794 research outputs found
Can Sodium Abundances of A-Type Stars Be Reliably Determined from Na I 5890/5896 Lines?
An extensive non-LTE abundance analysis based on Na I 5890/5896 doublet lines
was carried out for a large unbiased sample of ~120 A-type main-sequence stars
(including 23 Hyades stars) covering a wide v_e sin i range of ~10--300 km/s,
with an aim to examine whether the Na abundances in such A dwarfs can be
reliably established from these strong Na I D lines. The resulting abundances
([Na/H]_{58}), which were obtained by applying the T_eff-dependent
microturbulent velocities of \xi ~2--4 km/s with a peak at T_eff ~ 8000 K
(typical for A stars), turned out generally negative with a large diversity
(from ~-1 to ~0), while showing a sign of v_e sin i-dependence (decreasing
toward higher rotation). However, the reality of this apparently subsolar trend
is very questionable, since these [Na/H]_{58} are systematically lower by
~0.3--0.6 dex than more reliable [Na/H]_{61} (derived from weak Na I 6154/6161
lines for sharp-line stars). Considering the large \xi-sensitivity of the
abundances derived from these saturated Na I D lines, we regard that
[Na/H]_{58} must have been erroneously underestimated, suspecting that the
conventional \xi values are improperly too large at least for such strong
high-forming Na I 5890/5896 lines, presumably due to the depth-dependence of
\xi decreasing with height. The nature of atmospheric turbulent velocity field
in mid-to-late A stars would have to be more investigated before we can
determine reliable sodium abundances from these strong resonance D lines.Comment: 14 pages, 8 figures, accepted for publication in Publ. Astron. Soc.
Japan, Vol. 61, No. 5 (2009
Validation Test of Geant4 Simulation of Electron Backscattering
Backscattering is a sensitive probe of the accuracy of electron scattering
algorithms implemented in Monte Carlo codes. The capability of the Geant4
toolkit to describe realistically the fraction of electrons backscattered from
a target volume is extensively and quantitatively evaluated in comparison with
experimental data retrieved from the literature. The validation test covers the
energy range between approximately 100 eV and 20 MeV, and concerns a wide set
of target elements. Multiple and single electron scattering models implemented
in Geant4, as well as preassembled selections of physics models distributed
within Geant4, are analyzed with statistical methods. The evaluations concern
Geant4 versions from 9.1 to 10.1. Significant evolutions are observed over the
range of Geant4 versions, not always in the direction of better compatibility
with experiment. Goodness-of-fit tests complemented by categorical analysis
tests identify a configuration based on Geant4 Urban multiple scattering model
in Geant4 version 9.1 and a configuration based on single Coulomb scattering in
Geant4 10.0 as the physics options best reproducing experimental data above a
few tens of keV. At lower energies only single scattering demonstrates some
capability to reproduce data down to a few keV. Recommended preassembled
physics configurations appear incapable of describing electron backscattering
compatible with experiment. With the support of statistical methods, a
correlation is established between the validation of Geant4-based simulation of
backscattering and of energy deposition
Investigation of Geant4 Simulation of Electron Backscattering
A test of Geant4 simulation of electron backscattering recently published in
this journal prompted further investigation into the causes of the observed
behaviour. An interplay between features of geometry and physics algorithms
implemented in Geant4 is found to significantly affect the accuracy of
backscattering simulation in some physics configurations
Quantitative Test of the Evolution of Geant4 Electron Backscattering Simulation
Evolutions of Geant4 code have affected the simulation of electron
backscattering with respect to previously published results. Their effects are
quantified by analyzing the compatibility of the simulated electron
backscattering fraction with a large collection of experimental data for a wide
set of physics configuration options available in Geant4. Special emphasis is
placed on two electron scattering implementations first released in Geant4
version 10.2: the Goudsmit-Saunderson multiple scattering model and a single
Coulomb scattering model based on Mott cross section calculation. The new
Goudsmit-Saunderson multiple scattering model appears to perform equally or
less accurately than the model implemented in previous Geant4 versions,
depending on the electron energy. The new Coulomb scattering model was flawed
from a physics point of view, but computationally fast in Geant4 version 10.2;
the physics correction released in Geant4 version 10.2p01 severely degrades its
computational performance. Evolutions in the Geant4 geometry domain have
addressed physics problems observed in electron backscattering simulation in
previous publications.Comment: To be published in IEEE Trans. Nucl. Sc
Interfacial chemical bonding-mediated ionic resistive switching.
In this paper, we present a unique resistive switching (RS) mechanism study of Pt/TiO2/Pt cell, one of the most widely studied RS system, by focusing on the role of interfacial bonding at the active TiO2-Pt interface, as opposed to a physico-chemical change within the RS film. This study was enabled by the use of a non-conventional scanning probe-based setup. The nanoscale cell is formed by bringing a Pt/TiO2-coated atomic force microscope tip into contact with a flat substrate coated with Pt. The study reveals that electrical resistance and interfacial bonding status are highly coupled together. An oxygen-mediated chemical bonding at the active interface between TiO2 and Pt is a necessary condition for a non-polar low-resistance state, and a reset switching process disconnects the chemical bonding. Bipolar switching mode did not involve the chemical bonding. The nature of chemical bonding at the TiO2-metal interface is further studied by density functional theory calculations
The characteristic comparison of the accelerometer and the gyroscope in the measurement of human body sway
This study investigated the human body sway measuring instruments. An accelerometer and a gyroscope were used to examine patients with postural control related diseases in many studies. Some studies used either an accelerometer or a gyroscope attached to the head, chest, or waist to obtain the balance assessment parameters of body sway such as area, direction, etc. The purpose of this study is to identify the reliability between both sensors in human body sway analysis by assuming the human body sway as a simple pendulum model, and suggest an optimal measurement method using the acceleration and the gyroscope. The characteristic differences between the accelerometer and the gyroscope were illustrated, focusing mainly on the differences with respect to the position of the sensors. We confirmed that the magnitude, instead of three axis vector information, may be more useful in the body sway analysis
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