REVIEW 1 major objections 53 references
Stopping gradient propagation at unstable material boundaries enables usable derivatives for optimizing detector designs in radiation transport simulations.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.3
2026-06-30 23:35 UTC pith:MMLWVQVG
load-bearing objection The paper describes a heuristic to block gradients at unstable material boundaries in a differentiable Geant4-like transport code, but supplies no numbers or checks on whether the resulting gradients are actually usable. the 1 major comments →
Exploring the Boundaries of Differentiable Radiation Transport and Detector Simulation
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
When differentiating a Geant4-like radiation transport simulation with full electromagnetic physics, exploding gradients occur due to rare but extreme sensitivities at material boundaries which propagate through subsequent transport and shower development. To obtain usable derivatives for optimization, a targeted mitigation strategy is introduced that stops gradient propagation through boundary-crossing operations under identifiable unstable conditions while leaving the forward simulation unchanged. This enables stable, optimization-ready gradients in a detector-design problem.
What carries the argument
Targeted mitigation strategy that stops gradient propagation through boundary-crossing operations under identifiable unstable conditions.
Load-bearing premise
Unstable boundary-crossing conditions can be identified reliably and that selectively stopping gradient propagation at those points preserves the validity and usefulness of the resulting gradients for detector-design optimization.
What would settle it
A concrete detector-optimization run in which the mitigation either allows residual gradient explosions or produces a design whose performance is inferior to one obtained by non-gradient methods on the same forward simulator.
If this is right
- Gradients remain stable and usable for downstream optimization tasks.
- The forward primal simulation is left exactly unchanged.
- Stable derivatives become available for detector geometry and material optimization problems.
- The mitigation controls boundary-driven instabilities without altering the underlying physics model.
Where Pith is reading between the lines
- The same selective stopping rule could be applied to other Monte Carlo transport codes that encounter discrete boundary events.
- Detector design workflows could now incorporate gradient information alongside traditional sampling-based methods.
- Scaling the identification of unstable conditions to larger geometries with thousands of boundaries remains an open implementation question.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript applies automatic differentiation to a Geant4-like radiation transport simulation with full electromagnetic physics. It identifies exploding gradients arising from rare but extreme sensitivities at material boundaries during step-based transport and shower development. To address this, the authors introduce a targeted mitigation that halts gradient propagation through boundary-crossing operations only under identifiable unstable conditions, while leaving the forward (primal) simulation unchanged. They claim this produces stable, optimization-ready gradients and demonstrate the approach on a detector-design problem.
Significance. If the mitigation strategy can be shown to produce reliable gradients that preserve the validity of the underlying physics simulation and improve downstream optimization, the work would be significant for the field of differentiable Monte Carlo simulation in detector design. Enabling gradient-based optimization in full radiation transport codes without modifying the primal physics would address a recognized barrier in applying AD to complex particle-transport problems.
major comments (1)
- [Abstract] Abstract: The abstract states the problem and the proposed fix but supplies no quantitative results, error analysis, or description of the detector-design demonstration, preventing assessment of whether the data actually support the claim that stable, optimization-ready gradients are enabled. This is load-bearing for the central claim.
Simulated Author's Rebuttal
We thank the referee for their careful reading of the manuscript and for highlighting this important point about the abstract. We agree that strengthening the abstract will improve the paper.
read point-by-point responses
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Referee: [Abstract] Abstract: The abstract states the problem and the proposed fix but supplies no quantitative results, error analysis, or description of the detector-design demonstration, preventing assessment of whether the data actually support the claim that stable, optimization-ready gradients are enabled. This is load-bearing for the central claim.
Authors: We agree with the referee that the abstract would be strengthened by the inclusion of quantitative results, a brief error analysis summary, and a short description of the detector-design demonstration. In the revised manuscript we will expand the abstract to report key metrics such as the reduction in exploding-gradient events, typical gradient-norm stability before and after mitigation, and the optimization outcome (e.g., improvement in the design figure of merit). These additions will allow readers to assess the support for our central claim directly from the abstract. revision: yes
Circularity Check
No significant circularity
full rationale
The paper introduces a practical heuristic mitigation for exploding gradients in differentiable Geant4-like transport by halting propagation through boundary crossings under unstable conditions, while leaving the primal simulation unchanged. This is presented as an empirical engineering solution demonstrated on a detector-design optimization task rather than a first-principles derivation. No load-bearing equations, fitted parameters renamed as predictions, or self-citation chains reduce the central claim to its own inputs by construction. The approach is internally consistent with its stated goal of obtaining usable gradients and does not invoke uniqueness theorems or ansatzes that collapse into prior self-referential work.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption The Geant4-like simulation with full electromagnetic physics model produces physically meaningful forward results.
read the original abstract
We present an application of automatic differentiation for particle transport through matter using a Geant4-like radiation transport simulation with a full electromagnetic physics model. When differentiating this step-based transport, we observe exploding gradients driven by rare but extreme sensitivities at material boundaries, which propagate through subsequent transport and shower development. To obtain usable derivatives for optimization, we introduce a targeted mitigation strategy that stops gradient propagation through boundary-crossing operations under identifiable unstable conditions while leaving the forward (primal) simulation unchanged. We demonstrate that this enables stable, optimization-ready gradients in a detector-design problem.
