1,097 research outputs found
Photon noise correlations in millimeter-wave telescopes
Many modern millimeter and submillimeter (``mm-wave'') telescopes for
astronomy are deploying more detectors by increasing detector pixel density,
and with the rise of lithographed detector architectures and high-throughput
readout techniques, it is becoming increasingly practical to overfill the focal
plane. However, when the pixel pitch is small compared to the
product of the wavelength and the focal ratio , or
, the Bose term of the photon noise
correlates between neighboring detector pixels due to the Hanbury Brown & Twiss
(HBT) effect. When this HBT effect is non-negligible, the array-averaged
sensitivity scales with detector count less favorably than
the uncorrelated limit of . In this paper, we present
a general prescription to calculate this HBT correlation based on a quantum
optics formalism and extend it to polarization-sensitive detectors. We then
estimate the impact of HBT correlations on the sensitivity of a model mm-wave
telescope and discuss the implications for focal-plane design.Comment: 24 pages, 16 figure
Real-time Loss Estimation for Instrumented Buildings
Motivation. A growing number of buildings have been instrumented to measure and record
earthquake motions and to transmit these records to seismic-network data centers to be archived and
disseminated for research purposes. At the same time, sensors are growing smaller, less expensive to
install, and capable of sensing and transmitting other environmental parameters in addition to
acceleration. Finally, recently developed performance-based earthquake engineering methodologies
employ structural-response information to estimate probabilistic repair costs, repair durations, and
other metrics of seismic performance. The opportunity presents itself therefore to combine these
developments into the capability to estimate automatically in near-real-time the probabilistic seismic
performance of an instrumented building, shortly after the cessation of strong motion. We refer to
this opportunity as (near-) real-time loss estimation (RTLE).
Methodology. This report presents a methodology for RTLE for instrumented buildings. Seismic
performance is to be measured in terms of probabilistic repair cost, precise location of likely physical
damage, operability, and life-safety. The methodology uses the instrument recordings and a Bayesian
state-estimation algorithm called a particle filter to estimate the probabilistic structural response of
the system, in terms of member forces and deformations. The structural response estimate is then
used as input to component fragility functions to estimate the probabilistic damage state of structural
and nonstructural components. The probabilistic damage state can be used to direct structural
engineers to likely locations of physical damage, even if they are concealed behind architectural
finishes. The damage state is used with construction cost-estimation principles to estimate
probabilistic repair cost. It is also used as input to a quantified, fuzzy-set version of the FEMA-356
performance-level descriptions to estimate probabilistic safety and operability levels.
CUREE demonstration building. The procedure for estimating damage locations, repair costs, and
post-earthquake safety and operability is illustrated in parallel demonstrations by CUREE and
Kajima research teams. The CUREE demonstration is performed using a real 1960s-era, 7-story, nonductile
reinforced-concrete moment-frame building located in Van Nuys, California. The building is
instrumented with 16 channels at five levels: ground level, floors 2, 3, 6, and the roof. We used the
records obtained after the 1994 Northridge earthquake to hindcast performance in that earthquake.
The building is analyzed in its condition prior to the 1994 Northridge Earthquake. It is found that,
while hindcasting of the overall system performance level was excellent, prediction of detailed damage
locations was poor, implying that either actual conditions differed substantially from those shown on
the structural drawings, or inappropriate fragility functions were employed, or both. We also found
that Bayesian updating of the structural model using observed structural response above the base of
the building adds little information to the performance prediction. The reason is probably that
Real-Time Loss Estimation for Instrumented Buildings
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structural uncertainties have only secondary effect on performance uncertainty, compared with the
uncertainty in assembly damageability as quantified by their fragility functions. The implication is
that real-time loss estimation is not sensitive to structural uncertainties (saving costly multiple
simulations of structural response), and that real-time loss estimation does not benefit significantly
from installing measuring instruments other than those at the base of the building.
Kajima demonstration building. The Kajima demonstration is performed using a real 1960s-era
office building in Kobe, Japan. The building, a 7-story reinforced-concrete shearwall building, was not
instrumented in the 1995 Kobe earthquake, so instrument recordings are simulated. The building is
analyzed in its condition prior to the earthquake. It is found that, while hindcasting of the overall
repair cost was excellent, prediction of detailed damage locations was poor, again implying either that
as-built conditions differ substantially from those shown on structural drawings, or that
inappropriate fragility functions were used, or both. We find that the parameters of the detailed
particle filter needed significant tuning, which would be impractical in actual application. Work is
needed to prescribe values of these parameters in general.
Opportunities for implementation and further research. Because much of the cost of applying
this RTLE algorithm results from the cost of instrumentation and the effort of setting up a structural
model, the readiest application would be to instrumented buildings whose structural models are
already available, and to apply the methodology to important facilities. It would be useful to study
under what conditions RTLE would be economically justified. Two other interesting possibilities for
further study are (1) to update performance using readily observable damage; and (2) to quantify the
value of information for expensive inspections, e.g., if one inspects a connection with a modeled 50%
failure probability and finds that the connect is undamaged, is it necessary to examine one with 10%
failure probability
Does the effective Lagrangian for low-energy QCD scale?
QCD is not an approximately scale invariant theory. Hence a dilaton field is
not expected to provide a good description of the low-energy dynamics
associated with the gluon condensate. Even if such a field is introduced, it
remains almost unchanged in hadronic matter at normal densities. This is
because the large glueball mass together with the size of the phenomenological
gluon condensate ensure that changes to that condensate are very small at such
densities. Any changes in hadronic masses and decay constants in matter
generated by that condensate will be much smaller that those produced directly
by changes in the quark condensate. Hence masses and decay constants are not
expected to display a universal scaling.Comment: 7 pages (RevTeX), MC/TH 94/0
Cross-ladder effects in Bethe-Salpeter and Light-Front equations
Bethe-Salpeter (BS) equation in Minkowski space for scalar particles is
solved for a kernel given by a sum of ladder and cross-ladder exchanges. The
solution of corresponding Light-Front (LF) equation, where we add the
time-ordered stretched boxes, is also obtained. Cross-ladder contributions are
found to be very large and attractive, whereas the influence of stretched boxes
is negligible. Both approaches -- BS and LF -- give very close results.Comment: 11 pages, 7 figure
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