Polarization studies of radiative electron capture into highly-charged uranium ions

Recent advances in the development of 2D microstrip detectors open up new possibilities for hard x-ray spectroscopy, in particular for polarization studies. These detectors make ideal Compton polarimeters, which enable us to study precisely the polarization of hard x-rays. Here, we present recent results from measurements of Radiative Electron Capture into the K-shell of highly-charged uranium ions. The experiments were performed with a novel 2D Si(Li) Compton polarimeter at the Experimental Storage Ring at GSI. Stored and cooled beams of U91+ and U92+ ions, with kinetic energies of 43 MeV/u and 96 MeV/u respectively, were crossed with a hydrogen gasjet. The preliminary data analysis shows x-rays from the K-REC process, emitted perpendicularly to the ion beam, to be strongly linearly polarized.

Barrier height variations and interface properties of PtSi/Si structures

To study some of the interfacial properties of PtSi/Si diodes, Schottky structures were fabricated on (100) crystalline silicon substrates by conventional thermal evaporation of Pt on Si followed by annealing at different temperatures (from 400 degrees C to 700 degrees C) to form PtSi. The PtSi/n-Si diodes, all yielded Schottky barrier (SB) heights that are remarkably temperature dependent. The temperature range (20-290 K) over which the I-V characteristics were measured in the present study is broader with a much lower limit (20 K), than what is usually reported in literature. These variations in the barrier height are adequately interpreted by introducing spatial inhomogeneity into the barrier potential with a Gaussian distribution having a mean barrier of 0.76 eV and a standard deviation of 30 meV. Multi-frequency capacitance-voltage measurements suggest that the barrier is primarily controlled by the properties of the silicide-silicon interface. The forward C-V characteristics, in particular, show small peaks at low frequencies that can be ascribed to interface states rather than to a series resistance effect.

Fabrication and characterisation of high resistivity SOI substrates for monolithic high energy physics detectors

Silicon on Insulator (SOI) substrates offer a promising platform for monolithic high energy physics detectors with integrated read-out electronics and pixel diodes. This paper describes the fabrication and characterisation of specially-configured SOI substrates using improved bonded wafer ion split and grind/polish technologies. The crucial interface between the high resistivity handle silicon and the SOI buried oxide has been characterised using both pixel diodes and circular geometry MOS transistors. Pixel diode breakdown voltages were typically greater than 100V and average leakage current densities at 70 V were only 55 nA/ sq cm. MOS transistors subjected to 24 GeV proton irradiation showed an increased SOI buried oxide trapped charge of only 3.45x1011cn-2 for a dose of 2.7Mrad

Limiting withdrawal rate and maximum film thickness during dip-coating of titania sols onto a Si substrate

The article highlights new insights into production of thin titania films widely used as catalyst support in many modern reactors including capillary microreactors, microstructured fixed-bed reactors and falling film microreactors. Dip-coating of a Mania sol onto a Si substrate has been studied in the range of the sol viscosities of 1.5-2.5 mPa s and the sol withdrawal rates of 0.2-18 mm/s. Different viscosities of sols were created by addition of desired amounts of nitric acid to the synthesis mixture of titanium isopropoxide and Plutonic F127 in ethanol which allowed to control the rate of the condensation reactions. Uniform inesoporous titania coatings were obtained at the solvent withdrawal rates below 10 mm/s at sol viscosities in the range from 1.6 mPa s to 2.5 mPa s. There exists a limiting withdrawal rate corresponding to a capillary number of ca. 0.01 beyond which uniform titania films cannot be obtained. Below the limiting withdrawal rate, the coating thickness is a power function of the sol viscosity and withdrawal rate, both with an exponent of 2/3. The limiting withdrawal rate increases as the solvent evaporation rate increases and it decreases as the sol viscosity increases. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.

