The Scintillator Upgrade of IceTop: Performance of the prototype array

Chris Wendt (for the IceCube Collaboration); Delia Tosi; Matt Kauer; Thomas Huber

REVIEW 2 major objections 5 minor 1 cited by

The Scintillator Upgrade of IceTop: Performance of the prototype array

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The two scintillator prototype stations meet the requirements of the IceTop upgrade: stable timing, ~40 photoelectrons per particle, five times lower charge threshold than snow-covered tanks, and shower directions matching IceTop.

desk verdict A modest but solid prototype performance paper: the first year of in-situ data supports the feasibility claim, though the quantitative reporting is thinner than I would like. read the letter →

arxiv 1908.09860 v1 pith:NTDIUGOI submitted 2019-08-26 astro-ph.HE astro-ph.IM

Matt Kauer , Thomas Huber , Delia Tosi , Chris Wendt (for the IceCube Collaboration) This is my paper · ORCID

classification astro-ph.HEastro-ph.IM

keywords IceTopscintillatorupgradesurfacedetectorarraysiliconphotomultiplierairshowerreconstructionsnowaccumulationWhiteRabbittimingprototypestationcosmicrays

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

Snow accumulation on IceTop's buried tanks is the largest source of systematic uncertainty in cosmic-ray shower reconstruction and mass-composition measurements at the South Pole. To address this, the collaboration fielded two seven-panel scintillator prototype stations near the array center and operated them for more than a year. The paper establishes that the prototypes satisfy the upgrade requirements: White Rabbit timing stays stable, SiPM gain is held within 2% across Antarctic temperatures, the MIP light yield is 39–44 photoelectrons as simulated, and the stations reconstruct coincident air-shower directions in agreement with IceTop. The conclusion is that the deployed stations prove the detector design is feasible for a full surface instrumentation upgrade of IceTop.

What carries the argument

The load-bearing element is the 1.5 m² scintillator panel, built from 16 extruded polystyrene bars with a TiO2 reflector coating and wavelength-shifting fibers looped through the bars and read out by one 6×6 mm² silicon photomultiplier. Around the panel, the argument is carried by four mechanisms: White Rabbit GPS timing distributed over fiber links to synchronize the stations with IceTop; a per-panel temperature–voltage–gain plane fit that adjusts SiPM bias voltage to keep gain constant; a fast FFT-based determination of gain from the 'finger' charge spectrum; and a plane-front fit to the triggered scintillator signals that reconstructs the shower direction. These mechanisms together show that a low-mass, low-power station can provide calibrated, time-stable, snow-independent shower measurements.

What would settle it

Reconstruct the same coincident shower sample with an independent, snow-free direction measurement—for example the radio antennas proposed for the upgrade, or Monte Carlo showers with injected directions—and check whether the zenith/azimuth residual between scintillators and that reference grows with snow depth or with distance from array center. If an offset larger than the quoted Gaussian widths appears, the paper's accuracy claim would be refuted.

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Extended reading notes

Core claim

After more than a year of operation, the two prototype stations—each with seven 1.5 m² scintillator panels but different DAQ architectures—demonstrate the technical core of the planned IceTop scintillator upgrade. The paper shows that shower-trigger coincidences with IceTop carry the expected ~335 ns White Rabbit timing offset from cable delay; that temperature-compensated SiPM gain stays at the 30 ADC/PE setpoint within 2%; and that measured MIP light yields of 39–44 photoelectrons match GEANT4 predictions. A linear charge correlation with IceTop extends down to IceTop's threshold, and the scintillators' own threshold is about five times lower, directly mitigating the snow-threshold problem. Plane-front direction fits to the small prototype array agree with IceTop's Laputop reconstruction, which the paper takes as evidence that the timing quality is sufficient for air-shower detection. The paper's closing claim is that these stations are a proof of concept showing the design fulfills the requirements of a surface instrumentation upgrade of IceTop.

Load-bearing premise

The claim that the scintillator station reconstructs air-shower directions 'accurately' is established only by agreement with IceTop's own Laputop reconstruction; if that reference is itself biased by the snow effects the upgrade is meant to correct, the agreement does not prove absolute directional accuracy.

Editorial extensions

If this is right

A full array of up to 32 such stations would measure snow-driven threshold and efficiency changes in the IceTop tanks and correct the largest systematic uncertainties in shower and mass-composition analyses.
The year-long stable operation validates the surface field-hub and White Rabbit timing infrastructure planned for next-generation South Pole surface detectors.
With a charge threshold about five times lower than IceTop's, an upgraded surface array extends cosmic-ray measurements to smaller showers that snow-covered tanks currently miss.
The FFT-based SiPM gain monitor gives a quick, robust in-situ calibration that can be applied to other SiPM arrays without dedicated calibration runs.
The panel's weight below 50 kg and two-person transportability make the station design practical for remote polar deployment and expansion.

