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Point source

About: Point source is a research topic. Over the lifetime, 5077 publications have been published within this topic receiving 94091 citations.


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Journal ArticleDOI
Jianying Li1, Ronald J. Jaszczak1, Huili Wang1, Kim L. Greer1, R.E. Coleman1 
TL;DR: The experimental results indicate that the shift parameters determined in the same experiment with the point source located at different places are consistent but change from time to time, suggesting that calibration of the system is needed on a periodic basis.
Abstract: The difference between the displacement of the centre of rotation (mechanical shift, MS) and the electronic centring misalignment (electronic shift, ES) in cone beam SPECT is evaluated. A method is proposed to determine both MS and ES using the centroid of a projected point source sampled over 360 degrees and the Marquardt non-linear fitting algorithm. Both shifts are characterized by two orthogonal components. This method is verified using Monte Carlo simulated point source data with different combinations of mechanical and electronic shifts. Both shifts can be determined correctly. The authors have also applied the proposed method to their cone beam SPECT system to determine both shifts as well as the focal length. The determined ES parameters are then used to correct the projections and the MS parameters are incorporated into a reconstruction algorithm. The point source images are reconstructed and the image resolutions with and without the shift corrections are measured. The experimental results demonstrate that the image resolution is improved after shift corrections.

67 citations

Journal ArticleDOI
TL;DR: In this paper, the propagation of a broadband sound pulse in three-dimensional (3D) shallow water waveguides is investigated numerically using a Fourier synthesis technique based on the fully 3D parabolic equation based model 3DWAPE, recently modified to include a wide-angle paraxial approximation for the azimuthal component.
Abstract: In this paper, the propagation of a broadband sound pulse in three-dimensional (3D) shallow water waveguides is investigated numerically. Two cases are examined: (i) the 3D ASA benchmark wedge, and (ii) the 3D Gaussian canyon. The numerical method used to solve the four-dimensional acoustic problem is based on a Fourier synthesis technique. The frequency-domain calculations are carried out using the fully 3D parabolic equation based model 3DWAPE, recently modified to include a wide-angle paraxial approximation for the azimuthal component. A broadband sound pulse with a central frequency of 25 Hz and a bandwith of 40 Hz is considered. For both test cases, 3D results corresponding to a 25 Hz cw point source are first presented and compared with predictions from a 3D adiabatic modal model. Then, the acoustic problem is solved considering the broadband source pulse. The modal structure of the received signals is analyzed and exhibits multiple mode arrivals of the propagating signal.

67 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present a first systematic approach that takes the individual characteristics of these sources into account, as well as the impact of the extragalactic magnetic-field structures up to a distance of 120 Mpc.
Abstract: Radio galaxies are intensively discussed as the sources of cosmic rays observed above about $3\,{\times}\,10^{18}\,\text{eV}$, called ultra-high energy cosmic rays (UHECRs). We present a first, systematic approach that takes the individual characteristics of these sources into account, as well as the impact of the extragalactic magnetic-field structures up to a distance of 120 Mpc. We use a mixed simulation setup, based on 3D simulations of UHECRs ejected by observed, individual radio galaxies taken out to a distance of 120 Mpc, and on 1D simulations over a continuous source distribution contributing from beyond 120 Mpc. Additionally, we include the ultra-luminous radio galaxy Cygnus A at a distance of about $250\,$Mpc, as its contribution is so strong that it must be considered as an individual point source. The implementation of the UHECR ejection in our simulation setup is based on a detailed consideration of the physics of radio jets and standard first-order Fermi acceleration. We show that the average contribution of radio galaxies taken over a very large volume cannot explain the observed features of UHECRs measured at Earth. However, we obtain excellent agreement with the spectrum, composition, and arrival-direction distribution of UHECRs measured by the Pierre Auger Observatory, if we assume that most UHECRs observed arise from only two sources: The ultra-luminous radio galaxy Cygnus A, providing a mostly light composition of nuclear species dominating up to about $6\,{\times}\,10^{19}\,$eV, and the nearest radio galaxy Centaurus A, providing a heavy composition dominating above $6\,{\times}\,10^{19}\,$eV. Here we have to assume that extragalactic magnetic fields out to 250 Mpc, which we did not include in the simulation, are able to isotropize the UHECR events at about 8 EeV arriving from Cygnus A.

67 citations

Journal ArticleDOI
TL;DR: In this paper, the authors describe the development, evaluation, and application of a new plume-in-grid model for investigating the subgrid-scale effects, associated with NOx emissions from large elevated point sources, on O3 formation.
Abstract: [1] We describe the development, evaluation, and application of a new plume-in-grid model for investigating the subgrid-scale effects, associated with NOx emissions from large elevated point sources, on O3 formation. Traditional Eulerian air quality models cannot resolve the strong concentration gradients created by plumes emitted from large point sources. Although several plume-in-grid approaches have been used in the past to address this issue, they have been limited by their simplistic simulation of plume dispersion and/or chemistry and their lack of treatment of the effect of turbulence on plume chemistry. In the plume-in-grid model presented here, the embedded reactive plume model combines a state-of-the-science puff model with a gas-phase chemistry mechanism that is consistent with that used in the host grid model. The puff model uses a second-order closure scheme, allowing for a more accurate treatment of dispersion and the influence of turbulent concentration fluctuations on chemical rates. It also allows the splitting and merging of puffs to account for wind shear effects, varying chemistry across the plume, and interplume and intraplume interactions. The combined puff/chemistry model is embedded into an Eulerian grid model. Results from the application of this model to the northeastern United States, a domain containing some of the largest NOx-emitting power plants in the United States, show that the plume-in-grid treatment leads to significant differences in surface O3 and HNO3 concentrations.

67 citations

Journal ArticleDOI
TL;DR: In this article, the authors interpret the observed features in terms of sun glitter from the tilted facets of a Kelvin wake, and regard the present study as a step towards the interpretation of many unexplained naturally occurring features at the edge of the Sun glitter.
Abstract: Narrow V-shaped wakes extending some 20 km behind surface ships were first found on Synthetic Aperture Radar (SAR) images from SEASAT in 1978. The V-wake geometry differed strikingly from the traditional Kelvin wake geometry consisting of divergent and transverse wave components generated by a travelling pressure point. The SAR images can be accounted for in terms of Bragg scatter from relatively short waves generated by the surface vessel. An essential ingredient of this hypothesis is that the wave generation is by an intermittent rather than a steady point source. Optical images from a hand-held camera on a 1985 space shuttle mission revealed many V-like wakes behind surface ships. There is no Bragg scattering from the ocean surface at optical wavelengths, so an alternative hypothesis is called for. We can interpret the observed features in terms of sun glitter from the tilted facets of a Kelvin wake. An essential ingredient is the generation by complex sources rather than by a single point source. We regard the present study as a step towards the interpretation of many unexplained naturally occurring features at the edge of the Sun glitter.

67 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202350
2022133
2021103
2020135
2019123
2018133