États excités des isotopes de plomb de masse 202, 200 et 198

The low-lying excited levels of ^{198, 200, 202}Pb have been studied by observing γ- and β-rays and γ?γ coincidences in bismuth decay. Decay schemes have been proposed for these nuclei. Half-lives were determined for the 9^?, 7^?, 5^? and 4⁺ levels in ²⁰⁰Pb, and for the 5^? level in ¹⁹⁸Pb. Microscopic calculations have been made with Gaussian and surface delta interactions to account for level schemes. The results are discussed.     

Stereospecific access to branched-chain carbohydrate synthons.

Appropriate precursors for the synthesis of D-vancosamine and L-evernitrose have been stereospecifically obtained from methyl 4,6-O-benzylidene-2-deoxy-α- and β-D-threo-hexopyranosid-3-ulose.     

Connected functional working spaces: a framework for the teaching and learning of functions at upper secondary level

ZDM – Mathematics Education - This paper aims at contributing to remedy the narrow treatment of functions at upper secondary level. Assuming that students make sense of functions by working...     

Building an Optimal Portfolio in Discrete Time in the Presence of Transaction Costs

Abstract

Portfolio theory covers different approaches to the construction of a portfolio offering maximum expected returns for a given level of risk tolerance where the goal is to find the optimal investment rule. Each investor has a certain utility for money which is reflected by the choice of a utility function. In this article, a risk averse power utility function is studied in discrete time for a large class of underlying probability distribution of the returns of the asset prices. Each investor chooses, at the beginning of an investment period, the feasible portfolio allocation which maximizes the expected value of the utility function for terminal wealth. Effects of both large and small proportional transaction costs on the choice of an optimal portfolio are taken into account. The transaction regions are approximated by using asymptotic methods when the proportional transaction costs are small and by using expansions about critical points for large transaction costs.     

Structural study and crystal chemistry of the first stage calcium graphite intercalation compound

A novel and efficient synthesis method concerning the preparation of the first stage calcium graphite intercalation compound is provided. It makes use of a reaction between liquid metallic alloy and pyrolytic graphite. From now on it is especially easy to obtain bulk CaC₆ samples. Thanks to such samples, it was possible to study in detail the crystal structure of this binary intercalation compound. It has been entirely specified, so that we know that CaC₆ crystal is rhombohedral and belongs to the space group with the following parameters: and α=49.55°. The elemental unit cell contains one calcium atom and six carbon atoms. In this paper, we show also how the various MC₆ structures evolve according to the size of the intercalated element and to the bond nature that appears in the final compound. CaC₆ is unique, since all the other MC₆ compounds exhibit a hexagonal symmetry.     

Approaches for ultrafast imaging of transient materials processes in the transmission electron microscope

The growing field of ultrafast materials science, aimed at exploring short-lived transient processes in materials on the microsecond to femtosecond timescales, has spawned the development of time-resolved, in situ techniques in electron microscopy capable of capturing these events. This article gives a brief overview of two principal approaches that have emerged in the past decade: the stroboscopic ultrafast electron microscope and the nanosecond-time-resolved single-shot instrument. The high time resolution is garnered through the use of advanced pulsed laser systems and a pump-probe experimental platforms using laser-driven photoemission processes to generate time-correlated electron probe pulses synchronized with laser-driven events in the specimen. Each technique has its advantages and limitations and thus is complementary in terms of the materials systems and processes that they can investigate. The stroboscopic approach can achieve atomic resolution and sub-picosecond time resolution for capturing transient events, though it is limited to highly repeatable (>10(6) cycles) materials processes, e.g., optically driven electronic phase transitions that must reset to the material's ground state within the repetition rate of the femtosecond laser. The single-shot approach can explore irreversible events in materials, but the spatial resolution is limited by electron source brightness and electron-electron interactions at nanosecond temporal resolutions and higher. The first part of the article will explain basic operating principles of the stroboscopic approach and briefly review recent applications of this technique. As the authors have pursued the development of the single-shot approach, the latter part of the review discusses its instrumentation design in detail and presents examples of materials science studies and the near-term instrumentation developments of this technique.     

VIRGO mirrors: wavefront control

The Franco-Italian VIRGO program to detect gravitational waves requires large mirrors whose optical properties are extremely severe: absorption and scattering <1ppm at 1064nm, wavefront homogeneity of 9nm RMS on Ø100mm. To achieve this performance on the wavefront, we have developed an experimental method to correct the wavefront shape to be as plane as possible. A thin silica layer is added through a mask by sputtering, where it is necessary on the surface, on the last layer of the multilayer. A phase retardation is produced. The wavefront is measured before and after correction with a Zygo interferometer. The first test has been done on Ø80mm mirror at 633nm. The wavefront peak to valley goes from 34 to 14nm. At this level, we are limited by the interferometer accuracy and repeatability. The optical property modifications due to the corrective coating are small (at 1064nm) or easy to overcome. The corrected wavefront includes at the moment the reference flat of the interferometer, so the measurements are not absolute. But the feasibility of this correction method is proved and it is powerful.     

A powerful route to C-functionalised tetraazamacrocycles

The bisaminal template approach represents a powerful synthetic tool for the preparation of various C-functionalised tetraazacycloalkanes which are efficient precursors of bifunctional chelating agents.     

Complex formation of peroxouranyl (and uranyl) with polyaminocarboxylate ligands

Complexation of the uranyl ion (UO₂²⁺) and of the peroxouranyl species (UO₄) by some polyaminocarboxylate ligands has been investigated in solution (3M NaClO₄) at 25°C. The logarithms of the cumulative formation constants of the UO₂²⁺ chelates formed are: UO₂edta^2? (15.65), UO₂Hedta^? (18.59), (UO₂)₂edta (20.24); UO₂edda (16.02); UO₂Hnta (12.19); UO₂ida (9.63), UO₂H₂(ida)₂ (23.80). The equilibrium UO₂²⁺ + H₂O₂ ? UO₄ + 2H⁺ has a stability log K = ?3.99. The peroxocomplexes formed are UO₄Hedda^? (14.81, expressed from UO₂²⁺ and H₂O₂) and UO₄Hnta^2? (8.50). Solution structures of the chelates are proposed.