arXiv subjects

Loïc Thulliez

Publications and source records attributed to Loïc Thulliez.

4 recordsLinked to original sources

arXiv2026

Branching stochastic mechanics: Clustering and connected correlations within a branching-process representation of the Schrödinger equation

Can finite-range correlations hide in the statistics of an extended quantum state? The Schrödinger-Nagasawa transform represents the wave function by positive forward and backward diffusion fields whose product is the Born density. We promote them to branching superprocesses, $Φ_F$ and $Φ_B$: diffusion samples stochastic paths, whereas Bohm/Fisher-controlled branching generates genealogies of alternative continuations. With rates evaluated on the prescribed Born density, their means reproduce Schrödinger dynamics exactly. The connected sector exhibits supercritical, critical, and subcritical clustering in confinement and has critical dimension $d_c=2$ in free space. Stationary eigenmodes remain extended; subcritical clusters acquire the reduced de~Broglie scale. We then let the branching rate respond to the fluctuating product $Φ_FΦ_B$. In the reciprocal basis, the fields equal a smooth reference $R$ plus centered fluctuations $ψ_F,ψ_B$, defining the signed kernel $C_{\rm FB}(x,y)=\mathbb E_ω[ψ_F(x)ψ_B(y)]$. On the anticorrelated branch, $ρ_{\rm BSM}(x)=-C_{\rm FB}(x,x)$ is the positive organized connected weight. Stationary matching of this weight to the Born profile is imposed as a closure condition, while off-diagonal decay defines the correlation range. The pair equation splits into collective and relative sectors: spectral cancellation selects the Born collective mode, while suppression of the leading density fluctuation selects the anticorrelated source channel. With relative diffusivity $D_{\rm eff}$ and positive relaxation rate $μ_{\rm FB}$, correlations have screening length $ξ_{\rm FB}=\sqrt{D_{\rm eff}/μ_{\rm FB}}$. Within this reciprocal closure, an extended Born-shaped connected weight and a finite correlation range can coexist in one stochastic kernel.

cond-mat.stat-mech↗

arXiv2024

Calculation of crystal defects induced in CaWO$_{4}$ by 100 eV displacement cascades using a linear Machine Learning interatomic potential

We determine the energy stored in the crystal defects induced by $\mathcal{O}(10-100)$\,eV nuclear recoils in low-threshold CaWO$_{4}$ cryogenic detectors. A Machine Learning interatomic potential is developed to perform molecular dynamics simulations. We show that the energy spectra expected from Dark Matter and neutrino coherent scattering are affected by the crystal defects and we provide reference predictions. We discuss the special case of the spectrum of nuclear recoils induced by neutron capture, which could offer a unique sensitivity to the calculated stored energies.

physics.ins-det↗

arXiv2023

Generational variance reduction in Monte Carlo criticality simulations as a way of mitigating unwanted correlations

Monte Carlo criticality simulations are widely used in nuclear safety demonstrations, as they offer an arbitrarily precise estimation of global and local tallies while making very few assumptions. However, since the inception of such numerical approaches, it is well known that bias might affect both the estimation of errors on these tallies and the tallies themselves. In particular, stochastic modeling approaches developed in the past decade have shed light on the prominent role played by spatial correlations through a phenomenon called neutron clustering. This effect is particularly of great significance when simulating loosely coupled systems (i.e., with a high dominance ratio). In order to tackle this problem, this paper proposes to recast the power iteration technique of Monte Carlo criticality codes into a variance reduction technique called Adaptative Multilevel Splitting. The central idea is that iterating over neutron generations can be seen as pushing a sub-population of neutrons towards a generational detector (instead of a spatial detector as variance reduction techniques usually do). While both approaches allow for neutron population control, the former blindly removes or splits neutrons. In contrast, the latter optimizes spatial, generational, and spectral attributes of neutrons when they are removed or split through an adjoint flux estimation, hence tempering both generational and spatial correlations. This is illustrated in the present article with a simple case of a bare slab reactor in the one speed theory on which the Adaptive Multilevel Splitting was applied and compared to variations of the Monte Carlo power iteration method used in neutron transport. Besides looking at the resulting efficiency of the methods, this work also aims at highlighting the main mechanisms of the Adaptive Multilevel Splitting in criticality calculations.

cond-mat.stat-mech↗

arXiv2022

Neutron availability in the Complementary Experiments Hall of the IFMIF-DONES facility

The IFMIF-DONES facility will be dedicated to the irradiation of structural materials planned for the use in future fusion reactors such as DEMO (Demonstration Fusion Power Plant). The potentialities of the IFMIF-DONES facility to complement its principal purpose by other experiments that would open the facility to other communities is addressed in this work. It concerns a study based on simulations to evaluate the neutronic performances of IFMIF-DONES in an hall dedicated to complementary experiments where neutrons can be transported. With the simple beam tube geometry of 4.5~cm entrance diameter studied in this work we have shown that a collimated fast-neutron beam of about 2~10$^{10}$~n/cm$^2$/s is available in the hall. Adding a moderator in the hall with neutron extraction lines would allow to get thermal neutron beams of about ~10$^{6}$-10$^{7}$~n/cm$^2$/s for dedicated experiments. The results show that IFMIF-DONES has the potentialities to be a medium-flux neutron facility for most of the neutron applications.

physics.acc-ph↗