Figures
Reference graph
Works this paper leans on
-
[1]
Agostinelli Set al.2003 Geant4—a simulation toolkitNucl. Instrum. Methods Phys. Res., Sect. A506250–303 doi:10.1016/S0168-9002(03)01368-8
-
[2]
Allison Jet al.2006 Geant4 developments and applicationsIEEE Trans. Nucl. Sci.53270–278 doi:10.1109/TNS.2006.869826
-
[3]
Allison Jet al.2016 Recent developments in Geant4Nucl. Instrum. Methods Phys. Res., Sect. A835186–225 doi:10.1016/j.nima.2016.06.125
-
[4]
Zein S A, Bordage M C, Francis Z, Macetti G, Genoni A, Cappello C D, Shin W G and Incerti S 2021 Electron transport in DNA bases: An extension of the Geant4-DNA Monte Carlo toolkit Nucl. Instrum. Methods Phys. Res., Sect. B48870–82 doi:10.1016/j.nimb.2020.11.021
-
[5]
Dom´ ınguez-Mu˜ noz A D, Gallardo M I, Bordage M C, Francis Z, Incerti S and Cort´ es- Giraldo M A 2022 A model for Geant4-DNA to simulate ionization and excitation of liquid water by protons travelling above 100 MeVRadiat. Phys. Chem.199110363 doi:10.1016/j.radphyschem.2022.110363
-
[6]
Ganjgah A A and Taherparvar P 2024 The effects of cell displacement on DNA damages in targeted radiation therapy using Geant4-DNASci. Rep.1481863 doi:10.1038/s41598-024- 81863-4
-
[7]
Tabbakh F 2024 Significance of the proton energy loss mechanism to gold nanoparticles in proton therapy: a Geant4 simulationScientific Reports1476244 doi:10.1038/s41598-024-76244-w
-
[8]
Shin W Get al.2021 A Geant4-DNA Evaluation of Radiation-Induced DNA Damage on a Human FibroblastCancers134940 doi:10.3390/cancers13194940
-
[9]
Chappuis F, Tran H N, Zein S A, Bailat C, Incerti S, Bochud F and Desorgher L 2023 The general-purpose Geant4 Monte Carlo toolkit and its Geant4-DNA extension to investigate mechanisms underlying the FLASH effect in radiotherapy: Current status and challenges Physica Med.110102601 doi:10.1016/j.ejmp.2023.102601
-
[10]
Simhony Y, Segal A, Amrani O and Etzion E 2025 High-end space electronics: Active shielding to mitigate catastrophic single-event effectsAerosp. Sci. Technol.169111344 doi:10.1016/j.ast.2025.111344
-
[11]
Negm H and Ohgaki H 2024 Validation of GEANT4 simulation for active materials in- terrogation using nuclear resonance fluorescenceJ. Radiat. Res. Appl. Sci.17101209 doi:10.1016/j.jrras.2024.101209
-
[12]
MacFadden N, Peggs S and Gulliford C 2018 Development and Validation of a Geant4 Radiation Shielding Simulation Framework doi:10.2172/1515417 15
-
[13]
Baydin A G, Pearlmutter B A, Radul A A and Siskind J M 2018 Automatic differentiation in machine learning: a surveyJ. Mach. Learn. Res.181–43 doi:10.48550/arXiv.1502.05767
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.1502.05767 2018
-
[14]
Graphics371–12 doi:10.1145/3272127.3275109
Li T M, Aittala M, Durand F and Lehtinen J 2018 Differentiable Monte Carlo ray tracing through edge samplingACM Trans. Graphics371–12 doi:10.1145/3272127.3275109
-
[15]
Graphics381–17 doi:10.1145/3355089.3356498
Nimier-David M, Vicini D, Zeltner T and Jakob W 2019 Mitsuba 2: A Retargetable Forward and Inverse RendererACM Trans. Graphics381–17 doi:10.1145/3355089.3356498
-
[16]
Hu Y, Anderson L, Li T M, Sun Q, Carr N, Ragan-Kelley J and Durand F 2020 DiffTaichi: Differentiable Programming for Physical SimulationICLR 2020doi:10.48550/arXiv.1910.00935
-
[17]
Newbury R, Collins J, He K, Pan J, Posner I, Howard D and Cosgun A 2024 A Review of Differentiable SimulatorsIEEE Access1297581–97604 doi:10.1109/ACCESS.2024.3425448
-
[18]
Chen R T Q, Rubanova Y, Bettencourt J and Duvenaud D K 2018 Neural Ordi- nary Differential EquationsAdvances in Neural Information Processing Systemsvol 31 doi:10.48550/arXiv.1806.07366
