1,281 research outputs found
Introduction to Microwave Background Polarization
Microwave background polarization, though presently undetected, is a
fundamental prediction of any viable cosmological model. These lectures review
the theoretical description of polarization, its physical interpretation, and
potentially interesting polarization signals.Comment: Lectures given at the International School of Space Sciences,
L'Aquila, Italy, September 2-12, 1998. 18 pages with 2 figures; Elsevier tex
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Determining Alpha-Helix Correspondence for Protein Structure Prediction from Cryo-EM Density Maps, Master\u27s Thesis, May 2007
Determining protein structure is an important problem for structural biologists, which has received a significant amount of attention in the recent years. In this thesis, we describe a novel, shape-modeling approach as an intermediate step towards recovering 3D protein structures from volumetric images. The input to our method is a sequence of alpha-helices that make up a protein, and a low-resolution volumetric image of the protein where possible locations of alpha-helices have been detected. Our task is to identify the correspondence between the two sets of helices, which will shed light on how the protein folds in space. The central theme of our approach is to cast the correspondence problem as that of shape matching between the 3D volume and the 1D sequence. We model both the shapes as attributed relational graphs, and formulate a constrained inexact graph matching problem. To compute the matching, we developed an optimal algorithm based on the A*-search with several choices of heuristic functions. As demonstrated in a suite of real protein data, the shape-modeling approach is capable of correctly identifying helix correspondences in noise-abundant volumes with minimal or no user intervention
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Data Analysis for the E and B EXperiment and Instrumentation Development for Cosmic Microwave Background Polarimetry
The E and B EXperiment (EBEX) was a balloon-borne instrument designed to measure the polarization of the cosmic microwave background (CMB) while simultaneously characterizing Galactic dust emission. The instrument was based on a two-mirror ambient temperature Gregorian-Dragone telescope coupled with cooled refractive optics to a kilo-pixel array of transition edge sensor (TES) bolometeric detectors. To achieve sensitivity to both the CMB signal and Galactic foregrounds, EBEX observed in three signal bands centered on 150, 250, and 410 GHz. Polarimetry was achieved via a stationary wire-grid polarizer and a continuously rotating achromatic half-wave plate (HWP) based on a superconducting magnetic bearing (SMB). EBEX launched from McMurdo station, Antarctica on December 29, 2012 and collected ~ 1.3 TB of data during 11 days of observation.
This thesis is presented in two Parts. Part I reviews the data analysis we performed to transform the raw EBEX data into maps of temperature and polarization sky signals, with a particular focus on post-flight pointing reconstruction; time stream cleaning and map making; the generation of model sky maps of the expected signal for each of the three EBEX signal bands; removal of the HWP-synchronous signal from the detector time streams; and our attempts to identify, characterize, and correct for non-linear detector responsivity. In Part II we present recent developments in instrumentation for the next generation of CMB polarimeters. The developments we describe, including advances in lumped-element kinetic inductance detector (LEKID) technology and the development of a hollow-shaft SMB-based motor for use in HWP polarimetry, were motivated in part by the design for a prospective ground-based CMB polarimeter based in Greenland
On Open and Strong-Scaling Tools for Atom Probe Crystallography: High-Throughput Methods for Indexing Crystal Structure and Orientation
Volumetric crystal structure indexing and orientation mapping are key data
processing steps for virtually any quantitative study of spatial correlations
between the local chemistry and the microstructure of a material. For electron
and X-ray diffraction methods it is possible to develop indexing tools which
compare measured and analytically computed patterns to decode the structure and
relative orientation within local regions of interest. Consequently, a number
of numerically efficient and automated software tools exist to solve the above
characterisation tasks.
For atom probe tomography (APT) experiments, however, the strategy of making
comparisons between measured and analytically computed patterns is less robust
because many APT datasets may contain substantial noise. Given that general
enough predictive models for such noise remain elusive, crystallography tools
for APT face several limitations: Their robustness to noise, and therefore,
their capability to identify and distinguish different crystal structures and
orientation is limited. In addition, the tools are sequential and demand
substantial manual interaction. In combination, this makes robust uncertainty
quantifying with automated high-throughput studies of the latent
crystallographic information a difficult task with APT data.
To improve the situation, we review the existent methods and discuss how they
link to those in the diffraction communities. With this we modify some of the
APT methods to yield more robust descriptors of the atomic arrangement. We
report how this enables the development of an open-source software tool for
strong-scaling and automated identifying of crystal structure and mapping
crystal orientation in nanocrystalline APT datasets with multiple phases.Comment: 36 pages, 19 figures, preprin
CMBPol Mission Concept Study: Prospects for polarized foreground removal
In this report we discuss the impact of polarized foregrounds on a future
CMBPol satellite mission. We review our current knowledge of Galactic polarized
emission at microwave frequencies, including synchrotron and thermal dust
emission. We use existing data and our understanding of the physical behavior
of the sources of foreground emission to generate sky templates, and start to
assess how well primordial gravitational wave signals can be separated from
foreground contaminants for a CMBPol mission. At the estimated foreground
minimum of ~100 GHz, the polarized foregrounds are expected to be lower than a
primordial polarization signal with tensor-to-scalar ratio r=0.01, in a small
patch (~1%) of the sky known to have low Galactic emission. Over 75% of the sky
we expect the foreground amplitude to exceed the primordial signal by about a
factor of eight at the foreground minimum and on scales of two degrees. Only on
the largest scales does the polarized foreground amplitude exceed the
primordial signal by a larger factor of about 20. The prospects for detecting
an r=0.01 signal including degree-scale measurements appear promising, with 5
sigma_r ~0.003 forecast from multiple methods. A mission that observes a range
of scales offers better prospects from the foregrounds perspective than one
targeting only the lowest few multipoles. We begin to explore how optimizing
the composition of frequency channels in the focal plane can maximize our
ability to perform component separation, with a range of typically 40 < nu <
300 GHz preferred for ten channels. Foreground cleaning methods are already in
place to tackle a CMBPol mission data set, and further investigation of the
optimization and detectability of the primordial signal will be useful for
mission design.Comment: 42 pages, 14 figures, Foreground Removal Working Group contribution
to the CMBPol Mission Concept Study, v2, matches AIP versio
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