554 research outputs found
Hydrogen Embrittlement of Aluminum: the Crucial Role of Vacancies
We report first-principles calculations which demonstrate that vacancies can
combine with hydrogen impurities in bulk aluminum and play a crucial role in
the embrittlement of this prototypical ductile solid. Our studies of
hydrogen-induced vacancy superabundant formation and vacancy clusterization in
aluminum lead to the conclusion that a large number of H atoms (up to twelve)
can be trapped at a single vacancy, which over-compensates the energy cost to
form the defect. In the presence of trapped H atoms, three nearest-neighbor
single vacancies which normally would repel each other, aggregate to form a
trivacancy on the slip plane of Al, acting as embryos for microvoids and cracks
and resulting in ductile rupture along the these planes.Comment: To appear in Phys. Rev. Let
Quantum Invariants, Modular Forms, and Lattice Points II
We study the SU(2) Witten--Reshetikhin--Turaev invariant for the Seifert
fibered homology spheres with M-exceptional fibers. We show that the WRT
invariant can be written in terms of (differential of) the Eichler integrals of
modular forms with weight 1/2 and 3/2. By use of nearly modular property of the
Eichler integrals we shall obtain asymptotic expansions of the WRT invariant in
the large-N limit. We further reveal that the number of the gauge equivalent
classes of flat connections, which dominate the asymptotics of the WRT
invariant in N ->\infinity, is related to the number of integral lattice points
inside the M-dimensional tetrahedron
Pathogenic, Molecular, and Immunological Properties of a Virus Associated with Sea Turtle Fibropapillomatosis. Phase II : Viral Pathogenesis and Development of Diagnostic Assays
Research conducted under this RWO from July 1, 1997 through June 30, 2000 has
provided important new information about the pathogenesis, virology, and
immunology of marine turtle fibropapillomatosis. In particular, we have provided
strong evidence for the association of a herpesvirus with fibropapillomatosis of the
green turtle,Chelonia mydas, and the loggerhead turtle, Caretta caretta, in Florida. In
addition we have provided new evidence for the absence of papillomaviruses from
sea turtle fibropapillomas. Although unsuccessful, important new attempts were made
to cultivate the FP-associated herpesvirus in vitro in collaboration with the National
Wildlife Health Center. During this period of time, we completed publication of the first
comprehensive description of the comparative pathology and pathogenesis of
experimentally induced and spontaneous fibropapillomas of green turtles (Chelonia
mydas). We initiated innovative studies on the persistence of a Chelonian
herpesviruses in the marine environment demonstrating for the first time that the
environmental survivability of Chelonian herpesviruses makes them real threats to
marine turtle health. Finally, we explored development of a serological assay for FP
using synthetic herpesvirus peptides and developed methodologies for detection of
antibodies to LETV [Iung-eye-trachea virus] a disease-associated herpesvirus of the
green turtle, Chelonia mydas.. This last initiative is ongoing and will further our efforts
to develop specific immunological assays for the FP-associated herpesvirus and FP. (17 page document
Diffusion and jump-length distribution in liquid and amorphous CuZr
Using molecular dynamics simulation, we calculate the distribution of atomic
jum ps in CuZr in the liquid and glassy states. In both states
the distribution of jump lengths can be described by a temperature independent
exponential of the length and an effective activation energy plus a
contribution of elastic displacements at short distances. Upon cooling the
contribution of shorter jumps dominates. No indication of an enhanced
probability to jump over a nearest neighbor distance was found. We find a
smooth transition from flow in the liquid to jumps in the g lass. The
correlation factor of the diffusion constant decreases with decreasing
temperature, causing a drop of diffusion below the Arrhenius value, despite an
apparent Arrhenius law for the jump probability
The Heavy Photon Search beamline and its performance
The Heavy Photon Search (HPS) is an experiment to search for a hidden sector
photon, aka a heavy photon or dark photon, in fixed target electroproduction at
the Thomas Jefferson National Accelerator Facility (JLab). The HPS experiment
searches for the ee decay of the heavy photon with bump hunt and
detached vertex strategies using a compact, large acceptance forward
spectrometer, consisting of a silicon microstrip detector (SVT) for tracking
and vertexing, and a PbWO electromagnetic calorimeter for energy
