122792689
Computer animation of extensive air showers interacting with the MILAGRO water Cherenkov detector
I employ advanced computer animation to visualize the interaction of Extensive Air Showers (EAS) with the Milagro water reservoir. The animations help conceptualize the evolution of the EAS particle front as it hits a large volume of water and converts to a front of Cherenkov photons. Expected effects such as refraction and curvature are easily seen, as well as a number of novel behaviors. A simple model that explains the observed dynamics of the shower front is presented.
13859904
FOREGROUND CONTAMINATION IN INTERFEROMETRIC MEASUREMENTS OF THE REDSHIFTED 21 cm POWER SPECTRUM
Subtraction of astrophysical foreground contamination from “dirty” sky maps produced by simulated measurements of the Murchison Widefield Array (MWA) has been performed by fitting a third-order polynomial along the spectral dimension of each pixel in the data cubes. The simulations are the first to include the unavoidable instrumental effects of the frequency-dependent primary antenna beams and synthesized array beams. They recover the onedimensional spherically binned input redshifted 21 cm power spectrum within ∼ 1% over the scales probed most sensitively by the MWA (0.01 k 1M pc −1 ) and demonstrate that realistic instrumental effects will not mask the epoch of reionization signal. We find that the weighting function used to produce the dirty sky maps from the gridded visibility measurements is important to the success of the technique. Uniform weighting of the visibility measurements produces the best results, whereas natural weighting significantly worsens the foreground subtraction by coupling structure in the density of the visibility measurements to spectral structure in the dirty sky map data cube. The extremely dense uv-coverage of the MWA was found to be advantageous for this technique and produced very good results on scales corresponding to |u| 500λ in the uv-plane without any selective editing of the uv-coverage.
117096035
Quasinormal acoustic oscillations in the Michel flow
We study spherical and nonspherical linear acoustic perturbations of the Michel flow, which describes the steady radial accretion of a perfect fluid into a nonrotating black hole. The dynamics of such perturbations are governed by a scalar wave equation on an effective curved background geometry determined by the acoustic metric, which is constructed from the spacetime metric and the particle density and four-velocity of the fluid. For the problem under consideration in this paper the acoustic metric has the same qualitative features as an asymptotically flat, static and spherically symmetric black hole, and thus it represents a natural astrophysical analogue black hole. As for the case of a scalar field propagating on a Schwarzschild background, we show that acoustic perturbations of the Michel flow exhibit quasinormal oscillations. Based on a new numerical method for determining the solutions of the radial mode equation, we compute the associated frequencies and analyze their dependency on the mass of the black hole, the radius of the sonic horizon and the angular momentum number. Our results for the fundamental frequencies are compared to those obtained from an independent numerical Cauchy evolution, finding good agreement between the two approaches. When the radius of the sonic horizon is large compared to the event horizon radius, we find that the quasinormal frequencies scale approximately like the surface gravity associated with the sonic horizon.
113451453
Aging and conductivity of electrodes for high rate tRPCs from an ion conductivity approach
Resistive Plate Chambers (RPCs) are multipurpose ionizing gas detectors broadly used in Particle Physics research because of the outstanding time resolution they offer at a very affordable price, their robustness and their easy manufacture. However, to extend their use to a broader range of applications, some properties, specially the counting rate capability, need to be improved. The straightforward way to increase the rate capability is to develop new electrodes with resistivities in the range of 10 7 �10 8 .m offering, besides a “resistive” character, conductivity high enough for allowing faster charge depletion in the chamber. In this work, we have carried out electrical measurements on several materials that are currently used or, according to their conductivity values, are good candidates to be used as resistive plates in high rate RPCs. We have analysed their behaviour in order to identify and to better understand their conduction mechanisms and to determine the nature of the degradation processes.