3,532 research outputs found
Electromagnetic KY production from the proton in a Regge-plus-resonance approach
A Regge-plus-resonance (RPR) description of the p(\gamma,K)Y and p(e,e'K)Y
processes (Y = \Lambda, \Sigma^{0,+}) is presented. The proposed reaction
amplitude consists of Regge-trajectory exchanges in the t channel, supplemented
with a limited selection of s-channel resonance diagrams. The RPR framework
contains a considerably smaller number of free parameters than a typical
effective-Lagrangian model. Nevertheless, it provides an acceptable overall
description of the photo- and electroproduction observables over an extensive
photon energy range. It is shown that the electroproduction response functions
and polarization observables are particularly useful for fine-tuning both the
background and resonance parameters.Comment: 4 pages, 3 figures, Proceedings for IX International Conference on
Hypernuclear and Strange Particle Physics (HYP2006), October 10-14 2006,
Main
Cathodoluminescence of nanocrystalline Y2O3:Eu3+ with various Eu3+ concentrations
© The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.This article has been made available through the Brunel Open Access Publishing Fund.Herein a study on the preparation and cathodoluminescence of monosized spherical nanoparticles of Y2O3:Eu3+ having a Eu3+ concentration that varies between 0.01 and 10% is described. The luminous efficiency and decay time have been determined at low a current density, whereas cathodoluminescence-microscopy has been carried out at high current density, the latter led to substantial saturation of certain spectral transitions. A novel theory is presented to evaluate the critical distance for energy transfer from Eu3+ ions in S6 to Eu3+ ions in C2 sites. It was found that Y2O3:Eu3+ with 1–2% Eu3+ has the highest luminous efficiency of 16lm/w at 15keV electron energy. Decay times of the emission from 5D0 (C2) and 5D1 (C2) and 5D0 (S6) levels were determined. The difference in decay time from the 5D0 (C2) and 5D1 (C2) levels largely explained the observed phenomena in the cathodoluminescence-micrographs recorded with our field emission scanning electron microscope
Cathodoluminescence of Double Layers of Phosphor Particles
This article has been made available through the Brunel Open Access Publishing Fund.We present radiance measurements of particle layers of ZnO:Zn, Y2O3:Eu and Y2O2S:Eu bombarded with electrons at anode voltages between 1 and 15 kV. The layers described in this work refer to single component layers, double layers and two component mixtures. The phosphor layers are deposited on ITO-coated glass slides by settling; the efficiency of the cathodoluminescence is determined by summing the radiances and luminances in the reflected and transmitted modes respectively. The efficiency of a double layer of Y2O3:Eu on top of ZnO:Zn at high electron energy is significantly larger than the efficiency of a corresponding layer in which the two components are mixed. This result is interpreted in terms of the penetration-model, which predicts a larger efficiency for a high-voltage phosphor on top of a low-voltage phosphor. When a layer of the low-voltage phosphor ZnO:Zn is on top of the high-voltage phosphor Y2O3:Eu, we also observe a higher efficiency than that of the corresponding layer with both components mixed. In this case the efficiency increases due to suppression of charging in the Y2O3:Eu layer. Double layers of ZnO:Zn and Y2O2S:Eu did not show enhanced efficiency, because the size of the Y2O2S:Eu particles was too large to evoke the penetration effect.
© The Author(s) 2014. Published by ECS
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Cathodoluminescence studies of phosphors in a scanning electron microscope
Cathodoluminescence studies are reported of phosphors in a field emission scanning electron microscope (FESEM). A number of phosphor materials have been studied and exhibited a pronounced comet-like structure at high scan rates, because the particle continued to emit light after the beam had moved onto subsequent pixels. Image analysis has been used to study the loss of brightness along the tail and hence to determine the decay time of the materials. This technique provides a simple and convenient way to study the decay times of individual particles
Differential cross section analysis in kaon photoproduction using associated legendre polynomials
Angular distributions of differential cross sections from the latest CLAS
data sets \cite{bradford}, for the reaction have been analyzed using associated Legendre polynomials. This
analysis is based upon theoretical calculations in Ref. \cite{fasano} where all
sixteen observables in kaon photoproduction can be classified into four
Legendre classes. Each observable can be described by an expansion of
associated Legendre polynomial functions. One of the questions to be addressed
is how many associated Legendre polynomials are required to describe the data.
