192 research outputs found
Fermat's principle of least time in the presence of uniformly moving boundaries and media
The refraction of a light ray by a homogeneous, isotropic and non-dispersive
transparent material half-space in uniform rectilinear motion is investigated
theoretically. The approach is an amalgamation of the original Fermat's
principle and the fact that an isotropic optical medium at rest becomes
optically anisotropic in a frame where the medium is moving at a constant
velocity. Two cases of motion are considered: a) the material half-space is
moving parallel to the interface; b) the material half-space is moving
perpendicular to the interface. In each case, a detailed analysis of the
obtained refraction formula is provided, and in the latter case, an intriguing
backward refraction of light is noticed and thoroughly discussed. The results
confirm the validity of Fermat's principle when the optical media and the
boundaries between them are moving at relativistic speeds.Comment: 11 pages, 6 figures, RevTeX 4, comments welcome; V2: revised, Fig. 7
added; V3: several typos corrected, accepted for publication in European
Journal of Physics (online at: http://stacks.iop.org/EJP/28/933
Hierarchical model for the scale-dependent velocity of seismic waves
Elastic waves of short wavelength propagating through the upper layer of the
Earth appear to move faster at large separations of source and receiver than at
short separations. This scale dependent velocity is a manifestation of Fermat's
principle of least time in a medium with random velocity fluctuations. Existing
perturbation theories predict a linear increase of the velocity shift with
increasing separation, and cannot describe the saturation of the velocity shift
at large separations that is seen in computer simulations. Here we show that
this long-standing problem in seismology can be solved using a model developed
originally in the context of polymer physics. We find that the saturation
velocity scales with the four-third power of the root-mean-square amplitude of
the velocity fluctuations, in good agreement with the computer simulations.Comment: 7 pages including 3 figure
Ecological goods and services and agroforestry : the benefits for farmers and the interests for society
Paper presented at the 11th North American Agroforesty Conference, which was held May 31-June 3, 2009 in Columbia, Missouri.In Gold, M.A. and M.M. Hall, eds. Agroforestry Comes of Age: Putting Science into Practice. Proceedings, 11th North American Agroforestry Conference, Columbia, Mo., May 31-June 3, 2009.The main objective of this project is to estimate the social value of environmental goods and services (EG&S) generated by agroforestry practices and to evaluate the profitability of these practices for agricultural producers and for society. Two agroforestry practices are considered: riparian buffer zones and windbreaks. Moreover, the situations in two representative agricultural watersheds serve our analysis (Chateauguay and Fouquette watersheds). Among the numerous EG&S that are provided through agroforestry practices, nine have been chosen for this study: agriculture-related odors, aestheticism of the landscape, terrestrial biodiversity, surface water quality, carbon sequestration, road accidents, clearing snow from roads, treatment of drinking water and wild pollinating insects. Several economic valuation methods have been used, such as hedonic pricing value, benefit transfer, productivity method or experimental economics.Jean Nolet (1), Claude Sauve_ (1), Maria Olar (1), Maribel Hernandez (1), Marjolaine Mondon (1), Caroline Simard (1), Louis-Samuel Jacques (1), Andr� V�zina (2), Nathan De Baets (2), Maud Ablain (3), and Pierre Etcheverry (3) ; 1. EcoRessources Consultants, 825, rue Raoul-Jobin, Qu�bec, QC, Canada, G1N 1S6. 2. Activa Environnement Inc., 106, rue Industrielle, New Richmond (Qu�bec), G0C 2B0. 3. Centre d'expertise sur les produits agroforestiers (CEPAF), 235, route 230 ouest, La Pocati�re, QC, Canada, G0R 1Z0.try pIncludes bibliographical references
Characterization of an Ionization Readout Tile for nEXO
A new design for the anode of a time projection chamber, consisting of a
charge-detecting "tile", is investigated for use in large scale liquid xenon
detectors. The tile is produced by depositing 60 orthogonal metal
charge-collecting strips, 3~mm wide, on a 10~\si{\cm} 10~\si{\cm}
fused-silica wafer. These charge tiles may be employed by large detectors, such
as the proposed tonne-scale nEXO experiment to search for neutrinoless
double-beta decay. Modular by design, an array of tiles can cover a sizable
area. The width of each strip is small compared to the size of the tile, so a
Frisch grid is not required. A grid-less, tiled anode design is beneficial for
an experiment such as nEXO, where a wire tensioning support structure and
Frisch grid might contribute radioactive backgrounds and would have to be
designed to accommodate cycling to cryogenic temperatures. The segmented anode
also reduces some degeneracies in signal reconstruction that arise in
large-area crossed-wire time projection chambers. A prototype tile was tested
in a cell containing liquid xenon. Very good agreement is achieved between the
measured ionization spectrum of a Bi source and simulations that
include the microphysics of recombination in xenon and a detailed modeling of
the electrostatic field of the detector. An energy resolution =5.5\%
is observed at 570~\si{keV}, comparable to the best intrinsic ionization-only
resolution reported in literature for liquid xenon at 936~V/\si{cm}.Comment: 18 pages, 13 figures, as publishe
Sensitivity and discovery potential of the proposed nEXO experiment to neutrinoless double beta decay
The next-generation Enriched Xenon Observatory (nEXO) is a proposed
experiment to search for neutrinoless double beta () decay in
Xe with a target half-life sensitivity of approximately years
using kg of isotopically enriched liquid-xenon in a time
projection chamber. This improvement of two orders of magnitude in sensitivity
over current limits is obtained by a significant increase of the Xe
mass, the monolithic and homogeneous configuration of the active medium, and
the multi-parameter measurements of the interactions enabled by the time
projection chamber. The detector concept and anticipated performance are
presented based upon demonstrated realizable background rates.Comment: v2 as publishe
How and when plume zonation appeared during the 132 Myr evolution of the Tristan Hotspot
Increasingly, spatial geochemical zonation, present as geographically distinct, subparallel
trends, is observed along hotspot tracks, such as Hawaii and the Galapagos. The origin of this
zonation is currently unclear. Recently zonation was found along the last B70 Myr of the
Tristan-Gough hotspot track. Here we present new Sr–Nd–Pb–Hf isotope data from the older
parts of this hotspot track (Walvis Ridge and Rio Grande Rise) and re-evaluate published data
from the Etendeka and Parana flood basalts erupted at the initiation of the hotspot track. We
show that only the enriched Gough, but not the less-enriched Tristan, component is present in
the earlier (70–132 Ma) history of the hotspot. Here we present a model that can explain the
temporal evolution and origin of plume zonation for both the Tristan-Gough and Hawaiian
hotspots, two end member types of zoned plumes, through processes taking place in the
plume sources at the base of the lower mantle
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