3,614 research outputs found
Improved TPB-coated Light Guides for Liquid Argon TPC Light Detection Systems
Scintillation light produced in liquid argon (LAr) must be shifted from 128
nm to visible wavelengths in light detection systems used for liquid argon
time-projection chambers (LArTPCs). To date, LArTPC light collection systems
have employed tetraphenyl butadiene (TPB) coatings on photomultiplier tubes
(PMTs) or plates placed in front of the PMTs. Recently, a new approach using
TPB-coated light guides was proposed. In this paper, we report on light guides
with improved attenuation lengths above 100 cm when measured in air. This is an
important step in the development of meter-scale light guides for future
LArTPCs. Improvements come from using a new acrylic-based coating,
diamond-polished cast UV transmitting acrylic bars, and a hand-dipping
technique to coat the bars. We discuss a model for connecting bar response in
air to response in liquid argon and compare this to data taken in liquid argon.
The good agreement between the prediction of the model and the measured
response in liquid argon demonstrates that characterization in air is
sufficient for quality control of bar production. This model can be used in
simulations of light guides for future experiments.Comment: 25 pages, 20 figure
Normal State Magnetic Properties of Ni and Zn Substituted in YBa_{2}Cu_{3} O_{6+x}: Hole-Doping Dependence
We present SQUID susceptibility data on Zn and Ni substituted
YBa_{2}Cu_{3}O_{6+x}. Cross-checks with NMR yield an unprecedented accuracy in
the estimate of the magnetic susceptibility associated with the substituants,
from the underdoped to the lightly overdoped case. This allows us to determine
the Weiss temperature \theta for YBCO: its value is very small for all hole
dopings n_h. Since in conventional metals, the Kondo temperature,
, magnetic screening effects would not be expected for ; in contrast, increasing n_h produces a reduction of the small moment
induced by Zn^{2+} and a nearly constant effective moment for Ni^{2+}
corresponding to a spin 1/2 rather than to a spin 1.Comment: 4 pages, 5 figures, to be published in Europhysics Letter
The Effects of Dissolved Methane upon Liquid Argon Scintillation Light
In this paper we report on measurements of the effects of dissolved methane
upon argon scintillation light. We monitor the light yield from an alpha source
held 20 cm from a cryogenic photomultiplier tube (PMT) assembly as methane is
injected into a high-purity liquid argon volume. We observe significant
suppression of the scintillation light yield by dissolved methane at the 10
part per billion (ppb) level. By examining the late scintillation light time
constant, we determine that this loss is caused by an absorption process and
also see some evidence of methane-induced scintillation quenching at higher
concentrations (50-100 ppb). Using a second PMT assembly we look for visible
re-emission features from the dissolved methane which have been reported in
gas-phase argon methane mixtures, and we find no evidence of visible
re-emission from liquid-phase argon methane mixtures at concentrations between
10 ppb and 0.1%.Comment: 18 pages, 11 figures Updated to match published versio
Depolarization regions of nonzero volume in bianisotropic homogenized composites
In conventional approaches to the homogenization of random particulate
composites, the component phase particles are often treated mathematically as
vanishingly small, point-like entities. The electromagnetic responses of these
component phase particles are provided by depolarization dyadics which derive
from the singularity of the corresponding dyadic Green functions. Through
neglecting the spatial extent of the depolarization region, important
information may be lost, particularly relating to coherent scattering losses.
