A multi-PMT Optical Module for the IceCube Upgrade

Alexander Kappes; Lew Classen; Timo Karg (for the IceCube Collaboration)

REVIEW 3 major objections 5 minor 1 cited by

A multi-PMT Optical Module for the IceCube Upgrade

T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read A 24-photomultiplier glass module, the mDOM, is claimed to more than double the sensitive area of a single-tube module while recording photon directions and seeing supernovae to about 300 kiloparsecs.

desk verdict A solid, honest engineering progress report on the mDOM for IceCube Upgrade; the disclosed thermal-expansion problem is a real caveat but not a disqualifier at this stage. read the letter →

arxiv 1908.10802 v1 pith:65MZ4YAS submitted 2019-08-28 astro-ph.HE astro-ph.IM

Lew Classen , Alexander Kappes , Timo Karg (for the IceCube Collaboration) This is my paper · ORCID

classification astro-ph.HEastro-ph.IM

keywords multi-PMTopticalmoduleIceCubeUpgradeneutrinotelescopephotomultipliertubesupernovaneutrinosCherenkovlightcalibrationpressurevessel

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 argues that a multi-PMT Digital Optical Module (mDOM)—a glass sphere holding 24 three-inch photomultipliers—fits inside the size and power limits of the IceCube Upgrade and outperforms today's single-tube modules. Because each tube is digitized individually, one module records photon arrival times, amplitudes, and directions, and can trigger on multiple hits inside the same sphere. The authors report that the 24 tubes plus reflectors give more than twice the effective photosensitive area of the current optical module at about 5 W of power. They also simulate a 10,000-module array and find that the mDOM's local-coincidence trigger would detect supernovae out to about 300 kiloparsecs with one false discovery per century, compared with about 50 kiloparsecs for the current detector. A reader should care because the mDOM is the planned workhorse of the IceCube Upgrade, which aims to lower the energy threshold to a few GeV and improve ice calibration.

What carries the argument

The object that carries the argument is the mDOM itself: a roughly spherical pressure vessel containing 24 three-inch photomultipliers pointing in all directions, mounted behind a black 3D-printed support structure and coupled to the glass by curing silicone gel. The key functional mechanism is per-PMT digitization—each tube has an active Cockcroft-Walton high-voltage base and an analog front-end that sends one copy of the pulse to a comparator and another to a 100 MHz ADC—so every photon hit is individually time-stamped and shaped. That per-tube readout is what produces directional information, multi-hit triggering, and local coincidences, and those three capabilities are what the paper's sensitivity claims depend on.

What would settle it

Cool a fully assembled mDOM with its final support material to the lowest expected deployment temperature, then inspect for cracks in the pressure vessel, delamination of the optical gel, and shifts in photomultiplier alignment; any such damage would falsify the claim that the design meets the Upgrade's mechanical constraints.

Watch

Extended reading notes

Core claim

The central claim is that the mDOM design satisfies the constraints of the IceCube Upgrade: a 700-bar pressure vessel, a tight borehole diameter, low power, and reliable operation in deep ice. The authors state that the 24 PMTs, each with its own Cockcroft-Walton high-voltage base and a digitizing front-end channel, together provide more than twice the effective photosensitive area of the current single-PMT optical module, and that reflectors mounted at a 102-degree opening angle add about 20% sensitivity. The module also yields directional information per photon and supports local coincidences, which the authors use in a supernova study of a future 10,000-module array; that study reaches about 300 kiloparsecs with one false discovery per century. The paper further claims that in-situ calibration using radioactive decays in the pressure-vessel glass can synchronize individual PMTs to 0.1 ns, and that cameras, LEDs, and acoustic sensors will improve the understanding of the ice. Two demonstrator modules have been assembled, and the paper presents the design as ready for optimization and deployment.

