1,721,220 research outputs found
Direct kinetic energy extraction from neutron Compton profiles
Full text linkDeep inelastic neutron scattering experiments provide access to atomic
momentum distributions and mean kinetic energies. These quantities are
intimately connected to nuclear quantum effects associated to the
equilibrium ground state of condensed systems. The method to derive the
single particle mean kinetic energy, directly employing the sum rules
associated to the scattering functions, from a set of deep inelastic
neutron scattering spectra is discussed. This method does not make use
of nonlinear fitting of the scattering spectr
Deep inelastic neutron scattering on Pb-207 and NaHF2 as a test of detectors array on the VESUVIO spectrometer
Full text linkA prototype array of resonance detectors for deep inelastic neutron
scattering experiments has been installed on the VESUVIO spectrometer,
at the ISIS spallation neutron source. Deep inelastic neutron scattering
measurements on a reference lead sample and on NaHF2 molecular system
are presented. Despite on an explorative level, the results obtained for
the values of mean kinetic energy < E-k > are found in good agreement
with the theoretical predictions, thus assessing the potential
capability of the device for a routine use on the instrument. (C) 2007
Elsevier B.V. All rights reserved
Electron volt neutron spectrometers
Full text linkThe advent of pulsed neutron sources has made available intense fluxes of epithermal neutrons (500 meV ≤ E≤100 eV). The possibility to open new investigations on condensed matter with eV neutron scattering techniques, is related to the development of methods, concepts and devices that drive, or are inspired by, emerging studies at this energy scale. Electron volt spectrometers have undergone continuous improvements since the construction of the first prototype instruments, but in the last decade major breakthroughs have been accomplished in terms of resolution and counting statistics, leading, for example, to the direct measurement of the proton 3-D Born-Oppenheimer potential in any material, or to quantitatively probe nuclear quantum effects in hydrogen bonded systems. This paper reports on the most effective methods and concepts for energy analysis and detection, as well as devices for the optimization of electron volt spectrometers for different applications. This is set in the context of the progress made up to date in instrument development. Starting from early stages of development of the technique, particular emphasis will be given to the Vesuvio eV spectrometer at the ISIS neutron source, the first spectrometer where extensive scientific, as well as research and development programmes have been carried out. The potential offered by this type of instrumentation, from single particle excitations to momentum distribution studies, is then put in perspective into the emerging fields of eV spectroscopy applied to cultural heritages and neutron irradiation effects in electronics
Quesiti e soluzioni di Fisica Generale
No full textIl testo contiene una raccolta di circa 700 quesiti di Fisica Generale a risposta multipla, e relative soluzioni accompagnate da brevi spiegazioni. Domande teoriche ed esercizi risolvibili con pochi passaggi logico-matematici per verificare la generale comprensione degli argomenti trattati nei corsi universitari di Fisica. La raccolta dei quesiti è suddivisa per grandi aree tematiche: Richiami di matematica, misure di grandezze fisiche, errori di misura, statistica basilare, elementi di analisi vettoriale, Cinematica, Dinamica ed elementi di Gravitazione, Energia e Lavoro, Fluidodinamica, Termodinamica, Elettromagnetismo, Principi di Ottica
Measurement of proton momentum distributions using a direct geometry instrument
Full text linkWe report the results of inelastic neutron scattering measurements on bulk water and ice using the direct geometry SEQUOIA chopper spectrometer at the Spallation Neutron Source (U.S.A.), with incident energy E-i = 6 eV. In this set up the measurements allow to access the Deep Inelastic Neutron Scattering regime. The scattering is centred at the proton recoil energy given by the impulse approximation, and the shape of the recoil peak conveys information on the proton momentum distribution in the system. The comparison with the performance of inverse geometry instruments, such as VESUVIO at the ISIS source (U.K.), shows that complementary information can be accessed by the use of direct and inverse geometry instruments. Analysis of the neutron Compton profiles shows that the proton kinetic energy in ice at 271 K is larger than in room temperature liquid water, in agreement with previous measurements on VESUVIO
VESUVIO : a novel instrument for performing spectroscopic studies in condensed matter with eV neutrons at the ISIS facility
No full textThe VESUVIO project aims to provide unique prototype instrumentation at the ISIS-pulsed neutron source and to establish a routine experimental and theoretical program in neutron scattering spectroscopy at eV energies. This instrumentation will be specifically designed for high momentum, , and energy transfer inelastic neutron scattering studies of microscopic dynamical processes in materials and will represent a unique facility for EU researchers. It will allow to derive single-particle kinetic energies and single-particle momentum distributions, n(p), providing additional and/or complementary information to other neutron inelastic spectroscopic techniques.\ud
