Point source

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

X-ray focusing optics. I. Applications of wave optics to doubly curved crystals with a point x-ray source

Abstract: A wave optics approach to the calculations of the intensity distribution of an x‐ray point source image with a two‐dimensional (2D) focusing geometry is presented. Analytical formulas are derived to calculate the intensity distribution at the focal plane. This approach has taken into account the effects of x‐ray scattering within a 2D bent crystal, which includes the x‐ray refraction and absorption inside the crystal, and the effects of elastically deformed crystals described by the anisotropic elasticity theory. Based upon the elastic bending model, the modified Bragg law and 2D lens equations are discussed. In addition, the x‐ray extinction distance for curved crystals is found to be dependent on the size and the bending radius of bent crystals. For a monochromatic x‐ray point source, calculation of the intensity distribution with a 2D bent silicon crystal is given for both perfect and misaligned sources. The spatial resolution and the size of the image are determined.

Source spectra and RMS acceleration of Mexican subduction zone earthquakes

Abstract: Strong motion (SM) data of six Mexican subduction zone earthquakes (6.4≤MS≤8.1) recorded near the epicentral zone are analyzed to estimate their far-field source acceleration spectra at higher frequencies (f≥0.3 Hz). Apart from the usual corrections such as geometrical spreading (1/R), average radiation pattern (0.6), free surface amplification (a factor of 2), and equal partitioning of the energy into two orthogonal horizontal components (a factor of 1/\(\sqrt 2 \)), the observed spectra are corrected for a frequency dependentQ(Q=100f), a site dependent filter (e−πkf), and amplification ofS waves near the surface (a factor of about 2 atf≥2Hz). We takeR as the average distance from the rupture area to the site. If we model the high frequency plateau (f≥1 Hz) of the source spectra, by a point source ω−2-model, and interpret them in terms of Brune's model we obtain δσ between 50 and 100 bars for all earthquakes. The low-frequency broadband teleseismicP wave spectra, corrected witht*=1.0 s, agrees within a factor of two with SM source spectra near 1 Hz. The ω−2-model is inadequate to explain the observed source spectra in a broad frequency range; these resemble spectra given byGusev (1983) with some differences.

Laterally Confined Flow From a Point Source at the Surface of an Inhomogeneous Soil Column1

