arXiv subjects

David Regnier

Publications and source records attributed to David Regnier.

14 recordsLinked to original sources

arXiv2026

SINAPSE: A lightweight deep learning framework for accurate and explainable neutron-$\gamma$ discrimination

Traditionally, neutron-$\gamma$ discrimination in organic scintillators relies on techniques such as time-of-flight (ToF) selection and pulse-shape discrimination (PSD). However, particle identification through graphical cuts remains challenging in the low-charge regime due to poor signal-to-noise ratios (SNR). In this work, we propose SINAPSE, a lightweight deep learning framework for accurate and explainable neutron-$\gamma$ discrimination in the low-charge regime. The framework employs a dual-branch architecture that combines a 1-dimensional convolutional autoencoder for waveform denoising with a classifier for particle identification. Random augmentations are applied to high-SNR waveforms to simulate low-charge conditions, enabling robust extrapolation into regimes where conventional PSD labels are unreliable. We show that SINAPSE achieves superior denoising performance compared to conventional digital signal processing techniques, and outputs well-calibrated probabilities, consistent with traditional graphical cuts. Finally, we apply SHAP (SHapley Additive exPlanations) values to show that model decisions are driven by physically meaningful pulse-shape features, confirming consistency with established PSD principles.

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arXiv2026

Probability distribution of observables from a Bogoliubov vacuum projected onto good particle number: application to scission configurations of an actinide

Nuclear fission dynamics described within nuclear energy density functional frameworks (EDF) have seen substantial advances in the last decade. Part of this success stems from projection techniques, which allow the computation f probability distribution functions (pdf) for selected observables such as particle number and angular momentum of the fragments. Predicting the pdf of other observables, such as the total kinetic energy of the fragments, remains undone. This work proposes a method to determine the complete pdf of a new category of observables from a Bogoliubov vacuum projected onto good particle number. It relies on sampling nucleonic configurations in coordinate and intrinsic-spin representation. We assess the feasibility and convergence properties of the method and apply it to states representative of the scission of an actinide. Fluctuations in fragment shapes, inter-fragment Coulomb and nuclear interaction as well as the corresponding torques are analyzed. We find that a significant fraction of the fluctuation of several measured fission observables is already present within the mean-field picture.

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arXiv2024

Construction of continuous collective energy landscapes for large amplitude nuclear many-body problems

Several protocols are proposed to build continuous energy surfaces of many-body quantum systems, regarding both energy and states. The standard variational principle is augmented with constraints on state overlap, ensuring arbitrary precision on continuity. As an illustration, the lowest energy and excited state paths relevant for the $^{240}$Pu asymmetric fission are studied. The scission is clearly signed, with a neutron excess in the neck, the ultimate glue before its rupture. Our approach can potentially connect any couple of Hilbert space states, which opens up new horizons for various applications.

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arXiv2023

Multiconfigurational time-dependent density functional theory for atomic nuclei: Technical and numerical aspects

The nuclear time-dependent density functional theory (TDDFT) is a tool of choice for describing various dynamical phenomena in atomic nuclei. In a recent study, we reported an extension of the framework - the multiconfigurational TDDFT (MC-TDDFT) model - that takes into account quantum fluctuations in the collective space by mixing several TDDFT trajectories. In this article, we focus on technical and numerical aspects of the model. We outline the properties of the time-dependent variational principle that is employed to obtain the equation of motion for the mixing function. Furthermore, we discuss evaluation of various ingredients of the equation of motion, including the Hamiltonian kernel, norm kernel, and kernels with explicit time derivatives. We detail the numerical methods for resolving the equation of motion and outline the major assumptions underpinning the model. A technical discussion is supplemented with numerical examples that consider collective quadrupole vibrations in $^{40}$Ca, particularly focusing on the issues of convergence, treatment of linearly dependent bases, energy conservation, and prescriptions for the density-dependent part of an interaction.

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arXiv2023

Generative deep-learning reveals collective variables of Fermionic systems

Complex processes ranging from protein folding to nuclear fission often follow a low-dimension reaction path parameterized in terms of a few collective variables. In nuclear theory, variables related to the shape of the nuclear density in a mean-field picture are key to describing the large amplitude collective motion of the neutrons and protons. Exploring the adiabatic energy landscape spanned by these degrees of freedom reveals the possible reaction channels while simulating the dynamics in this reduced space yields their respective probabilities. Unfortunately, this theoretical framework breaks down whenever the systems encounters a quantum phase transition with respect to the collective variables. Here we propose a generative-deep-learning algorithm capable of building new collective variables highly representative of a nuclear process while ensuring a differentiable mapping to its Fermionic wave function. Within this collective space, the nucleus can evolve continuously from one of its adiabatic quantum phase to the other at the price of crossing a potential energy barrier. This approach applies to any Fermionic system described by a single Slater determinant, which encompasses electronic systems described within the density functional theory.

