43,965 research outputs found
Lunar International Observers Network operation during the Apollo 10 mission
Lunar International Observers Network operation during Apollo 10 missio
Advanced dosimetry systems for the space transport and space station
Advanced dosimetry system concepts are described that will provide automated and instantaneous measurement of dose and particle spectra. Systems are proposed for measuring dose rate from cosmic radiation background to greater than 3600 rads/hr. Charged particle spectrometers, both internal and external to the spacecraft, are described for determining mixed field energy spectra and particle fluxes for both real time onboard and ground-based computer evaluation of the radiation hazard. Automated passive dosimetry systems consisting of thermoluminescent dosimeters and activation techniques are proposed for recording the dose levels for twelve or more crew members. This system will allow automatic onboard readout and data storage of the accumulated dose and can be transmitted to ground after readout or data records recovered with each crew rotation
Universal features in sequential and nonsequential two-photon double ionization of helium
We analyze two-photon double ionization of helium in both the nonsequential
and sequential regime. We show that the energy spacing between the two emitted
electrons provides the key parameter that controls both the energy and the
angular distribution and reveals the universal features present in both the
nonsequential and sequential regime. This universality, i.e., independence of
photon energy, is a manifestation of the continuity across the threshold for
sequential double ionization. For all photon energies, the energy distribution
can be described by a universal shape function that contains only the spectral
and temporal information entering second-order time-dependent perturbation
theory. Angular correlations and distributions are found to be more sensitive
to the photon energy. In particular, shake-up interferences have a large effect
on the angular distribution. Energy spectra, angular distributions
parameterized by the anisotropy parameters, and total cross sections presented
in this paper are obtained by fully correlated time-dependent ab initio
calculations.Comment: 12 pages, 8 figure
Magnetic field induced finite size effect in type-II superconductors
We explore the occurrence of a magnetic field induced finite size effect on
the specific heat and correlation lengths of anisotropic type-II
superconductors near the zero field transition temperature Tc. Since near the
zero field transition thermal fluctuations are expected to dominate and with
increasing field strength these fluctuations become one dimensional, whereupon
the effect of fluctuations increases, it appears unavoidable to account for
thermal fluctuations. Invoking the scaling theory of critical phenomena it is
shown that the specific heat data of nearly optimally doped YBa2Cu3O7-x are
inconsistent with the traditional mean-field and lowest Landau level
predictions of a continuous superconductor to normal state transition along an
upper critical field Hc2(T). On the contrary, we observe agreement with a
magnetic field induced finite size effect, whereupon even the correlation
length longitudinal to the applied field H cannot grow beyond the limiting
magnetic length L(H). It arises because with increasing magnetic field the
density of vortex lines becomes greater, but this cannot continue indefinitely.
L(H) is then roughly set on the proximity of vortex lines by the overlapping of
their cores. Thus, the shift and the rounding of the specific heat peak in an
applied field is traced back to a magnetic field induced finite size effect in
the correlation length longitudinal to the applied field.Comment: 8 pages, 4 figure
Optical Probe of Quantum Shot Noise Reduction at a Single-Atom Contact
Visible and infra-red light emitted at a Ag-Ag(111) junction has been
investigated from tunneling to single atom contact conditions with a scanning
tunneling microscope. The light intensity varies in a highly nonlinear fashion
with the conductance of the junction and exhibits a minimum at conductances
close to the conductance quantum. The data are interpreted in terms of current
noise at optical frequencies, which is characteristic of partially open
transport channels
Unravelling the Mysteries of the Leo Ring: An Absorption Line Study of an Unusual Gas Cloud
Since the 1980's discovery of the large (2x10^9 Msun) intergalactic cloud
known as the Leo Ring, this object has been the center of a lively debate about
its origin. Determining the origin of this object is still important as we
develop a deeper understanding of the accretion and feedback processes that
shape galaxy evolution. We present HST/COS observations of three sightlines
near the Ring, two of which penetrate the high column density neutral hydrogen
gas visible in 21 cm observations of the object. These observations provide the
first direct measurement of the metallicity of the gas in the Ring, an
important clue to its origins. Our best estimate of the metallicity of the ring
is ~10% Zsun, higher than expected for primordial gas but lower than expected
from an interaction. We discuss possible modifications to the interaction and
primordial gas scenarios that would be consistent with this metallicity
measurement.Comment: 11 pages, 7 figures, accepted Ap
Query processing of spatial objects: Complexity versus Redundancy
The management of complex spatial objects in applications, such as geography and cartography,
imposes stringent new requirements on spatial database systems, in particular on efficient
query processing. As shown before, the performance of spatial query processing can be improved
by decomposing complex spatial objects into simple components. Up to now, only decomposition
techniques generating a linear number of very simple components, e.g. triangles or trapezoids, have
been considered. In this paper, we will investigate the natural trade-off between the complexity of
the components and the redundancy, i.e. the number of components, with respect to its effect on
efficient query processing. In particular, we present two new decomposition methods generating
a better balance between the complexity and the number of components than previously known
techniques. We compare these new decomposition methods to the traditional undecomposed representation
as well as to the well-known decomposition into convex polygons with respect to their
performance in spatial query processing. This comparison points out that for a wide range of query
selectivity the new decomposition techniques clearly outperform both the undecomposed representation
and the convex decomposition method. More important than the absolute gain in performance
by a factor of up to an order of magnitude is the robust performance of our new decomposition
techniques over the whole range of query selectivity
Conjunctive query inseparability of OWL 2 QL TBoxes
The OWL2 profile OWL 2 QL, based on the DL-Lite family of description logics, is emerging as a major language for developing new ontologies and approximating the existing ones. Its main application is ontology based data access, where ontologies are used to provide background knowledge for answering queries over data. We investigate the corresponding notion of query inseparability (or equivalence) for OWL 2 QL ontologies and show that deciding query inseparability is PSpace-hard and in ExpTime. We give polynomial-time (incomplete) algorithms and demonstrate by experiments that they can be used for practical module extraction
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