Atomic Number Distribution Research Articles

Molecular Dynamics Simulation of a Core−Shell Structured Metallic Nanoparticle

This study investigates the heating and cooling of a shell−core structured Ni−Al functional particle through a molecular dynamics simulation. The functional nanoparitcle is constructed from their perfect crystal measuring 5.6 nm in diameter with a shell thickness of 0.5 nm. The simulation is carried out within a Nose−Hoover thermostat scheme of the NVT canonical ensemble and performed by an Embedded-Atom-Method (EAM) force field. The thermodynamic properties and structure evolution during continuous heating and cooling processes are investigated through the characterization of the gyration radius, radial distribution function (RDF), atom number distribution, mean-square-distance (MSD), and layered potential energy distribution. Some unique behaviors related to nanometer scale functional particles are identified that include two-way diffusion of aluminum and nickel atoms, enhanced melting of the aluminum core through the shell structure, glass phase formation for the fast cooling rate, and the Ll2 Ni3Al ph...

Direct Observation of a Sub-Poissonian Number Distribution of Atoms in an Optical Lattice

We report single-site resolution in a lattice with tunneling between sites, allowing for an in situ study of stochastic losses. The ratio of the loss rate to the tunneling rate is seen to determine the number fluctuations, and the overall profile of the lattice. Sub-Poissonian number fluctuations are observed. Deriving the lattice beams from a microlens array results in perfect relative stability between beams.

Recent Developments and Applications of Radiation/Detection Technology in Tsinghua University

Nuclear technology applications have been very important research fields in Tsinghua University (THU) for more than 50 years. This paper describes two major directions and related projects running in THU concerning nuclear technology applications for radiation imaging and nuclear technology applications for astrophysics. Radiation imaging is a significant application of nuclear technology for all kinds of real world needs including security inspections, anti-smuggling operations, and medicine. The current improved imaging systems give much higher quality radiation images. THU has produced accelerating tubes for both industrial and medical accelerators with energy levels ranging from 2.5 ∼ 20 Mev . Detectors have been produced for medical and industrial imaging as well as for high energy physics experiments such as the MRPC with fast time and position resolutions. DR and CT systems for radiation imaging systems have been continuously improved with new system designs and improved algorithms for image reconstruction and processing. Two important new key initiatives are the dual-energy radiography and dual-energy CT systems. Dual-energy CT imaging improves material discrimination by providing both the electron density and the atomic number distribution of scanned objects. Finally, this paper also introduces recent developments related to the hard X-ray modulation telescope (HXMT) provided by THU.

Local icosahedral order and thermodynamics of simulated amorphous Fe

Abstract Local icosahedral order and thermodynamics of amorphous Fe have been analyzed in detail for models containing 3000 atoms, which were obtained by the molecular dynamics (MD) method. Models were obtained by cooling from the melt. Local order in models has been analyzed by using the technique proposed by Honeycutt and Andersen; we found an existence of icosahedral order in the system. Moreover, structural properties of models were also studied via radial distribution function (RDF), static structure factor, mean atomic distances, coordination number and bond-angle distributions. Glass transition temperature, heat capacity and potential energy of the system were found in addition to the evolution of structure and mean-squared displacement (MSD) of atoms upon cooling from the melt toward the glassy state. We found the glass transition temperature of simulated liquid Fe via temperature dependence of potential energy and it is close to that observed previously in the literature, i.e. T g ≈1070 K. Calculations showed that structural properties of amorphous Fe models with the Pak–Doyama interatomic potential agreed well with the experimental data.

Molecular dynamics simulations of simple monatomic amorphous nanoparticles

Monatomic amorphous nanoparticles were studied in a spherical model containing different numbers of atoms ranging from 1189 to 9093 by using molecular dynamics method under nonperiodic boundary conditions. We use the double-well interaction pair potential developed by Engel and Trebin, and amorphous nanoparticles were obtained by cooling from the melts. The structural properties of nanoparticles were studied via radial distribution function, mean atomic distances, coordination number, and bond-angle distributions. In addition, we also analyzed local order in nanoparticles by using the technique proposed by Honeycutt and Andersen, and we found the existence of icosahedral order in the system. We found strong size effects on the static properties of nanoparticles. Aging effects on the structure of nanoparticles were also observed and discussed. The radial atomic density profile of nanoparticles was found and discussed. On the other hand, the surface and core structures of nanoparticles were studied in detail. Moreover, we found the size dependence of several quantities such as the glass transition temperature $({T}_{g})$, the potential energy, and surface energies of nanoparticles. The mean-squared displacement of atoms was discussed.

