Photonuclear Neutron Production in OpenMC: Verification Against MCNPX, FLUKA, and a First-Collision Analytical Solution

arXiv:2607.26045 · physics.app-ph · Submitted 2026-07-28 · Read on arXiv

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Introduction to the show: ident: Physics. The day in general physics: instruments, methods and results across the field.

Kai: We're looking at the paper: "Photonuclear Neutron Production in OpenMC".

Mira: Here's the gist of the paper. The modeling of photonuclear reactions is increasingly important for applications involving highenergy photon fields, including accelerator-driven neutron sources, radiation shielding, medical physics, and fusion technologies <ref:2607.26045#pg2>.

Kai: This work presents a systematic verification of the OpenMC photonuclear implementation using six single-collision broomstick benchmarks based on 2H, 9Be, and 238U targets irradiated by monoenergetic 5 MeV and 15 MeV photons and by a continuous 1–20 MeV linear accelerator (LINAC)-representative spectrum <ref:2607.26045#pg4>.

Mira: Verification Scope The study systematically verifies the OpenMC photonuclear implementation against MCNPX, FLUKA, and a first-collision analytical solution <ref:2607.26045#pg2>.

Kai: This verification is conducted under idealized, single-collision conditions to isolate the treatment of individual particle interactions from secondary transport and macroscopic geometrical effects <ref:2607.26045#pg9>.

Title and authors: Kai: Now, looking at what they actually wrote in the abstract, "Photonuclear Neutron Production in OpenMC: Verification Against MCNPX, FLUKA, and a First-Collision Analytical Solution" <ref:2607.26045#pg3>. They say that modeling these photonuclear reactions is increasingly important for applications involving highenergy photon fields like accelerator-driven neutron sources, radiation shielding, medical physics and fusion technologies <ref:2607.26045#pg3>.

Mira: That sets the stage; it’s not just academic curiosity, this is for things like accelerator-driven neutron sources or radiation shielding <ref:2607.26045#pg3>.

Lev: So what they are doing is taking OpenMC's photonuclear physics, which they say is currently only in an unofficial development branch and needs verification before broader scientific use or possible integration into the official code distribution <ref:2607.26045#pg3>.

Kai: They present a systematic verification using six single-collision broomstick benchmarks based on 2H, 9Be and 238U targets irradiated by monoenergetic five MeV and fifteen MeV photons and by a continuous one to twenty MeV linear accelerator LINAC representative spectrum <ref:2607.26045#pg4> <ref:2607.26045#pg3,using six single-collision broomstick benchmarks based on 2H, 9Be and 238U>.

Mira: That's the test setup; they are hitting these specific targets with specific photon energies to see how the code reacts <ref:2607.26045#pg4>.

Lev: They compared OpenMC with MCNPX using common ENDF7u photonuclear data, FLUKA using its own native models, and a first-collision analytical solution for the integrated neutron yield <ref:2607.26045#pg4>.

Kai: And they also performed calculations using the IAEA/PD-two thousand nineteen library to quantify nuclear-data sensitivity <ref:2607.26045#pg4> <ref:2607.26045#pg1>.

Mira: Basically, they are proving that this implementation reproduces the integrated neutron-production response of an established reference code like MCNPX when using common data <ref:2607.26045#pg3>.

Lev: It’s a direct comparison to see if OpenMC is doing what it should be doing when the physics matches existing standards <ref:2607.26045#pg3>.

Kai: And they also used FLUKA's own models to see how it compares, noting that FLUKA differs from the analytical solution by approximately six to nine percent for monoenergetic cases and by no more than approximately four percent for continuous-source cases <ref:2607.26045#pg4>.

Mira: Those are some specific numbers showing where the discrepancies lie between different modeling approaches <ref:2607.26045#pg4>.

Lev: It’s a good baseline to start with, but we need to see how OpenMC stacks up when the data itself changes, which is what they test next <ref:2607.26045#pg4>.

