2,176 research outputs found
Performance of Geant4 in simulating semiconductor particle detector response in the energy range below 1 MeV
Geant4 simulations play a crucial role in the analysis and interpretation of
experiments providing low energy precision tests of the Standard Model. This
paper focuses on the accuracy of the description of the electron processes in
the energy range between 100 and 1000 keV. The effect of the different
simulation parameters and multiple scattering models on the backscattering
coefficients is investigated. Simulations of the response of HPGe and
passivated implanted planar Si detectors to \beta{} particles are compared to
experimental results. An overall good agreement is found between Geant4
simulations and experimental data
Measurement of the -asymmetry parameter of Cu in search for tensor type currents in the weak interaction
Precision measurements at low energy search for physics beyond the Standard
Model in a way complementary to searches for new particles at colliders. In the
weak sector the most general decay Hamiltonian contains, besides vector
and axial-vector terms, also scalar, tensor and pseudoscalar terms. Current
limits on the scalar and tensor coupling constants from neutron and nuclear
decay are on the level of several percent.
The goal of this paper is extracting new information on tensor coupling
constants by measuring the -asymmetry parameter in the pure Gamow-Teller
decay of Cu, thereby testing the V-A structure of the weak interaction.
An iron sample foil into which the radioactive nuclei were implanted was cooled
down to milliKelvin temperatures in a He-He dilution refrigerator. An
external magnetic field of 0.1 T, in combination with the internal hyperfine
magnetic field, oriented the nuclei. The anisotropic radiation was
observed with planar high purity germanium detectors operating at a temperature
of about 10\,K. An on-line measurement of the asymmetry of Cu
was performed as well for normalization purposes. Systematic effects were
investigated using Geant4 simulations.
The experimental value, = 0.587(14), is in agreement with the
Standard Model value of 0.5991(2) and is interpreted in terms of physics beyond
the Standard Model. The limits obtained on possible tensor type charged
currents in the weak interaction hamiltonian are -0.045
0.159 (90\% C.L.). The obtained limits are comparable to limits from other
correlation measurements in nuclear decay and contribute to further
constraining tensor coupling constants
Solar wind and seasonal influence on ionospheric currents from Swarm and CHAMP measurements
We present a new climatological model of the ionospheric current system, determined from magnetic measurements taken by the Challenging Minisatellite Payload (CHAMP) and Swarm satellites. The model describes the horizontal currents in the ionosphere, below the satellites, and the field-aligned (Birkeland) currents that connect the ionosphere with the magnetosphere. The model provides ionospheric current values at any location as continuous functions of solar wind speed, interplanetary magnetic field, dipole tilt angle, and the F10.7 index of solar flux. Geometric distortions due to variations in the Earth’s main magnetic field are taken into account, thus allowing for precise comparisons between the two hemispheres. The model is the first of its kind to describe the full 3-D electric currents and not only the field-aligned or the equivalent horizontal current. We use this capability to demonstrate a key difference between seasons: During winter, the total horizontal current is almost entirely confined to the auroral oval, for all interplanetary magnetic field orientations, where it connects upward and downward Birkeland currents. During more sunlit conditions, the horizontal current extends beyond the auroral oval and is a sum of currents connecting Birkeland currents and currents that circulate in the ionosphere. The westward electrojet is the only large-scale current structure that is persistent across seasons. Comparison with average convection maps suggests that it is comprised largely of Hall currents, which connect to Birkeland currents in the winter but not in summer.publishedVersio
Theoretical assessment of a repolarization time marker based on the intracardiac bipolar electrogram
The spatio-temporal organization of cardiac repolarization modulates the vulnerability to dangerous ventricular arrhythmias. Methodologies that provide accurate assessment of cardiac repolarization are of primary importance for a better understanding of cardiac electrophysiology and represent a potentially useful tool for clinical applications. The most commonly used repolarization time (RT) marker from extracellular recordings is derived from the unipolar electrogram (UEG). However, far field potentials and remote activity may in certain conditions bias this marker. In this paper, a RT marker based on the bipolar electrogram (BEG) is proposed. An analytical expression of the BEG based on a simple model of the cardiac extracellular potential is derived. According to the proposed analytical framework the BEG exhibits a repolarization wave whose extremum (maximum or minimum) corresponds to the average of the local RTs at the two electrodes of the bipole. The amplitude of this extremum is a function of the steepness of phase 3 of the action potentials, inter-electrode distance, conduction velocity and direction of wave-back propagation. A simulation study based on this analytical framework showed that for noisy to good signal quality (SNR of the UEG ≥ 10 dB), and for a typical inter-electrode distance of 2 mm, conduction velocity between 0.2 and 0.6 m/s, and an angle between conduction direction and the inter-electrode axis ≤ π/4, the median absolute error was lower than 6.8 ms while the median linear correlation between estimated and theoretical RT was higher than 0.91. Examples of RT derived from BEG recorded in a structurally normal heart in both the right and left ventricles demonstrate that the proposed procedure is feasible in human in-vivo studies
Visuocognitive Fluency Facilitating ECG Interpretation with Visual Metaphors and Expressive Tags
An accelerating high-latitude jet in Earth's core
