing yields 'substantial improvements in detector and signal uniformity.' The body, however, reports that the EUSO-SPB1 flight produced only a few hours of SiECA data and no cosmic-ray signatures, and the laboratory flat-field section gives only a qualitative uniformity statement, a dead channel, and an unexplained substantial loss of measured count rate relative to expectation.

Significance. If the central claims were fully supported, the paper would be a useful engineering contribution to the ongoing EUSO/POEMMA R&D effort: it documents a complete SiPM camera with ASIC readout, demonstrates resolvable single-photo-electron peaks in the parameter scans, and proposes a plausible per-channel bias-tuning flat-fielding method. These strengths are real and should be acknowledged. However, the paper's proof-level conclusion is not established by the presented evidence. The absence of in-flight validation is explicitly admitted in the introduction, and the laboratory data lack the quantitative error analysis and baseline comparisons needed to support the uniformity and single-photon-counting claims. The paper is better read as a hardware and calibration development report than as a demonstration of orbital viability, and the claims should be adjusted accordingly.

major comments (4)

[Abstract; Section 1; Section 5] The abstract's sentence 'SiECA proves SiPM are viable sensors to replace Multi-Anode PhotoMultiplier Tubes' is not supported by the body. Section 1 states that 'search for cosmic ray and other interesting signatures with SiECA was not possible' and that the EUSO-SPB1 flight produced 'only few hours of SiECA measurements,' while Section 5 describes the next step as deployment to a facility 'which maximizes the possibility to achieve the scientific goals.' The laboratory results demonstrate hardware readiness, but orbital viability remains an extrapolation. Please temper the abstract to 'supports the viability' or present in-flight validation data.
[Section 4, 'Illuminated Flat Field test'] The uniformity claim of 'variation of less than one photon per GTU' is reported without an RMS value, per-channel error bars, or any comparison against uniform biasing or a MAPMT baseline. Since the abstract's comparative claim ('in comparison to uniform biasing with MAPMTs... substantial improvements') depends on this measurement, the flat-field result must be quantified as a distribution with uncertainties and placed against a defined baseline before the improvement claim is meaningful.
[Section 4, 'Illuminated Flat Field test'] The text states that the expected rate is 7.5 photons/GTU and that 'the lower count rate indicates a substantial loss of signal in processing,' but it does not give the measured rate, the loss fraction, or any explanation or correction for the loss. Because the abstract's central claim rests on single-photon counting capability, this unexplained signal loss must be quantified and either corrected or explicitly bounded before the demonstrated capability can be assessed.
[Figure 2, right panel; Section 4] Channel 3B3 is reported as having zero count rate, i.e., it is a dead channel, yet the text does not discuss the failure mode or its effect on the flat-field and uniformity conclusions. A single dead channel in 256 is small, but it should be explicitly acknowledged as a known defect and either excluded from the reported uniformity statistics or included with a documented dead-channel fraction.

minor comments (5)

[Abstract; Section 3] The abstract states an integration time of 2.5 ms, while the body consistently uses a GTU of 2.5 µs (e.g., Section 3: '2.5 µs-GTU'); this is likely a typo and should be corrected.
[Section 4] The phrase '7.5 MHz − photon signal' has inconsistent units; it should read '7.5 photons per GTU' or, equivalently, an incident photon rate of 3 MHz for a 2.5 µs gate.
[Throughout] There are several typographical and formatting issues, including 'expereiences' in Section 5, 'F ront' in the Section 3 heading, and inconsistent use of 'Cerenkov' versus 'Cherenkov'; these should be cleaned up.
[Figure 2] The right panel lacks a clear description of the color scale and axis units in the text or caption; please add a caption that defines the plotted quantity and its units, and indicate how the 16x16 channel layout maps to the four MPPC arrays.
[Section 2.1] The text notes that temperature sensors were not used to regulate bias voltage during the EUSO-SPB1 deployment; it would be helpful to state explicitly how this affects the interpretation of the few hours of flight data, particularly regarding gain stability.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the SiECA paper is a hardware characterization report; its viability claim rests on laboratory measurements, not on a derivation from its own inputs.

