4,385 research outputs found
Portable Pulse Code Modulation /PCM/
Small, programmable, high speed PCM subsystem, supports the variety of signals inherent in sophisticated equipment. A signal generated by a transducer is first conditioned to the proper signal range, then sampled by an external multiplexer or by the subsystem directly and then converted and transmitted to a receiving station
Extended x-ray absorption fine structure study of porous GaSb formed by ion implantation
Porous GaSb has been formed by Ga ion implantation into crystalline GaSb substrates at either room temperature or â180 °C. The morphology has been characterized using scanning electron microscopy and the atomic structure was determined using extended x-ray absorption fine structure spectroscopy. Room-temperature implantation at low fluences leads to the formation of âŒ20-nm voids though the material remains crystalline. Higher fluences cause the microstructure to evolve into a network of amorphous GaSb rods âŒ15 nm in diameter. In contrast, implantation at â180 °C generates large, elongated voids but no rods. Upon exposure to air, the surface of the porous material is readily oxidized yielding GaâOâ and metallic Sb precipitates, the latter resulting from the reduction of unstable SbâOâ. We consider and discuss the atomic-scale mechanisms potentially operative during the concurrent crystalline-to-amorphous and continuous-to-porous transformations
Tests of Power Corrections for Event Shapes in e+e- Annihilation
A study of perturbative QCD calculations combined with power corrections to
model hadronisation effects is presented. The QCD predictions are fitted to
differential distributions and mean values of event shape observables measured
in e+e- annihilation at centre-of-mass energies from 14 to 189 GeV. We
investigate the event shape observables thrust, heavy jet mass, C-parameter,
total and wide jet broadening and differential 2-jet rate and observe a good
description of the data by the QCD predictions. The strong coupling constant
alpha_S(M_Z) and the free parameter of the power correction calculations
alpha_0(2 GeV) are measured to be alpha_S(M_Z) = 0.1171 +/- 0.0032/0.0020 and
alpha_0(2 GeV) = 0.513 +/- 0.066/0.045. The predicted universality of alpha_0
is confirmed within the uncertainties of the measurements.Comment: 28 pages, LaTeX2e, 21 .eps-files included, accepted by Eur. Phys. J.
A Study of Event Shapes and Determinations of alpha_s using data of e^+e^- Annihilations at sqrt{s} = 22 to 44 GeV
Data recorded by the JADE experiment at the PETRA e^+e^- collider were used
to measure the event shape observables thrust, heavy jet mass, wide and total
jet broadening and the differential 2-jet rate in the Durham scheme. For the
latter three observables, no experimental results have previously been
presented at these energies. The distributions were compared with resummed QCD
calulations (O(alpha_s^2)+NLLA), and the strong coupling constant alpha_s(Q)
was determined at different energy scales Q=sqrt{s}. The results,
\alpha_s(22 GeV) = 0.161 ^{+0.016}_{-0.011},
\alpha_s(35 GeV) = 0.143 ^{+0.011}_{-0.007},
\alpha_s(44 GeV) = 0.137 ^{+0.010}_{-0.007}, are in agreement with previous
combined results of PETRA albeit with smaller uncertainties. Together with
corresponding data from LEP, the energy dependence of alpha_s is significantly
tested and is found to be in good agreement with the QCD expectation.