measurement and fast triggering. To achieve large acceptance and good vertexing
resolution, the first layer of silicon detectors is placed just 10 cm
downstream of the target with the sensor edges only 500 m above and below
the beam. Placing the SVT in such close proximity to the beam puts stringent
requirements on the beam profile and beam position stability. As part of an
approved engineering run, HPS took data in 2015 and 2016 at 1.05 GeV and 2.3
GeV beam energies, respectively. This paper describes the beam line and its
performance during that data taking
Properties of THz radiation from grating generated by relativistic electron beam
In the present study, we have calculated the spectral characteristics of terahertz radiation based on diffraction radiation mechanism from periodic grating using previously written numerical code. The simulation results were analyzed for the possibility of creating a terahertz radiation source
Budget Processes: Theory and Experimental Evidence
This paper studies budget processes, both theoretically and experimentally. We compare the outcomes of bottom-up and top-down budget processes. It is often presumed that a top-down budget process leads to a smaller overall budget than a bottom-up budget process. Ferejohn and Krehbiel (1987) showed theoretically that this need not be the case. We test experimentally the theoretical predictions of their work. The evidence from these experiments lends strong support to their theory, both at the aggregate and the individual subject level
The HPS electromagnetic calorimeter
The Heavy Photon Search experiment (HPS) is searching for a new gauge boson, the so-called “heavy photon.” Through its kinetic mixing with the Standard Model photon, this particle could decay into an electron-positron pair. It would then be detectable as a narrow peak in the invariant mass spectrum of such pairs, or, depending on its lifetime, by a decay downstream of the production target. The HPS experiment is installed in Hall-B of Jefferson Lab. This article presents the design and performance of one of the two detectors of the experiment, the electromagnetic calorimeter, during the runs performed in 2015–2016. The calorimeter's main purpose is to provide a fast trigger and reduce the copious background from electromagnetic processes through matching with a tracking detector. The detector is a homogeneous calorimeter, made of 442 lead-tungstate (PbWO4) scintillating crystals, each read out by an avalanche photodiode coupled to a custom trans-impedance amplifier
Immunological characterization of chromogranins A and B and secretogranin II in the bovine pancreatic islet
Antisera against chromogranin A and B and secretogranin II were used for analysing the bovine pancreas by immunoblotting and immunohistochemistry. All three antigens were found in extracts of fetal pancreas by one dimensional immunoblotting. A comparison with the soluble proteins of chromaffin granules revealed that in adrenal medulla and in pancreas antigens which migrated identically in electrophoresis were present. In immunohistochemistry, chromogranin A was found in all pancreatic endocrine cell types with the exception of most pancreatic polypeptide-(PP-) producing cells. For chromogranin B, only a faint immunostaining was obtained. For secretorgranin II, A-and B-cells were faintly positive, whereas the majority of PP-cells exhibited a strong immunostaining for this antigen. These results establish that chromogranins A and B and secretogranin II are present in the endocrine pancreas, but that they exhibit a distinct cellular localization
The African Political Business Cycle: Varieties of Experience
We seek to understand both the incidence and the impact of the African political business cycle in the light of a literature which has argued that, with major extensions of democracy since the 1990s, the cycle has both become more intense and has made African political systems more fragile. With the help of country-case studies, we argue, first, that the African political business cycle is not homogeneous, and occurs relatively infrequently in so-called ‘dominant-party systems’ where a pre-election stimulus confers little political advantage. Secondly, we show that, in those countries where a political cycle does occur, it does not necessarily cause institutional damage. Whether it does or not depends not so much on whether there is an electoral cycle as on whether this cycle calms or exacerbates fears of an unjust allocation of resources. In other words, the composition of the pre-election stimulus, in terms of its allocation between different categories of voter, is as important as its size
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