1,608 research outputs found
Hierarchical structuring of Cultural Heritage objects within large aggregations
Huge amounts of cultural content have been digitised and are available
through digital libraries and aggregators like Europeana.eu. However, it is not
easy for a user to have an overall picture of what is available nor to find
related objects. We propose a method for hier- archically structuring cultural
objects at different similarity levels. We describe a fast, scalable clustering
algorithm with an automated field selection method for finding semantic
clusters. We report a qualitative evaluation on the cluster categories based on
records from the UK and a quantitative one on the results from the complete
Europeana dataset.Comment: The paper has been published in the proceedings of the TPDL
conference, see http://tpdl2013.info. For the final version see
http://link.springer.com/chapter/10.1007%2F978-3-642-40501-3_2
Tests of relativity using a microwave resonator
The frequencies of a cryogenic sapphire oscillator and a hydrogen maser are
compared to set new constraints on a possible violation of Lorentz invariance.
We determine the variation of the oscillator frequency as a function of its
orientation (Michelson-Morley test) and of its velocity (Kennedy-Thorndike
test) with respect to a preferred frame candidate. We constrain the
corresponding parameters of the Mansouri and Sexl test theory to and which is equivalent to the best previous result for the
former and represents a 30 fold improvement for the latter.Comment: 8 pages, 2 figures, submitted to Physical Review Letters (October 3,
2002
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New Collisional Ionization Equilibrium Calculations for Optically Thin Plasmas
Reliably interpreting spectra from electron-ionized laboratory and cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have carried out state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Mg-like ions of all elements from He to Zn as well as for Al- like to Ar-like ions of Fe. We have also carried out state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the most recently recommended electron impact ionization data, we present improved collisional ionization equilibrium (CIE) calculations. Here, as an example, we present our calculated fractional ionic abundances for iron using these data and compare them with those from the previously recommended CIE calculations
Dielectronic Recombination in Photoionized Gas. II. Laboratory Measurements for Fe XVIII and Fe XIX
In photoionized gases with cosmic abundances, dielectronic recombination (DR)
proceeds primarily via nlj --> nl'j' core excitations (Dn=0 DR). We have
measured the resonance strengths and energies for Fe XVIII to Fe XVII and Fe
XIX to Fe XVIII Dn=0 DR. Using our measurements, we have calculated the Fe
XVIII and Fe XIX Dn=0 DR DR rate coefficients. Significant discrepancies exist
between our inferred rates and those of published calculations. These
calculations overestimate the DR rates by factors of ~2 or underestimate it by
factors of ~2 to orders of magnitude, but none are in good agreement with our
results. Almost all published DR rates for modeling cosmic plasmas are computed
using the same theoretical techniques as the above-mentioned calculations.
Hence, our measurements call into question all theoretical Dn=0 DR rates used
for ionization balance calculations of cosmic plasmas. At temperatures where
the Fe XVIII and Fe XIX fractional abundances are predicted to peak in
photoionized gases of cosmic abundances, the theoretical rates underestimate
the Fe XVIII DR rate by a factor of ~2 and overestimate the Fe XIX DR rate by a
factor of ~1.6. We have carried out new multiconfiguration Dirac-Fock and
multiconfiguration Breit-Pauli calculations which agree with our measured
resonance strengths and rate coefficients to within typically better than
<~30%. We provide a fit to our inferred rate coefficients for use in plasma
modeling. Using our DR measurements, we infer a factor of ~2 error in the Fe XX
through Fe XXIV Dn=0 DR rates. We investigate the effects of this estimated
error for the well-known thermal instability of photoionized gas. We find that
errors in these rates cannot remove the instability, but they do dramatically
affect the range in parameter space over which it forms.Comment: To appear in ApJS, 44 pages with 13 figures, AASTeX with postsript
figure
Electron-ion recombination of Fe12+ forming Fe11+ : laboratory measurements and theoretical calculations
We have measured dielectronic recombination (DR) for Fe12 + forming Fe11 + using the heavy ion storage ring TSR located at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany. Using our results, we have calculated a plasma rate coefficient from these data that can be used for modeling astrophysical and laboratory plasmas. For the low temperatures characteristic of photoionized plasmas, the experimentally derived rate coefficient is orders of magnitude larger than the previously recommended atomic data. The existing atomic data were also about 40% smaller than our measurements at temperatures relevant for collisionally ionized plasmas. Recent state-of-the-art theory has difficulty reproducing the detailed energy dependence of the DR spectrum. However, for the Maxwellian plasma rate coefficient, recent theoretical results agree with our measurements to within about 30% for both photoionized and collisionally ionized plasmas
Dielectronic Recombination (via N=2 --> N'=2 Core Excitations) and Radiative Recombination of Fe XX: Laboratory Measurements and Theoretical Calculations
We have measured the resonance strengths and energies for dielectronic
recombination (DR) of Fe XX forming Fe XIX via N=2 --> N'=2 (Delta_N=0) core
excitations. We have also calculated the DR resonance strengths and energies
using AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four different
state-of-the-art theoretical techniques. On average the theoretical resonance
strengths agree to within <~10% with experiment. However, the 1 sigma standard
deviation for the ratios of the theoretical-to-experimental resonance strengths
is >~30% which is significantly larger than the estimated relative experimental
uncertainty of <~10%. This suggests that similar errors exist in the calculated
level populations and line emission spectrum of the recombined ion. We confirm
that theoretical methods based on inverse-photoionization calculations (e.g.,
undamped R-matrix methods) will severely overestimate the strength of the DR
process unless they include the effects of radiation damping. We also find that
the coupling between the DR and radiative recombination (RR) channels is small.