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.1806.07366 2018
-
[19]
Aehle M, Nov´ ak M, Vassilev V, Gauger N R, Heinrich L, Kagan M and Lange D 2024 Optimization Using Pathwise Algorithmic Derivatives of Electromagnetic Shower Simulations Comput. Phys. Commun.309109491 doi:10.1016/j.cpc.2024.109491
-
[20]
Nov´ ak M 2023 HepEmShow: a compact EM shower simulation application documentation, accessed 2026-01-20 URLhttps://hepemshow.readthedocs.io/en/latest/index.html
2023
-
[21]
Nov´ ak M, Hahnfeld J and Morgan B 2022 g4hepem: The G4HepEm R&D Project gitHub repository, accessed 2026-01-20 URLhttps://github.com/mnovak42/g4hepem
2022
-
[22]
Fabjan C W and Gianotti F 2003 Calorimetry for particle physicsRev. Mod. Phys.751243–1286 doi:10.1103/RevModPhys.75.1243
-
[23]
J.6971071–1102 doi:10.1088/0004-637X/697/2/1071
Atwood W Bet al.2009 The Large Area Telescope on the Fermi Gamma-Ray Space Telescope MissionAstrophys. J.6971071–1102 doi:10.1088/0004-637X/697/2/1071
-
[24]
Seuntjens J and Duane S 2009 Photon absorbed dose standardsMetrologia46S39–S58 doi:10.1088/0026-1394/46/2/S04
-
[25]
Aehle M, Nguyen X T, Nov´ ak M, Dorigo T, Gauger N R, Kieseler J, Klute M and Vassilev V 2024 Efficient Forward-Mode Algorithmic Derivatives of Geant4 doi:10.48550/arXiv.2407.02966
-
[26]
Kagan M and Heinrich L 2023 Branches of a Tree: Taking Derivatives of Programs with Discrete and Branching Randomness in High Energy Physics doi:10.48550/arXiv.2308.16680
-
[27]
Instrum.17P08021 doi:10.1088/1748-0221/17/08/p08021
Cheong S, Frisch J C, Gasiorowski S, Hogan J M, Kagan M, Safdari M, Schwartzman A and Vandegar M 2022 Novel light field imaging device with enhanced light collection for cold atom cloudsJ. Instrum.17P08021 doi:10.1088/1748-0221/17/08/p08021
-
[28]
Strong G Cet al.2024 TomOpt: differential optimisation for task- and constraint-aware design of particle detectors in the context of muon tomographyMach. Learn.: Sci. Technol.5035002 doi:10.1088/2632-2153/ad52e7 16
-
[29]
Gasiorowski S, Chen Y, Nashed Y, Granger P, Mironov C, Tsang K V, Ratner D and Terao K 2024 Differentiable simulation of a liquid argon time projection chamberMach. Learn.: Sci. Technol.5025012 doi:10.1088/2632-2153/ad2cf0
-
[30]
Alterkait O, Jes´ us-Valls C, Matsumoto R, de Perio P and Terao K 2026 End-to-end Differentiable Calibration and Reconstruction for Optical Particle Detectors doi:10.48550/arXiv.2602.24129
-
[31]
Aehle Met al.2025 Progress in end-to-end optimization of fundamental physics ex- perimental apparata with differentiable programmingReviews in Physics13100120 doi:10.1016/j.revip.2025.100120
-
[32]
Mokhtar F, Pata J, Garcia D, Wulff E, Zhang M, Kagan M and Duarte J 2025 Fine-tuning machine-learned particle-flow reconstruction for new detector geometries in future colliders Phys. Rev. D111doi:10.1103/physrevd.111.092015
-
[33]
Adelmann Aet al.2022 New directions for surrogate models and differentiable programming for High Energy Physics detector simulationSnowmass 2021doi:10.48550/arXiv.2203.08806
-
[34]
Shirobokov S, Belavin V, Kagan M, Ustyuzhanin A and Baydin A G 2020 Black-Box Optimization with Local Generative SurrogatesAdvances in Neural Information Pro- cessing SystemsURL https://proceedings.neurips.cc/paper_files/paper/2020/file/ a878dbebc902328b41dbf02aa87abb58-Paper.pdf
2020
-
[35]
Wozniak K A, Mulligana S, Kieseler J, Klute M, Fleuret F and Golling T 2025 End-to-End Optimal Detector Design with Mutual Information SurrogatesMach. Learn.: Sci. Technol.6 045047 doi:10.1088/2632-2153/ae1acb
-
[36]
Schmidt Ket al.2025 End-to-End Detector Optimization with Diffusion Models: A Case Study in Sampling CalorimetersParticles847 doi:10.3390/particles8020047
