A {\mu}-TPC detector for the characterization of low energy neutron fields

The AMANDE facility produces monoenergetic neutron fields from 2 keV to 20 MeV for metrological purposes. To be considered as a reference facility, fluence and energy distributions of neutron fields have to be determined by primary measurement standards. For this purpose, a micro Time Projection Chamber is being developed to be dedicated to measure neutron fields with energy ranging from 8 keV up to 1 MeV. In this work we present simulations showing that such a detector, which allows the measurement of the ionization energy and the 3D reconstruction of the recoil nucleus, provides the determination of neutron energy and fluence of these neutron fields

Observation of magnetic vortex pairs at room temperature in a planar {\alpha}-Fe2O3/Co heterostructure

Vortices are among the simplest topological structures, and occur whenever a flow field `whirls' around a one-dimensional core. They are ubiquitous to many branches of physics, from fluid dynamics to superconductivity and superfluidity, and are even predicted by some unified theories of particle interactions, where they might explain some of the largest-scale structures seen in today's Universe. In the crystalline state, vortex formation is rare, since it is generally hampered by long-range interactions: in ferroic materials (ferromagnetic and ferroelectric), vortices are only observed when the effects of the dipole-dipole interaction is modified by confinement at the nanoscale, or when the parameter associated with the vorticity does not couple directly with strain. Here, we present the discovery of a novel form of vortices in antiferromagnetic (AFM) hematite ($\alpha$-Fe$_2$O$_3$) epitaxial films, in which the primary whirling parameter is the staggered magnetisation. Remarkably, ferromagnetic (FM) topological objects with the same vorticity and winding number of the $\alpha$-Fe$_2$O$_3$ vortices are imprinted onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. Our data suggest that the ferromagnetic vortices may be merons (half-skyrmions, carrying an out-of-plane core magnetisation), and indicate that the vortex/meron pairs can be manipulated by the application of an in-plane magnetic field, H$_{\parallel}$, giving rise to large-scale vortex-antivortex annihilation.Comment: 16 pages, 4 figure

Modeling noise experiments performed at AKR-2 and CROCUS zero-power reactors

CORTEX is a EU H2020 project (2017-2021) devoted to the analysis of ’reactor neutron noise’ in nuclear reactors, i.e. the small fluctuations occurring around the stationary state due to external or internal disturbances in the core. One important aspect of CORTEX is the development of neutron noise simulation codes capable of modeling the spatial variations of the noise distribution in a reactor. In this paper we illustrate the validation activities concerning the comparison of the simulation results obtained by several noise simulation codes with respect to experimental data produced at the zero-power reactors AKR-2 (operated at TUD, Germany) and CROCUS (operated at EPFL, Switzerland). Both research reactors are modeled in the time and frequency domains, using transport or diffusion theory. Overall, the noise simulators managed to capture the main features of the neutron noise behavior observed in the experimental campaigns carried out in CROCUS and AKR-2, even though computational biases exist close to the region where the noise-inducing mechanical vibration was located (the so-called ”noise source”). In some of the experiments, it was possible to observe the spatial variation of the relative neutron noise, even relatively far from the noise source. This was achieved through reduced uncertainties using long measurements, the installation of numerous, robust and efficient detectors at a variety of positions in the near vicinity or inside the core, as well as new post-processing methods. For the numerical simulation tools, modeling the spatial variations of the neutron noise behavior in zero-power research reactors is an extremely challenging problem, because of the small magnitude of the noise field; and because deviations from a point-kinetics behavior are most visible in portions of the core that are especially difficult to be precisely represented by simulation codes, such as experimental channels. Nonetheless the limitations of the simulation tools reported in the paper were not an issue for the CORTEX project, as most of the computational biases are found close to the noise source

