Mechanism of adatom formation on Ag(1 1 0) studied by combined quasi-elastic He atom scattering and low energy ion scattering

Two complementary techniques, i.e. low-energy ion scattering in the neutral impact collision ion scattering spectroscopy (NICISS) mode and energy resolved He atom scattering (HAS) are combined to study the adatom formation mechanism on Ag(1 1 0). The NICISS spectra have been collected between 300 and 900 K along the 0 0 1, and azimuthal directions while HAS measurements of the diffuse elastic peak has been performed along the directions between 190 and 800 K. Both techniques are sensitive to surface disorder and HAS data have been corrected by using the NICISS results in particular on the anharmonicity estimated through a two atoms scattering model. In fact, above 500 K surface anharmonicity is clearly detected and accompanies the proliferation of thermal induced defects. Without this anharmonic correction the adatom formation energy E_a is too low and similar to the surface diffusion barrier. Instead, including the anharmonicity the estimate, E_a = (0.38 ± 0.03) eV, is in excellent agreement with the predictions of molecular dynamics simulations.     

Physics basis for the advanced tokamak fusion power plant, ARIES-AT

The advanced tokamak is considered as the basis for a fusion power plant. The ARIES-AT design has an aspect ratio of A≡R/a=4.0, an elongation and triangularity of κ=2.20,δ=0.90 (evaluated at the separatrix surface), a toroidal beta of β=9.1% (normalized to the vacuum toroidal field at the plasma center), which corresponds to a normalized beta of β_N≡100×β/(I_P(MA)/a(m)B(T))=5.4. These beta values are chosen to be 10% below the ideal MHD stability limit. The bootstrap-current fraction is f_BS≡I_BS/I_P=0.91. This leads to a design with total plasma current I_P=12.8  MA, and toroidal field of 11.1 T (at the coil edge) and 5.8 T (at the plasma center). The major and minor radii are 5.2 and 1.3 m. The effects of H-mode edge gradients and the stability of this configuration to non-ideal modes is analyzed. The current drive system consists of ICRF/FW for on-axis current drive and a Lower Hybrid system for off-axis. Transport projections are presented using the drift-wave based GLF23 model. The approach to power and particle exhaust using both plasma core and scrape-off-layer radiation is presented.     

Raman and optical absorption studies of proton bombarded CsI

CsI single crystals, which have the simple cubic structure, have been bombarded with 1 MeV protons at a temperature close to 300 K. Optical absorption and Raman studies have identified most of the defects created. These include F and F₂ centres, and V centres having absorption bands at 2.7 eV and 3.4 eV, which grow together during irradiation. The Raman studies identify these latter centres as having the structure. Isochronal and isothermal annealing experiments show a mutual decay of the F-type centres and these V centres in a second order reaction with an activation energy of 1.28 eV. The results are discussed in relation to the excitonic mechanism of defect production and the formation of interstitial iodine aggregates of various types in alkali iodides.     

Study of heavy ion range in different solid state nuclear track detector materials

The range of several heavy ions as ²³⁸U, ²⁰⁸Pb, ¹⁹⁷Au, ¹³⁹La, ⁵⁸Ni and ⁵⁶Fe in sodalime glass; ¹⁹⁷Au and ⁵⁸Ni in muscovite mica and Lexan polycarbonate; ²⁰⁹Bi and ¹⁹⁷Au in CR-39 have been determined experimentally. The calculations of range for these projectile–target combinations have been made using the Benton and Henke [10], Mukherjee and Nayak [11], Ziegler et al. [12] and Hubert et al. [14] semiempirical formulations. Finally a comparison has been made with the experimental results.     

Resonant coherent excitation of 2s electron of Li-like Fe ions to the n = 3 states

We observed resonant coherent excitation of the 2s electron to the n = 3 states of 83.5 MeV/u Li-like Fe²³⁺ ions planar-channeling in the plane of a Si crystal. A survival fraction of the Li-like ions was measured as a function of the angle between the incident beam and the [0 0 1] axis. Clear resonance dips corresponding to the transitions of a 2s electron to all the n = 3 states were observed. The transition of each resonance dip was identified by comparing with spectroscopic data. The resonance dips at the transition energies corresponding to the optically forbidden 2s_1/2–3s_1/2, 2s_1/2–3d_3/2 and 2s_1/2–3d_5/2 transitions were observed as well as the resonance dips at transition energies corresponding to the optically allowed 2s_1/2–3p_1/2 and 2s_1/2–3p_3/2 transitions.     

