Prompt (n,γ) Mass Measurements for the AVOGADRO Project

The aim of the AVOGADRO project is to replace the kilogram artefact by a high-purity, perfect single crystal of natural or isotope-enriched silicon. The isotopic composition and the impurities of the silicon crystal must, therefore, be known with highest possible accuracy and precision. The only method to obtain all this information without destruction of the massive samples is prompt (n,γ)-spectrometry. The measurements are performed at a thermal neutron guide of the ILL (Institut Max von Laue Paul Langevin) in Grenoble, France. The spectrometry of γ-radiation emitted by a nucleus promptly after thermal neutron capture allows a highly precise determination of atomic mass differences, as well as the determination of isotope abundances leading to the molar mass. The uncertainties assigned to the results for the respective atomic masses determined by the mass differences amount to up to 10⁻¹⁰, while the molar mass of an isotope-enriched Si single crystal has so far been determined with an uncertainty of 1 ⋅ 10⁻⁴. A direct comparison (for example, relative value of isotope abundances determined by (n,γ)-spectrometry omitting the thermal neutron cross section) furnishes a value of 7 ⋅ 10⁻⁵. The final aim of the AVOGADRO project is to provide a well specified crystal, which allows a more accurate value of the Avogadro constant to be determined. This constant is the key input parameter for tabulated values of fundamental constants and for a new definition of the unit of mass - the kilogram itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dipole soliton formation in a nematic liquid crystal in the nonlocal limit

The interaction of two symmetric solitary waves, termed nematicons, in a liquid crystal is considered in the limit of nonlocal response of the liquid crystal. This nonlocal limit is the applicable limit for most experimentally available liquid crystals. In this nonlocal limit, two separate cases for the initial separation of the nematicons are considered, these being large and small separation. Both spinning and nonspinning nematicons are considered. It is found that in the case of large initial separation, the nematicons can form a spinning or nonspinning bound state with a finite steady separation, this being called a nematicon dipole, when they are π out of phase. On the other hand, well separated, nonspinning, in-phase nematicons attract and merge, while well separated, spinning, in-phase nematicons can either form a bound state or merge into a single nematicon. In the limit of small initial separation, the nematicons rapidly merge when they are in phase. Modulation equations describing the nematicon interaction are derived via suitable trial functions in an averaged Lagrangian. These modulation equations are further modified to include the effect of the diffractive radiation shed as the nematicons evolve. Finally the modulation equations are approximated in order to investigate the various interaction regimes. Good to excellent agreement is found between their solutions and full numerical solutions of the nematicon equations.     

Development and preliminary results of an endoscopic Raman probe for potential in vivo diagnosis of lung cancers

A near-infrared Raman system was developed to collect real-time in vivo human lung spectra. The excitation light and the emission were guided to and from the tissue surface by a reusable fiber catheter passed down the instrument channel of a bronchoscope. Two-stage filtering was used to reduce laser noise, fluorescence, and Raman emissions from the fibers. A second fiber bundle guided the emission to a spectrometer where the fibers, in a round packing geometry, were spread out to form a parabolic arc that improved the signal-to-noise ratio 20-fold, facilitating real-time spectral measurements. Preliminary clinical tests show that clear and reliable Raman spectra can be obtained.     

Evidence for a reverse U-shaped conformation of single-chain bolaamphiphiles at the air-water interface

Infrared reflection absorption spectroscopy and X-ray reflectivity have been used to elucidate the molecular orientation and hydrocarbon chain conformation and packing of the symmetric long-chain bolaamphiphiles dotriacontane-1,1'-diyl-bis-[2-(trimethylammonio)ethylphosphate] (PC-C32-PC) and dotriacontane-1,1'-diyl-bis-[2-(dimethylammonio)ethylphosphate] (Me2PE-C32-Me2PE) at the air-water interface. At low surface pressures, these bipolar amphiphiles are found to lie flat on the water surface with a disordered chain. With increasing surface pressure, the alkyl chain becomes more ordered. Concomitantly, the chain is bent pointing into the air, whereas both polar headgroups keep contact with the water subphase. At an area of 0.9-1.1 nm2 per molecule, a surface pressure plateau is reached for both bolaamphiphiles, where the molecules adopt a reverse U-shaped conformation with a strongly tilted alkyl chain. Further compression leads to the formation of 3-D aggregates.