Observations of the change in Earth's magnetic field, the secular variation, provide information on the motion of liquid metal within the core that is responsible for its generation. The very latest high-resolution observations from ESA's Swarm satellite mission show intense field change at high-latitude localised in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we explain this feature with a localised, non-axisymmetric, westwards jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core. We find that the jet has increased in magnitude by a factor of three over the period 2000--2016 to about 40 km/yr, and is now much stronger than typical large-scale flows inferred for the core. The current accelerating phase may be a part of a longer term fluctuation of the jet causing both eastwards and westwards movement of magnetic features over historical periods, and may contribute to recent changes in torsional wave activity and the rotation direction of the inner core
Control over phase separation and nucleation using a laser-tweezing potential
Control over the nucleation of new phases is highly desirable but elusive. Even though there is a long history of crystallization engineering by varying physicochemical parameters, controlling which polymorph crystallizes or whether a molecule crystallizes or forms an amorphous precipitate is still a poorly understood practice. Although there are now numerous examples of control using laser-induced nucleation, the absence of physical understanding is preventing progress. Here we show that the proximity of a liquid–liquid critical point or the corresponding binodal line can be used by a laser-tweezing potential to induce concentration gradients. A simple theoretical model shows that the stored electromagnetic energy of the laser beam produces a free-energy potential that forces phase separation or triggers the nucleation of a new phase. Experiments in a liquid mixture using a low-power laser diode confirm the effect. Phase separation and nucleation using a laser-tweezing potential explains the physics behind non-photochemical laser-induced nucleation and suggests new ways of manipulating matter
Adenosine-mono-phosphate-activated protein kinase-independent effects of metformin in T cells
The anti-diabetic drug metformin regulates T-cell responses to immune activation and is proposed to function by regulating the energy-stress-sensing adenosine-monophosphate-activated protein kinase (AMPK). However, the molecular details of how metformin controls T cell immune responses have not been studied nor is there any direct evidence that metformin acts on T cells via AMPK. Here, we report that metformin regulates cell growth and proliferation of antigen-activated T cells by modulating the metabolic reprogramming that is required for effector T cell differentiation. Metformin thus inhibits the mammalian target of rapamycin complex I signalling pathway and prevents the expression of the transcription factors c-Myc and hypoxia-inducible factor 1 alpha. However, the inhibitory effects of metformin on T cells did not depend on the expression of AMPK in T cells. Accordingly, experiments with metformin inform about the importance of metabolic reprogramming for T cell immune responses but do not inform about the importance of AMPK
Nuclear Transparency to Intermediate-Energy Protons
Nuclear transparency in the (e,e'p) reaction for 135 < Tp < 800 MeV is
investigated using the distorted wave approximation. Calculations using
density-dependent effective interactions are compared with phenomenological
optical potentials. Nuclear transparency is well correlated with proton
absorption and neutron total cross sections. For Tp < 300 MeV there is
considerable sensitivity to the choice of optical model, with the empirical
effective interaction providing the best agreement with transparency data. For
Tp > 300 MeV there is much less difference between optical models, but the
calculations substantially underpredict transparency data and the discrepancy
increases with A. The differences between Glauber and optical model
calculations are related to their respective definitions of the semi-inclusive
cross section. By using a more inclusive summation over final states the
Glauber model emphasizes nucleon-nucleon inelasticity, whereas with a more
restrictive summation the optical model emphasizes nucleon-nucleus
inelasticity; experimental definitions of the semi-inclusive cross section lie
between these extremes.Comment: uuencoded gz-compressed tar file containing revtex and bbl files and
5 postscript figures, totalling 31 pages. Uses psfi
Impact of Type-2 Diabetes Mellitus on the Outcomes of Catheter Ablation of Atrial Fibrillation (European observational multicentre study)
Type-2 diabetes mellitus (DM) is associated with an increased risk of atrial fibrillation (AF). It is unclear whether DM is a risk factor for arrhythmia recurrence following catheter ablation of AF. We performed a nonrandomised, observational study in 7 high-volume European centres. A total of 2,504 patients who underwent catheter ablation of AF were included, and procedural outcomes were compared among patients with or without DM. Patients with DM (234) accounted for 9.3% of the sample, and were significantly older, had a higher BMI and suffered more frequently from persistent AF. Arrhythmia relapses at 12 months after AF ablation occurred more frequently in the DM group (32.0% vs 25.3%, p = 0.031). After adjusting for type of AF (i.e., paroxysmal vs persistent), during a median follow-up of 17 ± 16 months, atrial arrhythmia free-survival was lower in the diabetics with persistent AF (log-rank p = 0.003), and comparable for paroxysmal AF (log-rank p = 0.554). These results were confirmed in a propensity-matched analysis, and DM was also an independent predictor of AF recurrence on the multivariate analysis (hazard ratio 1.39; 95% confidence interval 95%1.07 to 1.88; p = 0.016). There was no significant difference in the rate of periprocedural complications among DM and non-DM patients (3.8% vs 6.3%, p = 0.128). Efficacy and safety of cryoballoon ablation were comparable to radiofrequency ablation in both DM and no-DM groups. In conclusion, catheter ablation of AF appears to be safe in patients with DM. However, DM is associated with higher rate of atrial arrhythmia relapse, particularly for patients with persistent AF
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