125,399 research outputs found
Analytic design of a 2.0 GHz space borne linear injected beam crossed field amplifier Final report
High efficiency design for crossed field amplifier for application in synchronous satellite
The Intrinsic Shapes of Molecular Cloud Fragments over a Range of Length Scales
We decipher intrinsic three-dimensional shape distributions of molecular
clouds, cloud cores, Bok globules, and condensations using recently compiled
catalogues of observed axis ratios for these objects mapped in carbon monoxide,
ammonia, through optical selection, or in continuum dust emission. We apply
statistical techniques to compare assumed intrinsic axis ratio distributions
with observed projected axis ratio distributions. Intrinsically triaxial shapes
produce projected distributions which agree with observations. Molecular clouds
mapped in CO are intrinsically triaxial but more nearly prolate than
oblate, while the smaller cloud cores, Bok globules, and condensations are also
intrinsically triaxial but more nearly oblate than prolate.Comment: 12 pages, 11 figures. Version with color figures can be found at
http://www.astro.uwo.ca/~cjones/ or http://www.astro.uwo.ca/~basu/. To appear
in ApJ, 10 April 2002, v. 569, no.
The Structure of the Nucleon: Elastic Electromagnetic Form Factors
Precise proton and neutron form factor measurements at Jefferson Lab, using
spin observables, have recently made a significant contribution to the
unraveling of the internal structure of the nucleon. Accurate experimental
measurements of the nucleon form factors are a test-bed for understanding how
the nucleon's static properties and dynamical behavior emerge from QCD, the
theory of the strong interactions between quarks. There has been enormous
theoretical progress, since the publication of the Jefferson Lab proton form
factor ratio data, aiming at reevaluating the picture of the nucleon. We will
review the experimental and theoretical developments in this field and discuss
the outlook for the future.Comment: arXiv admin note: text overlap with arXiv:1301.0905,
arXiv:hep-ph/0609004, arXiv:1411.6908 by other author
Macroscopic and microscopic studies of electrical properties of very thin silicon dioxide subject to electrical stress
The electrical characteristics of various size tunnel switch diode devices, composed of Al/SiO2/n-Si/p+-Si layers, which operate with a range of parameters (such as current densities in excess of 104 A/cm2) that stress the oxide layer far beyond the levels used in typical thin oxide metal-oxide semiconductor research have been examined. It is found that the first time a large current and electric field are applied to the device, a "forming" process enhances transport through the oxide in the vicinity of the edges of the gate electrode, but the oxide still retains its integrity as a tunnel barrier. The device operation is relatively stable to stresses of greater than 107 C/cm2 areally averaged, time-integrated charge injection. Duplication and characterization of these modified oxide tunneling properties was attempted using scanning tunneling microscopy (STM) to stress and probe the oxide. Electrical stressing with the STM tip creates regions of reduced conductivity, possibly resulting from trapped charge in the oxide. Lateral variations in the conductivity of the unstressed oxide over regions roughly 20–50 nm across were also found
Epitaxial silicon grown on CeO2/Si(111) structure by molecular beam epitaxy
Using electron beam evaporation, a Si/CeO2/Si(111) structure has been grown in a molecular beam epitaxy machine. In situ low energy electron diffraction, cross sectional transmission electron microscopy, selected area diffraction, and atomic force microscopy have been used to structurally characterize the overlying silicon layer and show it to be single crystalline and epitaxially oriented. Rutherford backscattering and energy dispersive x-ray analysis have been used to confirm the presence of a continuous 23 Ã… CeO2 layer at the interface. Rutherford backscattering and x-ray photoemission spectroscopy show an additional presence of cerium both at the exposed silicon surface and incorporated in low levels (~ 1%) within the silicon film, suggesting a growth mechanism with cerium riding atop the silicon growth front leaving behind small amounts of cerium incorporated in the growing silicon crystal
Opening angles, Lorentz factors and confinement of X-ray binary jets
We present a collation of the available data on the opening angles of jets in
X-ray binaries, which in most cases are small (less than 10 degrees). Under the
assumption of no confinement, we calculate the Lorentz factors required to
produce such small opening angles via the transverse relativistic Doppler
effect. The derived Lorentz factors, which are in most cases lower limits, are
found to be large, with a mean greater than 10, comparable to those estimated
for AGN and much higher than the commonly-assumed values for X-ray binaries of
2 to 5. Jet power constraints do not in most cases rule out such high Lorentz
factors. The upper limits on the opening angles show no evidence for smaller
Lorentz factors in the steady jets of Cygnus X-1 and GRS 1915+105. In those
sources in which deceleration has been observed (notably XTE J1550-564 and
Cygnus X-3), some confinement of the jets must be occurring, and we briefly
discuss possible confinement mechanisms. It is however possible that all the
jets could be confined, in which case the requirement for high bulk Lorentz
factors can be relaxed.Comment: 11 pages, 4 figures (2 colour), accepted for publication in MNRA
The Desktop Muon Detector: A simple, physics-motivated machine- and electronics-shop project for university students
This paper describes an undergraduate-level physics project that incorporates
various aspects of machine- and electronics-shop technical development. The
desktop muon detector is a self-contained apparatus that employs plastic
scintillator as a detection medium and a silicon photomultiplier for light
collection. These detectors can be used in conjunction with the provided
software to make interesting physics measurements. The total cost of each
counter is approximately $100.Comment: 29 pages, 14 figure
Bubble statistics and coarsening dynamics for quasi-two dimensional foams with increasing liquid content
We report on the statistics of bubble size, topology, and shape and on their
role in the coarsening dynamics for foams consisting of bubbles compressed
between two parallel plates. The design of the sample cell permits control of
the liquid content, through a constant pressure condition set by the height of
the foam above a liquid reservoir. We find that in the scaling state, all
bubble distributions are independent not only of time but also of liquid
content. For coarsening, the average rate decreases with liquid content due to
the blocking of gas diffusion by Plateau borders inflated with liquid. By
observing the growth rate of individual bubbles, we find that von Neumann's law
becomes progressively violated with increasing wetness and with decreasing
bubble size. We successfully model this behavior by explicitly incorporating
the border blocking effect into the von Neumann argument. Two dimensionless
bubble shape parameters naturally arise, one of which is primarily responsible
for the violation of von Neumann's law for foams that are not perfectly dry
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