1,721,251 research outputs found
The Harmonic Picture of Nuclear Mean Kinetic Energies in Heavy Water
Full text linkThis paper presents a study of the total mean kinetic energy, hEKi, and of
individual projections along a given molecular axis, hEKi ; for D and O nuclei in D2O, derived
using a harmonic model. Our theoretical approach assumes decoupling amongst translational,
rotational and vibrational modes. Resulting values of these dynamical quantities are discussed in
terms of the anisotropy of the quantum kinetic energy tensor, its relation to the local potential,
and deviations from the hypothesis of harmonicity and mode decoupling. Results are compared
with corresponding quantities obtained from Deep Inelastic Neutron Scattering experiments
performed on liquid and solid D2O, where the short-time dynamics and local environment of
D and O atoms were probed. The present study con rms an overall picture where even small
changes in the short-range environment of D and O nuclei have a strong in
uence on the quantum behaviour of heavy water
Neutron-resonance capture analysis on the VESUVIO spectrometer: Towards high-throughput material characterisation
No full textWe discuss the possibility to characterise nuclear quantum effects on the dynamics of heavy nuclei by means of neutron-resonance capture analysis on the VESUVIO spectrometer at ISIS. VESUVIO is equipped with yttrium-aluminium-perovskite gamma-sensitive detectors that can be used to record the Doppler-broadened line shape from a neutron-induced resonance in the neutron-energy range between 1 and 100 eV. The measurement of nuclear momentum distributions for heavy atoms using deep inelastic neutron scattering, the traditional technique for light-weight nuclei from hydrogen to fluorine, is currently severely limited by the resolution of the instrument. On the other hand, gamma-Dopplerimetry studies as the one presented here on gold allow for exquisite precision and short data-collection measurements, of the order of one hour, for the measurements, rendering the technique ideal for high-throughput investigations
Non-destructive Quantitation of Hydrogen via Mass-resolved Neutron Spectroscopy
No full textThis work introduces the use of mass-selective neutron spectroscopy as an analytical tool for the quantitative and non-destructive detection of hydrogen in bulk media. To this end, systematic measurements have been performed on a series of polyethylene standards of known thickness and density, in order to establish optimal data-acquisition protocols as well as associated limits of detection and quantitation. From this analysis, we conclude that state-of-the-art epithermal-neutron instrumentation enables the detection of aeral molar densities of bulk hydrogen in the mu mol cm(-2) range. We also discuss potential improvements on the horizon, with a view to broadening the scope of the technique across chemistry, materials science, and engineering
Nanocomposite materials as observed by mass-selective neutron spectroscopy
Full text linkThis work provides a current, critical view of the application of MAss-selective Neutron SpEctroscopy (MANSE) to nanocomposite materials. MANSE is a unique technique made possible owing to the existence of the pulsed neutron sources. At present, the only operating MANSE spectrometer in the world, VESUVIO, is located at the ISIS Neutron and Muon Source in the UK. We start by providing a brief description of the neutron Compton scattering, the anatomy of a mass-selective neutron spectrometer, and the experimental data treatment. We continue by briefly outlining the main quantum mechanical concepts, models and approximations relevant both to the ab initio prediction and experimental measurement of main MANSE observables. Next, we present several recent exemplars chosen to highlight the use of MANSE in the field of nanocomposites. Our examples include, in chronological order, encapsulated nanoparticles in amorphous silica gel, bioactive glass-ionomer cement, Cu-Ti-C composites, and sodium carboxymethyl starch-based binders in the presence of a mineral matrix. We close by providing our view of the ongoing and future challenges and opportunities in the mass-selective neutron investigation of NQEs in nanocomposite materials
Neutrons matter: VII international workshop on electron-Volt neutron spectroscopy – A preface to the workshop proceedings
Full text linkWe present here a collection of works reporting on the recent experimental and theoretical activities taking advantage of epithermal neutron spectroscopy, and in particular focusing on recent results presented during the VII International Workshop on Electron-Volt Neutron Spectroscopy held in Rome on 7-8 November 2018
The road to a station for epithermal and thermal neutron analysis