Synthesis of protective Mo-Si-B coatings in molten salts and their oxidation behavior in an air-water mixture

The formation of various phases during boronizing of silicided molybdenum substrates (MoSi2/Mo) was investigated. Boronizing treatments were conducted in molten salts under an inert gas atmosphere in the 700-1000 degrees C temperature range for 3-7 h. Depending on the process type (non-current or electrochemical) and molten salt temperature, the formation of different boride phases (MoB, Mo2B5, MoB2, MoB4) was observed. At the same time, substantial oxidation of the bulk molybdenum disilicide phase (MoSi2) to the Mo5Si3 phase was observed in non-current boronizing. The oxidation resistance of the coatings was investigated by the weight change in an air-water (2.3 vol.%) mixture at a temperature of 500 degrees C for a period up to 700 h. Results indicated that a two-phase microstructure consisting of the MoSi2, matrix phase with 12-15 wt.% of the MoB4 phase greatly improved the oxidation resistance of the molybdenum substrates. The weight gain rate observed was 6.5 center dot 10(-4) mg/cm(2) h. (c) 2006 Elsevier B.V. All rights reserved.

Quasiparticle calculations of the electronic properties of ZrO2 and HfO2 polymorphs and their interface with Si

Quasiparticle calculations are performed to investigate the electronic band structures of various polymorphs of Hf and Zr oxides. The corrections with respect to density-functional-theory results are found to depend only weakly on the crystal structure. Based on these bulk calculations as well as those for bulk Si, the effect of quasiparticle corrections is also investigated for the band offsets at the interface between these oxides and Si assuming that the lineup of the potential at the interface is reproduced correctly within density-functional theory. On the one hand, the valence-band offsets are practically unchanged with a correction of a few tenths of electron volts. On the other hand, conduction-band offsets are raised by 1.3-1.5 eV. When applied to existing calculations for the offsets at the density-functional-theory level, our quasiparticle corrections provide results in good agreement with the experiment.

Monte Carlo calculations of quantum yield in inhomogeneous PtSi/p-Si Schottky barriers

Monte Carlo calculations of quantum yield in PtSi/p-Si infrared detectors are carried out taking into account the presence of a spatially distributed barrier potential. In the 1-4 mu m wavelength range it is found that the spatial inhomogeneity of the barrier has no significant effect on the overall device photoresponse. However, above lambda = 4.0 mu m and particularly as the cut-off wavelength (lambda approximate to 5.5 mu m) is approached, these calculations reveal a difference between the homogeneous and inhomogeneous barrier photoresponse which becomes increasingly significant and exceeds 50% at lambda = 5.3 mu m. It is, in fact, the inhomogeneous barrier which displays an increased photoyield, a feature that is confirmed by approximate analytical calculations assuming a symmetric Gaussian spatial distribution of the barrier. Furthermore, the importance of the silicide layer thickness in optimizing device efficiency is underlined as a trade-off between maximizing light absorption in the silicide layer and optimizing the internal yield. The results presented here address important features which determine the photoyield of PtSi/Si Schottky diodes at energies below the Si absorption edge and just above the Schottky barrier height in particular.

Energy levels, radiative rates and electron impact excitation rates for transitions in Si II

Energies for the lowest 56 levels, belonging to the 3s² 3p, 3s 3p², 3p³, 3s² 3d, 3s 3p 3d, 3s² 4ℓ and 3s² 5ℓ configurations of Si II, are calculated using the General-purpose Relativistic Atomic Structure Package (GRASP) code. Analogous calculations have also been performed (for up to 175 levels) using the FlexibleAtomicCode (FAC). Furthermore, radiative rates are calculated for all E1, E2, M1 and M2 transitions. Extensive comparisons are made with available theoretical and experimental energy levels, and the accuracy of the present results is assessed to be better than 0.1Ryd. Similarly, the accuracy for radiative rates (and subsequently lifetimes) is estimated to be better than 20 per cent for most of the (strong) transitions. Electron impact excitation collision strengths are also calculated, with the Dirac Atomic R-matrix Code (DARC), over a wide energy range up to 13 Ryd. Finally, to determine effective collision strengths, resonances are resolved in a fine energy mesh in the thresholds region. These collision strengths are averaged over a Maxwellian velocity distribution and results listed over a wide range of temperatures, up to 10^5.5 K. Our data are compared with earlier R-matrix calculations and differences noted, up to a factor of 2, for several transitions. Although scope remains for improvement, the accuracy for our results of collision strengths and effective collision strengths is assessed to be about 20 per cent for a majority of transitions.