Radiation enhanced diffusion of implanted platinum in silicon guided by helium co-implantation for arbitrary control of platinum profile

The in-diffusion of platinum into a low-doped n-type float zone silicon guided and enhanced by radiation damage produced by co-implantation of helium ions was investigated. The implantation of 1 MeV platinum ions at different doses ranging from 5 × 10¹¹ to 5 × 10¹² cm⁻² was used to produce a finite source for platinum diffusion. Single and multiple energy implantation of helium ions with energies 7, 9, 11 and 13 MeV introducing different profiles of radiation defects were applied to enhance and shape the diffusion of platinum atoms performed by 20 min annealing at 725 °C in vacuum. The distribution of in-diffused platinum was studied by monitoring the acceptor level of substitutional platinum (E_C − E_T = 0.23 eV) by deep level transient spectroscopy. Results show that the helium co-implantation significantly enhances platinum diffusion and allows its control up to the depths of hundreds of micrometers. The resulting Pt_s distribution is given by the profile of radiation damage produced by helium ions while the amount of in-diffused Pt_s can be controlled by the dose of platinum implantation. It is also shown that an extra annealing at 685 °C performed prior to helium implantation substantially increases the amount of in-diffused platinum.     

Heavy ion irradiation effects in the rare-earth sesquioxide Dy2O3

Polycrystalline pellets of the rare-earth sesquioxide Dy₂O₃ with cubic C-type rare-earth structure were irradiated with 300 keV Kr²⁺ ions at fluences up to 5 × 10²⁰ Kr/m² at cryogenic temperature. Irradiation-induced microstructural evolution is characterized using grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). In previous work, we found a phase transformation from a cubic, C-type to a monoclinic, B-type (C2/m) rare-earth structure in Dy₂O₃ during Kr²⁺ ion irradiation at a fluence of less than 1 × 10²⁰ Kr/m². In this study, we find that the crystal structure of the top and middle regions of the implanted layer transform to a hexagonal, H-type (P6₃/mmc) rare-earth structure when the irradiation fluence is increased to 5 × 10²⁰ Kr/m²; the bottom of the implanted layer, on the other hand, remains in a monoclinic phase. The irradiation dose dependence of the C-to-B-to-H phase transformation observed in Dy₂O₃ appears to be closely related to the temperature and pressure dependence of the phases observed in the phase diagram. These transformations are also accompanied by a decrease in molecular volume (or density increase) of approximately 9% and 8%, respectively, which is an unusual radiation damage behavior.     

Investigation of neutron converters for production of optically stimulated luminescence (OSL) neutron dosimeters using Al2O3:C

This paper presents the optically stimulated luminescence (OSL) properties of neutron dosimeters in powder and in the form of pellets prepared with a mixture of Al₂O₃:C and neutron converters. The neutron converters investigated were high density polyethylene (HDPE), lithium fluoride (LiF), lithium fluoride 95% enriched with ⁶Li (⁶LiF), lithium carbonate 95% enriched with ⁶Li (⁶Li₂CO₃), boric acid enriched with 99% of ¹⁰B and gadolinium oxide (Gd₂O₃). The proportion of Al₂O₃:C and neutron converter in the mixture was varied to optimize the total OSL signal and neutron sensitivity. The neutron sensitivity and dose-response were determined for the OSL dosimeters using a bare ²⁵²Cf source and compared to the response of Harshaw TLD-600 and TLD-700 dosimeters (⁶LiF:Mg,Ti and ⁷LiF:Mg,Ti). The results demonstrate the possibility of developing an OSL dosimeter made of Al₂O₃:C powder and neutron converter with a neutron sensitivity (defined as the ratio between the ⁶⁰Co equivalent gamma dose and the reference neutron absorbed dose) and neutron–gamma discrimination comparable to the TLD-600/TLD-700 combination. It was shown that the shape of the OSL decay curves varied with the type of the neutron converter, demonstrating the influence of the energy deposition mechanism and ionization density on the OSL process in Al₂O₃:C.     