Load-bearing premise

The design's long-term viability depends on the internal support frame expanding and contracting with temperature in a way that does not crack the glass pressure vessel, pull the optical gel away from the photomultipliers, or misalign the tubes; the authors note that the demonstrator showed thermal-expansion problems and that the proposed fix is still under investigation.

Editorial extensions

If this is right

Each mDOM collects more than twice the light of a current optical module, so the IceCube Upgrade gains sensitivity without needing more modules.
Because every PMT is read out separately, a single module can distinguish photons arriving from different directions, improving event reconstruction.
Low-energy supernova neutrinos become detectable to about 300 kpc with a Gen2-scale array of 10,000 mDOMs, extending the current reach by a factor of about six.
The radioactive-decay calibration technique promises 0.1 ns synchronization between PMTs, which would tighten the ice-property systematics that limit directional reconstruction.

Reading between the lines

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

If the thermal-expansion fix holds, the mDOM's 24-PMT architecture could be reused in other deep-ice or deep-water neutrino detectors, since its geometry is not tied to a specific borehole.
The 102-degree reflector angle was optimized for a plane-wave Cherenkov front; other light distributions, such as point-like sources or heavily scattered light, could favor a different angle, so the ~20% gain should be re-checked for those event types.
The 0.1 ns calibration from glass radioactivity is a generic technique: any optical module with a glass pressure vessel could use it, not only the mDOM.
A near-term, low-cost test of the supernova local-coincidence idea would be to search for correlated multi-PMT hits in the first Upgrade string once deployed, during quiet periods or a candidate supernova, before a full Gen2 array exists.

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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

You should read this as a well-executed status report from the IceCube Upgrade hardware program, not as a final design validation. The genuinely new material is concrete: the 102-degree reflector optimization, the measured pressure-vessel deformation up to 700 bar, the Cockcroft-Walton active bases, the 5 W readout chain, and the local-coincidence supernova simulation. Those are real engineering results, supported by prototype builds, hyperbaric tests, and lab measurements. Credit is due for the honest disclosure in Sec. 2.6 that the demonstrator showed thermal-expansion problems with the support structure during cooling. That kind of failure reporting is exactly what a progress report should do. The stress-test note lands, but with proportion. The paper's abstract says the mDOM design "meets these challenges," and that claim is a bit ahead of the evidence because the thermal mismatch between support structure and glass vessel is not yet closed. The proposed glass-enhanced polyamide fix is unvalidated, and the differential contraction numbers in the stress-test note are plausible. This means the "more than twice effective area" claim should be read as conditional on the module surviving deployment and cooling intact. Still, this is not a hidden flaw or a fatal one; it is an admitted, in-progress item on a hardware timeline. In a conference proceedings, that is normal and acceptable, as long as the reader does not mistake the abstract for a finished qualification. The other soft spots are real but minor. The supernova reach of ~300 kpc is a forward simulation with no quoted uncertainties, and the paper explicitly lists the backgrounds not included (Michel electrons, correlated noise, channel cross-talk). That is a limitation, not a defect, because the goal is to show a new technique works in principle. The in-situ calibration method is lab-tested on a four-PMT setup and has a thesis behind it, so the 0.1 ns claim is grounded even if the details live outside the paper. The citation pattern is fine: self-citations point to theses and companion papers that supply the missing detail. Who gets value from this? Anyone tracking IceCube Upgrade instrumentation, people comparing multi-PMT module designs across KM3NeT and IceCube, and anyone planning simulation studies of local coincidences. It deserves a serious referee; I would send it to review, with a request that the authors soften the abstract's "meets these challenges" to reflect the open thermal issue and maybe add a sentence on the uncertainty of the supernova simulation. It is a conditional accept, not a reject.