High Resolution Modelling without Computation Slowdown for PETALE in CROCUS

International audienceThe PETALE experimental program was successfully carried out in the CROCUS reactor of EPFL in the fall of 2020, in a collaboration between EPFL and CEA. It consists of criticality and transmission experiments with four distinct metallic reflectors made of 304L stainless steel, iron, nickel, and chromium, to study the nuclear data of stainless steel. In addition to the experimental results, one of its outcomes is the production of C/E and their covariance matrices for the reaction rates of the dosimeters used in the transmission experiments, with the aim of allowing to constrain future nuclear data evaluations. For this purpose, the chosen methodology for uncertainty propagation is the computationally expensive Total Monte-Carlo method. A modified build of the Serpent2 Monte Carlo transport code is used to perform Variance Reduction and correlated sampling. In addition, high resolution modelling of the experiments is used to limit the presence of bias in the analysis, and to pave the way toward the submission of a high-quality benchmark to an NEA database. This paper presents advances in the modelling of the experiments, focusing on the metal reflectors, as installed at the periphery of CROCUS. It was possible to refine from a simple design model made of a few cuboids, to a fully detailed model, while keeping the loss in computational efficiency below 15%. Notably, all reflector sheets of PETALE were detailed into 121 voxels each, based on topological measurements, without impact on the calculation time, thanks to the use of Serpent2 3D lattices

High Resolution Modelling without Computation Slowdown for PETALE in CROCUS

International audienceThe PETALE experimental program was successfully carried out in the CROCUS reactor of EPFL in the fall of 2020, in a collaboration between EPFL and CEA. It consists of criticality and transmission experiments with four distinct metallic reflectors made of 304L stainless steel, iron, nickel, and chromium, to study the nuclear data of stainless steel. In addition to the experimental results, one of its outcomes is the production of C/E and their covariance matrices for the reaction rates of the dosimeters used in the transmission experiments, with the aim of allowing to constrain future nuclear data evaluations. For this purpose, the chosen methodology for uncertainty propagation is the computationally expensive Total Monte-Carlo method. A modified build of the Serpent2 Monte Carlo transport code is used to perform Variance Reduction and correlated sampling. In addition, high resolution modelling of the experiments is used to limit the presence of bias in the analysis, and to pave the way toward the submission of a high-quality benchmark to an NEA database. This paper presents advances in the modelling of the experiments, focusing on the metal reflectors, as installed at the periphery of CROCUS. It was possible to refine from a simple design model made of a few cuboids, to a fully detailed model, while keeping the loss in computational efficiency below 15%. Notably, all reflector sheets of PETALE were detailed into 121 voxels each, based on topological measurements, without impact on the calculation time, thanks to the use of Serpent2 3D lattices

High Resolution Modelling without Computation Slowdown for PETALE in CROCUS

International audienceThe PETALE experimental program was successfully carried out in the CROCUS reactor of EPFL in the fall of 2020, in a collaboration between EPFL and CEA. It consists of criticality and transmission experiments with four distinct metallic reflectors made of 304L stainless steel, iron, nickel, and chromium, to study the nuclear data of stainless steel. In addition to the experimental results, one of its outcomes is the production of C/E and their covariance matrices for the reaction rates of the dosimeters used in the transmission experiments, with the aim of allowing to constrain future nuclear data evaluations. For this purpose, the chosen methodology for uncertainty propagation is the computationally expensive Total Monte-Carlo method. A modified build of the Serpent2 Monte Carlo transport code is used to perform Variance Reduction and correlated sampling. In addition, high resolution modelling of the experiments is used to limit the presence of bias in the analysis, and to pave the way toward the submission of a high-quality benchmark to an NEA database. This paper presents advances in the modelling of the experiments, focusing on the metal reflectors, as installed at the periphery of CROCUS. It was possible to refine from a simple design model made of a few cuboids, to a fully detailed model, while keeping the loss in computational efficiency below 15%. Notably, all reflector sheets of PETALE were detailed into 121 voxels each, based on topological measurements, without impact on the calculation time, thanks to the use of Serpent2 3D lattices

Dosimetry modeling and experimental validation for the PETALE program in the CROCUS reactor

The PETALE experimental program in the CROCUS reactor intends to provide integral measurements on reactivity worth and dosimetry measurement to constrain nuclear data relative to stainless steel heavy reflectors. The experimental setup consists in eight successive plates of pure iron, pure nickel, pure chromium, or nuclear-grade stainless steel set at the close periphery of the core. The plates are interleaved with up to nine dosimeters that consist of thin activation foils with different possible materials to be sensitive to different ranges of the neutron spectrum. A precise measurement with a good estimation of the uncertainties and correlations is required, especially when comparing reaction rates, e.g. transmission measurement and/or spectral indices