Study of X-ray Radiation Damage in Silicon Sensors

The European X-ray Free Electron Laser (XFEL) will deliver 30,000 fully coherent, high brilliance X-ray pulses per second each with a duration below 100 fs. This will allow the recording of diffraction patterns of single complex molecules and the study of ultra-fast processes. Silicon pixel sensors will be used to record the diffraction images. In 3 years of operation the sensors will be exposed to doses of up to 1 GGy of 12 keV X-rays. At this X-ray energy no bulk damage in silicon is expected. However fixed oxide charges in the insulating layer covering the silicon and interface traps at the Si-SiO2 interface will be introduced by the irradiation and build up over time. We have investigated the microscopic defects in test structures and the macroscopic electrical properties of segmented detectors as a function of the X-ray dose. From the test structures we determine the oxide charge density and the densities of interface traps as a function of dose. We find that both saturate (and even decrease) for doses between 10 and 100 MGy. For segmented sensors the defects introduced by the X-rays increase the full depletion voltage, the surface leakage current and the inter-pixel capacitance. We observe that an electron accumulation layer forms at the Si-SiO2 interface. Its width increases with dose and decreases with applied bias voltage. Using TCAD simulations with the dose dependent parameters obtained from the test structures, we are able to reproduce the observed results. This allows us to optimize the sensor design for the XFEL requirements

A high-wavenumber boundary-element method for an acoustic scattering problem

In this paper we show stability and convergence for a novel Galerkin boundary element method approach to the impedance boundary value problem for the Helmholtz equation in a half-plane with piecewise constant boundary data. This problem models, for example, outdoor sound propagation over inhomogeneous flat terrain. To achieve a good approximation with a relatively low number of degrees of freedom we employ a graded mesh with smaller elements adjacent to discontinuities in impedance, and a special set of basis functions for the Galerkin method so that, on each element, the approximation space consists of polynomials (of degree $\nu$) multiplied by traces of plane waves on the boundary. In the case where the impedance is constant outside an interval $[a,b]$, which only requires the discretization of $[a,b]$, we show theoretically and experimentally that the $L_2$ error in computing the acoustic field on $[a,b]$ is ${\cal O}(\log^{\nu+3/2}|k(b-a)| M^{-(\nu+1)})$, where $M$ is the number of degrees of freedom and $k$ is the wavenumber. This indicates that the proposed method is especially commendable for large intervals or a high wavenumber. In a final section we sketch how the same methodology extends to more general scattering problems

Analysis of high-order finite elements for convected wave propagation

In this paper, we examine the performance of high-order finite element methods (FEM) for aeroacoustic propagation, based on the convected Helmholtz equation. A methodology is presented to measure the dispersion and amplitude errors of the p-FEM, including non-interpolating shape functions, such as ‘bubble’ shape functions. A series of simple test cases are also presented to support the results of the dispersion analysis. The main conclusion is that the properties of p-FEM that make its strength for standard acoustics (e.g., exponential p-convergence, low dispersion error) remain present for flow acoustics as well. However, the flow has a noticeable effect on the accuracy of the numerical solution, even when the change in wavelength due to the mean flow is accounted for, and an approximation of the dispersion error is proposed to describe the influence of the mean flow. Also discussed is the so-called aliasing effect, which can reduce the accuracy of the solution in the case of downstream propagation. This can be avoided by an appropriate choice of mesh resolution

On wave boundary elements for radiation and scattering problems with piecewise constant impedance

Discrete methods of numerical analysis have been used successfully for decades for the solution of problems involving wave diffraction, etc. However, these methods, including the finite element and boundary element methods, can require a prohibitively large number of elements as the wavelength becomes progressively shorter. In this paper, a new type of interpolation for the wave field is described in which the usual conventional shape functions are modified by the inclusion of a set of plane waves propagating in multiple directions. Including such a plane wave basis in a boundary element formulation is found in this paper to be highly successful. Results are shown for a variety of scattering/radiating problems from convex and nonconvex obstacles on which are prescribed piecewise constant Robin conditions. Notable results include a conclusion that, using this new formulation, only approximately three degrees of freedom per wavelength are required

