The eye of nuclear medicine

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High-pressure neutron diffraction apparatus for investigating the structure of liquids under hydrothermal conditions

A high-pressure setup is described for making neutron diffraction experiments on liquids under hydrothermal conditions. Designs are given for a modied Bridgman unsupported area seal, a fluid separator that keeps apart the liquid sample and pressurising fluid, and a pressure-cell made from the null-scattering alloy Ti0:676Zr0:324.Special attention is paid to the choice of construction materials used to avoid corrosion by the liquid sample under load at elevated temperatures. The apparatus is used to investigate the structure of heavy water at pressures up to 2 kbar and temperatures up to 250 degC

High-Pressure Transformation of SiO2 Glass from a Tetrahedral to an Octahedral Network:A Joint Approach Using Neutron Diffraction and Molecular Dynamics

International audienceA combination of in situ high-pressure neutron diffraction at pressures up to 17.5(5) GPa and moleculardynamics simulations employing a many-body interatomic potential model is used to investigate thestructure of cold-compressed silica glass. The simulations give a good account of the neutron diffractionresults and of existing x-ray diffraction results at pressures up to ∼60 GPa. On the basis of the moleculardynamics results, an atomistic model for densification is proposed in which rings are “zipped” by a pairingof five- and/or sixfold coordinated Si sites. The model gives an accurate description for the dependence ofthe mean primitive ring size hni on the mean Si-O coordination number, thereby linking a parameter that issensitive to ordering on multiple length scales to a readily measurable parameter that describes the localcoordination environment

Structure and properties of densified silica glass: characterizing the order within disorder

世界一構造秩序のあるガラスの合成と構造解析に成功 --ガラスの一見無秩序な構造の中に潜む秩序を抽出--. 京都大学プレスリリース. 2021-12-25.The broken symmetry in the atomic-scale ordering of glassy versus crystalline solids leads to a daunting challenge to provide suitable metrics for describing the order within disorder, especially on length scales beyond the nearest neighbor that are characterized by rich structural complexity. Here, we address this challenge for silica, a canonical network-forming glass, by using hot versus cold compression to (i) systematically increase the structural ordering after densification and (ii) prepare two glasses with the same high-density but contrasting structures. The structure was measured by high-energy X-ray and neutron diffraction, and atomistic models were generated that reproduce the experimental results. The vibrational and thermodynamic properties of the glasses were probed by using inelastic neutron scattering and calorimetry, respectively. Traditional measures of amorphous structures show relatively subtle changes upon compacting the glass. The method of persistent homology identifies, however, distinct features in the network topology that change as the initially open structure of the glass is collapsed. The results for the same high-density glasses show that the nature of structural disorder does impact the heat capacity and boson peak in the low-frequency dynamical spectra. Densification is discussed in terms of the loss of locally favored tetrahedral structures comprising oxygen-decorated SiSi4 tetrahedra

Dataset for "Structure of As-Se glasses by neutron diffraction with isotope substitution"

