4,874 research outputs found
Upgrade of the protection system for superconducting circuits in the LHC
Prior to the re-start of the Large Hadron Collider LHC in 2009 the protection system for superconducting magnets and bus-bars QPS will be substantially upgraded. The foreseen modifications will enhance the capability of the system in detecting problems related to the electrical interconnections between superconducting magnets as well as the detection of so-called aperture symmetric quenches in the LHC main magnets
Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene
For most optoelectronic applications of graphene a thorough understanding of
the processes that govern energy relaxation of photoexcited carriers is
essential. The ultrafast energy relaxation in graphene occurs through two
competing pathways: carrier-carrier scattering -- creating an elevated carrier
temperature -- and optical phonon emission. At present, it is not clear what
determines the dominating relaxation pathway. Here we reach a unifying picture
of the ultrafast energy relaxation by investigating the terahertz
photoconductivity, while varying the Fermi energy, photon energy, and fluence
over a wide range. We find that sufficiently low fluence ( 4
J/cm) in conjunction with sufficiently high Fermi energy (
0.1 eV) gives rise to energy relaxation that is dominated by carrier-carrier
scattering, which leads to efficient carrier heating. Upon increasing the
fluence or decreasing the Fermi energy, the carrier heating efficiency
decreases, presumably due to energy relaxation that becomes increasingly
dominated by phonon emission. Carrier heating through carrier-carrier
scattering accounts for the negative photoconductivity for doped graphene
observed at terahertz frequencies. We present a simple model that reproduces
the data for a wide range of Fermi levels and excitation energies, and allows
us to qualitatively assess how the branching ratio between the two distinct
relaxation pathways depends on excitation fluence and Fermi energy.Comment: Nano Letters 201
Photoconductivity of biased graphene
Graphene is a promising candidate for optoelectronic applications such as
photodetectors, terahertz imagers, and plasmonic devices. The origin of
photoresponse in graphene junctions has been studied extensively and is
attributed to either thermoelectric or photovoltaic effects. In addition, hot
carrier transport and carrier multiplication are thought to play an important
role. Here we report the intrinsic photoresponse in biased but otherwise
homogeneous graphene. In this classic photoconductivity experiment, the
thermoelectric effects are insignificant. Instead, the photovoltaic and a
photo-induced bolometric effect dominate the photoresponse due to hot
photocarrier generation and subsequent lattice heating through electron-phonon
cooling channels respectively. The measured photocurrent displays polarity
reversal as it alternates between these two mechanisms in a backgate voltage
sweep. Our analysis yields elevated electron and phonon temperatures, with the
former an order higher than the latter, confirming that hot electrons drive the
photovoltaic response of homogeneous graphene near the Dirac point
Effect of low perceived social support on health outcomes in young patients with acute myocardial infarction: results from the VIRGO (variation in recovery: role of gender on outcomes of young AMI patients) study
Background: Social support is an important predictor of health outcomes after acute myocardial infarction (AMI), but social support varies by sex and age. Differences in social support could account for sex differences in outcomes of young patients with AMI. Methods and Results: Data from the Variation in Recovery: Role of Gender on Outcomes of Young AMI Patients (VIRGO) study, an observational study of AMI patients aged ≤55 years in the United States and Spain, were used for this study. Patients were categorized as having low versus moderate/high perceived social support using the ENRICHD Social Support Inventory. Outcomes included health status (Short Form-12 physical and mental component scores), depressive symptoms (Patient Health Questionnaire), and angina-related quality of life (Seattle Angina Questionnaire) evaluated at baseline and 12 months. Among 3432 patients, 21.2% were classified as having low social support. Men and women had comparable levels of social support at baseline. On average, patients with low social support reported lower functional status and quality of life and more depressive symptoms at baseline and 12 months post-AMI. After multivariable adjustment, including baseline health status, low social support was associated with lower mental functioning, lower quality of life, and more depressive symptoms at 12 months (all P<0.001). The relationship between low social support and worse physical functioning was nonsignificant after adjustment (P=0.6). No interactions were observed between social support, sex, or country. Conclusion: Lower social support is associated with worse health status and more depressive symptoms 12 months after AMI in both young men and women. Sex did not modify the effect of social support.Emily M. Bucholz, Kelly M. Strait, Rachel P. Dreyer, Mary Geda, Erica S. Spatz, Hector Bueno, Judith H. Lichtman, Gail D'Onofrio, John A. Spertus, Harlan M. Krumhol
Bayesian Learning of Gas Transport in Three-Dimensional Fracture Networks
Modeling gas flow through fractures of subsurface rock is a particularly
challenging problem because of the heterogeneous nature of the material.
High-fidelity simulations using discrete fracture network (DFN) models are one
methodology for predicting gas particle breakthrough times at the surface, but
are computationally demanding. We propose a Bayesian machine learning method
that serves as an efficient surrogate model, or emulator, for these
three-dimensional DFN simulations. Our model trains on a small quantity of
simulation data and, using a graph/path-based decomposition of the fracture
network, rapidly predicts quantiles of the breakthrough time distribution. The
approach, based on Gaussian Process Regression (GPR), outputs predictions that
are within 20-30% of high-fidelity DFN simulation results. Unlike previously
proposed methods, it also provides uncertainty quantification, outputting
confidence intervals that are essential given the uncertainty inherent in
subsurface modeling. Our trained model runs within a fraction of a second,
which is considerably faster than other methods with comparable accuracy and
multiple orders of magnitude faster than high-fidelity simulations
An improved measurement of muon antineutrino disappearance in MINOS
We report an improved measurement of muon anti-neutrino disappearance over a
distance of 735km using the MINOS detectors and the Fermilab Main Injector
neutrino beam in a muon anti-neutrino enhanced configuration. From a total
exposure of 2.95e20 protons on target, of which 42% have not been previously
analyzed, we make the most precise measurement of the anti-neutrino
"atmospheric" delta-m squared = 2.62 +0.31/-0.28 (stat.) +/- 0.09 (syst.) and
constrain the anti-neutrino atmospheric mixing angle >0.75 (90%CL). These
values are in agreement with those measured for muon neutrinos, removing the
tension reported previously.Comment: 5 pages, 4 figures. In submission to Phys.Rev.Let
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Search for the disappearance of muon antineutrinos in the NuMI neutrino beam
We report constraints on antineutrino oscillation parameters that were obtained by using the two MINOS detectors to measure the 7% muon antineutrino component of the NuMI neutrino beam. In the Far Detector, we select 130 events in the charged-current muon antineutrino sample, compared to a prediction of 136.4 ± 11.7(stat)^(+10.2)_(-8.9)(syst) events under the assumption │Δm^2│ = 2.32 X 10^(-3) eV^2, sin^2(2θ) = 1.0
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Active to Sterile Neutrino Mixing Limits from Neutral-Current Interactions in MINOS
Results are reported from a search for active to sterile neutrino oscillations in the MINOS long-baseline experiment, based on the observation of neutral-current neutrino interactions, from an exposure to the NuMI neutrino beam of 7.07×10^(20) protons on target. A total of 802 neutral-current event candidates is observed in the Far Detector, compared to an expected number of 754±28(stat)±37(syst) for oscillations among three active flavors. The fraction f_s of disappearing ν_μ that may transition to ν_s is found to be less than 22% at the 90% C.L
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