Abstract: Solution of a linearized flow equation for steady, axisymmetric, laterally confined infiltration from a point source located at the soil surface is compared with pressure head patterns measured in an undisturbed column of sandy loam. The geometry approximates an array of trickle irrigation emitters. The hydraulic conductivity could be represented as an exponential function of both the pressure head and the depth in the column. This implies that steady, multidimensional flow in the column is described by a linear flow equation. Measured and predicted distributions of pressure head agreed most closely at an application rate of O.S cm/day. Increase in the size of a saturated zone about the point source at application rates higher than 0.5 cm/day caused isolines of pressure head to be distorted from the predicted shape. Flow patterns for homogeneous and heterogeneous soil are compared. Trickle irrigation systems are usually operated intermittently. Measured distributions of pressure heads under intermittent application were compared with steady infiltration patterns. A steadyflow solution will give an approximate prediction of intermittent pressure head patterns for continuously repeated application cycles over part of the flow region and during part of the time. Additional Index Words: water, infiltration, hydraulic conductivity, drip irrigation, trickle irrigation. Merrill, S. D., P. A. C. Raats, and C. Dirksen. 1978. Laterally confined flow from a point source at the surface of an inhomogeneous soil column. Soil Sci. Soc. Am. J. 42:851-857. D AND TRICKLE irrigation has become important in the production of many tree and vegetable crops. Drip irrigation systems usually are operated intermittently and consist of a series of point or line sources, which are usually arrayed and interacting. Salts in the irrigation water can build up between the sources. For more effective design and use of drip systems, we need to be able to predict water movement from interacting sources. Thus far, analytical solutions to equations describing unsteady, multidimensional flow in unsaturated soil have been found only when restrictive assumptions are imposed. For example, if the hydraulic conductivity is an exponential function of the pressure head and a linear function of the water content, then the equation for time-dependent, multidimensional flow is linearized, readily yielding solutions (e.g., Warrick, 1974). Numerical solutions are technically complex, and often considerably less convenient to use than analytical solutions. If a drip irrigation system is operated frequently, and water application periods are sufficiently long, the flow at 'Contribution from the U. S. Salinity Laboratory, Science and Education Administration, USDA, 4500 Glenwood Dr., Riverside, CA 92501. Received 6 Mar. 1978 Approved 26 June 1978. Formerly Physicist and formerly Soil Scientists. Current location of S. D. Merrill is Northern Great Plains Research Center, USDA-SEA, P. O. Box 459, Mandan, ND 58554; current location of P.A.C. Raats is Instituut voor Bodemvruchtbaarheid, Oosterweg 92, Haren (Gr.), The Netherlands; current location of C. Dirksen is Laboratory of Soils and Fertilizers, Agricultural State Univ. Wageningen, The Netherlands. some distance from the source approaches steady state. For steady flows, the exponential relationship between the hydraulic conductivity and the pressure head suffice to linearize the flow equation (Gardner, 1958; Raats, 1970). With increasing irrigation frequency, the range of water contents in the soil narrows (Rawlins and Raats, 1975) and the exponential conductivity relationship becomes more applicable. Specific solutions of the linearized, multidimensional, steady flow equation have been described by a number of workers. Philip (1968, 1972) and Raats (1971, 1972) gave solutions for flow from isolated point sources and cavities at arbitrary depth. Solutions for isolated line sources were discussed by Philip (1971, 1972). Raats (1970) gave solutions for an array of line sources at the soil surface. Zachman and Thomas (1973) discussed laterally confined flows from line sources at arbitrary depth. The latter geometry was also used in experimental studies by Thomas et al. (1974) without plants and by Thomas et al. (1977) with plants. Dirksen (1978) described steady and transient water content distibutions around buried, interacting line sources and compared them with theoretical distributions for steady sources. Raats (1977) gave a comprehensive set of solutions for laterally confined flows from point and line sources and to point and line sinks. The solution for axisymmetric, laterally confined flow from a point source at the soil surface is described in this paper. This type of axisymmetric flow approximates a square or hexagonal array of drip irrigation sources. An exact solution for flow from an array of point sources is approached by summation of a large number of solutions for isolated point sources, but this is awkward in derivation. The main objective of this study was to compare observed pressure head distributions under steady infiltration from a point source with the pattern predicted by theory. Measurements were made in a large undisturbed soil column, which was found to be inhomogeneous. The theory cited here assumes homogeneous, isotropic soils exhibiting no hysteresis. Hence, a second objective was to examine the influence of hydraulic conductivity variation over soil depth upon flow pattern. As the frequency of intermittent applications at a point source increases, flow approximately will become constant at shallower depths. If the amount of water applied during each cycle is the same, steady flow is approached or approximated during part of the application cycle. In view of this, a third objective of the study was to compare pressure head distributions under repeated, intermittent water application with those established by continuous application.

Beamforming with microphone arrays for directional sources.

Abstract: Beamforming is done with an array of sensors to achieve a directional or spatially-specific response by using a model of the arriving wavefront. Real acoustic sources may deviate from the conventional plane wave or monopole model, causing decreased array gain or a total breakdown of beamforming. An alternative to beamforming with the conventional source model is presented which avoids this by using a more general source model. The proposed method defines a set of “sub-beamformers,” each designed to respond to a different spatial mode of the source. The outputs of the individual sub-beamformers are combined in a weighted sum to give an overall output of better quality than that of a conventional (monopole) beamformer. It is shown that with appropriate weighting, the optimum array gain can be achieved. A simple method is demonstrated to estimate the weighted sum, based on the observed data. The variance and bias of the estimate in the presence of noise are evaluated. Simulation and experimentally measured results are shown for a simple directive source. In the experiment, the proposed method provides an array gain of about 11 dB while beamforming using a point source model achieves only −4 dB.

Erratum: ``Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. I. A Reverberation-based Measurement of the Black Hole Mass'' ( ApJ, 632, 799 [2005] )

Abstract: In the original version of this paper, we reported on a series of ultraviolet (UV) spectroscopic observations of the dwarf Seyfert 1 galaxy NGC4395,madewith the Space Telescope Imaging Spectrograph on theHubble Space Telescope (HST ). Unfortunately, a data processing error led to an incorrect flux calibration for these spectra. All STIS-based UV fluxes in the original paper are too high by a factor of 7.96 as a result of neglecting to adjust the flux-scaling algorithm from a diffuse source to a point source. Because most of the analysis in the original paper involved only relative flux changes, most of the scientific conclusions are unaffected by this correction, except for the slope of the broad-line region radius–luminosity relationship, as described below. Specific changes that result from this correction are: