751 research outputs found
The new small-angle neutron scattering instrument SANS-1 at MLZ—characterization and first results
AbstractA thorough characterization of the key features of the new small-angle neutron scattering instrument SANS-1 at MLZ, a joint project of Technische Universität München and Helmholtz Zentrum Geesthacht, is presented. Measurements of the neutron beam profile, divergency and flux are given for various positions along the instrument including the sample position, and agree well with Monte Carlo simulations of SANS-1 using the program McStas. Secondly, the polarization option of SANS-1 is characterized for a broad wavelength band. A key feature of SANS-1 is the large accessible Q-range facilitated by the sideways movement of the detector. Particular attention is hence paid to the effects that arise due to large scattering angles on the detector where a standard cos3 solid angle correction is no longer applicable. Finally the performance of the instrument is characterized by a set of standard samples
Photoinduced ordering and anchoring properties of azo-dye films
We study both theoretically and experimentally anchoring properties of
photoaligning azo-dye films in contact with a nematic liquid crystal depending
on photoinduced ordering of azo-dye molecules. In the mean field approximation,
we found that the bare surface anchoring energy linearly depends on the azo-dye
order parameter and the azimuthal anchoring strength decays to zero in the
limit of vanishing photoinduced ordering. From the absorption dichroism spectra
measured in the azo-dye films that are prepared from the azo-dye derivative
with polymerizable terminal groups (SDA-2) we obtain dependence of the dichroic
ratio on the irradiation dose. We also measure the polar and azimuthal
anchoring strengths in nematic liquid crystal (NLC) cells aligned by the
azo-dye films and derive the anchoring strengths as functions of the dichroic
ratio. Though linear fitting of the experimental data for both anchoring
strengths gives reasonably well results, it, in contradiction with the theory,
predicts vanishing of the azimuthal anchoring strength at certain nonzero value
of the azo-dye order parameter. By using a simple phenomenological model we
show that this discrepancy can be attributed to the difference between the
surface and bulk order parameters in the films.Comment: revtex4, 25 pages, 9 figure
The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later
We have used the VLA FIRST survey and the APM catalog of the POSS-I plates as
the basis for constructing a new radio-selected sample of optically bright
quasars. This is the first radio-selected sample that is competitive in size
with current optically selected quasar surveys. Using only two basic criteria,
radio-optical positional coincidence and optical morphology, quasars and BL
Lacs can be identified with 60% selection efficiency; the efficiency increases
to 70% for objects fainter than magnitude 17. We show that a more sophisticated
selection scheme can predict with better than 85% reliability which candidates
will turn out to be quasars.
This paper presents the second installment of the FIRST Bright Quasar Survey
with a catalog of 636 quasars distributed over 2682 square degrees. The quasar
sample is characterized and all spectra are displayed. The FBQS detects both
radio-loud and radio-quiet quasars out to a redshift z>3. We find a large
population of objects of intermediate radio-loudness; there is no evidence in
our sample for a bimodal distribution of radio characteristics. The sample
includes ~29 broad absorption line quasars, both high and low ionization, and a
number of new objects with remarkable optical spectra.Comment: 41 pages plus 39 gifs which contain all quasar spectra. Accepted for
publication in the Astrophysical Journal Supplement Serie
On the Absorption of High Energy Gamma-Rays by Intergalactic Infrared Radiation
We present a new calculation of the intergalactic -ray
pair-production absorption coefficient as a function of both energy and
redshift up to the redshift of 3C279, z = 0.54. In reexamining this problem, we
make use of new observational data on the intergalactic infrared radiation
field (IIRF), together with recent theoretical models of the galactic spectral
energy distributions of the IIRF from stars and dust reradiation and estimates
of the IIRF from galaxy counts and {\it COBE} results. We present our results
for two fairly well defined IIRF spectral energy distributions, one of which is
within of our previous estimate of the IIRF at m.