In this preliminary analysis, we used data models with different numbers of
associated Legendre polynomials. We then compared these models by calculating
posterior probabilities of the models. We found that the CLAS data set needs no
more than four associated Legendre polynomials to describe the differential
cross section data. In addition, we also show the extracted coefficients of the
best model.Comment: Talk given at APFB08, Depok, Indonesia, August, 19-23, 200
Statistical Properties of Interacting Bose Gases in Quasi-2D Harmonic Traps
The analytical probability distribution of the quasi-2D (and purely 2D) ideal
and interacting Bose gas are investigated by using a canonical ensemble
approach. Using the analytical probability distribution of the condensate, the
statistical properties such as the mean occupation number and particle number
fluctuations of the condensate are calculated. Researches show that there is a
continuous crossover of the statistical properties from a quasi-2D to a purely
2D ideal or interacting gases. Different from the case of a 3D Bose gas, the
interaction between atoms changes in a deep way the nature of the particle
number fluctuations.Comment: RevTex, 10pages, 4 figures, E-mail: [email protected]
A weakly nonlinear Alfvénic pulse in a transversely inhomogeneous medium
The interaction of a weakly nonlinear Alfvénic pulse with an Alfvén speed inhomogeneity in the direction perpendicular to the magnetic field is investigated. Identical to the phase mixing experienced by a harmonic Alfvén wave, sharp transverse gradients are generated in the pulse by the inhomogeneity. In the initial stage of the evolution of an initially plane Alfvénic pulse, the transverse gradients efficiently generate transversely propagating fast magnetoacoustic waves. However, high resolution full MHD numerical simulations of the developed stage of the pulse evolution show that the generation saturates due to destructive wave interference. It is shown that the weakly non-linear description of the generated fast magnetoacoustic wave is well described by the driven wave equation proposed in Nakariakov et al. (1997), and a simple numerical code (2D MacCromack), which solves it with minimal CPU resources, produces identical results to those obtained from the full MHD code (Lare2d, Arber et al. 2001). A parametric study of the phenomenon is undertaken, showing that, contrary to one's expectations, steeper inhomogeneities of the Alfvén speed do not produce higher saturation levels of the fast wave generation. There is a certain optimal gradient of the inhomogeneity that ensures the maximal efficiency of the fast wave generation
Witten's Invariants of Rational Homology Spheres at Prime Values of and Trivial Connection Contribution
We establish a relation between the coefficients of asymptotic expansion of
trivial connection contribution to Witten's invariant of rational homology
spheres and the invariants that T.~Ohtsuki extracted from Witten's invariant at
prime values of . We also rederive the properties of prime invariants
discovered by H.~Murakami and T.~Ohtsuki. We do this by using the bounds on
Taylor series expansion of the Jones polynomial of algebraically split links,
studied in our previous paper. These bounds are enough to prove that Ohtsuki's
invariants are of finite type. The relation between Ohtsuki's invariants and
trivial connection contribution is verified explicitly for lens spaces and
Seifert manifolds.Comment: 32 pages, no figures, LaTe
Optical Interferometry of early-type stars with PAVO@CHARA. I. Fundamental stellar properties
We present interferometric observations of 7 main-sequence and 3 giant stars
with spectral types from B2 to F6 using the PAVO beam combiner at the CHARA
array. We have directly determined the angular diameters for these objects with
an average precision of 2.3%. We have also computed bolometric fluxes using
available photometry in the visible and infrared wavelengths, as well as
space-based ultraviolet spectroscopy. Combined with precise \textit{Hipparcos}
parallaxes, we have derived a set of fundamental stellar properties including
linear radius, luminosity and effective temperature. Fitting the latter to
computed isochrone models, we have inferred masses and ages of the stars. The
effective temperatures obtained are in good agreement (at a 3% level) with
nearly-independent temperature estimations from spectroscopy. They validate
recent sixth-order polynomial (B-V)- empirical relations
\citep{Boyajian2012a}, but suggest that a more conservative third-order
solution \citep{vanBelle2009} could adequately describe the
(V-K)- relation for main-sequence stars of spectral type A0 and
later. Finally, we have compared mass values obtained combining surface gravity
with inferred stellar radius (\textit{gravity mass}) and as a result of the
comparison of computed luminosity and temperature values with stellar
evolutionary models (\textit{isochrone mass}). The strong discrepancy between
isochrone and gravity mass obtained for one of the observed stars,
\,Lyr, suggests that determination of the stellar atmosphere parameters
should be revised.Comment: 13 pages, 9 figures, accepted for publication in MNRA
Detection and Interpretation Of Long-Lived X-Ray Quasi-Periodic Pulsations in the X-Class Solar Flare On 2013 May 14
Quasi-periodic pulsations (QPP) seen in the time derivative of the GOES soft
X-ray light curves are analyzed for the near-limb X3.2 event on 14 May 2013.
The pulsations are apparent for a total of at least two hours from the
impulsive phase to well into the decay phase, with a total of 163 distinct
pulses evident to the naked eye. A wavelet analysis shows that the
characteristic time scale of these pulsations increases systematically from
25 s at 01:10 UT, the time of the GOES peak, to 100 s at 02:00 UT.
A second ridge in the wavelet power spectrum, most likely associated with
flaring emission from a different active region, shows an increase from
40 s at 01:40 UT to 100 s at 03:10 UT. We assume that the QPP that
produced the first ridge result from vertical kink-mode oscillations of the
newly formed loops following magnetic reconnection in the coronal current
sheet. This allows us to estimate the magnetic field strength as a function of
altitude given the density, loop length, and QPP time scale as functions of
time determined from the GOES light curves and RHESSI images. The calculated
magnetic field strength of the newly formed loops ranges from about 500 G
at an altitude of 24 Mm to a low value of 10 G at 60 Mm, in general
agreement with the expected values at these altitudes. Fast sausage mode
oscillations are also discussed and cannot be ruled out as an alternate
mechanism for producing the QPP
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