Large-scale quantum mechanical simulations of high-<Emphasis Type="Italic">Z</Emphasis> metals

High-Z metals constitute a particular challenge for large-scale ab initio electronic-structure calculations, as they require high resolution due to the presence of strongly localized states and require many eigenstates to be computed due to the large number of electrons and need to accurately resolve the Fermi surface. Here, we report recent findings on high-Z metals, using an efficient massively parallel planewave implementation on some of the largest computational architectures currently available. We discuss the particular architectures employed and methodological advances required to harness them effectively. We present a pair-correlation function for U, calculated using quantum molecular dynamics, and discuss relaxations of Pu atoms in the vicinity of defects in aged and alloyed Pu. We find that the self-irradiation associated with aging has a negligible effect on the compressibility of Pu relative to other factors such as alloying.     

Thermodynamics and the Elastic Moduli of Pu

Pu exhibits at least five structures below its melting point of 914 K. The lower-temperature phases have unusually low symmetry for an elemental metal, but at higher temperatures, Pu behaves like many other metals, going from fcc (face-centered cubic) to bcc (body-centered cubic) before melting. Electronic structure calculations appear to explain the lowest temperature monoclinic phase and the changes it undergoes with pressure (O. Eriksson, J. D. Baker, A. V. Balatsky, and J. M. Wills, J. Alloys Comp. 287, 1 (1999)), but the much simpler fcc phase of Pu remains difficult to understand ab initio. We show how the unusually small values of elastic moduli require that the effects of temperature and phonon entropy be included in any theory of fcc Pu, and we present new high-accuracy data for elastic moduli, that show that the elastic moduli of Ga-stabilized fcc Pu do not depend on Ga at 300 K, and that polycrystal and monocrystal elastic modulus data for Pu are in agreement.     

Measurements and modeling of linear and nonlinear effects in striplines

We present measurements of surface impedanceZ _S as a function of frequency, temperature, and rf magnetic field for high-quality epitaxial YBa₂Cu₃O_7–x thin films using a striplineresonator technique that measures theQ of the resonator vs. input power. The results have been modeled using Ginzburg-Landau theory at moderate fields, and the Bean critical state model at high fields. Good agreement has been obtained between the model and the measurements. We also discuss the current distributions in the stripline for both low power whenZ _S is linear and high power whenZ _S is nonlinear.     

Imaging of high-Z material for nuclear contraband detection with a minimal prototype of a muon tomography station based on GEM detectors

Muon Tomography based on the measurement of multiple scattering of atmospheric cosmic ray muons in matter is a promising technique for detecting heavily shielded high-Z radioactive materials (U, Pu) in cargo or vehicles. The technique uses the deflection of cosmic ray muons in matter to perform tomographic imaging of high-Z material inside a probed volume. A Muon Tomography Station (MTS) requires position-sensitive detectors with high spatial resolution for optimal tracking of incoming and outgoing cosmic ray muons. Micro Pattern Gaseous Detector (MPGD) technologies such as Gas Electron Multiplier (GEM) detectors are excellent candidates for this application. We have built and operated a minimal MTS prototype based on 30 cm×30 cm GEM detectors for probing targets with various Z values inside the MTS volume. We report the first successful detection and imaging of medium-Z and high-Z targets of small volumes (∼0.03 L) using GEM-based Muon Tomography.     

Antiferromagnetic-type spin fluctuations in actinide systems: Resistivity and magnetic susceptibility at high temperatures

A simple model describing antiferromagnetic-type spin fluctuations is developed to discuss the resistivity and magnetic susceptibility of actinide systems at high temperatures. It is found that the resistivity goes through a maximum and saturates at high temperatures, independently of the nature of the spin fluctuations. On the other hand, the magnetic susceptibility may be roughly temperature independent in the case of antiferromagnetic spin fluctuations, while it decreases rapidly in the ferromagnetic case. The first case applies to Np and Pu metals and some compounds, while the second case applies to compounds such as UAl ₂ , PuAl ₂ , NpRh ₃ , and USn ₃ .Laboratory associated with CNRS.     

