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arxiv: 2604.27399 · v1 · submitted 2026-04-30 · ⚛️ physics.ins-det · hep-ex

Recognition: unknown

Optical effects in Gaseous Electron Multipliers (GEMs)

D. Edgeman, D. Loomba, E. Tilly, F.M. Brunbauer, J. Schueler, K. Nikolopoulos, L. Millins, M. Gardner, P.A. Majewski, T. Marley, T. Neep, W. Thompson

Authors on Pith no claims yet

Pith reviewed 2026-05-07 08:43 UTC · model grok-4.3

classification ⚛️ physics.ins-det hep-ex
keywords GEMoptical readoutscintillation lighttrack reconstructionMIGDALOTPCGeant4glass substrate
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The pith

Scintillation light spreads through glass GEM substrates, increasing observed track intensity and width by up to 26% and 31%.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates why particle tracks in the MIGDAL optical time projection chamber appear brighter and wider than expected from charge simulations. It tests the idea that light from avalanches inside GEM holes travels through the substrate material and emerges from adjacent holes. Measurements on glass, ceramic, and FR4 GEMs show this effect is strongest in glass. Simulations incorporating this propagation reproduce the excess and suggest it accounts for the discrepancy in the experiment.

Core claim

Scintillation light produced inside a GEM hole during the avalanche propagates through the GEM substrate and exits neighboring holes. This optical broadening effect is strongest in glass GEMs. Lab measurements quantify this, and Geant4 simulations show that applying glass GEM effects to simulated tracks increases intensity by up to 26% and widths by 31%. This may explain the larger than expected intensity and track widths observed in the MIGDAL OTPC and is expected to be an observed effect in all GEM-based OTPCs.

What carries the argument

Optical propagation of scintillation light through the GEM substrate material, allowing photons to exit from neighboring holes and broaden the apparent track.

If this is right

  • Glass GEMs show the largest optical broadening among tested substrates.
  • The effect explains the systematic excess in intensity and width seen in MIGDAL's optical readout compared to charge simulations.
  • Similar optical effects are likely present in any GEM-based OTPC using glass substrates.
  • Simulations that include this propagation can better match experimental observations of track properties.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Future designs of high-resolution OTPCs may benefit from using GEM materials with lower light transmission to reduce track broadening.
  • Experiments relying on optical readout from GEMs should calibrate their simulations against lab measurements of light propagation.
  • Choosing ceramic or FR4 GEMs could minimize this effect if optical fidelity is prioritized over other properties.

Load-bearing premise

The light propagation measured using controlled external light sources matches the spectrum and directions of light actually produced by electron avalanches inside the GEM holes.

What would settle it

A direct measurement of light intensity in holes adjacent to a single active avalanche hole in a real GEM operating in the detector gas, compared to the lab propagation fractions.

Figures

Figures reproduced from arXiv: 2604.27399 by D. Edgeman, D. Loomba, E. Tilly, F.M. Brunbauer, J. Schueler, K. Nikolopoulos, L. Millins, M. Gardner, P.A. Majewski, T. Marley, T. Neep, W. Thompson.

