20,307 research outputs found
Calibration of a cylindrical RF capacitance probe
The calibration is considered of an RF antenna capacitance probe carried aboard the RAE-1 spacecraft and the correction of the probe for external effects, believed to be primarily due to local positive ion sheaths and/or photoelectron sheaths surrounding the antenna. The RAE-1 spacecraft was launched in July 1968 into a 5850-km. Circular orbit of 121-degree inclination and carried several antenna and radiometer systems covering a frequency range of 0.2 to 9.2 MHz for radio astronomical studies. The RF capacitance probe measurements discussed utilized a 37-meter electric dipole antenna formed by two monopoles made of silver-coated beryllium-copper alloy tapes formed into hollow cylindrical tubes 1.3 cm in diameter
Nuclear energy density functional from chiral pion-nucleon dynamics: Isovector terms
We extend a recent calculation of the nuclear energy density functional in
the framework of chiral perturbation theory by computing the isovector surface
and spin-orbit terms: (\vec \nabla \rho_p- \vec \nabla \rho_n)^2 G_d(\rho)+
(\vec \nabla \rho_p- \vec \nabla \rho_n)\cdot(\vec J_p-\vec J_n)
G_{so(\rho)+(\vec J_p-\vec J_n)^2 G_J(\rho) pertaining to different proton and
neutron densities. Our calculation treats systematically the effects from
-exchange, iterated -exchange, and irreducible -exchange with
intermediate -isobar excitations, including Pauli-blocking corrections
up to three-loop order. Using an improved density-matrix expansion, we obtain
results for the strength functions , and
which are considerably larger than those of phenomenological Skyrme forces.
These (parameter-free) predictions for the strength of the isovector surface
and spin-orbit terms as provided by the long-range pion-exchange dynamics in
the nuclear medium should be examined in nuclear structure calculations at
large neutron excess.Comment: 12 pages, 5 figure
Global stability analysis of birhythmicity in a self-sustained oscillator
We analyze global stability properties of birhythmicity in a self-sustained
system with random excitations. The model is a multi-limit cycles variation of
the van der Pol oscillatorintroduced to analyze enzymatic substrate reactions
in brain waves. We show that the two frequencies are strongly influenced by the
nonlinear coefficients and . With a random excitation, such as
a Gaussian white noise, the attractor's global stability is measured by the
mean escape time from one limit-cycle. An effective activation energy
barrier is obtained by the slope of the linear part of the variation of the
escape time versus the inverse noise-intensity 1/D. We find that the
trapping barriers of the two frequencies can be very different, thus leaving
the system on the same attractor for an overwhelming time. However, we also
find that the system is nearly symmetric in a narrow range of the parameters.Comment: 17 pages, 8 figures, to appear on Choas, 201
Large Scale Power Spectrum from Peculiar Velocities Via Likelihood Analysis
The power spectrum (PS) of mass density fluctuations, independent of
`biasing', is estimated from the Mark III catalog of peculiar velocities using
Bayesian statistics. A parametric model is assumed for the PS, and the free
parameters are determined by maximizing the probability of the model given the
data. The method has been tested using detailed mock catalogs. It has been
applied to generalized CDM models with and without COBE normalization.
The robust result for all the models is a relatively high PS, with at . An
extrapolation to smaller scales using the different CDM models yields . The peak is weakly constrained to the range
. These results are consistent with a direct
computation of the PS (Kolatt & Dekel 1996). When compared to galaxy-density
surveys, the implied values for () are of order
unity to within 25%.
The parameters of the COBE-normalized, flat CDM model are confined by a 90%
likelihood contour of the sort , where
and for models with and without tensor
fluctuations respectively. For open CDM the powers are and (no tensor fluctuations). A -shape model free of COBE
normalization yields only a weak constraint: .Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in The
Astrophysical Journa
Nuclear energy density functional from chiral pion-nucleon dynamics: Isovector spin-orbit terms
We extend a recent calculation of the nuclear energy density functional in
the systematic framework of chiral perturbation theory by computing the
isovector spin-orbit terms: . The calculation
includes the one-pion exchange Fock diagram and the iterated one-pion exchange
Hartree and Fock diagrams. From these few leading order contributions in the
small momentum expansion one obtains already a good equation of state of
isospin-symmetric nuclear matter. We find that the parameterfree results for
the (density-dependent) strength functions and agree
fairly well with that of phenomenological Skyrme forces for densities . At very low densities a strong variation of the strength functions
and with density sets in. This has to do with chiral
singularities and the presence of two competing small mass scales
and . The novel density dependencies of and
as predicted by our parameterfree (leading order) calculation should
be examined in nuclear structure calculations.Comment: 9 pages, 3 figure, published in: Physical Review C68, 014323 (2003
Steady-state signatures of radiation trapping by cold multilevel atoms
In this paper, we use steady-state measurements to obtain evidence of
radiation trapping in an optically thick a cloud of cold rubidium atoms. We
investigate the fluorescence properties of our sample, pumped on opened
transitions. The intensity of fluorescence exhibits a non trivial dependence on
the optical thickness of the media. A simplified model, based on rate equations
self-consistently coupled to a diffusive model of light transport, is used to
explain the experimental observations in terms of incoherent radiation trapping
on one spectral line. Measurements of atomic populations and fluorescence
spectrum qualitatively agree with this interpretation.Comment: 8 pages, 5 figure
"Oxide-free" tip for scanning tunneling microscopy
We report a new tip for scanning tunneling microscopy and a tip repair procedure that allows one to reproducibly obtain atomic images of highly oriented pyrolytic graphite with previously inoperable tips. The tips are shown to be relatively oxide-free and highly resistant to oxidation. The tips are fabricated with graphite by two distinct methods
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