Referee Report

3 major / 5 minor

Summary. The paper describes the design, prototyping status, and expected performance of the multi-PMT Digital Optical Module (mDOM) being developed for the IceCube Upgrade. The mDOM houses 24 three-inch-class PMTs with individual digitization and local-coincidence triggering, and is claimed to provide an effective photosensitive area more than twice that of the current IceCube optical module. The paper reports on PMT and reflector choices, a low-power Cockcroft-Walton active base and readout electronics, calibration devices, a support structure with optical gel, a pressure vessel rated at 700 bar, and construction of demonstrator modules. It also presents Monte Carlo studies of supernova detection with local coincidences, claiming a reach of about 300 kpc for an IceCube-Gen2-like detector of 10,000 mDOMs, and an in-situ calibration method using radioactive decays in the vessel glass.

Significance. If the design is validated, the mDOM is a central component of the IceCube Upgrade and a candidate for IceCube-Gen2, with directional information, multi-hit triggering, and calibration capabilities beyond the current single-PMT modules. The paper provides concrete engineering progress: pressure-vessel deformation measurements in a hyperbaric chamber, a built demonstrator, and prototype electronics. The claimed supernova reach and calibration precision, if substantiated, would be valuable for the community's planning. The paper is honest about several limitations, including the thermal-expansion problem and omitted background contributions in the supernova study. These disclosures strengthen the credibility of the report but also mean the central 'meets these challenges' claim is not yet fully closed.

major comments (3)

[Sec. 2.6 and Sec. 2.4] The demonstrator 'revealed problems with the thermal expansion of the support structure during cooling to low temperatures' (Sec. 2.6). This is directly load-bearing for the abstract claim that the design 'meets these challenges,' because mechanical integrity and optical coupling are prerequisites for the effective-area and sensitivity statements. The proposed remedy, glass-enhanced polyamide with about 30% lower expansion coefficient (Sec. 2.4), is still under investigation and no test or quantitative thermal-stress analysis is presented. Given the large coefficient-of-thermal-expansion mismatch between polyamide-type materials and borosilicate glass, the fix needs to be demonstrated before the module can be considered as meeting the mechanical and optical requirements. This issue should be resolved or explicitly reframed as an open design item.
[Sec. 3] The supernova detection claim of '~300 kpc with one false discovery per century' is based on a simulation for which no uncertainties are quoted, and the text explicitly lists omitted contributions: Michel electrons from atmospheric muons, correlated noise in an individual PMT, and cross-talk between PMT channels. These omissions could affect the event rate and background estimate, and thus the reach. The paper should state that the 300 kpc figure is a preliminary simulation result and should discuss the expected impact of the omitted terms, or provide a quantitative bound showing that they do not change the conclusion.
[Abstract and Sec. 1] The claim that 'the 24 PMTs provide an effective photosensitive area more than twice than that of the current IceCube optical module' is not supported by a derivation, simulation, or measurement in the paper. The effective area depends on the PMT photocathode coverage, reflector geometry (opening angle, reflectivity), shadowing by the support structure, and absorption in glass and gel, some of which are discussed qualitatively. Please provide the basis for this quantitative claim, or qualify it as a design expectation rather than a demonstrated property.

minor comments (5)

[Abstract] The phrase 'more than twice than that' is grammatically awkward; consider 'more than twice that of'.
[Sec. 2.4] The text says the support structure is 'subsequently died black with non-conductive color'; 'died' should be 'dyed'.
[Sec. 2.1] The reflector opening angle is quoted as 102 degrees following a Monte Carlo optimization for vertical photons, but the angular acceptance and overall sensitivity loss of '<1%' are given without error bars; stating the simulation model's assumptions would help reproducibility.
[Sec. 3] The sentence describing background contributions considered ('solar neutrino flux, dark-noise rate, radioactive decays in the vessel glass') is useful, but the text should explicitly state that the 300 kpc value is for the assumed LS220 model and 27-solar-mass progenitor; a reader might otherwise generalize the reach to all supernovae.
[References] The paper cites several M.Sc. theses and companion papers; for a proceedings, this is acceptable, but it would strengthen the report to give a single reference where the full mDOM mechanical design is documented in an archival source once available.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: the paper's claims are supported by forward simulations, lab measurements, and geometric arguments rather than by restating its inputs.