We present an extension to the strong-property-fluctuation theory in which
depolarization regions of nonzero volume and ellipsoidal geometry are
accommodated. Therein, both the size and spatial distribution of the component
phase particles are taken into account. The analysis is developed within the
most general linear setting of bianisotropic homogenized composite mediums
(HCMs). Numerical studies of the constitutive parameters are presented for
representative examples of HCM; both Lorentz-reciprocal and
Lorentz-nonreciprocal HCMs are considered. These studies reveal that estimates
of the HCM constitutive parameters in relation to volume fraction, particle
eccentricity, particle orientation and correlation length are all significantly
influenced by the size of the component phase particles
A Brief History of AGN
Astronomers knew early in the twentieth century that some galaxies have
emission-line nuclei. However, even the systematic study by Seyfert (1943) was
not enough to launch active galactic nuclei (AGN) as a major topic of
astronomy. The advances in radio astronomy in the 1950s revealed a new universe
of energetic phenomena, and inevitably led to the discovery of quasars. These
discoveries demanded the attention of observers and theorists, and AGN have
been a subject of intense effort ever since. Only a year after the recognition
of the redshifts of 3C 273 and 3C 48 in 1963, the idea of energy production by
accretion onto a black hole was advanced. However, acceptance of this idea came
slowly, encouraged by the discovery of black hole X-ray sources in our Galaxy
and, more recently, supermassive black holes in the center of the Milky Way and
other galaxies. Many questions remain as to the formation and fueling of the
hole, the geometry of the central regions, the detailed emission mechanisms,
the production of jets, and other aspects. The study of AGN will remain a
vigorous part of astronomy for the foreseeable future.Comment: 37 pages, no figures. Uses aaspp4.sty. To be published in
Publications of the Astronomical Society of the Pacific, 1999 Jun
Numerical modelling of transient cyclic vertical loading of suction caissons in sand
This paper presents numerical investigations of the monotonic and cyclic behaviours of suction caissons upon vertical transient loading. Both drained and partially drained conditions are investigated. Monotonic compression and traction simulations are carried out to qualitatively compare results with the literature and validate the model. They highlight the different modes of reaction of the caisson to both compression and traction loading. A sensitivity analysis points out the strong influence of some parameters on the resistance of the caisson but also on the failure mechanism. The transient behaviour of the caisson upon different kinds of cyclic load signals is analysed. Results reproduce the settlement and pore water pressure accumulations observed during experiments. The influence of the key design parameters on the settlement accumulation is also assessed. Finally a cyclic diagram is proposed to describe the evolution of the final settlement upon different magnitudes of loading
Predicted FeII Emission-Line Strengths from Active Galactic Nuclei
We present theoretical FeII emission line strengths for physical conditions
typical of Active Galactic Nuclei with Broad-Line Regions. The FeII line
strengths were computed with a precise treatment of radiative transfer using
extensive and accurate atomic data from the Iron Project. Excitation mechanisms
for the FeII emission included continuum fluorescence, collisional excitation,
self-fluorescence amoung the FeII transitions, and fluorescent excitation by
Lyman-alpha and Lyman-beta. A large FeII atomic model consisting of 827 fine
structure levels (including states to E ~ 15 eV) was used to predict fluxes for
approximately 23,000 FeII transitions, covering most of the UV, optical, and IR
wavelengths of astrophysical interest. Spectral synthesis for wavelengths from
1600 Angstroms to 1.2 microns is presented. Applications of present theoretical
templates to the analysis of observations are described. In particular, we
discuss recent observations of near-IR FeII lines in the 8500 Angstrom -- 1
micron region which are predicted by the Lyman-alpha fluorescence mechanism. We
also compare our UV spectral synthesis with an empirical iron template for the
prototypical, narrow-line Seyfert galaxy I Zw 1. The theoretical FeII template
presented in this work should also applicable to a variety of objects with FeII
spectra formed under similar excitation conditions, such as supernovae and
symbiotic stars.Comment: 33 pages, 15 postscript figure
Singular Modes of the Electromagnetic Field
We show that the mode corresponding to the point of essential spectrum of the
electromagnetic scattering operator is a vector-valued distribution
representing the square root of the three-dimensional Dirac's delta function.
An explicit expression for this singular mode in terms of the Weyl sequence is
provided and analyzed. An essential resonance thus leads to a perfect
localization (confinement) of the electromagnetic field, which in practice,
however, may result in complete absorption.Comment: 14 pages, no figure
Late Emission from the Type Ib/c SN 2001em: Overtaking the Hydrogen Envelope
The Type Ib/c supernova SN 2001em was observed to have strong radio, X-ray,
and Halpha emission at an age of about 2.5 yr. Although the radio and X-ray
emission have been attributed to an off-axis gamma-ray burst, we model the
emission as the interaction of normal SN Ib/c ejecta with a dense, massive (3
Msun) circumstellar shell at a distance about 7 x 10^{16} cm. We investigate
two models, in which the circumstellar shell has or has not been overtaken by
the forward shock at the time of the X-ray observation. The circumstellar shell
was presumably formed by vigorous mass loss with a rate (2-10) x 10^{-3}
Msun/yr at 1000-2000 yr prior to the supernova explosion. The hydrogen envelope
was completely lost, and subsequently was swept up and accelerated by the fast
wind of the presupernova star up to a velocity of 30-50 km/s. Although
interaction with the shell can explain most of the late emission properties of
SN 2001em, we need to invoke clumping of the gas to explain the low absorption
at X-ray and radio wavelengths.Comment: 26 pages, 4 figures, ApJ submitte
To âSketch-a-Scratchâ
A surface can be harsh and raspy, or smooth and silky, and everything in between. We are used to sense these features with our fingertips as well as with our eyes and ears: the exploration of a surface is a multisensory experience.
Tools, too, are often employed in the interaction with surfaces, since they augment our manipulation capabilities.
âSketch-a-Scratchâ is a tool for the multisensory exploration and sketching of surface textures. The userâs actions drive a physical sound model of real materialsâ response to interactions such as scraping, rubbing or rolling.
Moreover, different input signals can be converted into 2D visual surface profiles, thus enabling to experience them visually, aurally and haptically
- âŠ