3,452 research outputs found
Surface Chemical Reactions at the Atomic Scale: Gas Reactions with Semiconductors Studied with Scanning Tunneling Microscopy
The vacuum tunneling microscope has been extensively utilized in the study of the surface atomic configuration of conducting materials. Analysis of features in both the tunneling images and in the tunnel junction I-V characteristic yields insight into a wide variety of processes occurring at surfaces. In the last few years, elementary chemical reactions occurring at surfaces have been examined in this manner, principally adsorption of simple gas species such as H2, O2, and NH3 on semiconductors and metals. Adsorption sites have been deduced from changes brought about in surface configuration subsequent to gas exposure. The relationship of these sites with one another and their evolution as a function of exposure has been utilized to constrain mechanisms for the adsorption process.
More recently, work has been performed where the scanning tunneling microscope (STM) takes on an active role. Hydrogen terminated silicon surfaces have been prepared and imaged with the STM. The tunneling images and infrared absorption spectra showed that configurations of both the terraces and steps are radically changed due to hydrogen capping. Moreover, the low-energy high-current density electron source, which is formed by the STM tip, has been used to selectively desorb this species from the surface. This process results in configuration changes which are derived from both the desorption kinetics and the long-range configuration of the initial surface
Materials and technological processes for High-Gradient accelerating structures: new results from mechanical tests of an innovative braze-free cavity
Pure oxygen-free high-conductivity copper is a widely used material for manufacturing accelerating cavities working at room temperature. Several studies attempted to explain limitations associated with the maximum allowed field gradients and the behaviour of vacuum RF breakdown in copper accelerating structures through generation and movement of dislocations under stresses associated with RF electric and magnetic fields. Pure copper and also copper alloys undergo mechanical and thermal treatments to be hardened and strengthened during manufacturing, although their mechanical properties significantly change after heating above 590ˆC. High temperature brazing and diffusion bonding are assembly methods widely used to manufacture ultra-high vacuum accelerating devices. However, these processes, occurring at about 800-1000ˆC, significantly affect the mechanical properties of copper and copper alloys. We present here a novel Tungsten Inert Gas welding procedure, which is fast and keeps the high-gradient surfaces of the cavity and other components well below the copper annealing temperature. This process may be successfully used to manufacture copper-based accelerating components. This technology preserves the hardness and cleanliness of copper in order to achieve the maximum accelerating gradient
Molybdenum sputtering film characterization for high gradient accelerating structures
Technological advancements are strongly required to fulfill the demands of
new accelerator devices with the highest accelerating gradients and operation
reliability for the future colliders. To this purpose an extensive R&D
regarding molybdenum coatings on copper is in progress. In this contribution we
describe chemical composition, deposition quality and resistivity properties of
different molybdenum coatings obtained via sputtering. The deposited films are
thick metallic disorder layers with different resistivity values above and
below the molibdenum dioxide reference value. Chemical and electrical
properties of these sputtered coatings have been characterized by Rutherford
backscattering, XANES and photoemission spectroscopy. We will also present a
three cells standing wave section coated by a molybdenum layer 500 nm
thick designed to improve the performance of X-Band accelerating systems.Comment: manuscript has been submitted and accepted by Chinese Physics C
(2012
Mechanical Design and Characteristics of a Superconducting Insertion Quadrupole Model Magnet for the Large Hadron Collider
A superconducting insertion quadruple is being developed by KEK in collaboration with CERN for the Large Hadron Collider (LHC) project. The mechanical design of the magnet in which the pre-stress is applied to the coil through thin stainless steel collars inside the yoke, the two halves of which are held together by means of keys, has been validated experimentally by measurements on a short model. The 140 mm long model was assembled from real magnet components in order to simulate the magnet assembly and to evaluate the change in coil pre-stress during assembly and cool-down. A new technique using capacitance pressure transducers was used, which has enabled measurements of the stress distributions in the coil with high accuracy. This paper describes the mechanical design of the quadrupole magnet and results obtained from the short mechanical model
Effects of disorder in location and size of fence barriers on molecular motion in cell membranes
The effect of disorder in the energetic heights and in the physical locations
of fence barriers encountered by transmembrane molecules such as proteins and
lipids in their motion in cell membranes is studied theoretically. The
investigation takes as its starting point a recent analysis of a periodic
system with constant distances between barriers and constant values of barrier
heights, and employs effective medium theory to treat the disorder. The
calculations make possible, in principle, the extraction of confinement
parameters such as mean compartment sizes and mean intercompartmental
transition rates from experimentally reported published observations. The
analysis should be helpful both as an unusual application of effective medium
theory and as an investigation of observed molecular movements in cell
membranes.Comment: 9 pages, 5 figure
The impact of PARPs and ADP-ribosylation on inflammation and host–pathogen interactions
This work is licensed under a Creative Commons Attribution 4.0 International License.Poly-adenosine diphosphate-ribose polymerases (PARPs) promote ADP-ribosylation, a highly conserved, fundamental posttranslational modification (PTM). PARP catalytic domains transfer the ADP-ribose moiety from NAD+ to amino acid residues of target proteins, leading to mono- or poly-ADP-ribosylation (MARylation or PARylation). This PTM regulates various key biological and pathological processes. In this review, we focus on the roles of the PARP family members in inflammation and host–pathogen interactions. Here we give an overview the current understanding of the mechanisms by which PARPs promote or suppress proinflammatory activation of macrophages, and various roles PARPs play in virus infections. We also demonstrate how innovative technologies, such as proteomics and systems biology, help to advance this research field and describe unanswered questions
Effects of the pathogenic water mold Saprolegnia ferax on survival of amphibian larvae
Infectious diseases are a significant threat to worldwide biodiversity. Amphibian declines, a significant part of current biodiversity losses, are in many cases associated with infectious disease. Water molds are one group of pathogens affecting amphibians on a worldwide basis. Although water molds have been studied extensively for their effects on host embryos, little information is available about how they affect post-embryonic amphibians. We tested the effects of one species of water mold, Saprolegnia ferax, in a comparative study of larvae of 4 amphibian species: Pseudacris regilla (Pacific treefrog), Rana cascadae (Cascades frog), Ambystoma macrodactylum (long-toed salamander), and R. aurora (red-legged frog). S. ferax can kill amphibians at the embryonic and juvenile life history stages, depending on the amphibian species. In the present study, a 1 wk exposure to S. ferax killed P. regilla larvae and a 2 wk exposure killed R. aurora larvae. Larvae of the other host species were unaffected after 1 wk of exposure to S. ferax. Our results suggest that S. ferax can kill amphibian larvae and further suggest that evaluation of how pathogens affect amphibians at the population level requires investigation at various life stages
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