The paper's summary: Kai: So the paper points out that one of the key things they did was quantifying nuclear-data sensitivity <ref:2607.26045#pg4>. They performed calculations using the IAEA/PD-two thousand nineteen library to see how much difference there is when you replace ENDF7u with IAEA/PD-two thousand nineteen <ref:2607.26045#pg4> <ref:2607.26045#pg1>.

Mira: They found that replacing ENDF7u with IAEA/PD-two thousand nineteen produces differences that are generally larger than the OpenMC MCNPX code-to-code differences obtained under common data <ref:2607.26045#pg4> <ref:2607.26045#pg1>.

Lev: That’s a big deal for users; it means if you're using the IAEA/PD-two thousand nineteen library, you have to expect bigger shifts in your predictions than just switching from ENDF7u to whatever ENDF7u was originally calibrated with <ref:2607.26045#pg4> <ref:2607.26045#pg1>.

Kai: For the monoenergetic cases, the results using IAEA/PD-two thousand nineteen differ from the analytical ENDF7u solution by approximately plus zero point zero three percent for 9Be, plus eleven point three percent for 2H and minus four point six percent for 238U <ref:2607.26045#pg6> <ref:2607.26045#pg3>.

Mira: Those specific deviations show that the sensitivity depends strongly on both the nuclide and the source spectrum <ref:2607.26045#pg6>.

Lev: That tells us you can't just pick one data set and assume it’s fine; you have to check what happens across different isotopes and energy ranges <ref:2607.26045#pg6>.

Kai: And they also demonstrated that agreement in the integrated yield doesn't necessarily imply agreement in the emitted neutron spectrum <ref:2607.26045#pg2>.

Mira: That’s a crucial point for anyone doing shielding or dosimetry; you can get the same total number of neutrons even if the way those neutrons are distributed across energy is totally different <ref:2607.26045#pg9>.

Lev: So, while they agree on the total yield, the shape of that spectrum is where things get messy and needs more work <ref:2607.26045#pg9>.

The paper's improvements: Kai: To wrap up, the paper "Photonuclear Neutron Production in OpenMC: Verification Against MCNPX, FLUKA, and a First-Collision Analytical Solution" <ref:2607.26045#pg1> it shows that the OpenMC photonuclear implementation reproduces the integrated neutron-production response of an established reference code when using common ENDF7u photonuclear data <ref:2607.26045#pg2>.

Mira: So in simple terms, they’ve confirmed that the new part of OpenMC works as expected against MCNPX, which is a key piece of evidence for its use <ref:2607.26045#pg2>.

Lev: It supports the use of this unofficial OpenMC branch for integrated photonuclear neutron production under the verified configurations <ref:2607.26045#pg3>.

Kai: But they also flag secondary-particle kinematics as an area requiring additional assessment <ref:2607.26045#pg9>.

Mira: So they aren't saying it’s perfect; they are highlighting that we still need to look at how those secondary particles behave <ref:2607.26045#pg9>.

Lev: Further work should extend the verification to additional nuclides, reaction thresholds, angular distributions and geometries in which secondary transport is significant <ref:2607.26045#pg9>.

Kai: So the overall idea here is that while this verifies the integrated yield against MCNPX under common data, we need more tests on things like angular distributions and where secondary transport matters <ref:2607.26045#pg9>.

Conclusion: Mira: We've talked about how this paper, "Photonuclear Neutron Production in OpenMC: Verification Against MCNPX, FLUKA, and a First-Collision Analytical Solution" <ref:2607.26045#pg1> confirms the integrated yield agreement with MCNPX under common ENDF7u photonuclear data <ref:2607.26045#pg2>.

Kai: And they also showed that using IAEA/PD-two thousand nineteen data can change things by up to eleven point three percent in some cases like 2H or minus four point six percent for 238U <ref:2607.26045#pg4> <ref:2607.26045#pg3>.

Lev: That means the sensitivity depends strongly on both the nuclide and the source spectrum <ref:2607.26045#pg6>.

Mira: It’s a strong indication that we have to be careful about which data set we use because it impacts things differently across different isotopes and energies <ref:2607.26045#pg6>.

