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arxiv: 2605.17440 · v1 · pith:OR3X7UDAnew · submitted 2026-05-17 · ⚛️ physics.ins-det · physics.comp-ph

PyNMC: An Open-Source Framework for Neutron Multiplicity Counting Simulation Coupling OpenMC, FREYA, and ALPHANSO

Christopher Fichtlscherer This is my paper

Pith reviewed 2026-05-19 22:30 UTC · model grok-4.3

classification ⚛️ physics.ins-det physics.comp-ph
keywords neutron multiplicity countingnuclear safeguardsOpenMCFREYAALPHANSOsimulation frameworkplutonium assaybenchmark validation
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The pith

Open-source PyNMC framework simulates neutron multiplicity counting by coupling OpenMC, FREYA and ALPHANSO and matches ESARDA benchmark results.

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

The paper introduces PyNMC, a Python-native open-source framework that links OpenMC for neutron transport, FREYA for event-by-event prompt neutron emissions, and ALPHANSO for alpha-n source terms. It adds collision-level time-tagged recording and a shift-register post-processor to generate multiplicity data. Validation covers the ESARDA benchmark cases for bare 252Cf, low-multiplication Pu metal, and a PuO2 sample with alpha-n contributions. Simulated rates align with point-model predictions and the range of other participant codes. This matters for plutonium assay because it supplies a freely available alternative to export-controlled or institute-internal simulators.

Core claim

PyNMC couples OpenMC transport with FREYA correlated neutron emissions and ALPHANSO alpha-n estimates, together with custom time-tagged collision recording and a Python shift-register analyzer; the resulting multiplicity rates for bare 252Cf, Pu metal at M=1.12, and 10 g PuO2 match both analytic point-model expectations and the published scatter of ESARDA participant codes.

What carries the argument

The PyNMC coupling of OpenMC neutron transport, FREYA event-by-event emission, ALPHANSO alpha-n estimates, collision-level time-tagged recording, and Python shift-register post-processing.

If this is right

  • NMC simulations for safeguards and arms-control verification become possible without MCNP licenses or export-controlled code.
  • Python scripting allows custom detector geometries, source terms, and analysis pipelines to be added directly.
  • The same shift-register cross-check with ONMS enables consistent comparisons across open and closed simulation platforms.
  • Internal extension to higher-mass Pu samples at M=1.29 becomes feasible for exploring multiplication effects beyond the published benchmark range.

Where Pith is reading between the lines

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

  • The open licensing and Docker container could allow community extensions to additional detector materials or time-resolution models not covered in the current validation.
  • Integration with other Python-based data-analysis libraries might support automated uncertainty propagation for assay results on unknown samples.
  • The modular structure suggests straightforward replacement of individual components, such as swapping transport engines, to test sensitivity to specific physics models.

Load-bearing premise

The couplings between OpenMC transport, FREYA emissions, ALPHANSO alpha-n estimates, custom time-tagged recording, and the shift-register post-processor introduce no significant artifacts or biases relative to real detector physics.

What would settle it

A direct experimental measurement of neutron multiplicity rates from a calibrated 252Cf source or Pu sample in a real multiplicity counter, compared against PyNMC output for the same geometry and source, would falsify the validation if the rates differ beyond statistical uncertainty.

Figures

Figures reproduced from arXiv: 2605.17440 by Christopher Fichtlscherer.