Similarly, mean values of the observables were compared to analytic QCD
predictions where hadronisation effects are absorbed in calculable power
corrections.Comment: 36 pages, LaTeX2e, 34 .eps-files included, submitted to Z. Phys. C,
revised version, with comments of referee included and some typos corrected,
accepted for publicatio
Nanoscale density fluctuations in swift heavy ion irradiated amorphous SiO2
We report on the observation of nanoscale density fluctuations in 2 ÎŒm thick amorphous SiOâ layers irradiated with 185 MeV Au ions. At high fluences, in excess of approximately 5âĂâ10ÂčÂČ ions/cmÂČ, where the surface is completely covered by ion tracks, synchrotron small angle x-ray scattering measurements reveal the existence of a steady state of density fluctuations. In agreement with molecular dynamics simulations, this steady state is consistent with an ion track âannihilationâ process, where high-density regions generated in the periphery of new tracks fill in low-density regions located at the center of existing tracks.The authors acknowledge the Australian Research
Council and the Australian Synchrotron Research Program
for financial support and thank the staff at the ANU Heavy
Ion facility for their continued technical assistance. O.P.,
F.D., and K.N. acknowledge financial support from the
Academy of Finland under its Centre of Excellence program
as well as the OPNA project, and grants of computer
capacity from CSC
Study of moments of event shapes in e+e- annihilation using JADE data
Data from e+e- annihilation into hadrons collected by the JADE experiment at
centre-of-mass energies between 14 GeV and 44 GeV were used to study moments of
event shape distributions. The data were compared with Monte Carlo models and
with predictions from QCD NLO order calculations. The strong coupling constant
measured from the moments is alpha_S(M_Z) = 0.1286 +/- 0.0007 (stat) +/- 0.0011
(expt) +/- 0.0022 (had) +/- 0.0068 (theo), alpha_S(M_Z) = 0.1286 +/- 0.0072
(total error), consistent with the world average. However, systematic
deficiencies in the QCD NLO order predictions are visible for some of the
higher moments.Comment: JADE note 147 submitted as contributed paper to ICHEP 2004, corrected
statistical error of 6 observable average and several typo
Measurement of the Strong Coupling Constant alpha_S from the Four-Jet Rate in e+e- Annihilation using JADE data
Data from e+e- annihilation into hadrons collected by the JADE experiment at
centre-of-mass energies between 14 GeV and 44 GeV were used to study the
four-jet rate as a function of the Durham algorithm's resolution parameter
y_cut. The four-jet rate was compared to a QCD NLO order calculations including
NLLA resummation of large logarithms. The strong coupling constant measured
from the four-jet rate is alpha_S(M_Z) = 0.1169 +/- 0.0004 (stat) +/- 0.0012
(expt) +/- 0.0021 (had) +/- 0.0007 (theo), alpha_S(M_Z) = 0.1169 +/- 0.0026
(total error) in agreement with the world average.Comment: JADE note 146 submitted as contributed paper to ICHEP 200
Formation and structural characterization of Ni nanoparticles embedded in SiOâ
Face-centered cubic Ni nanoparticles were formed in SiOâ by ion implantation and thermal annealing. Small-angle x-ray scattering in conjunction with transmission electron microscopy was used to determine the nanoparticle size as a function of annealing temperature, whereas the local atomic structure was measured with x-ray absorption spectroscopy. The influence of finite-size effects on the nanoparticle structural properties was readily apparent and included a decrease in coordination number and bond length and an increase in structural disorder for decreasing nanoparticle size. Such results are consistent with the non-negligible surface-to-volume ratio characteristic of nanoparticles. In addition, temperature-dependent x-ray absorption spectroscopy measurements showed the mean vibrational frequency (as obtained from the Einstein temperature) decreased with decreasing nanoparticle size. This reduction was attributed to the greater influence of the loosely bound, under-coordinated surface atoms prevailing over the effects of capillary pressure, the former enhancing the low frequency modes of the vibrational density of statesThis work was financially supported by the Australian
Synchrotron and the Australian Research Council with access to equipment provided by the Australian Nanofabrication
Facility
A Measurement of the QCD Colour Factors using Event Shape Distributions at sqrt(s)=14 GeV to 189 GeV
Measurements of the QCD colour factors C_A and C_F and of the number of
active quark flavours n_f in the process e+e- -> hadrons at high energy are
presented. They are based on fits of O(alpha_S**2)+NLLA QCD calculations to
distributions of the event shape observables 1-T, C, B_T and B_W measured at
centre-of-mass energies from 14 GeV to 189 GeV. Hadronisation effects are
approximated with power correction calculations which also depend on the QCD
gauge structure. In this approach potential biases from hadronisation models
are reduced. Our results for individually measured quantities are n_f = 5.64 +-
1.35, C_A = 2.88 +- 0.27 and C_F = 1.45 +- 0.27 in good agreement with QCD
based on the SU(3) symmetry group where n_f=5 for the energies considered here,
C_A=3 and C_F=4/3. From simultaneous fits of C_A and C_F we find C_A = 2.84 +-
0.24 and C_F = 1.29 +- 0.18, which is also in good agreement with the QCD
expectation.Comment: accepted by European Journal of Physics
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