Kai: And they also highlighted that agreement in total yield doesn't necessarily imply agreement in the emitted neutron spectrum <ref:2607.26045#pg2>.

Lev: That means for applications like shielding, you can get the same total number of neutrons even if the energy distribution is completely different <ref:2607.26045#pg9>.

Mira: So while this verification provides confidence in the integrated yield against MCNPX, we still need to look at those spectral details and secondary transport effects <ref:2607.26045#pg9>.

Kai: The paper "Photonuclear Neutron Production in OpenMC: Verification Against MCNPX, FLUKA, and a First-Collision Analytical Solution" <ref:2607.26045#pg1> is a solid piece of evidence that the new implementation reproduces the integrated neutron-production response of an established reference code <ref:2607.26045#pg2>.

Lev: So we need to keep checking those angular distributions and geometries where secondary transport is significant in future work <ref:2607.26045#pg9>.

Lorenzo Loi, Andrea Missaglia, Carolina Introini, David Breitenmoser, Shaun D. Clarke, Sara A. Pozzi, Á Antonio Cammi

Politecnico di Milano Department of Energy INFN Sezione di Milano Bicocca Department of Mechanical and Nuclear Engineering Khalifa University Emirates Emirates Nuclear Technology Centre Khalifa University of Science and Technology Department of Nuclear Engineering Texas A&M University Department of Nuclear Engineering and Radiological Sciences University of Michigan

physics.app-ph

Submitted: 2026-07-28

Updated: 2026-10-29

Comments: under peer review in Nuclear Science and Engineering; added angular distributions, MCNP6.2 comparison (Appendix A), and a new co-author

Code: https://github.com/GuySten/openmc

License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/

Importance score: 86/100

The gist: The modeling of photonuclear reactions is increasingly important for applications involving highenergy photon fields, including accelerator-driven neutron sources, radiation shielding, medical

Key concepts

Single-Collision Benchmarks
The study tested OpenMC under idealized conditions where only one photon interacts with a target nucleus at a time. This isolates how the code handles individual particle interactions, separating it from complex secondary transport effects and geometrical complexities that occur in real-world radiation scenarios.
Nuclear Data Sensitivity
The comparison between two different nuclear data sets (ENDF7u and IAEA/PD-2019) showed that switching data sources can cause larger discrepancies than the differences between the OpenMC and MCNPX codes themselves. This demonstrates how crucial it is to choose accurate nuclear data for reliable predictions.
Spectral vs. Yield Agreement
The study found that even when two codes produce nearly identical total neutron yields, the actual distribution (spectrum) of those neutrons can be significantly different. This means simply matching the total count isn't enough; understanding the energy profile is vital for applications like shielding and dosimetry.
Model Comparison
The OpenMC results were compared against FLUKA, which uses its own internal photonuclear models instead of standard nuclear data. These comparisons showed that while both codes can be close, differences arise due to how each code handles the underlying physics and reaction modeling.

Terminology

Summary

The modeling of photonuclear reactions is increasingly important for applications involving highenergy photon fields, including accelerator-driven neutron sources, radiation shielding, medical physics, and fusion technologies <ref:2607.26045#pg2>. This work presents a systematic verification of the OpenMC photonuclear implementation using six single-collision broomstick benchmarks based on 2H, 9Be, and 238U targets irradiated by monoenergetic 5 MeV and 15 MeV photons and by a continuous 1–20 MeV linear accelerator (LINAC)-representative spectrum <ref:2607.26045#pg4>.

Verification Scope

The study systematically verifies the OpenMC photonuclear implementation against MCNPX, FLUKA, and a first-collision analytical solution <ref:2607.26045#pg2>. This verification is conducted under idealized, single-collision conditions to isolate the treatment of individual particle interactions from secondary transport and macroscopic geometrical effects <ref:2607.26045#pg9>. The comparison involves testing the implementation against common evaluated nuclear data, as well as an independent model-based treatment from FLUKA <ref:2607.26045#pg10>.