Application of electrochemical methods to aqueous reprocessing of spent nuclear fuel

Published data demonstrating the possibilities of electrochemical methods as applied to solving particular problems arising in various steps of industrial reprocessing of spent nuclear fuel using the Purex process are summarized. Attention is given to stabilization of U, Np, and Pu in required valence states in aqueous solutions and two-phase systems in the course of Pu stripping in the step of its separation from U, to efficient dissolution of PuO₂, to isolation of noble metals from high-level liquid waste, and to breakdown of spent organic solvents and complexing agents. Progress in the industrial use of electrolysis, in search for new electrode materials, and in studies of the mechanisms of electrocatalytic oxidation processes is noted.     

Band-limited image plane masks for the Terrestrial Planet Finder coronagraph: materials and designs for broadband performance

Coronagraphs for detection and characterization of exosolar earthlike planets require accurate masks with broadband performance in the visible and near infrared spectrum. Design and fabrication of image plane masks capable of suppressing broadband starlight to 10(-10) level contrast presents technical challenges. We discuss basic approaches, material choices, designs, and fabrication options for image plane masks with particular focus on material properties to obtain adequate spectral performance. Based on theoretical analysis, we show that metals such as Pt and Ni, and alloys such as Inconel, may be employed as promising mask materials that can meet broadband performance requirements.     

Disproportionation of Pu(V) in aqueous HCOOH solutions

The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH-H₂O system was studied by spectrophotometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO₄ changes on adding HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at 830.6 nm, corresponding to an f-f transition in the Pu₂²⁺ aqua ion, a new band arises with the maximum shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1: 1). The Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO₄ solution, which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately proportional to [Pu(V)]² and grows with an increase in [HCOOH]. The reaction products affect the reaction rate: Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cationcation complex. The disproportionation occurs via formation of a Pu(V)-Pu(V) cation-cation complex whose thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).     

Development of Decay Energy Spectroscopy for Radionuclide Analysis Using Cryogenic 4π Measurements

We report the recent progress in the development of decay energy spectroscopy for radionuclide analysis using a metallic magnetic calorimeter. In the present analysis, sample radionuclides were completely enclosed by a 4π steradian absorber. The use of a 4π absorber composed of gold foil guarantees that the total energy associated with radioactive decay is converted into thermal energy in the absorber. A paramagnetic temperature sensor was attached to the absorber to accurately measure the temperature change due to radioactive decay. The plutonium isotopes ²³⁸Pu, ²³⁹Pu, and ²⁴⁰Pu were readily identified in the decay energy spectrum because each isotope creates a single peak at its characteristic Q value. Two clear peaks were observed for ²³⁹Pu and ²⁴⁰Pu, and a 6.3?keV FWHM was obtained. The energy resolution of the method was affected by the low-energy tail of the spectrum at the left-hand side of the peaks. A 4.1?keV FWHM of a Gaussian fit was obtained for the right-hand side of the peak. Slow heat release to the absorber due to heat flow mechanisms is discussed to explain the low-energy tailing effect.     

Electronic Checkerboard Pattern in Striped Racetrack Domains: A Consistent Picture of Recent Neutron and STM Experiments

We discuss recent elastic neutron scattering and scanning tunneling experiments on high-T _c cuprates exposed to an applied magnetic field. In particular we show that a physical picture consisting of antiferromagnetic vortex cores operating as pinning centers for surrounding stripes is qualitatively consistent with the neutron data provided the stripes have the usual antiphase modulation. Further, we calculate the electronic structure in such a region using a T-matrix method, and find a checkerboard interference pattern consistent with recent scanning tunneling experiments.     

Quantum diffusion in solids

The lectures are devoted to the problems connected with the tunneling motion of particles in a solid in the presence of strong interaction with phonon and electron excitations. We examine the quantum diffusion in insulators, metals, and superconductors. The interaction with the barrier fluctuations as well as the intrawell interaction are taken into account. Special attention is paid to the role of adiabaticity, playing an important part in tunneling phenomena, revealing the decisive influence of nonadiabatic excitations. We consider the entire temperature interval: coherent diffusion at low T, suppression of the coherent phase correlations and the transition to the incoherent quantum diffusion, and then with the further increase in T the crossover to the tunneling diffusion induced by excitations. The detailed analysis of the quantum diffusion in imperfect crystals and the problems of localization, in particular, caused by the interparticle interaction is given. We discuss the wide spectrum of experimental results displaying the quantum diffusion in different systems.     