Figure 1
Figure 1. Figure 1: (a) Diagram of experimental setup used to measure the MH-PSF from a light source in a single GEM hole. Green corresponds to light leaving GEM toward the camera, Red corresponds to light traveling through GEM substrate. (b) Microscopic view isolating a single GEM hole with tape. (c) Microscopic view showing phosphorescent paint constrained to a single GEM hole. We isolated a hole by covering the surrounding… view at source ↗
Figure 2
Figure 2. Figure 2: Phosphorescent paint characteristics. (a) Spectrum peaking ∼ 520nm. (b) Intensity with time. The procedure described above worked for all except the standard thin GEM, whose hole diameter and pitch were too small to insert paint into a single isolated hole. For the remaining 3 GEMs (G-GEM, THGEM, M-THGEM), images were taken using the Andor iKon-L 936 BV5 CCD camera with a 2048×2048 array of 13.5×13.5 µm pi… view at source ↗
Figure 3
Figure 3. Figure 3: MH-PSF from single-hole light source results. (Top) G-GEM 600s exposure, (Middle) THGEM 30s exposure, (Bottom) M-THGEM 30s exposure. (Left) Camera image, (Center) Log intensity surface plots of camera images all at the same scale, (Right) 1D slice through GEM holes showing relative intensity with distance from the hole with the light source, also at the same scale. In camera images, slices are through the … view at source ↗
Figure 4
Figure 4. Figure 4: Geant4 GEM simulations. (a) A 21×21 GEM pitch portion of the G-GEM with dimensions matching view at source ↗
Figure 5
Figure 5. Figure 5: Slices through Geant4 MH-PSF G-GEM holes showing changes to GEM-lens distance (a) and offset perpendicular to GEM-lens distance (b). Using a known lens distance and diameter, a Python script can read the Geant4 output and calculate if each photon would hit or miss a camera lens, placed at a distance relative to the GEM, matching any given experiment. For each photon that hits the lens, the initial position… view at source ↗
Figure 6
Figure 6. Figure 6: Angular distribution of photons leaving GEM holes with distance traveling through substrate. (a) Radial-plane (inward/outward) from central GEM hole. (b) Tangential-plane perpendicular to central GEM hole direction. The Geant4 output direction is vital to exclude photons traveling at large 𝜃, the angle with respect to the lens direction view at source ↗
Figure 7
Figure 7. Figure 7: Geant4 G-GEM simulations vs MH-PSF measurements. 4.5. Realistic MIGDAL MH-PSF Both panels of view at source ↗
Figure 8
Figure 8. Figure 8: Adding measured G-GEM MH-PSF to simulated particle tracks. (a) Center of each GEM hole in particle track identified from known GEM pitch and hexagonal pattern. (b) Hexagon around simulated particle track GEM hole to measure intensity. (c) Simulated G-GEM MH-PSF intensity inside hexagon to normalize intensity to simulated particle track GEM hole, the area beyond the hexagon is the intensity to add. To apply… view at source ↗
Figure 9
Figure 9. Figure 9: Particle track changes are shown before (top) and after convolving a realistic 2 stacked G-GEM MH-PSF. The minimum increase (middle), without reflective surfaces beyond the G-GEM, and an example of adding reflective surfaces (bottom) are shown. Left is a 5.9 keV ER, center is a 200 kevee Flourine NR, right is a 600 kevee Carbon NR. The intensity distribution in the region outside the central hole hexagon o… view at source ↗
Figure 10
Figure 10. Figure 10: Measurements of simulated particle tracks include total intensity, number of pixels in the track, distance from interaction vertex to track edge (r1,r2,r3), and slice through the interaction vertex for Gaussian fitting to the track width and calculation of the standard deviation 𝜎. Comparisons of tracks without noise were performed to understand the true effect of the convolution process, which should res… view at source ↗
Figure 11
Figure 11. Figure 11: Effects on particle tracks before and after convolving a realistic 2 stacked G-GEM MH-PSF. The minimum increase (a) without reflective surfaces beyond the G-GEM and an example of adding reflective surfaces (b). Realistic changes are expected between these plots. 5.9 keV ERs are shown hollow and NRs solid. Error bars are given by the standard deviation of the distribution at each point. GEM-based optically… view at source ↗
Figure 12
Figure 12. Figure 12: Implications of the simulated G-GEM MH-PSF on Migdal effect searches. The mean ER peak intensity distance from interaction vertex is mostly unchanged after convolution, so only one blue diamond is shown. NR track widths, however, do change after convolution as shown by the red circles before convolution, the minimum increase expected without reflective surfaces beyond the G-GEM (green squares), and with r… view at source ↗
read the original abstract

Optical time projection chambers (OTPCs) are well suited for applications that require the highest spatial resolution for particle track reconstruction. The MIGDAL experiment uses a glass GEM-based OTPC and observes a systematic excess in both the intensity and width of particle tracks in its optical readout, when compared with charge readout simulations. One hypothesis is that scintillation light produced inside a GEM hole during the avalanche propagates through the GEM substrate and exits neighboring holes. We present lab measurements testing this hypothesized optical broadening effect in three types of GEM substrates: glass, ceramic, and FR4. Our observations quantify this optical broadening and demonstrate it to be strongest in glass GEMs. Additionally, we use Geant4 simulations to both reproduce our observations and quantify optical broadening effects in realistic charge avalanches. Applying our glass GEM effects to simulated particle tracks yields increases of track intensity and widths by up to around 26% and 31%, respectively. This may explain the larger than expected intensity and track widths observed in the MIGDAL OTPC and is expected to be an observed effect in all GEM-based OTPCs.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript investigates hypothesized optical broadening in GEM-based OTPCs, where scintillation light from avalanches in one GEM hole propagates through the substrate and exits neighboring holes. It reports laboratory measurements of this effect in glass, ceramic, and FR4 GEM substrates using controlled external light sources, complemented by Geant4 simulations that reproduce the lab data and extend the analysis to realistic charge avalanches. The central quantitative result is that applying the glass-GEM optical effects to simulated particle tracks increases track intensity by up to ~26% and widths by up to ~31%, which the authors suggest may explain the excess intensity and width observed in the MIGDAL OTPC relative to charge-readout simulations.

Significance. If the extrapolation from controlled-source lab data to operating avalanches is valid, the result identifies a measurable systematic that affects track reconstruction in all GEM-based optical TPCs. The multi-substrate comparison and use of Geant4 to propagate measured broadening into track-level predictions constitute a concrete, falsifiable contribution that could improve agreement between optical and charge readouts in MIGDAL and guide future detector design.

