385 research outputs found
Image Ellipticity from Atmospheric Aberrations
We investigate the ellipticity of the point-spread function (PSF) produced by
imaging an unresolved source with a telescope, subject to the effects of
atmospheric turbulence. It is important to quantify these effects in order to
understand the errors in shape measurements of astronomical objects, such as
those used to study weak gravitational lensing of field galaxies. The PSF
modeling involves either a Fourier transform of the phase information in the
pupil plane or a ray-tracing approach, which has the advantage of requiring
fewer computations than the Fourier transform. Using a standard method,
involving the Gaussian weighted second moments of intensity, we then calculate
the ellipticity of the PSF patterns. We find significant ellipticity for the
instantaneous patterns (up to more than 10%). Longer exposures, which we
approximate by combining multiple (N) images from uncorrelated atmospheric
realizations, yield progressively lower ellipticity (as 1 / sqrt(N)). We also
verify that the measured ellipticity does not depend on the sampling interval
in the pupil plane using the Fourier method. However, we find that the results
using the ray-tracing technique do depend on the pupil sampling interval,
representing a gradual breakdown of the geometric approximation at high spatial
frequencies. Therefore, ray tracing is generally not an accurate method of
modeling PSF ellipticity induced by atmospheric turbulence unless some
additional procedure is implemented to correctly account for the effects of
high spatial frequency aberrations. The Fourier method, however, can be used
directly to accurately model PSF ellipticity, which can give insights into
errors in the statistics of field galaxy shapes used in studies of weak
gravitational lensing.Comment: 9 pages, 5 color figures (some reduced in size). Accepted for
publication in the Astrophysical Journa
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Photon Collider Physics with Real Photon Beams
Photon-photon interactions have been an important probe into fundamental particle physics. Until recently, the only way to produce photon-photon collisions was parasitically in the collision of charged particles. Recent advances in short-pulse laser technology have made it possible to consider producing high intensity, tightly focused beams of real photons through Compton scattering. A linear e{sup +}e{sup -} collider could thus be transformed into a photon-photon collider with the addition of high power lasers. In this paper they show that it is possible to make a competitive photon-photon collider experiment using the currently mothballed Stanford Linear Collider. This would produce photon-photon collisions in the GeV energy range which would allow the discovery and study of exotic heavy mesons with spin states of zero and two
A Search for Scalar Chameleons with ADMX
Scalar fields with a "chameleon" property, in which the effective particle
mass is a function of its local environment, are common to many theories beyond
the standard model and could be responsible for dark energy. If these fields
couple weakly to the photon, they could be detectable through the "afterglow"
effect of photon-chameleon-photon transitions. The ADMX experiment was used in
the first chameleon search with a microwave cavity to set a new limit on scalar
chameleon-photon coupling excluding values between 2*10^9 and 5*10^14 for
effective chameleon masses between 1.9510 and 1.9525 micro-eV.Comment: 4 pages, 3 figure
Dilaton as a Dark Matter Candidate and its Detection
Assuming that the dilaton is the dark matter of the universe, we propose an
experiment to detect the relic dilaton using the electromagnetic resonant
cavity, based on the dilaton-photon conversion in strong electromagnetic
background. We calculate the density of the relic dilaton, and estimate the
dilaton mass for which the dilaton becomes the dark matter of the universe.
With this we calculate the dilaton detection power in the resonant cavity, and
compare it with the axion detection power in similar resonant cavity
experiment.Comment: 23 pages, 2 figure
A SQUID-based microwave cavity search for dark-matter axions
Axions in the micro eV mass range are a plausible cold dark matter candidate
and may be detected by their conversion into microwave photons in a resonant
cavity immersed in a static magnetic field. The first result from such an axion
search using a superconducting first-stage amplifier (SQUID) is reported. The
SQUID amplifier, replacing a conventional GaAs field-effect transistor
amplifier, successfully reached axion-photon coupling sensitivity in the band
set by present axion models and sets the stage for a definitive axion search
utilizing near quantum-limited SQUID amplifiers.Comment: 4 pages, 5 figures, submitted to PR
Move of a large but delicate apparatus on a trailer with air-ride suspension
When valuable delicate goods are shipped by truck, attention must be paid to
vibrations that may cause damage. We present a case study of moving an
extremely delicate 6230-kg superconducting magnet, immersed in liquid nitrogen,
from Livermore, CA to Seattle, WA showing the steps of fatigue analysis of the
load, a test move, and acceleration monitoring of the final move to ensure a
successful damage-free transport
Superdeformation in Po
The Yb(Si,5n) reaction at 148 MeV with thin targets was used
to populate high-angular momentum states in Po. Resulting rays
were observed with Gammasphere. A weakly-populated superdeformed band of 10
-ray transitions was found and has been assigned to Po. This is
the first observation of a SD band in the region in a nucleus
with . The of the new band is very similar to those of
the yrast SD bands in Hg and Pb. The intensity profile suggests
that this band is populated through states close to where the SD band crosses
the yrast line and the angular momentum at which the fission process dominates.Comment: 10 pages, revtex, 2 figs. available on request, submitted to Phys.
Rev. C. (Rapid Communications
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Experimental study of variations in background radiation and the effect on Nuclear Car Wash sensitivity
Error rates in a cargo screening system such as the Nuclear Car Wash [1-7] depend on the standard deviation of the background radiation count rate. Because the Nuclear Car Wash is an active interrogation technique, the radiation signal for fissile material must be detected above a background count rate consisting of cosmic, ambient, and neutron-activated radiations. It was suggested previously [1,6] that the Corresponding negative repercussions for the sensitivity of the system were shown. Therefore, to assure the most accurate estimation of the variation, experiments have been performed to quantify components of the actual variance in the background count rate, including variations in generator power, irradiation time, and container contents. The background variance is determined by these experiments to be a factor of 2 smaller than values assumed in previous analyses, resulting in substantially improved projections of system performance for the Nuclear Car Wash
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