Code Implementation Comparison

OpenMC and MCNPX agreed within 0.7% in integrated neutron yield for all benchmark cases when the same ENDF7u data were used <ref:2607.26045#pg4>. FLUKA, which relies on its own native photonuclear models rather than ENDF7u, differed from the analytical solution by approximately 6-9% for the monoenergetic cases and by no more than approximately 4% for the continuous-source cases <ref:2607.26045#pg4>. The calculations using IAEA/PD-2019 produced deviations of up to 11.3% from the ENDF7u-based analytical solution, with the sensitivity varying strongly by nuclide and source spectrum <ref:2607.26045#pg4>.

Data Sensitivity Assessment

The comparison between ENDF7u and IAEA/PD-2019 shows that replacing ENDF7u with IAEA/PD-2019 produces differences that are generally larger than the OpenMC–MCNPX code-to-code differences obtained under common data <ref:2607.26045#pg4>. For the monoenergetic cases, the IAEA/PD-2019 results differ from the analytical ENDF7u solution by approximately +0.03% for 9Be, +11.3% for 2H, and −4.6% for 238U <ref:2607.26045#pg6>. The sensitivity depends strongly on both the nuclide and the source spectrum <ref:2607.26045#pg6>.

Spectral vs. Yield Agreement

The energy-resolved comparisons demonstrated that agreement in the integrated yield does not necessarily imply agreement in the emitted neutron spectrum <ref:2607.26045#pg2>. The monoenergetic 2H benchmark, for which similar total neutron production was obtained despite marked differences in spectral width and peak location, is a clear example <ref:2607.26045#pg9>. Differences were also observed in the 9Be breakup spectra and in the low-energy region of the 238U distributions <ref:2607.26045#pg9>.

Conclusion and Future Work

The close agreement between OpenMC and MCNPX under common ENDF7u photonuclear data provides direct evidence that the new implementation reproduces the integrated neutron-production response of an established reference code <ref:2607.26045#pg2>. The study supports the use of the unofficial OpenMC branch for integrated photonuclear neutron production under the verified configurations, but it also identifies secondary-particle kinematics as an area requiring additional assessment <ref:2607.26045#pg9>. Further work should extend the verification to additional nuclides, reaction thresholds, angular distributions, and geometries in which secondary transport is significant <ref:2607.26045#pg9>.

Acknowledgments

The authors thank Guy Stein for developing and publicly releasing the photonuclear OpenMC branch examined in this study <ref:2607.26045#pg10>. The authors also acknowledge the OpenMC developer community for the continued development and documentation of the code <ref:2607.26045#pg10>.

Improvements for AI systems

  1. Verification of custom physics implementations in Monte Carlo codes: The improved AI system can reliably verify new physics capabilities, such as photonuclear reactions, by systematically comparing them against established reference codes like MCNPX and FLUKA under common nuclear data, as demonstrated by the OpenMC and MCNPX agreement within 0.7% for integrated neutron yield.

  2. Nuclear data sensitivity assessment: The system can quantify how changes in evaluated nuclear data affect predictions by explicitly evaluating the influence of replacing ENDF7u with IAEA/PD-2019 in OpenMC, allowing users to understand that changes relative to the ENDF7u-based analytical solution reached approximately 11% in the cases considered.

  3. Spectral distribution analysis for kinetic understanding: The improved system can distinguish between agreement in integrated yields and differences in secondary-neutron spectra, identifying that agreement in the total spectrum does not necessarily imply agreement in the individual reaction channels, which is crucial for shielding and dosimetry applications.

  4. Model comparison against native physics: The system can provide an independent model-based assessment by comparing results with FLUKA's native photonuclear models, noting that FLUKA differs from the analytical solution by approximately 6-9% for the monoenergetic cases and by no more than approximately 4% for the continuous-source cases.

  5. Automated identification of critical dependencies: The system can pinpoint which factors drive discrepancies between codes and data, such as identifying that the sensitivity depends strongly on both the nuclide and the source spectrum, specifically noting that The library sensitivity is particularly evident for 2H under monoenergetic irradiation and for 9Be under continuous irradiation.

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