Reading between the lines

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

The comparison with Laputop establishes agreement with IceTop, not absolute accuracy; if snow-related biases affect Laputop itself, the scintillators' true absolute pointing error remains unmeasured.
Because both prototype stations sit near the array center, the coincident-sample geometry is short-baseline; a production array with wider station spacing will be needed to test how angular resolution scales with lever arm.
The two DAQ variants are each validated by a single working instance; the inference that either architecture is production-ready would require a deployment decision and broader failure-mode statistics.
The 39–44 PE/MIP yield matches simulation for the first Antarctic year; long-term aging of the scintillator, fibers, and SiPM still needs a multi-year monitoring campaign to bound gradual degradation.

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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 genuinely new content here is the first year of operation of two IceTop scintillator prototype stations. The design and simulations were published before; this paper shows the deployed hardware actually works. Gain stability under temperature compensation holds to a couple of percent, the 39-44 PE/MIP light yield matches GEANT4, the charge response is linear down to IceTop's threshold, and the White Rabbit timing is verified with the known 335 ns cable delay. That is a legitimate, contained extension of an established program, and the FFT gain-determination method is a neat practical trick. I buy the proof-of-concept claim.

The soft spots are real but not load-bearing for a feasibility claim. The stress-test concern about Fig. 2 is fair: the timing-difference distribution looks broad and multi-peaked, and the text only quotes the mean delay. If there are multiple peaks or a width comparable to the timing precision, that matters for direction reconstruction, and the authors should quantify it. The same applies to Sec. 5.1: \"good correspondence\" is supported only by a figure, with no quoted fit widths, biases, event counts, or an explicit angular-resolution requirement. Using Laputop as the reference is acceptable for this stage, but since Laputop is itself affected by snow, the comparison does not establish absolute accuracy. None of this breaks the feasibility argument, but it is exactly the kind of quantitative detail a referee should ask for.

The citation pattern is fine - mostly IceCube collaboration papers on the design and simulations, which is appropriate for a proceedings paper from a large collaboration. No data release, which is typical for ICRC, but a bit disappointing.

This paper is for people working on the IceTop upgrade, large-surface scintillator arrays, and SiPM readout. It deserves a serious referee - the measurements are internally consistent and the claim is modest. If this were a journal submission, I would accept with minor revision: add numbers for the timing spread and the reconstruction comparison, and soften the word \"accurate\" to \"consistent with the reference.\" For a proceedings, I would still send it to review, because the prototype results will be cited by the collaboration's later upgrade papers. Recommend engaging with it.

Referee Report

2 major / 5 minor

Summary. The paper reports on two prototype scintillator stations deployed in 2017-2018 near the center of the IceTop array. Each station has seven 1.5 m^2 scintillator panels read out by SiPMs, with two different DAQ architectures (microDAQ and TAXI). The authors characterize the detector performance: White Rabbit timing relative to IceTop, temperature-compensated gain stability, MIP light yield, charge linearity with IceTop, and an initial comparison of air-shower direction reconstruction between the TAXI scintillator station and IceTop's Laputop reconstruction. The central conclusion is that the prototype stations demonstrate the feasibility of the scintillator surface upgrade of IceTop and its associated infrastructure.

Significance. If the prototype performance is taken at face value, the paper provides useful validation for the planned IceTop scintillator upgrade, particularly for snow-mitigation and for the surface field-hub and White Rabbit timing concepts that are also relevant to IceCube-Gen2. The paper's strengths include the use of an external reference (IceTop/Laputop and GEANT4 optical simulations) rather than circular self-comparison, long-term operation data, and a temperature-compensation scheme verified in situ. The significance is moderate: this is a proof-of-concept report on a small prototype, not a physics result, but it is directly relevant to the design choices of a major detector upgrade. The main weakness is that several load-bearing quantitative claims are presented only qualitatively or through figures without quoted numerical results.

major comments (2)

