A Sensitivity and Array-Configuration Study for Measuring the Power Spectrum of 21cm Emission from Reionization

Telescopes aiming to measure 21cm emission from the Epoch of Reionization must toe a careful line, balancing the need for raw sensitivity against the stringent calibration requirements for removing bright foregrounds. It is unclear what the optimal design is for achieving both of these goals. Via a pedagogical derivation of an interferometer's response to the power spectrum of 21cm reionization fluctuations, we show that even under optimistic scenarios, first-generation arrays will yield low-SNR detections, and that different compact array configurations can substantially alter sensitivity. We explore the sensitivity gains of array configurations that yield high redundancy in the uv-plane -- configurations that have been largely ignored since the advent of self-calibration for high-dynamic-range imaging. We first introduce a mathematical framework to generate optimal minimum-redundancy configurations for imaging. We contrast the sensitivity of such configurations with high-redundancy configurations, finding that high-redundancy configurations can improve power-spectrum sensitivity by more than an order of magnitude. We explore how high-redundancy array configurations can be tuned to various angular scales, enabling array sensitivity to be directed away from regions of the uv-plane (such as the origin) where foregrounds are brighter and where instrumental systematics are more problematic. We demonstrate that a 132-antenna deployment of the Precision Array for Probing the Epoch of Reionization (PAPER) observing for 120 days in a high-redundancy configuration will, under ideal conditions, have the requisite sensitivity to detect the power spectrum of the 21cm signal from reionization at a 3\sigma level at k<0.25h Mpc^{-1} in a bin of \Delta ln k=1. We discuss the tradeoffs of low- versus high-redundancy configurations.Comment: 34 pages, 5 figures, 2 appendices. Version accepted to Ap

Relaxation of strained silicon on virtual substrates

The relaxation of variable thickness strained silicon layers on 20% and 50% germanium composition virtual substrates have been quantified using two independent methods. High resolution X-ray diffraction offers a means to measure relaxation directly, and a defect etching technique has been developed from which relaxation can be determined by the measurement of dislocation densities. Comparisons between the relaxation of tensile strained silicon in this work and compressively strained Si1−xGex in other works, suggest that strained silicon is unusually stable to relaxation. Observation of dislocation structures with defect etching and transmission electron microscopy have shown that the additional stability arises from the interaction of dislocations which inhibits glide. Extended stacking faults, which only form when the strain is tensile (and are therefore absent in compressively strained Si1−xGex), are more effective at impeding dislocation glide and it is their increased density in thicker layers which yield enhanced stability, even after high temperature annealing

A PROFITABILITY ANALYSIS OF DAIRY FEEDING SYSTEMS IN THE NORTHEAST

This study analyzes the use and profitability of three distinct feeding systems; confinement feeding, traditional grazing, and management-intensive grazing from a randomly selected sample of northeastern dairy farms. The confinement feeding farms were significantly larger and produced more milk per cow, while the farms using management-intensive grazing incurred the lowest production costs. Both confinement feeding and management-intensive grazing generated significantly higher rates of return to farm assets relative to farms using a mixed system. Multiple regression analysis confirms the critical importance of herd size, milk production per cow, debt level and veterinary expenses to farm profitability in all production systems.Livestock Production/Industries,

Ontology-Driven Search and Triage: Design of a Web-Based Visual Interface for MEDLINE

Background: Diverse users need to search health and medical literature to satisfy open-ended goals such as making evidence-based decisions and updating their knowledge. However, doing so is challenging due to at least two major difficulties: (1) articulating information needs using accurate vocabulary and (2) dealing with large document sets returned from searches. Common search interfaces such as PubMed do not provide adequate support for exploratory search tasks. Objective: Our objective was to improve support for exploratory search tasks by combining two strategies in the design of an interactive visual interface by (1) using a formal ontology to help users build domain-specific knowledge and vocabulary and (2) providing multi-stage triaging support to help mitigate the information overload problem. Methods: We developed a Web-based tool, Ontology-Driven Visual Search and Triage Interface for MEDLINE (OVERT-MED), to test our design ideas. We implemented a custom searchable index of MEDLINE, which comprises approximately 25 million document citations. We chose a popular biomedical ontology, the Human Phenotype Ontology (HPO), to test our solution to the vocabulary problem. We implemented multistage triaging support in OVERT-MED, with the aid of interactive visualization techniques, to help users deal with large document sets returned from searches. Results: Formative evaluation suggests that the design features in OVERT-MED are helpful in addressing the two major difficulties described above. Using a formal ontology seems to help users articulate their information needs with more accurate vocabulary. In addition, multistage triaging combined with interactive visualizations shows promise in mitigating the information overload problem. Conclusions: Our strategies appear to be valuable in addressing the two major problems in exploratory search. Although we tested OVERT-MED with a particular ontology and document collection, we anticipate that our strategies can be transferred successfully to other contexts