39,689 research outputs found
Development of high resolution imaging detectors for x ray astronomy
This final report summarizes our past activities and discusses the work performed over the period of 1 April 1990 through 1 April 1991 on x-ray optics, soft x-ray (0.1 - 10 KeV) imaging detectors, and hard x-ray (10 - 300 KeV) imaging detectors. If microchannel plates (MCPs) can be used to focus x-rays with a high efficiency and good angular resolution, they will revolutionize the field of x-ray optics. An x-ray image of a point source through an array of square MCP pores compared favorably with our ray tracing model for the MCP. Initial analysis of this image demonstrates the feasibility of MCPs for soft x-rays. Our work continues with optimizing the performance of our soft x-ray MCP imaging detectors. This work involves readout technology that should provide improved MCP readout devices (thin film crossed grid, curved, and resistive sheets), defect removal in MCPs, and photocathode optimization. In the area of hard x-ray detector development we have developed two different techniques for producing a CsI photocathode thickness of 10 to 100 microns, such that it is thick enough to absorb the high energy x-rays and still allow the photoelectrons to escape to the top MCP of a modified soft x-ray imaging detector. The methods involve vacuum depositing a thick film of CsI on a strong back, and producing a converter device that takes the place of the photocathode
Relative potentials of concentrating and two-axis tracking flat-plate photovoltaic arrays for central-station applications
The purpose of this study is to assess the relative economic potentials of concenrating and two-axis tracking flat-plate photovoltaic arrays for central-station applications in the mid-1990's. Specific objectives of this study are to provide information on concentrator photovoltaic collector probabilistic price and efficiency levels to illustrate critical areas of R&D for concentrator cells and collectors, and to compare concentrator and flat-plate PV price and efficiency alternatives for several locations, based on their implied costs of energy. To deal with the uncertainties surrounding research and development activities in general, a probabilistic assessment of commercially achievable concentrator photovoltaic collector efficiencies and prices (at the factory loading dock) is performed. The results of this projection of concentrator photovoltaic technology are then compared with a previous flat-plate module price analysis (performed early in 1983). To focus this analysis on specific collector alternatives and their implied energy costs for different locations, similar two-axis tracking designs are assumed for both concentrator and flat-plate options
Qjets: A Non-Deterministic Approach to Tree-Based Jet Substructure
Jet substructure is typically studied using clustering algorithms, such as
kT, which arrange the jets' constituents into trees. Instead of considering a
single tree per jet, we propose that multiple trees should be considered,
weighted by an appropriate metric. Then each jet in each event produces a
distribution for an observable, rather than a single value. Advantages of this
approach include: 1) observables have significantly increased statistical
stability; and, 2) new observables, such as the variance of the distribution,
provide new handles for signal and background discrimination. For example, we
find that employing a set of trees substantially reduces the observed
fluctuations in the pruned mass distribution, enhancing the likelihood of new
particle discovery for a given integrated luminosity. Furthermore, the
resulting pruned mass distributions for (background) QCD jets are found to be
substantially wider than that for (signal) jets with intrinsic mass scales,
e.g. jets containing a W decay. A cut on this width yields a substantial
enhancement in significance relative to a cut on the standard pruned jet mass
alone. In particular the luminosity needed for a given significance requirement
decreases by a factor of two relative to standard pruning.Comment: Minor changes to match journal versio
Preliminary results of the University of California X-ray experiment on the OSO-3
Cosmic and solar X ray data obtained by Orbiting Solar Observatory /OSO-3
Effects of low energy electron irradiation on formation of nitrogen-vacancy centers in single-crystal diamond
Exposure to beams of low energy electrons (2 to 30 keV) in a scanning
electron microscope locally induces formation of NV-centers without thermal
annealing in diamonds that have been implanted with nitrogen ions. We find that
non-thermal, electron beam induced NV-formation is about four times less
efficient than thermal annealing. But NV-center formation in a consecutive
thermal annealing step (800C) following exposure to low energy electrons
increases by a factor of up to 1.8 compared to thermal annealing alone. These
observations point to reconstruction of nitrogen-vacancy complexes induced by
electronic excitations from low energy electrons as an NV-center formation
mechanism and identify local electronic excitations as a means for spatially
controlled room-temperature NV-center formation
All-Optical Depletion of Dark Excitons from a Semiconductor Quantum Dot
Semiconductor quantum dots are considered to be the leading venue for
fabricating on-demand sources of single photons. However, the generation of
long-lived dark excitons imposes significant limits on the efficiency of these
sources. We demonstrate a technique that optically pumps the dark exciton
population and converts it to a bright exciton population, using intermediate
excited biexciton states. We show experimentally that our method considerably
reduces the DE population while doubling the triggered bright exciton emission,
approaching thereby near-unit fidelity of quantum dot depletion.Comment: 5 pages, 3 figure
A characterization of quasi-rational polygons
The aim of this paper is to study quasi-rational polygons related to the
outer billiard. We compare different notions introduced, and make a synthesis
of those.Comment: 15 pages, 9 figure
Limits on the small scale structure of the diffuse cosmic X-rays
Autocorrelation functions for diffuse cosmic X ray
Anthropic prediction in a large toy landscape
The successful anthropic prediction of the cosmological constant depends
crucially on the assumption of a flat prior distribution. However, previous
calculations in simplified landscape models showed that the prior distribution
is staggered, suggesting a conflict with anthropic predictions. Here we
analytically calculate the full distribution, including the prior and anthropic
selection effects, in a toy landscape model with a realistic number of vacua,
. We show that it is possible for the fractal prior
distribution we find to behave as an effectively flat distribution in a wide
class of landscapes, depending on the regime of parameter space. Whether or not
this possibility is realized depends on presently unknown details of the
landscape.Comment: 13 page
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