[Sec. 4.1, Fig. 2] The timing verification of the White Rabbit system rests on a single quoted offset of about 335 ns consistent with the 60 m copper cable, but the distribution in Fig. 2 appears broad and multi-peaked. The paper does not report the width of this distribution, the number of coincident events, or an interpretation of the multiple peaks. Since the subsequent claim in Sec. 5.1 that the timing information is 'rather good' and sufficient for air-shower detection depends on the precision of relative timing, the observed timing jitter must be quantified. Without this, the timing verification, which is a load-bearing part of the feasibility claim, is not quantitatively established.
[Sec. 5.1, Fig. 10] The statement 'A good correspondence between the results of the reconstructions is obtained' and the concluding claim that 'The direction of incident air showers are reconstructed accurately' are not quantitatively supported in the text. Fig. 10 shows Gaussian fits with results in the top-left, but neither the fitted means and widths, the number of events, nor the angular acceptance cuts are reported in the text. The authors should quote these numbers and, ideally, compare them with the angular resolution required for the IceTop upgrade science goals. Absent that, the reader cannot assess whether the observed scatter is consistent with the stated aim of mitigating snow-related systematics or merely qualitatively consistent.

minor comments (5)

[Sec. 4.3] The MIP light yield is quoted as a range of 39-44 PE/MIP without uncertainties or the number of panels contributing to this range; please state the spread and how it was obtained.
[Sec. 4.4, Fig. 8] The linear charge correlation is shown graphically, but no fit parameters, correlation coefficient, or residuals are given; adding these would strengthen the linearity claim.
[Sec. 5] The trigger condition requiring three or more scintillators in coincidence within ±200 ns is introduced in Sec. 5; consider moving it to Sec. 3 alongside the DAQ description for clarity.
[References] Reference [6] is incomplete, with no title or publication venue; please add the full citation.
[Summary] The final sentence contains a subject-verb agreement issue: 'stations are a proof of concept ... and shows' should be 'and show'.

Circularity Check

0 steps flagged · score 1.0 of 10

No circular derivation: the prototype performance is measured against external references (IceTop timing and charge, Laputop reconstruction, GEANT4 forward simulation); same-collaboration citations are consistency checks, not load-bearing inputs.

full rationale

The paper's claims are empirical and self-contained: White Rabbit timing is checked against IceTop's GPS-based timing (Fig. 2), SiPM gain is measured independently by Gaussian and FFT fits (Sec. 4.3), MIP light yield is measured from pedestal-subtracted spectra (Fig. 7), energy linearity is checked against IceTop charge (Fig. 8), and shower-direction accuracy is assessed by comparing the scintillator plane-front fit with the external Laputop reconstruction (Fig. 10). None of these steps derives its conclusion from the conclusion itself, and no fitted parameter is renamed as a prediction. The gain setpoint of 30 ADC/PE is a calibration choice, while the reported values of 29.3 and 29.4 ADC/PE are measured, not forced. The same-collaboration citations, notably [5] for GEANT4 optical simulations, serve as a consistency comparison for the measured light yield; this is a forward-model check rather than an input that determines the measurement, so removing the citation would not alter the prototype data. The qualitative presentation of the Laputop comparison (Sec. 5.1) is a reporting-completeness caveat about external-reference accuracy, not a circularity. On this basis, no circular step is present; the minor self-citations do not raise the score beyond the no-significant-circularity band.

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

The central feasibility claim rests on (1) GEANT4 optical simulations as the light-yield benchmark, (2) IceTop Laputop as the directional reference, and (3) a plane-front approximation for a seven-detector station. The free parameters are operational settings, not fitted physics constants. No invented entities are introduced.

free parameters (3)

SiPM gain setpoint = 30 ADC/PE
Chosen to optimize microDAQ dynamic range; gain and light-yield results are quoted relative to this operating point.
Scintillator trigger threshold = ~0.2 MIP
Data-acquisition discriminator threshold; sets the event sample entering the analysis.
Scalar coincidence requirement = three or more panels within +/-200 ns
Event-selection cut for the performance results; affects the reconstructed sample.

assumptions (3)

domain assumption GEANT4 optical simulations predict a MIP light yield of roughly 40 to 45 PE/MIP.
Used as the benchmark for the measured light yield in Sec. 4.3 and Sec. 6; the systematic uncertainty of the simulation is not quantified here.
domain assumption IceTop Laputop reconstruction provides a sufficiently accurate reference for air-shower direction.
Used in Sec. 5.1 to conclude that the scintillator reconstruction is accurate; any Laputop bias is inherited.
domain assumption A plane-front fit is adequate for reconstructing showers with a station of seven detectors.
Used in Sec. 5.1; ignores shower-front curvature and may introduce bias for inclined events.

