Magnetic resonance imaging and pulsed Doppler sonography of poststenotic jets: Correlation between signal void and flow velocity distribution

To correlate the appearance of poststenotic jets on gradient echo images with features of localized Doppler spectra of the jets, we studied an in vitro model of steady flow-through stenoses of 86, 96, and 99% area reduction. As fluids, water and a 40% glycerol solution in water were used. MRI was performed with a 1.5 T whole body imager and gradient echo images were obtained in planes parallel to the direction of flow. Doppler spectra were acquired separately from the MR measurements at 1 cm intervals for a distance of 10 cm downstream from the stenosis. Poststenotic signal void was observed for water and for the 40% glycerol solution only if the mean velocity within the stenosis exceeded a limit of 50–60 cm/sec. On the MR images, the jets could be divided into two segments: A proximal jet segment of uniform width equal to the diameter of the stenosis, followed by a distal jet segment which was characterized by broadening and then dissipating signal void. Except for the 99% stenosis, a high signal intensity core was present within the proximal jet segment. In the proximal jet segment, the Doppler measurements showed a low temporal fluctuation of the maximal flow velocity and only little flow opposite to the main flow direction. In the distal jet segment, the velocity fluctuation and the intensity of reverse flow increased sharply. The high signal intensity core of the jet was associated with a poststenotic zone of constant maximal flow velocity. The results demonstrate a close relationship between characteristic features of poststenotic jets in MRI and pulsed Doppler sonography.     

Capabilities and limits of ICP-MS for direct determination of element traces in saline solutions

An ICP-MS method for the determination of ultra-traces of 24 Elements (Li, Be, Mg, Al, Cr, Mn, Co, Zn, Cu, Ga, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Ba, Tl, Pb, Bi, U) at ng/L to μg/L levels in highly saline solutions (up to 30 g/L NaCl) was developed. Calculated to the salt content of the samples, limits of detection at the sub-μg/kg level were obtained. This allows the measurement of samples like sea water, or clinical samples like urine, serum and whole blood, with few or without sample preparation and a drastic improvement to the limits of detection. Compared to the determination in solutions of 1 g/L NaCl the limits of detection in the original sample were improved by one order of magnitude. Improvements in instrumental stability are achieved by the use of an additional gas, that is introduced to the aerosol stream and avoids salt deposition in the aerosol tube of the torch, the use of High Matrix Content (HMC) cones, that show no clogging even at NaCl-concentrations up to 50 g/L. With this setup the long term stability for measurements with changing matrix concentrations is < 10% without and < 5% with use of an internal standard for the individual samples. Cleaning up is necessary after 2 days of operation. The direct analysis of the Nearshore Seawater Reference Material NRC-CNRC CASS3 showed a good agreement with the certified and measured concentrations. Elements, that do not suffer from an isobaric overlap of matrix compounds can be determined clearly at ng/L levels. Measurement of different matrix concentrations showed, that acceptable results can be achieved with a single calibration for concentrations from 5 g/L NaCl to 30 g/L NaCl, though matrix matching shows the best results.     

New AlDy(Ni,Co) glasses - the influence of pre-aging on decomposition and modelling of the crystallisation

The decomposition of AlDy₆Ni₁₀ and AlDy₈Ni₆Co₂ glasses prepared by melt spinning after different pre-aging regimes was investigated. The modelling of the transformation at the glass transition and the crystallisation was tested by a more general Johnson-Mehl-Avrami-Kolmogorov model and by the determination of the local kinetic parameter. The relaxation was tested with an lnt kinetic model after Gibbs.     

A new formula for the saddle-to-scission time

We propose a new formula for the saddle-to-scission time that is more general that the one based on Kramers' approach. Its validity and applicability is then studied in detail. Such a formula is useful for the evaluation of the fission time of very heavy nuclei.     

III-V compound semiconductor (001) surfaces

There has been renewed interest in the structure of III-V compound semiconductor (001) surfaces caused by recent experimental and theoretical findings, which indicate that geometries different from the seemingly well-established dimer models describe the surface ground state for specific preparation conditions. I review briefly the structure information available on the (001) surfaces of GaP, InP, GaAs and InAs. These data are complemented with first-principles total-energy calculations. The calculated surface phase diagrams are used to explain the experimental data and reveal that the stability of specific surface structures depends largely on the relative size of the surface constituents. Several structural models for the Ga-rich GaAs (001)(4×6) surface are discussed, but dismissed on energetic grounds. I discuss in some detail the electronic properties of the recently proposed cation-rich GaAs (001)ζ(4×2) geometry. Received: 18 May 2001 / Revised version: 23 July 2001 / Published online: 3 April 2002     

Isotopic production cross sections of residues in reactions induced by relativistic heavy ions with protons and deuterons

The isotopic production cross sections of heavy residues in relativistic heavy-ion collisions have been investigated in inverse kinematics. The primary reaction products were fully identified in mass and atomic number prior to beta decay using the fragment separator FRS. The huge collection of data obtained helps in the understanding of the two main reaction mechanisms involved: fragmentation and fission. These data provide basic information for future radioactive ion beam facilities and for technical applications like intense neutron sources by means of spallation targets.