Figure 6
Figure 6. Figure 6: contains sub-panels for c2-10, c3s, and c4s only. [PITH_FULL_IMAGE:figures/full_fig_p016_6.png] view at source ↗
read the original abstract

Neutron multiplicity counting (NMC) underpins plutonium assay in nuclear safeguards, arms control, and disarmament verification, but existing simulation tools are essentially limited to MCNPX-PoliMi [1] (export-controlled, MCNP license required) and ONMS [2] (open-source but built on Geant4 with no scripting API); other codes (RMC, MCNP-PTA) are institute-internal. We present PyNMC, an open-source, Python-native NMC simulation framework that couples OpenMC for transport with FREYA for event-by-event correlated prompt-neutron emission and ALPHANSO for native ($\alpha$, n)-source estimates, together with collision-level time-tagged event recording and a Python shift-register post-processor cross-validated against ONMS. The framework is validated against the ESARDA Neutron Multiplicity Benchmark on bare $^{252}$Cf (c2-10, c2-100), the low-multiplication Pu metal case c3s (M = 1.12 from an independent k-eigenvalue calculation; ESARDA spec M = 1.08), and a 10 g PuO2 sample with an ($\alpha$, n)-source term (c4s); an internal stress-test extension to a $\approx 100$ g Pu metal sample at M = 1.29 is reported alongside but lies beyond the ESARDA participant range. For c4s, ALPHANSO gives $\alpha$ = 0.78 with modern cross-section data; the reported benchmark comparison rescales the ($\alpha$,n) rate to the ESARDA value $\alpha$ = 0.853. Simulated rates agree with point-model predictions for all cases, and with the published ESARDA participant-code scatter where participant results exist. The framework is shipped as a Docker container under the MIT license and is openly available on GitHub at github.com/cfichtlscherer/nmc.

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

1 major / 2 minor

Summary. The manuscript presents PyNMC, an open-source Python-native framework for neutron multiplicity counting simulations that couples OpenMC for neutron transport, FREYA for event-by-event correlated prompt-neutron emission, ALPHANSO for native (α,n)-source estimates, collision-level time-tagged recording, and a Python shift-register post-processor. It reports validation against the ESARDA Neutron Multiplicity Benchmark for bare 252Cf (c2-10, c2-100), low-multiplication Pu metal (c3s, M=1.12 from independent k-eigenvalue calculation), and 10 g PuO2 with (α,n) source (c4s), with simulated rates agreeing with point-model predictions and ESARDA participant-code scatter; for c4s the (α,n) rate is rescaled post-hoc from ALPHANSO's α=0.78 to the ESARDA value α=0.853, and an internal stress-test extension to ~100 g Pu metal at M=1.29 is included.

Significance. If the couplings introduce no significant artifacts, the work provides a valuable open-source, accessible alternative to export-controlled tools such as MCNPX-PoliMi for NMC simulations in nuclear safeguards and verification applications. The Docker container, MIT license, and GitHub availability (github.com/cfichtlscherer/nmc) are explicit strengths supporting reproducibility and broader community adoption.

major comments (1)
  1. [Abstract] Abstract: The post-hoc rescaling of the (α,n) rate for the c4s PuO2 case from ALPHANSO's native α=0.78 (with modern cross sections) to the ESARDA-specified α=0.853 means the reported benchmark agreement tests OpenMC transport, FREYA emission correlations, time-tagging, and the shift-register post-processor but does not directly validate integration of the native ALPHANSO alpha-n estimates without rate biases relative to the benchmark physics. Because the central claim rests on the full set of couplings (including ALPHANSO) producing no significant artifacts, this adjustment is load-bearing for the validation evidence.
minor comments (2)
  1. The manuscript would benefit from explicit quantitative tables (with uncertainties) comparing simulated multiplicity rates, point-model predictions, and ESARDA participant scatter for all benchmark cases to allow direct assessment of agreement.
  2. [Abstract] Clarify in the text how the independent k-eigenvalue M=1.12 for c3s relates to the ESARDA specification M=1.08 and whether this difference affects the validation interpretation.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive evaluation of PyNMC and for the constructive comment on the c4s validation. We address the point below and will revise the manuscript accordingly.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The post-hoc rescaling of the (α,n) rate for the c4s PuO2 case from ALPHANSO's native α=0.78 (with modern cross sections) to the ESARDA-specified α=0.853 means the reported benchmark agreement tests OpenMC transport, FREYA emission correlations, time-tagging, and the shift-register post-processor but does not directly validate integration of the native ALPHANSO alpha-n estimates without rate biases relative to the benchmark physics. Because the central claim rests on the full set of couplings (including ALPHANSO) producing no significant artifacts, this adjustment is load-bearing for the validation evidence.

