1,895 research outputs found
Vibrational States of Glassy and Crystalline Orthotherphenyl
Low-frequency vibrations of glassy and crystalline orthoterphenyl are studied
by means of neutron scattering. Phonon dispersions are measured along the main
axes of a single crystal, and the corresponding longitudinal and transversal
sound velocities are obtained. For glassy and polycrystalline samples, a
density of vibrational states is determined and cross-checked against other
dynamic observables. In the crystal, low-lying zone-boundary modes lead to an
excess over the Debye density of states. In the glass, the boson peak is
located at even lower frequencies. With increasing temperature, both glass and
crystal show anharmonicity.Comment: 7 pages of LaTeX (svjour), 2 tables, 10 figures accepted in Eur.
Phys. J.
Helicity amplitudes and electromagnetic decays of strange baryon resonances
We present results for the helicity amplitudes of the lowest-lying hyperon
resonances Y*, computed within the framework of the Bonn constituent-quark
model, which is based on the Bethe-Salpeter approach. The seven parameters
entering the model are fitted against the best known baryon masses.
Accordingly, the results for the helicity amplitudes are genuine predictions.
Some hyperon resonances are seen to couple more strongly to a virtual photon
with finite Q^2 than to a real photon. Other Y*'s, such as the S_{01}(1670)
Lambda resonance or the S_{11}(1620) Sigma resonance, have large
electromagnetic decay widths and couple very strongly to real photons. The
negatively-charged and neutral members of a Sigma* triplet may couple only
moderately to the Sigma(1193), while the positively-charged member of the same
Sigma* triplet displays a relatively large coupling to the Sigma^+(1193) state.
This illustrates the necessity of investigating all isospin channels in order
to obtain a complete picture of the hyperon spectrum.Comment: 4 pages, 1 figure, 1 table, Proceedings of the Conference
"International Workshop on the Physics of Excited Baryons NSTAR 05",
Tallahassee, Florida (USA), Oct. 2005, contributed tal
Comparing Simulations and Observations of the Lyman-Alpha Forest I. Methodology
We describe techniques for comparing spectra extracted from cosmological
simulations and observational data, using the same methodology to link
Lyman-alpha properties derived from the simulations with properties derived
from observational data. The eventual goal is to measure the coherence or
clustering properties of Lyman-alpha absorbers using observations of quasar
pairs and groups. We quantify the systematic underestimate in opacity that is
inherent in the continuum fitting process of observed spectra over a range of
resolution and signal-to-noise ratio. We present an automated process for
detecting and selecting absorption features over the range of resolution and
signal-to-noise of typical observational data on the Lyman-alpha "forest".
Using these techniques, we detect coherence over transverse scales out to 500
h^{-1}_{50} kpc in spectra extracted from a cosmological simulation at z = 2.Comment: 52 pages, includes 14 figures, to appear in ApJ v566 Feb 200
Annealing tests of in-pile irradiated oxide coated UâMo/AlâSi dispersed nuclear fuel
Authors do acknowledge the MERARG team for their experimental work (CEA) and F. Charollais, J. Noirot and finally B. Kapusta for their advices and comments. This study was supported by a combined Grant (FRM0911) of the Bundesministerium fĂŒr Bildung und Forschung (BMBF) and the Bayerisches Staatsministerium fĂŒr Wissenschaft, Forschung und Kunst (StMWFK).UâMo/Al based nuclear fuels have been worldwide considered as a promising high density fuel for the conversion of high flux research reactors from highly enriched uranium to lower enrichment. In this paper, we present the annealing test up to 1800°C of in-pile irradiated UâMo/AlâSi fuel plate samples. More than 70% of the fission gases (FGs) are released during two major FG release peaks around 500°C and 670°C. Additional characterisations of the samples by XRD, EPMA and SEM suggest that up to 500°C FGs are released from IDL/matrix interfaces. The second peak at 670°C representing the main release of FGs originates from the interaction between UâMo and matrix in the vicinity of the cladding
Harmonic behavior of metallic glasses up to the metastable melt
