arXiv subjects

Vladimir Sobes

Publications and source records attributed to Vladimir Sobes.

8 recordsLinked to original sources

arXiv2026

Resonance Statistics -Informed Fitting Applied to Automated Cross Section Evaluation

This work investigates the use of resonance statistics for resonance evaluation to inform spin group assignment and an alternative fitting objective function beyond the commonly used chi-squared statistic. Resonance statistics -informed methods are applied to the automated resonance fitting framework, developed by N. Walton et al. In this automated framework, the utility of resonance statistics is largely unexplored. The new resonance statistics -informed spin group shuffling algorithm reduces spin group frequency bias seen in the base fitting algorithm. Although resonance statistics -informed optimization produces negligible changes in pointwise cross section agreement, it significantly improves consistency with Wigner level-spacing statistics and stabilizes the fitted resonance density in the presence of model imperfections.

physics.comp-ph↗

arXiv2024

Validating Automated Resonance Evaluation with Synthetic Data

The integrity and precision of nuclear data are crucial for a broad spectrum of applications, from national security and nuclear reactor design to medical diagnostics, where the associated uncertainties can significantly impact outcomes. A substantial portion of uncertainty in nuclear data originates from the subjective biases in the evaluation process, a crucial phase in the nuclear data production pipeline. Recent advancements indicate that automation of certain routines can mitigate these biases, thereby standardizing the evaluation process, reducing uncertainty and enhancing reproducibility. This article contributes to developing a framework for automated evaluation techniques testing, emphasizing automated fitting methods that do not require the user to provide any prior information. This approach simplifies the process and reduces the manual effort needed in the initial evaluation stage. It highlights the capability of the framework to validate and optimize subroutines, targeting the performance analysis and optimization of the fitting procedure using high-fidelity synthetic data (labeled experimental data) and the concept of a fully controlled computational experiment. An error metric is introduced to provide a clear and intuitive measure of the fitting quality by quantifying the accuracy and performance across the specified energy. This metric sets a scale for comparison and optimization of routines or hyperparameter selection, improving the entire evaluation process methodology and increasing reproducibility and objectivity.

physics.comp-ph↗

arXiv2024

Incorrect Resonance Escape Probability in Monte Carlo Codes due to the Threshold Approximation of Temperature-Dependent Scattering

Monte Carlo-transport codes are designed to simulate the complex neutron transport physics associated with nuclear systems. These codes are tasked with simulating phenomena such as temperature effects on cross-sections, thermo-physical effects, reaction rates, and kinematics. It is not computationally possible to simulate the physics of a system exactly. However, many of the approximations made by modern simulation codes have been well validated. This article investigates an impactful simulation error caused by an approximation made in many Monte Carlo-transport codes. The approximation that target-at-rest is valid for neutrons at energies 400 times that of the thermal energy of the target particle is found to be inaccurate in certain scenarios. This paper identifies such cases, notably TRISO [1] fuel and instances where fuel infiltrates the pores of graphite in Molten Salt Reactors. The breakdown of this approximation occurs particularly when there exists a small length scale between fuel, a material with absorption resonances, and moderator, a scattering material. When threshold values are too small, resonance escape probabilities can deviate by as much as 1% per resonance, forming a baseline defect. Furthermore, two distinct anomalies were observed upon temperature variation, directly attributed to the transition between target-at-rest and target-in-motion physics. Equations provided in this study offer predictions for the temperature ranges within which these anomalies occur, based on system temperature and threshold value. The recommendations put forth in this paper advocate for incorporating the threshold value as a user-defined variable in transport Monte Carlo codes employing this approximation. Additionally, users are advised to conduct convergence studies to ensure that the chosen threshold value is sufficiently high to mitigate the influence of baseline defects and anomalies.

physics.comp-ph↗

arXiv2023

Methodology for physics-informed generation of synthetic neutron time-of-flight measurement data

Accurate neutron cross section data are a vital input to the simulation of nuclear systems for a wide range of applications from energy production to national security. The evaluation of experimental data is a key step in producing accurate cross sections. There is a widely recognized lack of reproducibility in the evaluation process due to its artisanal nature and therefore there is a call for improvement within the nuclear data community. This can be realized by automating/standardizing viable parts of the process, namely, parameter estimation by fitting theoretical models to experimental data. This automation effort could greatly benefit from a synthetic data resource. This work leverages problem-specific physics, Monte Carlo sampling, and a general methodology for data synthesis to generate unlimited, labelled experimental cross-section data that is statistically indistinguishable to the observed data. Heuristic and, where applicable, rigorous statistical comparisons to observed data support this claim. The demonstration is based on/limited to transmission measurements at Rensselaer Polytechnic Institute (RPI) and energy-differential cross sections in the resolved resonance region (RRR). An open-source software is published alongside this article that executes the complete methodology to produce high-utility synthetic datasets. The goal of this work is to provide an approach and corresponding tool that will allow the evaluation community to begin exploring more data-driven, ML-based solutions to long-standing challenges in the field.