full rationale

The paper makes three main quantitative claims: the mDOM's 24 PMTs provide more than twice the effective photosensitive area of the current IceCube DOM; the mDOM design meets the Upgrade's size, power, and pressure constraints; and local coincidences would let IceCube-Gen2 detect supernovae out to ~300 kpc. None of these reduce to a fitted parameter or a self-defined quantity. The effective-area claim is geometric: 24 three-inch PMTs sum to more sensitive area than a single ten-inch PMT, with reflectors adding about 20% in simulated sensitivity. The supernova reach is a forward simulation with explicitly stated inputs: a 27-solar-mass Type IIp supernova model with the LS220 equation of state, a detector of 10,000 mDOMs, and backgrounds from solar neutrinos, PMT dark noise, and radioactive decays in the glass. The calibration method is validated in first lab tests on a four-PMT configuration. References to collaboration theses and companion papers are used as sources of detail and prior studies, not as definitions of the present paper's outputs. The paper is transparent about an unresolved engineering issue -- the thermal expansion of the support structure during cooling, reported in Sec. 2.6 -- but this is a correctness or readiness risk, not a circularity. No equation or claim in the paper is shown to be equivalent to its own input by construction.

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

No new physical entities are postulated. The free parameters are engineering optimizations (reflector angle, trigger threshold). The axioms are the modeling choices and environmental assumptions behind the supernova simulation and the thermal integrity of the module.

free parameters (2)

Reflector opening angle = 102 degrees
Optimized in a Monte Carlo study to maximize sensitivity for vertical photons (Sec. 2.1). It is an optimized design choice rather than a first-principles derivation.
Comparator threshold = ~0.2 photoelectrons
Chosen trigger threshold in the analog front-end (Sec. 2.2). It sets the trade-off between detection efficiency and dark-rate rejection, but no sensitivity scan is shown.

assumptions (4)

domain assumption The simulated supernova flux from a 27 solar mass progenitor with the LS220 equation of state is representative for the claimed detection reach.
Sec. 3: 'modelled the neutrino flux from a type IIp supernova with a progenitor mass of 27 solar masses (using the LS220 equation of state model from [9])'. Different flux models could change the reach.
domain assumption The neglected backgrounds (Michel electrons from atmospheric muons, correlated noise in a single PMT, cross-talk between PMT channels) do not affect the supernova conclusions.
Sec. 3 explicitly states 'Not considered are Michel electrons..., correlated noise..., as well as potential cross-talk...'. If these are non-negligible, the false-discovery-rate and reach figures would change.
domain assumption The future detector for the supernova study consists of 10,000 mDOMs (IceCube-Gen2-like), not the ~700 modules of the IceCube Upgrade.
Sec. 3: 'simulated a future detector comprising 10,000 mDOMs, similar to the possible design of IceCube-Gen2 [10]'. The 300 kpc reach applies to this scenario only.
domain assumption The support-structure material can be made to survive the in-situ temperature cycle without damaging the optical gel or pressure vessel.
Sec. 2.6 reports thermal-expansion problems during demonstrator cooling; Sec. 2.4 proposes glass-enhanced polyamide as a fix, which is still under investigation.

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Cite this review

Pith. "Pith review of A multi-PMT Optical Module for the IceCube Upgrade." pith.science (2026). https://pith.science/paper/65MZ4YAS