Concentration dependence of the charge of atoms adsorbed on single-sheet graphene

Simple schemes for calculating the dependences of the charge Z _a of adatoms on their coverage of a single graphene sheet are developed with allowance for the dipole-dipole repulsion. The proposed schemes are applied to the adsorption of alkali metals and atomic hydrogen. It is established that an increase in the adsorption coverage leads to a decrease in the adatom charge, which concerns both the band and local contributions to this value.     

Statistical Performance of a Control Chart for Individual Observations Monitoring the Ratio of Two Normal Variables

Statistical Process Control monitoring of the ratio Z of two normal variables X and Y has received too little attention in quality control literature. Several applications dealing with monitoring the ratio Z can be found in the industrial sector, when quality control of products consisting of several raw materials calls for monitoring their proportions (ratios) within a product. Tables about the statistical performance of these charts are still not available. This paper investigates the statistical performance of a Phase II Shewhart control chart monitoring the ratio of two normal variables in the case of individual observations. The obtained results show that the performance of the proposed chart is a function of the distribution parameters of the two normal variables. In particular, the Shewhart chart monitoring the ratio Z outperforms the (p = 2) multivariate T² control chart when a process shift affects the in‐control mean of X or, alternatively, of Y and the correlation among X and Y is high and when the in‐control means of X and Y shift contemporarily to opposite directions. The sensitivity of the proposed chart to a shift of the in‐control dispersion has been investigated, too. We also show that the standardization of the two variables before computing their ratio is not a good practice due to a significant loss in the chart's statistical performance. An illustrative example from the food industry details the implementation of the ratio control chart. Copyright © 2013 John Wiley & Sons, Ltd.     

Electronic and Structural Engineering of Atomically Dispersed Isolated Single-Atom and Alloy Architectures

Precise configurations of isolated metal atoms in nitrogen-doped carbon materials with 2D single or multilayers and 3D nanoarchitectures are gaining attention owing to their good stability and activity at high current densities. Atomic metal–N_x moieties, which utilize maximum atoms to attain high intrinsic activity and novel electronic architecture of support materials, facilitate strong interaction between the central metal atom and support matrix. However, resource consumption is considerably high due to the inferior atomic utilization of active sites. Therefore, energy-efficient electrochemical processes are needed to develop advanced isolated single-atom architecture, which would provide high atom-utilization and good durability. Herein, the concepts of atomically dispersed metal sites in single-atom and alloy architectures and their electronic features associated with structural evolution are discussed. Opportunities and challenges associated with the use of isolated single-atoms in 2D materials are discussed based on their unique electronic defects, low-valence central metals, mechanical flexibility, and maximum access to metal sites. This insightful revisit into the engineering of single-atom and alloy architectures would provide a profound understanding of electronic modulations and regulation of geometric characteristics, and unravels potential directions for electrochemical energy conversion, charge storage, and sensing processes.     

Book Reviews–10-Year Index 1959–1968

The dependence structure of a stationary time series {X_t } is usually discussed in terms of the observed variables X_t . Let F(x) be the common marginal c.d.f. of the X_t We propose a model involving only the transformed series {Z_t }, where Z_t = F(X_t ). The conditional distribution of Z_t given Z _t–1, Z _t–2, … is expressed in terms of Z_t , Z _t1, … and parameters β₁, β₂, …. The case where Z_t depends only on Z _t–1 is considered in detail. Estimators for the β's are found, and an example using a normal Markov process is given.     

Statistical properties of a 2D granular material subjected to cyclic shear

This work focuses on the evolution of structure and stress for an experimental system of 2D photoelastic particles that is subjected to multiple cycles of pure shear. Throughout this process, we determine the contact network and the contact forces using particle tracking and photoelastic techniques. These data yield the fabric and stress tensors and the distributions of contact forces in the normal and tangential directions. We then find that there is, to a reasonable approximation, a functional relation between the system pressure, P, and the mean contact number, Z. This relationship applies to the shear stress τ, except for the strains in the immediate vicinity of the contact network reversal. By contrast, quantities such as P, τ and Z are strongly hysteretic functions of the strain, ε. We find that the distributions of normal and tangential forces, when expressed in terms of the appropriate means, are essentially independent of strain. We close by analyzing a subset of shear data in terms of strong and weak force networks.     

