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Curtarolo S.,Duke University | Hart G.L.W.,Duke University | Hart G.L.W.,Brigham Young University | Nardelli M.B.,Duke University | And 8 more authors.
Nature Materials | Year: 2013

High-throughput computational materials design is an emerging area of materials science. By combining advanced thermodynamic and electronic-structure methods with intelligent data mining and database construction, and exploiting the power of current supercomputer architectures, scientists generate, manage and analyse enormous data repositories for the discovery of novel materials. In this Review we provide a current snapshot of this rapidly evolving field, and highlight the challenges and opportunities that lie ahead. © 2013 Macmillan Publishers Limited. All rights reserved.


Argaman N.,Nuclear Research Center - Negev
Physics Letters, Section A: General, Atomic and Solid State Physics | Year: 2010

A number of anomalies have been reported in molten Bi, including a first-order liquid-liquid transition at 1010 K and ambient pressure, which is irreversible at cooling rates of several degrees per minute. An interpretation of these effects as due to long-range bond-orientational order is suggested. Significant evidence for directionality in liquid Bi, albeit only at temperatures close to melting, is available in experiments made circa 1930. Further experimentation is called for. © 2010 Elsevier B.V. All rights reserved.


El Abd A.,Nuclear Research Center - Negev
Journal of Radioanalytical and Nuclear Chemistry | Year: 2010

The thermal neutron cross-sections and resonance integrals of the 186W (n,γ) 187W and 98Mo (n,γ) 99Mo reactions in the thermal and 1/E regions, respectively, of a thermal reactor neutron spectrum have been experimentally determined by the activation method using 197Au (n,γ) 198Au reaction as a single comparator. The high purity natural W, Mo, and Zr foils; and Au wire diluted in aluminum, were irradiated without Cd shield in two neutron irradiation sites, characterized with different values for the thermal-to-epithermal flux ratios, f at the Second Egyptian Research Reactor (ETRR-2). The induced activities in the samples were measured by high-resolution γ-ray spectrometry with a calibrated germanium detector. Thermal neutron cross-sections for 2200 m/s neutrons and resonance integrals for the 186W (n,γ) 187W and 98Mo (n,γ) 99Mo reactions have been obtained relative to the reference values, σ0 = 98.65 ± 0.09 b and I 0 = 1500 ± 28 b for the 197Au (n,γ) 198Au reaction. The necessary correction factors for thermal neutron and resonance neutron self-shielding effects, and the epithermal flux index (α) were taken into account in the determinations. The results obtained were: σ0 = 38.43 ± 0.4 b and I 0 = 502 ± 65 b for 186W (n,γ) 187W, and σ0 = 0.137 ± 0.014 band I 0 = 6.47 ± 0.8 for 98Mo (n,γ) 99Mo. These results are discussed and compared with previous measurements and evaluated data in literature. The traditional method of determining thermal cross-sections and resonance integrals via neutron irradiation with and without Cd shield in one irradiation position was avoided in this work by neutron irradiation without Cd shield in at least two different neutron irradiation positions. This method provides alternative way for determining thermal cross-sections and resonance integrals simultaneously. © 2010 Akadémiai Kiadó.


Medhat M.E.,Nuclear Research Center - Negev
Journal of Radioanalytical and Nuclear Chemistry | Year: 2012

The total mass attenuation coefficients for natural beryl, corundum, garnet, pearl, and tourmaline gemstones were measured at 81, 356.5, 661.6, 1173.2, and 1332.5 keV photon energies. The samples were irradiated with 133Ba, 137Cs and 60Co radioactive point sources using gamma ray transmission method. Total atomic and electronic cross-sections, effective atomic numbers and electron densities were determined experimentally and theoretically. The experimental values were compared with the calculated values for all samples. The calculations were extended for total photon interactions in a wide energy range (1 keV- 100 GeV) using WinXCom program of the most commonly irradiated gemstones with different sources of ionized radiation. The values of these parameters have been found to vary with photon energy and chemical composition of the gemstones. All variations of these parameters against energy are shown graphically for total photon interactions. © Akadémiai Kiadó, Budapest, Hungary 2012.


Pazy E.,Nuclear Research Center - Negev | Argaman N.,Nuclear Research Center - Negev
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Employing a thermodynamic interpretation of gravity based on the holographic principle and assuming underlying particle statistics, fermionic or bosonic, for the excitations of the holographic screen leads to modified Newtonian dynamics (MOND). A connection between the acceleration scale a 0 appearing in MOND and the Fermi energy of the holographic fermionic degrees of freedom is obtained. In this formulation the physics of MOND results from the quantum-classical crossover in the fermionic specific heat. However, due to the dimensionality of the screen, the formalism is general and applies to two-dimensional bosonic excitations as well. It is shown that replacing the assumption of the equipartition of energy on the holographic screen by a standard quantum-statistical-mechanics description wherein some of the degrees of freedom are frozen out at low temperatures is the physical basis for the MOND interpolating function μ∼. The interpolating function μ∼ is calculated within the statistical mechanical formalism and compared to the leading phenomenological interpolating functions, most commonly used. Based on the statistical mechanical view of MOND, its cosmological implications are reinterpreted: the connection between a 0 and the Hubble constant is described as a quantum uncertainty relation; and the relationship between a 0 and the cosmological constant is better understood physically. © 2012 American Physical Society.