@misc{pith2026190810802,
  author       = {Pith},
  title        = {Pith review of: A multi-PMT Optical Module for the IceCube Upgrade},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/65MZ4YAS}},
  note         = {Machine review of arXiv:1908.10802}
}

read the original abstract

Following the first observation of an astrophysical high-energy neutrino flux with the IceCube Neutrino Observatory in 2013 and the identification of a first cosmic high-energy neutrino source in 2017, the detector will be upgraded with about 700 new advanced optical sensors. This will expand IceCube's capabilities both at low and high neutrino energies. A large fraction of the upgrade modules will be multi-PMT Digital Optical Modules, mDOMs, each featuring 24 three-inch class photomultiplier tubes (PMTs) pointing uniformly in all directions, thereby providing an almost homogeneous angular coverage. The signal from each PMT is digitized individually, providing directional information for the incident photons. Together, the 24 PMTs provide an effective photosensitive area more than twice than that of the current IceCube optical module. The main mDOM design challenges arise from the constraints on the module size and power needed for the 24-channel high-voltage and readout systems. This contribution presents an mDOM design that meets these challenges and discusses the sensitivities expected from these modules.

Figures

Figures reproduced from arXiv: 1908.10802 by the authors.

**Figure 1.** mDOM overview: Left: Demonstrator module (see Sec. 2) Right: Exploded view featuring main components. 1. The multi-PMT Digital Optical Module (mDOM) Located in the deep glacial ice of Antarctica, IceCube [1] is the world’s largest neutrino telescope. Originally designed for the investigation of the neutrino sky on the TeV to PeV energy scale and beyond, the energy threshold was lowered to ∼10GeV by the DeepCore [2]… view at source ↗

**Figure 2.** Currently considered IceCube Upgrade string configurations in comparison to the strings deployed in IceCube and the DeepCore extension. Each marker represents an optical module. ing freeze-in up to more than twice the static pressure and a tight power budget per module. The mDOM design (see [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗

**Figure 3.** Candidate PMTs for the multi-PMT Digital Optical Module (mDOM). layer located on the outside surface of the glass tube surrounding the electron multiplier system, which is electrically connected to the photocathode. Reflectors mounted around the entrance window of the PMTs increase the effective area for vertical (on-axis) illumination and compensate for photons lost due to shadowing of the support structure (see Se… view at source ↗

Figures from the paper (2 more)

**Figure 4.** Figure 4: Left: Test setup featuring a Hamamatsu PMT equipped with an active base read-out by the analog front-end test board. Right: mDOM Test Mainbaord (mDOT). The board features the final main board form factor and four different analog front-end designs. converter (ADC) data…

**Figure 5.** Figure 5: Horizontal and vertical (violet line) shrinkage of an mDOM pressure vessel as function of external pressure in a hyperbaric chamber. The horizontal deformation was measured separately in the upper (two sensors, blue and green lines, partially hidden) and lower (red …

Discussion (0). Continue with ORCID to comment.

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

Estimating the sensitivity of the IceCube Upgrade to probe the interior of the Earth using atmospheric neutrino oscillations
hep-ex 2026-08 conditional novelty 5.0 of 10

A Monte Carlo sensitivity study projects that the IceCube Upgrade with DeepCore can detect Earth matter effects at 5.5-7.1 sigma, reject a uniform Earth at 2.4 sigma, and measure the Earth's mass to about 10% precision.

Reference graph

Works this paper leans on

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Pith tools

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

[1] [1]

IceCube Collaboration, M. G. Aartsen et al., JINST 12 (2017) P03012

2017

[2] [2]

Abbasi et al., APP 35 (2012) 615

IceCube Collaboration, R. Abbasi et al., APP 35 (2012) 615

work page 2012

[3] [3]

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KM3NeT Collaboration, H. Löhner et al., Nucl. Inst. Meth. A718 (2013) 513–515

work page 2013

[4] [4]

IceCube Collaboration, PoS(ICRC2019)1022 (these proceedings)

work page

[5] [5]

M. A. Unland Elorrieta et al., JINST 14 (2019) P03015

work page 2019

[6] [6]

IceCube Collaboration, PoS(ICRC2019)928 (these proceedings)

[7] [7]

IceCube Collaboration, PoS(ICRC2019)1030 (these proceedings)

work page

[8] [8]

Makino, EPJ Web Conf

Y . Makino, EPJ Web Conf. 207 (2019) 06005

work page 2019