A. De Francesco

Magnetic properties and 57Fe Mössbauer spectroscopy of Mediterranean prehistoric obsidians for provenance studies

Abstract Samples from the Mediterranean source-islands: Gyali, Lipari, Melos, Palmarola, Pantelleria and Sardinia (Monte Arci) had their 57Fe Mossbauer spectra and magnetizations measured. The saturation magnetization (Ms) depends on the island of provenance and it reaches a maximum ∼0.3 emu/g for Palmarola obsidians, indicating a larger magnetic phase content. The coercive forces (Hc) range from 46 to 372 Oe for respectively samples from Pantelleria and Palmarola islands. Our analyses show that in a Mr/Ms vs. Hc/Hm plot the data points accumulate in areas that depend on obsidian provenance (Mr, remanent magnetization; Hm, maximum applied field). The Mossbauer spectra are mainly composed of broad asymmetric doublets, which were fitted assuming two Fe2+ and one Fe3+ sites. In addition, the obsidians of Melos and Palmarola present a magnetic component attributed to magnetite and/or hematite. Binary diagrams comparing different Mossbauer hyperfine parameters for the Fe3+ and Fe2+ species gave indications of similarities and differences between obsidians of different origins. Since this kind of plot does not give an unambiguous characterization of samples from each island, a detailed analysis of the Mossbauer results is in progress using statistical method of clustering analysis as has been shown by Scorzelli et al. [Comptes Rendus Acad. Sci., Serie II, 332 (2001) 769] and Poupeau et al. [IAOS Bull. 28 (2001) 2] in previous studies.

Collective Dynamics of Intracellular Water in Living Cells

Water dynamics plays a fundamental role for the fulfillment of biological functions in living organisms. Decades of hydrated protein powder studies have revealed the peculiar dynamical properties of hydration water with respect to pure water, due to close coupling interactions with the macromolecule. In such a framework, we have studied coherent collective dynamics in protein and DNA hydration water. State-of-the-art neutron instrumentation has allowed us to observe the propagation of coherent density fluctuations within the hydration shell of the biomolecules. The corresponding dispersion curves resulted to be only slightly affected by the coupling with the macromolecules. Nevertheless, the effects of the interaction appeared as a marked increase of the mode damping factors, which suggested a destructuring of the water hydrogen-bond network. Such results were interpreted as the signature of a glassy dynamical character of macromolecule hydration water, in agreement with indications from measurements of the density of vibrational states. Extending the investigations to living organisms at physiological conditions, we present here an in-vivo study of collective dynamics of intracellular water in Escherichia coli cells. The cells and water were fully deuterated to minimise the incoherent neutron scattering background. The water dynamics observed in the living cells is discussed in terms of the dynamics of pure bulk water and that of hydration water measured in powder samples.

Density of states from mode expansion of the self-dynamic structure factor of a liquid metal

We show that by exploiting multi-Lorentzian fits of the self-dynamic structure factor at various wave vectors it is possible to carefully perform the Q→0 extrapolation required to determine the spectrum Z(ω) of the velocity autocorrelation function of a liquid. The smooth Q dependence of the fit parameters makes their extrapolation to Q=0 a simple procedure from which Z(ω) becomes computable, with the great advantage of solving the problems related to resolution broadening of either experimental or simulated self-spectra. Determination of a single-particle property like the spectrum of the velocity autocorrelation function turns out to be crucial to understanding the whole dynamics of the liquid. In fact, we demonstrate a clear link between the collective mode frequencies and the shape of the frequency distribution Z(ω). In the specific case considered in this work, i.e., liquid Au, analysis of Z(ω) revealed the presence, along with propagating sound waves, of lower frequency modes that were not observed before by means of dynamic structure factor measurements. By exploiting ab initio simulations for this liquid metal we could also calculate the transverse current-current correlation spectra and clearly identify the transverse nature of the above mentioned less energetic modes. Evidence of propagating transverse excitations has actually been reported in various works in the recent literature. However, in some cases, like the present one, these modes are difficult to detect in density fluctuation spectra. We show here that the analysis of the single-particle dynamics is able to unveil their presence in a very effective way. The properties here shown to characterize Z(ω), and the information in it contained therefore allow us to identify it with the density of states (DoS) of the liquid. We demonstrate that only nonhydrodynamic modes contribute to the DoS, thus establishing its purely microscopic origin. Finally, as a by-product of this work, we provide our estimate of the self-diffusion coefficient of liquid gold just above melting.

BRISP: The New Brillouin Spectrometer at ILL's High Flux Reactor

The new Time-of-Flight (TOF) Brillouin Spectrometer (BRISP) for inelastic neutron scattering has been developed and constructed at ILLs High Flux Reactor (HFR) for investigating a host of open scientific problems in the low-momentum region and relatively high incoming neutron energies with a dedicated instrument [1]. It studies the high-frequency dynamics of atomic nuclei and electron spins in disordered systems, ranging from liquids to compressed gases, including magnetic and biological systems.

A high-flux upgrade for the BRISP spectrometer at ILL

To date, the BRISP spectrometer represents the state-of-the-art for every instrument aiming to perform Brillouin neutron scattering. Exploiting accurate ray-tracing McStas simulations, we investigate an improved configuration of the BRISP primary spectrometer to provide a higher flux at the sample position, while preserving all the present capabilities of the instrument. This configuration is based on a neutron guide system and is designed to fit the instrument platform with no modifications of the secondary spectrometer. These evaluations show that this setup can achieve a flux gain factor ranging from 3 to 6, depending on the wavelength. This can expand the experimental possibilities of BRISP towards smaller samples, possibly using also complex sample environments.

Bayesian approach to the analysis of neutron Brillouin scattering data on liquid metals

When the dynamics of liquids and disordered systems at mesoscopic level is investigated by means of inelastic scattering (e.g., neutron or x ray), spectra are often characterized by a poor definition of the excitation lines and spectroscopic features in general and one important issue is to establish how many of these lines need to be included in the modeling function and to estimate their parameters. Furthermore, when strongly damped excitations are present, commonly used and widespread fitting algorithms are particularly affected by the choice of initial values of the parameters. An inadequate choice may lead to an inefficient exploration of the parameter space, resulting in the algorithm getting stuck in a local minimum. In this paper, we present a Bayesian approach to the analysis of neutron Brillouin scattering data in which the number of excitation lines is treated as unknown and estimated along with the other model parameters. We propose a joint estimation procedure based on a reversible-jump Markov chain Monte Carlo algorithm, which efficiently explores the parameter space, producing a probabilistic measure to quantify the uncertainty on the number of excitation lines as well as reliable parameter estimates. The method proposed could turn out of great importance in extracting physical information from experimental data, especially when the detection of spectral features is complicated not only because of the properties of the sample, but also because of the limited instrumental resolution and count statistics. The approach is tested on generated data set and then applied to real experimental spectra of neutron Brillouin scattering from a liquid metal, previously analyzed in a more traditional way.

Toward the commissioning phase of the BRIllouin spectrometer BRISP

The construction project of the novel thermal neutron Brillouin spectrometer (BRISP) is entering its commissioning period. After a brief review of the scientific motivation and the technical components of the instrument, we present here a historical overview of the installation steps that have gradually brought such a difficult project into the present important phase.