Full text linkDespite the large variety of research interests and themes motivating the current neutron research included in this collection, we have found common denominators characterising the manner in which the chosen research methodology tries to tackle the envisaged scientific questions. This article attempts to characterise those trends in current research with the aim of identifying the main mid-to-long term opportunities faced by electron-Volt neutron spectroscopy. The main realisation from this exercise is that the scientific community seems eager to combine neutron-based techniques over a broad energy range. To this end, the most natural choice seems to be to resort to neutron instruments where such capabilities are already present from the outset, with the most prominent example being the VESUVIO spectrometer at the ISIS pulsed neutron and muon source in the UK. However broad the operational basis of the existing neutron beamline infrastructure may be, progress, achievable only through further instrument upgrades, is the only way forward. The need to move forward is clearly seen within the community and is well documented by the research presented in this collection. This need for a substantial upgrade has crystallised in the form of a proposition to build a station rather than a conventional beamline, for Epithermal and Thermal Neutron Analysis station, hereafter ETNA
A McStas simulation of the incident neutron beam on the VESUVIO spectrometer
Full text linkWe present a Monte Carlo simulation of the incident neutron beam on the VESUVIO spectrometer at the ISIS Facility using the McStas code. As VESUVIO allows for concurrent measurements of neutron diffraction, neutron transmission, and deep inelastic neutron scattering, both incident and transmitted beams are characterized by a broad energy range, spanning over several orders of magnitude from fractions of meV to tens of keV. A transport simulation in the case of the VESUVIO spectrometer is a challenging task, for the McStas code has been traditionally applied to cold and thermal neutrons, and never used in the modelling of electron-volt neutron spectrometers, to the best of our knowledge. In this simulation study, we discuss the modelling of the collimation stages along the primary flight path so as to reproduce the absolute intensity of the incident neutron beam and its shape, both recently characterized experimentally. Finally, we show some preliminary results employing incoherent scattering samples so as to compare the epithermal component of the simulated backscattering spectra to experimental results from Pb. © Published under licence by IOP Publishing Ltd
Fractal dimension as a scaling law for nuclear quantum effects: a neutron Compton scattering study on carbon allotropes
No full textIn this work, we tackle the problem of the sensitivity of neutron Compton scattering, measured through the widths of nuclear momentum distributions, to the degree of complexity and ordering of the structural motif characterising the surrounding environment felt by a particular nucleus (carbon). In doing so, we replace the usual concept of the bond strength categorised in terms of its thermodynamical or electronic properties with a novel observable inspired by the language of mathematical topology, the Hausdorff-Besicovitch fractal dimension. We derive a relatively straightforward connection between the fractal dimension of a given system under consideration and the nuclear kinetic energy. To achieve this, we modify the concept of the energy equipartition theorem for solid-state systems composed of carbon atoms where the atom-ordering topology does not follow a simple two or three-dimensional order, but rather atoms are placed along curves in space that have an intermediate dimension related to the varying amounts of information they contain. A series of results from past neutron Compton scattering studies, as well as new results on Buckminsterfullerene (C-60), correlate with the topological measures of surface roughness and bending, as categorised quantitatively by the fractal dimension of the system. Namely, for the same formal chemical binding motif (sp(2) C) and with decreasing system dimensionality from nearly 3 towards 1, the quantum nature of the system becomes more pronounced. The simple scaling law developed in this work allows for relatively simple assessment of the nuclear "quantumness" of a given system with potentially important ramifications in the ab initio modelling of nuclear quantum effects in condensed matter
Nuclear kinetic energies from final-state effects in the harmonic limit
No full textThis work shows a simple, yet quite powerful, data pre-treatment protocol for mass-resolved neutron spectroscopy, aimed at a better estimation of the main quantum observable linked to a nuclear-momentum distribution, its second moment. From a methodological point of view, the immediate benefit of having such a protocol is twofold. Firstly, a good estimate of a second moment of a nuclear-momentum distribution, and hence nuclear kinetic energy, provided as input for subsequent data fitting, accelerates the convergence of a data fit and minimises the likelihood of it being stuck in a non-physical solution. Secondly, it provides a simple screening tool in the search for quantum systems exhibiting statistically significant departures for a classical behaviour of equipartition of kinetic energy at any given temperature. This second benefit renders the presented protocol an important data screening tool in the search of materials exhibiting exotic properties, possibly attributed to nuclear quantum effects
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