The influence of calcium ions on the development of acidity in corrosion product deposits on SIMFUEL, UO2

In order to clarify the influence of groundwater constituents on the formation of corrosion products and secondary phase deposits on corroding/dissolving nuclear fuel surfaces under waste disposal conditions we have investigated the influence of Ca²⁺, present as CaCl₂. The influence of calcium ions on the anodic dissolution of SIMFUEL (doped uranium dioxide) has been characterized over the potential range 0–500 mV (vs. SCE). Through the use of X-ray photoelectron spectroscopy (XPS) the surface composition over this potential range has been determined. Ca²⁺ was found not to influence the conversion of U^IVO₂ to , but to suppress the subsequent formation of a U^VI surface species which lead to the formation of a hydrated deposit, UO₃ · yH₂O. The adsorption of Ca²⁺ on the UO₂ surface is believed to inhibit fuel dissolution either via inhibiting the stabilization of the cation precursor (UO₂(OH)₂)_ads or by blocking the O²⁻ anion transfer reaction from the fuel surface.     

Interface mixing induced by swift heavy ions at metal-oxide/silicon interfaces

It was observed previously that ceramic/ceramic bilayers were very sensitive with respect to the electronic stopping power S_e, i.e. strong interface mixing, scaling with , occurred if a threshold S_ec was exceeded. The threshold seemed to be determined by the higher track formation threshold of two constituents forming the bilayer. Although no track formation has been observed in crystalline Si even for Uranium projectiles, interface mixing was observed previously for some Si-multilayers.

In this paper we report on the interface mixing of NiO, Fe₂O₃, TiO₂ on Si due to irradiation with 90–350 MeV Ar-, Kr-, Xe- and Au-ions at 80 K at fluences up to 9E15 ions/cm². Interface mixing, analyzed by means of Rutherford Backscattering Spectrometry (RBS), is found for these bilayers, too. But the threshold for intermixing is significantly higher compared to the ceramic/ceramic bilayers. This observation could be an evidence for the threshold being determined by the Si-layer. In contrast to NiO/Si and Fe₂O₃/Si, where an usual random walk mixing Δσ² = kΦ was observed, the interface broadening Δσ² for TiO₂/Si is found to scale nonlinearly with the ion fluence, which indicates that mixing is driven by a chemical solid-state reaction. At higher fluences plateaus form at the low energy Ni-edge of the RBS spectra. The plateaus indicate phase formation. X-Ray diffraction spectra does not show any evidence for new crystalline phases.     

Extending the operable time of the Syrian MNSR

The possibility of extending the operable time of the Syrian MNSR is investigated through a 3-D detailed model of the reactor constructed for this purpose. Good agreement between calculated and measured reactor parameter values were obtained for the reactor before modification. The operable time is increased by increasing the initial available excess reactivity. The latter is increased by adding Top Beryllium Shims in the Shim Tray. The increased initial excess reactivity is compensated for by increasing control rod worth by substituting the actual cadmium absorber by ₅B¹⁰ absorber. The shut down margin is also enhanced and safer reactor is obtained.     

Micro-PIXE and channeling PIXE analysis of Ag-doped YBa2Cu3O7−δ thin films

Nuclear Microscopy, utilizing a 2 MeV He⁺ beam for channeling Rutherford Backscattering (RBS) and PIXE analysis, was used to characterise Ag-doped YBa₂Cu₃O_7−δ thin films and measure the lateral distribution of the Ag. The samples were prepared by in situ two-beam pulsed laser deposition in order to investigate the effects of such dopings on critical current densities [1 and 2]. Films deposited at temperatures above 650°C form needle-like surface structures with a length of up to 100 μm; these tend to align with in-plane a–b axis. Results for a sample prepared at a substrate temperature of 730°C and a maximum Ag concentration of 5 at.% are discussed. The needle-like structures were found to be rich in Ag and Cu, and the YBa₂Cu₃O_7−δ film contained 0.02 at.% Ag. Broad beam PIXE-channeling results indicate that 19% of the Ag is substitutional.     

