1,630 research outputs found
A Relic Neutrino Detector
Probably the most promising way of detecting cosmic neutrinos is measuring
the mechanical force exerted by elastic scattering of cosmic neutrinos from
macroscopic targets. The expected acceleration is for
Dirac neutrinos of mass and local density . A
novel torsion balance design is presented. which addresses the
sensitivity-limiting factors of existing balances, such as seismic and thermal
noise, and angular readout resolution and stability.Comment: 4 pages, 2 figures. In proceedings of the COSMO-98 Int'l Workshop on
Particle Physics and the Early Universe, edited by D. Caldwell (AIP Press,
New York, 1999
A two-stage magnetic refrigerator for astronomical applications with reservoir temperatures above 4 K
We propose a novel adiabatic demagnetization refrigerator (ADR) to produce temperatures as low as 100 mK starting from a high temperature reservoir between 4 and 8 K. The high temperature reservoir for the ADR can be provided by a mechanical cooler or an unpumped liquid helium bath. This refrigerator can be used to cool bolometric infrared detectors for low background astronomy from mountain tops, balloons or satellites as well as to cool cryogenic x-ray detectors. The two-stage ADR consists of a single magnet with a paramagnetic chromic-cesium-alum (CCA) salt pill to produce the low temperature and paramagnetic gadolinium-gallium-garnet (GGG) as the first stage to intercept heat from the high temperature reservoir. Thermal contact between the paramagnets and the reservoir during magnetization is made with a mechanical heat switch. The ADR is suspended with Kevlar chords under tension for high mechanical stiffness and low parasitic heat leak. In a single cycle, the ADR maintains a temperature of 100 mK for 10 to 100 hours. This time depends strongly on the magnetic field and reservoir temperature but not on the volume of the paramagnetic material as long as the heat leak is dominated by the suspension
Axions from wall decay
We discuss the decay of axion walls bounded by strings and present numerical
simulations of the decay process. In these simulations, the decay happens
immediately, in a time scale of order the light travel time, and the average
energy of the radiated axions is for . is found to increase approximately linearly with
. Extrapolation of this behaviour yields in axion models of interest.Comment: 6 pages, 7 figures, to be published in the Proc. of the 5th IFT Axion
workshop Gainesville FL, Mar 13-15 199
Probing Axions with Radiation from Magnetic Stars
Recent experiments suggest that polarized photons may couple significantly to
pseudoscalar particles such as axions. We study the possible observational
signatures of axion-photon coupling for radiation from magnetic stars, with
particular focus on neutron stars. We present general methods for calculating
the axion-photon conversion probability during propagation through a varying
magnetized vacuum as well as across an inhomogeneous atmosphere. Partial
axion-photon conversion may take place in the vacuum region outside the neutron
star. Strong axion-photon mixing occurs due to a resonance in the atmosphere,
and depending on the axion coupling strength and other parameters, significant
axion-photon conversion can take place at the resonance. Such conversions may
produce observable effects on the radiation spectra and polarization signals
from the star. We also apply our results to axion-photon propagation in the Sun
and in magnetic white dwarfs. We find that there is no appreciable conversion
of solar axions to photons during the propagation.Comment: 12 pages, 11 figures. Minor changes. PRD accepte
A broadband THz receiver for low background space applications
We have developed a sensitive bolometric receiver for low background space applications. In a 10 percent bandwidth at 1 THz, this receiver is approximately 100 times more sensitive than a quantum limited heterodyne receiver with a 1 GHz IF bandwidth. This receiver is designed to be used for the long wavelength band (200-700 microns) in the MIPS instrument on NASA's SIRTF satellite. The bolometers are cooled to 100 mK by an adiabatic demagnetization refrigerator. Roughly 60 g of cesium chrome alum salt is partially demagnetized to 100 mK, followed by a slow regulated downramp to compensate for the heat leak. The hold time of the ADR system is about 18 hours with a temperature stability of delta T(sub rms) approx. equals 10 micro-K. The composite bolometers have electrical responsivities of 10(exp 9)V/W and electrical NEP's of about 3x10(exp -17) W/square root of Hz. The bolometer signals are read out by JFET preamplifiers located on the helium plate and operated at 120 K. We have addressed a number of space qualification issues, such as the development of an analog magnet controller, construction of a cryogenic shake-table for bolometers and selection of the paramagnetic salt CCA which can survive a bakeout at 50 C. The receiver is scheduled to be flown in the spring of 1992 on a balloon telescope. This flight has a dual purpose. One is to provide realistic test of the capabilities of the new receiver. The other is to search for anisotropies in the cosmic microwave background on scales of a few degrees
The Rydberg-Atom-Cavity Axion Search
We report on the present progress in development of the dark matter axion
search experiment with Rydberg-atom-cavity detectors in Kyoto, CARRACK I and
CARRACK II. The axion search has been performed with CARRACK I in the 8 % mass
range around , and CARRACK II is now ready for the search in
the wide range . We have also developed
quantum theoretical calculations on the axion-photon-atom system in the
resonant cavity in order to estimate precisely the detection sensitivity for
the axion signal. Some essential features on the axion-photon-atom interaction
are clarified, which provide the optimum experimental setup for the axion
search.Comment: 8 pages, 2 figures, Invited talk presented at the Dark2000,
Heidelberg, Germany,10-15 July, 200
Can one predict DNA Transcription Start Sites by studying bubbles?
It has been speculated that bubble formation of several base-pairs due to
thermal fluctuations is indicatory for biological active sites. Recent
evidence, based on experiments and molecular dynamics (MD) simulations using
the Peyrard-Bishop-Dauxois model, seems to point in this direction. However,
sufficiently large bubbles appear only seldom which makes an accurate
calculation difficult even for minimal models. In this letter, we introduce a
new method that is orders of magnitude faster than MD. Using this method we
show that the present evidence is unsubstantiated.Comment: 4 pages, 3 figures, accepted for publication in physical review
letter
Evaluating 35 Methods to Generate Structural Connectomes Using Pairwise Classification
There is no consensus on how to construct structural brain networks from
diffusion MRI. How variations in pre-processing steps affect network
reliability and its ability to distinguish subjects remains opaque. In this
work, we address this issue by comparing 35 structural connectome-building
pipelines. We vary diffusion reconstruction models, tractography algorithms and
parcellations. Next, we classify structural connectome pairs as either
belonging to the same individual or not. Connectome weights and eight
topological derivative measures form our feature set. For experiments, we use
three test-retest datasets from the Consortium for Reliability and
Reproducibility (CoRR) comprised of a total of 105 individuals. We also compare
pairwise classification results to a commonly used parametric test-retest
measure, Intraclass Correlation Coefficient (ICC).Comment: Accepted for MICCAI 2017, 8 pages, 3 figure
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