19,842 research outputs found
NonâRayleigh Statistics of Ultrasonic Backscattered Echo from Tissues
The envelope of the backscattered signal from tissues can exhibit nonâRayleigh statistics if the number density of scatterers is small or if the variations in the scattering cross sections are random. The K distribution which has been used extensively in radar, is introduced to model this nonâRayleigh behavior. The generalized K distribution is extremely useful since it encompasses a wide range of distributions such as Rayleigh, Lognormal, and Rician. Computer simulations were conducted using a simple oneâdimensional discrete scatteringmodel to investigate the properties of the echo envelope. In addition to cases of low number densities, significant departures from Rayleigh statistics were seen as the scattering cross sections of the scatterers become random. The validity of this model was also tested using data from tissue mimicking phantoms. Results indicate that the density function of the envelope can be modeled by the K distribution and the parameters of the K distribution can provide information on the nature of the scattering region in terms of the number density of the scatterers as well as the scattering cross sections of the scatterers in the range cell. [Work was supported by NSF Grant No. BCSâ9207385.
The Megamaser Cosmology Project. V. An Angular Diameter Distance to NGC 6264 at 140 Mpc
We present the direct measurement of the Hubble constant, yielding the direct
measurement of the angular-diameter distance to NGC 6264 using the HO
megamaser technique. Our measurement is based on sensitive observations of the
circumnuclear megamaser disk from four observations with the Very Long Baseline
Array, the Green Bank Telescope and the Effelsberg Telescope. We also monitored
the maser spectral profile for 2.3 years using the Green Bank Telescope to
measure accelerations of maser lines by tracking their line-of-sight velocities
as they change with time. The measured accelerations suggest that the systemic
maser spots have a significantly wider radial distribution than in the
archetypal megamaser in NGC 4258. We model the maser emission as arising from a
circumnuclear disk with orbits dominated by the central black hole. The best
fit of the data gives a Hubble constant of 689 km s
Mpc, which corresponds to an angular-diameter distance of 14419
Mpc. In addition, the fit also gives a mass of the central black hole of
(3.090.42) . The result demonstrates the
feasibility of measuring distances to galaxies located well into the Hubble
flow by using circumnuclear megamaser disks.Comment: 29 pages, 8 figures, Accepted by Ap
Modeling of chemical processes in the low pressure capacitive RF discharges in a mixture of Ar/C2H2
We study the properties of a capacitive 13.56 MHz discharge properties with a
mixture of Ar/C2H2 taking into account the plasmochemistry and growth of heavy
hydrocarbons. A hybrid model was developed to combine the kinetic description
for electron motion and the fluid approach for negative and positive ions
transport and plasmochemical processes. A significant change of plasma
parameters related to injection of 5.8% portion of acetylene in argon was
observed and analyzed. We found that the electronegativity of the mixture is
about 30%. The densities of negatively and positively charged heavy
hydrocarbons are sufficiently large to be precursors for the formation of
nanoparticles in the discharge volume.Comment: 11 pages, 14 figure
The Megamaser Cosmology Project: IV. A Direct Measurement of the Hubble Constant from UGC 3789
In Papers I and II from the Megamaser Cosmology Project (MCP), we reported
initial observations of water masers in an accretion disk of a supermassive
black hole at the center of the galaxy UGC 3789, which gave an angular-diameter
distance to the galaxy and an estimate of Ho with 16% uncertainty. We have
since conducted more VLBI observations of the spatial-velocity structure of
these water masers, as well as continued monitoring of its spectrum to better
measure maser accelerations. These more extensive observations, combined with
improved modeling of the masers in the accretion disk of the central
supermassive black hole, confirm our previous results, but with signifcantly
improved accuracy. We find Ho = 68.9 +/- 7.1 km/s/Mpc; this estimate of Ho is
independent of other methods and is accurate to +/-10%, including sources of
systematic error. This places UGC 3789 at a distance of 49.6 +/- 5.1 Mpc, with
a central supermassive black hole of (1.16 +/- 0.12) x 10^7 Msun.Comment: to appear in Ap
Theory of Nonequilibrium Spin Transport and Spin Transfer Torque in Superconducting-Ferromagnetic Nanostructures
Spin transport currents and the spin-transfer torques in voltage-biased
superconducting-ferromagnetic nanopillars (SFNFS point contacts) are computed.