In two amorphous alloys ZrTiCuNiBe and ZrAlNiCu coherent neutron scattering has been measured over five decades in energy, including measurements in the metastable melt of a metallic alloy more than 80 K above Tg. In the vibrational spectra a pronounced "boson" peak is found: Even in crystallized samples the density of states exceeds the Debye Ï2 model, and in the amorphous state low-frequency vibrations are further enhanced. The peak position shows no dispersion in q, while intensities are strongly correlated with the static structure factor. Over the full energy range the temperature dependence is strictly harmonic. From high-energy resolution measurements we establish lower bounds for the temperatures at which structural α and fast ÎČ relaxation become observable
Static observables of relativistic three-fermion systems with instantaneous interactions
We show that static properties like the charge radius and the magnetic moment
of relativistic three-fermion bound states with instantaneous interactions can
be formulated as expectation values with respect to intrinsically defined
wavefunctions. The resulting operators can be given a natural physical
interpretation in accordance with relativistic covariance. We also indicate how
the formalism may be generalized to arbitrary moments. The method is applied to
the computation of static baryon properties with numerical results for the
nucleon charge radii and the baryon octet magnetic moments. In addition we make
predictions for the magnetic moments of some selected nucleon resonances and
discuss the decomposition of the nucleon magnetic moments in contributions of
spin and angular momentum, as well as the evolution of these contributions with
decreasing quark mass.Comment: 13 pages, including 2 figures and 3 tables, submitted to Eur.Phys.J.
Centaurus A at Hard X-rays and Soft Gamma-rays
Centaurus A, at a distance of less than 4 Mpc, is the nearest radio-loud AGN.
Its emission is detected from radio to very-high energy gamma-rays. Despite the
fact that Cen A is one of the best studied extragalactic objects the origin of
its hard X-ray and soft gamma-ray emission (100 keV < E < 50 MeV) is still
uncertain. Observations with high spatial resolution in the adjacent soft X-ray
and hard gamma-ray regimes suggest that several distinct components such as a
Seyfert-like nucleus, relativistic jets, and even luminous X-ray binaries
within Cen A may contribute to the total emission in the MeV regime that has
been detected with low spatial resolution. As the Spectral Energy Distribution
of Cen A has its second maximum around 1 MeV, this energy range plays an
important role in modeling the emission of (this) AGN. As there will be no
satellite mission in the near future that will cover this energies with higher
spatial resolution and better sensitivity, an overview of all existing hard
X-ray and soft gamma-ray measurements of Cen A is presented here defining the
present knowledge on Centaurus A in the MeV energy range.Comment: 8 pages, 5 figures, article for conference proceedings "The Many
Faces of Centaurus A"; accepted for publication in PAS
Quasars as Absorption Probes of the J0053+1234 Region
We present UBRI photometry and spectra for 60 quasars found within one square
degree centered on the J0053+1234 region, which has been the subject of the
Caltech Faint Galaxy Redshift Survey. Candidate quasars were selected by their
ultraviolet excess with respect to the stellar locus, and confirmed
spectroscopically. The quasars span a wide range in brightness (17.5<B<21.6)
and redshift (0.43<z<2.38). These new quasars comprise a grid of absorption
probes that can be used to study large-scale structure as well as the
correlation between luminous galaxies, non-luminous halos, and Lyman-alpha
absorbers in the direction of the deep pencil-beam galaxy survey. Spectra of 14
emission line galaxies found using the same technique are also presented.Comment: Accepted for publication in the Astronomical Journal; 29 pages,
including 6 tables and 7 figure
Sintering characteristics of nanocrystalline TiO2âA study combining small angle neutron scattering and nitrogen absorption-BET
Small angle neutron scattering (SANS) was employed to characterize the pore structure of nanophase TiO2 ceramic materials compacted at different temperatures. Nanophase samples, produced by inert gas condensation, were compacted at 25, 290, 413, and 550 °C using a pressure of 1 GPa. The pore size distribution of the sample compacted at room temperature was very broad, with sizes ranging from 3-30 nm and pores comprising 38% of the sample volume. Compaction at 290 and 413 °C reduced the pore volume to 25% and 20%, respectively, by eliminating pores at both the small and large ends of the distribution. Compaction at 550 °C resulted in a pore volume that was less than 8%. Complications in the SANS analysis arising from the scattering from grain boundaries are discussed. The results from SANS are compared with those derived from nitrogen absorption, BET, measurement
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