The interaction of antihydrogen with simple atoms and molecules

In this paper, we describe calculations that we have carried out of cross sections for rearrangement processes in very low-energy helium + antihydrogen scattering that result in or or . A significantly more accurate method from that used previously [E.A.G. Armour, S. Jonsell, Y. Liu, A.C. Todd, Nucl. Instr. and Meth. B 247 (2006) 127] is used to calculate the entrance channel wave function. Results are presented for the first two processes. Mention is made of the use of the method in calculations of low-energy e⁺H₂ scattering.     

Study of the electronic structures of oxygen doped in LiBaF3 crystal

The most likely substituting positions of impurity oxygen ions in LiBaF₃ crystals are studied using the general utility lattice program (GULP). The calculated results indicate that the main defect model is [] in the O:LiBaF₃ crystal. The electronic structures of the LiBaF₃ crystal with the defect [] are calculated using the DV-Xα method. It can be concluded from the electronic structures that the LiBaF₃ crystal with the defect [] will exhibit a 217-280 nm absorption band and the impurity oxygen will decrease core-valence luminescence yield.     

Morphology and interface structure of α-Fe2O3 islands grown on sapphire by ion-implantation and annealing

The morphology and interface structure of α-Fe₂O₃ islands grown on α-Al₂O₃ single crystals (sapphire) by Fe-ion-implantation and annealing in an oxidizing atmosphere have been studied using transmission electron microscopy. The α-Fe₂O₃ islands have the orientation relationship of and with sapphire. The typical outline of α-Fe₂O₃ islands consists of two (0 0 0 1) and six planes. The interfaces between α-Fe₂O₃ islands and sapphire are semicoherent, that is coherent regions separated by misfit dislocations at the interfaces. When imaged along the direction, the projected Burgers vector is determined to be . When imaged along the direction, the projected Burgers vector is determined to be . These misfit dislocations form a network structure at the interface to accommodate the mismatch between the lattices of the α-Fe₂O₃ and the α-Al₂O₃.     

380 keV proton irradiation effects on photoluminescence of Eu-doped GaN

The effect of 380 keV proton irradiation on the photoluminescence (PL) properties has been investigated for undoped and Eu-doped GaN. As the proton irradiation exceeds , a drastic decrease of PL intensity of the near band-edge emission of undoped GaN was observed. On the other hand, for Eu-doped GaN, the PL emission corresponding to the ⁵D₀→⁷F₂ transition in Eu³⁺ kept the initial PL intensity after the proton irradiation up to . Present results, together with our previous report on electron irradiation results, suggest that Eu-doped GaN is a strong candidate for light emitting devices in high irradiation environment.     

Radiation induced defects in BaBPO5:Ce and their role in thermally stimulated luminescence reactions: EPR and TSL investigations

Defect centers induced by gamma irradiation in Ce doped BaBPO₅ were investigated using EPR spectroscopy. From EPR studies, three phosphorous centered radicals were characterized on the basis of observed ³¹P hyperfine splitting and g values as , and radicals. In addition to this, two types of boron oxygen hole centers (BOHC) and O⁻ were also formed at room temperature. An intense broad signal in sample annealed in argon (g_⊥ = 1.9258 and g_‖ = 1.8839) was assigned to Ce³⁺ ions associated with the electron trapped at anion vacancy or nearby lattice defect. TSL studies showed two glow peaks, a relatively weaker one at 425 K and an intense one at 575 K. Spectral studies of the TSL glow peaks have shown that Ce³⁺ ion acts as emission center. From the temperature dependence of the EPR spectra of gamma irradiated samples, the glow peaks at 425 K and 575 K were attributed to thermal destruction of /O⁻ and BOHC, respectively, by trapping of electrons from elsewhere. The energy released in electron hole recombination process is used for the excitation of Ce³⁺ ions resulting in these glow peaks at 425 K and 575 K. The spectral studies of the TSL glow peaks have shown emission at 330 nm indicating Ce³⁺ acts as the luminescent centre. The trap depth and the frequency factor for the 425 K and 575 K peaks were determined using different heating rates method.     

First-principles study on electronic structures and absorption spectra for BaWO4 crystal containing barium vacancy

The electronic structures, dielectric function and absorption spectra for the perfect BaWO₄ (BWO) crystal and the BWO crystal containing barium vacancy () have been studied using density functional theory code CASTEP with the lattice structure optimized. The results indicate that the optical properties of the BWO crystal exhibit anisotropy and its optical symmetry coincide with lattice structure geometry of the BWO crystal. For the BWO crystal containing , there exhibit four absorption bands peaking at 0.71 eV (1751 nm), 1.85 eV (672 nm), 3.43 eV (362 nm) and 3.85 eV (322 nm), respectively. The origins of the 370 nm absorption band should be related to the .     