The Critical Temperature of Aluminum

The critical temperature, T _c, of metals is a fundamental point when vaporization due to high energy exchanges occurs. Although aluminum is a metal often studied as a benchmark for theories, its critical temperature is not known with a high degree of accuracy. Its determination by experiment is difficult as a result of its high value. This paper reviews the existing data and proposes new ones resulting from recent measurements of particular physical properties and recent theoretical approaches. These new estimates lead to the recommended value of T _c = (6700 ± 800) K.     

3He Transverse Excited State Occupation in Thin 3He-4He Films

The discovery that ³ He was occupying transverse excited states at submonolayer coverages in ³ He-⁴ He mixture films on a Nuclepore substrate, was a surprise. In this note we discuss the relationship between theory and experiment in attempting to understand the physics of this behavior. We first discuss various single-atom-limit calculations of the level spacing between the ground-state and first excited state. We then introduce a free, quasi-particle picture for analyzing experimental magnetization step data and compare those results with the single-atom-limit calculations. The experiments clearly show excited state occupation at submonolayer coverages in contradistinction with the calculations. We then briefly discuss a microscopic, semi-phenomenological theory which, in agreement with experiment, yields ³ He occupation of the first excited state at submonolayer coverages. The mechanism is a model ³ He-³ He effective interaction due to one ripplon exchange. This effective interaction is density dependent and very long ranged. It strongly modifies the small-k properties of the ³ He self-energy and, in particular, causes the ground-state to first excited state level spacing to decrease with increasing ³ He areal density.     

Model Assessment of the Migration Capability of Transuranium Radionuclides (<Superscript>241</Superscript>Am and Pu Isotopes) and <Superscript>152</Superscript>Eu in the System Bottom Sediments-Yenisei River Water by Chemical Fractionation Technique

Sorption of radiotracers ²⁴¹Am and ²⁴²Pu and weighable amounts of uranium and stable Eu on bottom sediments in the simulated system bottom sediments—Yenisei river water was studied. At a contact time of 40 min these metals are completely sorbed on the bottom sediments, presumably via association with organomineral complexes and hydrated gels. Along with sorption, these metals can form soluble complexes and stable colloids (pseudocolloids). The distribution factors of ²⁴¹Am and ²⁴²Pu, and stable Eu in the simulated system between bottom sediments and liquid phases were calculated from the results of sorption experiments. The introduced ²⁴¹Am and ¹⁵²Eu initially present in radioactively contaminated bottom sediments of the Yenisei river show similar pattern of distribution over differently mobile fractions. The distribution of weighable amounts of stable Eu significantly differs from that of initially present ¹⁵²Eu and tracer ²⁴¹Am due to decelerated sorption of stable Eu and its considerably higher concentration. A significant part of ²⁴²Pu, ²⁴¹Am, and ¹⁵²Eu is associated with mobile fractions of bottom sediments, which, under certain environmental conditions, can pass into river water as migrating species.__________Translated from Radiokhimiya, Vol. 47, No. 4, 2005, pp. 379–384.Original Russian Text Copyright © 2005 by Bondareva, Bolsunovskii, Sukhorukov, Kazbanov, Makarova, Legler.     

Sorption of Pu(IV) in the polymeric colloidal form on rock typical of Mayak Production Association area

Sorption of colloids of polymeric Pu from simulated groundwater on a rock typical of Mayak Production Association area was studied. In 20 days, polymeric Pu with the particle size exceeding 220 nm is 99% sorbed by the rock with the distribution coefficient K _d = 1880. Desorption performed for 5 days allows no more than 40% of the sorbed Pu to be transferred into the solution, even with such strong complexing agents as 0.05 M hydroxyethylidenediphosphonic acid in 0.1 M HNO₃ and 0.1 M Tamm solution.