physics.comp-ph↗

arXiv2020

Scattering matrix pole expansions for complex wavenumbers in $R$-matrix theory

In this follow-up article to [Shadow poles in the alternative parametrization of R-matrix theory, Ducru (2020)], we establish new results on scattering matrix pole expansions for complex wavenumbers in R-matrix theory. In the past, two branches of theoretical formalisms emerged to describe the scattering matrix in nuclear physics: R-matrix theory, and pole expansions. The two have been quite isolated from one another. Recently, our study of Brune's alternative parametrization of R-matrix theory has shown the need to extend the scattering matrix (and the underlying R-matrix operators) to complex wavenumbers. Two competing ways of doing so have emerged from a historical ambiguity in the definitions of the shift $\boldsymbol{S}$ and penetration $\boldsymbol{P}$ functions: the legacy Lane \& Thomas "force closure" approach, versus analytic continuation (which is the standard in mathematical physics). The R-matrix community has not yet come to a consensus as to which to adopt for evaluations in standard nuclear data libraries, such as ENDF. In this article, we argue in favor of analytic continuation of R-matrix operators. We bridge R-matrix theory with the Humblet-Rosenfeld pole expansions, and unveil new properties of the Siegert-Humblet radioactive poles and widths, including their invariance properties to changes in channel radii $a_c$. We then show that analytic continuation of R-matrix operators preserves important physical and mathematical properties of the scattering matrix -- cancelling spurious poles and guaranteeing generalized unitarity -- while still being able to close channels below thresholds.

nucl-th↗

arXiv2020

Shadow poles in Brune parametrization of R-matrix theory

Our collective knowledge of nuclear cross sections is recorded as resonance parameters in nuclear data libraries. To evaluate these parameters, campaigns of measurements are fitted with a parametric model of nuclear cross sections called R-matrix theory. In order to remove the arbitrary boundary parameters in the Wigner-Eisenbud R-matrix parametrization, the community is considering converting all nuclear data libraries to Brune parameters. In this article, we show there are more Brune parameters than previously thought. Below the channel threshold, we prove there exists two types of additional 'shadow poles' - branch shadow poles and analytic shadow poles - depending on how we continue R-matrix operators to complex wavenumbers (to do so we establish Mittag-Leffler expansions of R-matrix operators). This entails there are more Brune resonance energies than levels. Yet, we also prove that choosing any subset of Brune poles will yield the same cross sections than using the entire set of Brune poles, as long as it has at least the number of levels. In practice, this means that shadow Brune poles can be discarded from the new nuclear data libraries. Many isotopes are evaluated with the Reich-Moore approximation, introducing complex resonance energies to eliminate certain channels. We generalize Brune's parameterization to encompass the Reich-Moore approximation and the additional shadow poles, and show that all Brune parameters depend on what convention we choose to continue the R-matrix operators to complex wavenumbers. To convert nuclear data libraries to Brune parameters, the nuclear scientists community must thus first decide on such a convention. The authors argue in favor of analytic continuation. The first evidence of shadow poles in Brune's alternative parametrization of R-matrix theory is observed in isotope xenon-134, spin-parity group 1/2(-).

nucl-th↗

arXiv2020

Windowed multipole representation of R-matrix cross sections

Nuclear cross sections are basic inputs to any nuclear computation. Campaigns of experiments are fitted with the parametric R-matrix model of quantum nuclear interactions, and the resulting cross sections are documented - both point-wise and as resonance parameters (with uncertainties) - in standard evaluated nuclear data libraries (ENDF, JEFF, BROND, JENDL, CENDL, TENDL): these constitute our common knowledge of fundamental nuclear physics. In the past decade, a collaborative effort has been deployed to establish a new nuclear cross section library format - the Windowed Multipole Library - with the goal of considerably reducing the cost of cross section calculations in nuclear transport simulations. This article lays the theoretical foundations underpinning these efforts. From general R-matrix scattering theory, we derive the windowed multipole representation of nuclear cross sections. Though physically and mathematically equivalent, the windowed multipole representation is particularly well suited for subsequent temperature treatment of angle-integrated cross sections: we show that accurate Doppler broadening can be performed analytically up to the first reaction threshold; and we derive cross sections temperature derivatives to any order. Furthermore, we here establish a way of converting the R-matrix resonance parameters uncertainty (covariance matrices) into windowed multipole parameters uncertainty. We show that generating stochastic nuclear cross sections by sampling from the resulting windowed multipole covariance matrix can reproduce the cross section uncertainty in the original nuclear data file. Through this foundational article, we hope to make the Windowed Multipole Representation accessible, reproducible, and usable for the nuclear physics community, as well as provide the theoretical basis for future research on expanding its capabilities.

nucl-th↗

arXiv2019

Scattering matrix pole expansions & invariance with respect to R-matrix parameters

Nuclear data libraries (ENDF, JEFF, JENDL, CENDL, etc.) document our phenomenological knowledge of nuclear cross sections as interpreted by R-matrix theory. The R-matrix scattering model can parameterize the energy dependence of the scattering matrix in different ways. This article establishes new results on three such sets of parameters: the Wigner-Eisenbud, the Brune, and the Siegert-Humblet parameters. We show how the latter two arise from invariance to the arbitrary boundary condition, and how they can trade-off more complex parameters for a simpler energy dependence parameterization: the scattering matrix pole expansion of Humblet and Rosenfeld. We establish that the scattering matrix's invariance to channel radius sets a partial differential equation on the widths of the Kapur-Peierls operator, which enables us to derive an explicit transformation of the Siegert-Humblet radioactive residue widths under a change of channel radius. Considering the continuation of the scattering matrix to complex wavenumbers, several new results are established. We unveil there exist more Brune parameters than previously thought, depending on the way the scattering matrix is continued to complex energies. This points to a broader conundrum in the field: how to analytically continue the scattering matrix while closing sub-threshold channels. We argue that, contrary to the Lane and Thomas legacy of force-closing sub-threshold channels which introduces spurious poles, analytic continuation is the physically correct way of defining the scattering matrix for complex wave numbers. To back this claim we establish thee results: analytic continuation cancels spurious poles, enforces the generalized unitarity conditions of Eden and Taylor, and closes massive particle channels below threshold by both quantum tunneling and from the definition of the cross section as the ratio of probability currents.

nucl-th↗