319 research outputs found
Continuum elastic sphere vibrations as a model for low-lying optical modes in icosahedral quasicrystals
The nearly dispersionless, so-called "optical" vibrational modes observed by
inelastic neutron scattering from icosahedral Al-Pd-Mn and Zn-Mg-Y
quasicrystals are found to correspond well to modes of a continuum elastic
sphere that has the same diameter as the corresponding icosahedral basic units
of the quasicrystal. When the sphere is considered as free, most of the
experimentally found modes can be accounted for, in both systems. Taking into
account the mechanical connection between the clusters and the remainder of the
quasicrystal allows a complete assignment of all optical modes in the case of
Al-Pd-Mn. This approach provides support to the relevance of clusters in the
vibrational properties of quasicrystals.Comment: 9 pages without figure
Yttria and Ceria Doped Zirconia Thin Films Grown by Pulsed Laser Deposition
The Yttria stabilized Zirconia (YSZ) is a standard electrolyte for solid oxide fuel cells (SOFCs), which are potential candidates for next generation portable and mobile power sources. YSZ electrolyte thin films having a cubic single phase allow reducing the SOFC operating temperature without diminishing the electrochemical power density. Films of 8 mol % Yttria stabilized Zirconia (8YSZ) and films with addition of 4 weight % Ceria (8YSZ + 4CeO2) were grown by pulsed laser deposition (PLD) technique using 8YSZ and 8YSZ + 4CeO2 targets and a Nd-YAG laser (355 nm). Films have been deposited on Soda-Calcia-Silica glass and Si(100) substrates at room temperature. The morphology and structural characteristics of the samples have been studied by means of X-ray diffraction and scanning electron microscopy. Films of a cubic-YSZ single phase with thickness in the range of 1-3 µm were grown on different substrates.Fil: Saporiti, F.. Universidad de Buenos Aires. Facultad de IngenierĂa. Departamento de IngenierĂa Mecánica. Grupo de Materiales Avanzados; Argentina;Fil: Juarez, R. E.. Universidad de Buenos Aires. Facultad de IngenierĂa. Departamento de IngenierĂa Mecánica. Grupo de Materiales Avanzados; Argentina;Fil: Audebert, Fernando Enrique. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de TecnologĂas y Ciencias de la IngenierĂa; Argentina; Universidad de Buenos Aires. Facultad de IngenierĂa. Departamento de IngenierĂa Mecánica. Grupo de Materiales Avanzados; Argentina;Fil: Boudard, M.. Centre National de la Recherche Scientifique. Laboratoire des MatĂ©riaux et du GĂ©nie Physique; Francia
Sound modes broadening for Fibonacci one dimensional quasicrystals
We investigate vibrational excitation broadening in one dimensional Fibonacci
model of quasicrystals (QCs). The chain is constructed from particles with two
masses following the Fibonacci inflation rule. The eigenmode spectrum depends
crucially on the mass ratio. We calculate the eigenstates and eigenfunctions.
All calculations performed self-consistently within the regular expansion over
the three wave coupling constant. The approach can be extended to three
dimensional systems. We find that in the intermediate range of mode coupling
constants, three-wave broadening for the both types of systems (1D Fibonacci
and 3D QCs) depends universally on frequency. Our general qualitative
conclusion is that for a system with a non-simple elementary cell phonon
spectrum broadening is always larger than for a system with a primitive cell
(provided all other characteristics are the same).Comment: 2o pages, 15 figure
Measurement of nuclide cross-sections of spallation residues in 1 A GeV 238U + proton collisions
The production of heavy nuclides from the spallation-evaporation reaction of
238U induced by 1 GeV protons was studied in inverse kinematics. The
evaporation residues from tungsten to uranium were identified in-flight in mass
and atomic number. Their production cross-sections and their momentum
distributions were determined. The data are compared with empirical
systematics. A comparison with previous results from the spallation of 208Pb
and 197Au reveals the strong influence of fission in the spallation of 238U.Comment: 20 pages, 10 figures, background information at
http://www-wnt.gsi.de/kschmidt
Production of medium-mass neutron-rich nuclei in reactions induced by 136Xe projectiles at 1 A GeV on a beryllium target
Production cross sections of medium-mass neutron-rich nuclei obtained in the
fragmentation of 136Xe projectiles at 1 A GeV have been measured with the
FRagment Separator (FRS) at GSI. 125Pd was identified for the first time. The
measured cross sections are compared to 238U fission yields and model
calculations in order to determine the optimum reaction mechanism to extend the
limits of the chart of the nuclides around the r-process waiting point at N=82.Comment: 9 pages, 6 figure
Fission-Residues Produced in the Spallation Reaction 238U+p at 1 A GeV
Fission fragments from 1 A GeV 238U projectiles irradiating a hydrogen target
were investigated by using the fragment separator FRS for magnetic selection of
reaction products including ray-tracing and DE-ToF techniques. The momentum
spectra of 733 identified fragments were analysed to provide isotopic
production cross sections, fission-fragment velocities and recoil momenta of
the fissioning parent nuclei. Besides their general relevance, these quantities
are also demanded for applications. Calculations and simulations with codes
commonly used and recently developed or improved are compared to the data.Comment: 60 pages, 21 figures, 4 tables, 2 appendices (15 pages
Evaporation residues produced in spallation of 208Pb by protons at 500A MeV
The production cross sections of fragmentation-evaporation residues in the
reaction Pb+p at 500A MeV have been measured using the inverse-kinematics
method and the FRS spectrometer (GSI). Fragments were identified in nuclear
charge using ionisation chambers. The mass identification was performed
event-by-event using the B-rho - TOF - Delta-E technique. Although
partially-unresolved ionic charge states induced an ambiguity on the mass of
some heavy fragments, production rates could be obtained with a high accuracy
by systematically accounting for the polluting ionic charge states. The
contribution of multiple reactions in the target was subtracted using a new,
partly self-consistent code. The isobaric distributions are found to have a
shape very close to the one observed in experiments at higher energy. Kinematic
properties of the fragments were also measured. The total and the isotopic
cross sections, including charge-pickup cross sections, are in good agreement
with previous measurements. The data are discussed in the light of previous
spallation measurements, especially on lead at 1 GeV
Coincidence measurement of residues and light particles in the reaction 56Fe+p at 1 GeV per nucleon with SPALADIN
The spallation of Fe in collisions with hydrogen at 1 A GeV has been
studied in inverse kinematics with the large-aperture setup SPALADIN at GSI.
Coincidences of residues with low-center-of-mass kinetic energy light particles
and fragments have been measured allowing the decomposition of the total
reaction cross-section into the different possible de-excitation channels.
Detailed information on the evolution of these de-excitation channels with
excitation energy has also been obtained. The comparison of the data with
predictions of several de-excitation models coupled to the INCL4 intra-nuclear
cascade model shows that only GEMINI can reasonably account for the bulk of
collected results, indicating that in a light system with no compression and
little angular momentum, multifragmentation might not be necessary to explain
the data.Comment: 4 pages, 5 figures, revised version accepted in Phys. Rev. Let
On contribution of three-body forces to interaction at intermediate energies
Available data on large-angle nucleon-deuteron elastic scattering
below the pion threshold give a signal for three-body forces. There is a
problem of separation of possible subtle aspects of these forces from off-shell
effects in two-nucleon potentials.
By considering the main mechanisms of the process, we show qualitatively that
in the quasi-binary reaction with the final spin singlet
NN-pair in the S-state the relative contribution of the 3N forces differs
substantially from the elastic channel.
It gives a new testing ground for the problem in question.Comment: 9 pages, Latex, 3 Postscript figure
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