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Cite this review

Pith. "Pith review of The Scintillator Upgrade of IceTop: Performance of the prototype array." pith.science (2026). https://pith.science/paper/NTDIUGOI

@misc{pith2026190809860,
  author       = {Pith},
  title        = {Pith review of: The Scintillator Upgrade of IceTop: Performance of the prototype array},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/NTDIUGOI}},
  note         = {Machine review of arXiv:1908.09860}
}

abstract

The IceCube Collaboration foresees to upgrade IceTop, the present surface array, with scintillator detectors augmented by radio antennas. As one of several goals the scintillator detectors will be used to measure and mitigate the effects of snow accumulation on the IceTop tanks: the increasing energy threshold and efficiency loss are nowadays the sources of the largest systematic uncertainties in shower reconstruction and mass composition analysis. In addition, the upgrade will provide useful experience for the development of next generation neutrino detectors proposed for the South Pole. In the Austral summer season, 2017-2018 two full "stations" were installed near the center of the IceTop array. Each station features custom-designed electronics and consists of seven detectors, each having an active area of 1.5m$^{2}$ plastic scintillator and wavelength shifting fibers read out by a Silicon Photomultiplier. In this contribution we review the detector design and performance, and show results from more than one year of operation of the prototype stations. During that year several thousand air shower events have been measured in coincidence with IceTop.

Figures

Figures reproduced from arXiv: 1908.09860 by the authors.

**Figure 1.** Deployment of the 14 scintillator panels at the South Pole. There are 2 scintillator panels deployed at each location A-G. The nearby IceTop tank positions are shown in circles. The overlapping of the circles with the scintillator locations indicates where the panels are placed on top of an IceTop tank. 2 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗

**Figure 2.** Timing difference between coincident scintillator and IceTop events. Each of the two prototype stations has a different DAQ concept. One station’s DAQ was developed by the Wisconsin IceCube Particle Astrophysics Center (WIPAC) and utilizes digital communications between the scintillator and surface field hub for increased bandwidth. This is known as the microDAQ station. The other station’s DAQ was developed by … view at source ↗

**Figure 3.** An example plane fit to the temperature vs voltage vs measured gain of the SiPM. The gain and photo-detection efficiency (PDE) of SiPMs is temperature dependent. The on-board scintillator microDAQ mitigates this effect at the software level by periodically measuring the temperature of the SiPM and adjusting the bias voltage to maintain a constant gain. The temperature dependence of each individual scintillato… view at source ↗

Figures from the paper (7 more)

**Figure 4.** Figure 4: Gaussian fits to the finger spectrum give an average PE separation of 29.3 ± 0.4 ADC/PE which is consistent with the FFT result shown in [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]

**Figure 5.** Figure 5: The FFT of charge histogram returns a gain of 29.4 ± 0.1 ADC/PE which is consistent with the Gaussian fit method shown in [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]

**Figure 6.** Figure 6: Example temperature-compensated gain stability of one scintillator panel over a broad range of operating temperatures. The estimated light yield from minimally ionizing particles (MIPs) is determined by an exponential plus Gaussian fit to the spectra after pedestal su…

**Figure 7.** Figure 7: An example fit to the MIP peak. The pedestal is in purple, the exponential dark noise component is green, the MIP component estimated with a Gaussian is in blue, and the fit total is in red. In this example the gain was measured to be 29.4 ADC/PE and the MIP was fit to…

**Figure 8.** Figure 8: Coincident events are found between IceTop station 33 (tank A) and scintillator location B and IceTop station 43 (tank B) and scintillator location F as shown in Fig.1. Plots show the charge of scintillator events in ADC counts vs charge of coincident IceTop events in …

**Figure 9.** Figure 9: Distributions of the reconstructed zenith and azimuth angles of events detected by the TAXI scintillator station (Sec. 3) which are coincident with IceTop events [PITH_FULL_IMAGE:figures/full_fig_p007_9.png]

**Figure 10.** Figure 10: Histograms of the differences between the TAXI scintillator station (Sec. 3) event reconstruction angles and IceTop Laputop reconstructions. Only IceTop events with Laputop reconstructed core positions within the scintillator area are considered. The Gaussian fits are…

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Forward citations

Cited by 1 Pith paper

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Reference graph

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

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" write newline "" before.all 'output.state := FUNCTION blank.sep after.quote 'output.state := FUNCTION fin.entry output.state after.quoted.block = 'skip 'add.period if write newline FUNCTION new.block output.state before.all = 'skip output.state after.quote = after.quoted.block 'output.state := after.block 'output.state := if if FUNCTION new.sentence out...

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