Estimation of equilibrium surface precipitation constants for trivalent metal sorption onto hydrous ferric oxide and calcite

Equilibrium constants for modeling surface precipitation of trivalent metal cations (M³⁺) onto hydrous ferric oxide and calcite were estimated from linear correlations of standard state Gibbs free energies of formation, () of the surface precipitates. The surface precipitation reactions were derived from Farley et. al. [K.J. Farley, D.A. Dzombak, F.M.M. Morel, J. Colloid Interface Sci. 106 (1985) 226] surface precipitation model, which are based on surface complexation model coupled with solid solution representation for surface precipitation on the solid surface. The values were correlated through the following linear free energy relations and where ‘ss’ stands for the end-member solid component of surface precipitate, is in kJ/mol, r_M³⁺ is the Shannon-Prewitt radius of M³⁺ in a given coordination state (nm), and is the non-solvation contribution to the Gibbs free energy of formation of the aqueous M³⁺ ion. Results indicate that the above surface precipitation correlations are useful tools where experimental data are not available.     

Crystallographic measurement of the β to α phase transformation and δ-hydride precipitation in a laser-welded Zircaloy-2 tube by electron backscattering diffraction

Crystallographic measurement of the β to α phase transformation and δ-hydride precipitation in a laser-welded Zircaloy-2 ferrule tube were carried out using an electron backscattering diffraction pattern (EBSP). A basket-weave structure with sub-micron lath width caused by quenching from the β to α phase was observed in the heat-affected and fusion zones, and mainly showed a grain boundary misorientation angle of 60° with an <1 1  0> rotation axis. This result is consistent with the Burgers orientation relationship of {1 1 0}_β//(0 0 0 1)_α and <1 1 1>_β//<1 1  0>_α for the β to α phase transformation. The texture of the quenched α′ phase was strongly inherited from the original α phase, having a radial (0 0 0 1) basal pole and axial {1 1  0} textures, even in the fusion zone. The primary hydride habit plane in the welded Zircaloy-2 was (0 0 0 1)_α//{1 1 1}_δ, matching previously obtained results for recrystallized cladding tubes. In addition to the primary habit plane, secondary habit planes were observed for the other low-index planes {1 0  0} and {1 0  1} in the fusion zone. The heterogeneous accumulation of hydrides in the transition zone between heat-affected and unaffected zones was mainly due to the residual stress distribution in the narrow region.     

Gamma radiolysis of NaCl brine: Effect of dissolved radiolysis gases on the radiolytic yield of long-lived products

Five moles per litre NaCl solution is γ-irradiated in autoclaves at 35 °C and 90 °C. H₂, O₂ and are formed as long-lived products and reach equilibrium concentrations at a gas partial pressure of some 10 bar. Since formation of a gas phase is completely suppressed, H₂ and O₂ are kept dissolved. The equilibrium concentration level depends on dose rate and pH. Experiments at 90 °C and with added H₂ show no gas production at all. From the kinetic simulation of the experiments we deduce that the reaction of OH and radicals with H₂ limits or inhibits sustaining solution decomposition. If species are present which compete with the radicals for H₂, (such as Br⁻) the decomposition is promoted. In such a case the gas partial pressure exceeds 100 bar.     

Secondary ion emission from Cu(1 0 0) surfaces with atomic adsorbates (N, O, Cl, S and Br)

Several targets that consist of atomic species X (X = N, O, Cl, S, Br) adsorbed at hollow sites on the Cu(1 0 0) surface have been examined with low-fluence secondary ion mass spectrometry (SIMS). The positive and negative secondary ion (SI) abundance distributions, which show a range of characteristics, have been discussed with the aid of thermochemical data derived from ab initio calculations. In positive SIMS, CuX⁺ is never observed, while the only heteronuclear (mixed-atom) SI that is observed for all five systems is Cu₂X⁺. In negative SIMS, the dominant heteronuclear species for all systems is , except for N/Cu(1 0 0), which produces no , ions. Cu⁻ emission is observed only for O/Cu(1 0 0). By analogy with results from laser ablation studies of O/Cu targets, it is conjectured that Cu⁻ is a daughter product of the gas-phase dissociation of polyatomic Cu-O anion clusters.     

Ion implantation in 4H-SiC

Silicon carbide offers unique applications as a wide bandgap semiconductor. This paper reviews various aspects of ion implantation in 4H-SiC studied with a view to optimise ion implantation in silicon carbide. Al, P and Si ions with keV energies were used. Channelling effects were studied in both a-axis and c-axis crystals as a function of tilts along major orthogonal planes and off the major orthogonal planes. Major axes such as [0 0 0 1] and the and minor axis like the showed long channelling tails and optimum tilts for minimising channelling are recommended. TEM analyses of the samples showed the formation of (0 0 0 1) prismatic loops and the loops as well,in both a and c-cut crystals. We also note the presence of voids only in P implanted samples implanted with amorphising doses. The competing process between damage accumulation and dynamic annealing was studied by determining the critical temperature for the transition between crystalline and amorphous SiC and an activation energy of 1.3 eV is extracted.     

Atomistic simulations of nanometric dislocation loops in bcc tungsten

Small dislocation loops formed from self-interstitial atoms (SIAs) are commonly found in irradiated metals. These defects significantly influence the mechanical properties of the materials. Atomistic simulations are used to describe nanometric circular dislocation loops with Burger’s vectors , and in bcc tungsten. Particular attention is paid to the habit plane of the loop. Two different embedded atom model (EAM) potentials are used. The energetics and geometry of the loops are studied as a function of their size.