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arXiv2023

Quantum fluctuations induce collective multiphonons in finite Fermi liquids

We show that collective multiphonon states in atomic nuclei emerge at high excitation energies when quantum fluctuations in the collective space are included beyond the independent-particle approximation. The quadrupole response of a nucleus is studied using an extension of the nuclear time-dependent density-functional theory that mixes several many-body trajectories. While a single trajectory can account for the excitation of the first collective quantum, the second and the third quanta emerge due to the interference between trajectories. The collective spectrum, found as nearly harmonic, is in excellent agreement with the experimentally observed three quanta of the isoscalar giant quadrupole resonance in $^{40}$Ca. This study offers guidance for multiphonon searches in other self-bound systems and demonstrates the resistance to internal excitation of finite Fermi liquids.

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arXiv2022

Building Surrogate Models of Nuclear Density Functional Theory with Gaussian Processesand Autoencoders

From the lightest Hydrogen isotopes up to the recently synthesized Oganesson (Z=118), it is estimated that as many as about 3000 atomic nuclei could exist in nature. Most of these nuclei are too short-lived to be occurring on Earth, but they play an essential role in astrophysical events such as supernova explosions or neutron star mergers that are presumed to be at the origin of most heavy elements in the Universe. Understanding the structure, reactions, and decays of nuclei across the entire chart of nuclides is an enormous challenge because of the experimental difficulties in measuring properties of interest in such fleeting objects and the theoretical and computational issues of simulating strongly-interacting quantum many-body systems. Nuclear density functional theory (DFT) is a fully microscopic theoretical framework which has the potential of providing such a quantitatively accurate description of nuclear properties for every nucleus in the chart of nuclides. Thanks to high-performance computing facilities, it has already been successfully applied to predict nuclear masses, global patterns of radioactive decay like $\beta$ or $\gamma$ decay, and several aspects of the nuclear fission process such as, e.g., spontaneous fission half-lives. Yet, predictive simulations of nuclear spectroscopy or of nuclear fission, or the quantification of theoretical uncertainties and their propagation to applications, would require several orders of magnitude more calculations than currently possible. However, most of this computational effort would be spent into generating a suitable basis of DFT wavefunctions. Such a task could potentially be considerably accelerated by borrowing tools from the field of machine learning and artificial intelligence. In this paper, we review different approaches to applying supervised and unsupervised learning techniques to nuclear DFT.

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arXiv2022

Theory of Nuclear Fission

Atomic nuclei are quantum many-body systems of protons and neutrons held together by strong nuclear forces. Under the proper conditions, nuclei can break into two (sometimes three) fragments which will subsequently decay by emitting particles. This phenomenon is called nuclear fission. Since different fission events may produce different fragmentations, the end-products of all fissions that occurred in a small chemical sample of matter comprise hundreds of different isotopes, including $\alpha$ particles, together with a large number of emitted neutrons, photons, electrons and antineutrinos. The extraordinary complexity of this process, which happens at length scales of the order of a femtometer, mostly takes less than a femtosecond but is not completely over until all the lingering $\beta$ decays have completed - which can take years - is a fascinating window into the physics of atomic nuclei. While fission may be more naturally known in the context of its technological applications, it also plays a pivotal role in the synthesis of heavy elements in astrophysical environments. In both cases, experimental measurements are not sufficient to provide complete data. Simulations are needed, yet at levels of accuracy and precision that pose formidable challenges to nuclear theory. The goal of this article is to provide a comprehensive overview of the theoretical methods employed in the description of nuclear fission.

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arXiv2021

Microscopic Calculation of Fission Product Yields with Particle Number Projection

Fission fragments' charge and mass distribution is an important input to applications ranging from basic science to energy production or nuclear non-proliferation. In simulations of nucleosynthesis or calculations of superheavy elements, these quantities must be computed from models, as they are needed in nuclei where no experimental information is available. Until now, standard techniques to estimate these distributions were not capable of accounting for fine-structure effects, such as the odd-even staggering of the charge distributions. In this work, we combine a fully-microscopic collective model of fission dynamics with a recent extension of the particle number projection formalism to provide the highest-fidelity prediction of the primary fission fragment distributions for the neutron-induced fission of $^{235}$U and $^{239}$Pu. We show that particle number projection is an essential ingredient to reproduce odd-even staggering in the charge yields and benchmark the performance of various empirical probability laws that could simulate its effect. This new approach also enables for the first time the realistic determination of two-dimensional isotopic yields within nuclear density functional theory.

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arXiv2020

Future of Nuclear Fission Theory

There has been much recent interest in nuclear fission, due in part to a new appreciation of its relevance to astrophysics, stability of superheavy elements, and fundamental theory of neutrino interactions. At the same time, there have been important developments on a conceptual and computational level for the theory. The promising new theoretical avenues were the subject of a workshop held at the University of York in October 2019; this report summarises its findings and recommendations.