major comments (2)
  1. [Application to simulated tracks / Geant4 extension] The 26% intensity and 31% width increases (final paragraph of the abstract and the corresponding application section) are obtained by feeding lab-measured optical transmission into Geant4 and applying the result to simulated tracks. This procedure assumes that the spectrum, angular distribution, and wavelength-dependent propagation of light from the external controlled source match those of actual scintillation produced inside GEM holes during avalanches. No direct measurement of light emission from operating avalanches (same gas, gain, and hole geometry as MIGDAL) is reported; any mismatch scales directly into the quoted correction factors and their explanatory power for the MIGDAL excesses.
  2. [Geant4 simulations and validation] The manuscript states that Geant4 reproduces the laboratory light distributions, yet provides no quantitative metrics (e.g., goodness-of-fit, residual distributions, or uncertainty on optical material parameters) for that agreement, nor a sensitivity study varying the scintillation spectrum or isotropy. Because the free parameters in the optical model directly determine the propagated 26%/31% figures, the absence of these checks leaves the robustness of the central claim unquantified.
minor comments (2)
  1. The abstract and results sections should report the number of GEM samples tested, number of repetitions per configuration, data-exclusion criteria, and explicit uncertainties (including systematic contributions) on the quoted 26% and 31% values.
  2. Clarify the precise definitions of 'track intensity' and 'track width' used when applying the optical correction to simulated tracks, and state whether these definitions match the MIGDAL analysis pipeline.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful and constructive review of our manuscript. We address each major comment below, indicating revisions that will be incorporated to improve clarity and robustness.

read point-by-point responses

  1. Referee: [Application to simulated tracks / Geant4 extension] The 26% intensity and 31% width increases (final paragraph of the abstract and the corresponding application section) are obtained by feeding lab-measured optical transmission into Geant4 and applying the result to simulated tracks. This procedure assumes that the spectrum, angular distribution, and wavelength-dependent propagation of light from the external controlled source match those of actual scintillation produced inside GEM holes during avalanches. No direct measurement of light emission from operating avalanches (same gas, gain, and hole geometry as MIGDAL) is reported; any mismatch scales directly into the quoted correction factors and their explanatory power for the MIGDAL excesses.

    Authors: We acknowledge that our laboratory setup employs an external light source rather than direct scintillation from operating avalanches, and that the spectrum and angular distribution are therefore matched by design rather than measured in situ. The source wavelength range was selected to overlap with the expected scintillation spectrum of the MIGDAL gas mixture, and the Geant4 model uses the measured wavelength-dependent transmission through each substrate. We agree this introduces an assumption whose impact is not fully quantified in the current text. In the revised manuscript we will add an explicit discussion of these assumptions, including a qualitative assessment of possible mismatches, and we will flag the need for future in-situ optical measurements during avalanche operation as a limitation. The reported 26% and 31% figures will be presented as estimates under the stated conditions rather than as definitive corrections. revision: partial

  2. Referee: [Geant4 simulations and validation] The manuscript states that Geant4 reproduces the laboratory light distributions, yet provides no quantitative metrics (e.g., goodness-of-fit, residual distributions, or uncertainty on optical material parameters) for that agreement, nor a sensitivity study varying the scintillation spectrum or isotropy. Because the free parameters in the optical model directly determine the propagated 26%/31% figures, the absence of these checks leaves the robustness of the central claim unquantified.

    Authors: We thank the referee for highlighting this gap. The current manuscript demonstrates agreement primarily through visual overlay of measured and simulated light profiles. In the revised version we will add quantitative validation metrics, including chi-squared per degree of freedom for the light-distribution comparisons and example residual plots. We will also perform and report a sensitivity study in which the scintillation spectrum, isotropy, and key optical parameters (refractive index, absorption length) are varied within physically motivated ranges; the resulting spread in the derived track intensity and width increases will be quoted. These additions will directly address the robustness of the central quantitative results. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central results derive from independent lab measurements

full rationale

The paper's derivation begins with direct laboratory measurements of optical propagation through physical GEM samples (glass, ceramic, FR4) using controlled external light sources. These empirical broadening factors are then fed into Geant4 simulations that first reproduce the lab data and subsequently apply the same propagation model to separate simulated charge avalanches and particle tracks. The reported 26% intensity and 31% width increases are therefore outputs of this forward propagation, not parameters fitted to the MIGDAL OTPC observations themselves. No equation or step in the chain defines a quantity in terms of the target excess or renames a fit as a prediction; the MIGDAL comparison remains a post-hoc consistency check rather than an input. The chain is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The claim rests on the assumption that scintillation light is generated inside GEM holes and that a measurable fraction propagates laterally through the substrate. Geant4 optical transport requires material refractive indices, absorption lengths, and surface properties that are either taken from literature or tuned to match the lab data.

free parameters (1)
  • Geant4 optical material parameters
    Refractive index, absorption length, and surface roughness for glass, ceramic, and FR4 are required by the simulation and may be adjusted to reproduce the measured light leakage.
axioms (2)
  • domain assumption Scintillation light is produced inside GEM holes during electron avalanche
    Invoked in the hypothesis and in the Geant4 modeling of light generation.
  • standard math Light propagation through the GEM substrate can be modeled by standard optical transport in Geant4
    The simulation framework assumes classical ray tracing and Fresnel reflection at interfaces.

pith-pipeline@v0.9.0 · 5532 in / 1639 out tokens · 71483 ms · 2026-05-07T08:43:26.720963+00:00 · methodology

discussion (0)

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Reference graph

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