-
[37]
Toward the end-to-end optimization of particle physics instruments with differentiable programming
Dorigo T (MODE Collaboration) 2023 Toward the end-to-end optimization of par- ticle physics instruments with differentiable programmingRev. Phys.10100085 doi:10.1016/j.revip.2023.100085
-
[38]
Dorigo T (MODE Collaboration) 2023 Toward artificial-intelligence assisted design of experi- mentsNucl. Instrum. Methods Phys. Res., Sect. A1047167873 doi:10.1016/j.nima.2022.167873
-
[39]
Dorigo T (MODE Collaboration) 2026 On the Codesign of Scientific Experiments and Industrial Systems doi:10.48550/arXiv.2603.26613
-
[40]
Instrum.15P05009 doi:10.1088/1748-0221/15/05/P05009
Cisbani Eet al.2020 AI-optimized detector design for the future Electron-Ion Collider: the dual-radiator RICH caseJ. Instrum.15P05009 doi:10.1088/1748-0221/15/05/P05009
-
[41]
Instrum.19C07001 doi:10.1088/1748-0221/19/07/C07001
Diefenthaler M (AID(2)E Collaboration) 2024 AI-assisted detector design for the EIC (AID(2)E) J. Instrum.19C07001 doi:10.1088/1748-0221/19/07/C07001
-
[42]
Roussel Ret al.2024 Bayesian optimization algorithms for accelerator physicsPhys. Rev. Accel. Beams27(8) 084801 doi:10.1103/PhysRevAccelBeams.27.084801
-
[43]
Fanelli Cet al.2023 AI-assisted optimization of the ECCE tracking system at the Electron Ion ColliderNucl. Instrum. Methods Phys. Res., Sect. A1047167748 doi:10.1016/j.nima.2022.167748 17
-
[44]
Qasim S R, Owen P and Serra N 2025 Physics Instrument Design with Reinforcement Learning Mach. Learn.: Sci. Technol.035033 doi:10.1088/2632-2153/adf7ff
-
[45]
Kortus T, Keidel R, Gauger N R and Kieseler J (Bergen pCT) 2026 Constrained collaborative optimization of charged particle tracking with multi-agent reinforcement learningMach. Learn.: Sci. Technol.7015021 doi:10.1088/2632-2153/ae352b
-
[46]
Zoccheddu S, Qasim S R, Owen P and Serra N 2026 Large Language Models for Physics Instrument Design doi:10.48550/arXiv.2601.07580
-
[47]
Nguyen X T, Chen L, Dorigo T, Gauger N R, Vischia P, Nardi F, Awais M, Hanif H, Abbas S and Kapoor R 2026 Differentiable Surrogate for Detector Simulation and Design with Diffusion Models doi:10.48550/arXiv.2601.07859
work page internal anchor Pith review Pith/arXiv arXiv doi:10.48550/arxiv.2601.07859 2026
-
[48]
Hill J and Ryoo H J 2026 GRACE: an Agentic AI for Particle Physics Experiment Design and Simulation doi:10.48550/arXiv.2602.15039
-
[49]
SciCompKL 2024 CoDiPack (Code Differentiation Package) gitHub repository, accessed 2026- 01-20 URLhttps://github.com/SciCompKL/CoDiPack
2024
-
[50]
Methods Software331207–1231 doi:10.1080/10556788.2018.1471140
Sagebaum M, Albring T and Gauger N R 2018 Expression templates for primal value taping in the reverse mode of algorithmic differentiationOptim. Methods Software331207–1231 doi:10.1080/10556788.2018.1471140
-
[51]
M Sagebaum T Albring N G 2019 High-Performance Derivative Computations using CoDiPack ACM Trans. Math. Software (TOMS)45doi:10.1145/3356900
-
[52]
com/mnovak42/hepemshow
Nov´ ak M 2023 HepEmShow gitHub repository, accessed 2026-01-20 URL https://github. com/mnovak42/hepemshow
2023
-
[53]
Urb´ an L 2006 A Model for Multiple Scattering in GEANT4 Tech. Rep. CERN-OPEN-2006-077 CERN URLhttps://cds.cern.ch/record/1004190 A Supplementary Results 18 fPropagation Stopgrad Fraction (%) Energy Steps Tracks 0.0 Track-only 9.3 20.2 11.8 Track + desc. 15.1 26.9 32.3 0.05 Track-only 9.6 20.4 11.9 Track + desc. 15.5 27.2 32.8 0.1 Track-only 9.9 20.7 12.2...
discussion (0)
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