480 research outputs found
Adjustability of valves for shunting hydrocephalus is luxury, progress or necessity? Our personal experience to stimulate debate
Temporal variability of accumulation at Neumayer station, Antarctica, from stake array measurements and a regional atmospheric model
In this study we investigate the ability of the regional atmospheric model RACMO to represent temporal variations of Antarctic accumulation using weekly stake array measurements from Neumayer Station. The model uses ECMWF reanalyses data to force the atmospheric variables at the lateral boundaries of the model domain. Accumulation is defined as precipitation minus sublimation. Generally the model represents the synoptic situations that lead to precipitation reasonably well. The amounts of accumulation are, however, usually lower in the model than in the measurements. It cannot be distinguished whether the model underestimates precipitation or whether this effect is due to the redistribution of snow by the wind, which is not taken into account in the model, but affects the accumulation at the measuring site. Significant differences between model and measurements also occur in cases of net ablation due to wind erosion or when accumulation was due to snowdrift from southwest without precipitation observed
A new technology for adjustability and MR-resistance of shunt-valves â experience after implantation of 54 proGAVs
Detection of Xe from the Fukushima nuclear power plant in the upper troposphere above Germany
After the accident in the Japanese Fukushima Dai-ichi nuclear power plant in
March 2011 large amounts of radioactivity were released and distributed in the
atmosphere. Among them were also radioactive noble gas isotopes which can be
used as tracers to test global atmospheric circulation models. This work
presents unique measurements of the radionuclide Xe from Fukushima in
the upper troposphere above Germany. The measurements involve air sampling in a
research jet aircraft followed by chromatographic xenon extraction and
ultra-low background gas counting with miniaturized proportional counters. With
this technique a detection limit of the order of 100 Xe atoms in
litre-scale air samples (corresponding to about 100 mBq/m) is achievable.
Our results provide proof that the Xe-rich ground level air layer from
Fukushima was lifted up to the tropopause and distributed hemispherically.
Moreover, comparisons with ground level air measurements indicate that the
arrival of the radioactive plume at high altitude over Germany occurred several
days before the ground level plume.Comment: 8 pages, 5 figure
Seasonal variations of accumulation and the isotope record in ice cores: a study with surface snow samples and firn cores from Neumayer station, Antarctica
CornforthâEvans Transition States in Stereocontrolled Allylborations of Epoxy Aldehydes
Allylboration reactions rank among the most reliable tools in organic synthesis. Herein, we report a general synthesis of trifunctionalized allylboronates and systematic investigations of their stereocontrolled transformations with substituted aldehyde substrates, in order to efficiently access diverse, highly substituted target substrates. A peculiar transition in stereocontrol was observed from the polar FelkinâAnh (PFA) to the CornforthâEvans (CE) model for alkoxyâ and epoxyâsubstituted aldehydes. CEâtype transition states were uniformly identified as minima in advanced, DFTâbased computational studies of allylboration reactions of epoxy aldehydes, conforming well to the experimental data, and highlighting the underestimated relevance of this model. Furthermore, a mechanismâbased rationale for the substitution pattern of the epoxide was delineated that ensures high levels of stereocontrol and renders α,ÎČâepoxy aldehydes generally applicable substrates for target synthesis
Localized states in anthrylpolyenes : influence of geometry
Combining group theoretical arguments and (extended) HĂŒckel calculations, it is shown that terminally anthryl-substituted polyenes exhibit states almost completely localized at the substituents. These localized states are related to characteristic optical transitions in anthracene which also are observed in anthrylpolyenes independent of the length of the polyene chain
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