full rationale

The paper does not derive a theoretical result or use fitted parameters to predict a quantity that was part of the fit. The central content is the construction and laboratory characterization of a 256-channel SiPM camera. The flat-fielding procedure tunes per-channel bias and ASIC settings and then evaluates uniformity under illumination, but this is an engineering calibration-verification loop, not a circular derivation: the authors explicitly describe the resulting improvement in conditional terms ('should result in much more uniform sensitivity') rather than presenting the measured uniformity as an independent prediction. The single-photon counting capability is supported by parameter scans showing separated photo-electron peaks, which is independent evidence. Self-citations to earlier SiECA R&D and a single-channel readout paper are background references and are not load-bearing: the paper's claims rest on the measurements reported here, not on an imported uniqueness theorem or ansatz. The abstract's 'proves SiPM are viable sensors' is stronger than the body's evidence, especially given the paper's own admission that no cosmic-ray signatures were obtained during the EUSO-SPB1 flight, but that is an evidentiary/correctness concern, not circularity. No specific reduction of a claimed result to its inputs can be exhibited, so the appropriate finding is no significant circularity, score 0.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The central claim rests on standard device physics assumptions about MPPC and ASIC behavior, plus the representativeness of lab calibration for orbital conditions. No new entities are introduced; the only fitted quantities are per-channel operating parameters whose values are not reported.

free parameters (2)

Per-channel MPPC bias voltage = not reported
Eight bias generators and Citiroc fine-tune DACs are set per channel to flatten gain and PDE; the uniformity improvement claim depends on these settings, but exact values are not listed.
Per-channel ASIC gain and discriminator threshold = not reported
Selected from the parameter scan to sit at photo-electron peaks; the single-photon counting and flat-field results depend on these settings.

assumptions (4)

domain assumption MPPC gain scales linearly with overvoltage and PDE behavior is as specified by Hamamatsu for the S13361-3050AS-08.
Flat-fielding by bias voltage assumes the manufacturer's gain/overvoltage relation holds for all channels; no independent measurement of breakdown voltage per channel is shown.
domain assumption The Citiroc 1A ASIC discriminator and peak-counting logic correctly convert MPPC pulses to per-GTU counts with negligible dead time.
The single-photon counting claim depends on the ASIC's threshold response and on the FPGA trigger handling; the paper provides no direct measurement of counting linearity or dead time.
domain assumption The 3-photon-per-pulse, 1 MHz LED source provides a known absolute illumination for calibration.
The expected 7.5 photons per GTU is used to interpret the flat-field count rate; the paper reports no independent calibration of the light source intensity.
domain assumption The 2.5 µs GTU and JEM-EUSO geometry parameters are appropriate for detecting UHECR fluorescence tracks from orbit.
The camera timing is set to these parameters, so the viability claim inherits the assumption that this GTU is adequate, without simulation or end-to-end validation in this paper.

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Cite this review

Pith. "Pith review of Silicon Photomultipliers for Orbital Ultra High Energy Cosmic Ray Observation." pith.science (2026). https://pith.science/paper/HTZNNVD6

@misc{pith2026190809136,
  author       = {Pith},
  title        = {Pith review of: Silicon Photomultipliers for Orbital Ultra High Energy Cosmic Ray Observation},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/HTZNNVD6}},
  note         = {Machine review of arXiv:1908.09136}
}

read the original abstract

Development of the Silicon photomultiplier Elementary Cell Add-on camera (SiECA) has provided extensive information regarding the use of SiPMs for future cosmic ray detection systems. We present the technical aspects of sensor readout development utilizing Citiroc ASIC chips from Weeroc controlled by a Xilinx FPGA to process and package events from four 64 channel Hamamatsu MPPC S13361 arrays generating 128 frame events with an integration time of 2.5ms (parameters are based on JEM-EUSO geometry but can be easily adjusted). With single photon counting capability, SiECA proves SiPM are viable sensors to replace Multi-Anode PhotoMultiplier Tubes in future devices, especially when high luminosity exposure is possible potentially damaging MAPMT based systems. Complementary to the technical aspects, computational and analysis methods for sensor array characterization and in depth device flat-fielding are presented. Provided channel by channel biasing, in comparison to uniform biasing with MAPMTs, fine tuning of operating parameters with MPPC arrays allows for substantial improvements in detector and signal uniformity.

Figures

Figures reproduced from arXiv: 1908.09136 by the authors.

**Figure 2.** Left: Signal-to-Noise (SNR) ratio plot for Vbias = 55 V; Right: Example flat field results with EUSO-SPB1 flight parameters energetically unlikely. The next dark band is the 2-PE amplitude threshold which is notably darker due to the substantially decreased dark count rate achieving this charge amplitude. Subsequently higher PE peaks are distinct for low to moderate gains but become indistinct due to the non-lineari… view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

16 extracted references · 16 canonical work pages

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Pith tools

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

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M. Renschler, W. Painter, F. Bisconti, A. Haungs, T. Huber, M. Karus, H. Schieler, A.Weindl, Nucl. Instrum. Meth. A888 (2018) 257

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