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We present the K-edge X-ray absorption near edge spectra of hexagonal-closed packed iron at pressure and temperature conditions relevant to Earth's mantle conditions. The calculated spectra have been obtained using the first-principles scheme based on the continued-fraction approach and norm-conserving pseudopotentials. The atomic configurations used for the X-ray absorption near edge spectroscopy calculations were obtained from classical molecular dynamics simulations, using an optimized embedded-atom potential. We compare our calculated spectra to recently available experiment results (R. Boehler, H.G. Musshoff, R. Ditz, G. Aquilanti, and A. Trapananti, Rev. Sci. Instrum. 80 (2009), pp. 045103–045108) and identify the main features of the spectra that may indicate onset of melting in iron.

The electronic structures of hexagonal closed-packed (h.c.p) titanium containing a vacancy and krypton impurity atoms at various insertion sites are calculated by first-principles methods in the framework of the density-functional theory (DFT). The density of states (DOS) for titanium containing a vacancy defect shows resonance-like features. Also, the bulk electron density decreases from similar to 0.15/angstrom(3) to similar to 0.05/angstrom(3) at the vacancy centre. Electronic structure calculations have been performed to investigate what underlies the krypton site preference in titanium. The DOS of the nearest-neighbour (NN) titanium atoms to the octahedral krypton appears to be less distorted (relative to pure titanium) when compared to the NN titanium atoms to the tetrahedral krypton. The electronic density deformation maps show that polarization of the titanium atoms is stronger when the krypton atom is located at the tetrahedral site. Since krypton is a closed-shell atom, thus precluding any bonding with the titanium atoms, we may conclude that the polarization of the electrons in the vicinity of the inserted krypton atoms and the distortion of the DOS of the NN titanium atoms to the krypton serve to indicate which defect site is preferred when a krypton atom is inserted into titanium. Based on these considerations, we conclude that the substitutional site is the most favourable one, and the octahedral is the preferred interstitial site, in agreement with recent DFT calculations of the energetics of krypton impurity sites. (C) 2011 Elsevier Ltd. All rights reserved.

The mechanical properties of mesoporous silica films were characterized by x-ray reflectivity measurements. The measurements provide information on the deformation of the pores and the walls induced by the adsorption of water in the pores. The analysis of the nanoscaled deformations supplies a method to determine the elastic modulus E of thin porous films. The nanodeformation of the porous network during its filling with water is interpreted in three regimes of isotherm sorptions.

A magnetically guided slow positron beam is being constructed at Qatar University and is currently being optimised for regular operation. This is the first positron beam in the Middle East, as well as being the first Arabic positron beam. Novel features in the design include a purely magnetic in-line deflector, working in the solenoid guiding field, to eliminate un-moderated positrons and block the direct line of sight to the source. The impact of this all-magnetic transport on the Larmor radius and resultant beam characteristics are studied by SIMION simulations for both ideal and real life magnetic field variations. These results are discussed in light of the coupled effect arising from electrostatic beam extraction.

Positron beam timing spectroscopy has been used to investigate positron diffusion in hydrogenated amorphous silicon between 85 and 350 K. The diffusivity is determined from both the contribution of the surface, where the positrons have a different characteristic annihilation rate, and its effect on the shape of the bulk annihilation rate distribution. We observe a single positron state with a temperature independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present the first ever direct observation of hopping diffusion of positrons in solids. The migration enthalpy for positrons in this state is found to be 17.7(3) meV.