Search

A new method for directly sampling the neutron resonance upscattering effect is presented. Alternatives have relied on inefficient rejection sampling techniques or large tabular storage of relative velocities. None of these approaches, which require pointwise energy data, are particularly well suited to the windowed multipole cross section representation. The new method called multipole analytic resonance scattering (MARS) overcomes these limitations by inverse transform sampling from the target relative velocity distribution where the cross section is expressed in the multipole formalism. The closed form relative speed distribution contains a novel special function we deem the incomplete Faddeeva function: $$ w(z, x) = \frac{i}{\pi} \int_{-\infty}^x \frac{e^{-t^2} dt}{z-t}. $$ We present the first results on its efficient numerical evaluation.

In recent years, radiation mapping has attracted widespread research attention and increased public concerns on environmental monitoring. In terms of both materials and their configurations, radiation detectors have been developed to locate the directions and positions of the radiation sources. In this process, algorithm is essential in converting detector signals to radiation source information. However, due to the complex mechanisms of radiation-matter interaction and the current limitation of data collection, high-performance, low-cost radiation mapping is still challenging. Here we present a computational framework using Tetris-inspired detector pixels and machine learning for radiation mapping. Using inter-pixel padding to increase the contrast between pixels and neural network to analyze the detector readings, a detector with as few as four pixels can achieve high-resolution directional mapping. By further imposing Maximum a Posteriori (MAP) with a moving detector, further radiation position localization is achieved. Non-square, Tetris-shaped detector can further improve performance beyond the conventional grid-shaped detector. Our framework offers a new avenue for high quality radiation mapping with least number of detector pixels possible, and is anticipated to be capable to deploy for real-world radiation detection with moderate validation., Comment: 29 pages, 20 figures. Ryotaro Okabe and Shangjie Xue contributed equally to this work

The development of next-generation autonomous control of fission systems, such as nuclear power plants, will require leveraging advancements in machine learning. For fission systems, accurate prediction of nuclear transport is important to quantify the safety margin and optimize performance. The state-of-the-art approach to this problem is costly Monte Carlo (MC) simulations to approximate solutions of the neutron transport equation. Such an approach is feasible for offline calculations e.g., for design or licensing, but is precluded from use as a model-based controller. In this work, we explore the use of Artificial Neural Networks (ANN), Gradient Boosting Regression (GBR), Gaussian Process Regression (GPR) and Support Vector Regression (SVR) to generate empirical models. The empirical model can then be deployed, e.g., in a model predictive controller. Two fission systems are explored: the subcritical MIT Graphite Exponential Pile (MGEP), and the critical MIT Research Reactor (MITR). Findings from this work establish guidelines for developing empirical models for multidimensional regression of neutron transport. An assessment of the accuracy and precision finds that the SVR, followed closely by ANN, performs the best. For both MGEP and MITR, the optimized SVR model exhibited a domain-averaged, test, mean absolute percentage error of 0.17 %. A spatial distribution of performance metrics indicates that physical regions of poor performance coincide with locations of largest neutron flux perturbation -- this outcome is mitigated by ANN and SVR. Even at local maxima, ANN and SVR bias is within experimental uncertainty bounds. A comparison of the performance vs. training dataset size found that SVR is more data-efficient than ANN. Both ANN and SVR achieve a greater than 7 order reduction in evaluation time vs. a MC simulation., Comment: 20 pages, 7 figures

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., Comment: Under review in Phys. Rev. C. 23 pages, 1 figure, 1 table. arXiv admin note: substantial text overlap with arXiv:1903.02661

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., Comment: 35 pages, 6 figues, 3 tables, article in review at Phys. Rev. C

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(-)., Comment: arXiv admin note: substantial text overlap with arXiv:1903.02661

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.

We report on the recording of 3D optical memories in sol-gel materials by using a non-linear absorption effect. This effect induces a local change of the optical properties of the material which is read and quantified with a high resolution full-field Optical Coherence Tomography setup. It is the first time that this technique is used for this purpose. Data recording was performed by focused picosecond (ps) single-pulse irradiation at 1064 nm with energy densities of 10 and 33 J/cm2 per pulse., Comment: 19 pages, 7 figures