Point source

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

Geometrical corrections in angular correlation measurements

Abstract: It is shown that given a point-point angular correlation, the experimentally measured angular correlation using finite-sized detectors of arbitrary shape and efficiency distributions can be obtained in terms of Legendre polynomials. The manner in which detector symmetries of various types affect the form of the measured angular correlation is discussed; in particular it is shown that if the efficiency function of both counters is invariant to reflection about both horizontal and vertical axes, the measurement angular correlation will contain no P1's of higher order than the P1 of highest order appearing in the point-point correlation. The formula for the measured angular correlation is also shown to apply if an axially-extended source instead of a point source is used, the detector coefficients simply being replaced by a new set of suitably averaged coefficients. Tables of correction factors to fourth order in detector and axial source size are included for the special cases of rectangular and circular detectors of constant efficiency.

Exact Ray Theory and its Application to the Reflection of Elastic Waves from Vertically Inhomogeneous Media

Abstract: Summary The possibilities and limitations of the theoretical investigation of elastic waves in horizontally layered media with the aid of the exact ray theory are studied. In order to simplify the problem as far as possible, it is restricted to the propagation of sound waves from a line source in a layered-liquid medium. Three different types of the transition between two homogeneous liquids are investigated: a first-order discontinuity, a transition consisting of a prescribed number of homogeneous layers, and a transition layer with linear variation of sound velocity and density with depth. Theoretical seismograms are computed by exact formulas and by wave front approximations for the reflected waves in these models. From these results, the main conclusions about the applicability of the exact ray theory to studies of wave propagation in more complicated media are as follows: (1) The exact ray theory is applicable to the investigation of vertical and near-vertical reflections from inhomogeneous media without difficulties; and (2) continuously refracted waves in such media can be investigated by an approximation which is sufficient for many practical cases. The final aim of this study was to test whether the exact ray theory can be used to compute theoretical seismograms for seismological applications. Therefore, some approximations were developed which are useful for computations for realistic models of the Earth's crust and upper mantle. An Earth-flattening approximation is given which accounts for the influence of the Earth's curvature on the propagation of body waves, and the approximate computation of point source seismograms from line source seismograms is described. An example of theoretical seismograms is presented for an upper mantle model, published by Julian & Anderson (1968).

Hamiltonian Monte Carlo Inversion of Seismic Sources in Complex Media.

Abstract: We present a probabilistic seismic point source inversion, taking into account 3-D heterogeneous Earth structure. Our method rests on (1) reciprocity and numerical wavefield simulations in complex media and (2) Hamiltonian Monte Carlo sampling that requires only a small amount of test models to provide reliable uncertainty information on the timing, location, and mechanism of the source. Using spectral element simulations of 3-D, viscoelastic, anisotropic wave propagation, we precompute receiver side strain tensors in time and space. This enables the fast computation of synthetic seismograms for any hypothetical source within the volume of interest, and thus a Bayesian solution of the inverse problem. To improve efficiency, we developed a variant of Hamiltonian Monte Carlo sampling. Taking advantage of easily computable derivatives, numerical examples indicate that Hamiltonian Monte Carlo can converge to the posterior probability density with orders of magnitude less samples than the derivative-free Metropolis-Hastings algorithm, which we use for benchmarking. Exact numbers depend on observational errors and the quality of the prior. We apply our method to the Japanese Islands region where we previously constrained 3-D structure of the crust and upper mantle using full-waveform inversion with a minimum period of 15 s.

Refinements to the geometry factor used in the AAPM Task Group Report No. 43 necessary for brachytherapy dosimetry calculations

Abstract: Determination of the geometry factor is necessary for brachytherapydosimetry calculations as recommended by the AAPM Task Group No. 43 (TG-43). The equivalence and errors associated with use of a point source approximation for an extended line segment source are examined. For all angles, the error using the point source approximation is less than 2% for distances in which the ratio of radius to active source length, (r/L), exceed about 3.6. A novel approach to determining the geometry factor using Monte Carlo methods is discussed in which the particle flux emanates from the active source and streams with no interactions occurring within the source or phantom. This method was performed for determining the geometry factor along the transverse axis for six brachytherapy sources. Differences in the geometry factor exceeding 2% between the point source approximation and that obtained using Monte Carlo methods occurred at distances ranging from 0.5 to 5 mm from the source center along the transverse plane. The merits of the Monte Carlo approach for solving the geometry factor are discussed in light of using a point or line source approximation for calculating additional brachytherapydosimetry parameters.

Solar wind motion within 30 R solar masses - Spacecraft radio scintillation observations

Abstract: New remote-sensing observations are reported of the solar wind motion within about 30 earth radii. Use is made of the interplanetary scintillation (IPS) spaced receiver technique with the radio source being a spacecraft signal (rather than a natural radio source as in previous spaced receiver studies). The spacecraft used are Helios A and B and the Viking orbiters. The purposes of the study are (1) to augment the scarce estimates of solar wind bulk flow speed near the sun and in the ecliptic with measurements made using spacecraft signals, and (2) to estimate random velocity components and identify the region where the random velocity is a significant fraction of the mean velocity. In addition, the radial evolution of speed and random velocity is compared with that of the plasma density fluctuation spectrum. Also reported are the first accurately normalized IPS scintillation index measurements using a monochromatic point source.