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He-4 adsorbed in cylindrical silica nanopores: Effect of size on the single-atom mean kinetic energy
Full text linkThis paper reports a study of the short-time dynamics of helium confined in silica nanopores (xerogel powder), with average pore diameters of 24 and 160 Å. The longitudinal momentum distribution of helium adsorbed in xerogels has been determined via deep inelastic neutron scattering (DINS) measurements performed on the VESUVIO spectrometer at the ISIS spallation source. DINS measurements, in the attosecond time scale, (i.e., 10−16–10−15 s), were performed at a temperature of T=2.5 K and saturated vapor pressure conditions, with 95% pore volume filling. The average wave-vector transfer q was about 130 Å−1. For confined helium, significant changes in the values of the single-particle mean kinetic energies ⟨EK⟩ are found in the bulk phase. These are 32.6±8.7 K for the 24 Å and 24.4±5.3 K for the 160 Å pore diameters, remarkably higher than ⟨EK⟩=16.2±0.4 K, the value of normal liquid 4He at T=2.5 K and saturated vapor pressure conditions. The results are interpreted in terms of a model where 4He atoms are arranged in concentric annuli along the cylindrical pore axis, with ⟨EK⟩ mainly dependent on the ratio between the atomic “effective” diameter and the pore diameter. The number of solid layers close to pore surface is found to be strongly pore-size dependent with one single solid layer for 24 Å diameter pore and three solid layers for 160 Å diameter pore
Mean kinetic energy of helium atoms in fluid He-3 and He-3-He-4 mixtures
Full text linkMomentum distributions and mean kinetic energies of helium atoms, in pure
fluid 3He and 3He–4He mixtures, at T = 2 K and 3He concentrations of
x = 0.20 and x = 1.00, are presented. The experimental technique employed
is deep inelastic neutron scattering measurements in the eV energy range, with
wavevector transfer of typically 100 °A−1
< q < 250 °A−1. Single-particle
dynamical properties of 3He–4He mixtures are discussed in the context of
previous results on mixtures at different concentrations and pure 3He and 4He. In the pure fluids, the kinetic energy of 3He and 4He are remarkably similar
for molar volumes above 25 cm3 mol−1, while for smaller molar volumes, upon approaching the liquid–solid transition, the kinetic energy is larger in 3He
than 4He. On the other hand, the short-time dynamics of the helium mixture reveal quite a different picture with respect to the pure 3He and 4He: the momentum distribution and mean kinetic energy of the light helium component are independent of the molar volume and concentration
Molecular specificity in neutron imaging: the case of hydrogen adsorption in metal organic frameworks
No full textNeutron imaging is the technique of choice for a number of in situ and operando applications, where a high penetration power is required. White-beam neutron imaging and energy-resolved Bragg edge imaging are successful techniques, the former for the detection of specific elements characterized by strong neutron attenuation and the latter for studying crystal structures. Here we discuss the capabilities of energy-selective neutron imaging taking advantage of the incoherent and inelastic scattering interactions in hydrogenous materials, as a way to obtain molecular-specific information about the composition of a given sample. While few examples from the available literature are discussed, a worked example is presented based on new experimental data on molecular-hydrogen adsorption and conversion in the HKUST-1 metal organic framework
Chemometrics tools for Advanced Spectroscopic Analyses
Full text linkAny system is described by several variables, often in the form of hidden information, able to describe and explain functional mechanisms for the majority of the processes which can be evaluated analytically only when we consider entire complex datasets. The relationship between those variables is the key to identify and quantify correlations among the parameters describing the data in a strictly model-free manner. In chemometrics one uses mathematical and statistical methods to improve the understanding of chemical information through the correlation of physical parameters or properties to analytical instrument data. This approach is currently used across chemistry, materials science, biology, with a growing impact is the field of spectroscopy. This paper presents the ability of chemometric technique applied to Advanced Spectroscopic Analyses, examples include spectroscopic data collected from both the High- resolution neutron Spectrometer TOSCA, operating at the ISIS pulsed Neutron and Muon Source (UK) and X-ray fluorescence (XRF) spectroscopy. This work demonstrates the high-resolution of the Principal Component Analysis (PCA) to a spectroscopic data-set dealing with the determination of marker bands from Inelastic Neutron Scattering (INS) spectra of a large data- set, the presence of a probably additional transition phase of one globular molecule and evidencing the metallic nature of the black/brownish inscriptions on daily-use textiles used in ancient Egypt. This study will pave the way for the analysis of multi-parametric, high-throughput INS data, now within reach using state-of-the-art chemical neutron spectrometers such as VESPA
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