Cavity QED with cold atoms trapped in a double-well potential

We investigate the interplay dynamics of a cavity QED system, where the two-level atoms are trapped in a double-well potential, and the cavity mode, with a frequency largely detuned from the atomic level splitting, is driven by a probe laser. The interaction between the center-of-mass motion of the atoms and the cavity mode is induced by the position-dependent atom-field coupling. The dynamics of the system is characterized by two distinct time scales, the inverse of the atomic interwell tunneling rate and the inverse of the cavity loss rate. The system shows drastically different (quasi)steady behaviors in the short- and long-time intervals, and the detection of the statistics of atom number distribution from the transmission spectra is available only in the short-time interval.

Structure study of multi-component borosilicate glasses from high-Q neutron diffraction measurement and RMC modeling

A neutron diffraction structure study has been performed on multi-component borosilicate glasses with compositions (65−x)SiO2·xB2O3·25Na2O·5BaO·5ZrO2, x=5–15mol%. The structure factor has been measured up to a rather high momentum transfer value of 30Å−1, which made high r-space resolution available for real space analyses. Reverse Monte Carlo simulation was applied to calculate the partial atomic pair correlation functions, nearest neighbor atomic distances and coordination number distributions. The Si–O network consists of tetrahedral SiO4 units with characteristic first neighbor distances at rSi–O=1.60Å and rSi–Si=3.0Å. The boron environment contains two well-resolved B–O distances at 1.40 and 1.60Å and both 3- and 4-fold coordinated B atoms are present. A chemically mixed network structure is proposed including [4]B–O–[4]Si and [3]B–O–[4]Si chain segments. The O–O and Na–O distributions suggest partial segregation of silicon and boron rich regions. The highly effective ability of Zr to stabilize glassy and hydrolytic properties of sodium-borosilicate materials is interpreted by the network-forming role of Zr ions.

The ground state solutions of two-component Bose-Einstein condensates in Bessel optical lattices

The ground state solutions of two-component Bose-Einstein condensates (BECs) in Bessel optical lattices (BLs) are studied by means of the balance condition between BLs strength and inter-atomic interactions. We consider a quasi-two-dimensional (2D) BECs system，strongly confined in longitudinal direction and weakly trapped in the radial direction in the transverse plane，which obeys 2D Gross-Pitaevskii equation (GPE) derived from its 3D counterpart. Analytically we obtained the atom number distribution，atom number and energy of ground state and give the parameter ranges. Compared to single-component BEC，two-component BECs exhibit a rich variety of ground state structures. These structures depend upon various parameters，in particular the inter-atomic interactions and the BLs strength. Neglecting the inter-component atomic interaction，the corresponding results of single-component situation are given.

Finite Size Effects on Static and Dynamic Properties of Non-periodic Boundary Condition Supercooled Liquids

Finite size effects on static and dynamic properties of Al 2 O 3 ·2SiO 2 spherical models with different diameters of 2, 3, 4, and 5 nm have been studied via molecular dynamics (MD) simulation under non-periodic boundary conditions (NPBC). Models have been obtained by cooling from the melts at constant density of 2.60 g/cm 3 . We found strong finite size effects on both static and dynamic properties of the system. Static properties have been studied via partial radial distribution function (PRDF), mean atomic distances and coordination number distributions. Dynamic properties of the system with different sizes have been found and discussed via the mean-squared displacement (MSD) of atoms and temperature dependence of diffusion constant. Moreover, size dependence of glass transition temperature has been found and discussed.

Scattered X-ray radiation in diagnosis of mammary gland diseases

The possibility of estimating the effective atomic number distribution from the ratio of the scattering and total absorption coefficients is considered. To illustrate the use of this technique in mammology, the results of measurement of microcalcinate and cholesterol distribution are discussed. It is suggested that tomographic examination of mammary gland should include detection of scattered radiation.