Medhat M.E.,Nuclear Research Center - Negev
Annals of Nuclear Energy | Year: 2012

The objective of the proposed work was to apply gamma-ray transmission method for studying the properties of cultivated soil. In this study, mass attenuation coefficients, bulk density, moisture content, porosity, and field capacity are determined. Five soil samples are collected from different agriculture zones in Egypt. A high-resolution gamma-ray spectrometry based on HPGe (high purity germanium) detector was employed to measure attenuation of strongly collimated monoenergetic gamma beam through soil samples. The radioactive sources used in the experiment were 241Am, 133Ba, 137Cs and 60Co. The mass attenuation coefficients of soil samples were calculated from the transmission measurements. WinXCom program was used to calculate mass attenuation coefficients of soil samples. There is a satisfactory agreement between experimental and theoretical values. It was observed that gamma-ray transmission method is an efficient tool for studying soil parameters for agriculture purposes. © 2011 Elsevier Ltd. All rights reserved.


Medhat M.E.,Nuclear Research Center - Negev
Annals of Nuclear Energy | Year: 2012

Artificial neural network (ANN) represents one of artificial intelligence methods in the field of modeling and uncertainty in different applications. The objective of the proposed work was focused to apply ANN to identify isotopes and to predict uncertainties of their activities of some natural radioactive sources. The method was tested for analyzing gamma-ray spectra emitted from natural radionuclides in soil samples detected by a high-resolution gamma-ray spectrometry based on HPGe (high purity germanium). The principle of the suggested method is described, including, relevant input parameters definition, input data scaling and networks training. It is clear that there is satisfactory agreement between obtained and predicted results using neural network. © 2012 Elsevier Ltd. All rights reserved.


Medhat M.E.,Nuclear Research Center - Negev
Annals of Nuclear Energy | Year: 2012

Expressions for calculating the elemental concentrations of composite materials based on a gamma absorption technique are derived. These expressions provide quantitative information about elemental concentrations of materials. Calculations are carried out for estimating the concentrations of copper and gold in some alloys of bronze and gold. The method was also applied for estimating the concentrations of some heavy elements in a water matrix highlighting the differences with photon attenuation measurements. Theoretical mass attenuation coefficient values were obtained using the WinXCom program. A high-resolution gamma-ray spectrometry based on high purity germanium detector (HPGe) was employed to measure the attenuation of a strongly collimated monoenergetic gamma beam through samples. © 2012 Elsevier Ltd. All rights reserved.


Livneh T.,Nuclear Research Center - Negev | Sterer E.,Nuclear Research Center - Negev
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

We report on the temperature and pressure dependence of Stokes and anti-Stokes Raman spectra of a single crystal of 2H -MoS2 as the energies of the A1 and B1 excitons, EA1 and EB1, are tuned to resonate with an exciting laser at E L=1.96 eV. Pressure- and temperature-dependent intensity ratio analysis of the resonant A1g phonon and the E2g 1 phonon is complemented by the calculation of resonance Raman probability profiles of the former, which well agree with experiments. The temperature-dependent proximity of EA1 and EB1 to E L is reflected in the formation of Stokes dominated A1 and anti-Stokes dominated B1 temperature "zones" with a midpoint positioned at T∼260 K. The shift in the frequency of the Stokes two-phonon dispersive band relative to that of the anti-Stokes band is explained as due to changing in the order of participation of the quasiacoustic phonon in the scattering process. © 2010 The American Physical Society.


El-Khayatt A.M.,Nuclear Research Center - Negev
Annals of Nuclear Energy | Year: 2011

This work is concerned with a construction and use of NXcom computer program for calculating the removal and attenuation coefficients of transmitted fast neutrons and γ-rays, respectively, through mixtures, composites, concretes and compounds. The program uses only one input data file for neutrons and γ-rays calculations. For γ-ray attenuation, the program predictions were tested by comparing them with the well-known WinXcom program results and an excellent agreement was noticed. Also, it has been used for calculating the values of macroscopic effective removal cross-sections ΣR (cm-1) for five new published polyamide and anhydride composites designed for shielding mixed neutron and γ-rays. The obtained values for ΣR using the program and the reported attenuation thicknesses which were based on the Monte Carlo N-Particle (MCNP) code showed the same trend. The NXcom program can be used as a preliminary effective tool for testing the shielding material against fast neutrons and γ-rays. © 2010 Elsevier Ltd. All rights reserved.

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