We develop and implement an algorithm based on the Ricatti formulation of the
quasiclassical theory of superconductivity to solve the time-dependent boundary
conditions for the nonequilibrium Green's functions for spin transport through
the ferromagnetic interfaces. A signature of the nonequilibrium torque is a
component perpendicular to the plane spanned by the two ferromagnetic moments.
The perpendicular component is absent in normal-metal-ferromagnetic nanopillars
(NFNFN) contacts, but is shown to have the same order of magnitude as the
in-plane torque for non-equilibrium SFNFS contacts. The out-of-plane torque is
due to the rotation of quasiparticle spin by the exchange fields of the
ferromagnetic layers. In the ballistic limit the equilibrium torque is related
to the spectrum of spin-polarized Andreev bound states, while the {\sl ac}
component, for small bias voltages, is determined by the nearly adiabatic
dynamics of the Andreev bound states. The nonlinear voltage dependence of the
non-equilibrium torque, including the subharmonic gap structure and the
high-voltage asymptotics, is attributed to the interplay between multiple
Andreev reflections, spin filtering and spin mixing. These properties of spin
angular momentum transport may be exploited to control the state of
nanomagnets.Comment: 15 pages, 14 figure
Arrival direction distribution of cosmic rays of energy 10 (18) eV
The Haverah Park air-shower experiment recorded over 8500 events with primary energy 10 to the 18th power eV between 1963 and 1983. An analysis of these events for anisotropies in celestial and galactic coordinates is reported. No very striking anisotropies are observed
Two liquid states of matter: A new dynamic line on a phase diagram
It is generally agreed that the supercritical region of a liquid consists of
one single state (supercritical fluid). On the other hand, we show here that
liquids in this region exist in two qualitatively different states: "rigid" and
"non-rigid" liquid. Rigid to non-rigid transition corresponds to the condition
{\tau} ~ {\tau}0, where {\tau}is liquid relaxation time and {\tau}0 is the
minimal period of transverse quasi-harmonic waves. This condition defines a new
dynamic line on the phase diagram, and corresponds to the loss of shear
stiffness of a liquid at all available frequencies, and consequently to the
qualitative change of many important liquid properties. We analyze the dynamic
line theoretically as well as in real and model liquids, and show that the
transition corresponds to the disappearance of high-frequency sound,
qualitative changes of diffusion and viscous flow, increase of particle thermal
speed to half of the speed of sound and reduction of the constant volume
specific heat to 2kB per particle. In contrast to the Widom line that exists
near the critical point only, the new dynamic line is universal: it separates
two liquid states at arbitrarily high pressure and temperature, and exists in
systems where liquid - gas transition and the critical point are absent
overall.Comment: 21 pages, 8 figure
Worldsheet boundary conditions in Poisson-Lie T-duality
We apply canonical Poisson-Lie T-duality transformations to bosonic open
string worldsheet boundary conditions, showing that the form of these
conditions is invariant at the classical level, and therefore they are
compatible with Poisson-Lie T-duality. In particular the conditions for
conformal invariance are automatically preserved, rendering also the dual model
conformal. The boundary conditions are defined in terms of a gluing matrix
which encodes the properties of D-branes, and we derive the duality map for
this matrix. We demonstrate explicitly the implications of this map for
D-branes in two non-Abelian Drinfel'd doubles.Comment: 20 pages, Latex; v2: typos and wording corrected, references added;
v3: three-dimensional example added, reference added, discussion clarified,
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