9,860 research outputs found
Neutron and muon-induced background studies for the AMoRE double-beta decay experiment
AMoRE (Advanced Mo-based Rare process Experiment) is an experiment to search
a neutrinoless double-beta decay of Mo in molybdate crystals. The
neutron and muon-induced backgrounds are crucial to obtain the zero-background
level (< counts/(keVkgyr)) for the AMoRE-II experiment,
which is the second phase of the AMoRE project, planned to run at YEMI
underground laboratory. To evaluate the effects of neutron and muon-induced
backgrounds, we performed Geant4 Monte Carlo simulations and studied a
shielding strategy for the AMORE-II experiment. Neutron-induced backgrounds
were also included in the study. In this paper, we estimated the background
level in the presence of possible shielding structures, which meet the
background requirement for the AMoRE-II experiment
Mechanism of enhanced light output in InGaN-based microlight emitting diodes
Micro-light emitting diode (LED) arrays with diameters of 4 to 20 mum have been fabricated and were found to be much more efficient light emitters compared to their broad-area counterparts, with up to five times enhancement in optical power densities. The possible mechanisms responsible for the improvement in performance were investigated. Strain relaxation in the microstructures as measured by Raman spectroscopy was not observed, arguing against theories of an increase in internal quantum efficiency due to a reduction of the piezoelectric field put forward by other groups. Optical microscope images show intense light emission at the periphery of the devices, as a result of light scattering off the etched sidewalls. This increases the extraction efficiency relative to broad area devices and boosts the forward optical output. In addition, spectra of the forward emitted light reveal the presence of resonant cavity modes [whispering gallery (WG) modes in particular] which appear to play a role in enhancing the optical output
Nonequilibrium perturbation theory for spin-1/2 fields
A partial resummation of perturbation theory is described for field theories
containing spin-1/2 particles in states that may be far from thermal
equilibrium. This allows the nonequilibrium state to be characterized in terms
of quasiparticles that approximate its true elementary excitations. In
particular, the quasiparticles have dispersion relations that differ from those
of free particles, finite thermal widths and occupation numbers which, in
contrast to those of standard perturbation theory evolve with the changing
nonequilibrium environment. A description of this kind is essential for
estimating the evolution of the system over extended periods of time. In
contrast to the corresponding description of scalar particles, the structure of
nonequilibrium fermion propagators exhibits features which have no counterpart
in the equilibrium theory.Comment: 16 pages; no figures; submitted to Phys. Rev.
InGaN nano-ring structures for high-efficiency light emitting diodes
A technique based on the Fresnel diffraction effect for the fabrication of nano-scale site-controlled ring structures in InGaN/GaN multi-quantum well structures has been demonstrated. The ring structures have an internal diameter of 500 nm and a wall width of 300 nm. A 1 cm-1 Raman shift has been measured, signifying substantial strain relaxation from the fabricated structure. The 9 nm blueshift observed in the cathodoluminescence spectra can be attributed to band filling and/or screening of the piezoelectric field. A light emitting diode based on this geometry has been demonstrated
Zero range model of traffic flow
A multi--cluster model of traffic flow is studied, in which the motion of
cars is described by a stochastic master equation. Assuming that the escape
rate from a cluster depends only on the cluster size, the dynamics of the model
is directly mapped to the mathematically well-studied zero-range process.
Knowledge of the asymptotic behaviour of the transition rates for large
clusters allows us to apply an established criterion for phase separation in
one-dimensional driven systems. The distribution over cluster sizes in our
zero-range model is given by a one--step master equation in one dimension. It
provides an approximate mean--field dynamics, which, however, leads to the
exact stationary state. Based on this equation, we have calculated the critical
density at which phase separation takes place. We have shown that within a
certain range of densities above the critical value a metastable homogeneous
state exists before coarsening sets in. Within this approach we have estimated
the critical cluster size and the mean nucleation time for a condensate in a
large system. The metastablity in the zero-range process is reflected in a
metastable branch of the fundamental flux--density diagram of traffic flow. Our
work thus provides a possible analytical description of traffic jam formation
as well as important insight into condensation in the zero-range process.Comment: 10 pages, 13 figures, small changes are made according to finally
accepted version for publication in Phys. Rev.
Camera for QUasars in EArly uNiverse (CQUEAN)
We describe the overall characteristics and the performance of an optical CCD
camera system, Camera for QUasars in EArly uNiverse (CQUEAN), which is being
used at the 2.1 m Otto Struve Telescope of the McDonald Observatory since 2010
August. CQUEAN was developed for follow-up imaging observations of red sources
such as high redshift quasar candidates (z >= 5), Gamma Ray Bursts, brown
dwarfs, and young stellar objects. For efficient observations of the red
objects, CQUEAN has a science camera with a deep depletion CCD chip which
boasts a higher quantum efficiency at 0.7 - 1.1 um than conventional CCD chips.
The camera was developed in a short time scale (~ one year), and has been
working reliably. By employing an auto-guiding system and a focal reducer to
enhance the field of view on the classical Cassegrain focus, we achieve a
stable guiding in 20 minute exposures, an imaging quality with FWHM >= 0.6"
over the whole field (4.8' * 4.8'), and a limiting magnitude of z = 23.4 AB mag
at 5-sigma with one hour total integration time.Comment: Accepted for publication in PASP. 26 pages including 5 tables and 24
figure
Particle Number Fluctuations in Relativistic Bose and Fermi Gases
Particle number fluctuations are studied in relativistic Bose and Fermi
gases. The calculations are done within both the grand canonical and canonical
ensemble. The fluctuations in the canonical ensemble are found to be different
from those in the grand canonical one. Effects of quantum statistics increase
in the grand canonical ensemble for large chemical potential. This is, however,
not the case in the canonical ensemble. In the limit of large charge density a
strongest difference between the grand canonical and canonical ensemble results
is observed.Comment: 13 pages, 6 figure
3D-Printed Microfluidic Device for the Detection of Pathogenic Bacteria Using Size-based Separation in Helical Channel with Trapezoid Cross-Section
A facile method has been developed to detect pathogenic bacteria using magnetic nanoparticle clusters (MNCs) and a 3D-printed helical microchannel. Antibody-functionalized MNCs were used to capture E. coli (EC) bacteria in milk, and the free MNCs and MNC-EC complexes were separated from the milk using a permanent magnet. The free MNCs and MNC-EC complexes were dispersed in a buffer solution, then the solution was injected into a helical microchannel device with or without a sheath flow. The MNC-EC complexes were separated from the free MNCs via the Dean drag force and lift force, and the separation was facilitated in the presence of a sheath flow. The concentration of the E. coli bacteria was determined using a light absorption spectrometer, and the limit of detection was found to be 10 cfu/mL in buffer solution and 100 cfu/mL in milk.open119188sciescopu
Relativistic theories of interacting fields and fluids
We investigate divergence-type theories (DTT) describing the dissipative
interaction between a field and a fluid. We look for theories which, under
equilibrium conditions, reduce to the theory of a Klein-Gordon scalar field and
a perfect fluid. We show that the requirements of causality and positivity of
entropy production put non-trivial constarints to the structure of the
interaction terms. These theories provide a basis for the phenomonological
study of the reheating period.Comment: 17 pages, no figures, minor corrections mad
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