We then apply our results to the -ray spectrum of Mrk 421, and obtain
good agreement with the observational data, including the recent results of the
{\it HEGRA} group.Comment: Plain TeX file with text in abs96.tex, 3 postscript figures (Fig. 1 -
ircobe.eps, Fig. 2 - taupl.eps, Fig. 3 - mki.eps), stylefile epsf.sty
included, accepted for publication in the Astrophysical Journa
Photoinduced 3D orientational order in side chain liquid crystalline azopolymers
We apply experimental technique based on the combination of methods dealing
with principal refractive indices and absorption coefficients to study the
photoinduced 3D orientational order in the films of liquid crystalline (LC)
azopolymers. The technique is used to identify 3D orientational configurations
of trans azobenzene chromophores and to characterize the degree of ordering in
terms of order parameters. We study two types of LC azopolymers which form
structures with preferred in-plane and out-of-plane alignment of
azochromophores, correspondingly. Using irradiation with the polarized light of
two different wavelengths we find that the kinetics of photoinduced anisotropy
can be dominated by either photo-reorientation or photoselection mechanisms
depending on the wavelength. We formulate the phenomenological model describing
the kinetics of photoinduced anisotropy in terms of the isomer concentrations
and the order parameter tensor. We present the numerical results for absorption
coefficients that are found to be in good agreement with the experimental data.
The model is also used to interpret the effect of changing the mechanism with
the wavelength of the pumping light.Comment: uses revtex4 28 pages, 10 figure
Metastable Dynamics above the Glass Transition
The element of metastability is incorporated in the fluctuating nonlinear
hydrodynamic description of the mode coupling theory (MCT) of the liquid-glass
transition. This is achieved through the introduction of the defect density
variable into the set of slow variables with the mass density and
the momentum density . As a first approximation, we consider the case
where motions associated with are much slower than those associated with
. Self-consistently, assuming one is near a critical surface in the MCT
sense, we find that the observed slowing down of the dynamics corresponds to a
certain limit of a very shallow metastable well and a weak coupling between
and . The metastability parameters as well as the exponents
describing the observed sequence of time relaxations are given as smooth
functions of the temperature without any evidence for a special temperature. We
then investigate the case where the defect dynamics is included. We find that
the slowing down of the dynamics corresponds to the system arranging itself
such that the kinetic coefficient governing the diffusion of the
defects approaches from above a small temperature-dependent value .Comment: 38 pages, 14 figures (6 figs. are included as a uuencoded tar-
compressed file. The rest is available upon request.), RevTEX3.0+eps
Citricultura biodinâmica: princípios e insumos para nutrição de plantas.
bitstream/item/131884/1/Documento-380-com-capa.pd
Nonlinear Hydrodynamics of a Hard Sphere Fluid Near the Glass Transition
We conduct a numerical study of the dynamic behavior of a dense hard sphere
fluid by deriving and integrating a set of Langevin equations. The statics of
the system is described by a free energy functional of the
Ramakrishnan-Yussouff form. We find that the system exhibits glassy behavior as
evidenced through stretched exponential decay and two-stage relaxation of the
density correlation function. The characteristic times grow with increasing
density according to the Vogel-Fulcher law. The wavenumber dependence of the
kinetics is extensively explored. The connection of our results with
experiment, mode coupling theory, and molecular dynamics results is discussed.Comment: 34 Pages, Plain TeX, 12 PostScript Figures (not included, available
on request
Identification of candidate pelagic marine protected areas through a seabird seasonal-, multispecific- and extinction risk-based approach
With increasing pressure on the oceans from environmental change, there has been a global call for improved protection of marine ecosystems through the implementation of marine protected areas (MPAs). Here, we used species distribution modelling (SDM) of tracking data from 14 seabird species to identify key marine areas in the southwest Atlantic Ocean, valuing areas based on seabird species occurrence, seasonality and extinction risk. We also compared overlaps between the outputs generated by the SDM and layers representing important human threats (fishing intensity, ship density, plastic and oil pollution, ocean acidification), and calculated loss in conservation value using fishing and ship density as cost layers. The key marine areas were located on the southern Patagonian Shelf, overlapping extensively with areas of high fishing activity, and did not change seasonally, while seasonal areas were located off south and southeast Brazil and overlapped with areas of high plastic pollution and ocean acidification. Non-seasonal key areas were located off northeast Brazil on an area of high biodiversity, and with relatively low human impacts. We found support for the use of seasonal areas depending on the seabird assemblage used, because there was a loss in conservation value for the seasonal compared to the non-seasonal approach when using ‘cost’ layers. Our approach, accounting for seasonal changes in seabird assemblages and their risk of extinction, identified additional candidate areas for incorporation in the network of pelagic MPAs
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