    Authors: We agree that the post-hoc rescaling means the c4s benchmark comparison validates the full simulation chain (OpenMC transport, FREYA correlations, time-tagging, and shift-register post-processor) when the (α,n) source strength is set to the ESARDA-specified value rather than directly testing ALPHANSO's native estimate against the benchmark's implicit physics. The rescaling is performed solely to enable an apples-to-apples comparison with the published ESARDA participant results and point-model predictions, which all adopt α=0.853. ALPHANSO's native α=0.78 arises from its use of modern evaluated cross sections and is not an artifact of the coupling. In the revised version we will update the abstract and the c4s section to state explicitly that the reported agreement demonstrates consistency of the coupled framework under the benchmark's specified source strength, while noting that ALPHANSO's native rate reflects updated nuclear data and can be used directly in applications preferring modern libraries. This clarification addresses the load-bearing nature of the claim without requiring new simulations. revision: yes

Circularity Check

0 steps flagged

No circularity: external benchmark validation of coupled simulation codes

full rationale

The paper presents a software framework that couples existing external codes (OpenMC, FREYA, ALPHANSO) with custom post-processing and validates the results against the independent ESARDA Neutron Multiplicity Benchmark plus point-model calculations. No derivations, predictions, or first-principles results are claimed that reduce by construction to quantities defined by the authors' own fitted parameters, self-citations, or ansatzes. The rescaling of the (α,n) rate for the c4s case to the ESARDA-specified α = 0.853 is an explicit adjustment to enable direct comparison with benchmark specifications rather than a self-definitional or fitted-input step. The central claims rest on agreement with external participant-code scatter and independent models, rendering the chain self-contained against external references.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central contribution is a software integration rather than new physical postulates; the framework inherits modeling assumptions from OpenMC, FREYA, and ALPHANSO.

free parameters (1)
  • alpha-n rate rescaling for c4s case
    ALPHANSO produces alpha = 0.78 but the benchmark comparison rescales to the ESARDA value of 0.853
axioms (1)
  • domain assumption OpenMC, FREYA, and ALPHANSO provide sufficiently accurate models of neutron transport, correlated emission, and alpha-n sources for the benchmark cases
    The validation rests on these external codes being correct for the physics regimes tested

pith-pipeline@v0.9.0 · 5904 in / 1368 out tokens · 39987 ms · 2026-05-19T22:30:07.802785+00:00 · methodology

discussion (0)

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Reference graph

Works this paper leans on

26 extracted references · 26 canonical work pages · 1 internal anchor

  1. [1]

    MCNP-PoliMi: a Monte-Carlo Code for Correlation Measurements,

    S. A. Pozzi, E. Padovani, and M. Marseguerra, “MCNP-PoliMi: a Monte-Carlo Code for Correlation Measurements,” Nuclear Instruments and Methods in Physics Research, Section A, vol. 513, no. 3, pp. 550–558, 2003

    work page 2003

  2. [2]

    Simulation of Neutron Multiplicity Measurements using Geant4 — Open Source Software for Nuclear Arms Control,

    M. Kütt, “Simulation of Neutron Multiplicity Measurements using Geant4 — Open Source Software for Nuclear Arms Control,” Ph.D. dissertation, Technische Universität Darmstadt, 2016

    work page 2016

  3. [3]

    Application Guide to Neutron Multiplicity Counting,

    D. G. Langner, J. E. Stewart, M. M. Pickrell, M. S. Krick, N. Ensslin, and W. C. Harker, “Application Guide to Neutron Multiplicity Counting,” Los Alamos National Laboratory, LA-13422- M, 1998

    work page 1998

  4. [4]

