Monochromateur | EPFL Graph Search

Un monochromateur est un dispositif utilisé en optique pour sélectionner une gamme la plus étroite possible de longueurs d'onde à partir d'un faisceau lumineux de gamme de longueurs d'onde plus large. Le nom monochromateur est issu du grec mono (seul) et chroma (couleur). Bien que le principe soit radicalement différent, la fonction est similaire à celle des filtres utilisés pour sélectionner une fréquence d'un signal électromagnétique dans un récepteur radio. Principes Un dispositif permettant d'obtenir un faisceau lumineux monochromatique a de nombreuses applications en sciences, que ce soit en chimie, en physique ou en optique, car de nombreux phénomènes dépendent de la longueur d'onde de la lumière. Parmi les différents appareils utilisant des monochromateurs, on peut notamment citer les spectromètres utilisés pour des dosages ou de l'analyse chimique ou structurale. Pour produire une lumière monochromatique il est possible de réaliser une source monochromatique à la l

A new transmission based monochromator for energy-selective neutron imaging at the ICON beamline

Steven Luc X Peetermans

A new type of monochromator has been developed for energy-selective neutron imaging at continuous sources. It combines the use of a mechanical neutron velocity selector with pyrolytic graphite crystals of different mosaicity. The beam can be monochromatized to similar levels as a standard double crystal monochromator. It can flexibly produce different desired spectral shapes, even an asymmetric one Intrinsically, no higher order contamination of the spectrum is present. Working with the transmitted beam, the beam divergence (and thus the spatial resolution) is uncompromisecl. The device has been calibrated, characterized and its performance demonstrated with the measurement of Bragg edges for iron and lead, resolving them more sharply than if solely a mechanical velocity selector was used. (C) 2014 Elsevier B.V. All rights reserved

Elsevier Science Bv2014

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.

EPFL2015

Ultrafast Momentum-Resolved Free-Electron Probing of Optically Pumped Plasmon Thermal Dynamics

Fabrizio Carbone, Vahagn Mkhitaryan

Current advances in ultrafast electron microscopy make it possible to combine optical pumping of a nanostructure and electron beam probing with sub-Angstrom and femtosecond spatiotemporal resolution. We present a theory predicting that this technique can reveal a rich out-of-equilibrium dynamics of plasmon excitations in graphene and graphite samples. In a disruptive departure from the traditional probing of nanoscale excitations based on the identification of spectral features in the transmitted electrons, we show that the measurement of angle-resolved, energy-integrated inelastic electron scattering can trace the temporal evolution of plasmons in these structures and provide momentum-resolved mode identification, thus avoiding the need for highly monochromatic electron beams and the use of electron spectrometers. This previously unexplored approach to study the ultrafast dynamics of optical excitations can be of interest to understand and manipulate polaritons in 2D semiconductors and other materials exhibiting a strong thermo-optical response.

AMER CHEMICAL SOC2021

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

EPFL2015