24,923 research outputs found
Holography utilizing surface plasmon resonances
A holographic recording medium with a substrate which has a diffraction grating composed of a number of spaced line ridges on the surface is discussed. The first layer consists of a metal with a plasma wavelength shorter than that of the selected light source. A second layer, or coating, on top of the first layer consists of a thin film of photosensitive emulsion. A diagram of the device is provided. The principles of operation are explained
A study of space contamination by means of the surface plasma resonance effect in grating diffraction
Surface plasma resonance effect in diffraction gratings and relation of effect to space contamination by spacecraft instrument
The Early Formation, Evolution and Age of the Neutron-Capture Elements in the Early Galaxy
Abundance observations indicate the presence of rapid-neutron capture (i.e.,
r-process) elements in old Galactic halo and globular cluster stars. These
observations demonstrate that the earliest generations of stars in the Galaxy,
responsible for neutron-capture synthesis and the progenitors of the halo
stars, were rapidly evolving. Abundance comparisons among several halo stars
show that the heaviest neutron-capture elements (including Ba and heavier) are
consistent with a scaled solar system r-process abundance distribution, while
the lighter such elements do not conform to the solar pattern. These
comparisons suggest two r-process sites or at least two different sets of
astrophysical conditions. The large star-to-star scatter observed in the
neutron-capture/iron ratios at low metallicities -- which disappears with
increasing [Fe/H] -- suggests an early, chemically unmixed and inhomogeneous
Galaxy. The stellar abundances indicate a change from the r-process to the slow
neutron capture (i.e., s-) process at higher metallicities in the Galaxy. The
detection of thorium in halo and globular cluster stars offers a promising,
independent age-dating technique that can put lower limits on the age of the
Galaxy.Comment: 6 pages, 3 figures; To appear in the proceedings of the 20th Texas
Symposium on Relativistic Astrophysics, J. C. Wheeler & H. Martel (eds.
Nuclear Chronometers
Observations of metal-poor Galactic halo stars indicate that the abundance
pattern of the (heaviest) neutron-capture elements is consistent with the
scaled solar system r-process abundances. Utilizing the radioactive (r-process)
element thorium, age determinations have been made for several of these same
stars, placing constraints on both Galactic and cosmological age estimates.Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic
Evolution'
Galactic Cosmochronometry from Radioactive Elements in the Spectra of Very Old Metal-Poor Stars
In a short review of neutron-capture elemental abundances in Galactic halo
stars, emphasis is placed on the use of these elements to estimate the age of
the Galactic halo. Two prominent characteristics of neutron-capture elements in
halo stars are their large star-to-star scatter in the overall abundance level
with respect to lighter elements, and the dominance of r-process abundance
patterns at lowest stellar metallicities. The r-process abundance signature
potentially allows the direct determination of the age of the earliest Galactic
halo nucleosynthesis events, but further developments in r-process theory, high
resolution spectroscopy of very metal-poor stars, and in basic atomic data are
needed to narrow the uncertainties in age estimates. Attention is brought to
the importance of accurate transition probabilities in neutron-capture element
cosmochronometry. Recent progress in the transition probabilities of rare earth
elements is discussed, along with suggestions for future work on other species.Comment: 19 pages, 5 figures; To appear in Physica Script
Neutron-Capture Element Trends in the Halo
In a brief review of abundances neutron-capture elements (Z > ~30) in
metal-poor halo stars, attention is called to their star-to-star scatter, the
dominance of r-process synthesis at lowest metallicities, the puzzle of the
lighter members of this element group, and the possibility of a better
r-/s-process discriminant.Comment: 6 pages, 2 figures. To appear in the Proceedings of ``Cosmic
Evolution'
Fuselage upwash effects on RSRA rotor systems
The effects of RSRA fuselage configurations on rotor performance and loads have been quantified analytically by means of currently available potential flow and rotor analysis. Four configurations of the Rotor Systems Research Aircraft (RSRA) were considered in this study. They were: (1) fuselage alone (conventional helicopter); (2) fuselage with auxiliary propulsion; (3) fuselage with wings (auxiliary lift); and (4) fuselage with both auxiliary lift propulsion. The rotor system investigated was identical to a CH-47D front rotor except that it had four instead of three blades. Two scaled-down versions of the same rotor were also analyzed to determine the effect of rotor scale on the fuselage upwash effects. The flight conditions considered for the upwash study are discussed. The potential flow models for the RSRA configuration, with and without the wings and the auxiliary propulsion system, are presented. The results of fuselage/wing/propulsion system upwash on performance and loads are also presented
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