E. Guarini

A honeycomb collimator for the neutron Brillouin scattering spectrometer BRISP

A new collimator for thermal neutrons, based on the honeycomb design, has been produced in a few prototype versions. The method of construction is simple and the overall cost of the device is rather low. Development of this collimator has been stimulated by the technical specifications of the spectrometer BRISP for thermal neutron Brillouin scattering, presently under construction. The performances of the prototypes have been measured on the monochromatic beam of a steady source and exploiting the time-of-flight technique at a pulsed neutron source. The expected response of the device has been calculated by means of a Monte Carlo simulation of the experiments. This paper describes the method of construction and reports on the results of the neutron test measurements in comparison with the Monte Carlo simulation.

Brillouin spectroscopy of protein hydration water: new experimental potentialities opened up by the thermal neutron spectrometer BRISP

Neutron Brillouin spectroscopy is a powerful tool for studying the microscopic dynamics of soft matter. At the same time, it is an intrinsically difficult technique: for this reason the collective dynamics of some complex systems is still waiting for deeper investigations. An important such case is the study of coherent collective modes in protein hydration water, a subject of major interest for both soft-matter physics and protein science. Thanks to the capabilities of the new Brillouin spectrometer BRISP, which recently became operative at the Institut Laue-Langevin (Grenoble, France), we can present here the first reliable experimental determination of the neutron Brillouin spectrum of protein hydration water.

Structure of liquid n-hexane

The structure of deuterated liquid n-hexane has been investigated at room temperature by neutron diffraction and molecular dynamics simulations. By carrying out a careful analysis of the measurements, experimental data were obtained in very good agreement with the simulated data. This allowed a thorough analysis of the simulation results aiming at the evaluation of the partial, intra-, and intermolecular components of the n-hexane structure. We finally compare the intramolecular differential cross sections calculated from the most probable n-hexane molecular configurations with the measured and simulated data.

Intermolecular structure of liquid deuterium : a new neutron diffraction investigation

The static centre of mass structure factor of liquid deuterium, already determined by means of time of flight neutron diffraction (as reported in Physical Review E in 1993), has been re-investigated with a two-axis diffractometer at a reactor source. A detailed analysis of the data is presented, with special attention devoted to the correction for inelastic scattering effects. The comparison of the two sets of data reveals the presence of discrepancies, which cannot be attributed to statistical inaccuracies. The density and the temperature derivatives of the structure factor have also been measured with high accuracy. For these quantities, the two experiments are in good agreement with each other. This suggests that the discrepancy in the structure factor is due to systematic effects which vanish in a differential measurement.

Dynamic structure of He-Ne mixtures by molecular dynamics simulation: From hydrodynamic to fast and slow sound modes

Molecular dynamics (MD) results for the dynamic structure of a He(0.77)Ne(0.23) gas mixture at two densities (15.8 and 36.1 nm(-3)) show a clear crossover from hydrodynamic modes to distinct excitations for the two species. The higher density dispension curve neatly shows high- and low-frequency branches setting on with a rather localized transition. The lower density results agree very well with existing neutron scattering data and, in particular, display hydrodynamic behavior up to k approximately 2 nm(-1), in contrast with the conclusions of previous simulation studies. A smooth transition to fast sound is shown to take place for 2<k/nm(-1)<5, where the present MD data fill the existing gap in the experimental results.

The hydrogen-bond collective dynamics in liquid methanol

The relatively simple molecular structure of hydrogen-bonded (HB) systems is often belied by their exceptionally complex thermodynamic and microscopic behaviour. For this reason, after a thorough experimental, computational and theoretical scrutiny, the dynamics of molecules in HB systems still eludes a comprehensive understanding. Aiming at shedding some insight into this topic, we jointly used neutron Brillouin scattering and molecular dynamics simulations to probe the dynamics of a prototypical hydrogen-bonded alcohol, liquid methanol. The comparison with the most thoroughly investigated HB system, liquid water, pinpoints common behaviours of their THz microscopic dynamics, thereby providing additional information on the role of HB dynamics in these two systems. This study demonstrates that the dynamic behaviour of methanol is much richer than what so far known, and prompts us to establish striking analogies with the features of liquid and supercooled water. In particular, based on the strong differences between the structural properties of the two systems, our results suggest that the assignment of some dynamical properties to the tetrahedral character of water structure should be questioned. We finally highlight the similarities between the characteristic decay times of the time correlation function, as obtained from our data and the mean lifetime of hydrogen bond known in literature.