Single-session anodal transcranial direct current stimulation to enhance sport-specific performance in athletes: A systematic review and meta-analysis

BackgroundTranscranial direct current stimulation (tDCS) has emerged as a promising and feasible method to improve motor performance in healthy and clinical populations. However, the potential of tDCS to enhance sport-specific motor performance in athletes remains elusive.ObjectiveWe aimed at analyzing the acute effects of a single anodal tDCS session on sport-specific motor performance changes in athletes compared to sham.MethodsA systematic review and meta-analysis was conducted in the electronic databases PubMed, Web of Science, and SPORTDiscus. The meta-analysis was performed using an inverse variance method and a random-effects model. Additionally, two subgroup analyses were conducted (1) depending on the stimulated brain areas (primary motor cortex (M1), temporal cortex (TC), prefrontal cortex (PFC), cerebellum (CB)), and (2) studies clustered in subgroups according to different sports performance domains (endurance, strength, visuomotor skill).ResultsA total number of 19 studies enrolling a sample size of 258 athletes were deemed eligible for inclusion. Across all included studies, a significant moderate standardized mean difference (SMD) favoring anodal tDCS to enhance sport-specific motor performance could be observed. Subgroup analysis depending on cortical target areas of tDCS indicated a significant moderate SMD in favor of anodal tDCS compared to sham for M1 stimulation.ConclusionA single anodal tDCS session can lead to performance enhancement in athletes in sport-specific motor tasks. Although no definitive conclusions can be drawn regarding the modes of action as a function of performance domain or stimulation site, these results imply intriguing possibilities concerning sports performance enhancement through anodal M1 stimulation

Potential Application of the Modulated Scatterer Technique to Multilayered Material Evaluation and Health Monitoring

Modulated scatterer technique (MST) is based on illuminating a small antenna, usually a dipole, loaded with a PIN diode, with an electromagnetic wave. The scattered (or reflected) wave from the probe may then be used to determine dielectric properties of the material in which the probe is located or embedded. The PIN diode is turned on and off which not only changes the impedance of the probe, but also modulates (with the same rate) the reflection from the probe. A major challenge associated with MST is detecting and distinguishing the desired probe response in the ever-present reflections from surrounding structures and materials. This challenge can be overcome by incorporating a swept-frequency method into the measurements. A swept-frequency technique allows the use of the Fourier Transform method which results in separate detection of the reflection from the probe (similar to pulsed methods). Having the ability to discriminate the probe response renders the MST technique useful for multilayer structure applications as well. The probe can be placed in a given layer of a material, and the properties of that layer can be monitored (regardless of the presence of other layers). Additionally, the probe can be placed at an interface and changes in that interface (such as disbonding) can be detected. The ratio of the reflection from a probe, between the on and off states, has been shown to be a unique technique for evaluating properties of materials. This paper presents the basis and some results of applying swept-frequency MST for inspecting layered materials

Local muscle metabolic demand induced by neuromuscular electrical stimulation and voluntary contractions at different force levels: a NIRS study

Functional Muscle metabolic demand during contractions evoked by neuromuscular electrical stimulation (NMES) has been consistently documented to be greater than voluntary contractions (VOL) at the same force level (10-50% maximal voluntary contraction-MVC). However, we have shown using a near-infrared spectroscopy (NIRS) technique that local muscle metabolic demand is similar between NMES and VOL performed at MVC levels, thus controversy exists. This study therefore compared biceps brachii muscle metabolic demand (tissue oxygenation index-TOI and total hemoglobin volume-tHb) during a 10s isometric contraction of the elbow flexors between NMES (stimulation frequency of 30Hz and current level to evoke 30% MVC) and VOL at 30% MVC (VOL-30%MVC) and MVC (VOL-MVC) level in 8 healthy men (23-33-y). Greater changes in TOI and tHb induced by NMES than VOL-30%MVC confirm previous studies of a greater local metabolic demand for NMES than VOL at the same force level. The same TOI and tHb changes for NMES and VOL-MVC suggest that local muscle metabolic demand and intramuscular pressure were similar between conditions. In conclusion, these findings indicate that NMES induce a similar local muscle metabolic demand as that of maximal VOL