ENTROPY AND ENTANGLEMENT OF THE SUPERPOSITIONS OF DISPLACED FOCK STATES WITH A TWO-LEVEL ATOM

The properties of the displaced Fock states (DFS's) superpositions are reviewed. The interaction of these states with a two-level atom in cavity with the presence of additional Kerr medium is studied. Exact general matrix elements of the time-dependent operators of a Jaynes-Cummings model (JCM), in the presence of a Kerr medium, with these states are derived. The atomic inversion and photon number distribution are discussed. The quantum entropy and the entanglement of the atom-field are investigated. The exact results are employed to perform a careful investigation of the temporal evolution of the entropy. The connection between the field entropy and the collapses and revivals of the atomic inversion has been established. The general conclusions reached are illustrated by numerical results.

A tunable monochromatic hard X-ray CT composed of an X-band linear accelerator and a Q-switched laser

A Compton scattering hard X-ray source for 10–80keV is under construction, which consists of an X-band (11.424GHz) electron linear accelerator and a YAG laser. The main advantages are to produce tunable quasi-monochromatic hard X-rays (10–80keV) with the intensities of 108−10photons/s (at several stages) and its tabletop size. The second important aspect is to decelerate the beams before the beam dumper to reduce the noise radiation there. This enables a 3rd generation synchrotron radiation source to be constructed in small facilities without heavy shielding. The final goal is to load this hard X-ray source on a moving gantry. This device will realize a tunable monochromatic X-ray CT and dual energy X-ray imaging, which determines 3D distribution of electron density and equivalent atomic number. This provides real physical information to diagnostics and curative program.

Correlations and Counting Statistics of an Atom Laser

We demonstrate time-resolved counting of single atoms extracted from a weakly interacting Bose-Einstein condensate of 87Rb atoms. The atoms are detected with a high-finesse optical cavity and single atom transits are identified. An atom laser beam is formed by continuously output coupling atoms from the Bose-Einstein condensate. We investigate the full counting statistics of this beam and measure its second order correlation function g((2))(tau) in a Hanbury Brown-Twiss type experiment. For the monoenergetic atom laser we observe a constant correlation function g((2))(tau)=1.00 +/- 0.01 and an atom number distribution close to a Poissonian statistics. A pseudothermal atomic beam shows a bunching behavior and a Bose distributed counting statistics.

X-Ray scattering tomography

This paper discusses issues related to the development of reconstructed tomography based on scattered X-ray radiation. For most X-ray tomography, based on collimated initial and scattered radiation, the distribution of the object density (linear scattering coefficient) is reconstructed by the number of counted photons scattered in the reconstructed voxel (volumetric pixel) where density distribution is received only visually and not numerically. Heterogeneous structures along the initial path and trajectory of counted scattered photons cause significant errors in imaging. Collimation of the initial and scattered radiation allows the detection of relatively few scattered photons, and this is the reason why the statistics relative to this process are very poor. The paper presents mathematical algorithms that improve images resulting from this type of tomography and demonstrates that the imaging based on scattered collimated radiation does not provide the quality of resolution comparable to conventional transmission tomography. One approach to generate high-quality scattering tomography at high resolution is the use of a large non-collimated detector. This allows counting almost all of the scattered photons leaving the object in the direction of the detector. It provides the opportunity to reconstruct the linear scattering coefficient distribution numerically. The combined use of transmission and scattering tomography present the opportunity to identify an effective atomic number and density distribution.

INTERACTION OF THE SUPERPOSITIONS OF SQUEEZED DISPLACED FOCK STATES WITH ATOMIC SYSTEMS

The properties of the squeezed displaced Fock states (SDFS) superpositions are reviewed. The interaction of these states with a two-level atom in cavity with the presence of additional Kerr medium is studied. Exact general matrix elements of the time-dependent operators of a Jaynes–Cummings model (JCM), in the presence of a Kerr medium, with these states are derived. The atomic inversion and photon number distribution are discussed.