    Nuclear Warhead Verification: A Review of Attribute and Template Systems,

    J. Yan and A. Glaser, “Nuclear Warhead Verification: A Review of Attribute and Template Systems,” Science & Global Security, vol. 23, no. 3, pp. 157–170, 2015

    work page 2015

  5. [5]

    Reducing Neutron Multiplicity Counting Bias for Plutonium Warhead Authentica - tion,

    M. Göttsche, “Reducing Neutron Multiplicity Counting Bias for Plutonium Warhead Authentica - tion,” Ph.D. dissertation, Universität Hamburg, 2015

    work page 2015

  6. [6]

    The Effect of Multiplication on the Quantitative Determination of Spontaneously Fissioning Isotopes by Neutron Correlation Analysis,

    K. Böhnel, “The Effect of Multiplication on the Quantitative Determination of Spontaneously Fissioning Isotopes by Neutron Correlation Analysis,” Nuclear Science and Engineering, vol. 90, no. 1, pp. 75–82, 1985

    work page 1985

  7. [7]

    On the Factorial Moments of the Neutron Multiplicity Distribution of Fission Cascades,

    W. Hage and D. M. Cifarelli, “On the Factorial Moments of the Neutron Multiplicity Distribution of Fission Cascades,” Nuclear Instruments and Methods in Physics Research, Section A , vol. 236, no. 1, pp. 165–177, 1985

    work page 1985

  8. [8]

    Models for a Three-Parameter Analysis of Neutron Signal Correlation Measurements for Fissile Material Assay,

    D. M. Cifarelli and W. Hage, “Models for a Three-Parameter Analysis of Neutron Signal Correlation Measurements for Fissile Material Assay,” Nuclear Instruments and Methods in Physics Research, Section A, vol. 251, no. 3, pp. 550–563, 1986

    work page 1986

  9. [9]

    Multiplicity Theory Beyond the Point Model,

    I. Pázsit and L. Pál, “Multiplicity Theory Beyond the Point Model,” Annals of Nuclear Energy, vol. 154, p. 108119, 2021

    work page 2021

  10. [10]

    On the Theory of Stochastic Processes in Nuclear Reactors,

    L. Pal, “On the Theory of Stochastic Processes in Nuclear Reactors,” Nuovo Cimento Supplemento, vol. 7, pp. 25–42, 1958

    work page 1958

  11. [11]

    On the Stochastic Theory of Neutron Transport,

    G. I. Bell, “On the Stochastic Theory of Neutron Transport,” Nuclear Science and Engineering, vol. 21, no. 3, pp. 390–401, 1965

    work page 1965

  12. [12]

    MCNPX-PoliMi for Nuclear Nonproliferation Applications,

    S. A. Pozzi et al., “MCNPX-PoliMi for Nuclear Nonproliferation Applications,” Nuclear Instruments and Methods in Physics Research, Section A, vol. 694, pp. 119–125, 2012. 22 / 23

    work page 2012

  13. [13]

    MCNP User's Manual, Code Version 6.2,

    C. J. Werner, Ed., “MCNP User's Manual, Code Version 6.2,” Los Alamos National Laboratory, LA-UR-17-29981, 2017

    work page 2017

  14. [14]

    MCNP Version 6.2 Release Notes,

    C. J. Werner et al., “MCNP Version 6.2 Release Notes,” Los Alamos National Laboratory, LA- UR-18-20808, 2018

    work page 2018

  15. [15]

    Monte Carlo simulation of neutron counters for safeguards applications,

    M. Looman, P. Peerani, and H. Tagziria, “Monte Carlo simulation of neutron counters for safeguards applications,” Nuclear Instruments and Methods in Physics Research, Section A, vol. 598, no. 2, pp. 542–550, 2009

    work page 2009

  16. [16]