We have used our experimental and theoretical results to produce
Maxwellian-averaged rate coefficients for Delta_N=0 DR of Fe XX. For kT>~1 eV,
which includes the predicted formation temperatures for Fe XX in an optically
thin, low-density photoionized plasma with cosmic abundances, our experimental
and theoretical results are in good agreement. We have also used our R-matrix
results, topped off using AUTOSTRUCTURE for RR into J>=25 levels, to calculate
the rate coefficient for RR of Fe XX. Our RR results are in good agreement with
previously published calculations.Comment: To be published in ApJS. 65 pages with 4 tables and lots of figure
Ultrasonic treatment of Cryptosporidium oocysts
Abstract The effect of 20 kHz ultrasound on the viability of Cryptosporidium oocysts was investigated. More than 90% of the dispersed Cryptosporidium oocysts could be deactivated in about 1.5 min of continuous sonication. In order to apply this technique to large quantities of contaminated water, quantitative filtration and redispersion of Cryptosporidium oocysts were investigated and found to be easily achievable. The estimated cost of sonication showed that the ultrasound treatment of Cryptosporidium oocysts contaminated water could be a very effective means of "deactivating" Cryptosporidium oocysts
Recombination of W19+ ions with electrons : absolute rate coefficients from a storage-ring experiment and from theoretical calculations
Experimentally measured and theoretically calculated rate coefficients for the recombination of W19+ ([Kr] 4d10 4f9) ions with free electrons (forming W18+) are presented. At low electron-ion collision energies, the merged-beams rate coefficient is dominated by strong, mutually overlapping, recombination resonances as already found previously for the neighboring charge-state ions W18+ and W20+. In the temperature range where W19+ is expected to form in a collisionally ionized plasma, the experimentally derived recombination rate coefficient deviates by up to a factor of about 20 from the theoretical rate coefficient obtained from the ADAS database. The present calculations, which employ a Breit-Wigner redistributive partitioning of autoionizing widths for dielectronic recombination via multi-electron resonances, reproduce the experimental findings over the entire temperature range
Anisotropic fragmentation in low-energy dissociative recombination
On a dense energy grid reaching up to 75 meV electron collision energy the
fragmentation angle and the kinetic energy release of neutral dissociative
recombination fragments have been studied in a twin merged beam experiment. The
anisotropy described by Legendre polynomials and the extracted rotational state
contributions were found to vary on a likewise narrow energy scale as the
rotationally averaged rate coefficient. For the first time angular dependences
higher than 2 order could be deduced. Moreover, a slight anisotropy at
zero collision energy was observed which is caused by the flattened velocity
distribution of the electron beam.Comment: 8 pages, 4 figures; The Article will be published in the proceedings
of DR 2007, a symposium on Dissociative Recombination held in Ameland, The
Netherlands (18.-23. July 2008); Reference 19 has been published meanwhile in
S. Novotny, PRL 100, 193201 (2008
Dielectronic Recombination of Ground-State and Metastable Li+ Ions
Dielectronic recombination has been investigated for Delta-n = 1 resonances
of ground-state Li+(1s^2) and for Delta-n = 0 resonances of metastable Li+(1s2s
^3S). The ground-state spectrum shows three prominent transitions between 53
and 64 eV, while the metastable spectrum exhibits many transitions with
energies < 3.2 eV. Reasonably good agreement of R-matrix, LS coupling
calculations with the measured recombination rate coefficient is obtained. The
time dependence of the recombination rate yields a radiative lifetime of 52.2
+- 5.0 s for the 2 ^3S level of Li+.Comment: Submitted to Phys. Rev. A; REVTeX, 4 pages, 3 figure
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