A study on the medium range order in moltenFe3Si and FeSi alloys

Prepeaks are observed in the x-ray diffraction patterns of moltenFe3Si and FeSi alloys and the mechanism of their microstructures is discussed. The distance in realspace corresponding to the prepeak positions resembles that of the Si–Si distances in theD03 typecrystal of Fe3Si within a deviation of 2.4%. Furthermore, by the reverse Monte Carlo(RMC) simulation, the prepeak is only found in the partial structureSSiSi(Q) formolten Fe3Si alloy, while in molten FeSi alloy prepeaks are found in bothSSiSi(Q) and SFeFe(Q) with similar height. The Gaussian distribution is found in the partialcoordination number distribution of Fe atoms around a Si atom in the twoalloys. The dominant 7-coordination in molten FeSi alloy suggests thatFe7Si and FeSi7 type clusters are kept in the molten state from FeSi crystal. Si–Si coherentpacking should be responsible for the medium range order (MRO) of moltenFe3Si while Fe–Fe and Si–Si coherent packing for that of molten FeSi.

How Sensitive Is CAT to Porosity?

Abstract Computer Axial Tomography (CAT) maps the distribution of the linear attenuation coefficient (LAC) over a section or over an entire object. LAC represents the product between density and mass attenuation coefficient (MAC). These parameters are influenced in complete different manners by porosity provided that pore sizes are much smaller than voxels: density lineally decreases with porosity while MAC is almost independent of it. Consequently, in the case of single energy tomography, CAT images will be strongly influenced by the porosity. In the case of dual-energy tomography, when it results for the same section two CAT images, one depicting density and the other one the effective atomic number distributions, only the first one will reflect the sample porosity while the second one remains independent of the specimen porosity. This fact could be very useful in investigating complex samples such as rocks or soils.

3-D dual gamma-ray computer axial tomography investigation of polymetallic nodules

Polymetallic nodules (PN) collected from the north-east Pacific Clarion-Clipperton region were investigated with a 192Ir dual-energy gamma-ray computer tomograph. Tomographic images depicting density and effective atomic number distribution of the same section were obtained. These images present a linear response for a broad range of sample densities (0.97–6.22 g/cm 3), and likewise effective atomic numbers (5.48–29). Sixty-five successive digital sections, each with a thickness of 0.8 mm, reveal the 3-D density as well as the effective atomic number distribution function over the entire nodule volume, allowing one to obtain virtual sections across any desired plane. The tomograms show many details of internal structure of the investigated nodule such as an old nucleus, fissure networks or layer structures. The total concentration of iron and manganese determined by this method was equal to 19.4±1.7%. Gamma-ray computer tomography considerably extends the quantitative analysis capability of classic X-ray computer tomography.

Momentum dependent interactions and the asymmetry of the reaction: Multifragmentation as an example

We discuss the importance of momentum dependent interactions in explaining the multifragmentation in asymmetric reactions by comparing the results with ${}^{16}\mathrm{O}\ensuremath{-}\mathrm{induced}$ emulsion data. The reactions are simulated within a quantum molecular dynamics model and the clusterization is performed with the minimum spanning tree method. Our results clearly indicate that the inclusion of momentum dependent forces improves the agreement with the measured atomic number distribution. The simple static interaction fails to explain the experimental data. We also discuss the universal dependence of the effect of momentum dependent interactions on the asymmetry of a reaction.

Quantitative analysis of reservoir rocks by microfocus X-ray computerised tomography

Abstract Microfocus Computer Tomography (μCT) is a non-destructive imaging technique that allows visualisation of internal features within non-transparent objects such as sedimentary rocks. With resolution as good as 10 μm in three dimensions, this technique is highly superior to medical CT widely used in geology and material sciences. Superior resolution is achieved by decreasing the X-ray spot size, which allows a decrease in the penumbra effect and an increase in primary magnification by placing the sample close to the X-ray source. Since polychromatic X-ray sources are used, the technique is not free of artefacts. Application of filtering techniques is recommended to circumvent these artefacts and to achieve good quality images for quantitative analysis. Application of a dual energy approach allows quantification of density as well as effective atomic number distribution of internal features. As the technique acquires data in 3D, μCT provides an ideal tool to link 2D thin-section petrography to 3D petrophysical measurements or to be used within an upscaling approach. A number of applications and the use of dual energy density and effective atomic number characterisations are presented. Attention is also paid to some of the limitations of this technique.