    CGMF & FREYA Verification in Monte Carlo Code RMC,

    Y. Gou, Z. Liu, C. Jia, and K. Wang, “CGMF & FREYA Verification in Monte Carlo Code RMC,” EPJ Web of Conferences, vol. 302, p. 13001, 2024

    work page 2024

  17. [17]

    Simulation of Neutron Multiplicity Counting Based on Monte Carlo Code RMC,

    Y. Gou, C. Jia, Z. Liu, and K. Wang, “Simulation of Neutron Multiplicity Counting Based on Monte Carlo Code RMC,” Nuclear Science and Engineering, vol. 199(sup1), pp. 485–499, 2024

    work page 2024

  18. [18]

    Simulation of Neutron and Gamma Ray Emission from Fission and Photofission. LLNL Fission Library 2.0.2,

    J. M. Verbeke, C. Hagmann, and D. Wright, “Simulation of Neutron and Gamma Ray Emission from Fission and Photofission. LLNL Fission Library 2.0.2,” Lawrence Livermore National Laboratory, UCRL-AR-228518-REV-1, 2016

    work page 2016

  19. [19]

    OpenMC: A State-of-the-Art Monte Carlo Code for Research and Development,

    P. K. Romano, N. E. Horelik, B. R. Herman, A. G. Nelson, B. Forget, and K. Smith, “OpenMC: A State-of-the-Art Monte Carlo Code for Research and Development,” Annals of Nuclear Energy, vol. 82, pp. 90–97, 2015

    work page 2015

  20. [20]

    ALPHANSO: Open-Source Modeling of (alpha,n) Neutron Source Terms,

    D. Rawal, A. J. Nelson, W. Zywiec, and D. Siefman, “ALPHANSO: Open-Source Modeling of (alpha,n) Neutron Source Terms,” arXiv preprint, Art. no. arXiv:2603.17719, 2026

    work page internal anchor Pith review arXiv 2026

  21. [21]

    Fission Reaction Event Yield Algorithm FREYA 2.0.2,

    J. M. Verbeke, J. Randrup, and R. Vogt, “Fission Reaction Event Yield Algorithm FREYA 2.0.2,” Computer Physics Communications, vol. 222, pp. 263–266, 2018

    work page 2018

  22. [22]

    Prompt Neutron Emission Multiplicity Distribution and Average Values (Nubar) at 2200 m/s for the Fissile Nuclides,

    N. E. Holden and M. S. Zucker, “Prompt Neutron Emission Multiplicity Distribution and Average Values (Nubar) at 2200 m/s for the Fissile Nuclides,” Nuclear Science and Engineering, vol. 98, no. 2, pp. 174–181, 1988

    work page 1988

  23. [23]

    Fission Fragment Decay Simulations with the CGMF Code,

    P. Talou, I. Stetcu, P. Jaffke, M. E. Rising, A. E. Lovell, and T. Kawano, “Fission Fragment Decay Simulations with the CGMF Code,” Computer Physics Communications, vol. 269, p. 107923, 2021

    work page 2021

  24. [24]

    ESARDA Multiplicity Benchmark Exercise — Final Report,

    P. Peerani and M. Swinhoe, “ESARDA Multiplicity Benchmark Exercise — Final Report,” ESARDA Bulletin, no. 34, pp. 3–32, 2006

    work page 2006

  25. [25]

    ESARDA Multiplicity Benchmark Exercise — Phases III and IV,

    P. Peerani, M. Swinhoe, A. L. Weber, and L. G. Evans, “ESARDA Multiplicity Benchmark Exercise — Phases III and IV,” ESARDA Bulletin, no. 42, pp. 2–25, 2009

    work page 2009

  26. [26]

    Statistics and Nuclear Reactions,

    V. Weisskopf, “Statistics and Nuclear Reactions,” Physical Review, vol. 52, no. 4, pp. 295–303, 1937. 23 / 23

    work page 1937