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arXiv2020

The time-dependent generator coordinate method in nuclear physics

The emergence of collective behaviors and the existence of large amplitude motions are both central features in the fields of nuclear structure and reactions. From a theoretical point of view, describing such phenomena requires increasing the complexity of the many-body wavefunction of the system to account for long-range correlations. One of the challenges, when going in this direction, is to keep the approach tractable within our current computational resources while gaining a maximum of predictive power for the phenomenon under study. In the Generator Coordinate Method (GCM), the many-body wave function is a linear superposition of (generally non-orthogonal) many-body states (the generator states) labeled by a few collective coordinates. Such a method has been widely used in structure studies to restore the symmetries broken by single-reference approaches. In the domain of reactions, its time-dependent version (TDGCM) has been developed and applied to predict the dynamics of heavy-ion collisions or fission where the collective fluctuations play an essential role. In this review, we present the recent developments and applications of the TDGCM in nuclear reactions. We recall the formal derivations of the TDGCM and its most common approximate treatment, the Gaussian Overlap Approximation. We also emphasize the Schrödinger Collective-Intrinsic Model (SCIM) variant focused on the inclusion of quasiparticle excitations into the description. Finally, we highlight several exploratory studies related to a TDGCM built on time-dependent generator states.

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arXiv2019

Combining phase-space and time-dependent reduced density matrix approach to describe the dynamics of interacting fermions

The possibility to apply phase-space methods to many-body interacting systems might provide accurate descriptions of correlations with a reduced numerical cost. For instance, the so--called stochastic mean-field phase-space approach, where the complex dynamics of interacting fermions is replaced by a statistical average of mean-field like trajectories is able to grasp some correlations beyond the mean-field. We explore the possibility to use alternative equations of motion in the phase-space approach. Guided by the BBGKY hierarchy, equations of motion that already incorporate part of the correlations beyond mean-field are employed along each trajectory. The method is called Hybrid Phase-Space (HPS) because it mixes phase-space techniques and the time-dependent reduced density matrix approach. The novel approach is applied to the one-dimensional Fermi-Hubbard model. We show that the predictive power is improved compared to the original stochastic mean-field method. In particular, in the weak-coupling regime, the results of the HPS theory can hardly be distinguished from the exact solution even for long time.

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arXiv2018

Microscopic description of pair transfer between two superfluid Fermi systems: combining phase-space averaging and combinatorial techniques

In a mean-field description of superfluidity, particle number and gauge angle are treated as quasi-classical conjugated variables. This level of description was recently used to describe nuclear reactions around the Coulomb barrier. Important effects of the relative gauge angle between two identical superfluid nuclei (symmetric collisions) on transfer probabilities and fusion barrier have been uncovered. A theory making contact with experiments should at least average over different initial relative gauge-angles. In the present work, we propose a new approach to obtain the multiple pair transfer probabilities between superfluid systems. This method, called Phase-Space combinatorial (PSC) technique, relies both on phase-space averaging and combinatorial arguments to infer the full pair transfer probability distribution at the cost of multiple mean-field calculations only. After benchmarking this approach in a schematic model, we apply it to the collision $^{20}$O+$^{20}$O at various energies below the Coulomb barrier. The predictions for one pair transfer are similar to results obtained with an approximated projection method whereas significant differences are found for two pairs transfer. Finally, we investigated the applicability of the PSC method to the contact between non-identical superfluid systems. [...] The applicability of the PSC method to asymmetric nuclear collisions is investigated for the $^{14}$O+$^{20}$O collision and it turns out that unrealistically small single- and multiple-pair transfer probabilities are obtained. [...] We conclude that the best ground for probing gauge-angle effects in nuclear reaction and/or for applying the proposed PSC approach on pair transfer is the collisions of identical open-shell spherical nuclei.

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arXiv2012

Numerical search of discontinuities in self-consistent potential energy surfaces

Potential energy surfaces calculated with self-consistent mean-field methods are a very powerful tool, since their solutions are, in theory, global minima of the non-constrained subspace. However, this minimization leads to an incertitude concerning the saddle points, that can sometimes be no more saddle points in bigger constrained subspaces (fake saddle points), or can be missing on a trajectory (it missing saddle points). These phenomena are the consequences of discontinuities of the self-consistent potential energy surfaces (SPES). These discontinuities may have important consequences, since they can for example hide the real height of an energy barrier, and avoid any use of a SPES for further dynamical calculations, barrier penetrability estimations, or trajectory predictions. Discontinuities are not related to the quality of the production of a SPES, since even a perfectly converged SPES with an ideally fine mesh can be discontinuous. In this paper we explain what are the discontinuities, their consequences, and their origins. We then propose a numerical method to detect and identify discontinuities on a given SPES, and finally we discuss what are the best ways to transform a discontinuous SPES into a continuous one.

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