Evidence for the coexistence of two density fluctuation modes in molten Li30Bi70 as probed by neutron scattering

We report on a novel simultaneous observation of two inelastic doublets in the dynamic structure factor of the liquid alloy Li30Bi70. This result establishes the coexistence of two density fluctuation modes in a liquid binary system with disparate mass components. The measurements of the dynamic structure factor of the alloy have been performed on the hot neutron three- axis spectrometer IN1 of the Institut Laue-Langevin with an unprecedented energy resolution in the small wave vector domain. Analysis of the dynamical parameters characterizing the two excitations reveals the acoustic nature of the low-frequency mode and suggests an optic-like nature for the high-frequency mode. A comparison with recent neutron scattering results on pure liquid Bi suggests that the low-frequency mode in the alloy is closely related to the heavier component dynamics.

Velocity autocorrelation in liquid parahydrogen by quantum simulations for direct parameter-free computations of neutron cross sections

Accurate knowledge of the single-molecule (self) translational dynamics of liquid para-H2 is an essential requirement for the calculation of the neutron scattering properties of this important quantum liquid. We show that, by using Centroid Molecular Dynamics (CMD) quantum simulations of the velocity autocorrelation function, calculations of the total neutron cross section (TCS) remarkably agree with experimental data at the thermal and epithermal incident neutron energies where para-H2 dynamics is actually dominated by the self contributions. This result shows that a proper account of the quantum nature of the fluid, as provided by CMD, is a necessary and very effective condition to obtain the correct absolute-scale cross section values directly from first-principle computations of the double differential cross section, and without the need of introducing any empirically adjusted quantity. At subthermal incident energies, appropriate modeling of the para-H2 intermolecular (distinct) dynamics also becomes crucial, but quantum simulations are not yet able to cope with it. Existing simple models which account for the distinct part provide an appropriate correction of self-only calculations and bring the computed results in reasonable accord with TCS experimental data available until very recently. However, if just published cross section measurements in the cold range are considered, the agreement turns out to be by far superior and very satisfactory. The possible origin of slight residual differences will be commented and suggest further computational and experimental efforts. Nonetheless, the ability to reproduce the total cross section in the wide range between 1 and 900 meV represents an encouraging and important validation step of the CMD method and of the present simple algorithm.

The microscopic structure of the hydrogen liquids

We have measured the microscopic structure of liquid para-hydrogen by means of a neutron diffraction experiment on the D4C liquids diffractometer at Institute Laue-Langevin (Grenoble, France). This is the first direct neutron diffraction measurement of the static structure factor of hydrogen. The present determination of the microscopic structure of hydrogen is consistent with previous experimental determinations carried out on liquid deuterium and with path integral Monte Carlo simulations. The comparison with recent x-ray determinations is also satisfactory.

Optimizing the Setup of the BRISP Spectrometer by Upgraded McStas Simulations

Here we report on new results obtained by ad hoc developed McStas simulations of the time-of-flight thermal neutron Brillouin spectrometer BRISP, presently under construction at the ILL. In such a spectrometer a double-curvature focusing monochromator is combined with a multi-beam converging collimator. This configuration, first exploited for BRISP, has been carefully analyzed by developing new specific McStas codes which enable a reliable simulation of the real components, including the detailed features of the monochromator, the background disk-chopper and the honeycomb converging collimator. The present McStas calculations are also used as a guide to refine technical details, such as the optimal arrangement of the crystals on the monochromator surface as a function of the measured mosaic spread. The simulation results are consistent with the expected estimates of both intensity and resolution, and show that BRISP can provide new opportunities for advanced investigations of the low-Q dynamics of disorde...