Data sets used to prepare Figures 1-7 in the Journal of Chemical Physics article entitled "Structure of As-Se glasses by neutron diffraction with isotope substitution." The data sets refer to the measured or modelled structure of As-Se glasses with compositions at or near to As_{0.30}Se_{0.70}, As_{0.35}Se_{0.65} and As_{0.40}Se_{0.60}. Figure 1 shows the total structure factors F(k) for as-prepared glassy (a) As_{0.30}Se_{0.70}, (b) As_{0.35}Se_{0.65} and (c) As_{0.40}Se_{0.60}. The points with vertical error bars show the measured functions and the solid curves show spline fits. The error bars are smaller than the line thickness at most k values. The GEM data sets extend to k_{max} = 40 A^{-1} but are shown over a smaller k-range for clarity of presentation. In (c) a comparison is made between the ^{nat}F(k) functions measured using D4c (black curve) versus GEM (red curve). Figure 2 shows the total pair-distribution functions G(r) for glassy (a) As_{0.30}Se_{0.70}, (b) As_{0.35}Se_{0.65} and (c) As_{0.40}Se_{0.60}. The broken curves show the Fourier transforms of the spline-fitted F(k) functions shown in Fig. 1. The solid curves show the same functions after the low-r oscillations have been set to the G(0) limit and the GEM data beyond the first peak have been smoothed by Fourier transforming F(k) after the application of a Lorch modification function with k_{max} = 40 A^{-1}. In (c) a comparison is made between the ^{nat}G(r) functions measured using D4c (black curves) versus GEM (red curves). Figure 3 shows the difference functions Delta F_{gamma}(k) for glassy (a) As_{0.30}Se_{0.70}, (b) As_{0.35}Se_{0.65} and (c) As_{0.40}Se_{0.60}. The points with vertical error bars show the measured functions and the solid curves show the back Fourier transforms of the Delta G_{gamma}(r) functions given by the solid curves in Fig. 4. The error bars are smaller than the line thickness at most k values. Figure 4 shows the difference functions Delta G_{gamma}(r) for glassy (a) As_{0.30}Se_{0.70}, (b) As_{0.35}Se_{0.65} and (c) As_{0.40}Se_{0.60}. The broken curves show the Fourier transforms of the spline-fitted Delta F_{gamma}(k) functions shown in Fig. 3. The solid curves show the same functions after the low-r oscillations have been set to the Delta G_{gamma}(0) limit and the data beyond the first peak have been smoothed by Fourier transforming Delta F_{gamma}(k) after the application of a Lorch modification function with k_{max} = 30 A^{-1} (GEM) or 23.45 A^{-1} (D4c). Figure 5 shows a comparison between the difference functions (a) Delta F_{Se}(k), (b) Delta F_X(k) and (c) Delta F_{As}(k) obtained from FPMD (solid red curves), AXS-RMC (broken blue curves) and neutron diffraction (solid black curves). In the AXS-RMC work, the difference functions do not extend beyond k_{max} = 11.4 A^{-1}, and the curves labelled As_{0.30}Se_{0.70} and As_{0.35}Se_{0.66} correspond to actual compositions of As_{0.29}Se_{0.71} and As_{0.33}Se_{0.67}, respectively. Several of the curves have been offset vertically for clarity of presentation and the magnitude of the offset is indicated in parenthesis. Figure 6 shows a comparison between the difference functions (a) Delta G_{Se}(r), (b) Delta G_X(r) and (c) Delta G_{As}(r) obtained from FPMD (solid red curves), AXS-RMC (broken blue curves) and neutron diffraction. In the AXS-RMC work, the curves labelled As_{0.30}Se_{0.70} and As_{0.35}Se_{0.66} correspond to actual compositions of As_{0.29}Se_{0.71} and As_{0.33}Se_{0.67}, respectively. Several of the curves have been offset vertically for clarity of presentation and the magnitude of the offset is indicated in parenthesis. Figure 7 shows differences between the measured coordination numbers bar{n} or bar{n}_{gamma} and those calculated using the CON (black markers) and RCN (red markers) models for glassy As_{0.30}Se_{0.70} (squares), As_{0.35}Se_{0.65} (circles) and As_{0.40}Se_{0.60} (triangles). The bar{n} values for the samples containing ^{nat}Se and ^{76}Se are denoted by bar{n}_{nat} and bar{n}_{76}, respectively, and are highlighted in yellow and blue, respectively. The bar{n}_{Se}, bar{n}_X and bar{n}_{As}$ values are highlighted in green, cyan and magenta, respectively.The data sets were collected using the methods described in the published paper.The data sets were analysed using the methods described in the published paper.Figures 1 - 6 were prepared using QtGrace (https://sourceforge.net/projects/qtgrace/). The data set corresponding to a plotted curve within an QtGrace file can be identified by clicking on that curve. Figure 7 was prepared using Origin (http://www.originlab.com/). The data set corresponding to a plotted curve within an Origin file can be identified by clicking on that curve.The files are labelled according to the corresponding figure numbers. The units for each axis are identified on the plots

Structure of As-Se glasses by neutron diffraction with isotope substitution

The method of neutron diffraction with selenium isotope substitution is used to measure the structure of glassy As0.30Se0.70, As0.35Se0.65 and As0.40Se0.60. The method delivers three difference functions for each sample in which either the As-As, As-Se or Se-Se correlations are eliminated. The measured coordination numbers are consistent with the "8-N" rule and show that the As0.30Se0.70network is chemically ordered, a composition near to which there is a minimum in the fragility index and a boundary to the intermediate phase. Chemical ordering in glassy As0.35Se0.65 and As0.40Se0.60 is, however, broken by the appearance of As-As bonds, the fraction of which increases with the arsenic content of the glass. For the As0.40Se0.60 material, a substantial fraction of As-As and Se-Se defect pairs (~11%) is frozen into the network structure on glass formation

Structure and dynamics of aqueous NaCl solutions at high temperatures and pressures

The structure of a concentrated solution of NaCl in D2O was investigated by in situ high-pressure neutron diffraction with chlorine isotope substitution to give site-specific information on the coordination environment of the chloride ion. A broad range of densities was explored by first increasing the temperature from 323 to 423 K at 0.1 kbar and then increasing the pressure from 0.1 to 33.8 kbar at 423 K, thus mapping a cyclic variation in the static dielectric constant of the pure solvent. The experimental work was complemented by molecular dynamics simulations using the TIP4P/2005 model for water, which were validated against the measured equation of state and diffraction results. Pressure-induced anion ordering is observed, which is accompanied by a dramatic increase in the Cl–O and O–O coordination numbers. With the aid of bond-distance resolved bond-angle maps, it is found that the increased coordination numbers do not originate from a sizable alteration to the number of either Cl⋯D–O or O⋯D–O hydrogen bonds but from the appearance of non-hydrogen-bonded configurations. Increased pressure leads to a marked decrease in the self-diffusion coefficients but has only a moderate effect on the ion–water residence times. Contact ion pairs are observed under all conditions, mostly in the form of charge-neutral NaCl0 units, and coexist with solvent-separated Na+–Na+ and Cl−–Cl− ion pairs. The exchange of water molecules with Na+ adopts a concerted mechanism under ambient conditions but becomes non-concerted as the state conditions are changed. Our findings are important for understanding the role of extreme conditions in geochemical processes