51,295 research outputs found
Experimental active and passive dosimetry systems for the NASA Skylab program
Active and passive dosimetry instrumentation to measure absorbed dose, charged particle spectra, and linear energy transfer spectra inside the command module and orbital workshop on the Skylab program were developed and tested. The active dosimetry system consists of one integral unit employing both a tissue equivalent ionization chamber and silicon solid state detectors. The instrument measures dose rates from 0.2 millirad/hour to 25 rads/hour, linear energy transfer spectra from 2.8 to 42.4 Kev/micron, and the proton and alpha particle energy spectra from 0.5 to 75 Mev. The active dosimeter is equipped with a portable radiation sensor for use in astronaut on-body and spacecraft shielding surveys during passage of the Skylab through significant space radiations. Data are transmitted in real time or are recorded by onboard spacecraft tape recorder for rapid evaluation of the radiation levels. The passive dosimetry systems consist of twelve (12) hard-mounted assemblies, each containing a variety of passive radiation sensors which are recoverable at the end of the mission for analysis
Finite temperature superfluid density in very underdoped cuprates
The combination of a large superconducting gap, low transition temperature,
and quasi two-dimensionality in strongly underdoped high temperature
superconductors severely constrains the behavior of the ab-plane superfluid
density \rho with temperature T. In particular, we argue that the contribution
of nodal quasiparticles to \rho(T) is essential to account both for the
amplitude of, and the recently observed deviations from, the Uemura scaling. A
relation between T_c and \rho(0) which combines the effects of quasiparticle
excitations at low temperatures and of vortex fluctuations near the critical
temperature is proposed and discussed in light of recent experiments.Comment: 5 RevTex pages, 4 figures (one new); more discussion and comparison
with experiment; version to appear in Phys. Rev.
Virtual chemical reactions for drug design
Two methods for the fast, fragment-based combinatorial molecule assembly were developed. The software COLIBREE® (Combinatorial Library Breeding) generates candidate structures from scratch, based on stochastic optimization [1]. Result structures of a COLIBREE design run are based on a fixed scaffold and variable linkers and side-chains. Linkers representing virtual chemical reactions and side-chain building blocks obtained from pseudo-retrosynthetic dissection of large compound databases are exchanged during optimization. The process of molecule design employs a discrete version of Particle Swarm Optimization (PSO) [2]. Assembled compounds are scored according to their similarity to known reference ligands. Distance to reference molecules is computed in the space of the topological pharmacophore descriptor CATS [3]. In a case study, the approach was applied to the de novo design of potential peroxisome proliferator-activated receptor (PPAR gamma) selective agonists. In a second approach, we developed the formal grammar Reaction-MQL [4] for the in silico representation and application of chemical reactions. Chemical transformation schemes are defined by functional groups participating in known organic reactions. The substructures are specified by the linear Molecular Query Language (MQL) [5]. The developed software package contains a parser for Reaction-MQL-expressions and enables users to design, test and virtually apply chemical reactions. The program has already been used to create combinatorial libraries for virtual screening studies. It was also applied in fragmentation studies with different sets of retrosynthetic reactions and various compound libraries
Experimental probing of the anisotropy of the empty p states near the Fermi level in MgB2
We have studied the Boron K-edge in the superconductor MgB2 by electron
energy loss spectroscopy (EELS) and experimentally resolved the empty p states
at the Fermi level that have previously been observed within an energy window
of 0.8eV by soft x-ray absorption spectroscopy. Using angular resolved EELS, we
find that these states at the immediate edge onset have pxy character in
agreement with predictions from first-principle electronic structure
calculations.Comment: 15 pages, 5 figure
Low-energy local density of states of the 1D Hubbard model
We examine the local density of states (DOS) at low energies numerically and
analytically for the Hubbard model in one dimension. The eigenstates represent
separate spin and charge excitations with a remarkably rich structure of the
local DOS in space and energy. The results predict signatures of strongly
correlated excitations in the tunneling probability along finite quantum wires,
such as carbon nanotubes, atomic chains or semiconductor wires in scanning
tunneling spectroscopy (STS) experiments. However, the detailed signatures can
only be partly explained by standard Luttinger liquid theory. In particular, we
find that the effective boundary exponent can be negative in finite wires,
which leads to an increase of the local DOS near the edges in contrast to the
established behavior in the thermodynamic limit.Comment: 6 pages, 4 figures, more information can be found at
http://www.physik.uni-kl.de/eggert/papers/index.htm
Phase sensitive Brillouin scattering measurements with a novel magneto-optic modulator
A recently reported phase sensitive Brillouin light scattering technique is
improved by use of a magnetic modulator. This modulator is based on Brillouin
light scattering in a thin ferrite film. Using this magnetic modulator in time-
and space Brillouin light scattering measurements we have increased phase
contrast and excluded influence of optical inhomogeneities in the sample. We
also demonstrate that the quality of the resulting interference patterns can be
improved by data postprocessing using the simultaneously recorded information
about the reference light
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