Calcium ion selectivity and isotope effects studied by using benzo-18-crown-6 resins

The chromatography experiments using benzo-18-crown-6 resin were carried out to study isotope effects in calcium adsorption. By the elution technique with hydrochloric acid solution, it was found that the adsorption of calcium depends on the concentration of hydrochloric acid in solution and that 9 M HCl concentration is the appropriate condition for the adsorption of calcium to the cavity of benzo-18-crown-6 of which cavity size is a little larger than the diameter of calcium ion. Thereafter, the isotope fractionation of calcium in the complex formation reaction with 18-crown-6 ether was investigated by using the breakthrough technique of chromatography with above-mentioned synthesized resin and 9 M hydrochloric acid solution. The observed isotope separation coefficient ε=−1 was 1.9 × 10⁻³ for the isotopic pair ⁴⁰Ca/⁴⁸Ca.     

Spectroscopic and microscopic study of the corrosion of iron–silicon steel by lead–bismuth eutectic (LBE) at elevated temperatures

The performance of iron–silica alloys with different silicon composition was evaluated after exposure to an isothermal bath of lead–bismuth eutectic (LBE). Four alloys were evaluated: pure iron, Fe–1.24%Si, Fe–2.55%Si and Fe–3.82%Si. The samples were exposed to LBE in a dynamic corrosion cell for periods from 700 to 1000 h at a temperature of 550 °C. After exposure, the thickness and composition of the oxide layer were examined using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectrometry (XPS), including sputter depth profiling. Particular attention was paid to the role, spatial distribution, and chemical speciation of silicon. Low-binding-energy silicon (probably silicates or ) was found in the oxide; while elemental silicon (Si) was found in the metal as expected, and silica (SiO₂) was found at the bottom of the oxide layer, consistent with the formation of a layer between the oxide and the metal. Alloys with low concentrations of Si contained only silicate in the oxide. Alloys with higher concentrations of Si contained a layer of silica at the boundary between the oxide and the bulk metal. All of the alloys examined showed signs of oxide failure. This study has implications for the role of silicon in the stability of the oxide layer in the corrosion of steel by LBE.     

Standard Gibbs energy of formation of Mo3Te4 by emf measurements

The emf of the galvanic cells Pt, Mo, MoO₂¦8 YSZ¦‘FeO’, Fe, Pt (I) and Pt, Fe,‘FeO’ ¦8 YSZ¦MoO₂, Mo₃Te₄, MoTe₂(), C, Pt (II) were measured over the temperature ranges 837 to 1151 K and 775 to 1196 K, respectively, using 8 mass% yttria-stabilized zirconia (8 YSZ) as the solid electrolyte. From the emf values, the partial molar Gibbs energy of solution of molybdenum in Mo₃Te₄/MoTe₂(), was found to be

Full-size image

. Using the literature data for the Gibbs energy of formation of MoTe₂(). the expression ΔG°_f(Mo₃Te_4,s) ± 5.97 (kj/mol) = −253.58 + 0.09214T(K) was derived for the range 775 to 1196 K. A third-law analysis yielded a value of −209 ± 10 kJ/mol for ΔH°_f.298^o of Mo₃Te₄(s).     

Performance of oxygen sensor in lead–bismuth at high temperature

Performance of yttria-stabilized zirconia solid electrolyte oxygen sensor with a reference electrode of Bi/Bi₂O₃ was investigated. The oxygen sensor was tested in alumina vessel in order to prevent generating of impurities. The oxygen potential in the melt was controlled by injecting steam–hydrogen gas mixture (P_H2/P_H2O) into stagnant LBE. The electromotive force (EMF) of the sensor was compared with the theoretical EMF derived from the Nernst equation at various LBE temperatures (550–700 °C). The influences of various injection gas temperatures (200–500 °C) on the sensor output were also investigated. It was found that the sensor signals of the oxygen potential in LBE have not been affected by the injection gas temperature. The results also showed that the measured EMFs were in good agreement with the theoretical values of the EMF. The material aspects were investigated as well. The SEM (Scanning Electron Microscope) devices were used to analyze the cross-section of oxygen sensors after the exposition to LBE at 700 °C for 1000 h. The SEM micrograph showed that the yttria-stabilized zirconia solid electrolyte had an excellent corrosion resistance to the high temperature LBE as the working fluid and high temperature bismuth as the reference fluid.